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Faster, Hotter MHD-Driven Jets Using RF Pre-Ionization

Author: Vernon H. Chaplin
Requested Type: Poster Only
Submitted: 2012-12-06 17:15:40

Co-authors: Paul M. Bellan, Krishnan Chander

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

Abstract Text:
We are investigating MHD-driven jets relevant to solar physics, astrophysical jets, and laboratory fusion plasmas with open magnetic field lines. Previous experiments at Caltech have focused on plasmas created by breaking down neutral gas using high voltage. In these experiments, loops of plasma are created along an externally generated poloidal magnetic field linking a pair of coaxial planar electrodes. The high voltage used to break down the plasma drives a current along these poloidal field lines, injecting magnetic helicity and creating 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, Ipol. ~ 100 kA, Bpol. ~ 0.2 T, β ≤ 1, lmfp ~ 10 μm, Lundquist number ~ 10 – 100.

The Paschen criterion governing neutral gas breakdown sets an undesirable lower limit for the jet density in these experiments. To overcome this constraint and access a new regime with lower density, faster, hotter jets, we have designed and constructed a pre-ionization system powered by a pulsed 3 kW 13.56 MHz class D RF power amplifier. The RF amplifier is mounted on a compact printed circuit board and powered by AA batteries, allowing it to float at the high voltage of the center electrode of the jet experiment. Battery operation is feasible because the source is pulsed for < 1 ms, so the total electrical energy in a pulse is on the order of 1 J.

Plasma is pre-ionized in a tube behind the electrodes (pre-ionized plasma density ~ 10^18 – 10^19 m^-3) and flows into the chamber along the poloidal bias field lines, then the main capacitor bank is fired to create an MHD-driven jet, as in the original experiment sequence. The lower density jets created with RF pre-ionization are expected to be less collisional and have higher Lundquist numbers than jets created by Paschen breakdown.

Experiments utilizing pre-ionization in combination with neutral gas puffing have so far achieved argon jet velocities a factor of ~2 higher than was possible without pre-ionization. Results of these experiments will be presented, along with an update on ongoing efforts to produce a low-density jet from pre-ionized plasma alone. A new IGBT-based driver circuit for the ignitron that switches the main capacitor bank will also be described.

Characterization: 4.0

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