Deuterium Input / Pressure Question

Every fusor and fusion system seems to need a vacuum. This area is for detailed discussion of vacuum systems, materials, gauging, etc. related to fusor or fusion research.
Post Reply
User avatar
Anze A Ursic
Posts: 157
Joined: Thu Mar 10, 2022 9:28 pm
Real name: Anze A Ursic

Deuterium Input / Pressure Question

Post by Anze A Ursic »

Hello everyone,

another day, another problem, but this one does not seem too bad. I am doing some preliminary testing on the vacuum chamber before we try actual fusion next week and I have the following problem; I have an MKS GM50A MFC that I just power it on with 15-24VDC and then use another power supply to input 0-5V on the "Setpoint Input" pin to control the flow.

So here's the issue during my test, I use my old dying rotary pump to get to about 60-70 micron, turn on the turbo which gets me to roughly 0.4 micron and then I open the Deuterium valve ever so slightly just to get some gas in on the other side of the MFC (at this point, input voltage is still 0V so its valve is completely closed). Then, while carefully monitoring the pressure gauge at around 0.4 micron, I start going up in voltage with the setpoint input, this raises the pressure as one would expect, since Deuterium is now being added into the system.

Here's the issue, at 5V (max input voltage, thus max flowrate), the pressure is just 5mTorr. I suppose that's not too good, I'd need about 10-15mTorr to properly fuse, correct? But at the same time, if I recall correctly from my previous background gas plasma runs, I noticed the second a plasma strikes the pressure tends to go up. Is that what I will be relying on in an actual fusion run to get me to the correct pressure or must I find another MFC with higher flowrate or something?

As is the case every single time I post on this forum, your help is greatly appreciated and I cannot thank you enough for the help you have provided so far! Cheers!

Anze
User avatar
Liam David
Posts: 518
Joined: Sat Jan 25, 2014 5:30 pm
Real name: Liam David
Location: PPPL

Re: Deuterium Input / Pressure Question

Post by Liam David »

If you don't have a leak, I would very much suggest a bakeout to drop your base pressure, if possible.

Do you have a valve between the turbo and chamber? What is the max flow rate of your MFC? If you're flowing more than a few sccm you're using much more than necessary. In the ideal case you would throttle your turbo, but that likely won't even be possible given your high base pressure unless you completely forsake gas purity.

There's no rule stating that 10-15 mtorr is the ideal range for fusion. That depends entirely on chamber and cathode size/shape, gas purity, etc. Crudely speaking, due to Paschen's law your large chamber will operate at a lower pressure for a given voltage and current than, say, a 2.75" cross. Glow discharge fusors operating anywhere between a few 0.1 mtorr and some 50 mtorr have been built over the years by amateurs and universities alike. Also, remember that some vacuum gauges are gas-dependent.

The pressure increases when plasma strikes due to ion/electron/fast neutral heating of surfaces (outgassing) and sputtering of the absorbed gases.
User avatar
Anze A Ursic
Posts: 157
Joined: Thu Mar 10, 2022 9:28 pm
Real name: Anze A Ursic

Re: Deuterium Input / Pressure Question

Post by Anze A Ursic »

Might be a dumb one, but does dropping base pressure with just the mechanical pump also drop the final pressure the turbo will achieve?

Anze
User avatar
Liam David
Posts: 518
Joined: Sat Jan 25, 2014 5:30 pm
Real name: Liam David
Location: PPPL

Re: Deuterium Input / Pressure Question

Post by Liam David »

That depends on the type of turbo, and in general the answer to your question is yes. Compound turbomolecular drag pumps will fare better with higher backing pressures than a standard turbomolecular pump due to viscous flow; the compression ratio drops dramatically above a certain foreline pressure. Take the following graph for compound pumps and shift it a decade or so to the left (from https://www.idealvac.com/files/literatu ... _Pumps.pdf)

turbo.png

However, a turbo should pull below 0.4 mtorr with a backing pressure of 60 mtorr (only a compression ratio of ~100) so I would suspect a bad leak and/or really bad outgassing. My older, standard turbomolecular pump will do that well or better in a deuterium atmosphere if I let in enough gas to raise the foreline to 60 mtorr, and note how the compression ratio and pumping speed of hydrogen compares to N2.
Chris Seyfert
Posts: 37
Joined: Sun Mar 29, 2009 2:51 am
Real name:

Re: Deuterium Input / Pressure Question

Post by Chris Seyfert »

Hello Anze,

Mass flow controllers require a minimum pressure differential in order to function correctly. The datasheet for the GM50A requires 10 psid, "d" for "delta" or pressure drop across the flow controller. Your approach of "opening the deuterium valve ever so slightly" is likely not putting enough pressure across the MFC for proper operation. You should monitor the output voltage signal from the MFC, as that will report what the measured actual flow rate. If the setpoint and the feedback aren't in agreement, something isn't working right.

Also, one cannot assume that an MFC valve is closed because the input is at 0V. The setpoint is referred to a mass flow, not the valve position. Without pressure across the MFC, the controller can't regulate and valve is in an indeterminate position.

Lastly, there may be gas correction factors that may play a role in accurate measurements, if your MFC wasn't calibrated for D2 gas from the factory. Fortunately, it just so happens that an MFC calibrated for nitrogen is also calibrated for deuterium. Other gas corrections can be found at MKS' web site, search for "gas flow corrections for thermal-based flow".
Post Reply

Return to “Vacuum Technology (& FAQs)”