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Interaction of multiple shocks plays a critical role in setting up an adiabatic compression of megabar pressure in nanosecond timescale in inertial confinement fusion. In this paper, we present observations of dynamic behavior and interaction of multiple shocks in polystyrene (CH) planar targets driven by a single-ramp pulse of 2.5 ns at the SG-II laser facility with a specially designed velocity interferometer system for any reflector (VISAR). A maximum pressure of [Formula: see text] and a mass density of [Formula: see text] are measured, respectively. Radiation-hydrodynamic simulations reveal the interaction process of the multiple shocks and are in good agreement with the measurements. A theoretical model is proposed to invert the space-time history of the shock generation with the VISAR data. Moreover, an optimized double-slope ramp pulse is proposed for further compression experiments. The improved multiple-shock coalescence is expected to effectively enhance both density and velocity for an initial compression of the CH target.

Fusion reactions in a plasma environment are fundamental issues with general interest in high energy density sciences. The reaction rate in an astro-system, which may differ from cold matter, is an important subject in the ambiguous problems of elemental abundance. In addition, the stopping of charged particle in plasma has a considerable impact on the design of nuclear fusion reactors as it is related to the α-particle heating process and ion-driven fast ignition, but still needs better understanding. In this research, an experiment on laser-driven D–D fusion reactions (D + D → 3He + n) has been carried out to investigate the effects of ion stopping power in plasma on fusion reactivities. The neutron yields, plasma density, and deuteron energy loss in the plasma have been measured simultaneously, and the plasma temperature has been analyzed from simulations. It is experimentally demonstrated that the fusion reaction yield is closely correlated with ion beam transportation in the plasma. As a cold target heated to plasma, the reaction probabilities from a deuteron beam and deuterated target interactions can be enhanced or suppressed, which is ascribed to the deuteron stopping power variation in the plasma. The results show the importance of considering the temperature adjusted ion stopping power to correctly model the fusion reaction yields. This work has an impact on understanding the fusion reactions in plasma environment, which is also likely to help achieve higher neutron yields.

The interaction of two interfered picosecond laser pulses with overdense plasma and the resulting hot electron generation are studied by particle-in-cell simulation. We find that the yield and temperature of forward hot electrons can be significantly increased when laser interference fringes have a period around [Formula: see text] and the angle between the two pulses is about [Formula: see text]. The enhancements result from local intensity increase at laser interference fringes and the plasma surface structure formed by laser pulses. The optimal angle and fringe period are analyzed, and the dependence between the optimal period and plasma density scale length is discussed. This work could be applied in hot electron generation and the resulting ion acceleration, fast ignition of laser fusion, etc.

The Zeeman splitting effect is observed in a strong magnetic field generated by a laser-driven coil. The expanding plasma from the coil wire surface is concentrated at the coil center and interacts with the simultaneously generated magnetic field. The Cu I spectral lines at wavelengths of 510.5541, 515.3235, and 521.8202 nm are detected and analyzed. The splittings of spectral lines are used to estimate the magnetic field strength at the coil center as ∼31.4 ± 15.7 T at a laser intensity of ∼5.6 × 1015 W/cm2, which agrees well with measurements using a B-dot probe. Some other plasma parameters of the central plasma disk are also studied. The temperature is evaluated from the Cu I spectral line intensity ratio, while the electron density is estimated from the Stark broadening effect.

The absolute response of the GE Amersham Typhoontm imaging plate scanner is studied in this paper. The sensitivity function of the scanner with different photomultiplier tube voltages was obtained by using a pre-calibrated Cu Kα x-ray tube. The results showed that the sensitivity function decreases exponentially with higher voltage and is also affected by the scanning pixel size. The spatial resolution and the fading effect of the imaging plate system on x rays were also investigated and compared with the previous scanner models.

