Evidence of anomalous electron heating in laboratory quasi-perpendicular collisionless shock experiments

We present results from laboratory experiments on supercritical magnetized collisionless shocks at conditions relevant to planetary bow-shocks. We report the first observation of fully developed shocks (x4 compression ratio and a downstream region decoupled from the piston) probed after seven upstream ion gyration periods. The data indicate the presence of a foot ahead of the density discontinuity, where both electrons and ions exhibit significant super-adiabatic heating. We directly measure x4 electron heating from an initial upstream with electron temperature T_e ~ 100 eV and indirectly infer a downstream ion temperature T_i ~ 400 eV from energy conservation across the shock. This ion temperature is consistent with the broadening of the optical Thomson scattering ion-acoustic wave feature for a proton-dominated downstream plasma. As a consequence, we infer that electrons and ions are in equipartition, with an order-unity electron-proton downstream temperature ratio.