Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA-9mA

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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Leak raised the fusor pressure to about 500mTorr after about 12 hours, pumped down to 3.1e-4 torr in 3 hours.

Tried the dust off spray at 3.1e-4 torr(I gave everything a solid spraying,), still haven't found the leak.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Jerry Biehler »

Very strange. Sure the turbo is running at full speed? On my sem which has a similar volume and turbo size, I can be down to the -5 in minutes, even after the system has been up to air for a while. And that is all o-rings, not a metal seal on it.

Maybe you have some internal component that is porous and is holding a whole bunch of air or water vapor. Any blind holes inside the system with non-vented screws?
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Dennis P Brown »

Wait, 12 hour and the system rose up to just half a torr? That isn't a very large leak if it is a leak ... if the system had been exposed to air, that could be normal out gassing. Maybe its the turbo (not up to speed as mentioned) but are you certain the ion gauge is correct? My system too, gets to mid 10^-5 torr in minutes and I have a small turbo and lots of O-rings. To be on the safe side, I have two independent ion gauges for pressure check so maybe (?) the gauge is lying? My system was bottoming out in the very low 10^-5 torr range and the "Dust Off" spray detected a leak that keep me from reaching mid 10^-6 torr so the stuff works - if you are reading no leaks maybe you have no leaks! Check the accuracy of the ion gauge.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Jim Kovalchick »

I agree with Dennis. Pressure rise to only a half torr in 12 hours is not bad at all. If possible, you should check your turbo and instrumentation by dead heading your pumps onto just the instrumentation without all the chamber attached.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

I brought the chamber up to air and swapped ion gauge systems.
Deadheading the gauge against just the turbo brought it down to the low E-5 range pretty rapidly(minutes).

Leaving the fusor pumping down over night resulted is a vacuum of 2.1e-4 after the gauge tube swap.

The turbo pump is a pfeiffer TPU 055, pumps about 30L/s in the 1e-4 range.

Leak rate by pressure rise method is a pretty constant 1E-5 TL/s
Leak rate by He leak checker was about 3E-8 TL/s

Things that I suspect for the gas source(other then potential leaks):
--------------------------------------------------------------------------------
The grid system:
It has some trapped volume(but not tightly trapped) There is a 0.75" OD, 0.5"ID, 6" long quartz tube with a BN insert sitting on the top
The grid insulator ceramics: there are 95% Boron Nitride (BN), 5% Boric Oxide
If memory serves correct, the "leak" was there without the grid was mounted

The ion sources:
They have NdFeB magnets
The casings are 410 stainless(sulfur free though)

Plastic insulator:
There is an acetal copolymer standoff(2" length, 1.5" ID)
It was baked in a vacuum oven for 1 week in the 10s of mTorr to outgas any condensable volatiles


pumpdown-gauge2.jpg
pumpdown-gauge2-pressure-rise.jpg
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Jerry Biehler »

Plastics are generally a bad idea inside the chamber. Some are usable like teflon (internally only, it is somewhat permeable by air). It is best to stick to ceramics.

Boron Nitride is just fine.

Magnets and the stainless should be fine.

A not terrible list of vacuum compatible materials is here: https://en.wikipedia.org/wiki/Materials ... s_to_avoid
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Latest pictures:
There was an ion beam hitting Pyrex view port and causing it to fluoresce, magnets were added to sweep away the beam resulting in a much clearer picture.
Still no progress with the leak/outgassing Contemplating replacing the acetal insulator with a PEEK/stainless insulator and possibly re-cleaning the grid ceramics and ion injector parts.
SAM_4400.JPG
SAM_4402.JPG
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Dennis P Brown »

OK - still don't think it is the chamber; now the turbo works (but as you know, speed is the issue and ultimate chamber pressure) but what about the fore pump and/or lines? A leak back against the turbo will surely look like a chamber leak and/or does the fore pump work well? Have you tested just the fore pump on the chamber? If its performance is very poor (can't pump fast, or oil out gassing or a leak in the fore line system) the turbo will not pump your system down against a high fore pressure. Not necessarily the problem but very well worth looking into.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

I think it may be outgassing.

The turbo has a valve on the inlet, and the chamber also has a valve on the inlet (both nw16 valves), they are connected by a 2ft nw16 stainless flex hose. Deadheaded against the ion gauge with no chamber, the turbo will pull down into the low 10^-5 torr range very fast so I don't think it's the turbo or backing pump. Closing the valve on the chamber or the turbo will cause the pressure to rise at about the same rate, so I don't think it an issue with the lines connecting the turbo station to the fusor.

Valves are pictued here:
http://www.rtftechnologies.org/physics/ ... hub-v2.htm

I'm sure I take a rather substantial hit in conductance from the valves, but it does seem that there is a gas source inside the chamber.

