Some Diffusion pump Questions

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.
Lukas Bochtler
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Re: Some Diffusion pump Questions

Post by Lukas Bochtler » Wed Jun 28, 2017 5:51 pm

That wont be a problem.

I was at my microscope today (its not in my house) and found out some interesting things. First off, the microscope still contained a specimen in the sample chamber (i looked in threw the load lock), second the data recorder camera still had a roll of film in it (thanks to me being familiar with medium format cameras, i retrieved without damaging it in any way). So, my guess is that the last time it was used, the person running it left in a hurry, or forgot his sample. Anyway, i will have the film developed and am curious to see what, if anything is on it.

Some other things, i went to the microscope to take some measurements, and check some things. I have taken the broken cathode cable with me, in order to have it potentially restored by that aforementioned company. Though the column connector seems to be potted as well (which is a bit of a bummer). But possibly the company can depot it, alternatively i can make a new one, as its some basic turning in plastic. (not sure which plastic yet though, My gut tells me its PTFE, which explains how i can pull part of the potting out by hand (and some genital force).

Some other things, the ion pump power cables are all severed, they are coax style cables terminated in the plugs seen in the picture. Im not sure if i can pull them or not. Im afraid that ill break the cathode or something if they are vacuum tight or something.
20170628_175746.jpg
The 3 Ion pump power connectors.
I have measured the cables to be 8.2mm OD, with 6.8mm OD HV insulation, and a 1.6mm diameter conductor (im guessing 14 AWG) made of smaller conductors. Still working on getting the original power supply...
20170628_174421.jpg
The Ion pump cables
Also good thing i checked the diffusion pump, its not 500W its 700W. the second image is the name plate of the diffusion pump
20170628_165617.jpg
Diffusion pump

I also have this oddity in the column, and im not sure what this is, or what to connect the pipes to:
20170628_170425.jpg
Unkown part of the SEM
Fascinatingly enough, the 2 roughing pumps where right next to the column, something i did not expect. (based on the vacuum hoses, which still had the clamps on both ends, and the hoses being very short.)

Also, i found out that the microscope has a after market vacuum gauge on it, a Leybold PR25. I do recall having a manual for a meter for it, but im not sure anymore. Any information on this gauge would be nice. As well as, with what controllers dose it work with?

Also, with this diffusion pump, how do i check the oil level, and how do i get the oil in, if it needs changing? I have not found any way to do either.

edit:
I have another question, will this pump be enough to pump down the column to the point of me beeing able to test the gun? The original pumps where 145 L/min pumps, where as the one in the image is about 120L/min. (Its a Leybold Trivac S4A)
20170628_192041.jpg
Leybold Trivac S4B
Though running the pump will be difficult from the microscope. Since the originals where 100V AC, and this is 220V 3Phase. I think i shall use a transformer to drive a AC relay to connect the pump to power.

John Futter
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Re: Some Diffusion pump Questions

Post by John Futter » Wed Jun 28, 2017 6:58 pm

The ion pumps look like standard Varian type connectors
You just pull them in the direction of the cable exit to undo
The cable is standard RG8, RG213 COAX any ham would have some this cable we use at work up to 70kV so the 10kV your ion pumps use is well inside it5s spec

Lukas Bochtler
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Re: Some Diffusion pump Questions

Post by Lukas Bochtler » Wed Jun 28, 2017 7:44 pm

Thanks for the tip that the ables are RG-213. I do wonder, will it be a good idea to try and replace the cables by trying to remove them from the plugs, or just attach a high voltage BNC connector to the cable ends, and connect the cables to the power supply that way?

I fear that if i try and remove the cables from the ion pump power connectors (the one on the top pump is larger btw, and seems to be screw in as well). i will not get the new one in, unless i build (or buy, if thats even possible) new connectors.
Also, i do trust you with your voltage rating, i will call my favorite cable manufacturer (Sommer Kable) to see what they think about it. (they make a RG-213 cable)

Also, the pumps in the S800 appear to run off of 3.6KV. (a 1.8KV 500W transformer (well 500W fuse anyway) connected to a full wave voltage doubler. (with quite frankly an absurd amount of monitoring to boot).

Also, what am i missing with the ion pumps, why would one use a coax cable to power them, when the secondary electron detector (i think) is powered by a standard 20Kv unshielded cable. (well im guessing the accelerator is, not the PMT). Dose it have to do with voltage stability due to interference? Or is it to supply a better ground return path then the ground straps of the column?

ian_krase
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Re: Some Diffusion pump Questions

Post by ian_krase » Wed Jun 28, 2017 9:22 pm

Coax cable provide a safety ground. Intrinsically safe, and for modestly high voltage good coax is often much cheaper and more available than High voltage wire.

