International Journal of Modern Physics B

The welcoming speech to Joint International Workshop on Laser and Beam Interactions and Laser and Plasma Accelerators held on December 12-16, 2005.

Laser plasma accelerators produce today ultra short, quasi-monoenergetic and collimated electron beams with potential applications in material science, chemistry and medicine. The laser plasma accelerator used to produce such an electron beam is presented. The design of a laser based accelerator designed to produce more energetic electron beams with a narrow relative energy spread is also proposed here. This compact approach should permit a miniaturization and cost reduction of future accelerators and associated X-Free Electrons Lasers (XFEL).

A periodically bunched electron beam is useful for generating high-brightness electron superradiance. This paper studies the generation and acceleration of density-modulated electron beams from a photocathode electron gun driven by a laser beat wave. Computer simulation shows the feasibility of accelerating and preserving the density-modulated electron beam in an accelerator. This paper also details the implementation of a beat-wave laser system with a variable beat frequency for driving a photocathode electron gun.

The interaction of high-intensity laser pulses with matter releases instantaneously ultra-large currents of highly energetic electrons, leading to the generation of highly-transient, large-amplitude electric and magnetic fields. We report results of recent experiment in which such phenomena have been studied by using proton probing techniques able to provide maps of the electrostatic fields with high spatial and temporal resolution. The dynamics of ponderomotive channelling in underdense plasmas have been studied with this technique, as also the process of Debye sheath formation and MeV ion front expansion at the rear of laser-irradiated thin metallic foils.

Ultra high energy cosmic rays (UHECR) have been observed but their sources and production mechanisms are yet to be understood. We envision a laboratory astrophysics program that will contribute to the understanding of cosmic accelerators with efforts to: 1) test and calibrate UHECR observational techniques, and 2) elucidate the underlying physics of cosmic acceleration through laboratory experiments and computer simulations. Innovative experiments belonging to the first category have already been done at the SLAC FFTB. Results on air fluorescence yields from the FLASH experiment are reviewed. Proposed future accelerator facilities can provided unprecedented high-energy-densities in a regime relevant to cosmic acceleration studies and accessible in a terrestrial environment for the first time. We review recent simulation studies of non-linear plasma dynamics that could give rise to cosmic acceleration, and discuss prospects for experimental investigation of the underlying mechanisms.

Laser ion production has been studied for downsizing of the accelerator dedicated for cancer therapy. For optimization of various parameters such as pre-pulse condition, target position, laser spot size on the target, laser pulse width and so on, time of flight (TOF) measurement utilizing the detected signal by a plastic scintillation counter played an essential role for real time measurement. Protons up to ~900 keV and ~600 keV are produced from the thin foil targets of Ti 3 μm and 5 μm in thickness, respectively. Modification of the energy distribution of the laser-produced ions with Maxwell distribution by utilizing an RF electric field synchronized to the pulse laser, which is the rotation of the ion beam in the longitudinal phase space (Phase Rotation), has been demonstrated for the first time. By using the Ti Sapphire laser of the wave length and pulse duration of 800 nm and a few hundreds fs, respectively, intensity increase of factor 3 in a certain energy bins is attained creating energy peaks with the energy spread about 7 %, which is found to be reproduced with good probability.

A structure-based laser accelerator harnesses technological progress developed by the laser and optical fiber industries, potentially facilitating a compact and efficient system. In the optical regime, dielectrics sustain higher electric fields and gradients of the order of a few GV/m may become available, but the acceleration structures are different than those used in the microwave regime. Various dielectric structures have been analyzed, and from the pure accelerator parameters perspective (gradient, interaction impedance, group velocity, wake-fields), their performance is of great promise. Operation similar to current linear accelerators may lead to a prohibitively low efficiency. Therefore, including a feedback attached to each module may improve the efficiency from a few percents to higher than 90% – in fact, the efficiency is limited only by the constraints on the stability of the optical system. Single mode operation in the optical regime imposes that at least one of the dimensions of each micro-bunch ought to be sub-micronic leading to a stringent constraint on the emittance and thus on the luminosity. Attempting to increase the latter, imposes high energy density in the vacuum tunnel as well as in its adjacent dielectric layer(s). This, in turn, is bounded by the maximum stress, temperature increase and heat dissipation, dielectrics can sustain at these scales.

