1. Improvements to the Periodically Oscillating Plasma Sphere Experiment
Title of poster in Arial bold, size pt.
First initial, author’s last name, affiliation in Arial, italic, 54 pt; with group designation
A.M. McEvoy, Y. Kim, H.W. Herrmann, (LANL)
ABSTRACT
The Periodically Oscillating Plasma Sphere (POPS), 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. 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.
Recent funding proposals have been submitted to address several design improvements that will significantly improve the operation of the existing POPS IEC device. The greatest improvement in electron confinement will arise from a reconfiguration of the electron emitter geometry. The existing electron guns are located at the chamber wall and utilize an extractor grid to inject electrons from the hot cathode surface to the inner grid. Most of the electrons injected in this manner make one pass through the inner grid and are lost to the chamber wall on the opposite side, significantly reducing the electron confinement efficiency of the device.
Additional improvements will provide enhanced control over virtual cathode formation and increased potential well lifetimes. A recently completed 10 stage Marx-bank power supply will provide the capability to dynamically ramp the electron injection current, which will compensate for electron losses and extend the virtual cathode lifetime. A POPS frequency feedback control system will allow for dynamic tuning of the applied driving frequency to match the fluctuating resonant frequency of the POPS plasma. Redesigned electron guns will remove the extractor grids, which currently sink many more electrons than are injected to the virtual cathode, and will include focusing apertures for enhanced control of the electron injection profile.
Proposed Improvements
Existing electron gun arrangement inhibits electron confinement (Fig. 1a) and significantly limits achievable potential well depths by ensuring only single pass electron trajectories
Improved electron emitters that extend away from the chamber wall will allow for enhanced electron confinement (Fig. 1b)
Replacing, modifying or removing the outer grid will be necessary to provide room for the improved electron guns
Upgraded Pierce-type electron guns will exclude extractor grids, which act as a prohibitive electron sink, and will include tunable focusing optics for increased electron injection control
Real-time POPS frequency feedback control will monitor changes in potential well depth and adjust the applied driving frequency accordingly (Fig. 2)
10-stage Marx bank power supply will provide ramped electron injection capability to dynamically control virtual cathode electron density profile (Fig. 3)
Inner Grid
Chamber Wall
Figure 1a: Electrons born at the chamber wall are lost to the opposite wall after one pass
Project Goals
Experimental validation for the POPS-based fusion concept
Formation of virtual cathodes with deep potential wells and long lifetimes
Maintain phase-locking between POPS plasma and external RF oscillation
Maximize compression of POPS plasma during collapse phase of oscillation
Ultimate Goal: Demonstrate feasibility of Q > 1 operation of an advanced IEC device utilizing POPS
Previous Results and Observations
POPS driven plasma oscillations were shown to extend virtual cathode lifetimes
Predicted POPS frequency formulation was verified for several ion species over a range of potential well depths
Figure 1b: Electrons born inside the chamber wall can re-circulate
Arbitrary voltage control
channels
voltage
Electron
emitter
Emissive probe
Grid holder
Outer
grid
Inner
Los Alamos
POPS-based
IEC device
Figure 3: Ramped voltage profile of 10-stage Marx bank electron emitter power supply
Figure 2: Gradual decay of virtual cathode changes the resonant driving frequency of the plasma
Contact Information
Aaron M. McEvoy
Phone: (505)
Work supported by DOE Office of Fusion Energy Sciences Innovative Confinement Concepts Program
LA-UR