H. Aoki

Collisionless shock generation in high-speed counterstreaming plasma flows by a high-power laser

The experimental demonstration of the formation of a strong electrostatic (ES) collisionless shock has been carried out with high-speed counterstreaming plasmas, produced by a high-power laser irradiation, without external magnetic field. The nearly four times density jump observed in the experiment shows a high Mach-number shock. This large density jump is attributed to the compression of the downstream plasma by momentum transfer by ion reflection of the upstream plasma. Particle-in-cell (PIC) simulation shows the production of a collisionless high Mach-number ES shock with counterstreaming interaction of two plasma slabs with different temperatures and densities, as pointed out by Sorasio et al. [Phys. Rev. Lett. 96, 045005 (2006)]. It is speculated that the shock discontinuity is balanced with the momentum of incoming and reflected ions and the predominant pressure of the electrons in the downstream with PIC simulation.

JET FORMATION IN COUNTERSTREAMING COLLISIONLESS PLASMAS

Plasma jet formation was observed in counterstreaming plasmas in a laboratory experiment. In order to model an ambient plasma of astrophysical jets, the counterstreaming plasmas were created by irradiating a double CH-plane target with a high-power laser system. Since the mean free paths of the ions in terms of the counterstreaming motion were larger than the scale length of the experiment, the two-stream interaction of the plasmas was essentially collisionless. The time evolution of the jet collimation was obtained over several shots with different timing by shadowgraphy. When a single CH-plane target was irradiated, no jet collimation was observed. The counterstreaming plasma as an ambient plasma is essential for the jet plasma to collimate.

Laboratory investigations on the origins of cosmic rays

We report our recent efforts on the experimental investigations related to the origins of cosmic rays. The origins of cosmic rays are long standing open issues in astrophysics. The galactic and extragalactic cosmic rays are considered to be accelerated in non-relativistic and relativistic collisionless shocks in the universe, respectively. However, the acceleration and transport processes of the cosmic rays are not well understood, and how the collisionless shocks are created is still under investigation. Recent high-power and high-intensity laser technologies allow us to simulate astrophysical phenomena in laboratories. We present our experimental results of collisionless shock formations in laser-produced plasmas.

Collisionless shock generation by a high-power laser

We report the experimental generation of an electrostatic collisionless shock in counter-streaming plasmas without an external magnetic field. The collisionless counter-streaming plasmas were created by a high-power laser. The expansion of plasmas was measured with interferometry and shadowgraphy. Large density jump was observed in both diagnostics. The density jump is the collisionless shock because the width of this jump is much shorter than the ion-ion mean-free-path calculated from flow velocities and densities. Particle-in-cell simulations suggest the formation of the electrostatic collisionless shock in the counter-streaming plasmas. In our experiment, considering the shock width, the observed shock is the electrostatic shock.

Characterization of electrostatic shock in laser-produced optically-thin plasma flows using optical diagnostics

We present a method for evaluating the properties of electrostatic shock in laser-produced plasmas by using optical diagnostics. A shock is formed by a collimated jet in counter-streaming plasmas in nearly collisionless condition, showing the steepening of the transition width in time. In the present experiment, a streaked optical pyrometry was applied to evaluate the electron density and temperatures in the upstream and downstream regions of the shock so that the shock conditions are satisfied, by assuming thermal bremsstrahlung emission in optically thin plasmas. The derived electron densities are nearly consistent with those estimated from interferometry.

Jet formation in counterstreaming plasmas produced by high-power laser beams

We report the experimental results of a plasma jet formation in counterstreaming plasmas created by irradiating a double CH-plane target with a high-power laser system. The time evolution of the jet collimation was obtained over several shots, each shot taking a shadowgraphy image at a different timing. Since no jet formation was observed when a single CH-plane target was irradiated, the presence of the counterstreaming plasma, as an ambient plasma, is essential to the jet plasma collimation.

Optical pyrometer system for collisionless shock experiments in high-power laser-produced plasmas

A temporally and spatially resolved optical pyrometer system has been fielded on Gekko XII experiments. The system is based on the self-emission measurements with a gated optical imager (GOI) and a streaked optical pyrometer (SOP). Both detectors measure the intensity of the self-emission from laser-produced plasmas at the wavelength of 450 nm with a bandpass filter with a width of ~10 nm in FWHM. The measurements were calibrated with different methods, and both results agreed with each other within 30% as previously reported [T. Morita et al., Astrophys. Space Sci. 336, 283 (2011)]. As a tool for measuring the properties of low-density plasmas, the system is applicable for the measurements of the electron temperature and density in collisionless shock experiments [Y. Kuramitsu et al., Phys. Rev. Lett. 106, 175002 (2011)].

Laboratory experiment to study collisionless shock

We report the experimental results of collisionless shock formation in counterstreaming plasmas produced by a high-power laser system. The experiment was performed with Gekko XII HIPER laser system at the Institute of Laser Engineering. In order to model collisionless shocks in the universe, supersonic counterstreaming plasma flows were generated using a CH double-plane target. By using the self-emission measurements, we observed the emission increase toward the shock through the downstream. We also observed the density jump associated with the emission increase. The width of the transition region is shorter than the ion-ion mean-free-path calculated from the measured plasma velocity and density.