There is considerable interest in the propagation dynamics of intense electron and photon neutrino beams in a background dispersive medium such as dense plasmas, particularly in the search for a mechanism to explain the dynamics of type II supernovae. Neutrino interactions with matter are usually considered as single particle interactions. All the single particle mechanisms describing the dynamical properties of neutrino's in matter are analogous with the processes involving single electron interactions with a medium such as Compton scattering, and Cerenkov radiation etc. However, it is well known that beams of electrons moving through a plasma give rise to a new class of processes known as collective interactions such as two stream instabilities which result in either the absorption or generation of plasma waves. Intense photon beams also drive collective interactions such as modulational type instabilities. In both cases relativistic electron beams of electrons and photon beams can drive plasma wakefields in plasmas. Employing the relativistic kinetic equations for neutrinos interacting with dense plasmas via the weak force we explore collective plasma streaming instabilities driven by Neutrino electron and photon beams and demonstrate that all three types of particles can drive wakefields.

In the forthcoming decades, the physics of massive neutrinos will pose unprecedented challenges to traditional acceleration techniques. In this talk we revise the main motivations for pursuing neutrino oscillation studies with novel ν sources and, particularly, we consider the opportunities offered by laser-plasma accelerators. The existence of efficient ion acceleration regimes in collective laser-plasma interactions opens up the possibility to develop neutrino and, more generally, high-energy physics facilities in conjunction with projects for inertial confinement nuclear fusion (ICF) and neutron spallation sources. Moreover, parametric amplification techniques allow pulse compression implementations that do not fall in contradiction with current designs for ICF drivers. We discuss the conditions under which these efficient regimes can be put into operation and the perspectives for their empirical verification.

Plasma channels have been widely used to guide intense laser pulses over many Rayleigh lengths. Using optimized segmented capillary discharges, we demonstrated guided propagation of ultra short (100 fs) high intensity (10¹⁶ W/cm^-2, limited by the laser system) pulses over distances up to 12.6 cm and intensities above 10¹⁸W/cm² for 1.5cm boron nitride capillary. Both radial and longitudinal density profiles of plasma channels were studied under various discharge conditions. A new diagnostic technique is presented in which the transport of a guided laser pulse at different delay times from the initiation of the discharge is sampled on a single discharge shot. Using external, 10 nsec Nd YAG laser of several tenths of milijoules to ignite polyethylene capillaries we have demonstrated channels of various length in density range of 10¹⁷ - 10¹⁹cm^-3 and up to 25% deep. The longitudinal profiles were found to be remarkably uniform in both short and long capillaries. The Boron Nitride capillary has provided a guiding medium that can withstand more than 1000 shots. Using these capillaries we have guided laser intensities above 10¹⁸W/cm². The laser ignition of capillary discharge provided reliable almost jitter free approach. The concerns related to influence of relatively high current density flow through capillary on the injected electrons were studied extensively by us both theoretically and experimentally using a simple injection method. The method is based on the interaction of a high intensity laser pulse with a thin wire placed near capillary entrance. The influence of magnetic fields was found to be insignificant. Using this method we have studied transport of electrons though capillary discharge.

Dielectric structures promise to support high field, especially for short wakefield pulses produced by a high charged electron beam traveling in a dielectric tube. To push the gradient higher, we have tested two structures using recent upgraded Argonne wakefield accelerator facility that capable of producing up to 100 nC charge and bunch length of < 13ps (FWHM). Here we report on the experiment results that more than 80 nC beam passes through a 14 GHz dielectric loaded wakefield structure that produced an accelerating field of ~ 45 MV/m. The two structures consist of a cylindrical ceramic tube (cordierite) with a dielectric constant of 5, inner and outer radii of 5 mm and 7.49 mm, respectively, and with length of 102 mm and 23 mm long. We present measurements made with single electron bunches and also with two bunches separated by 1.5 ns. As a next step in these experiments, another structure, with an output coupler, has been designed and is presently being fabricated.

