H. Kiriyama

On some theoretical problems of laser wake-field accelerators

Enhancement of the quality of laser wake-field accelerated (LWFA) electron beams implies the improvement and controllability of the properties of the wake waves generated by ultra-short pulse lasers in underdense plasmas. In this work we present a compendium of useful formulas giving relations between the laser and plasma target parameters allowing one to obtain basic dependences, e.g.xa0the energy scaling of the electrons accelerated by the wake field excited in inhomogeneous media including multi-stage LWFA accelerators. Consideration of the effects of using the chirped laser pulse driver allows us to find the regimes where the chirp enhances the wake field amplitude. We present an analysis of the three-dimensional effects on the electron beam loading and on the unlimited LWFA acceleration in inhomogeneous plasmas. Using the conditions of electron trapping to the wake-field acceleration phase we analyse the multi-equal stage and multiuneven stage LWFA configurations. In the first configuration the energy of fast electrons is a linear function of the number of stages, and in the second case, the accelerated electron energy grows exponentially with the number of stages. The results of the two-dimensional particle-in-cell simulations presented here show the high quality electron acceleration in the triple stage injection–acceleration configuration.

Burst intensification by singularity emitting radiation in multi-stream flows

Burst Intensification by Singularity Emitting Radiation (BISER) is proposed. Singularities in multi-stream flows of emitting media cause constructive interference of emitted travelling waves, forming extremely localized sources of bright coherent emission. Here we for the first time demonstrate this extreme localization of BISER by direct observation of nano-scale coherent x-ray sources in a laser plasma. The energy emitted into the spectral range from 60 to 100u2009eV is up to ~100 nJ, corresponding to ~1010 photons. Simulations reveal that these sources emit trains of attosecond x-ray pulses. Our findings establish a new class of bright laboratory sources of electromagnetic radiation. Furthermore, being applicable to travelling waves of any nature (e.g. electromagnetic, gravitational or acoustic), BISER provides a novel framework for creating new emitters and for interpreting observations in many fields of science.

Scintillator-based transverse proton beam profiler for laser-plasma ion sources

A high repetition rate scintillator-based transverse beam profile diagnostic for laser-plasma accelerated proton beams has been designed and commissioned. The proton beam profiler uses differential filtering to provide coarse energy resolution and a flexible design to allow optimisation for expected beam energy range and trade-off between spatial and energy resolution depending on the application. A plastic scintillator detector, imaged with a standard 12-bit scientific camera, allows data to be taken at a high repetition rate. An algorithm encompassing the scintillator non-linearity is described to estimate the proton spectrum at different spatial locations.

Laser wakefield accelerated electron beam monitoring and control

We will discuss our participation in the ImPACT project, which has as one of its goals the development of an ultra-compact electron accelerator using lasers (< 1 GeV, < 10 m) and the generation of an x-ray beam from the accelerated electrons. Within this context we will discuss our investigation into electron beam monitoring and control. Since laser accelerated electrons will be used for x-ray beam generation combined with an undulator, we will present investigation into the possibilities of the improvement of electron beam emittance through cooling.

J-KAREN-P laser facility at QST: High contrast, high intensity petawatt OPCPA/Ti: Sapphire hybrid laser system

The J-KAREN-P laser facility [1] can provide FW peak power at 0.1 Hz. It is based on the generation of short pulses of 30 fs and energy of 30 J after compression. The contrast of the generated pulses is better than 1012 and the final focused intensity is higher than W/cm2. Such performance in high field science will give rise to the birth of new applications and breakthroughs, which include relativistic particle acceleration, bright x-ray source generation, and nuclear activation.

Latest achivements at the J-KAREN-P laser facility at QST

We report on a high-contrast, high-intensity Ti:sapphire chirped-pulse amplification system that incorporates a nonlinear preamplifier based on optical parametric chirped-pulse amplification (OPCPA). Chirped-pulses are amplified to 63 J at 0.1 Hz and compressed down to 30 fs. The temporal contrast is better than 3 × 10−12 on the sub-nanosecond timescale. A peak intensity of W/cm2 on target is reached by focusing a wavefront corrected 0.3 PW laser beam with an f/1.3 off-axis parabolic mirror.

High resolution X-ray spectra of stainless steel foils irradiated by femtosecond laser pulses with ultra-relativistic intensities

We report on the spectra of x-rays emitted from dense plasma generated via irradiation of thin stainless steel foils by ultra-relativistic femtosecond laser pulses (intensities ~3 × 1021 W/cm2). Kinetic modelling was used to estimate electron plasma density and temperature, demonstrating Te ~2.1 keV for Ne ~5 × 1022 cm−3 in the hottest emission region. Thus, it is experimentally demonstrated for the first time that the laser pulse of over 1021 W/cm2 intensity is absorbed neither in the solid density plasma nor in a pre-plasma of a common critical density, but in the matter of so called relativistic critical density.

Research on Laser Acceleration and Coherent X-Ray Generation Using J-KAREN-P Laser

We present the progress on the upgrade status of the J-KAREN-P , which is a Ti: Sapphire laser aiming at the intensity of 1022 W/cm2 at the repetition rate of 0.1 Hz. The upgrade includes two pilot experiments in order to show the laser performance on target. The first experiment is to generate high-energy ions from thin-foil target. The second experiment is the high-order harmonic at a relativistic intensity. Currently, laser acceleration of protons is being tested and we have obtained 32 MeV protons from a 5-µm stainless steel target irradiated by a 14-J, 30-fs laser pulse. In addition, a joint program toward compact X-ray free-electron laser based on laser electron acceleration is presented briefly and the corresponding J-KAREN-P work is presented.

High-Order Harmonic Generation by Relativistic Plasma Singularities: The Driving Laser Requirements

We discuss the new regime of high-order harmonic generation by relativistic -irradiance multi-terawatt femtosecond lasers focused onto gas jet targets [PRL, 108, 135004, 2012; NJP, 16, 093003, 2014]. The laser induces multi-stream relativistic plasma flow resulting in the formation of density singularities: structurally stable, oscillating electron spikes coherently emitting high-frequency radiation. Here we analyse the dependence of the harmonic yield on the focal spot quality and derive the required laser parameters for efficient harmonics generation. We show the status of the J-KAREN-P laser [IEEE J. Sel. Topics Quantum Electron., 21, 1601118, 2015] and report on the progress towards satisfying these requirements.

X-ray emission from relativistically moving electron density cusps

We report on novel methods to generate ultra-short, coherent, X-rays using a laserplasma interaction. Nonlinear interaction of intense laser pulses with plasma creates stable, specific structures such as electron cusps. For example, wake waves excited in an underdense plasma by an intense, short-pulse laser become dense and propagate along with the laser pulse. This is called a relativistic flying mirror. The flying mirror can reflect a counter-propagating laser pulse and directly convert it into high-frequency radiation, with a frequency multiplication factor of ∼ 4γ2 and pulse shortening with the same factor. After the proof-of-principle experiments, we observed that the photon number generated in the flying mirror is close to the theoretical estimate. We present the details of the experiment in which a 9 TW laser pulse focused into a He gas jet generated the Flying Mirror, which partly reflected a 1 TW pulse, giving up to ∼ 1010 photons, 60 nJ (1.4×1012 photons/sr) in the XUV spectral region (12.8-22 nm).