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Well over 100 diagnostics can operate on the National Ignition Facility (NIF) as a result of several decades of development on NIF, and before that on Nova, OMEGA, and earlier LLNL lasers. A subset of these have guided the approach to achieving ignition on the NIF in 2022 [H. Abu-Shawareb et al. (Indirect Drive ICF Collaboration), Phys. Rev. Lett. 129(7), 075001 (2022)]. Achieving ignition on NIF has required many types of experiments with this core set of diagnostics, some constraining known unknowns and some revealing surprises—arguably unknown unknowns. Early design work realized that the extreme precision required for ignition on NIF would require fine-tuning by experiment, that is, measuring and adjusting known unknowns. Many examples are given where the use of the core set of ignition diagnostics in experimental arrangements called platforms demonstrated control of the key theoretical parameters defined as shape, adiabat, velocity, and mix. The direction of the adjustments to input conditions is found either by trend analysis or, in many cases, by observing from the diagnostic data the direction to make an adjustment. In addition, diagnostics have revealed some unexpected or neglected known issues, which degrade performance, or unexpected issues, unknown unknowns. Some of these factors had been previously considered, but underestimated or difficult to calculate at the time. The overall methodology can be described as a variant of Popper's falsifiability philosophy [K. Popper, The Logic of Scientific Discovery (Hutchinson, 1974)]. This paper summarizes the role of ignition diagnostics in terms of falsification or validation of theory or experimental setup as well as uncovering unexpected issues. The journey to ignition started in the seventies with a 1-µm wavelength laser producing disastrous results. Diagnostics have guided us to the recent multi-decadal goal of demonstrating ignition and burn in the laboratory. [ABSTRACT FROM AUTHOR]

Collimation control of energetic plasma beams is crucial in the laser–plasma field. In this paper, we report on a self-collimated acceleration scheme for a plasma beam using an ultra-intense Laguerre–Gaussian (LG) laser irradiating a solid target. Three-dimensional (3D) particle-in-cell simulations show that a plasma beam with a high current density is stably formed by the radiation pressure of the hollow LG laser. The initial interaction of LG laser with solid target can be approximately researched by a deformable mirror model. Under the effect of the ponderomotive force of the LG laser, the plasma converges in the center axis to form a narrow beam. An elongated strong-magnetic tunnel (B ∼ 2 kT) is self-generated around the plasma beam, capable of trapping some electrons in a region with a radius of less than 500 nm (r < 500 nm). Compared with the case driven by the conventional Gaussian laser, the beam radius size is dramatically reduced from the microscale to hundreds on the nanoscale. The beam density is increased by at least ten times. Such an interesting scheme can provide a feasible and efficient way to achieve and enhance the collimation of energetic particle beams, which may benefit the general applications of fast ignition in inertial fusion, radiotherapy, realization of high-energy density states, and so on. [ABSTRACT FROM AUTHOR]

Quantum electrodynamic (QED) plasmas, describing the intricate interplay of strong-field QED and collective pair plasma effects, play pivotal roles in astrophysical settings like those near black holes or magnetars. However, the creation of observable QED plasmas in laboratory conditions was thought to require ultra-intense lasers beyond the capabilities of existing technologies, hindering experimental verification of QED plasma theories. This paper provides a comprehensive review of recent studies outlining a viable approach to create and detect observable QED plasmas by combining existing electron beam facilities with state-of-the-art lasers. The collision between a high-density 30 GeV electron beam and a 3 PW laser initiates a QED cascade, resulting in a pair plasma with increasing density and decreasing energy. These conditions contribute to a higher plasma frequency, enabling the observation of ∼ 0.2 % laser frequency upshift. This solution of the joint production-observation problem should facilitate the near-term construction of ultra-intense laser facilities both to access and to observe the realm of strong-field QED plasmas. [ABSTRACT FROM AUTHOR]

This paper presents spatially resolved laser Thomson scattering measurements in a cathode presheath and describes a new approximation for the electric field at the plasma-sheath boundary that accounts for collisional and ionization effects. The approximation is derived from a 1D sheath model using asymptotic theory, and the approximation is validated against the exact solution of the 1D model. The approximation of the electric field at the plasma-sheath boundary is examined with experimentally measured profiles of electron properties in the cathode presheath and negative glow of a glow discharge. The measurements are made with a noninvasive laser Thomson scattering system applied to a 20 kV pulsed plasma with an electrode gap of 26.53 mm at neutral pressures of 2, 2.5, and 25 Torr for argon, krypton, and helium, respectively. Peak electron densities for the He, Ar, and Kr plasmas are around 9 × 1018, 2.4 × 1019, and 7.5 × 1019 m−3, respectively. For all the gases, the electrons in the negative glow are approximately isothermal at electron temperatures between 1 and 2 eV, and for He and Ar, the electrons were not isothermal in the cathode presheath. The electron density profile in the cathode presheath was nonmonotonic, and calculations of the ionization rate indicate that a sharp increase in the ionization rate may produce a nonmonotonic density profile in the cathode presheath. The insights gained from spatially-resolved noninvasive measurements of electron properties in a cathode sheath reveal the need for more detailed cathode sheath models. [ABSTRACT FROM AUTHOR]

Direct laser acceleration of electrons is an important energy deposition mechanism for laser-irradiated plasmas that is particularly effective at relativistic laser intensities in the presence of quasi-static laser-driven plasma electric and magnetic fields. These radial electric and azimuthal magnetic fields provide transverse electron confinement by inducing betatron oscillations of forward-moving electrons undergoing laser acceleration. Electrons are said to experience a betatron resonance when the frequency of betatron oscillations matches the average frequency of the laser field oscillations at the electron position. In this paper, we show that the modulation of the laser frequency as seen by an electron performing betatron oscillations can be another important mechanism for net energy gain that is qualitatively different from the betatron resonance. Specifically, we show that the frequency modulation experienced by the electron can lead to net energy gain in the regime where the laser field performs three oscillations per betatron oscillation. There is no net energy gain in this regime without the modulation because the energy gain is fully compensated by the energy loss. The modulation slows down the laser oscillation near transverse stopping points, increasing the time interval during which the electron gains energy and making it possible to achieve net energy gain. [ABSTRACT FROM AUTHOR]

Strongly magnetizing a plasma adds a range of waves that do not exist in unmagnetized plasmas and enlarges the laser-plasma interaction (LPI) landscape. In this paper, we use particle-in-cell simulations to investigate strongly magnetized LPI in one dimension under conditions relevant for magneto-inertial fusion experiments, focusing on a regime where the electron-cyclotron frequency is greater than the plasma frequency and the magnetic field is at an oblique angle with respect to the wave vectors. We show that when electron-cyclotron-like hybrid wave frequency is about half the laser frequency, the laser light resonantly decays to magnetized plasma waves via primary and secondary instabilities with large growth rates. These distinct magnetic-field-controlled instabilities, which we collectively call two-magnon decays, are analogous to two-plasmon decays in unmagnetized plasmas. Since additional phase mixing mechanisms are introduced by the oblique magnetic field, collisionless damping of large-amplitude magnetized waves substantially broadens the electron distribution function, especially along the direction of the magnetic field. During this process, energy is transferred efficiently from the laser to plasma waves and then to electrons, leading to a large overall absorptivity when strong resonances are present. The enhanced laser energy absorption may explain hotter-than-expected temperatures observed in magnetized laser implosion experiments and may also be exploited to develop more efficient laser-driven x-ray sources. [ABSTRACT FROM AUTHOR]

