Anomalous electron heating in laboratory magnetized quasi-perpendicular collisionless shocks

Abstract: We present laboratory results from supercritical, magnetized collisionless shock experiments ($M_A \lesssim 10$, $\beta\sim 1$). We report the first observation of fully-developed shocks ($R=4$ compression ratio and a downstream region decoupled from the piston) after seven upstream ion gyration periods. A foot ahead of the shock exhibits super-adiabatic electron and ion heating. We measure the electron temperature $T_e = 115$ eV and ion temperature $T_i = 15$ eV upstream of the shock; whereas, downstream, we measure $T_e=390$ eV and infer $T_i=340$ eV, consistent with both Thomson scattering ion-acoustic wave spectral broadening and Rankine-Hugoniot conditions. The downstream electron temperature has a $30$-percent excess from adiabatic and collisional electron-ion heating, implying significant collisionless anomalous electron heating. Furthermore, downstream electrons and ions are in equipartition, with a unity electron-ion temperature ratio $T_e/T_i = 1.2$.