We report the development of laboratory astrophysics program in Taiwan. We begin with outlining Taiwan's participation in the FLASH collaboration for measuring the fluorescence yield using SLAC 28.5 GeV electron beam. We then report the domestic effort of studying cosmic ray shower properties using NSRRC 1.5 GeV electron beam.

An energy gain of the monoenergetic component of electrons γ were successfully increased to 25 MeV by decreasing an electron density n_e to 4 × 10¹⁹cm^-3 and increasing a laser power P_L to 4 TW. An empirical scaling law of the energy gain of γ ∝ P_L/n_e was derived from the experiment. A forward scattering spectrum of the laser pulse showed anti-Stokes satellite peaks up to third-order harmonics around a second harmonic light and a Stokes satellite peak around a laser wavelength, which indicated that a wakefield was excited in a self-modulation laser wakefield regime. A density modulation of wakefield was estimated at 60 ± 10 % from intensity ratios of satellite peaks. The empirical scaling law well agrees with results obtained at other institutes by using higher laser power of 10 ~ 30 TW.

A relativistic electron bunch with a large charge (~2 nC) was produced from a self-modulated laser wakefield acceleration configuration. In this experiment, an intense laser pulse with a peak power of 2 TW and a duration of 700 fs was focused in a nitrogen gas jet, and multi-MeV electrons were observed from the strong laser-plasma interaction. By passing the electrons through a small pinhole-like collimator of cone f/70, we observed a narrowing in the electron beam's energy spread. The beam clearly showed a small energy-spread behavior with a central energy of 4.8 MeV and a charge of 115 pC. The acceleration gradient was estimated to be about 20 GeV/m.

We have developed a multi-terawatt Ti:Sapphire laser system to study on the laser driven particle accelerators. We have also developed a relevant instruments that contains a pointing stabilizer and single-shot selector to perform a lot of experimental studies. After development, we are studying on the electron acceleration. A quasi-monoenergetic electron beam is observed in the forward direction. The peak energy of a quasi-monoenergetic component of the electron beam is 20 MeV with a energy spread of 20% at the plasma density of 4.7 × 10¹⁹cm^-3. The table-top laser driven fs quasi-monoenergetic electron beam which is applicable to variety of fields is described.

An electron beam generated in a radio-frequency photo-injector illuminated by a laser "comb" pulse evolves towards a homogenous electron beam with an energy modulation. The density modulation in fact is transformed into an energy modulation, with a saw-tooth like shape, by the longitudinal space charge forces. Such an energy distribution can be exploited to restore the initial density profile by means of a RF accelerating structure operating in the velocity bunching mode. It results a train of short electron bunches suitable for advanced PWFA experiments. The initial laser comb can be obtained with a pulse shaping device inserted into the laser system. In this paper we discuss the beam dynamics features of such a beam.

Technologies for rf gun-based high brightness injector are being developed at NSRRC. An X-band photo-cathode rf gun has been constructed and operated successfully for coherent electromagnetic radiation research at U.C. Davis. Recent measurement of a 1.5-2 MeV beam from this gun with bunch charge at 25 pC showed that it has a normalized emittance at 1.65 mm-mrad. As a successive project, a 100 MeV electron beam injector system with a thermionic rf gun and a photo-cathode rf gun that share the same 2998 MHz rf linac is being designed as the pre-injector of the TPS (Taiwan Photon Source) booster as well as beam injector for novel light source research. For evaluation purposes, a prototype thermionic rf gun system is now under construction at NSRRC with the existing 2856 MHz high power microwave system as the driver source.