In direct-drive inertial confinement fusion, laser imprint can cause areal density perturbations on the target shell that seed the Rayleigh–Taylor instability and further degrade the implosion. To mitigate the effect of laser imprint, a foam overcoating layer outside the target shell has been suggested to increase the thermal smoothing of the conduction region (between the ablation front and the critical density surface) and mass ablation of the ablation front. In this paper, we use a two-dimensional radiation hydrodynamic code FLASH to investigate the laser imprint mitigation performance and find other physical mechanisms of foam overcoatings. First, radiation ablation dynamically modulates density distribution not only to increase the frequency of the perturbed ablation front oscillation but also to decrease the amplitude of the oscillation. Second, a larger length of the shocked compression region reduces the amplitude of the perturbed shock front oscillation. The areal density perturbations decrease with the decrease in the perturbations of the ablation front and shock front. Based on the abovementioned physical mechanisms, we propose the optimal ranges of foam parameters to mitigate laser imprint with the aid of dimensional analysis: the foam thickness is about two to three times that of the perturbation wavelength, and the foam density is about 1/2–3/2 times that of the critical density. [ABSTRACT FROM AUTHOR]

Ultra-intense lasers are a promising source of energetic ions for various applications. An interesting approach described in Ferri et al. [Commun. Phys. 2, 40 (2019)] argues from particle-in-cell simulations that using two laser pulses of half energy (half intensity) arriving with close to 45° angle of incidence is significantly more effective at accelerating ions than one pulse at full energy (full intensity). For a variety of reasons, at the time of this writing, there has not yet been a true experimental confirmation of this enhancement. In this paper, we perform 2D particle-in-cell simulations to examine if a millijoule class, 5 × 10 18 W cm−2 peak intensity laser system could be used for such a demonstration experiment. Laser systems in this class can operate at a kHz rate which should be helpful for addressing some of the challenges of performing this experiment. Despite investigating a 3.5 times lower intensity than Ferri et al. [Commun. Phys. 2, 40 (2019)] did, we find that the double pulse approach enhances the peak proton energy and the energy conversion to protons by a factor of about three compared to a single laser pulse with the same total laser energy. We also comment on the nature of the enhancement and describe simulations that examine how the enhancement may depend on the spatial or temporal alignment of the two pulses. [ABSTRACT FROM AUTHOR]

The effects of the subwavelength nanowire target on the enhanced laser absorption, heating of electrons, and acceleration and control of energetic ions are investigated by using two-dimensional particle-in-cell simulations. Compared with the flat target, the conversion efficiency and acceleration of target normal sheath acceleration can be improved remarkably. In the condition considered in this paper, the conversion efficiency from the laser to electrons can be increased by about four times (14.74% to 65.78%), and the cutoff energy of electrons can be raised by 1.5 times. Furthermore, the cutoff energies of both protons and carbon ions are increased by almost two times. The dependence of this effect for different nanowire widths is discussed by numerical simulations. It is found that the efficiency from the laser to electrons reaches the highest value when the nanowire width is d = 0.2 μ m. The optimum width for C6+ ions is d = 0.3 μ m , while d = 0.8 μ m is better for proton acceleration. Thus, the laser absorption, electron heating, and ion acceleration could be controlled by selecting the width of subwavelength nanowires. [ABSTRACT FROM AUTHOR]

The magnetized liner inertial fusion (MagLIF) scheme relies on coupling laser energy into an underdense fuel raising the fuel adiabat at the start of the implosion. To deposit energy into the fuel, the laser must first penetrate a laser entrance hole (LEH) foil which can be a significant energy sink and introduce mix. In this paper, we report on experiments investigating laser energy coupling into MagLIF-relevant gas cell targets with LEH foil thicknesses varying from 0.5 lm to 3 lm. Two-dimensional (2D) axisymmetric simulations match the experimental results well for 0.5 lm and 1 lm thick LEH foils but exhibit whole-beam self-focusing and excessive penetration of the laser into the gas for 2 lm and 3 lm thick LEH foils. Better agreement for the 2 lm-thick foil is found when using a different thermal conductivity model in 2D simulations, while only 3D Cartesian simulations come close to matching the 3 lm-thick foil experiments. The study suggests that simulations may over-predict the tendency for the laser to self-focus during MagLIF preheat when thicker LEH foils are used. This effect is pronounced with 2D simulations where the azimuthally symmetric density channel effectively self-focuses the rays that are forced to traverse the center of the plasma. The extra degree of freedom in 3D simulations significantly reduces this effect. The experiments and simulations also suggest that, in this study, the amount of energy coupled into the gas is highly correlated with the laser propagation length regardless of the LEH foil thickness. [ABSTRACT FROM AUTHOR]

This paper details and examines the origins of radiation drive asymmetries present during the initial High Foot implosion experiments. Such asymmetries are expected to reduce the stagnation pressure and the resulting yield of these experiments by several times. Analysis of reemission and dual axis shock timing experiments indicates that a flux asymmetry, with a P2/P0 amplitude that varies from -10% to -5%, is present during the first shock of the implosion. This first shock asymmetry can be corrected through adjustments to the laser cone fraction. A thin shell model and more detailed radiation hydrodynamic calculations indicate that an additional negative P2/P0 asymmetry during the second or portions of the third shock is required to reach the observed amount of asymmetry in the shape of the ablator at peak implosion velocity. In conjunction with symmetry data from the x-ray self emission produced at stagnation, these models also indicate that after the initially negative P2/P0 flux asymmetry, the capsule experiences a positive P2/P0 flux asymmetry that develops at or before ~2 ns into the peak of the laser power. Here, direct evidence for this inference, using measurements of the x-ray emission produced by the lasers irradiating the hohlraum, is presented. These data indicate that the reduction in the transmitted inner laser cone energy results from impeded propagation through the plasma associated with the ablation of the capsule target. This paper also correlates measurements of the outer cone laser deposition location with variations in the observed x-ray self emission shape from experiments conducted with nominally the same input conditions. [ABSTRACT FROM AUTHOR]

Focusing petawatt class laser beams to a variety of spot sizes for different applications is expensive in cost, labor, and space. In this paper, we propose a plasma lens to flexibly resize the laser beam by utilizing the self-focusing effect of laser in plasmas. Using a fixed conventional focusing system to focus the laser a short distance in front of the plasma, we can adjust the effective laser beam waist within a certain range, with the plasma lens acting as an adjustable eyepiece in a telescope. Such a setup is a powerful tool for laser wakefield accelerator experiments in state-of-the-art petawatt laser projects and allows for scanning focal spot parameters. [ABSTRACT FROM AUTHOR]