2 other things that i thought of:

I have a pair of ZnSe viewports, made out of AR coated lenses for high power CO2 lasers, though ZnSe in it self shouldn't out gas.
http://www.rtftechnologies.org/physics/ ... ometer.htm

There is a woven alumina fabric shield for the chamber o-ring to prevent plasma bombardment, though it was baked out at very high temperature(red hot)
bottom of:
http://www.rtftechnologies.org/physics/ ... sphere.htm
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Dennis P Brown »

From the picture your turbo is attempting to pump your chamber through a very long, very small diameter (16 mm) line. While for a well sealed (your's appears to be), clean system that will work it is really creating unnecessary issues. Why not connect the turbo directly to the chamber with as large coupling as possible? Adding a 16 mm valve just makes it worse. While not necessarily the cause, that just makes any minor leaks/out-gassing very slow to pump. My turbo has a 16 mm diameter opening for the fore line port but I use an adapter and connect my fore pump to the turbo using a 40 mm diameter SS tube. As for my fusor, I use a water cooled DF pump and the DF connects directly to my fusor chamber via a gate valve that is the same diameter as the DF pump mouth. My chamber is, however, very much bigger; despite this, it pumps down in under five minutes to mid 10^-5 torr once my DF pump is at temp.
You have checked the fore line pump and it is getting under 10 microns?
As for your insulators you show in a pic - exactly what do they accomplish? That is, once you strike a plasma in your chamber, the insulator section is worthless since all metal parts connected to the insulator are now connected directly to the plasma and will share in the voltage.
By the way, you have one extremely professional looking system! Your machine shop work is absolutely fantastic and the results really show it.
Also, unless your metal flanges have a scratch, I agree it is highly unlikely they are leaking. Out gassing in a long, small diameter system with a large number of components can really take time but if it has been under vacuum for a number of days, then really, a clean system will not out gas significantly from then on even if exposed for short periods to air - do consider an air dryer for admitted air. I have one and it really makes a difference. My accelerator when back filled for an hour or so with dry air then pumps down to mid 10^-5 torr in under 5 minutes.
Can't really tell but do you do have a oil vapor anti-back streaming component for your fore line pump?
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

The hose used while pumping the fusor is larger then the one shown in that picture, I use a 2', 0.75" ID stainless hose to connect the turbo station to the fusor.
SAM_3401.JPG
The reason for the dual valves is to allow the turbo station to be disconnected from the fusor without bringing the fusor up to air so the station can be connected to other hardware. The turbo side valve allows the turbo to be isolated from whatever it is connected and the roughing pump so the vacuum system can be brought up to air/roughed out without stopping the turbo. Hopefully someday I'll have gate valves instead of the 16mm solenoid valves.

As far as an air drier, I'm thinking about buying one of these:
https://secure.drierite.com/catalog3/page5.cfm
How important is it to add a fine filter on the drier outlet(eg a 40um) to prevent dririte particles from entering the system?

As far as outgassing there seems to be a thermal component associated with the heating of the grid boron nitride insulator ceramics. As they get hot, the fusor pressure rises.

The plastic insulator electrically separates the CF cube from the fusor shell (at very low pressures where there are predominantly ion beams rather then a diffuse plasma filling the fusor), both hemispheres are electrically insulated from each other as well to allow a future upgrade that can monitor collected shell current in each hemisphere independently.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Dennis P Brown »

I just use aquarium grade filter floss to filter my drying agent; as for the insulator - a fusor operating at 5 microns is in a plasma state, not isolated ions and all parts will then conduct electric power from the plasma very well; yes, significantly below 10^-3 torr it starts to become very non-conductive ions but I didn't know that was were you were operating (exactly how can it operate as a fusor, then?) Good idea to show the current picture when asking questions on problems. I find it difficult to believe your system leaks that badly - then the only possible reason is the turbo or fore pump. As for the fore pump, if the turbo fore line pressure is below 100 microns than all that remains is the turbo. That said, a bad turbo with "zero" load can make 10^-6 torr but fail with a small out gassing. I suggest you check the fore line pressure (we had a SS tube like yours have a pin hole leak!) and then, the turbo on a smaller system that has more limited out gassing. If the turbo is somewhat better but still in the 10^-4 range than it is the turbo ... . Maybe your system has a tiny bit of liquid water in which case out gassing could continue for days ... just guessing here.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Here is the roughing line pump down with the turbo on and the fusor open. Backing line at about 15mTorr
roughing-line-turbo-on-chamber-open.jpg
Turbo pump down with turbo pumping on gauge only (pumps down to about 8e-5)
turbo-pumpdown1.jpg
Turbo pump down with turbo pumping on gauge only (pumps down to about 8e-5)
turbo-pumpdown2.jpg
Gauge valved off
gauge-valved-off.jpg
This leads me to believe the gas load is somehow coming from the inside of the gauge. I have suspected this for some time but have no way of proving it. Two different gauges give similar results. Is it possible the gauges are contaminated?

The turbo spins up fine to full speed. Two different turbo pumps have produced similar results on this system. Is it possible the turbo is contaminated?
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull »

One reason I like the diff pump is that water in the system can and does wind up in the diff pump and its oil and is pumped out of it as the boiler reaches its operating range between 140-180 degees C. Once a first pumping and long run occurs, the water is pretty much out of my system unless I leave it un-attended for a month or more. (common after HEAS)

The common water signature and assurance that it is gone is as folllows:

Turn on the forepump and open the basllast valve fully and evacuate the fore line to about 15 microns. Open the valve to the diff pump and pump it down to a foreline indication of about 20 microns. Open the fusor valve and this raises the foreline pressure to almost a full torr, but instantly plunges back to about 25 microns and stabilizes there. Now turn on the diff pump boiler. As the boiler temp reaches about 80 deg C. the foreline pressure rises, (yes, I have a PID controller on the boiler to read the temp and control the fan)

This pressure continues to rise to about 40-50 microns as the boiler temp goes above 120 deg C. (volitiles and water being removed from the diff pump and its oil) The pressure in the fusor doesn't rise more than about 2 microns during all this as the jets in diff pump go active and the mechanical pump ditches the water. Quickly, within seconds the fusor chamber pressure plunges to 0.5 microns or (5X10e-4 torr). At this point I let the boiler continue to heat to about 160 deg C and by this time the foreline is back to about 20 microns and I shut the ballast off.
As the ballast is closed I'll usually see about another 3 or 4 micron drop in the foreline to 15 microns or so. Ultimately, the diff pump drags the system down to the 7X10e-5 range and fusor operation with flowing D2 commences.

