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Biassed Magnet

The stability of the electrode discharge was a function of both the location and voltage of the electrode, which precluded a more general analysis of the system. We found that we could eliminate the necessity of the electrode entirely if we biassed the magnet directly, in essence, making it a very large electrode. At first blush, one would have thought that the electric field, generated by spatial gradients in the potential surrounding the electrode, would be critical in ionizing the background gas and producing the glow discharge, so that biassing the magnet would require even greater voltages to compensate for its weak gradients. However the magnet confined the plasma so well that secondary collisions became the most important source of ionization, and in fact, glow discharge was sustained at a lower working voltage.

In table [*] we list the minimum voltage for a sustainable glow discharge from three different aspect ratio Nd-B magnets. In figure [*] we plot the current-voltage behavior of each system. Note that especially at lower pressures, these curves exhibit hysteresis, and in fact, will often oscillate spontaneously. That is, they grow brighter and brighter until either they trip the current limit, or they discharge and drop in intensity. We attribute both hysteresis and oscillation to a positive feedback in the production of ions through secondary collisions.


next up previous
Next: Space Charge Up: Dipole Equilibrium Previous: Electrode Injection
Robert Sheldon 2002-02-07