Simulations predict that directly driven multi-shell targets can provide a robust alternative to conventional high-convergence implosion concepts by coupling two to three times more energy into the final igniting thermonuclear fuel assembly than indirect-drive concepts. The three-shell directly driven Revolver concept [K. Molvig, M. J. Schmitt, B. J. Albright, E. S. Dodd, N. M. Hoffman, G. H. McCall, and S. D. Ramsey, Phys. Rev. Lett. 116, 255003 (2016)] utilizes a design that maximizes laser energy conversion into inward kinetic energy of the outermost ablator shell (∼9%) while minimizing the DT fuel convergence (∼9) to reduce the mixing of material from the innermost shell into the fuel. Inherent in this design concept is the use of 192 narrow beams (with a 1/e laser beam-to-capsule diameter ratio of 0.33) from the National Ignition Facility laser pointed in a polar direct drive laser configuration. In this paper, we demonstrate that low average laser intensity at the capsule surface (≤300 TW/cm2) limits the measured laser backscatter, indicating that a greater amount of laser energy is coupled into the target. Omega experiments have been performed to determine the coupling of laser energy to the outermost shell of a scaled Revolver target (i.e., the ablator shell) by measuring capsule implosion trajectories and scattered-light fractions for two different drive configurations. Comparisons of simulated shell trajectory and velocity profiles with experimental data obtained from self-emission images show good agreement and are consistent with measured scattered light data. Moreover, the low levels of scattered light measured are consistent with post-shot simulation results that show high hydro-coupling efficiency. These results strengthen the case for using narrow beams at low intensity to drive large ablator capsules for future direct-drive, multi-shell ignition concepts. [ABSTRACT FROM AUTHOR]

III, the sound speed equation is missing a division symbol "/" and should be HT ht In the caption of Table I, the magnetization parameter I i is missing a division symbol "/" and should be HT ht The above typos did not affect any results in the published paper. Erratum: "Kinetic simulation study of magnetized collisionless shock formation on a terawatt laser system" [Phys. REFERENCES 1 Y. Zhang, J. R. Davies, P. V. Heuer, and C. Ren, " Kinetic simulation study of magnetized collisionless shock formation on a terawatt laser system", Phys. [Extracted from the article]

The hohlraum is very crucial for indirect laser driven Inertial Confinement Fusion. Usually, its shape is designed as sphere, cylinder, or rugby with some kind of fixed functions, such as ellipse or parabola. Recently, a spherical hohlraum with octahedral 6 laser entrance holes (LEHs) has been presented with high flux symmetry [Lan et al., Phys. Plasmas 21, 010704 (2014); 21, 052704 (2014)]. However, there is only one shape parameter, i.e., the hohlraum to capsule radius ratio, being optimized. In this paper, we build the hohlraum with octahedral 6LEHs with a unified freeform representation, in which, by varying additional shape parameters: (1) available hohlraum shapes can be uniformly and accurately represented, (2) it can be used to understand why the spherical hohlraum has higher flux symmetry, (3) it allows us to obtain a feasible shape design field satisfying flux symmetry constraints, and (4) a synthetically optimized hohlraum can be obtained with a tradeoff of flux symmetry and other hohlraum performance. Finally, the hohlraum with octahedral 6LEHs is modeled, analyzed, and then optimized based on the unified free-form representation. The results show that a feasible shape design field with flux asymmetry no more than 1% can be obtained, and over the feasible design field, the spherical hohlraum is validated to have the highest flux symmetry, and a synthetically optimal hohlraum can be found with closing flux symmetry but larger volume between laser spots and centrally located capsule. [ABSTRACT FROM AUTHOR]

In this paper, atmospheric laser filament initiation is simulated under an external field 5.7 kV/cm, which is far below the conventional breakdown threshold (25–30 kV/cm). The time-dependent model is based on a self-consistent solution to a coupled electron energy distribution function, vibrational kinetics, gas thermal balance, as well as detailed air chemistry. The model is validated with the experiment results on plasma density decay both with and without an external field. The slow decay of plasma density is ascribed to the decreased electron recombination and increased detachment rate under an external field. The initiation process is facilitated by the increase in the electron temperature, gas heating, and efficient vibrational excitation under the external field. We have varied the detachment rate between oxygen negative ions and nitrogen vibrational states to study its role on the initiation process, and found that larger detachment rates lead to shorter breakdown delay times. If the detachment rate is taken to be 3 × 10−11 cm3 s−1 as an upper limit, the breakdown threshold and delay time are consistent with the previous experiment study at least semiquantitatively. This work is beneficial for studying the initiation of a preformed laser filament under an external field as well as potential applications in lightning initiation and guiding below the conventional gas breakdown threshold. [ABSTRACT FROM AUTHOR]

Plasmas may be used as gain media for amplifying intense lasers, and external magnetic fields may be applied to improve the performance. For midinfrared lasers, the requisite magnetic field is on the megagauss scale, which can already be provided by current technologies. Designing the laser amplifier requires knowing the magnetized three-wave coupling coefficient, which is mapped out systematically in this paper. By numerically evaluating its formula, we demonstrate how the coupling coefficient depends on the angle of wave propagation, laser polarization, magnetic field strength, plasma temperature, and plasma density in the backscattering geometry. Since the mediation is now provided by magnetized plasma waves, the coupling can differ significantly from unmagnetized Raman and Brillouin scatterings. [ABSTRACT FROM AUTHOR]

Silver (Ag) wire arrays were recently introduced as efficient x-ray radiators and have been shown to create L-shell plasmas that have the highest electron temperature (>1.8 keV) observed on the Zebra generator so far and upwards of 30 kJ of energy output. In this paper, results of single planar wire arrays and double planar wire arrays of Ag and mixed Ag and Al that were tested on the UNR Zebra generator are presented and compared. To further understand how L-shell Ag plasma evolves in time, a time-gated x-ray spectrometer was designed and fielded, which has a spectral range of approximately 3.5-5.0Å . With this, L-shell Ag as well as cold Lα and Lβ Ag lines was captured and analyzed along with photoconducting diode (PCD) signals (>0.8 keV). Along with PCD signals, other signals, such as filtered XRD (>0.2 keV) and Si-diodes (SiD) (>9 keV), are analyzed covering a broad range of energies from a few eV to greater than 53 keV. The observation and analysis of cold Lα and Lβ lines show possible correlations with electron beams and SiD signals. Recently, an interesting issue regarding these Ag plasmas is whether lasing occurs in the Ne-like soft x-ray range, and if so, at what gains? To help answer this question, a non-local thermodynamic equilibrium (LTE) kinetic model was utilized to calculate theoretical lasing gains. It is shown that the Ag L-shell plasma conditions produced on the Zebra generator at 1.7 maximum current may be adequate to produce gains as high as 6cm-1 for various 3p → 3s transitions. Other potential lasing transitions, including higher Rydberg states, are also included in detail. The overall importance of Ag wire arrays and plasmas is discussed. [ABSTRACT FROM AUTHOR]