We have computed a suite of simulations of propagating three-dimensional relativistic jets involving substantial ranges of initial jet Lorentz factors and ratios of jet density to external medium density. These allow us to categorize the respective active galactic nuclei (AGN) into Fanaroff–Riley (FR) class I (jet dominated) and FR class II (lobe dominated) based upon the stability and morphology of the simulations. We used the Athena code, and more recently, the Athena++ code, to produce a collection of large 3D variations of jets, many of which propagate stably and quickly for over 100 jet radii, but others of which eventually become unstable and fill up slowing advancing lobes. Comparing the times when some jets become unstable to these initial parameters allows us to find a threshold where radio-loud AGNs transition from class II to class I. With our highest resolution, fully 3D relativistic simulations we can represent the jets more accurately and thus improve upon and refine earlier results that were based on both our now high-resolution 3D and 2D simulations.

Polarization shaped terahertz sources play a key factor in terahertz wireless communications, biological sensing, imaging, coherent control in fundamental sciences, and so on. Recently developed spintronic terahertz emitters have been considered as one of the next-generation promising high performance broadband terahertz sources. However, until now, polarization control, especially for twisting the circularly polarized terahertz waves at the spintronic terahertz source, has not yet been systematically explored and experimentally achieved. In this work, we not only demonstrate the generation of circularly polarized terahertz waves in cascade spintronic terahertz emitters via delicately engineering the amplitudes, applied magnetic field directions, and phase differences in two-stage terahertz beams but also implement the manipulation of the chirality, azimuthal angle, and ellipticity of the radiated broadband terahertz waves. We believe our work can help with further understanding of the ultrafast optical magnetic physics and may have valuable contributions for developing advance terahertz sources and optospintronic devices.

We demonstrate a control of spin-polarized currents and a nonlinear response to the excited state of magnetic films through measuring the terahertz waves generated by illuminating a W/CoFeB/Pt structure with a pair of femtosecond laser pulses. By changing the delay time of the two laser pulses, the waveforms and spectra of the terahertz waves are efficiently modulated. The terahertz wave generated by one pulse is obviously influenced by the other. The experimental results can be explained by a microscopic three-temperature model, which serves as proof for spin-flip scattering contributing to ultrafast demagnetization. Our experiment offers a new method for the investigation of ultrafast magnetism and terahertz spintronics.

An ultrafast pulsed magnetic field from a two-loop solenoid is generated by a femtosecond (fs) laser. High temporal resolution is needed to measure the magnetic field. We describe an improved Faraday-rotation measurement to evaluate the evolution of the magnetic field with a resolution of ∼3.3 picoseconds (ps) in a single shot, with an uncompressed chirped pulse from a Ti:sapphire laser as the optical probe. A magnetic field of 0.52 T with a rise time of 20.8 ps has been measured with this chirped Faraday probe. In addition, we demonstrate the magnetic field strength driven by the femtosecond laser can be modified by adjusting the focal spot size.An ultrafast pulsed magnetic field from a two-loop solenoid is generated by a femtosecond (fs) laser. High temporal resolution is needed to measure the magnetic field. We describe an improved Faraday-rotation measurement to evaluate the evolution of the magnetic field with a resolution of ∼3.3 picoseconds (ps) in a single shot, with an uncompressed chirped pulse from a Ti:sapphire laser as the optical probe. A magnetic field of 0.52 T with a rise time of 20.8 ps has been measured with this chirped Faraday probe. In addition, we demonstrate the magnetic field strength driven by the femtosecond laser can be modified by adjusting the focal spot size.

Proton emissions from vacuum-gapped cascaded-ultrathin-foil targets irradiated with low-contrast intense picosecond laser pulses were measured. The maximum energy of the proton beam and the laser-to-proton energy conversion efficiency were both increased in comparison with those from the single-layer reference targets. A transition from plateau to exponential profile in proton energy spectral shape was found for the target with a front-foil thickness of above 500 nm. The measured annular x-ray emissions from both target front and rear sides indicate that the proton enhancement could be attributed to the modified preplasma distribution. A simple model and hydrodynamic simulations further show that the optimal acceleration occurs when the front shutter foil is right swelled onto the front surface of the rear source foil by the prepulses at the arrival of the main laser pulse. This cascaded thin-foil target design can be popularized in improving laser-driven proton beams for wide applications.

