Development and Operation of a Supersonic Nonequilibrium MHD Channel

Abstract

The results of the present work demonstrate the Lorentz force effect on the supersonic boundary layer in M=3 flow of nitrogen ionized by a high-power transverse RF discharge (up to 1 kW) in the presence of a B=1.5 T magnetic field. Boundary layer density fluctuation spectra are measured using the Laser Differential Interferometry (LDI) diagnostics. In particular, decelerating Lorentz force applied to the flow produces a well reproduced increase of the density fluctuation intensity by up to 10-20% (1-2 dB), compared to the accelerating force of the same magnitude applied to the same flow. The effect is detected for two possible combinations of the magnetic field and current directions producing the same Lorentz force direction (both for accelerating and decelerating force). The effect is observed to increase with the flow conductivity. On the other hand, the effect of Joule heating on the fluctuation spectra appears insignificant. Experiments using transverse repetitively pulsed, short pulse duration, high voltage discharge sustained in M=4 flows of nitrogen and air at magnetic fields of up to B=1.75 T showed that it produces stable, diffuse, and uniform plasmas. The peak voltage and current in the pulsed discharge are 10-15 kV and 150-300 A, with the voltage pulse duration (FWHM) of approximately 20 ns. Magnetic field was found to considerably improve the load impedance matching of the pulse discharge, increasing the pulse power coupled to the flow from about 8 mJ at B=0 to 56 mJ at B=1.5 Tesla. Operation of a crossed discharge (pulser + DC sustainer) in supersonic flows of air and nitrogen at the pulse repetition rate of 40 kHz demonstrated that such discharge produces a stable plasma, with the lifetime of the order of the time between the high voltage pulses (both in nitrogen and dry air). The time-average DC current achieved in such discharges is up to at least 0.8 A in nitrogen (conductivity of σ=0.18 mho/m) and up to 0.4 A in air (σ=0.09 mho/m). These results demonstrate feasibility of the use of the repetitively pulsed discharge as an efficient and stable ionization source producing uniform high electrical conductivity in supersonic nonequilibrium MHD flows. 1