A supersonic arc jet

In the plasma of a cascaded arc, with arc currents from 40 up to 100 A, arc voltage of about 100 V and an argon pressure of about 0.5 bar, the gas temperature equals the electron temperature of about 12000K. The electron density ranges from 10(21) to 10(23) m/sup -3/, ionization degree is about 10%. The equilibrium of this kind of plasmas is well understood. In the authors' experiment this arc plasma expands into a vacuum system through a hole with a diameter of 4 mm in the anode. To obtain a background pressure of roughly 1 mBar, large pumping speeds are needed. The expansion clearly shows supersonic behaviour and is limited by a shock front (Barrel-shock and Mach-disk). The experiments were done in a mixture of argon (70 cc/sec) and hydrogen (0,7 cc/sec