Fusion will heat the fusor to about 100 deg C and water molecules are continuously boiled out of the fusor walls. This is, inturn, pumped out of the system. Thus a three day group of separate runs will have the bulk of the water out of the system.

Note I never use S.I. pressure units and also refuse to use torr in my discussion unless well below 1 micron. The fusor in operation and in most pump down scenarios is always in the "micron range" It is a micron device. I religiously go to scientific notation torr for diff pump bottoming figures. However the fusor system never, ever dwells there long as fusor operation shoots back to the multi-micron level of real fusion operations.

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Fusion is the energy of the future....and it always will be
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Nick Peskosky »

Andrew,

I think I might have an answer to your virtual leak problem for you from experience. I just looked at the way you have the MKS gauge head connected to your vacuum head CF-cube. It looks like you're using a Conflat->VCR->KF->KF flange sequence to connect the gauge head to the chamber itself. You might be facing a conductance issue with reducing the throat of the head from KF-to-VCR in the molecular regime, in effect differentially pumping the sensor. I did something similar on my chamber when I had my combination Pirani/Ion gauge hooked to the small male 1/4" VCR via a ~5" length of 1/4" ID SS tubing->KF->KF. I could never get the chamber pressure (as measure during this setup) to dip below the mid E-4 Torr range but when I dead-headed the gauge my turbo would quickly drop the pressure below E-6 Torr. When I re-plumbed the gauge to a KF->CF nipple on my 2.75" tee the measured pressures dropped well into the lower E-5 Torr range after minimal pumping. The leak probably appears to come from the gauge because it isn't bottoming out at the chamber's ultimate pressure (you wouldn't see this when you dead head the gauge because in-effect it is your total 'chamber' volume). Hope this helps!
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

The picture with the VCR adapter is pretty old, currently it is a 2.75" CF to NW40 adapter on the cube(the gauge has a NW40 flange).
When deadheading the gauge it is an NW40-NW16 adapter.

I'm going to install some bake out heater tape on the fusor shell to assist with pumpdown.
Would it work to install a small(~2l/s) ion pump on the CF cube on the fusor for maintaining low pressure on a valved off system when not running(so the backing/turbo pump don't have to be continuously run)?
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Ion beam deflecting magnets have been added to all 4 view ports on the fusor. In addition to preventing burn in on the view ports, it also eliminates view port fluorescence under ion beam bombardment leading to considerably clearer pictures.

the magnets used are:
https://www.kjmagnetics.com/proddetail. ... CS&cat=173

The aluminum clamps are modified from:
http://www.ebay.com/itm/CBM-10923-Bille ... 70&vxp=mtr

Modified and unmodified clamps
SAM_4408a.jpg
Clamps and magnets
SAM_4411a.jpg
Mounted clamps
SAM_4413a.jpg
View port with magnets
SAM_4414a.jpg
Viewport without magnets
SAM_4417a.jpg
Any guesses to the dendritic pattern around the edge of the view port under ion bombardment?
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Jerry Biehler »

That delrin has got to go. I thought you were just using it internally, not between atmosphere and delrin. I talked to a prof friend of mine and plastic just has no place in a vacuum system.

Also you need at least a KF40 line going from the turbo to the chamber and the line really should be smooth bore. Your conductance like you have it set up is just awful.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Jerry Biehler »

Andrew Seltzman wrote: I'm going to install some bake out heater tape on the fusor shell to assist with pumpdown.
Would it work to install a small(~2l/s) ion pump on the CF cube on the fusor for maintaining low pressure on a valved off system when not running(so the backing/turbo pump don't have to be continuously run)?
2 l/s is probably a bit small. They are good for keeping really small spaces down. Look at the size of ion pumps they put on field emission SEMs, that is closer to what you want. They just keep the pressure down in the column, it needs to be in the -9 to get good life out of the tip. And ion pumps can be very finicky, sometimes they won't start till down to the -6, -7 range.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

We use delrin / acetal insulation brakes fairly extensively on Madison symetric torus, all the vacuum engineers agree it is fine as long as there is no direct plasma bombardment, and it is not operated at very high temperature. They say it has similar outgassing properties as viton.

It is however critical to pick the correct type of acetal casting/extrusion to eliminate porosity leaks: an acetal round will have axial porosity down the center(useful as the type of insulating break I have, since the center is bored out, useless as a blank-off), while an acetal plate will have porosity along the center of plane(useful as a blankoff, useless as round insulating break since it would have radial porosity)

Outgassing mass loss percentages are listed here, with the plastic Semitron® ESd 225 noted as filled acetal (1.00% TML, 0.05% CVCM and 0.60 % WVR), it as not that much more then PEEK ( 0.31% TML, 0.00% CVCM, 0.06 % WVR)
http://www.boedeker.com/outgas.htm

To quote from a previous thread where I discussed the Acetal insulator
To further eliminate problems, the acetal copolymer insulator was baked out at 110C for 24H under 30" of vacuum in a vacuum oven. The mass of the insulator before and after baking was measured and the total mass lost was determined to be 0.4%. This includes condemnable volatiles (out gassing mass) and absorbed water. Mass changed from 135.8g to 135.5g. Additionally a stainless plasma limiter was added to the insulator to prevent plasma bombardment of the material.