An S-band thermionic 1.5-cell rf gun injector system is being built at NSRRC for Taiwan Photon Source (TPS) booster. At optimized field ratio, an electron beam with linear energy chirp can be generated by the rf gun. The beam is then compressed by an alpha magnet such that ultra-short electron bunches at femto-second bunch length can be obtained. Particle dynamics has been studied for an rf gun with nose cone at the cathode, simulation results show that 74 pC bunches at about 97 fs is achievable. Some of the critical components such as the rf gun and alpha magnet have been fabricated and being tested in house. Experiments for intense coherent THz radiations and tunable femto-second X-ray from ultra-short bunches are being proposed as future applications.

The seminal work of J. M. Dawson on sheet crossing and wave breaking is extended to laser wakefield scenarios. We derive a quantitative estimate for the sheet crossing time. We find that the time for the onset of sheet crossing is reduced for lower densities, higher laser intensities and smaller spot sizes. Furthermore, we show that a preformed parabolic plasma channel can decrease the time required for sheet crossing.

Pulse radiolysis spectroscopically studies the initial stage of atomic and molecular reactions induced by electron beams. Single-bunched beams with bunch lengths on the order of femtosecond or shorter are fervently requested for this purpose. The laser wake field acceleration is readily applicable now, which is able to realize beams with bunch length of 1 fs, energy of 30 MeV, repetition rate of 1 Hz, bunch charge of 10 pC, energy width of 10%, etc.

The Laser Plasma Division at the Centre for Advanced Technology is engaged in a variety of research and development activities on laser-plasma interaction with special emphasis on laser-matter interaction at ultra-high intensities. An important aspect of our future work is studies in laser-plasma based acceleration using an elaborate infrastructural set-up of ultra-fast laser and plasma diagnostic systems and recently acquired 10TW, 50fs Ti:Sapphire laser system. This paper presents outline of the planned studies in this field.

A compact, high-brightness x-ray source has been developed through Thomson scattering between photons and relativistic electrons. 33keV energy photons (maximum) were generated in a 165-degree interaction configuration with 38MeV electrons and 800nm-wavelength Ti:sapphire laser light. The number of total photons generated at an interaction point was 10⁶ photons/pulse for a 0.8nC electron bunch charge and 150mJ laser pulse energy. In a 90-degree interaction configuration, 10⁵ photons/pulse total photons were obtained (maximum). Transverse profiles of x-ray intensity and energy were measured by an x-ray CCD camera. These experiment profiles agreed with the analytical results. Imaging using this x-ray source was demonstrated as an application. X-ray images for some objects were taken with various lengths between the objects and the camera. As a result, the refraction contrast images were observed with 17keV x-rays.

Measurement of X-rays produced in nonlinear Thomson scattering of high power laser and electron beams is a good test of electrodynamics in the high field region. We performed an experiment of head-on collision between sub-terawatt CO₂ laser and 60-MeV electron beams at Brookhaven National Laboratory, Accelerator Test Facility. Angular distributions of X-rays produced in Thomson scattering were measured and contribution of the second order nonlinear interaction was observed by comparing the distributions with Monte Carlo simulations based on the semi-classical theory of photon-electron interactions.

Study of Compton backscattering with relativistic high-intense electron beam and single-pass free electron laser (FEL) is carried out to produce high-brightness short X-ray pulse. The single-pass FEL such as SASE is high power coherent light source and the wavelength of the FEL can be tuned changing magnetic field strength of wiggler or undulator continuously. In our study, the relativistic electron beam is generated using a linear accelerator, which is a driver for the FEL. The electron beam is used for both the Compton backscattering and the generation of SASE light. The preliminary experiment of X-ray generation based on Compton backscattering with high-intensity electron beam and infrared SASE light is planed using the L-band linear accelerator at the Institute of Scientific and Industrial Research (ISIR), Osaka University. We will describe the preliminary experiment and the result of numerical studies.

