Dynamics of the boundary layer created by the explosion of a dense object in an ambient dilute gas triggered by a high power laser

The dynamics of the boundary layer in between two distinct collisionless plasmas created by the interaction between a dense object modeling a cluster and a short laser pulse in the presence of an ambient gas is studied with two dimensional relativistic particle-in-cell simulations, which are found to be described by three successive processes. In the first phase, a collisionless electrostatic shock wave, launched near the cluster expansion front, reflects the ambient gas ions at a contact surface as a moving wall, which allows a particle acceleration with a narrower energy spread. In the second phase, the contact surface disappears and the compressed surface of the ambient gas ions passes over the shock potential, forming an overlapping region between the cluster expansion front and the compressed surface of the ambient gas. Here, another type of nonlinear wave is found to be evolved, leading to a relaxation of the shock structure, while continuing to reflect the ambient gas ions. The nonlinear wave exhibits a bipolar electric field structure that is sustained for a long timescale coupled with slowly evolving ion dynamics, suggesting that a quasistationary kinetic equilibrium dominated by electron vortices in the phase space is established. In the third phase, a rarefaction wave is triggered and evolves at the compressed surface of ambient gas. This is because some of the ambient gas ions tend to pass over the potential of the bipolar electric field. Simultaneously, a staircase structure, i.e., a kind of internal shock, is formed in the cluster due to the deceleration of cluster ions. Such structure formations and successive dynamics accompanied by the transitions from the shock wave phase through the overlapping phase to the rarefaction wave phase are considered to be a unique nature at the boundary layer created by an explosion of a dense plasma object in an ambient dilute plasma

Generation of quasi-monoenergetic protons exceeding 200 MeV via intra-cluster collisionless shocks in a laser-irradiated micron-size H2 cluster

In laser-driven ion accelerations, the recent advancements in acceleration techniques using thin foil targets now allow the maximum proton energies close to 100 MeV. However, the generation of ion beams with low bandwidth and low divergence at a high repetition rate still remains a critical issue. In addition, from a view point of practical applications, high-purity proton beams are quite advantageous. In experiments using thin foil targets, however, protons from surface contaminants along with the high-z component materials are accelerated together, making the production of impurity-free proton beams unrealistic. Here we propose a new way to produce highly-directional, highly-reproducible, impurity free, quasi-monoenergetic proton beams exceeding 200 MeV using micron-size cluster targets. Interaction processes of a PW class, ultrashort (∼40 fs) laser pulses and micron-size hydrogen clusters are investigated using 3D-PIC simulations. We found a special parameter regime that highly directional (divergence angle ~8.5 degree) and quasi-monoenergetic (ΔE/E∼7%) protons with energies up to 290 MeV are accelerated by collisionless shocks propagating inside the micron-size clusters in the relativistically-induced transparency regime. The proposed acceleration mechanism can be considered as a future candidate in laser-driven proton sources exceeding 200 MeV with the upcoming advanced multi-PW lasers.International Conference on Inertial Fusion Sciences and Applications (IFSA 2017

Design of the energy spectrometer for laser-accelerated protons using stacked CR-39 detector

In the laser-driven ion acceleration experiment, CR-39 track detectors have been used for the measurement of laser-accelerated ions. This is because CR-39 detectors are insensitive for X-rays and energetic electrons, which are simultaneously generated with ions by the interaction between intense laser pulse and the target material. To evaluate the energy spectrum of laser-accelerated protons, which has the broad energy spectrum, the stacked CR-39 detectors are required because the energetic protons penetrate through the single layer of CR-39. For example, in the case of HARZLAS (TD-1) type CR-39 detector, which can detect up to 20 MeV protons, with the thickness of 0.9 mm, more than 9.63 MeV protons penetrate through the single layer. In other words, the protons with the energies more than 9.63 MeV create the etch pits not only on first layer but also on second layer. In such case, the accurate energy spectrum is not able to obtain by the numbers of etch pits on each layer of CR-39. In the present study, to measure the precise energy spectrum of laser-accelerated protons, we have designed the stacked detector using HARZLAS (TD-1) and energy moderators. Particle and Heavy Ion Transport Code System (PHITS) has been used for the optimization of the thickness of energy moderator and proof-of-principal calculation of the designed detector. We have applied polytetrafluoroethylene (PTFE) as the energy moderators because PTFE has the largest stopping power in the plastics. The thickness of PTFE has been determined as 1.8 mm to avoid a 20 MeV proton entering into the second layer of HARZLAS (TD-1). Therefore, the repetition of 0.9 mm thick HARZLAS (TD-1) and 1.8 mm thick PTFE can obtain the accurate energy spectrum of broad energy spread proton beams. In order to confirm the capability of the designed stacked detector, we have tried to reconstruct the model energy spectrum using the PHITS code simulation. Figure 1 shows the comparison between the model spectrum and the calculated spectrum. From the results of this simulation, the obtained energy spectrum almost reconstructed the model energy spectrum. Thus, the designed stacked detector can be applied to laser-driven ion acceleration experiments as the energy spectrometer for laser-accelerated protons.The 12th International Workshop on Ionizing Radiation Monitorin

