I adjusted the filament current so I could determine if I was producing enough electrons. It turns out that at pressures below about 5 mTorr, where operation is smooth and well controlled, I need all the filament emission I can get. I found that to increase the drive current I could keep turning up the ionization grid voltage or increase the filament current.
I started out running the filaments at 3A, but when I was testing the system at pressures of about 2 mTorr I felt it was requiring too much voltage to get it to draw any ion current. I increased the filament current to 3.5A and the ion current went up by a factor of about 7. Still, at a filament current 3.5A I had put over 1500V on the ionization grid to get the system to draw an ion current of a couple of milliamps when the acceleration voltage was up close to 40kV.
There is a threshold voltage on the ionization grid below which the system won't draw any current. As the acceleration voltage on the inner grid is increased, the threshold voltage also increases. At operating voltages over 20kV, it would take my system something like 500V on the ionization grid just to get it to begin drawing current. The current draw of the ionization grid is somewhat higher the current to the inner grid, maybe as much as twice as high, I can't remember exactly since I wasn't too concerned about its exact value.
I will have to wait until I can hit lower pressures to see I if I can get enough ionization. If not, I'll keep cranking up the filament current until they fall apart at the spot welds. If that still isn't enough larger filaments will be needed. Does anyone have any idea what kind of filament wattage was used in the system described in the Hirsh/Meeks patent?
Joe Zambelli
Created on Thursday, January 18, 2001 5:01 PM EDT by Joe Zambelli