Enhancement of collisionless shock ion acceleration by electrostatic ion two-stream instability in the upstream plasma

Abstract

Ion acceleration in electrostatic collisionless shocks is driven by the interaction of the high-power laser with specially tailored near-relativistic critical density plasma. 2D EPOCH particle-in-cell simulations show that the ion acceleration is dependent on the target material used. In materials with low charge-tomass ratio hZ=Ai, proton beams with high flux and low energy spread are generated. In multi-ion plasmas the ions with different hZ=Ai acquire different velocities under a non-oscillating component of electrostatic field in the upstream region. This relative drift between the protons (hZ=Ai ¼ 1) and the lower hZ=Ai ions leads to the excitation of electrostatic ion two-stream instability. This in turn generates a low-velocity component in the upstream expanding protons. The velocity distribution of the upstream expanding protons is further broadened toward the higher velocity by the electrostatic ion two-stream instability between reflected protons, which results in large number of protons being accelerated by the shock.

Ion acceleration at two collisionless shocks in a multicomponent plasma

“…At a 0 = 3.35, as the relativistic electrons move through the plasma, the inertia of the more massive ions sets up an electrostatic field E x . The exponentially decreasing density profile on the rear side of the target results in an electrostatic field or target-normal-sheath-acceleration field, E TNSA [59]. This TNSA field occurs in the upstream region and results in the upstream ions moving at velocity v 0 in the longitudinal direction [72].…”

“…velocities above the lower threshold. This further increases the ion population available for CSA [59]. In a multicomponent C 2 H 3 Cl plasma, both RI-EITI and HI-EITI play essential roles in enabling the acceleration of C 6+ ions.…”

“…The ordering of velocities results from differences in the average charge-to-mass ratio Z / A , that is as V i sh and v i m are predominantly determined by the ion-acoustic velocity c i s and velocity of ions due to E TNSA , respectively. Differences in the hole-boring velocity, which depends on √ Z / A ,e xplains why V P H > V P C 2 H 3 Cl and V C C > V C C 2 H 3 Cl [59]. As a result, the shock velocity in a single-component H plasma (with Z / A =1) is larger than that in C 2 H 3 Cl ( Z / A =0.48), and shock velocity for C 6+ ion in a single-component C plasma ( Z / A =0.50) is larger than that in C 2 H 3 Cl.…”

“…These sources may prove useful for many applications [32][33][34]. Among the many ion acceleration mechanisms being pursued [35][36][37][38][39][40][41][42][43][44][45][46], collisionless shock acceleration (CSA) of ions [47][48][49][50][51][52][53][54][55][56][57][58][59] is of particular relevance to space and astrophysical shocks. With CSA, ions located ahead of an unmagnetized electrostatic (and collisionless) shock [7] are reflected by the electrostatic potential of the shock to twice the shock velocity [49].…”

“…In laser-plasma experiments hydrogen and carbon are ever-present on the surfaces of solid targets and inevitably result in multicomponent plasmas. A number of studies [53,54,56,58] specifically use multicomponent thin-foil targets such as plastic (CH) or Mylar (C 10 H 8 O 4 ), and in Kumar et al [59] we reported on how target composition influences CSA by comparing C 2 H 3 Cl, CH, He 3 H, and H. Inclusion of a high atomic-number element like Cl, results in partial ionization, to Cl 15+ , enabling the study of a material with Z / A < 0.5. An electrostatic ion two-stream instability (EITI) excited in the multicomponent plasma is central to the ion acceleration process with CSA accelerating protons [59] and heavier ions to the same velocity.…”