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Beam charge control in the staging of laser wakefield acceleration (LWFA) is a crucial technique for developing full-optical jitter-free high-energy electron accelerators. Precise control of total charge in pre-accelerated electron bunches is necessary to achieve practical electron beam characteristics in the final acceleration stage(s). In contrast to the well-known cathode techniques in conventional accelerators, in LWFA the electron injection results from non-linear processes originating from plasma wave breaking. Therefore, the development of charge control requires a deep understanding of the electron self-injection processes and applications of non-trivial tools. The use of asymmetric gas-jet nozzles seems to be a promising way in developing charge control via tuning the target parameters such as plasma density, density slope, and acceleration length. Here, we demonstrate and characterize controllable electron self-injection, owing to a parametric resonance in slantwise density gas jets irradiated by 50 TW femtosecond laser pulses. The measured characteristics of the electron bunches, in which charge and energy distribution depend on the gas density and gas density gradient, agree well with those obtained by multidimensional particle-in-cell simulation and confirm the possibility of charge control.

1.Why temporal diagnostics are necessary in LWFA 2.Electro-optic (EO) spatial decoding technique 3.EO spatial decoding on the Coulomb field of electron bunches from LWFA (Emission timing) 4.EO spatial decoding on the coherent transition radiation (CTR) from electron bunches in LWFA (Bunch duration, timing outside plasma) 5.Summary, 第５回 RIKEN-RAP and QST-KPSI Joint Seminar

High power laser-driven electron sources [1,2], with their inherent ultrafast character, are considered to have potentials for applications in fast ignition [3], pump-probe study [4], and the construction of a table-top x-ray free-electron laser (XFEL) [5]. For such applications, the timing jitter and the bunch duration of the electron bunches are important parameters that are barely investigated experimentally. We established electro-optic sampling techniques [6] adequate for the diagnostics of electron temporal information in laser wakefield acceleration (LWFA) [2]. For the investigation of electron timing jitter and injection process, electro-optic spatial decoding (EOSD) on the electron Coulomb field was performed. The spherical wavefront of the Coulomb field and plasma-density-dependent electron emission timing were discovered [7,8,9]. For the determination of electron bunch durations, EOSD on the coherent transition radiation (CTR) produced when electrons passing through a metal foil was conducted. An electron beam peak current up to 10 kA was demonstrated. This research not only showed the capability of EO sampling serving as a real-time electron temporal diagnostic for laser-driven sources but also demonstrated the ultra-fast and high-brightness nature of laser-driven electron sources for various applications., The 30th International Toki Conference on Plasma and Fusion Research

A novel method to fabricate undulator magnets of a-few-millimetre-period length is being explored. Plate-type magnets, 100 mm-long with 4 mm-period length, have been successfully fabricated. They produce an undulator field of approximately 3 kG at a gap of 1.6 mm. Prototype undulators based on this technology have been constructed. Field measurements and characterization show that the quality of the undulator field of these plate magnets is sufficient for an undulator light source, and the calculated spectrum shows that the fundamental radiation emitted from this field is quite satisfactory. Test experiments for light generation using a real electron beam have been carried out at a test accelerator at the Research Center for Electron Photon Science (ELPH), Tohoku University, Japan, which is able to realize optics conditions to accept a very short gap of ∼1.6 mm. First observation and characterization of blue light was successfully accomplished.

European physical journal special topics 229(24), 3675 - 4284 (2020). doi:10.1140/epjst/e2020-000127-8, This report presents the conceptual design of a new European research infrastructure EuPRAXIA. The concept has been established over the last four years in a unique collaboration of 41 laboratories within a Horizon 2020 design study funded by the European Union. EuPRAXIA is the first European project that develops a dedicated particle accelerator research infrastructure based on novel plasma acceleration concepts and laser technology. It focuses on the development of electron accelerators and underlying technologies, their user communities, and the exploitation of existing accelerator infrastructures in Europe. EuPRAXIA has involved, amongst others, the international laser community and industry to build links and bridges with accelerator science — through realising synergies, identifying disruptive ideas, innovating, and fostering knowledge exchange. The Eu-PRAXIA project aims at the construction of an innovative electron accelerator using laser- and electron-beam-driven plasma wakefield acceleration that offers a significant reduction in size and possible savings in cost over current state-of-the-art radiofrequency-based accelerators. The foreseen electron energy range of one to five gigaelectronvolts (GeV) and its performance goals will enable versatile applications in various domains, e.g. as a compact free-electron laser (FEL), compact sources for medical imaging and positron generation, table-top test beams for particle detectors, as well as deeply penetrating X-ray and gamma-ray sources for material testing. EuPRAXIA is designed to be the required stepping stone to possible future plasma-based facilities, such as linear colliders at the high-energy physics (HEP) energy frontier. Consistent with a high-confidence approach, the project includes measures to retire risk by establishing scaled technology demonstrators. This report includes preliminary models for project implementation, cost and schedule that would allow operation of the full Eu-PRAXIA facility within 8—10 years., Published by Springer, Heidelberg