Compact hard X-ray source via laser Compton sattering has been developed in SHI and AIST. Our system has the injector and the linac and the high power laser system. The injector has a photo-cathode rf gun with a solenoid magnet. To enhance the X-ray yeild, we are planning to increase electron beam charge up to 5 nC/bunch and to make multi-bunch beam. The beam tracking simulation in the injector have been performed by changing laser spot size, laser pulse width, rf phase and solenoid field to optimize the distance between the injector and the linac for 5 nC/bunch high charge beam. In addition, high charge multi-bunch beam simulation in rf-gun cavity have been carried out to investigate the influence by the beam loading and the wake field.

By focusing an ultraintense laser onto a helium gaz jet, a collimated beam of ultrafast broadband X-ray radiation can now be generated. The X-ray radiation results from the betatron oscillations of relativistic electrons in the laser created plasma channel. Thus, just as in a synchrotron, the spectral and flux properties of the X-ray beam can be linked to the electron beam through the plasma wiggler strength. The radiation has been observed within 1-10 keV with filters and presents a divergence of as low as 20 mrad. In addition, this source possesses the unique properties to be ultrafast and perfectly synchronized with the laser system, which opens the way toward new types of pump probe experiments.

Besides the exploitation of the Helios 2 superconducting compact storage ring for user operations and for its own R&D work, SSLS envisions a 4^th generation synchrotron light source to complement and, eventually, replace the current facility. This vision includes an accelerator system based on a superconducting linear accelerator with up to 5 recirculation loops for energy multiplication and recovery with the distinguishing feature that the light would be generated by superconducting mini- and micro-undulators. Under this programme, SSLS is pursuing the development of superconducting miniundulators including a proof-of-technology electron beam experiment in co-operation with the Shanghai Institute of Applied Physics and the analytical and numerical study of the fabrication tolerances of superconducting miniundulators with respect to their finite length field errors and mechanical errors.

Synchrotron radiation produced either in storage rings or SASE-FELs is longitudinally incoherent. In this paper a way to produce short longitudinally coherent X-ray pulses is discussed. In addition it is investigated if these sources can be modified to use them as light sources for vacuum electron accelerators.

We have proposed a new scheme for polarized positron production, consisting of two quantum processes i.e. inverse Compton scatterings of circularly polarized laser light on high-quality electron beams and successive electron-positron pair creations. Using a circularly polarized laser beam of 532 nm scattered off a high-quality electron beam with the energy of 1.28 GeV, we have demonstrated for the first time productions of highly polarized positrons with an intensity of 2×10⁴e⁺/bunch and an average energy of 36 MeV. Hence an average polarization of positrons was determined as 73±15(stat)±19(syst)% by means of a newly designed positron polarimeter.

Companion development of ultrafast electron beam diagnostics capable of noninvasively resolving single bunch detail is essential for the development of high energy, high brightness accelerator facilities and associated beam-based light source applications. Existing conventional accelerators can exhibit timing-jitter down to the 100 femtosecond level which exceeds their single bunch duration capability. At the other extreme, in relatively jitterless environments, laser-plasma wakefield accelerators (LWFA) can generate single electron bunches of duration estimated to be of order 10 femtoseconds making this setting a valuable testbed for development of broadband electron bunch diagnostics. Characteristics of electro-optic schemes and laser-induced reflectance are discussed with emphasis on temporal resolution.

The discussion how to accelerate electrons to GeV leads us to the concept of the capillary. The 40 TW laser with prepulses, guided into a 1 cm-long capillary has accelerated the electrons to 100 MeV. The prepulse-free PW laser was also guided into the longer capillary, which formed a 3 cm plasma channel, but the energy gain was only 40 MeV. The prepulse-free laser made it clear that the cone works as an electron seeder. Application of the capillary accelerator was finally discussed.