Previous discussion threads on vacuum use of plastics are here:
viewtopic.php?f=6&t=2968&hilit=acetal
viewtopic.php?f=6&t=2943&p=18165&hilit=acetal#p18165

Hopefully I'll swap out the acetal with a short stainless spool with a PEEK insulator to make sure, and it will also increase conductivity by allowing the removal of the baffle that shields the acetal from plasma bombardment.

----------------------------------------
Current times / pressures are:
3min from 160mtorr to 2.8mtorr
20min from 2.8mtorr to 1.2mtorr
30min from 2.8mtorr to 3e-4torr
3hours from 3e-4torr to 1.8e-4torr
Best vacuum 1.2e-4torr

Once valved off, pressure rise from 1.2e-4torr to 9e-3torr in 20 min

Stable plasma discharge in the 3e-4torr range with slow rise as components heat up.
To be clear, I have not done a high temperature(of the shell) run of the fusor, so there still might be significant trapped water on the walls.

------------------

I've noticed a steady pressure rise during operation when starting at 3e-4torr. I think the very tip of the cone may get extremely hot due to plasma bombardment, and it may be vaporizing the boric oxide binder which is then decomposing in the plasma and contributing to the pressure rise.
pump5.jpg
I've tracked down the vapor pressures of the boron nitride and boric oxide binder at various temperatures (I'm using HBN grade boron nitride for the insulator)
https://www.momentive.com/WorkArea/Down ... x?id=27480

Significant heating of the top of the insulator confirmed by thermal imaging, but I lack the resolution to differentiate the temperature at the very tip.
Note: the temperature readout is incorrect due the the attenuation of the ZnSe viewport; the insulator is actually much hotter.
https://www.youtube.com/watch?v=8DfPozHL37A
SAM_0350a.jpg
SAM_0314a.jpg
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Bern's logon is not currently working, but he sent me some useful suggestions:
----------------------------------------------------------------------------------------------

Hi Andrew,

I read your recent fusor.net post. My login is not working there tonight, but I thought I would provide some food for thought for you to check out this weekend regarding your vacuum issues. I believe you are chasing not one, but several issues and the combined effects are what you are seeing.

1) It does look like you have a oil contamination problem from your Pfeiffer Duo 1.5A backing pump that has affected your MKS Quattro 999 gage. This appears to be the most likely significant problem. The reasons that backing pump oil contamination are suspected are: a) you do not have a metal sieve trap between your backing pump and your TPH 055 turbo pump and b) the reading you are getting on your MKS Quattro 999 gage when connected directly to the TPH 055 turbo pump (with a specified ultimate vacuum pressure of 1E-6 mbar or 7.5E-7 Torr) is only 8E-5 Torr after ten minutes of operation. Also, in addition to reviewing your current setup, from reviewing the older images of your early ion injectors, it looks like you may have had oil contamination issues plaguing you for a while. The brown residue on the old ion injectors and in the chamber sure looks like deposited oil film.


You're probably thinking that the backing pump oil is rated at a vapor pressure of 1E-6 Torr, so that probably isn't an issue. Think again. That rating is at room temperature and the backing pump gets hot when operating for a while. If you take the hose connecting your backing pump to your turbo pump off, I am sure that you will find it has a bit of an oily smell on the inside and this is a not good thing. When you bought the preconfigured system, it likely didn't have a metal sieve trap, so you are probably thinking it shouldn't need one. It never ceases to amaze me where some manufacturers cut corners to keep pricing down on an initial system. Anyway, you will save yourself a lot of grief and continual cleaning of oil film out of your system by putting a metal sieve trap with a stainless steel gauze filter (for filtering hydrocarbons) between your backing pump and your turbo pump. Either that or buy and install a very expensive dry pump capable of operating at vacuum pressures required by your turbo pump. The metal sieve trap is a much less expensive solution. You should also install a anti-suck-back valve between the metal sieve trap and the turbo pump. Even if the backing pump has a built in valve or holds vacuum pressure well enough after being turned off, the trapped oil in the metal sieve trap filter may eventually escape in small amounts. What works well is to use a anti-suck-back valve between the metal sieve filter and the tubing to the turbo pump. You will save yourself a lot of headaches in hunting for problems and cleaning oil out of the rest of the system by installing these items. As examples, check out the MKS Vacuum Sentry and Norcal metal sieve traps.


Once you have a compatible metal sieve trap with stainless steel gauze in place and anti-suck-back valve, then put a known good MKS Quattro 999 (or similar) gage on the end of the turbo pump and test the vacuum pressure. Your last reading was 8E-5 Torr after 10 minutes. You should see something closer to 7E-6 Torr (or a bit lower) and you should see a continued drop to 7.5E-7 Torr. Oil contamination affects both ion gages and capacitance-based gages, usually causing them to read a bit over an order of magnitude higher pressure than they should be. Don't go through the trouble of trying to clean the turbo pump, except as an absolute last resort. See what you can improve in the system by cleaning everything else first - you probably won't need to touch the turbo pump. You should be able to find instructions on cleaning the MKS Quattro 999. You will find it far easier to take a part and work on than a turbo pump. Also, assuming the vacuum gage is now registering where it should be, consider cleaning the inside of the vacuum chamber to remove any oil residue prior to testing again with the vacuum chamber.