In this paper, the effect of the wiggler magnetic field strength on the self-focusing of an intense laser pulse propagating through a magnetized non-Maxwellian plasma is studied. By using a relativistic fluid model, a non-linear equation describing the spot-size of the laser pulse for right- and left-hand polarizations has been derived. The evaluation of the spot-size in the self-focusing of laser pulse with the respect to the kind of polarization is investigated. Besides, the effects of wiggler magnetic field and a non-Maxwellian distribution of charge density on the evolution of the laser spot-size have been discussed. The results show that in the right-hand polarization, the existence of superthermal particles and wiggler magnetized field simultaneously cause improvement in the self-focusing quality. In addition, for the left-hand polarization, the self-focusing in the presence of wiggler field increases with an increase in the ki. Furthermore, the self-focusing decreases with an increase in the wiggler amplitudes for the left-hand polarization. Moreover, it is observed that an increase in the normalized laser frequency leads to an increase in the variation in the laser spot-size, and then the laser pulse becomes more focused. [ABSTRACT FROM AUTHOR]

Intermittent convective-like plasma transport associated with mesoscale coherent structures extended along the magnetic field lines (“blobs”) is often dominant at the edge of tokamaks, stellarators, and linear devices. Blobs can travel a large distance toward the wall (∼10 cm and larger) and strongly enhance both edge plasma energy and particle transport and plasma-wall interactions. The dynamics of blobs and blob generation mechanisms are discussed in this paper. [ABSTRACT FROM AUTHOR]

This paper presents a computational study of the impact of preheating, in advance of shock heating, on a structured interface and on the subsequent postshock instability evolution. The study was performed by applying a method, described previously, of evaluating radiative effects using a multidimensional, front-tracking hydrodynamic code with input from a one-dimensional, radiation-hydrodynamic code. The method is general and could be applied to a range of laser-driven shock experiments. Results of simulations are shown for both high and low levels of preheat, conducted using a robust front-tracking algorithm in the presence of a radiation energy source. In the low-preheat case, which represents the minimum to be anticipated in laboratory experiments, some impact of preheat on both preshock conditions and postshock evolution are observed. In the high-preheat case, which represents one potential result of preheating by increased radiation and/or energetic electrons, the preheat alters the spectral content of the interface structure. In this case, before the shock reaches the interface, higher-order harmonic modes are induced, the interface position is shifted, and the perturbation amplitude is reduced. Furthermore, the postshock evolution of the interface is affected by the amount of preheat and by whether radiative heating after the laser pulse is also included. Such a numerical assessment of preheating can be important to the design and analysis of laboratory experiments. The initial conditions for the interaction of any shock wave with structures in the target may be altered by the presence of preheating. This poses a challenge to the laser experimental study of fluid mixing. Numerical simulations can serve as a useful tool to guide decisions regarding control and/or measurement of this effect. [ABSTRACT FROM AUTHOR]

In the fast ignition (FI) scheme, at first, high-density fuel core plasma is assembled by implosion laser, and is then heated by petawatt laser to achieve a fusion burning condition. The formation of high-density fuel core plasma is one of the key issues for FI. A typical target for FI is a shell fitted with a reentrant gold cone to make a pass for heating laser. The ablated plasma of gold cone interferes with the implosion dynamics, which is quite different from that of the conventional central-hot-spot approach. Therefore, the dynamics of a nonspherical implosion must be controlled to assemble high density and high areal density. Numerical simulations are performed to study radiation hydrodynamics of cone-guided implosions. In the results, the effect of the cone on implosion dynamics is clarified. The cone surface is irradiated by the radiation and ablated plasma affects the imploding shell. Coating on the cone, which tamps the gold plasma, is effective to improve the implosion performance, although the result does not satisfy the condition of core plasma for the first stage of the Fast Ignition Realization Experiment [K. Mima et al., Proceedings of the IAEA Fusion Energy Conference, Lyon, 2002 (IAEA, Vienna, 2002), Paper No. IAEA-CN-94/IF/03]. [ABSTRACT FROM AUTHOR]

In this paper, new results on radiative shocks generated by a high power laser in a xenon gas cell are presented. Several shock parameters were measured: temperature, radial expansion and velocity, as well as the electron density in the radiative precursor and its velocity. Multiple laser shot allowed the investigation of physical trends changing initial conditions (laser energy and initial gas pressure). Results are compared with one- and two-dimensional radiative hydrodynamic simulations. The experiments were carried out at the LULI Laboratory. [ABSTRACT FROM AUTHOR]

In this paper the generation of electrostatic wake field and electron acceleration in this field by intense laser pulse propagating in plasma in the presence of external magnetic field is investigated. It is shown that if the change in the laser pulse wave amplitude (approximation of the constant laser pulse wave field) is not taken into account, it is possible by switching to Lagrangian variables to reduce the system of equations describing excitation of the wake field and electron acceleration to a simplified system of ordinary differential equations (ODE), which can be easily integrated. This makes it possible to find the energy, transferred to electron oscillations in the wake field for different angles between the direction of laser pulse propagation and magnetic field. Stability of the laser pulse propagating with arbitrary angle to external magnetic field towards excitation of parametric and decay instabilities with involvement of upper hybrid and lower hybrid waves is investigated, as well. Equations describing these instabilities are derived and growth rates are found analytically for a small amplitude wake field and with the help of numerical calculations in the case of the finite wake field amplitude. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In this paper, we describe the development of electron beam pumped KrF lasers for inertial fusion energy. KrF lasers are an attractive driver for fusion, on account of their demonstrated very high beam quality, which is essential for reducing imprint in direct drive targets; their short wavelength (248 nm), which mitigates the growth of plasma instabilities; and their modular architecture, which reduces development costs. In this paper we present a basic overview of KrF laser technology as well as current research and development in three key areas: electron beam stability and transport; KrF kinetics and laser propagation; and pulsed power. The work will be cast in context of the two KrF lasers at the Naval Research Laboratory, The Nike Laser (5 kJ, single shot), and The Electra Laser (400-700 J repetitively pulsed). [ABSTRACT FROM AUTHOR]

Advanced z-pinch accelerators require precise timing of multiple mega-ampere drivers to deliver terawatt power. The triggering of these drivers is now largely initiated by laser ionization of gas switches. In this paper, we discuss detailed fully kinetic simulation of the Z laser-triggered gas switch involving detailed finite-difference time-domain particle-in-cell Monte Carlo modeling of the trigger section of the switch. Other components of the accelerator from the Marx bank through the pulse-forming line are described as circuit elements. The simulations presented here build on a recently developed model of electro-negative gas breakdown and streamer propagation that included photons produced from de-excited neutrals. New effects include multi-photon ionization of the gas in a prescribed laser field. The simulations show the sensitivity of triggering to laser parameters including focal plane within the anode-cathode gap of the trigger section of the switch, intensity at focus, and laser pulse length. Detailed electromagnetic simulations of the trigger section with circuit modeling of the upstream and downstream components are largely in agreement with Z data and demonstrate a new capability. [ABSTRACT FROM AUTHOR]