Laser-driven collisonless electrostatic shock formation and the subsequent ion acceleration have been studied in near critical density plasmas. Particle-in-cell simulations show that both the speed of laser-driven collisionless electrostatic shock and the energies of shock-accelerated ions can be greatly enhanced due to fast laser propagation in near critical density plasmas. However, a response time longer than tens of laser wave cycles is required before the shock formation in a near critical density plasma, in contrast to the quick shock formation in a highly overdense target. More important, we find that some ions can be reflected by the collisionless shock even if the electrostatic potential jump across the shock is smaller than the ion kinetic energy in the shock frame, which seems against the conventional ion-reflection condition. These anomalous ion reflections are attributed to the strongly time-oscillating electric field accompanying laser-driven collisionless shock in a near critical density plasma., Comment: 9 figures,\

We demonstrate a novel plasma device for magnetic reconnection, driven by Gekko XII lasers irradiating a double-turn Helmholtz capacitor-coil target. Optical probing revealed an accumulated plasma plume near the magnetic reconnection outflow. The background electron density and magnetic field were measured to be approximately 1018 cm−3 and 60 T by using Nomarski interferometry and the Faraday effect, respectively. In contrast with experiments on magnetic reconnection constructed by the Biermann battery effect, which produced high beta values, our beta value was much lower than one, which greatly extends the parameter regime of laser-driven magnetic reconnection and reveals its potential in astrophysical plasma applications.

We have studied the generation of terahertz (THz) radiation via the interaction of intense femtosecond laser pulses with solid targets at a small incidence angle. It is found that preplasma with a moderate density gradient can enhance the emission. We also observe saturation of the THz output with the driving laser energy. We find that THz emission is closely related to the 3/2 harmonics of the driving laser. Particle-in-cell simulations indicate that under the present experimental conditions, the THz emission could be attributed to the transient currents at the plasma-vacuum interface, mainly formed by the two-plasmon-decay instability.

A simple scheme to produce strong magnetic fields due to cold electron flow in an open-ended coil heated by high power laser pulses is proposed. It differs from previous generation of magnetic fields driven by fast electron current in a capacitor-coil target [S. Fujioka et al., Sci. Rep. 3, 1170 (2013)]. The fields in our experiments are measured by B-dot detectors and proton radiography, respectively. A 205 T strong magnetic field at the center of the coil target is generated in the free space at Iλ2 of 6.85 × 1014 W cm−2 μm2, where I is the laser intensity, and λ is the laser wavelength. The magnetic field strength is proportional to Iλ2. Compared with the capacitor-coil target, the generation mechanism of the magnetic field is straightforward and the coil is easy to be fabricated.

Filamentary discharges in dielectric barrier discharge operating in a glow regime are studied by use of two-dimensional particle-in-cell simulation with Monte Carlo collisions included. The formation of multiple filaments and the involved electric fields, electric potentials, plasma densities, and particle temperatures are presented. It is found that the filaments are not ignited simultaneously, and they expire with the shift of the plasma sheaths. “Dark discharges”, which produce ion densities lower by one order of magnitude than a typical filamentary discharge, are observed while the filaments from earlier discharges are decaying.

We propose a scheme to generate 10 MeV-level quasimonoenergetic proton bunches using proton-doped heavy-ion targets irradiated by intense lasers via target normal sheath acceleration. The heavy substrate ions provide a long-life quasi-stable sheath field to accelerate the doped protons at the target rear and consequently a quasimonoenergetic proton bunch is produced. The scheme is demonstrated by two-dimensional particle-in-cell simulations. An exemplificative simulation with parameters of targets made by ion-implant technique, a kind of modern doping process, gives a quasimonoenergetic bunch with peak energy ∼13MeV, energy spread ∼24%, and ∼nC charge at the focused laser intensity 1020W/cm2.

It is shown that there is an upper-limit laser power for self-focusing of a laser pulse in plasma in addition to the well-known lower-limit critical power set by the relativistic effect. This upper limit is caused by the transverse ponderomotive force of the laser, which tends to expel plasma electrons from the laser propagating area. Furthermore, there is a lower-limit plasma density for a given laser spot size, below which self-focusing does not occur for any laser power. Both the lower-limit density and the upper-limit power are derived theoretically and verified by two-dimensional and three-dimensional particle-in-cell simulations. It is also found that plasma channels may be unfavorable for stable guiding of lasers above the upper-limit power.