The structure of the wakefield is studied in a gas filled capillary under conditions of a quasi-monomode laser pulse propagation in a capillary tube. Self-consistent numerical simulations show that in the central bulk part of a plasma where the laser intensity exceeds the ionization threshold, the wakefield structure is similar to that of an infinite homogeneous plasma. The efficient generation of a regular wakefield over long distances suitable for the laser wakefield accelerators is shown. The modification of the spectrum of a frequency doubled probe pulse traveling in a plasma wave created in the wake of a pump pulse guided inside a capillary tube is analyzed for the cases of narrow or broad chirped probe spectra. It is shown that in both cases the measurement of the spectrum can be used as a diagnostic for the amplitude of the plasma wave. The results of full scale numerical modeling confirm the developed analytical theory of wakefield diagnostics.

Compton scattering hard X-ray source which consists of an X-band (11.424 GHz) electron linear accelerator and YAG laser is under construction at Nuclear Professional School, the University of Tokyo (UTNS). Monochromatic hard X-rays are required for variety of medical and biological applications. Our scheme of the hard X-ray source is to produce a monochromatic hard X-ray via collision between 35 MeV electron beam and 2.5 J/10 nsec Nd:YAG laser. In order to increase the efficiency of the X-ray yield, we adopt a laser pulse circulation system. In our case, the laser pulse circulation system can increase the X-ray intensity of up to 50 times. Main features of our scheme are to produce monochromatic tunable hard (10-40 keV) X-rays with the intensities of 10⁸-10⁹ photons/sec. In addition, X-ray energy can be changed with rapidly by 40 ms by introducing two different wavelength lasers (YAG fundamental (1064 nm), 2nd harmonic (532 nm)) and optical switch. This quick energy change is indispensable to living specimens and very difficult by a large SR light source and others. We designed a laser pulse circulation system to increase the X-ray yield 10 times higher (up to 10⁸ photons/RF pulse, 10⁹ photons/sec). It can be proved that the laser total increases 10 times higher by principle experiment with lower energy laser (25 mJ/pulse). Dual-energy X-ray CT and subtraction X-ray CT are available to determine 3D distribution of atomicc number density and electron density, and specified atomic distribution, respectively. Here, the construction status of the X-band beam line and the application plan of the hard X-ray will be reported.

Development status of a double-line table top beat wave electron laser acceleration system is discussed. The advantage of beat wave driven laser accelerator is described here using 1D-PIC simulation comparing with a single laser wake-field acceleration concerning an electric field excitation. As for experimental status, double line different color pulses (central wavelength with 795 nm and 805 nm, respectively) were successfully amplified to 30 mJ as a single overlapping beam with a hybrid chirped pulse amplification system, Optical Parametric Chirped Pulse Amplification (OPCPA) and Ti-Sap multi-pass amplifier pumped by YAG lasers, and then compressed into ps time duration. Amplified double-color pulse drove beating pulse bunches of 300 fs time duration which was identified by an autocorrelation trace. In order to investigate plasma sustainment in cm order, amplified beam illuminated a 2 cm in-length glass capillary and then the laser penetration in the capillary was confirmed.

The acceleration of ions in the interaction of ultrashort, high intensity, circularly polarized laser pulses with overdense plasmas has been theoretically investigated. By using particle-in-cell (PIC) simulations it is found that high-density, short duration ion bunches moving into the plasma are promptly generated at the laser-plasma interaction surface. This regime is qualitatively different from ion acceleration regimes driven by fast electrons such as sheath acceleration at the rear side of the target. A simple analytical model accounts for the numerical observations and provides scaling laws for the ion bunch velocity and generation time as a function of pulse intensity and plasma density. The ion bunches have moderate energies (100 keV-1 MeV) but very high density and low beam divergence, and might be of interest for problems of ultrafast compression, acceleration or heating of high–density matter. In particular, we have studied their application to the development of compact sources of fusion neutrons. We analyzed two target schemes showing that intense neutron bursts with femtosecond duration are produced.

The acceleration of high-energy ion beams (up to several tens of MeV per nucleon) following the interaction of short (t<1ps) and intense (Iλ² > 10¹⁸W cm^-2μm²) laser pulses with solid targets has been one of the most active areas of research in the last few years. The exceptional properties of these beams (high brightness and high spectral cutoff, high directionality and laminarity, short burst duration) distinguish them from those of the lower energy ions accelerated in earlier experiments at moderate laser intensities. In view of these properties, laser-driven ion beams can be employed in a number of groundbreaking applications in the scientific, technological and medical areas. This paper reviews the current state-of-the-art, highlights recent developments and indicate future directions of this research area.