The Dublin City University (DCU) laser ion source (LIS) is a 'compact high-pressure' laser ion source utilizing a table top Q-switched laser. The DCU-LIS combines high laser fluence (F>4 kJ cm-2), high laser intensity (I>1011 W cm-2) with a short field free region (L=48 mm) and high source potential (Vext>40 kV) in order to offset recombination losses within the plasma and maximize the proportion of highly charged ions which are extracted from the plasma plume. Such a configuration also provides high peak currents (Ip>3 mA), high current densities (J>5 mA cm-2), and high charge states (Cu6+) in the extracted ion-bunch train. However, to obtain and utilize these parameter values in a high pressure LIS requires characterization and control of a number of processes related to ion dynamics and space charge effects on the extracted ions at the plasma plume-anode-extraction gap interface. Relevant issues include electric field distortion, Debye shielding, beam divergence, overfocusing, and perveance (P) in addition to current density profiles for the extracted ion beam. In this paper we focus on these issues and their impact on charge particle extraction and acceleration with a view to elucidating the parameter regimes within which the DCU-LIS performance envelope is optimal. [ABSTRACT FROM AUTHOR]

New advances in fabrication of low-density high-Z-doped foams have opened new windows on understanding how materials that are not in local thermodynamic equilibrium (LTE) are heated and radiate. Simulations are discussed in this paper of the x-ray spectral emissions from laser-irradiated very low-density Ge-doped silica aerogel targets using a two-dimensional radiation-hydrodynamics code incorporating a modern non-LTE superconfiguration atomic model. Details of the computational model are presented, and it is shown that, for the long-scale-length, subcritical-density, ∼2–3 keV electron temperature plasmas created in experiments at the Omega laser facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)], the simulations provide a close match to both the measured Ge L-shell emission (∼1–1.5 keV) and the measured Ge K-shell emission (∼10–11 keV), but only by accounting properly for nonlocal thermal conduction. The older average-atom atomic model is shown to be inadequate for these non-LTE plasmas. [ABSTRACT FROM AUTHOR]

The development of new laser systems at the 10 Petawatt range will push laser wakefield accelerators to novel regimes, for which theoretical scalings predict the possibility to accelerate electron bunches up to tens of GeVs in meter-scale plasmas. Numerical simulations will play a crucial role in testing, probing, and optimizing the physical parameters and the setup of future experiments. Fully kinetic simulations are computationally very demanding, pushing the limits of today’s supercomputers. In this paper, the recent developments in the OSIRIS framework [R. A. Fonseca et al., Lect. Notes Comput. Sci. 2331, 342 (2002)] are described, in particular the boosted frame scheme, which leads to a dramatic change in the computational resources required to model laser wakefield accelerators. Results from one-to-one modeling of the next generation of laser systems are discussed, including the confirmation of electron bunch acceleration to the energy frontier. [ABSTRACT FROM AUTHOR]

Rugby-shaped hohlraums have been proposed as a method for x-ray drive enhancement for indirectly driven capsule implosions. This concept has recently been tested in a series of shots on the OMEGA laser facility [T. R. Boehly, D. L. Brown, R. S. Craxton et al., Opt. Commun. 133, 495 (1997)]. In this paper, experimental results are presented comparing the performance of D2-filled capsules between standard cylindrical Au hohlraums and rugby-shaped hohlraums. The rugby hohlraums demonstrated 18% more x-ray drive energy as compared with the cylinders, and the high-performance design of these implosions (both cylinder and rugby) also provided ≈20× more deuterium (DD) neutrons than any previous indirectly driven campaign on OMEGA and ≈3× more than ever achieved on NOVA [E. M. Campbell, Laser Part. Beams 9, 209 (1991)] implosions driven with nearly twice the laser energy. This increase in performance enables, for the first time, a measurement of the neutron burn history and imaging of the neutron core shapes in an indirectly driven implosion. Previous DD neutron yields had been too low to register this key measurement of capsule performance and the effects of dynamic mix. A wealth of additional data on the fuel areal density from the suite of charged particle diagnostics was obtained on a subset of the shots that used D 3He rather than D2 fuel. Comparisons of the experimental results with numerical simulations are shown to be in very good agreement. The design techniques employed in this campaign, e.g., smaller laser entrance holes and hohlraum case-to-capsule ratios, provide added confidence in the pursuit of ignition on the National Ignition Facility [J. D. Lindl, P. Amendt, R. L. Berger et al., Phys. Plasmas 11, 339 (2004)]. [ABSTRACT FROM AUTHOR]

This paper reports an experimental study on preheating of laser-irradiated targets. We performed temperature measurements at the rear surface of laser-irradiated targets under conditions of two different laser wavelengths (0.35 or 0.53 μm) and several intensities (2×1013–1×1014 W/cm2) in order to verify an effect of radiation and nonlocal electron heat transport. The preheating temperature was evaluated by observing self-emission, reflectivity, and expansion velocity at the rear surface of planar polyimide foils. The experimental results show that the x-ray radiation is dominant for preheating for 0.35-μm laser irradiation, but contribution of nonlocal electron heat transport is not negligible for 0.53-μm laser irradiation conditions. [ABSTRACT FROM AUTHOR]

Inertial confinement fusion targets must be carefully designed to ignite their central hot spots and burn. Changes in the optimal implosion could reduce the fusion energy or even prevent ignition. Since there are unavoidable uncertainties due to technological defects and not perfect reproducibility from shot to shot, the fusion energy will remain uncertain. The degree with which a target can tolerate larger specifications than specified, and the probability with which a particular yield is exceeded, are possible measures of the robustness of that design. This robustness must be assessed in a very high-dimensional parameter space whose variables include every characteristics of the given target and of the associated laser pulse shape, using high-fidelity simulations. Therefore, these studies would remain computationally very intensive. In this paper we propose an approach which consist first of constructing an accurate metamodel of the yield on the whole parameter space with a reasonable data set of simulations. Then the robustness is very quickly assessed for any set of specifications with this surrogate. The yield is approximated by a neural network, and an iterative method adds new points in the data set by means of D-optimal experimental designs. The robustness study of the baseline Laser Mégajoule target against one-dimensional defects illustrates this approach. A set of 2000 simulations is sufficient to metamodel the fusion energy on a large 22-dimensional parameter space around the nominal point. Furthermore, a metamodel of the robustness margin against all specifications has been obtained, providing guidance for target fabrication research and development. [ABSTRACT FROM AUTHOR]