2) Non-optimized turbo pump and backing pump combination - it also amazes me how many manufacturers put out turbo pumping stations with less than adequate backing pumps. Oh sure, they work well enough, but the manufacture is not always concentrating on performance, but in some cases looks more at the bottom line on cost (usually due to competition). So take a look at the manual for your TPH 055. It is a TPH 055, because it has a NW63 port and not a conflat flange (also, the images of the placard on the unit on your web site show it to be a TPH 055). You can find the manual here http://www.idealvac.com/files/ManualsII ... Manual.pdf . So take a look at the graph in figure 2.2 Volume Flow Rate on p. 9. Note that the ultimate pressure rating on your Pfeiffer Duo 1.5A is 6E-3 mbar and that means that the maximum volume flow rate you can get out of the TPH 055 connected to it is right at about 30l/s. Hmmmmm ... that sure sounds like exactly what you measured. Now you know why and that the volume flow rate from the turbo pump won't get any better, unless you change the backing pump. If you still have that Leybold D2.5 sitting around, it has an ultimate pressure of about 2E-3 mbar and has approximately the same pumping speed as the Pfeiffer Duo 1.5A. Because it has a lower ultimate pressure and assuming it will get close to that, you should be able to get a volume flow rate out of the TPH 055 of around 45 l/s when using it (that is 15 l/s more!). You will still need the metal sieve trap in place to keep oil vapor from the backing pump from contaminating the rest of the system. One thing to note on backing pumps. Do check the vacuum pressure going to the turbo pump and know the rating of the oil that the bearings on your turbo pump use. I don't think it will be an issue for your TPH 055 turbo pump given the ratings indicated in the graph of figure 2.2, but some turbo pumps have bearing oil with a vapor pressure of around 5E-3 Torr, so pulling the exhaust port vacuum pressure lower on some turbo pumps can reduce the amount of oil on the bearings over time and freeze up the bearings. Given that the manufacturer is indicating that their TPH 055 turbo pump needs at least 1E-3 mbar (7.5E-4 Torr) backing vacuum pressure to get to its rated volume flow rate of 55l/s, then you would expect the bearings to be in good shape with a backing pump vacuum pressure down to that; however, you should always check with the manufacturer to be sure. Anyway, it looks like you can improve the volume flow rate on the TPH 055 from 30 l/s to 55 l/s by changing the backing pump to one better matched for the TPH 055 operation.


3) Losses through the tubing. It is best to keep all vacuum tubing as short as possible. As far as the diameter used, this will depend on cost, mechanical fit in your system, and the amount of time you want to spend waiting for the system to pump down. Given your background I suspect you are already familiar with the wealth of information on this subject on the Internet and in text books, so I don't plan to cover it here. However, do note that some improvements can be had at reasonable cost. For example, you have a 2ft tube connecting the turbo pump to the vacuum chamber. You could change this to a 19 inch tube (standard size or possibly shorter) and gain some benefit. An option that many find reasonably economical and sufficient is to use CF2.75 connections with a relatively short piece of 1.5" dia. tubing. To keep vibration down to the turbo pump, it is best to use bellows tubing as a connection directly from the turbo pump exhaust port towards the backing pump.


4) That big o-ring connecting each half of your chamber. Note that the reason the TPH 055 has an ultimate pressure rating of only1E-6 mbar instead of a far lower ultimate pressure (orders of magnitude lower) is that it uses a viton o-ring as a seal on its NW 63 port instead of a metal-to-metal conflat flange seal. So consider that your even larger viton o-ring sealing the two halves of your vacuum chamber is causing more of an issue. I know that for some reason you were not measuring an appreciable leak near the big o-ring you are using, but if you do some research on atmospheric gas permeation through viton o-rings, you will wonder why you didn't. If reaching a higher vacuum is more important than measuring current separately on each half of your vacuum chamber, then you may want to consider putting a metal seal between your vacuum chamber halves instead of a viton o-ring. Also, if you have not already done so, research the amount of outgassing you can get from new viton o-rings.


5) Ionizer impact on the cold cathode ionization gage. You did some nice work on developing your ionizers. However, consider that these can affect your vacuum gages, particularly, if they are ionization gages (such as cold cathode gages). Your MKS Quattro 999 appears to have a KF25 connection. You can acquire KF25 o-rings with a stainless steel screen and use these to connect your vacuum pressure gauge to the KF25 port on your chamber. While this may not totally eliminate errors in vacuum pressure readings caused by your ionizers, it will likely reduce the issue quite a bit.


6) Impact of ionized gas and high voltage electrostatic forces on polymers, epoxies, and sealants in the vacuum chamber. Even if you have plastics, polymers, and sealants rated not to outgas down to very low vacuum pressures, these ratings do not apply when you subject these substances to high electrostatic charges. For example, if you apply some epoxies designed for use in sealing vacuum chambers in an application to hold your cathode of your fusor in place (attached to the cathode itself), they will outgas like crazy when you turn up the voltage to 20kV or so, even though they don't outgas at all otherwise. So it would be reasonable to expect that plastics, polymers, and sealants could do the same when exposed to the beams from your ionizers (probably what you are seeing when you have the ion beam pointed at your viewport is outgassing from viewport sealant). You also noted that increased temperatures tend to cause the plastics to outgas. This sounds plausible as well. So in your selection of plastics, polymers, and sealants you not only should consider their operation under a vacuum, but also their operation under a vacuum combined with high temperature and high electrostatic voltages. So that you are not chasing outgassing issues as much when you are working to achieve a lower overall vacuum pressure, leave the ionizers and the cathode grid voltage off, until you get the vacuum level to a suitably low pressure. At least that way you can tell where the leaks are and then once the leaks are resolved, you will know that what is left are outgassing issues to track down (that will likely be more prevalent when you turn on the ionizers and high voltage to the cathode).


