While reading http://www.talk-polywell.org/bb/viewtopic.php?t=326 which is about the collage fusor, I saw a note by MSimon:
"There is also the compression ratio problem. Turbo pumps have a compression ratio for H2 on the order of 1,000. So you need two pumps in series to get the outlet pressure up into the backing pump range (1 torr). "
This is the first time I've heard anyone suggest this. My backing pump was a two stage rotary pump, which easily pumped down into the millitorr range. Are they using two turbo pumps in series because they have a wimpy backing pump?
Can anyone shed any light on this?
D2 and turbo pumps?
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Carl Willis
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Re: D2 and turbo pumps?
This advice is dubious and unheard of in my experience.
It is true that turbo pumps are not as good at pumping the light gases as they are for air.
The pumps should be bought with an eye for good compression ratio if they are to be used with light gases in continuous flow systems. The solution is never to use one in series with the other (Steve Hansen, have you ever seen that??). The outlet on a turbopump is a very small orifice and as I see it will absolutely choke the pumping speed of the system (e.g. from possibly hundreds of LPS, to the single digit LPS) if you do that.
Modern turbopumps have H2 compression ratios higher than 1000. 10,000-20,000 is more like it for recent hardware. Also, for D2 a turbopump can be expected to have a compression ratio about 1.5 times that quoted for H2, according to formulae in O'Hanlon's "User's Guide to Vacuum Technology," which sits on my desk at work. In some of our linacs at work we have used old, surplus TMP340M pumps that are only good for K = 2,500 to pump big D2 ion sources. No problem!
Finally, the foreline pressure should be better than 1 torr. A decent and properly-sized roughing pump should do two orders of magnitude lower than this--even with some reasonable flow-through gas load.
-Carl
It is true that turbo pumps are not as good at pumping the light gases as they are for air.
The pumps should be bought with an eye for good compression ratio if they are to be used with light gases in continuous flow systems. The solution is never to use one in series with the other (Steve Hansen, have you ever seen that??). The outlet on a turbopump is a very small orifice and as I see it will absolutely choke the pumping speed of the system (e.g. from possibly hundreds of LPS, to the single digit LPS) if you do that.
Modern turbopumps have H2 compression ratios higher than 1000. 10,000-20,000 is more like it for recent hardware. Also, for D2 a turbopump can be expected to have a compression ratio about 1.5 times that quoted for H2, according to formulae in O'Hanlon's "User's Guide to Vacuum Technology," which sits on my desk at work. In some of our linacs at work we have used old, surplus TMP340M pumps that are only good for K = 2,500 to pump big D2 ion sources. No problem!
Finally, the foreline pressure should be better than 1 torr. A decent and properly-sized roughing pump should do two orders of magnitude lower than this--even with some reasonable flow-through gas load.
-Carl
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John Futter
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Re: D2 and turbo pumps?
Light gases are the bane of high vacuum work.
Carl is right about only one turbo ie no to more in series. Also the older pumps like the TMp340 maglev stall at 2 Torr ie they stop pumping when the backing pressure rises to this or above.
Our vacuum protection systems switch off the turbos @ 2 x e-1 torr.
If a turbo is left on above stall pressure it gets very very hot and damage the motor windings / seals
Carl is right about only one turbo ie no to more in series. Also the older pumps like the TMp340 maglev stall at 2 Torr ie they stop pumping when the backing pressure rises to this or above.
Our vacuum protection systems switch off the turbos @ 2 x e-1 torr.
If a turbo is left on above stall pressure it gets very very hot and damage the motor windings / seals
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Carl Willis
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Re: D2 and turbo pumps?
In case the Peninsula crew reads this thread and wants to press forth into cleaner vacuum, I'll reiterate my plug for the humble diffusion pump, which is what I use. That would be betting less money on equipment that, if obtained used, has better reliability than a turbo and is far more robust.
Think of it this way: the money saved by buying a diff pump over a turbo will easily subsidize a stronger HV supply and a nice NIM instrumentation system.
Like turbo pumps, the light gas compression "problem" (which is not really a problem--read on) occurs with diff pumps also. This problem is simply irrelevant for fusor vacuum systems. Why? Because the pressure regime for most fusor oepration is no different from the base pressure provided by most mechanical vacuum pumps, so by that logic the compression ratio is pretty unimportant. Also, the gases one is most concerned about removing are air, water vapor, and organic vapors. Deuterium does not need to be pulled out of the system; after all, it's wanted there to do fusion! To sum this all up, in a fusor the high vacuum pump (be it turbomolecular or diffusion) is mainly advantageous for increasing pumping speed far beyond what a mechanical pump can provide near its ultimate pressure, and for removing contaminant gases and vapors. Providing high compression of the fusion gas is not in the job description.
-Carl
Think of it this way: the money saved by buying a diff pump over a turbo will easily subsidize a stronger HV supply and a nice NIM instrumentation system.
Like turbo pumps, the light gas compression "problem" (which is not really a problem--read on) occurs with diff pumps also. This problem is simply irrelevant for fusor vacuum systems. Why? Because the pressure regime for most fusor oepration is no different from the base pressure provided by most mechanical vacuum pumps, so by that logic the compression ratio is pretty unimportant. Also, the gases one is most concerned about removing are air, water vapor, and organic vapors. Deuterium does not need to be pulled out of the system; after all, it's wanted there to do fusion! To sum this all up, in a fusor the high vacuum pump (be it turbomolecular or diffusion) is mainly advantageous for increasing pumping speed far beyond what a mechanical pump can provide near its ultimate pressure, and for removing contaminant gases and vapors. Providing high compression of the fusion gas is not in the job description.
-Carl