Petawatt class femtosecond lasers and x-ray free electron lasers (XFEL) open up a new page in research fields related to space and vacuum physics. One of fundamental principles can be explored by these new instruments is the equivalence principle, saying that gravitation and acceleration should be treated equivalently. If it is true this must lead to the appearance of the Unruh effect analogous to Hawking’s black hole radiation. It says that a detector moving with a constant acceleration w sees a boson’s thermal bath with its temperature T w = ℏ w / 2 π k B c . Practical detection of the Unruh effect requires extremely strong acceleration and fast probing sources. Here we demonstrate that x-rays scattering from highly accelerated electrons can be used for such detection. We present two, feasible for the Unruh effect, designs for highly accelerated systems produced in underdense plasma irradiated by high intensity lasers pulses. The first is Thomson scattering of a XFEL pulse from plasma wave excited by an intense laser pulse. Properly chosen observation angles enable us to distinguish the Unruh effect from the normal Doppler shift with a reasonable photon number. The second is a system consisting of electron bunches accelerated by a laser wake-field, as sources and as detectors, which move in a constantly accelerating reference frame (Rindler space) and are probed by x-ray free electron laser pulses. The numbers of photons is shown to be enough to observe reproducible results.

We report on an experimental investigation of the jitter of electrons from laser wakefield acceleration. The relative arrival timings of the generated electron bunches were detected via electro-optic spatial decoding on the coherent transition radiation emitted when the electrons pass through a 100 μm thick stainless steel foil. The standard deviation of electron timing was measured to be 7 fs at a position outside the plasma. Preliminary analysis suggested that the electron bunches might have durations of a few tens of femtoseconds. This research demonstrated the potential of laser wakefield acceleration for femtosecond pump–probe studies.

Effects of ionization injection in low and high Z gas mixtures for the laser wake field acceleration of electrons are analyzed with the use of balance equations and particle-in-cell simulations via test probe particle trajectories in realistic plasma fields and direct simulations of charge loading during the ionization process. It is shown that electrons appearing at the maximum of laser pulse field after optical ionization are trapped in the first bucket of the laser pulse wake. Electrons, which are produced by optical field ionization at the front of laser pulse, propagate backwards; some of them are trapped in the second bucket, third bucket and so on. The efficiency of ionization injection is not high, several pC/mm/bucket. This injection becomes competitive with wave breaking injection at lower plasma density and over a rather narrow range of laser pulse intensity., Comment: 12 figures

Figure 20.1 was not correct in the published article. The original article has been corrected. The published apologizes for the inconvenience.

Particle energy chirp is shown to be a useful instrument in the staging laser wakefield acceleration directed to generation of high-quality dense electron beams. The chirp is a necessary tool to compensate non-uniformity of acceleration field in longitudinal direction and achieve essential reduction of energy dispersion. This is demonstrated via particle-in-cell simulations exploiting the splitting technique for plasma and beam electrons. Properly chosen beam chirps allow a decrease in the energy dispersion of order of magnitude in every single stage during acceleration to the GeV energy range.

Staging laser wake-field acceleration is considered to be a necessary technique for developing full-optical jitter-free high energy electron accelerators. Splitting of the acceleration length into several technical parts and with independent laser drivers allows not only the generation of stable, reproducible acceleration fields but also overcoming the dephasing length while maintaining an overall high acceleration gradient and a compact footprint. Temporal and spatial coupling of pre-accelerated electron bunches for their injection in the acceleration phase of a successive laser pulse wake field is the key part of the staging laser-driven acceleration. Here, characterization of the coupling is performed with a dense, stable, narrow energy band of

The multi-stage technique for laser driven acceleration of electrons become a critical part of full-optical, jitter-free accelerators. Use of several independent laser drivers and shorter length plasma targets allows the stable and reproducible acceleration of electron bunches (or beam) in the GeV energies with lower energy spreads. At the same time the charge coupling, necessary for efficient acceleration in the consecutive acceleration stage(s), depends collectively on the parameters of the injected electron beam, the booster stage, and the non-linear transverse dynamics of the electron beam in the laser pulse wake. An unmatched electron beam injected in the booster stage(s), and its non-linear transverse evolution may result in perturbation and even reduction of the field strength in the acceleration phase of the wakefield. Analysis and characterization of charge coupling in multi-stage laser wakefield acceleration (LWFA) become ultimately important. Here, we investigate two-stage LWFA via fully relativistic multi-dimensional particle-in-cell simulations, and underlying the most critical parameters, which affect the efficient coupling and acceleration of the electron beam in the booster stage., 10 figures