The precise measurement of laser-accelerated ions using CR-39 detectors

The laser-driven ion acceleration via the interaction of ultrashort, intense laser pulses with matter, known as laser-plasma acceleration, is featured by its high accelerating electric fields and short pulse length compared to conventional rf-accelerators. The precise quantitative characterization of laser-accelerated ion beams in both space and energy domains is a critical issue in understanding the physics of the laser-driven ion acceleration process. To characterize the ion beam, CR-39 detectors have been used for a long time in laser-driven ion acceleration experiments as the most reliable detector. This is because CR-39 detectors have the great advantage of being insensitive to high-energy photons and electrons. In addition, CR-39 detectors are robust against electromagnetic pulses (EMP) from intense laser-plasma interactions, thus, it would be suitable for the future ion acceleration experiments using multi-PW class lasers. The potential abilities of CR-39 detectors enable us to discuss the laser-driven ion acceleration mechanism based on qualitative information on laser-accelerated ions provided by the analysis of etch pit structures on CR-39 detectors. Therefore, CR-39 detectors are the most qualified candidate detector for characterization of the laser accelerated ions. In the poster presentation, the measurement methods for laser-accelerated ions using CR-39 detector will be discussed with the outline of the laser-driven ion acceleration experiment and the other ion beam diagnosis technique.The 12th International Workshop on Ionizing Radiation Monitorin

The precise measurement of laser-accelerated MeV/n-class high-Z ions and protons using CR-39 detectors

The precise quantitative characterization of laser-accelerated ion beams in both space and energy domains is a critical issue in understanding the physics of the laser-driven ion acceleration process. To characterize the ion beam, CR-39 detectors have been used for a long time in laser-driven ion acceleration experiments as the most reliable detector. This is because CR-39 detectors have the great advantage of being insensitive to high-energy photons and electrons. In addition, CR-39 detectors are robust against electromagnetic pulses (EMP) from intense laser-plasma interactions, therefore, it would be suitable for the future ion acceleration experiments using multi-PW class lasers. For example, the multi-step etching technique, in which the etching and the microscopic observations are repeated every certain period of the chemical etching time, has been applied to laser-driven ion acceleration experiments to demonstrate a tenfold improvement in the accuracy of determination of the maximum proton energy with uncertainty E = 0.1 MeV.In the present study, to demonstrate the capability of CR-39 for precise measurements in the energy and spatial distributions of laser accelerated ions, CR-39 detectors have been applied to the laser-driven ion acceleration experiment using cluster-gas target, which consists of CO2 clusters embedded in background H2 gas, with the J-KAREN laser (1 J, 40 fs) at QST-KPSI. In order to obtain the high resolution energy spectra of each ion species, the new diagnosis method, which can separately measure the precise energy spectra of the laser-accelerated MeV/n class high-Z ions and that of MeV protons, has been developed. By a careful analysis of etch pit structures using the multi-step etching technique, the maximum energies of carbon/oxygen ions (from clusters) and protons (from background gas) were determined as 1.1±0.1 MeV and 1.6±0.1 MeV/n, respectively. The shapes of energy spectra revealed that the number of carbon/oxygen ions sharply decreased at the maximum energy and that of protons gradually decreased with increasing energy. In addition, in the same experiment series, to obtain the spatial distribution of laser-accelerated ions, CR-39 detectors were installed as to surround the laser focal spot. The CR-39 detectors, which were installed at 45̊ and 90̊ from laser propagation direction, showed a homogeneous etch pit spatial distribution. On the other hand, an inhomogeneous etch pit spatial distribution was observed on CR-39 installed at the laser propagation direction. The inhomogeneous distribution suggests that the spatial distribution of the ion beam could be modulated by the effect of electromagnetic structures created in the laser-plasma. The potential abilities of CR-39 detectors enable us to discuss the laser-driven ion acceleration mechanism based on qualitative information on laser-accelerated ions provided by the analysis of etch pit structures on CR-39 detectors, therefore, CR-39 detectors are the most qualified candidate detector for characterization of the laser accelerated ions.International Symposium on Ultrafast Intense Laser Science 1