Now for some comparisons. It is usually helpful to have something to compare to in order to get some idea what to reasonably expect.

Take a look at the chamber configuration at: viewtopic.php?f=18&t=10032

The backing pump has since been changed to a Welch 8912 (3.8 CFM/Ultimate Pressure 1E-4 Torr). As you can see in the picture, the chamber has 5 total ports (1 KF25 and 4 CF2.75). The more ports, the more chances for leakage, so the number of ports was kept to a minimum. All seals are metal except the KF25 on the MKS421 (viton o-ring with SS metal screen) and the NW63 (viton o-ring with SS metal screen) on the MDP-5010 (7.5l/s and ultimate vacuum pressure of 1E-6 mbar or 7.5E-7 Torr).

In that configuration:

From atmospheric pressure (at 643 AMSL) to 5E-3 Torr using only the backing pump takes about 17 minutes.

The MDP-5010 is then turned on and in 10 more minutes, the vacuum pressure in the chamber goes from 5E-3 Torr to 5.9E-5 Torr. In 50 more minutes, the 6 inch diameter chamber is at 1E-6 Torr.

Just my two cents.

Kind Regards,

Bern
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

TLDR: Conductance of pumping system is terrible(~0.35L/s), gas load in fusor predominantly(about a factor of 10 over the o-rings) due to delrin surface outgassing, insulator will need to be replaced(cheaper then re-working the turbo station valves).

To address Bern's suggestions <thanks Bern, very well thought out suggestions :) >

1) oil contamination
--------------------------------------------------------------
I bought the gauges(3 of them) used on e-bay, used, unknown history. I got them for $50 ea, but they each cost $1120 new
http://www.capovani.com/iinfo.cfm?itemno=172700
http://www.mksinst.com/product/Product. ... ductID=448
so replacement is not really a practical option. 2 were working, 1 had a burnt out ion gauge, the 2 that are working have similar performance.

I do not necessarily suspect oil contamination in the fusor since I turn on the roughing and turbo pumps on at the same time(roughing from the 1 torr range or thereabouts) and rough through the turbo, that way the turbo will have spun up by the time the roughing line is at a pressure where oil back streaming would occur and back streaming through a running turbo is unlikely.

To prevent oil back streaming when the pump is shut down, the pump has in integral anti-suck back/vent system(identical in function to the MKS Vacuum Sentry) upon pump shutdown, the pump is vented to atmospheric pressure, and the roughing line is isolated (pictures here http://www.rtftechnologies.org/general/ ... uo15a.html). As you noted, this may still leak and oil and a sieve trap would still backstream

To prevent this I have an additional high vacuum valve to isolate the turbo connected directly to the backing flange on the turbo, upon shutdown the roughing line is also vented to atmosphere, leaving the turbo isolated. The high vacuum valve on the turbo inlet and the valve on the fusor is closed as well. Eg, for low pressures(under ~100mTorr), the fusor never sees anything but a running turbo, or closed high vacuum valve, it is never left with a direct line to the roughing pump.

In as far as the backing line sieves go, this seems like a worthwhile addition, I definitely like the nor-cal right angle versions:
https://www.n-c.com/ECommerceSite/media ... eTraps.pdf
these are on ebay, what do you think:
http://www.ebay.com/itm/331264350423?_t ... EBIDX%3AIT
the other option is a metal sieve filled roughing line(which I already have)
SAM_4432.JPG

2) Non-optimized turbo pump and backing pump combination
---------------------------------------------------
I no longer have the Leybold D2.5, nor would I go back to it, I like the pfeiffer better as the oil lasts longer. I also rebuilt it after buying it so it's in very good condition. The TPU 055 manual specifies a 1.5m^3/h (the rate of the Pfeiffer Duo 1.5A) pumping rate as sufficient.
I think you are misreading figure 2.2 Volume Flow Rate on p. 9 (http://www.idealvac.com/files/ManualsII ... Manual.pdf), that specifies pumping speed vs turbo intake pressure, not outlet pressure.


3) Losses through the tubing
---------------------------------------------------
Right on the nose there, conductance is absolutely dip, see calculations below. I might go to a nw25 flex hose and valves, hopefully gate valves if I can find them.


4) That big o-ring connecting each half of your chamber
---------------------------------------------------
The big o-ring stays, I'd love if they made non-conductive copper o-rings, but they don't. Also not nearly the predominant gas load in the chamber(the 16x 2.75CF orings are greater length), and the outgassing of the delrin is a factor of 10 greater then the total oring permeation rate. see calculations below


5) Ionizer impact on the cold cathode ionization gage.
---------------------------------------------------
The MKS Quattro 999 is actually a hot cathode bayard-alpert type. It also has a built in fine mesh stainless screen on the flange(an nw40)


6) Impact of ionized gas and high voltage electrostatic forces on polymers
---------------------------------------------------
The main gas load in the chamber though is due to the delrin surface outgassing combined with the very low pumping rate, see calculations below.

