I have a series of bits on reactor pressure control at:
http://iecfusiontech.blogspot.com/2007/ ... alves.html
http://iecfusiontech.blogspot.com/2007/ ... izing.html
http://iecfusiontech.blogspot.com/2007/ ... ntrol.html
http://iecfusiontech.blogspot.com/2007/ ... uages.html
The tighter we can control reactor pressure the higher the pressure we can run the reactor at. Tight pressure control (better than 1% precision, better than 10% accuracy) will help a lot.
Pressure Control
Turbo pumps and base pressure
I saw your comments about turbo pumps on your "power and control" blog. An effective way to increase the pumping capacity of a turbo pump to reach a lower base pressure is to add a cryotrap between the main valve and the turbo pump. This will make it easier to get to a lower base pressure, say 10-9 torr, which is necessary if your operating pressure (with the gases flowing) is 10-7 torr.
10-9 toor is considered "near" UHV conditions, which are rather difficult to achieve with trubo pumps, even with the cryotrap. The UHV systems I have worked with (MBE and CBE deposition systems) all used cryopumps and required a multi-day "bake out" period to get to their final base pressures (10-11 torr).
Cryotraps and cryopumps are the only pumps that are useful when using hydrogen gas. The small molecular weight of hydrogen gas reduces the effectiveness of both turbo pumps and diffusion pumps.
Diffusion pumps are very reliable IF used properly, but are a bitch if mishandled (like opening the main valve to the chamber when the roughing pressure is not low enough). They are also a bitch to clean, which has to be done from time to time. They are, however, very cheap compared to turbo pumps.
We used both ion gauge and Baratron (MKS) with our PVD coating systems. Of course, we were running at an operating pressure of 10-3 torr during deposition. Ion guages should work down to the 10-9 torr level.
10-9 toor is considered "near" UHV conditions, which are rather difficult to achieve with trubo pumps, even with the cryotrap. The UHV systems I have worked with (MBE and CBE deposition systems) all used cryopumps and required a multi-day "bake out" period to get to their final base pressures (10-11 torr).
Cryotraps and cryopumps are the only pumps that are useful when using hydrogen gas. The small molecular weight of hydrogen gas reduces the effectiveness of both turbo pumps and diffusion pumps.
Diffusion pumps are very reliable IF used properly, but are a bitch if mishandled (like opening the main valve to the chamber when the roughing pressure is not low enough). They are also a bitch to clean, which has to be done from time to time. They are, however, very cheap compared to turbo pumps.
We used both ion gauge and Baratron (MKS) with our PVD coating systems. Of course, we were running at an operating pressure of 10-3 torr during deposition. Ion guages should work down to the 10-9 torr level.
Kurt,
Most helpful.
I think we are stuck with turbo pumps because of the need to extract helium. Cryo pumps do not do well with helium.
Compression with turbo molecular pumps (TMPs) for light gases is about 1E4 per stage. At 1E-7 torr operating pressure that gives a roughing pump pressure of 10 torr - very respectable. Dr. B says arcing starts at 3E-6 Torr. Then you are up to 300 torr. That is practically high pressure. The whole system is inherently stable because the inherent pumping capacity in molecules per second increases directly with pressure. Raise the flow and the pressure goes up proportionately. A 10% increase in flow causes a 10% increase in pressure. Nice and linear.
I think that means two TMPs in series followed by a roughing pump.
I have done some more gas flow stuff at:
http://iecfusiontech.blogspot.com/2007/ ... esign.html
and
http://iecfusiontech.blogspot.com/2007/ ... tings.html
The whole system needs to be at minimum PLC automated in terms of sequencing. That kind of automation is really cheap.
One way to get longer operating times is to pre-calibrate (just before the run) the valve position for maximum gas flow and then run your gas controller accordingly. You might want to do a profile to optimize rate of rise.
There really needs to be a valve design program.
I have some ideas.
Most helpful.
I think we are stuck with turbo pumps because of the need to extract helium. Cryo pumps do not do well with helium.
Compression with turbo molecular pumps (TMPs) for light gases is about 1E4 per stage. At 1E-7 torr operating pressure that gives a roughing pump pressure of 10 torr - very respectable. Dr. B says arcing starts at 3E-6 Torr. Then you are up to 300 torr. That is practically high pressure. The whole system is inherently stable because the inherent pumping capacity in molecules per second increases directly with pressure. Raise the flow and the pressure goes up proportionately. A 10% increase in flow causes a 10% increase in pressure. Nice and linear.
I think that means two TMPs in series followed by a roughing pump.
I have done some more gas flow stuff at:
http://iecfusiontech.blogspot.com/2007/ ... esign.html
and
http://iecfusiontech.blogspot.com/2007/ ... tings.html
The whole system needs to be at minimum PLC automated in terms of sequencing. That kind of automation is really cheap.
One way to get longer operating times is to pre-calibrate (just before the run) the valve position for maximum gas flow and then run your gas controller accordingly. You might want to do a profile to optimize rate of rise.
There really needs to be a valve design program.
I have some ideas.
MSimon, forgive me if I'm dropping into the conversation late and am missing something.
I know you have calculated the amount of fusion won't produce enough helium to sneeze at, at least for the present. That's not why the UHV level is not the real problem.
