[Sneap] FW: Alphtross Toubles

[Van Deusen, Stuart B]

-----Original Message-----
From: Buller, Daniel L
Sent: Tuesday, June 10, 2008 5:56 AM
To: Van Deusen, Stuart B
Subject: RE: [Sneap] Alphtross Toubles

This is caused by a change in the Rubidium vapor density, probably due to an oxide layer on the Rubidium that prohibits uniform evaporation.

Dan

-----Original Message-----
From: Van Deusen, Stuart B
Sent: Monday, June 09, 2008 8:51 AM
To: Buller, Daniel L
Subject: FW: [Sneap] Alphtross Toubles

I'm not sure if this is a problem you've had and fixed or not.

Stuart


-----Original Message-----
From: sneap-bounces at tunl.duke.edu [mailto:sneap-bounces at tunl.duke.edu] On Behalf Of david weisser [david.weisser at anu.edu.au]
Sent: Friday, June 06, 2008 8:27 PM
To: SNEAP (SNEAP)
Subject: [Sneap] Alphtross Toubles

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