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Investigation into reducing the mass influx associated with helicity injection

Author: Paul M. Bellan
Requested Type: Poster Only
Submitted: 2009-12-08 22:47:32

Co-authors: V. H. Chaplin

Contact Info:
1200 E. California Blvd
Pasadena, CA   91125

Abstract Text:
Spheromak formation physics, astrophysical jets, and solar coronal loops all involve electric current flow between bounding surfaces (electrodes) intercepting an open magnetic flux tube. This current has an associated azimuthal magnetic field that vectorially adds to the original axial magnetic field resulting in a twisted magnetic field. Ramping up the current flowing between the electrodes is equivalent to injecting azimuthal magnetic flux linking the already-existing axial flux thereby increasing the magnetic helicity of the flux tube. The ideal MHD frozen-in flux condition implies that ramping up the electric current and the concomitant injection of azimuthal magnetic flux necessitates an injection of plasma from an electrode into the flux tube. This ingestion of plasma from an electrode into the flux tube requires a plasma source at the electrode. In our experiments ionization of gas puffed from a nozzle (hole) at the electrode provides this plasma source. The ingested plasma is accelerated by MHD forces into the flux tube resulting in a supersonic inflow, i.e., jet, [1] that fills the flux tube. In addition, axial compression of the frozen-in moving azimuthal flux pinches the plasma jet so it becomes collimated and dense. The velocity, density, and magnetic profile of experimentally observed collimated jets depend on the mass influx, the normal magnetic flux, and the electric current. Current ramp-up in our existing experiments is achieved by application of high voltage across the electrodes. From Faraday’s law the rate of flux injection is just the applied voltage. The applied high voltage must first perform the task of ionizing the injected neutral gas, a task governed by the Paschen breakdown criterion which imposes an undesirable lower bound for the mass influx in our existing experiments. The Paschen constraint could in principle be overcome by pre-ionizing the gas in the injection nozzles. Pre-ionization would enable access to regimes having lower densities, faster jets, and hotter plasmas. Investigations have begun on using rf breakdown inside the gas nozzle with the goal of having the high voltage operate on a pre-existing plasma and so be relieved of the task of ionizing neutral gas subject to the Paschen constraint. The goal is thus to have “plasma puffing” instead of “gas puffing”. A battery-powered 13.56 MHz rf source capable of producing over 3.5 kW pulsed output has been constructed and is being tested to see if it can provide a suitable source plasma. Battery operation permits floating the rf system at arbitrarily high voltage so that pre-ionized plasma can be produced at a nozzle located on the center electrode (high voltage electrode) of a coaxial magnetized plasma source.

[1] D. Kumar and P. M. Bellan, Phys. Rev. Lett. 105, art 105003 (2009)

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