[Amps] Ion Pumps - current levels

[Peter Chadwick]

The Siemens YH1045 TWT ( bit old now - used in the Marconi P2000 ground station tx) ran 10kW out at around 6GHz: it ran depressed collector at 18kV, 2.7 amps or so, and the helix was at 22kV and about 0.5 amp. Its ion pump ran with around 3kV. Before the heater was fired up, the ion pump current had to be below 25 microamps: after the heater was on, it shot up. If it exceeded 100 micoramps, the tube had to be tripped off. Once the ion pump current was below 2 microamps, you could apply the HV: if the ion pump current rose to 25 microamps, you tripped the HV, and if it hit 100 microamps, the heater went off as well.
Doing those trips was great fun in middle 1960s: no monolithic op amps (even then bias current might be a problem), no CMOS op amps either. Eventually, we did it discretely using a dual unprotected MOSFET for the input to get over the problems of offset and bias current.

73
Peter G3RZP