[Sneap] Alphtross Toubles

[david weisser]

SNEAPers,

 

We have had a fantastic run with our NEC Alphatross helium charge exchange
ion source - more than 2 micro-amps for months and steady as a rock.  For
almost all of this time, the source was not turned off though occasionally
the rubidium oven heater was turned off for long weekends. Just as I was
bragging about it and preparing to share our expertise with others, it went
to, oh your know!

 

Just before that, it was turned off for a week for maintenance to the lab
air conditioning and when it was turned on again, the discharge quartz tube
filled with Rubidium.  So, of course, we stripped and cleaned the entire
source, including the exchange chamber, and loaded a fresh rubidium load.
However, now the output is far from steady - the negative ion current varies
by +- 10% over about 5 minutes.  So too does the Bias current, that is the
current in the power supply that accelerates the negative ion beam from the
rubidium exchange canal to ground.

 

We investigated and "fixed" everything we could think of.  The Probe
current, that is the current to the 6 kV electrode in the RF discharge
bottle is steady and so, we infer, was the positive ion current.  The plate+
screen current is also steady. Nevertheless, we eliminated the possibility
that the gas pressure in the source was subtly varying by eliminating the
gas pressure regulator on the helium bottle and reverting to the small,
fixed bottle pressure. No help.  We reverted to variac control to the
rubidium oven heater and even replaced the heater cartridge - no help.  The
room temperature was steady too.  We stripped and cleaned the rubidium from
the Bias electrode - also no help.

 

We have run out of ideas.  The RBS machine is still useable so the problem
is not operationally debilitating but it has taken a toll on my ego.  

 

All ideas welcome,

 

David

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