Characterization of magnetoplasma-dynamic compressors with self-triggering circuits

Erosion-type magnetoplasmadynamic compressors (MPC´s)¹ are of interest as UV pump lamps for gas lasers,² as shock-wave generators,³ and as sources of plasma flow.⁴ These are very low impedance devices and the electrical efficiency is greatly degraded when external switchgear is employed. A new triggering scheme is reported where the MPC device acts both as load and switch, which significantly reduces the external electrical loss. The improved electrical efficiency, defined as the percentage of stored electrical energy that is deposited into the plasma during the first half-cycle of the current pulse, is typically η ≈ 12-17%. Electrical, radiative, and blast-wave characteristics were investigated for compact MPC´s of various designs operating in laboratory air. Poly-oxymethylene, which ablates cleanly and without surface carbonization, is the preferred insulator material and sintered tungsten-copper alloy is selected for the cathode material. MPC´s are an extremely intense source of UV and VUV radiation with considerable enhancement of the continuum spectrum by line emission from vaporized cathode and insulator species. The plasma is optically dense for λ > 250 nm and is characterized by an equivalent blackbody temperature of up to T_BB ≈ 40 kK. The peak optical intensity in the UV occurs concurrently with the peak current and instant of magnetic pinch. Efficient generation of powerful shock waves requires a very low-impedance driver capable of high-dI/dt operation. Impulses greater than 0.4 N-s were produced using modest stored energies of ≈4 kJ. Measured shock velocities correlate well with a planar Chapman-Jouget detonation where the peak electrical power is assumed to be uniformly distributed over a surface equal to the MPC throat area. The life expectancy of these devices is limited primarily by erosion of the cathode and secondarily by ablation of the insulator.