7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Ion acceleration in a high-intensity laser-driven collisionless shock
Y. Sakawa1, R. Kumar2, M. Hanano2, A. Morace1, L. N.K. Döhl3, N. Woolsey3, Y. Ohira4, T. Sano11
Institute of Laser Engineering, Osaka University, Japan
2
Graduate School of Science, Osaka University, Japan
3
York Plasma Institute, Department of Physics, University of York, United Kingdom
4Department of Earth and Planetary Science, The University of Tokyo, Japane-mail (speaker): sakawa-y@ile.osaka-u.ac.jp
Collisionless shock is a shock wave generated in acollisionless plasma, in which Coulomb collisions arenegligible. Whereas the shock-front thickness ofcollisional shock is of the order of Coulomb
mean-free-path (mfp), that of collionless shock is muchsmaller than the mfp, and wave-particle interactions andcollective effects of electric and magnetic fields playessential roles in the shock formation. Collisionlessshock is ubiquitous in space and astrophysical plasmas,
such as Earth’s bow shock and supernova remnant
shocks, and is believed to be a source of cosmic rays.
Thanks to the development of high-power lasers, a new
method of studying high-energy astrophysics, such as theformation and evolution of collisionless shocks, in thelaboratory, Laser Astrophysics, is emerging [1-3].
Intense laser-plasma interactions are an essential tool
for the laboratory study of ion acceleration at acollisionless shock. With two-dimensional (2D)particle-in-cell (PIC) calculations of a multicomponent
plasma we observe two electrostatic collisionless shocks
at two distinct longitudinal positions when driven with alaser at normalized laser vector potential a0 that exceeds
10 [4,5]. These shocks, associated with protons andcarbon ions, accelerate ions to different velocities in an
expanding upstream with higher flux than in asingle-component hydrogen or carbon plasma. This
results from an electrostatic ion two-stream instability
caused by differences in the charge-to-mass ratio ofdifferent ions.
Electrostatic two-stream instabilities play essential
roles in an electrostatic collisionless shock formation.
They are a key dissipation mechanism and result in ion
heating and acceleration. 2D PIC simulations in a
multicomponent plasma reveal excitation of alongitudinally propagating electrostatic instability due toa non-oscillating component of the electrostatic field in
the upstream region of the shock, and generation of upand down-shifted velocity components within the
expanding-ion components [6]. A linear analysis of theinstabilities for a C2H3Cl plasma using the 1D
electrostatic plasma dispersion function, which includes
electron and ion temperature effects, shows that the most
unstable mode is the electrostatic ion-beam two-stream

instability (IBTI), which is weakly dependent on theexistence of electrons [6]. The IBTI is excited byvelocity differences between the expanding protons andcarbon-ion populations. There is an electrostatic
electron-ion two-stream instability with a much smaller
growth rate associated with a population of protons
reflecting at the shock. The excitation of the fast-growing
IBTI associated with laser-driven collisionless shockincreases the brightness of a quasimonoenergetic ion
beam.
Particle acceleration in collisionless shocks inmulticomponent plasma are ubiquitous in space andastrophysics, and these calculations identify thepossibility for studying these complex processes in thelaboratory.
References[1] Y. Sakawa, et al, Collisionless electrostatic shock
generation using high-energy laser systems, Advances inPhysics: X 1, 425-443 (2016).
[2] T. Morita, Y. Sakawa, et al, Characterization ofelectrostatic shock in laser-produced optically-thinplasma flows using optical diagnostics, Physics ofPlasmas 24, 072701-1-7 (2017).
[3] Y. Sakawa, et al, Generation of counter-streaming
plasmas for collisionless shock experiment, High Energy
Density Physics 23, 207-211 (2017).
[4] R. Kumar, Y. Sakawa, et al, Enhancement ofcollisionless shock ion acceleration by electrostatic ion
two-stream instability in the upstream plasma, PhysicalReview Accelerators and Beams 22, 043401-1-8 (2019).
[5.] R. Kumar, Y. Sakawa, et al, Ion acceleration at twocollisionless shocks in a multicomponent plasma,
Physical Review E 103, 043201-1-10 (2021).
[6] Y. Sakawa, et al, Identification of electrostatic
two-stream instabilities associated with a laser-driven
collisionless shock in a multicomponent plasma, PhysicalReview E 104, 055202-1-13 (2021).