Jim Stead
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Re: Some Diffusion pump Questions

Post by Jim Stead » Thu Jun 29, 2017 1:22 am

The "oddity" in your column looks like a cold trap to me. Based on the connectors, I would think the left side supports an LN2 feed, and the right side hose is the vent.

John Futter
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Re: Some Diffusion pump Questions

Post by John Futter » Thu Jun 29, 2017 2:08 am

It is a cold trap to limit any contamination of the FE source.
So initial pumpdown would be rough out column fill cold trap with liquid N2 start diffusion pump pump until 10 to minus 6 torr then attempt to start ion pumps keeping diff pump going until vacuum starts getting deeper
I presume you can isolate the diffusion pump when the ion pumps are working

Jerry Biehler
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Re: Some Diffusion pump Questions

Post by Jerry Biehler » Thu Jun 29, 2017 4:42 am

Like the other have said, cold trap. Mostly to protect the lenses from building up oil vapor which means the column needs to be disassembled and thats no fun on a FESEM.

The pumps were not next to the column. There were two weighted isolators that go inline to the pumps. Basically a piece of pipe though a chunk of cast iron or concrete. It dampens the vibrations from the roughing pumps that are transmitted through the hoses.

There is a reason for a lot of stuff in the ion power supply, the supplies not only run the pump they measure the current on the pumps which is proportional to the pressure and control lock out to make sure HV is not applied to the emitter under insufficient vacuum. And yes, the ion pumps stay on all the time. A lot of FESEMs have battery backup for the ion pumps that keeps them going during a power outage or even moving the machine.

You may have a very hard time getting these pumps started. Honestly, without the proper resources or lots of money this machine is going to be boat anchor.

Lukas Bochtler
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Re: Some Diffusion pump Questions

Post by Lukas Bochtler » Wed Jul 05, 2017 12:16 pm

A quick update on the pumping down of the microscope. I have managed to make contact with someone who knows this microscope well, and he stated some interesting things that i thought would be good to know.

The microscope needs both roughing pumps present to begin pumping down the column, this is due to the initial pup-down of the sample chamber and column being done via the Load Locks pump, once itd down to the pressure needed to start the diffusion pump, it will start, together with its roughing pump. Once thats had time to warm up, it will begin to pull high vacuum. During the whole process the bypass valves are open allowing pump down of hte ion pumps. Once the diffusion pump reaches a high vacuum i can try to start the ion pumps. If they stay on great, if not, baking time, during which i try and start the pumps again. Once they start i should close the bypass valves.

As to the LN2 cold baffle, it appears i only need it when pumping down the chamber to the vacuum needed to achieve the microscopes maximum resolution, but is not needed when using lower resolution.

Also apparently, if i do need the LN2 baffle, the nitrogen lasts about 8 hours.

Apparently the whole pump down of the column will take roughly a week.

Lukas Bochtler
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Re: Some Diffusion pump Questions

Post by Lukas Bochtler » Wed Jul 12, 2017 11:24 am

I have another question about diffusion pumps and general vacuum systems.

I was told by someone experienced with vacuum systems, that i should not discharge the air from the vacuum pumps into the air of my microscope location. But instead discharge it threw some sort of exhaust outside the building.

So, i was wondering, how i should have that made (well what shape diameter etc..)


I was thinking of using a 1" (roughly) pipe (edit: Make that 2") (probably stainless steel or aluminum, in a L shape, pointing down, with a screen on the bottom (as to not have insects enter it). This would go threw the wall of the microscopes building (its not purpose built, there are other things in the room). I would then connect all 3 Vacuum pumps (the 2 of the microscope, and 1 from the sputter coater) as well as the nitrogen exhausts to this pipe, via multi T manifold connected via a KF flange or similar.

Would this be a good idea?

I would be relying on the pressure generated by the vacuum pumps and nitrogen exhausts to shove the air out of this. I don't really want to build a chimney for the microscope. If i need more pressure i can add a fan to push the air out. I would have this exhaust at around a hight of 1 1/2 meters, with a 25cm pipe going pointing down.

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Dennis P Brown
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Re: Some Diffusion pump Questions

Post by Dennis P Brown » Wed Jul 12, 2017 4:04 pm

Roughing pumps can create an oil mist so many of us vent it outside. Any small pipe will do; yes, it will self exhaust by the roughing pumps - and when the scope is roughed out, the pumps will exhaust very, very little - really just a faint trace of oil mist. Except for roughing down the scope after it is at atm pressure, the vent line will never handle any real volumes of gas. I use an off-the-self flexible garden water hose (using a nice KF to hose adapter on the exhaust port of the roughing pump.)

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