Compact telemeter for plasma losses on isolated surfaces

Two infrared light-beam telemeters for reading currents between 1x10-6 and 5x10-4 A at frequencies from dc to 100 kHz are described. One design is currently in use aboard the isolated rings of the Wisconsin levitated octupole for measuring the flux of plasma to collectors on the ring surface from the stable confinement region around the ring. This device is triggered from outside the vacuum vessel and applies a 20 V bias to the striped plasma collectors. Each telemeter is completely contained within a 1 cm diam by 12 cm long case and, although buried in the solid aluminum rings, can be removed and serviced through airlocks. A second telemeter design, a more versatile but much more complex predecessor, is powered by capacitors that are charged by a bank of solar cells, which are illuminated by a light source outside the plasma containment zone in controlled patterns to select bias from 0 to 50 V in 10 steps. Elaborate precautions were taken to avoid anticipated possible interference from plasma light, large magnetic fields, and high microwave fields