Cold neutrons are good probe for nano meter scale objects. While some huge facilities as spallation neutron sources are under construction, small local (satellite) neutron sources will help to test new ideas on research. Such small ones are considered to be complementary parts of the huge facilities. Because the threshold energy of the d-d reactions is fairly low, irradiation of deuterated polyethylene (CD₂) target by laser induced energetic deuterons may generate neutrons. A preliminary experiment performed with 1TW laser irradiating CD₂ foil just before another thick CD₂ target is described.

This is a duplicate pubication and the authors submitted and published the same article in APL.

A study of the propagation of intense short laser pulses in air and the interaction of these pulses with distant targets is described. It is shown that the beam filamentation pattern can be controlled by introducing beam astigmatism. In addition, it is demonstrated that the collapse distance of intense femtosecond laser beams scales as P^-1/2 for input powers that are moderately above the critical power for self focusing, and that at higher powers the collapse distance scales as P^-1. Related to the interaction of intense short pulses with distant targets, it is measured that the threshold fluence for optical damage in wide gap materials is lower by up to 20% for negatively chirped pulses than for positively chirped, at pulse durations ranging from 60 fs to 1 ps.

Gamma-Ray Burst (GRB) prompt emission can, for specific conditions, be so powerful and short-pulsed to strongly influence any surrounding plasma. In this paper, we briefly discuss the possibility that a very intense initial burst of radiation produced by GRBs satisfy the intensity and temporal conditions to cause stochastic wake-field particle acceleration in a surrounding plasma of moderate density. We consider a simple but realistic GRB model for which particle wake-field acceleration can first be excited by a very strong low-energy precursor, and then be effective in producing the observed prompt X-ray and gamma-ray GRB emission.

We solve analytically the out-of-equilibrium initial stage that follows the injection of a radially finite electron beam into a plasma at rest and test it against particle-in-cell simulations. For initial large beam edge gradients and not too large beam radius, compared to the electron skin depth, the electron beam is shown to evolve into a ring structure. For low enough transverse temperatures, filamentation instability eventually proceeds and saturates when transverse isotropy is reached. The analysis accounts for the variety of very recent experimental beam transverse observations.

The growth rates and the saturation points of the self-generated magnetic field in our simulation are in good agreement with the results of our theoretical calculations.

Particle-in-Cell (PIC) simulations of fast particles produced by a short laser pulse with duration of 40 fs and an intensity of 10²⁰W/cm² interacting with a foil target are performed. The experimental process is numerically simulated by considering a triangular concave target illuminated by an ultraintense laser. We have demonstrated increased acceleration and higher proton energies for triangular concave targets. We also determined the optimum target plasma conditions for maximum proton acceleration. The results indicated that a change in the plasma target shape directly affects the degree of contraction accelerated proton bunch.

We review recent PIC simulation results which show that double-sided irradiation of a thin over-dense plasma slab by ultra-intense laser pulses from both sides can lead to sustained comoving acceleration of surface electrons to energies much higher than the conventional ponderomotive limit. The acceleration stops only when the electrons drift transversely out of the laser beam. We show latest 2.5D results of parameter studies based on finite laser spot size and discuss future laser experiments that can be used to test these results.

A strong electromagnetic field may modify the refractive index of the normal vacuum due to nonlinear effects caused by photon-photon interactions. In order to test the increase of the refractive index, Cherenkov radiations associated with relativistic electrons into the strong electromagnetic field can be utilized as an experimental probe. We will present a result of the experimental search for the Cherenkov emissions with a laser intensity of 10¹⁷W/cm² with the wavelength of 800nm and the electron beam of 35.4MeV/c, where photons with kinematically expected Compton energies and a symptom of visible ray emissions were observed.