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Observations of magnetic field amplification when a magnetized plasma jet impacts a target cloud

Author: Auna L. Moser
Requested Type: Consider for Invited
Submitted: 2009-12-10 13:39:50

Co-authors: P. M. Bellan

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

Abstract Text:
The Caltech spheromak formation experiment creates a dense magnetized plasma jet via a high voltage capacitor bank across coplanar electrodes. We recently replaced the existing 120 uF capacitor bank, which supplied a ~100 kA current pulse with a FWHM of ~10 us, with a pulse forming network (PFN) with 2 parallel sections of 5 120 uF capacitors, designed to produce a peak current of ~150kA with a FWHM of ~50 us. Preliminary experiments with the PFN show that the plasma jet extends to 55 cm, ~1.5 times the length of the jet produced by the old capacitor bank and can develop 2 visible twists upon onset of the kink instability. Future experiments will use the old capacitor bank for plasma breakdown and the PFN for sustainment.

A series of experiments in which we inject a localized cloud of neutral gas into the path of the axially expanding plasma jet allow us to study the effect of various species of neutrals on the evolution of the jet. The magnetized plasma jet is dense, ~10^21 m^(-3) and travels at velocities up to 50 km/s, depending on the mass of the plasma species. The neutral cloud is much less dense, and stationary on the time scale of the experiment. In experiments performed to date, qualitatively different behavior is seen in two different cases. In the first case, a light-species plasma, usually H, collides with a heavy-species cloud, usually Ar. At 50 km/s, the H travels faster than the neutral Ar's critical ionization velocity. In these experiments, the front of the jet slows significantly and fast camera images and 12-channel spectrometer data show the H plasma thickening in the region where the jet first encounters the Ar. A multi-element magnetic probe with channels along the axial direction shows deformation of the magnetic field in the region of plasma-neutral interaction. The magnetic field is amplified in the region of plasma pile-up. Farther from the electrode, past the plasma pile-up region, the magnetic field is diminished. In a second type of experiment, the jet is a heavy species (again, usually Ar) and the neutral gas is light (H). In this case the jet travels at ~20 km/s, below the neutral H's critical ionization velocity. In these experiment the jet is minimally affected by the neutral gas.

Characterization: E1,E5

If not selected for an oral, please have poster grouped with other Caltech presentations.

Princeton University

Innovative Confinement Concepts Workshop
February 16-19, 2010
Princeton, New Jersey

ICC 2010