Particle Acceleration in Quasi-Perpendicular Hall-Mediated Collisionless Shocks

Measurements of the earth’s bowshock are made difficult by both the non-stationarity of collisionless shock physics and the irrepeatability of a satellite shock crossing. A high-beta ($\beta>5$) laboratory theta-pinch experiment provides for a repeatable system to explore the collisionless shock formation process. A target plasma ($T_e \sim 2\ eV$, $n_e\sim1\times 10^{18}\ m^{-3}$) was rapidly compressed with an axial magnetic field produced from four low-inductance coils. This magnetic piston induces an opposing toroidal current in the plasma that propagates radially inward at the downstream Alfv$\'en speed ($30--100\ km\ s^-1). The upstream sound and Alfven speeds are adjustable via control over the plasma density and background field strength, but are nominally 15 and 10 $km s^-1$. The piston propagates inward with an upstream magnetosonic Mach number of $M_{MS}\sim 3--10$. Similar to the earth’s bowshock, specularly reflected ions are launched ahead of the shock, returning to the shock only on ion larmor scales. The magnetized electrons, confined to the field lines, are energized via parallel electric fields that develop to ensure $\nabla \cdot \vec{J} = 0$. Cylindrical VPIC simulations confirm the interpretation of the experiment. Complete measurements of the experiment, including ion doppler spectroscopy, will be shown.