Measurement of Light scattered from Density Fluctuations within a Collisionless Shock

Abstract

WE have previously described a collisionless shock with low Alfvén Mach number (M_A), which propagates perpendicular to a magnetic field^1–3. The shock is produced by the radial compression of an initial hydrogen plasma within a linear z-pinch. The initial plasma is 85 per cent ionized with electron density $n_{{\text{e}}_1}$ = 6.4 × 10²⁰ m⁻³, and temperatures $T_{{\text{e}}_1}$ = $T_{{\text{i}}_1}$ = 1.2 eV, and is in an axial magnetic field $B_{{\text{z}}_1}$ = 0.12 Wb m⁻². The shock propagates radially inwards through the initial plasma with a steady velocity V_s = 240 km s⁻¹ (M_A = 2.5), and with a steady structure of width L_s = 1.4 mm and compression ratio F = 2.5. The measured electron temperature behind the shock, $T_{{\text{e}}_2}$ = 44 eV, implies a resistivity within the shock about two orders of magnitude greater than the classical collisional value⁴. This demonstrates the collisionless nature of the shock. The effective mean resistivity (η̄), obtained from the power balance equation corresponds to a colision frequency The currently accepted theories of such shocks^3,5,6 invoke ion wave turbulence⁷ to explain the structure and collisionless heating.