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icc_poster_2011-1.pdf2011-08-25 12:59:42Vernon Chaplin

Development of a Compact RF Pre-Ionization System for an MHD-Driven Jet Experiment

Author: Vernon H. Chaplin
Requested Type: Poster Only
Submitted: 2011-06-06 18:06:56

Co-authors: P. M. Bellan

Contact Info:
Caltech
1200 E California Blvd
Pasadena, CA   91125
USA

Abstract Text:
We are investigating MHD-driven jets with a coaxial planar electrode geometry. External coils are used to create a poloidal magnetic field linking the inner and outer electrodes, and neutral gas is injected through nozzles in the electrodes. A high voltage is applied across the electrodes, breaking down the neutral gas and driving a current along the poloidal field lines. The ramping up of this current corresponds to the injection of magnetic helicity, as the associated toroidal magnetic field adds to the poloidal bias field to create a twisted field structure. MHD forces cause the current loops linking the electrodes to merge along the central axis, driving a collimated jet of plasma away from the electrodes. The jet formation process is analogous to the mechanism believed to drive astrophysical jets perpendicular to accretion disks. As the jet lengthens, it goes kink unstable, detaching from the electrodes and evolving toward a spheromak-like force-free equilibrium. Nominal jet parameters are n ~ 10^22 m^-3, T ~ 2 eV, v ~ 10-50 km/s, I_pol. ~ 100 kA, B_pol. ~ 0.2 T, β ≤ 1. The density and flow velocity are constrained by the Paschen criterion for neutral gas breakdown under the applied high voltage: if the gas pressure is too low, plasma will not form. In order to overcome this constraint and access a new regime with lower density, faster, hotter jets, we are developing a scheme for puffing pre-ionized plasma into the chamber in place of neutral gas. Pre-ionization will be achieved using a pair of 13.56 MHz RF sources capable of outputting over 3 kW pulsed power each. The RF sources utilize power MOSFET switches and are each mounted on a single 3 inch by 7.5 inch printed circuit board. Impedance matching is accomplished with adjustable arrays of capacitors, also mounted on printed circuit boards; one RF source is tuned to output a high voltage and initiate breakdown, while the other is tuned to sustain the pre-ionized plasma by maximizing the power delivered from the source. We are investigating helicon waves as a means for efficiently coupling RF power to the plasma. The nozzle where pre-ionization takes place will be located directly behind the center electrode, which is at 2-5 kV during the main discharge. In order to protect the RF sources from this high voltage, the pre-ionization system must be able to float at the potential of the center electrode. This is achieved by using AA batteries to power the RF source. Battery operation is feasible because the sources are pulsed for < 1 ms, so the total electrical energy in a pulse is on the order of 1 J. Characterization of the RF source behavior and initial measurements of the pre-ionized plasma properties will be presented.

Characterization: D1,D5

Comments:
Please group poster with other Caltech posters.

University of Washington

Workshop on Innovation in Fusion Science (ICC2011) and
US-Japan Workshop on Compact Torus Plasma
August 16-19, 2011
Seattle, Washington

ICC 2011