The big chamber in the old EMC2 lab had turbos, and a couple of the turbos had a chiller device on them. This was not a full cyrotrap, it was simply a vaned device that could freeze out water vapor. The chamber was electropolished, and brand new, without a bakeout, could pull down to the mid e-8 range on one of its 6 turbos. The manifold was pulled down by two roughing pumps with Roots blowers to improve the throughput. All the turbos were on big gate valves, and the whole system was rigged so that it could be used with half the turbos down for service and any other component out of action.
And I should have had more pumps!
I don't judge the crossover to UHV by the pressure. For me, you are in UHV when water vapor is a trace gas, and you are dominated by hydrogen. Cryotraps are good for reducing water vapor, but if you need the cyrotrap, that means the system is still producing water vapor. You are much better off if the system is inherently clean enough that has stopped producing H2O, and also hydrocarbons if those are present. A bakeout at up to 400C is the traditional answer. UV lamps are supposed to also be very good at getting water out of the machine. If a system can get under 1e-7 and the dominant background gas is hydrogen, you are getting into the UHV range. Better, you have kept the machine clean and run plasma it until the dominant background gas is deuterium, you're where you want to be.
But operating these machines, it is not your ultimate pressure that is the problem, it is dealing with huge gas loads evolved from metal parts when the plasma lights off. You need to get rid of that to keep the desired conditions. Aside from opening high-conductance pipes to the hard vacuum of space, I think turbos will probably be the answer. Lots and lots of them, with attention paid to conductance.
I know you have calculated the amount of fusion won't produce enough helium to sneeze at, at least for the present. That's not why the UHV level is not the real problem.
The big chamber in the old EMC2 lab had turbos, and a couple of the turbos had a chiller device on them. This was not a full cyrotrap, it was simply a vaned device that could freeze out water vapor. The chamber was electropolished, and brand new, without a bakeout, could pull down to the mid e-8 range on one of its 6 turbos. The manifold was pulled down by two roughing pumps with Roots blowers to improve the throughput. All the turbos were on big gate valves, and the whole system was rigged so that it could be used with half the turbos down for service and any other component out of action.
And I should have had more pumps!
I don't judge the crossover to UHV by the pressure. For me, you are in UHV when water vapor is a trace gas, and you are dominated by hydrogen. Cryotraps are good for reducing water vapor, but if you need the cyrotrap, that means the system is still producing water vapor. You are much better off if the system is inherently clean enough that has stopped producing H2O, and also hydrocarbons if those are present. A bakeout at up to 400C is the traditional answer. UV lamps are supposed to also be very good at getting water out of the machine. If a system can get under 1e-7 and the dominant background gas is hydrogen, you are getting into the UHV range. Better, you have kept the machine clean and run plasma it until the dominant background gas is deuterium, you're where you want to be.
But operating these machines, it is not your ultimate pressure that is the problem, it is dealing with huge gas loads evolved from metal parts when the plasma lights off. You need to get rid of that to keep the desired conditions. Aside from opening high-conductance pipes to the hard vacuum of space, I think turbos will probably be the answer. Lots and lots of them, with attention paid to conductance.
Tom,
Agree on all your points.
Were you responding to some one else? As to eqpt, bake out. Critical.
All I was working on with the gas flow stuff was to try and come up with a better gas delivery system than puff gas. That seems like a weak link now.
As far as I can tell there are no low flow low pressure servo valves on the market. Thus the design effort.
Agree on all your points.
Were you responding to some one else? As to eqpt, bake out. Critical.
All I was working on with the gas flow stuff was to try and come up with a better gas delivery system than puff gas. That seems like a weak link now.
As far as I can tell there are no low flow low pressure servo valves on the market. Thus the design effort.
Tom,
I got to thinking more about your pumping problems and I think you were not getting the best possible use of your pumps for light gases.
For light gases there is a compression ratio problem.
Say your compression ratio for H2 was 1E4 and your fore pump could get you down to 1E-2 torr. Your H2 is going to stay at 1E-6 torr.
With N2 the compression ratio is 1E8 so a fore pump of 1E-2 gets you down to 1E-10. Very nice.
Two 1E4 compression turbo molecular pumps in series also gets you down to 1E-10 for light gases.
Your problem may not have been inadequate gross capacity as much as not enough capacity for light gases.
I looked at this question long and hard in order to do the valve design right. It is one reason I tackled the pump question before running the numbers on the valve question.
With six pumps I would have two sets of 2 in series and the remaining two as individual pumps. Use 4 pumps to get you down to 1E-5 torr or so and then turn on your two series pumps to take care of H2, He, D2, etc.
I got to thinking more about your pumping problems and I think you were not getting the best possible use of your pumps for light gases.
For light gases there is a compression ratio problem.
Say your compression ratio for H2 was 1E4 and your fore pump could get you down to 1E-2 torr. Your H2 is going to stay at 1E-6 torr.
With N2 the compression ratio is 1E8 so a fore pump of 1E-2 gets you down to 1E-10. Very nice.
Two 1E4 compression turbo molecular pumps in series also gets you down to 1E-10 for light gases.
Your problem may not have been inadequate gross capacity as much as not enough capacity for light gases.
I looked at this question long and hard in order to do the valve design right. It is one reason I tackled the pump question before running the numbers on the valve question.
With six pumps I would have two sets of 2 in series and the remaining two as individual pumps. Use 4 pumps to get you down to 1E-5 torr or so and then turn on your two series pumps to take care of H2, He, D2, etc.