Only polymers in the vacuum chamber are the viton o-rings(hemisphere midplane shielded with a ceramic limiter, all others well out of the line of sight of the plasma) and the delrin insulator(also shielded from direct bombardment). The viewports use no sealants(epoxy, torrseal, etc) other then direct contact with the 2.75 CF viton o-rings. The fusor runs are done at very low pressure(and consequently low power) and the chamber does not heat up much, only slightly warm to the touch. I do believe the pressure increace during operation is a temperature driven outgassing, but of the Boric Oxide binder in the HBN grade boron nitride grid insulator.

Comparisons
---------------------------------------------------
Very nice chamber and system Bern, thanks for the well though through input

=============================================================================================================
OK, and now for some math

Problem: sub-optimal final pressure, and poor pumping rate, chamber pressure rise when valved off.

Fusor volume:
--------------------------------------------
Main 6" diameter sphere: 1.85 L
13x 1.5" diameter, 2.5" length cylindrical segments(10x half nipple, insulator and cube): 0.9411L
Total volume: 2.79L

Gas source rate
----------------------------------------
Pressure rise when valved off: 2E-3 Torr in 250 sec
Gas source rate into fusor volume: 2.24e-05 TL/s

Viton o-ring atmospheric permeation
-------------------------------------------
Oring quantity: 1x 6.5", 16x 1.875", 1x kf16, 1x kf40
total_oring_length: 122.5"
permiation rate for viton=2.5E-8 TL/s/(linear inch) (http://www.vacuumlab.com/Articles/Gas%2 ... 0Seals.pdf)
o-ring permeation rate: 3.06e-06 TL/s

Delrin insulator outgassing
-------------------------------------------
Insulator internal dimentions: 1.5" dia, 1.5" length
delrin_area: 45.6 cm^2
Surface outgassing rate: 8e-7 TL/s/(cm^2) (from acetal: http://vacuumcursus.nl/casussen/Vacuum_Systems_UHV.pdf)
Gas load rate: 3.65e-05 TL/s

gas load
-------------------------------------------
assuming dominated by permeation of orings and delrin outgassing
total_gas_load=delrin_outgassing_rate+oring_leak_rate
Total gas load: 3.95e-05 TL/s

Pumping system conductance
-------------------------------------------------
Edwards PV16 valve conductance: 2 L/s (https://shop.edwardsvacuum.com/Viewers/ ... &lcid=2057)
Valve quantity in series: 2
Conductance for a 20" long 2cm ID straight smooth tube: 1.57 L/s (http://www.physics.ucdavis.edu/~kliu/Phy250/Vacuum.pdf)
Conductance of 20" long, 2cm ID bellows: 0.52 (using a fudge factor of 1/3 the conductance of the straight tube)
conductance=1/(N_valve/C_valve+1/C_bellows)
Total system conductance from turbo to fusor: 0.34 L/s

Ultimate pressure (calculated)
-------------------------------------------------
ultimate_pressure=(total_gas_load)/conductance
calculated final pressure: 1.15e-04 torr
Actual measured pressure after about 10 hours of pumping: 1.2e-4 torr

Math checks out, it's the delrin+low conductance
disclaimer: fudge factor assuming conductance of bent bellows is 1/3 that of straight smooth tube of same dimensions

=============================================================================================
Solution: the delrin has to go, the insulation break must be maintained, it will have to be replaced with PEEK+ stainless or ceramic

A new ceramic break can be bought for about $200
http://www.lesker.com/newweb/feedthroug ... fm?pgid=cf
but it's a bit long to fit my system at 2.65" length, I'll replace the 1.5" long delrin break with a 1.5" stainless short nipple+ a 0.25" PEEK insulator which should fit fine, though it will add 1 2.75" viton o-ring to the system.

Gas source from delrin is 10x that of viton o-ring permeation, the o-rings can stay. I'd be happy to pump down to a final pressure of 1e-5 torr(calculated 8.9e-6 torr with the delrin removed and no changes to the conductance) and operate the fusor at a pressure of 5e-4 torr

Matlab calculations:
===================================================

%gas permiation rate

format COMPACT
clear all
clc

vol_sphere=4/3*pi*(3*0.0254).^3*1e3 %vol of sphere in L
vol_ports=pi*(1.5/2*0.0254).^2*(2.5*0.0254)*13*1e3 %vol of 10 ports+cube in L
volume_fusor=vol_sphere+vol_ports %vol of fusor in L
measured_leak_rate=2E-3*volume_fusor/250

%-----------------------------------
rate_viton=2.5E-8; %TL/s/(linear inch)

N_275=16;
N_600=1;
N_kf16=1;
N_kf40=1;

D_275=1.875;
D_600=6.5;
D_kf16=1;
D_kf40=1.5;

total_oring_length=pi*(N_275*D_275 + N_600*D_600 + N_kf16*D_kf16 + N_kf40*D_kf40)

oring_leak_rate=total_oring_length*rate_viton

%-----------------------------------
delrin_outgassing=8e-7; %TL/s/(cm^2)
delrin_area=pi*(1.5*2.54)*(1.5*2.54) %(cm^2)
delrin_outgassing_rate=delrin_outgassing*delrin_area

%-----------------------------------
total_gas_load=delrin_outgassing_rate+oring_leak_rate

%-----------------------------------
C_valve=2; %valve conductance l/s
N_valve=2; %valve number
C_tube=10*(2^3/(20*2.54)) %straight smooth tube conductance l/s
C_bellows=C_tube/3

conductance=1/(N_valve/C_valve+1/C_bellows)