25th International Conference on the Application of Accelerators in Research and Industry, CAARI 2018, Texas, USA, 12 Aug 2018 - 17 Aug 2018; AIP conference proceedings 2160(1), 040012-1 - 040012-9 (2019). doi:https://doi.org/10.1063/1.5127692, Plasma accelerators present one of the most suitable candidates for the development of more compact particle acceleration technologies, yet they still lag behind radiofrequency (RF)-based devices when it comes to beam quality, control, stability and power efficiency. The Horizon 2020-funded project EuPRAXIA (“European Plasma Research Accelerator with eXcellence In Applications”) aims to overcome the first three of these hurdles by developing a conceptual design for a first international user facility based on plasma acceleration. In this paper we report on the main features, simulation studies and potential applications of this future research infrastructure., Published by AIP Publishing, Melville, NY

Propagation of short and ultraintense laser pulses in a semi-infinite space of overdense hydrogen plasma is analyzed via fully relativistic, real geometry particle-in-cell (PIC) simulations including radiation friction. The relativistic transparency and hole-boring regimes are found to be sensitive to the transverse plasma field, backward light reflection, and laser pulse filamentation. For laser intensities approaching I ∼ 1024 W/cm2, the direct laser acceleration of protons, along with ion Coulomb explosion, results in their injection into the acceleration phase of the compressed electron wave at the front of the laser pulses. The protons are observed to be accelerated up to 10–20 GeV with densities around a few times the critical density. The effect qualitatively depends on initial density and laser intensity, disappearing with the initial density increase or intensity decrease.

Numerical and experimental demonstrations are presented to reduce the distortion induced by intense terahertz field in a cross-polarized terahertz time-domain spectroscopy system. A quarter-wave plate is used to increase the initial birefringence of the probe beam. With an optimized angle, the distortion can be removed effectively, while the signal-to-noise ratio of the system is also improved.

Radially polarized intense terahertz (THz) radiation behind a thin foil irradiated by ultrahigh-contrast ultrashort relativistic laser pulse is recorded by a single-shot THz time-domain spectroscopy system. As the thickness of the target is reduced from 30 to 2 \textmu{}m, the duration of the THz emission increases from 5 to over 20 ps and the radiation energy increases dramatically, reaching $\ensuremath{\sim}10.5\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}$ per pulse, corresponding to a laser-to-THz radiation energy conversion efficiency of $1.7%$. The efficient THz emission can be attributed to reflection (deceleration and acceleration) of the laser-driven hot electrons by the target-rear sheath electric field. The experimental results are consistent with that of a simple model as well as particle-in-cell simulation.

Radially polarized intense terahertz (THz) radiation is measured experimentally behind a thin foil irradiated by ultrashort relativistic intense laser pulse of ultrahigh contrast. As the target thickness is reduced from 30 to 2 μm, the energy of the THz emission increases dramatically, which even reaches up to 10.5 mJ per pulse, corresponding to energy conversion efficiency of 1.7%. This efficient THz emission is attributed to bremsstrahlung radiation of the energetic electrons rapidly refluxing between the target front and back electrostatic sheaths fields created during the laser-target interaction. Analytical modeling and particle-in-cell simulation confirms this interpretation.

Single-shot terahertz time-domain spectroscopy is presented with directly encoded spatial resolution. A single reflective echelon and multiple semi-cylindrical lenses are used to obtain both the temporal waveform and the spatial distribution of the terahertz field. This system can be used to rapidly characterize terahertz pulses generated by high power pulsed laser systems, which themselves suffer from large pulse energy and spectrum fluctuations.

Pinch dynamics of the imploding plasma and its relations with the 13.5 nm extreme ultraviolet (EUV) emissions have been experimentally investigated for a laser assisted Sn based discharge produced plasma (LA-DPP) EUV source. Plasma behaviors during the discharge are clarified using the laser aided shadowgraphic technique. Temporally and spatially resolved electron density distributions obtained by using Nomarski interferometry reveal that the maximum EUV emission corresponds to the electron density of (2.0–3.0)×1018 cm−3. The ion fraction and electron temperature of the pre-pinch plasma are estimated using a stationary collisional-radiative model.

