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chaplin_epr_poster_2014.pdf2014-08-11 20:27:12Vernon Chaplin

Battery-Powered Inductively Coupled Plasma Source for Pre-Ionization in an MHD-Driven Jet Experiment

Author: Vernon H. Chaplin
Requested Type: Poster Only
Submitted: 2014-05-29 16:34:03

Co-authors: Paul M. Bellan

Contact Info:
Caltech
1200 E California Blvd
Pasadena, California   91125
United States

Abstract Text:
A novel radiofrequency (RF) pre-ionization plasma source has been developed to enable neutral gas breakdown at lower pressures and create faster jets at the Caltech MHD-driven jet experiment. Jets are formed by driving a current along an externally generated poloidal magnetic field linking a pair of coaxial planar electrodes; the geometry is similar to a coaxial helicity injection system. In previous experiments, the Paschen breakdown criterion has set an undesirable lower limit on the jet mass; pre-ionization of plasma in a tube behind the electrodes allows us to overcome this constraint.

The RF plasma source uses a customized pulsed 3 kW 13.56 MHz class D RF power amplifier that is powered by AA batteries, allowing it to safely float at 3-6 kV with the cathode of the jet experiment. Peak argon ion densities of nearly 4e19 m^-3 have been achieved in the source tube. The RF power is coupled to the plasma through a helicon antenna; however, scaling measurements and comparisons of the experimentally obtained plasma parameters with the predictions of a global discharge model revealed that the dominant energy input mechanism was inductive coupling rather than wave heating. The observed lack of helicon mode operation may have been a result of the small discharge tube radius (1.1 cm) and short pulse time (<1 ms).

Analytical and numerical modeling showed that the field-aligned transport of the RF plasma out of the antenna region was well-described by ambipolar diffusion with a dominant ionization term. Fast camera images of the expanding plasma were often misleading because the plasma transitioned from an ionizing phase with depressed neutral emission to a recombining phase with enhanced emission during the course of the experiment. Under some conditions, the overall level of optical emission was highest after RF power turn-off (in the afterglow), and the argon plasma density also rose by a factor of 2 or more, probably due to metastable-metastable ionization.

Characterization: 4.0

Comments:
Please group Caltech posters together if possible (posters by Chaplin, Zhai, Chai, Haw, and Bellan).

Workshop on Exploratory Topics in Plasma and Fusion Research (EPR) and US-Japan Compact Torus (CT) Workshop
August 5-8, 2014
Madison, Wisconsin

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