This paper presents a computational and theoretical study of the yields for the 12C(p,γ)13N and 12C(d,n)13N nucleosynthesis reactions driven by high-energy (MeV) Coulomb explosion (CE) of (CH4)n and (CD4)n nanodroplets (n=104–107, radii R0=100–500 Å) in ultraintense near-infrared Gaussian laser fields (peak intensities IM=1018–1019 W cm-2, pulse length τ=25 fs). Efficient nucleosynthesis with 12C nuclei requires the production of protons and deuterons in the energy range of 0.5–2.0 MeV, which is accomplished by CE of nanodroplets. The energy distributions of the nuclei were obtained from scaled molecular dynamics simulations and were utilized to calculate the energy averaged cross sections and the reaction yields for nucleosynthesis. Two distinct reaction modes were established pertaining to reactions inside the plasma filament volume (IF), and to reactions outside the plasma filament volume (OF). The dominating contribution to the total yields originates from the OF mode. The time resolved γ-ray emission for 12C(p,γ)13N will be exhibited in distinct time intervals of 2–100 ps for the OF mode and of 65 fs–3 ps for the IF mode. Estimates of the yields were provided by incorporating the effects of nanodroplet size distributions that result in a slow size dependence of the yields in the range R0>250 Å, and the effects of laser intensity attenuation in an assembly of nanodroplets that result in the decrease of the OF and IF yields due to the decrease of the effective plasma filament volume. For 400 Å nanodroplets at IM=4×1018–1019 W cm-2, under realistic experimental conditions, the 12C(p,γ)13N nucleosynthesis driven by CE of (CH4)n nanodroplets is characterized by a yield (per laser pulse) of ∼100, while the yield for the 12C(d,n)13N nucleosynthesis driven by CE of (CD4)n clusters is ∼9×104. Table-top nucleosynthesis driven by nanodroplet CE is amenable to experimental observation. [ABSTRACT FROM AUTHOR]

A multiwave interaction formulation of coaxial Bragg structure with either one or both of the conductors sinusoidally corrugated is presented to describe all the forward and backward waves of various propagating modes within the structure. The validity of the formulation is examined in terms of the reported experiments, and good agreement of the theoretical results with the experimental measurements is demonstrated. Comparison of the present formulation with the previous two-wave interaction treatment shows substantial difference, and confirms the significance of the multiwave interaction formulation presented in this paper. Based on the multiwave interaction formulation, interesting information is revealed that a higher-order mode (such as TE6,1) operation at a frequency of hundreds of gigahertz in a coaxial Bragg reflector can be achieved due to the suppression of spurious modes. This peculiarity provides potential application in constructing a high-Q coaxial Bragg resonator for a high-power cyclotron autoresonance maser or a free-electron laser oscillator in the millimeter and submillimeter wave ranges. [ABSTRACT FROM AUTHOR]

Recent experimental results demonstrate that the forming of plasma jets is a fundamental process accompanying the laser-produced plasma expansion, if a massive planar target with relatively high atomic number is irradiated by a defocused laser beam. In this paper some new results on the influence of target irradiation conditions on plasma jet parameters are presented. The experiment was carried out at the Prague Asterix Laser System (PALS) iodine laser [K. Jungwirth, A. Cejnarova, L. Juha, B. Kralikova, J. Krasa, E. Krousky, P. Krupickova, L. Laska, K. Masek, A. Prag, O. Renner, K. Rohlena, B. Rus, J. Skala, P. Straka, and J. Ullschmied, Phys. Plasmas 8, 2495 (2001)]. with the third harmonic beam of the pulse duration of 250 ps. The beam energies varied in the range of 13–160 J. The planar massive targets used in the experiment were made of copper. For measurements of the electron density evolution a three frame interferometric system was employed. The jets were produced in the whole range of the laser energy used. Calculations of the efficiency of the plasma jet production show that it decreases with increasing the laser energy. [ABSTRACT FROM AUTHOR]

In the past few years collisionally pumped extreme ultraviolet (XUV) lasers utilizing a capillary discharge were demonstrated. An intense current pulse is applied to a gas-filled capillary, inducing magnetic collapse (Z pinch) and formation of a highly ionized plasma column. Usually, a small current pulse (prepulse) is applied to the gas in order to preionize it prior to the onset of the main current pulse. In this paper we investigate the effects of the prepulse on a capillary discharge Ne-like Ar XUV laser (46.9 nm). The importance of the prepulse in achieving suitable initial conditions of the gas column and preventing instabilities during the collapse is demonstrated. Furthermore, measurements of the amplified spontaneous emission (ASE) properties (intensity and duration) in different prepulse currents revealed unexpected sensitivity. Increasing the prepulse current by a factor of 2 caused the ASE intensity to decrease by an order of magnitude and to nearly disappear. This effect is accompanied by a slight increase in the lasing duration. We attribute this effect to axial flow in the gas during the prepulse. [ABSTRACT FROM AUTHOR]

