Experimental Study of Collision-Free Shock Waves and Anomalous Electron Heating

Cylindrically converging magnetic compression waves have been observed propagating into a low density hydrogen plasma (ne = 3 × 1014, kTe = 1.4 eV) in a fast theta pinch discharge with the compressing field and initial field both parallel and antiparallel. For parallel fields, the waves are nonstationary in this experiment and cylindrical convergence effects cannot be ruled out. The thickness of the transition is greater than 1 cm which is less than the ion gyroradius and the ion mean free path for ions travelling with the wave front across the initial magnetic field. The electron heating rate (≃10 eV/nsec) and the transition thickness are both greater than one would expect for resistive heating with the Spitzer-Härm resistivity. In the case where the compressing field and the initial field are antiparallel, a wave flow develops which appears to be an almost plane and nearly stationary collision-free shock. For an initial field of −450 G the Alfvén Mach number is estimated between 4 and 9. The magnetic transition has a slowly rising front part (>1 cm) followed by a faster rising part (l ≃ 3 mm).