[Sneap] Alphatross troubles - resolved

[David Weisser]

SNEAPers,

Thanks to all of you who have shared experience and advice about the
instability problem we were having with the Alphatross output.  I think that
a concentrated session on the Alphatross would be a good idea for Ovidiu to
include in the next meeting.

The instability problem has been fixed. The source output is back to 1.5 to
3.0 micro-amps and stable. Those of you who pointed to Rb vapour density
variations are right.  What eliminated the instability was the
re-installation of the aluminium foil around the oven heater and
thermocouple.  Howard Wallace had wrapped the oven assembly in foil quite a
while ago to cope with the apparent temperature differentials between a red
hot heater and the dark stainless steel Rb reservoir.  I removed the foil
following the discovery of Rb in the quartz ion source bottle thinking that
the Rb got too hot. In retrospect, we realize that the instability started
then.

Had Howard still been in the lab, we would not have struggled in this trap
for so long. Unfortunately, Howard no longer is able to come to work.

It is easier to report the success than to explain it. Those of you with
standard NEC installations, which control the heat to the oven with a
variac, would not have the problem we had since we installed a temperature
controller.  As the temperature controller switches the AC off and on to
maintain a preset temperature, the beam output changes even though the
thermocouple remains apparently constant to 1 C. A significant part of the
heat transfer is radiative which has a much faster response than conduction
though the rather poor clamp.  Enclosing the heater, thermocouple and bottom
of the reservoir, places them in an oven which moderates the radiative heat
transfer.

So it looks like the Rb is not too oxidized since we have the same load in
and that the mesh wick is not an issue since it has not been in use for more
than a year.

We did have a problem with the alignment of the ion source bottle because I
installed it rather than Howard or Tom and I wasn't careful enough.  This
misalignment resulted in poorer transmission because the center of the
plasma was offset to the source exit canal.  The misalignment symptom also
showed up as their being a peak in the source output at a source solenoid
magnet current below the 4 A maximum.  This corresponded to spreading the
hot core of the plasma to better overlap the exit canal when maximum plasma
density was not lined up with the canal.

We checked on the oxygen beam output and it was immeasurably small when the
Helium minus was 1.5 micro-amps.

There is a consensus with which we agree, that keeping the source going all
the time or at least keeping the magnet on all the time, is correlated with
best performance.  I wish I understood why.

Thanks again to you all and I look forward to a strong session on the
Alphatross in October.

David & Tom



********************************************
D.C. Weisser
Nuclear Physics Department
Research School of Physical Sciences and Engineering
Australian National University
Canberra A.C.T. 0200
AUSTRALIA

Tel:    + 61 (0)2 6125 2080
Fax:    + 61 (0)2 6125 0748
Mobile: + 61 (0)414 249 209

david.weisser at anu.edu.au