Regular electron bunches undergoing extremely high acceleration emulating the behavior of electrons near black holes are shown to occur in underdense plasmas irradiated by multi-PW laser pulses with the intensity over 1022 W/cm2. The interaction of multi-PW laser pulses with underdense plasma, in the regime of strong relativistic wave-breaking, is investigated via fully relativistic 3D particle-in-cell simulations including ion motion and radiation reaction. The effect of a transverse cylindrical plasma wave in laser pulse wakes on the formation of regular electron bunches under extremely high acceleration, w, suitable for the detection of Hawking-Unruh temperature T H = ℏ w / 2 π k B c is shown and analyzed.Regular electron bunches undergoing extremely high acceleration emulating the behavior of electrons near black holes are shown to occur in underdense plasmas irradiated by multi-PW laser pulses with the intensity over 1022 W/cm2. The interaction of multi-PW laser pulses with underdense plasma, in the regime of strong relativistic wave-breaking, is investigated via fully relativistic 3D particle-in-cell simulations including ion motion and radiation reaction. The effect of a transverse cylindrical plasma wave in laser pulse wakes on the formation of regular electron bunches under extremely high acceleration, w, suitable for the detection of Hawking-Unruh temperature T H = ℏ w / 2 π k B c is shown and analyzed.

A way toward a quasicontinuous extreme ultraviolet (EUV) radiation source is proposed and explored. Tin and lithium vapor discharges with the lateral laser-ablation injection are experimentally studied as possible efficient sources of quasicontinuous emission of EUV radiation at a wavelength of 13.5 nm. It is shown that the time-of-flight control of optimal plasma parameters by means of varying ablating laser pulse parameters provides a considerable elongation of maximal-power EUV emission with an overall efficiency of 0.1% and with an energy output exceeding 1% of the energy deposited in the discharge plasma. Along with a high average power and a stable position, such an emitter may have its size small enough to be used in the projection lithography.

Acceleration of plasma electrons by laser wake-filed is the most promising process to produce the next generation of compact accelerators because of its ultrahigh acceleration gradient. To achieve the efficient and stable mono-energetic acceleration of plasma electrons, we investigated the laser-plasma interaction and its correlation to electron acceleration through single shot measurements. We observed a thin laser channel when a mono-energetic spectrum was generated.

The fluctuation of the electron bunch duration due to energy spectrum instability in a laser plasma cathode has been examined. Previous experiments clearly proved that a laser plasma cathode can generate ultrashort electron bunches with a bunch duration of 130 fs (FWHM) and a geometrical emittance 0.07π mm mrad. The effect of temporal elongation of electron bunches due to their energy spread is estimated and the results are in good agreement with previous experiments. It is also clarified that the instability of the energy spectrum not only leads to a fluctuation of the bunch shape but also to a time-of-flight jitter, affecting possible future applications of a laser plasma cathode.

Propagation of relativistically intense azimuthally or radially polarized laser pulses (RPP) in underdense plasmas is demonstrated to be unstable, via 3D particle-in-cell simulation and disregarding the Kerr non-linearity. Strong pulse filamentation occurs for RPP in transversely uniform plasma with an increment, $\Gamma$, close to the well-known one depending on acceleration, $\alpha$, and modulated density gradient length, $L$, as $\Gamma \approx (\alpha/L)^{1/2}$. In deep plasma channels the instability vanishes. Electron self-injection and acceleration by the resulting laser pulse wake is explored., Comment: The paper need to be written in a more proper way

We report, for the first time, that the energy of femtosecond optical laser pulses, E, with relativistic intensities I > 1021 W/cm2 is efficiently converted to X-ray radiation, which is emitted by “hot” electron component in collision-less processes and heats the solid density plasma periphery. As shown by direct high-resolution spectroscopic measurements X-ray radiation from plasma periphery exhibits unusual non-linear growth ~E4–5 of its power. The non-linear power growth occurs far earlier than the known regime when the radiation reaction dominates particle motion (RDR). Nevertheless, the radiation is shown to dominate the kinetics of the plasma periphery, changing in this regime (now labeled RDKR) the physical picture of the laser plasma interaction. Although in the experiments reported here we demonstrated by observation of KK hollow ions that X-ray intensities in the keV range exceeds ~1017 W/cm2, there is no theoretical limit of the radiation power. Therefore, such powerful X-ray sources can produce and probe exotic material states with high densities and multiple inner-shell electron excitations even for higher Z elements. Femtosecond laser-produced plasmas may thus provide unique ultra-bright X-ray sources, for future studies of matter in extreme conditions, material science studies and radiography of biological systems.

