[Sneap] Some comments about the ion source gas supply used at Lowell

[Gunter Kegel]

Hi Sneapies,

Some of your comments caused me to write up the attached story. Feel free to
call me (978 934 3280) or to send me an e-mail (Gunter_Kegel at uml.edu), if
you have any questions.

Gunter Kegel

 

 

 

Some comments about the ion source gas supply used at Lowell.

 

 We banished all rubber (Viton) O-rings from our gas supply system with one
exception: the O-ring at the ion source gas inlet. (One might use an indium
seal there.) We use VCR fittings with copper gaskets and soft annealed
stainless steel tubing with 1/8 in. diameter throughout.

 

We buy commercial gas bottles (lecture bottles) with research grade hydrogen
(deuterium, helium) for the ion source. We do not use reducing valves or
pressure gauges. If the gas pressure exceeds 300 psi we bleed the excess gas
into an evacuated evaporator bell jar.

 

We only use adjustable thermo-mechanical leaks (no Pd leaks) made by
Potentials, Inc., because they have both, gas inlet and outlet on the same
side. We remove the rubber O-ring and modify the leak by brazing a VCR gland
to it. The set screw in the leak is gently tightened while the leak is hot
and the (cold) leak is tested by applying 300 psi of He to the inlet while
the outlet is hooked up to a He leak detector. No He should be detected.

 

Each one of our three leaks has a by-pass valve attached to it to quickly
evacuate the dead space between gas cylinder and leak.

 

There is a shut-off valve between ion source and gas supply system so we do
not pollute the gas supply system when changing ion sources nor do we let
the accelerator system up to air when changing gas cylinders. Do not make
our mistake: We use a spring-loaded valve, closed with a mall lever and
opened by spring action. Our accelerator pressure, when gassed up, is about
200 psi, sufficient to overcome the spring action and to close the leak.

 

We installed a light pipe of 1" dia. Lucite (optical grade), appropriately
shaped, from the ion source to a point which can be seen with the periscope
mounted below the base plate. This tells us if or not he source is lit and -
to some degree - what color it has. The guide deforms easily when hot. A hot
air gun works well.

 

With these changes we get ion source lives between 1000 and 1500 hours.  

 

 

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