Third-harmonic emission from laser filament intercepted by plasma spot is studied by numerical simulations. Significant enhancement of the third-harmonic generation is obtained due to the disturbance of the additional plasma. The contribution of the pure plasma effect and the possible plasma-enhanced third-order susceptibility on the third-harmonic generation enhancement are compared. It is shown that the plasma induced cancellation of destructive interference [Y. Liu et al., Opt. Commun. 284, 4706 (2011)] of two-colored filament is the dominant mechanism of the enhancement of third-harmonic generation.

We experimentally investigated electron acceleration from submicron size argon clusters-gas target irradiated by a 100 fs, 10 TW laser pulses having a high-contrast. Electron beams are observed in the longitudinal and transverse directions to the laser propagation. The measured energy of the longitudinal electron reaches 600 MeV and the charge of the electron beam in the transverse direction is more than 3 nC. A two-dimensional particle-in-cell simulation of the interaction has been performed and it shows an enhancement of electron charge by using the cluster-gas target.

Using opened reentrant cone silicon targets, we have demonstrated the effect of micro focusing of fast electrons generated in intense laser-plasma interactions. When an intense femtosecond laser pulse is focused tightly onto one of the side walls of the cone, fast electron beam emitted along the side wall is observed. When a line focus spot, which is long enough to irradiate both of the side walls of the cone simultaneously, is used, two electron beams emitted along each side wall, respectively, are observed. The two beams should cross each other near the open tip of the cone, resulting in micro focusing. We use a two-dimensional Particle-In-Cell code to simulate the electron emission both in opened and closed cone targets. The simulation results of the opened cone targets are in agreement with the experimental observation while the results of the closed cone targets do not show the micro focusing effect.

It is shown by analysis and simulations that an extremely powerful terahertz (THz) radiation can be produced by a few-cycle laser pulse in a tenuous plasma. The THz amplitude scales linearly with the laser amplitude as well as with the sine of the laser carrier-envelope phase, and in particular, it increases exponentially with the decrease of the laser duration. For example, the THz amplitude increases by near 2 orders of magnitude as the laser duration decreases from one and a half cycles to one cycle; a single-cycle laser of 200 TW can drive the THz radiation of 1 GW with the energy conversion efficiency higher than 10−4.

The experimental demonstration of the formation of a strong electrostatic (ES) collisionless shock has been carried out with high-speed counterstreaming plasmas, produced by a high-power laser irradiation, without external magnetic field. The nearly four times density jump observed in the experiment shows a high Mach-number shock. This large density jump is attributed to the compression of the downstream plasma by momentum transfer by ion reflection of the upstream plasma. Particle-in-cell (PIC) simulation shows the production of a collisionless high Mach-number ES shock with counterstreaming interaction of two plasma slabs with different temperatures and densities, as pointed out by Sorasio et al. [Phys. Rev. Lett. 96, 045005 (2006)]. It is speculated that the shock discontinuity is balanced with the momentum of incoming and reflected ions and the predominant pressure of the electrons in the downstream with PIC simulation.

It is shown theoretically and numerically that a dc/ac bias field applied over tenuous plasma can be converted efficiently into electromagnetic (EM) waves at the plasma frequency ωp, with the amplitude and polarization determined by the bias. The initial phase of the EM waves can be controlled by the triggering time of the bias and therefore circularly/elliptically polarized EM waves can be obtained by applying two bias fields perpendicular to each other. When the bias frequency is near ωp, the resonance appears and the EM waves are generated with the intensity enhanced considerably as compared with the dc-bias case. This approach provides a potential way to produce tunable far-infrared EM waves such as terahertz waves. Limits of this approach by available parameters of plasmas and bias are also discussed.

