[time-nuts] HP5061B High Ion Current/Tubes Out of Cesium

[Donald E. Pauly]

https://www.febo.com/pipermail/time-nuts/2017-April/thread.html

I spoke to Corby on the phone a few days ago about our first HP5061B
that locks fine but has ion current on the order of 76 μA.  It has not
gone down significantly in two months of pump operation.  He mentioned
that the electrodes in the pump are made from titanium and that sharp
whiskers can form on the electrodes from metal migration.  They
apparently cause corona and keep the ion current high indefinitely.
According to Corby, the vacuum in the tube may be fine and the leakage
current make it appear otherwise.

We performed an experiment using our second HP5061B that we suspect of
being out of cesium.  When we first got it, we had about 10 μA of ion
current and within a day it went down to nearly zero.  Today we
jumpered R12 on the cesium oven board to raise the oven temperature.
We previously had checked all waveforms for normal operation on the
board.  This includes measuring cold resistance of the cesium heater
at 2.6 Ω and hot wire ionizer of 0.1 Ω.  Power to each was close to
the nominal 2.6 Watts and 4 Watts respectively.

We bumped oven heater voltage up to 11 Volts with the short on R12.
This could have put up to 48 Watts into the oven heater unless its
resistance went up significantly.  After a couple of minutes the oven
150° C  overtemp circuit shut down the switching regulator.  We saw no
increase of beam current even though normal oven temperature is 85°
according to the tube data plate.  We let the tube cool down and
repeated the experiment several times.  We had turned the beam current
adjust all the way up to -2,880 V.  On our good instrument we can get
20 μA beam current with only -1,700 V or so out of the -2,500 V
supply.  We therefore concluded that the beam tube was hopeless and
decided on the risky experiment.

We removed the +3,500 V supply on the suspected bad tube from the ion
pump and connected the -2,500 V supply to the pump.  We left the
-2,500 supply on the electron multiplier as well.  We saw no drop in
its -2,880 Voltage.  We would easily have seen 200 M Ω worth of
leakage on the ion pump.  Therefore the ion pump will work with either
polarity of voltage.  We have  decided to take the risk of reversing
the diodes on the +3,500 V supply on our good instrument and watch the
ion current.  We hope that the reverse polarity will burn out the
whiskers or other leakage caused by long application of positive
voltage.  We have devised a test that will show up to 1,000 M Ω of any
resistive leakage on the tube before we apply reverse voltage to it.

πθ°μΩω±√·Γλ
WB0KVV