Single-shot measurement of plasma parameters is very important in laser wakefield acceleration research aiming to generate compact high-quality femtosecond electron beams. To achieve the efficient and stable monoenergetic acceleration of plasma electrons, effects of laser prepulses, plasma density gradients, wave-breaking processes, laser pulse instability, and energy spectra of accelerated electrons have to be measured simultaneously, granting reproducible laser pulse parameters. A strong correlation between the laser prepulse forming the plasma cavity and electron acceleration has been experimentally confirmed.

In this article, the present status of radiation therapy in Japan and updated medical accelerators are reviewed. In addition, the potential of laser plasma acceleration as a future medical accelerator is discussed. The updated results of laser plasma cathode experiment by the University of Tokyo are described.

We present the results of electron generation experiments conducted at the Advanced Photon Research Center, Japan Atomic Energy Research Institute, using 23-fs relativistically intense 20-TW tightly focused laser pulses with underdense plasma. We observed electron energies up to 40 MeV characterized by a two-temperature Maxwell distribution. With the help of particle-in-cell simulations, we found that these are due to different plasma wave-breaking processes. A charge of 5 nC/shot was obtained at a small solid angle, which corresponds to high peak current generation.

Femtosecond electron beams are novel tool for pump-probe analysis of condensed matter. Progress in developing femtosecond electron beams with the use of both conventional accelerators and laser-plasma accelerators is discussed. In conventional accelerators, the critical issue is timing jitter and drift of the linac-laser synchronization system. Sophisticated electronic devices are developed to reduce the jitter to 330 fs (rms); the precise control of temperature at several parts of the accelerator lessens the drift to 1 ps (rms). We also report on a full-optical X-ray and e-beam system based on the laser-plasma cathode by using a 12 TW 50 fs laser, which enables 40 MeV (at maximum), 40 fs (cal.), 100 pC and quasi-monochromatic single electron bunches. Since the synchronization is done by a passive optical beam-splitter, this system intrinsically has no jitter and drift. It could achieve tens of femtoseconds time-resolved analysis in the near future.

We are developing two spatial filters for our 12 TW–50 fs laser system in order to improve a spatial profile of the laser. In our scheme of a laser plasma cathode, electrons are injected into further wake fields by relativistic wave breaking. Previous experiments clearly showed production of well collimated ∼40 MeV electrons, which is suggested having 40 fs pulse duration by a particle in cell (PIC) calculation. It was found from our recent research that the stability of the electron beam strongly depends on the plasma dynamics. With these spatial filters we expect that we can stabilize the femto-second electron bunch generation and plasma dynamics.

Density effects on the dynamics of a cavity produced in the wake of an ultraintense (a0=eE/mcω≫1) and short (ωplτ/π

Formation of a plasma cavity with a shock wave in gas jets irradiated by tightly focused femtosecond laser pulses causes the wave break of the laser wake field at the front of the shock wave and, as a result, the injection of electrons into the acceleration phase of the wake-field wave. A strong crescentlike deformation of the cavity and a change in electron signal are observed with gas density growth. It is attributed to a mutual effect of the cavity on the laser pulse propagation and break of the plasma wake field due to refraction of the laser pulse.

Recently, it is strongly required to develop ultra-short electron beam sources for ultra-fast probe analysis of matter. Among a number of concepts of ultra-short electron bunch production the laser wake-field acceleration (LWFA) provides one of the most promising approaches. In this paper, updated achievements of generation of an ultra-short electron bunch by self-injected LWFA are summarized. We describe latest results of plasma cathode by Univ. of Tokyo, also review the ones by Univ. of Michigan, LBNL (Lawrence Berkeley National Laboratory), and LOA (Laboratoire d’Optique Appliquee).

Underdense plasma produced in gas jets by low intensity laser prepulses in the presence of a static magnetic field, $B\ensuremath{\sim}0.3\text{ }\text{ }\mathrm{T}$, is shown experimentally to become an optical element allowing steering of tightly focused high power femtosecond laser pulses within several degrees along with essential enhancement of pulse's focusability. Strong laser prepulses form a density ramp perpendicularly to magnetic field direction and, owing to the light refraction, main laser pulses propagate along the magnetic field even if it is tilted from the laser axis. Electrons generated in the laser pulse wake are well collimated and follow in the direction of the magnetic field; their characteristics are measured to be not sensitive to the tilt of magnetic field up to angles $\ifmmode\pm\else\textpm\fi{}5\ifmmode^\circ\else\textdegree\fi{}$.