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Space-charge Neutralization for Improved IEC-POPS Operation

Author: Yongho Kim
Requested Type: Poster Only
Submitted: 2010-01-13 16:39:48

Co-authors: A.M.McEvoy, H.W.Herrmann

Contact Info:
Los Alamos National Laboratory
Plasma Physics Group (P-24)
Los Alamos, NM   87545
US

Abstract Text:
The Periodically Oscillating Plasma Sphere (POPS) [1], originally proposed by D. Barnes and R. Nebel of Los Alamos National Laboratory (LANL), is a novel approach to inertial electrostatic confinement (IEC) fusion that exploits the natural resonant frequency of the IEC plasma to maintain stability and thermodynamic equilibrium. Electrons are injected to the center of the POPS IEC device, using a combination of hot cathode electron emitters and positively biased grids, which produces a spherical, uniform electron-density virtual cathode [2]. The resulting potential well is driven by a sinusoidal voltage on the innermost grid that drives the plasma like a harmonic oscillator: phase locking a radial plasma expansion and compression oscillation while resonantly heating the confined ions.

During the POPS experiments, however, virtual cathode instability and degradation was observed, likely due to background ionization. In the first few milliseconds following virtual cathode formation, the well depth gets smaller and then disappears very rapidly. 1D simulation indicates that the accumulation of ions in the center leads to virtual cathode loss, which can be prevented by sufficient space-charge neutralization. In order to enhance electron injection during the compression phase, and thereby neutralize ion build up, we will continually ramp up the emitter bias voltage. In this way, we should be able to maintain the virtual cathode depth while simultaneously increasing the ion density to the quasi-neutral limit. A pulsed high-voltage power supply is an essential element to ramp up emitter bias voltage and so a scaled prototype modulator for IEC POPS operation was designed and constructed. In addition, a POPS frequency feedback control system is under implementation which allows for dynamic tuning of the applied driving frequency to match the fluctuating resonant frequency of the POPS plasma.

[1] R.A. Nebel and D.C. Barnes, “The Periodically Oscillating Plasma Sphere,” Fusion Technology 38, 28 (1998).
[2] J. Park, R.A. Nebel, S. Stange, and S.K. Murali, “Experimental Observation of a Periodically Oscillating Plasma Sphere in a Gridded Inertial Electrostatic Confinement Device,” Physical Review Letters 95, 015003 (2005).

Characterization: C

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