Nonlinear interactions between ultraintense laser pulses and plasmas reveals a great variety of important collective phenomena. Advances in laser technology have made possible compact terawatt lasers capable of generating subfemtosecond pulses at ultrahigh intensities. These ultrahigh intensities result in highly relativistic nonlinear electron dynamics. This paper addresses stationary stage of the interaction of an ultraintense short laser pulse with a plasma including finite spot size effects. The study is based on the approach that accounts for the transversal effects in an average manner. Following the standard method, a reduced wave equation for the slowly varying laser envelope is derived, including nonlinear effects associated with relativistic electron mass variation and relativistic ponderomotive force. Localized and periodic solutions of the envelope equation are presented and compared with the case when the transversal distribution is infinite. It is found that the finite transverse size of laser spot has significant effects on the propagation characteristics of the stationary profiles. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In ongoing experiments performed on the OMEGA laser [J. M. Soures et al., Phys. Plasmas 5, 2108 (1996)] at the University of Rochester Laboratory for Laser Energetics, nanosecond laser pulses are used to drive strong blast waves into two-layer targets. Perturbations on the interface between the two materials are unstable to the Richtmyer–Meshkov instability as a result of shock transit and the Rayleigh–Taylor instability during the deceleration-phase behind the shock front. These experiments are designed to produce a strongly shocked interface whose evolution is a scaled version of the unstable hydrogen–helium interface in core-collapse supernovae such as SN 1987A. The ultimate goal of this research is to develop an understanding of the effect of hydrodynamic instabilities and the resulting transition to turbulence on supernovae observables that remain as yet unexplained. The authors are, at present, particularly interested in the development of the Rayleigh–Taylor instability through the late nonlinear stage, the transition to turbulence, and the subsequent transport of material within the turbulent region. In this paper, the results of numerical simulations of two-dimensional (2D) single and multimode experiments are presented. These simulations are run using the 2D Arbitrary Lagrangian Eulerian radiation hydrodynamics code CALE [R. T. Barton, Numerical Astrophysics (Jones and Bartlett, Boston, 1985)]. The simulation results are shown to compare well with experimental radiography. A buoyancy-drag model captures the behavior of the single-mode interface, but gives only partial agreement in the multimode cases. The Richtmyer–Meshkov and target decompression contributions to the perturbation growth are both estimated and shown to be significant. Significant dependence of the simulation results on the material equation of state is demonstrated, and the prospect of continuing the experiments to conclusively demonstrate the transition to turbulence is discussed. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The 1990 National Academy of Science final report of its review of the Inertial Confinement Fusion Program recommended completion of a series of target physics objectives on the 10-beam Nova laser at the Lawrence Livermore National Laboratory as the highest-priority prerequisite for proceeding with construction of an ignition-scale laser facility, now called the National Ignition Facility (NIF). These objectives were chosen to demonstrate that there was sufficient understanding of the physics of ignition targets that the laser requirements for laboratory ignition could be accurately specified. This research on Nova, as well as additional research on the Omega laser at the University of Rochester, is the subject of this review. The objectives of the U.S. indirect-drive target physics program have been to experimentally demonstrate and predictively model hohlraum characteristics, as well as capsule performance in targets that have been scaled in key physics variables from NIF targets. To address the hohlraum and hydrodynamic constraints on indirect-drive ignition, the target physics program was divided into the Hohlraum and Laser–Plasma Physics (HLP) program and the Hydrodynamically Equivalent Physics (HEP) program. The HLP program addresses laser–plasma coupling, x-ray generation and transport, and the development of energy-efficient hohlraums that provide the appropriate spectral, temporal, and spatial x-ray drive. The HEP experiments address the issues of hydrodynamic instability and mix, as well as the effects of flux asymmetry on capsules that are scaled as closely as possible to ignition capsules (hydrodynamic equivalence). The HEP program also addresses other capsule physics issues associated with ignition, such as energy gain and energy loss to the fuel during implosion in the absence of alpha-particle deposition. The results from the Nova and Omega experiments approach the NIF requirements for most of the important ignition capsule parameters, including drive temperature, drive symmetry, and hydrodynamic instability. This paper starts with a review of the NIF target designs that have formed the motivation for the goals of the target physics program. Following that are theoretical and experimental results from Nova and Omega relevant to the requirements of those targets. Some elements of this work were covered in a 1995 review of indirect-drive [J. D. Lindl, “Development of the indirect-drive approach to inertial confinement fusion and the target physics basis for ignition and gain,” Phys. Plasmas 2, 3933 (1995)]. In order to present as complete a picture as possible of the research that has been carried out on indirect drive, key elements of that earlier review are also covered here, along with a review of work carried out since 1995. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Table-top capillary discharge soft x-ray lasers combine the advantages of a small size and a high repetition rate with an extremely high brightness similar to that of their laboratory-size predecessors. When utilized to probe high density plasmas their short wavelength results in a higher critical density, reduced refraction, decreased free-electron absorption, and higher resolution as compared to optical probes. These characteristics allow the design of experiments capable of measuring the evolution of plasmas with density-scale length products that are outside the reach of optical lasers. This paper reviews the use of a 46.9 nm wavelength Ne-like Ar capillary discharge table-top laser in dense plasma diagnostics, and reports soft x-ray laser interferometry results of spot-focus Nd:YAG laser plasmas created at moderate irradiation intensity (∼7 × 10[SUP12] W cm[SUP-2]) with ∼13 ns pulse width duration laser pulses. The measurements produced electron density maps with densities up to 0.9 × 10[SUP21] cm[SUP-3] that show the development of a concave electron density profile that differ significantly from those of a classical expansion. This two-dimensional behavior, that was recently also observed in line-focus plasmas, is analyzed here for the case of spot-focus plasmas with the assistance of hydrodynamic model simulations. The results demonstrate the use of a table-top soft x-ray laser interferometer as a new high resolution tool for the study of high density plasma phenomena and the validation of hydrodynamic codes. [ABSTRACT FROM AUTHOR]

As tabletop lasers continue to reach record levels of peak power, the interaction of light with matter has crossed a new threshold, in which plasma electrons at the laser focus oscillate at relativistic velocities. The highest forces ever exerted by light have been used to accelerate beams of electrons and protons to energies of a million volts in distances of only microns. Not only is this acceleration gradient up to a thousand times greater than in radio-frequency-based sources, but the transverse emittance of the particle beams is comparable or lower. Additionally, laser-based accelerators have been demonstrated to work at a repetition rate of 10 Hz, an improvement of a factor of 1000 over their best performance of just a couple of years ago. Anticipated improvements in energy spread may allow these novel compact laser-based radiation sources to be useful someday for cancer radiotherapy and as injectors into conventional accelerators, which are critical tools for x-ray and nuclear physics research. They might also be used as a spark to ignite controlled thermonuclear fusion. The ultrashort pulse duration of these particle bursts and the x rays they can produce, hold great promise as well to resolve chemical, biological or physical reactions on ultrafast (femtosecond) time scales and on the spatial scale of atoms. Even laser-accelerated protons are soon expected to become relativistic. The dense electron–positron plasmas and vast array of nuclear reactions predicted to occur in this case might even help bring astrophysical phenomena down to Earth, into university laboratories. This paper reviews the many recent advances in this emerging discipline, called high-field science. [ABSTRACT FROM AUTHOR]

Execution and modeling of drive symmetry experiments in gas-filled hohlraums have been pursued to provide both a better understanding of radiation symmetry in such hohlraums and to verify the accuracy of the design tools which are used to predict target performance for the National Ignition Facility (NIF) [J. Lindl, Phys. Plasmas 2, 3933 (1995)]. In this paper, the results of a series of drive symmetry experiments using gas-filled hohlraums at the Nova laser facility [C. Bibeau et al., Appl. Opt. 31, 5799 (1992)] at Lawrence Livermore National Laboratory are presented. A very important element of these experiments was the use of kineform phase plates (KPP) to smooth the Nova beams. The effect of smoothing the ten Nova beams with KPP phase plates is to remove most of the beam bending which had been observed previously, leaving a residual bending of only 1.5°, equivalent to a 35 μm pointing offset at the hohlraum wall. The results show that the symmetry variation with pointing of implosions in gas-filled hohlraums is consistent with time integrated modeling. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Accurate modeling is necessary to support precision experiments investigating strong-field QED phenomena. This modeling is particularly challenging in the transition between the perturbative and nonperturbative regimes, where the normalized laser amplitude a0 is comparable to unity and wavelength-scale interference is significant. Here, we describe how to simulate nonlinear Compton scattering, Breit–Wheeler pair creation, and trident pair creation in this regime, using the Monte Carlo particle-tracking code Ptarmigan. This code simulates collisions between high-intensity lasers and beams of electrons or γ rays, primarily in the framework of the locally monochromatic approximation. We benchmark our simulation results against full QED calculations for pulsed plane waves and show that they are accurate at the level of a few per cent, across the full range of particle energies and laser intensities. This work extends our previous results to linearly polarized lasers and arbitrary polarized γ rays. [ABSTRACT FROM AUTHOR]

