Laser-driven multi-MeV high-purity proton acceleration via anisotropic ambipolar expansion of micron-scale hydrogen clusters

強力なレーザーを使ってエネルギーがそろった純度100％の陽子ビーム発生に成功 --レーザー駆動陽子ビーム加速器の実現へ向けて大きく前進--. 京都大学プレスリリース. 2022-10-13.Multi-MeV high-purity proton acceleration by using a hydrogen cluster target irradiated with repetitive, relativistic intensity laser pulses has been demonstrated. Statistical analysis of hundreds of data sets highlights the existence of markedly high energy protons produced from the laser-irradiated clusters with micron-scale diameters. The spatial distribution of the accelerated protons is found to be anisotropic, where the higher energy protons are preferentially accelerated along the laser propagation direction due to the relativistic effect. These features are supported by three-dimensional (3D) particle-in-cell (PIC) simulations, which show that directional, higher energy protons are generated via the anisotropic ambipolar expansion of the micron-scale clusters. The number of protons accelerating along the laser propagation direction is found to be as high as 1.6 ±0.3 × 10⁹/MeV/sr/shot with an energy of 2.8 ±1.9 MeV, indicating that laser-driven proton acceleration using the micron-scale hydrogen clusters is promising as a compact, repetitive, multi-MeV high-purity proton source for various applications

Recent progress in x-ray spectral diagnostics of contrast dependent plasma at relativistic intensities of ~10^22 W/cm^2

At PW-class laser facilities pulse intensities of the order of 10^21 – 10^22 W/cm^2 can be reached by focusing high quality laser beams to spots of a few microns size. Under such conditions, during fs- time intervals a huge amount of optical energy is concentrated into a tiny volume and by the interaction with solid targets creates a plasma with high energy density. Such plasma can be sources of non-linear phenomena, among known and predicted by theory there is the acceleration to high energy of protons, ions, electrons and neutrons, generation of HOH and gamma-flare, creation of exotic states of hollow ions and WDM. The observed phenomena are defined by the type of target, laser energy and strongly depend on the prepulse condition. Achieving highly ionized states for elements with medium Z allows to effectively perform x-ray diagnostics of plasma parameters and preplasma conditions by means of high-resolution spectroscopy based on the analysis of spectra of H-like and He-like ions.Here we l present a review on the recent results on application of x-ray spectroscopic diagnostics of plasmas generated in the interaction of J-KAREN-P ultra-high intensity laser pulses with solid and clusters targets under conditions of different intrinsic laser contrast and under the cases of specially incorporated prepulse for controlled preplasma formation. Plasma parameters were evaluated by means of time integrated x-ray spectroscopic measurements and kinetic modeling using the code PrismSPECT. Certain line intensity relations, including the relations (Ly_a / He_b), (Ly_a / Ly_a satellites), relation between Ly_a satellites, as well as broadening and the shape of line, were used as the main tools for evaluation of plasma parameters. Each of the tools has preferred dependences on plasma electron temperature, electron density, electric and magnetic field strength. Four types of experiments are under consideration.3rd International Conference on Nuclear Photonics NP202

X-ray spectroscopy evidence of solid-density ultra-relativistic laser plasma in renewable micron-scale cryogenic clusters targets

Recently it was shown that injection of gas preliminary cooled down to cryogenic temperatures is a promising way to produce micron-scale clusters. For example, in hydrogen clusters with the size up to ≈2 μm were produced via a conical nozzle at 25 K, 6 MPa. Micron scale clusters can be also created with another pre-cooled gases, for example Ar or Kr, but for other temperature values. So large clusters diameter value becomes comparable with a waist width of a laser beam produced by the J-KAREN-P laser facility. As a result, an experimental case can occur, when most of the laser energy is absorbed in a single cluster. Such situation can be considered as an interaction of a laser pulse with matter, state of which is close to solid-state. In the presentation plasma X-ray spectra features indicating the described case of high-intensity laser and large-scale-cluster interaction are discussed. They were observed experimentally during irradiation of cryogenic (T = 140 K – 220 K) Ar flows by ultra-intensive (I = 10^22 W/cm^2) femtosecond laser pulses generated by the J-KAREN-P laser. Registered X-ray spectra of produced plasma contains Ar XVIII Ly-α and Ar XVII He-α of comparable intensities. It should be noted that the Ar XVIII Ly-α line has been for the first time observed in laser-matter interaction experiments. It indicates that the density of irradiated matter is high enough to provide a rate of impact ionization process, which is sufficient to create a significant amount of H-like Ar ions. It is a challenge to reach such condition for a pure gas or small clusters. Thus, the observed spectra are considered as an evidence of a large micron cluster existing in the beam waist region.International Conference on High Energy Density Sciences (HEDS2021