The temporally and spatially resolved characteristics of silicon dioxide aerogel plasmas were studied using x-ray spectroscopy. The plasma was generated in the near-Planckian radiation environment within gold hohlraum targets irradiated by laser pulses with a total energy of 2.4 kJ in 1 ns. The contributions of silicon ions at different charge states to the specific components of the measured absorption spectra were also investigated. It was found that each main feature in the absorption spectra of the measured silicon dioxide aerogel plasmas was contributed by two neighboring silicon ionic species.

Terahertz radiation generated by focusing the fundamental laser pulse and its second harmonic into ambient air strongly saturates with increasing pump laser energy. We demonstrate a simple method to control the Gaussian pump laser beam to improve the output of terahertz radiation with an adjustable aperture. With the optimal aperture-limited pump laser beams, the terahertz wave amplitudes can be enhanced by more than eight times depending on the pump laser parameters than those of aperture-free cases.

Photoionized plasma was studied experimentally under laboratory conditions by means of high intensity short pulse lasers. The experiment consists of a gold cavity filled with nitrogen gas. Six laser beams were focused on the inner surface of the gold cavity, thereby generating an almost black-body radiation having temperature of 80eV inside the cavity. This radiation heats the nitrogen gas mainly by means of photoionization. L-shell emissions from N V to N VII have been observed in the wavelength range between 90 and 200A. A time-dependent Detailed Configuration Accounting computer program has been developed to analyze the experimental spectra. In contrast to standard analysis of astrophysical observations, the evidence for photoionization is inferred from the spectral lines ratios. Comparison between the experimental and simulated line spectra indicates that the radiation heated nitrogen attains temperature of 20–30eV, much lower than the source radiation temperature. Paradoxically, it is also shown that...

The spatial and energy distributions of fast electrons emitted from foil targets irradiated by ultrashort intense laser pulses are measured. Four groups of collimated emissions of fast electrons along the front and rear target surfaces are observed for an incidence angle of

Based on particle-in-cell simulations, ion acceleration by collisionless electrostatic shock waves in the interaction of intense laser pulses with solid targets containing two ion species of different charge-to-mass ratios R is studied. The acceleration processes of the ions with different R and located in different regions of the target have been considered. Simulations also show that ions with larger R can be reflected and accelerated dramatically between two oppositely propagating shocks generated in plasma dominated by ions with smaller R. The studies suggest a new way to generate high energy ions. The involved phenomena may also occur in space and astrophysical plasmas.

Anomalous observations using the fast ignition for laser driven fusion energy are interpreted and experimental and theoretical results are reported which are in contrast to the very numerous effects usually observed at petawatt-picosecond laser interaction with plasmas. These anomalous mechanisms result in rather thin blocks (pistons) of these nonlinear (ponderomotive) force driven highly directed plasmas of modest temperatures. The blocks consist in space charge neutral plasmas with ion current densities above 1010A∕cm2. For the needs of applications in laser driven fusion energy, much thicker blocks are required. This may be reached by a spherical configuration where a conical propagation may lead to thick blocks for interaction with targets. First results are reported in view of applications for the proton fast igniter and other laser-fusion energy schemes.

Collisionless electrostatic shock (CES) generation and subsequent ion acceleration in laser plasma interaction are studied numerically by particle-in-cell simulations. Usually a CES is composed of a high ion density spike surrounded by a bipolar electric field. Ions in front of it can be either submerged or reflected by the shock front. The submerged ions experience few oscillations before becoming part of the shock itself, while the reflected ions are accelerated to twice the shock speed. The effects of the target thickness, density, ion mass, preplasma conditions, as well as the laser intensity on the shock generation are examined. Simulations show that such shocks can be formed in a wide range of laser and target conditions. The characteristic of the shock propagation through a plane interface between two targets with different properties is also investigated. These results are useful for future experimental studies of shock generation and acceleration.

A scheme is proposed to produce high-quality quasi-monoenergetic attosecond electron bunches based on laser ponderomotive-force acceleration along the surface of wire or slice targets. Two- and three-dimensional particle-in-cell simulations demonstrate that the electron energy depends weakly on the target density. A simple analytical model shows that the electron energy scales linearly with the laser field amplitude, in good agreement with the simulation results. Electron bunches produced by this scheme are suitable for applications such as coherent x-ray radiation, radiography, and injectors in accelerators, etc.