We discuss that a scheme of nonperturbative pair production by high energy photons (ω ≳ m) in a strong external field is achievable at the next high intensity laser experiments. The pair momentum is boosted, and for ω ≳ 1.2 m , the pair yield is increased when the external field is formed by two laser pulses converging at a small angle. These characteristics are nonperturbative in origin and related to the presence of magnetic field in addition to electric field. By enhancing the signal over perturbative backgrounds, these features allow the employment of above-threshold photons ω > 2 m , which further increases the pair yield. We note the close relation of this photon-pair conversion mechanism to spontaneous pair creation, recommending it as an accessible stepping stone experiment using state-of-the-art or soon-to-be laser technology. [ABSTRACT FROM AUTHOR]

The feasibility of ionization injection for Direct Laser Acceleration (DLA) of electrons up to hundreds of MeV has been studied analytically. Criteria for effective injection determining a range of background and in-channel plasma parameters, laser intensity, etc., were found using phase portraits of the system deduced from the simplified analytical model. The found optimal trajectory in the phase space corresponds to the electron with low (few eV) initial energy experiencing ∼109 times energy gain. For this to occur, electron density should be a few percent of the critical density, while the in-channel electron density should be ∼3 times lower. The analytically obtained dependence of the energy gain on the initial electron longitudinal and transverse momenta corresponds well to the results of exact numerical simulations of an electron motion in the plasma channel. To test the theory, a series of PIC simulations were carried out. PIC simulation confirms the model if the plasma channel has appropriate parameters. The developed approach can form the basis for further studies of electron injection in DLA varying plasma and laser parameters as well as initial electron energies. [ABSTRACT FROM AUTHOR]

Spin-polarized positron beams have widely been utilized in applications ranging from fundamental physical studies to material processing. Preparing highly polarized positron beams for accurate probing is a long-standing issue. Here, we put forward a method to produce ultra-relativistic polarized positrons with unprecedented purity in a femtosecond timescale employing a few-PW circularly polarized laser pulse. The fully spin-resolved QED Monte Carlo method is used for simulating the two successive QED processes during the interaction, i.e., nonlinear Compton scattering and nonlinear Breit–Wheeler pair production. As the photons emitted in a circularly polarized laser field are symmetrically polarized, the polarization of the intermediate gamma photon beam averages out to zero, which is advantageous for improving the polarization of positrons. Meanwhile, the moderate laser intensity suppresses the depolarization of the new-born positrons induced by radiation reaction effect. As a result, the polarization of the positrons can reach up to ≳ 90%, the highest among the laser-driven polarization schemes conceived hitherto. Furthermore, our method relaxes the requirement on laser intensity to few-PW level, offering a promising way of preparing polarized positrons with current-generation laser facilities. [ABSTRACT FROM AUTHOR]

In this work, the third harmonic generated by Gaussian laser pulse propagating in magnetized plasma under the impact of tapered magnetostatic wiggler field is studied. Fundamental equations for analysis of third-harmonic generation by employing the equation of motion, the continuity equation, and the perturbation technique for Gaussian laser pulse in a weakly relativistic regime and the attendance of a wiggler magnetic field are derived. Additionally, the influences of laser strength, wiggler field, plasma frequency, and the δ-parameter on the third-harmonic generation are investigated. The results show that the amount of wiggler frequency required to generate the third harmonic decreases with an increase in the laser strength parameter. Aside from that, the plasma frequency is enhanced with increasing wiggler frequency. Furthermore, it is observed that the efficiency of the generated third harmonic decreases with increasing δ-parameter. In addition, it is found that the efficiency of the generated third harmonic also depends on the time, and the maximum efficiency occurs while the phase matching condition is satisfied. Furthermore, it is seen that the efficiency of the generated third harmonic enhances with increasing plasma frequency. Moreover, it is revealed that the Gaussian laser pulse amplitude has a negligible effect on the third-harmonic efficiency at very low intensities. [ABSTRACT FROM AUTHOR]

Laser pulse propagation can be strongly influenced by nonlinearity effects (relativistic and/or atomic electrons), ionization processes, and finite pulse length effects. In this paper these processes are included in the analysis of the propagation and stability of intense laser pulses in plasmas. An envelope equation, which includes ionization and nonlinear effects, is derived and the spot size is found to be unstable to an ionization-modulation instability. Introducing a quasiparaxial approximation to the wave equation, a pair of coupled envelope-power equations including finite pulse length effects, as well as nonlinearities, is derived and analyzed. In addition, short laser pulses propagating in plasma channels are found to undergo an envelope modulation that is always damped in the front and initially grows in the back of the pulse. Finite pulse length effects are also shown to modify nonlinear focusing processes. Finally, it is shown that, in a partially stripped plasma, the bound electrons can significantly alter the stability of laser pulses. In the presence of both free and bound electrons, an atomic modulation instability develops that can have a growth rate substantially higher than either the conventional relativistic modulational instability or the forward Raman instability. The filamentation instability is shown to be enhanced by bound electrons while the backward Raman instability is unaffected. An example of laser wakefield acceleration to electron energies greater than 2.5 GeV in a plasma channel is described. [ABSTRACT FROM AUTHOR]

In this paper we discuss some of the important issues pertaining to laser acceleration in vacuum, neutral gases, and plasmas. The limitations of laser vacuum acceleration as they relate to electron slippage, laser diffraction, material damage, and electron aperture effects, are discussed. An inverse Cherenkov laser acceleration configuration is presented in which a laser beam is self-guided in a partially ionized gas. Optical self-guiding is the result of a balance between the nonlinear self-focusing properties of neutral gases and the diffraction effects of ionization. The stability of self-guided beams is analyzed and discussed. In addition, aspects of the laser wakefield accelerator are presented and laser-driven accelerator experiments are briefly discussed. [ABSTRACT FROM AUTHOR]

Understanding drive symmetry in gas-filled hohlraums is currently of interest because the baseline design of the indirect drive ignition target for the planned National Ignition Facility uses a gas-filled hohlraum. This paper reports on the results of a series of experiments performed at the Nova laser@C. Bibeau et al. Appl. Opt. 31, 5799 ~1992!# facility at Lawrence Livermore National Laboratory with the goal of understanding time-dependent drive symmetry in gas filled hohlraums.Time-dependent symmetry data from capsule implosions and reemission targets in gas-filled hohlraums are discussed. Results of symmetry measurements using thin wall gas-filled hohlraums are also discussed. The results show that the gas is effective in impeding the motion of the wall blowoff material, and that the resulting implosion performance of the capsule is not significantlydegraded from vacuum results. The implosion symmetry in gas differs from vacuum results with similar laser pointing indicating a shift in beam position on the hohlraum wall and hotter drive at the capsule's poles than at the equator. A theory has been proposed to explain the observed shift as a plasma physics effect: beam steering due to filamentation and transverse plasma flows.. [ABSTRACT FROM AUTHOR]