Investigation of magnetofluiddynamic acceleration of subsonic inductively coupled plasma

Electromagnetic acceleration uses electromagnetic body force produced by the interactions of currents carried in plasma which is either externally applied or self-induced magnetic fields to accelerate the whole body of gas. Historically, these plasmas sources have been arc jets, shock tube and microwaves. Never has an electromagnetic accelerator been powered by an inductively coupled plasma (ICP) source.

The von Karman Institute has experimentally investigated the acceleration of an electrically conductive fluid produce by a subsonic ICP source. This ICP source was powered with a 15 kW and 27.1 MHz radio frequency facility called the Minitorch. The electromagnetic acceleration was accomplished with the design, fabrication and testing of a linear Hall current magnetofluiddynamic accelerator (MFDA) channel. The channel was geometrically orientated into the Hall configuration to accounts for the large Hall Effect. This channel used a single pair of copper annulus electrodes powered by a 10 kW direct current power supply. The channel was water cooled and contained various diagnostics to provide greater insight to the electromagnetic acceleration process. This was the first successful magnetofluiddynamic acceleration of an ICP source and validates the proof of concept.

One-dimensional MFD modeling was formulated and used to determine the necessary performance requirements of the MFDA channel E and B field subsystems. An interaction parameter of approximately 2.25 was required for the doubling of an inlet velocity of 300 m/sec. The required subsystem need to provide a current density was 6 Amps/cm2 with a magnetic field strength of 0.50 Tesla over an acceleration length of 0.1 meters. Additional the most critical constraint was the thermal management subsystem which was designed to overcome large heat transfer fluxes to achieve a steady state condition over a test run of 10 minutes.

The dynamic pressure measured increase the inlet velocity 101% for an argon plasma flowing at 1.01 g/s at a magnetic field strength of 0.49 Tesla. his strong acceleration of the plasma was most notable near the region of the electrodes at the exit of the 0.1 m long channel. The central region of the plasma has less dynamic pressure increase corresponding to only a maximum of 15% increase in velocity at a magnetic strength of 0.49 Tesla. Experimental results showed that axial discharge voltages increased with increased magnetic fields, indicating a strong Hall Effect in the accelerator as expected.

Theoretical analysis was accomplished using the one-dimensional equation of motion and was compared to utilizing only the momentum equation. Experimental force fluxes were compared to the calculated values of the one-dimensional equation of motion and momentum equation. The reference area for the current density was selected from intensity measurement using a high speed camera with the MFDA channel on. There was significant error in the analysis concerning using the momentum Lorentz force only versus the one-dimensional equations of motion; which included joule heating. This analysis summarized the necessity to include joule heating in the formulation of the problem.