%-----------------------------------

ultimate_pressure=(total_gas_load)/conductance


format LOOSE
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Dennis P Brown »

Really great posts and as I warned earlier - a oil vapor anti-back flow device is critical on any fore pump. All I will add is that you keep saying the plastic components are fine since you did "X" but your system isn't getting to any were acceptable vacuum - so, when in doubt, remove all suspect components and test your system without them. If the problem is solved, slowly, carefully, reinstall removed components one at a time, testing each time. This will remove that unknown. As pointed out, a fusor with its intense plasma is no place for plastics and even teflon should be considered questionable.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Bern Bareis »

Andrew,

1) If you are not obtaining measurements reasonably close to the ultimate vacuum pressure of the TPH 055 when you have the MKS Quattro 999 attached at the input (i.e. only elements in the system being the backing pump and possibly a metal sieve trap, the TPH 055, the MKS Quattro 99, and reasonably high conductance tubing/couplings in between), then there is something that is not quite right in that system. If you are indicating that you are getting 8E-5 Torr in that configuration, you should be obtaining a reading that is quite a bit lower, given that the ultimate vacuum pressure for the TPH 055 is 7.5E-7 Torr. My thought is that the most likely cause would be some sort of contamination in the MKS gauge.

Regarding the metal sieve trap, if you can find a larger diameter and can still fit that on your system, it would not need to be cleaned as often, if you will be using your system a lot. That being said, the 2 inch diameter metal sieve trap with stainless steel gauze/mesh works well on the system that I provided the link for earlier. The 2 inch diameter metal sieve trap appears to have considerably more filtering than the metal sieve filled roughing line that you provided a picture of. By putting in a metal sieve trap and a valve between the metal sieve trap and the exhaust port of the turbomolecular pump on your system you should be able to pretty much eliminate the potential for oil vapor from the backing pump contaminating the rest of the system. The reason that a Vacuum Sentry was applied on the system from the link I provided earlier is that in the event of a power failure or simply shutting down the backing pump, the line between the metal sieve trap and the MDP-5010 is closed off within 30 milliseconds and the side leading to the backing pump is vented to atmosphere from the Vacuum Sentry towards the backing pump. Consider that if you are using a manual valve and the vacuum is not properly vented between the manual valve and the valve in the backing pump when the pump is turned off, then you will maintain a vacuum across the metal sieve trap and if it is still quite hot from running the backing pump for a long time you may have some amount of oil vapor awaiting entry into your system the next time you open up the manual valve. The Vacuum Sentry eliminates this potential since it both closes off the path to the chamber and vents the path towards the backing pump.

I am making the assumption that when you bring your vacuum chamber up to atmospheric pressure and are venting it you are doing this from the vacuum chamber end of your system and not the other way around. Otherwise, you could end up sucking oil vapor from tubing leading to the backing pump, if present, into the chamber and gauges.

2) I agree. Though if you can find a Inlet Pressure versus Fore Pressure graph for the TPH 055, this may provide some additional useful information. After reviewing a few of these graphs for other turbomolecular pumps I am familiar with, I am convinced that you would have to be pretty far off on the Fore Pressure requirement not to get close to the ultimate vacuum pressure, so this is probably not an issue.

3) Looks like you have a good handle on the conductance issues.

4) Depends on how low you want to take the vacuum pressure in the chamber as to whether or not at some point you will decide to remove the o-ring. At your target vacuum pressure, it does not look like an issue. Given the length of time you have been operating your vacuum chamber and producing results with that o-ring in the design, I suspect you have a good idea where its shortcomings are. A check of the permeability of the o-ring was useful.

5) Since the MKS Quattro 999 has a built in screen to help isolate any impact the voltage and current transferred through the plasma may have, then I agree that the difference in the measurement between the ionizers on versus off must be predominantly from something else.

6) You are exploring new territory. Keep at it.

Kind Regards,

Bern
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Dennis P Brown »

Hold on a minute. I am very confused by this post:
The big o-ring stays, I'd love if they made non-conductive copper o-rings, but they don't.
If this is a fusor there is absolutely no way that the two halves will not be in full electrical contact in a plasma in the micron range. If your goal is to build an ultra high vac system and not do fusion then ok. If you are doing fusor fusion, then an insulating vacuum seal between these components is useless.
Its possible that your gauges are out-of-calibration and you are getting to the low 10^-6 torr? I really have doubts about the gauges because you posted:
Turbo pump down with turbo pumping on gauge only (pumps down to about 8e-5)
Your turbo just on the gauge should easily bottom out the 10^-6 torr range in a few minutes - my entire system (a small chamber and "leaky accelerator tube and 1 meter of teflon tubing) pump down to 5*10^-5 torr in under five minutes and I use a near identical turbo! Your turbo is neither not working properly or you have very bad contamination in the turbo/chamber (which I doubt) or the gauges/unit is not reading correctly.
Your pressure rise is sufficiently slow - this tends to tell me that your system isn't really contaminated (2 microns in 30 seconds isn't bad at all and your top pressure after a 12 hours was a very acceptable half torr.) If that is true then I suspect that your turbo is at fault or the gauge system - and frankly, high end of 10^-5 torr with no load is very poor performance for a turbo. There is really no way you will determine your problem until you calibrate your ion gauge somehow - sorry but until you know for certain the gauges (with control box) is working, you will not solve this problem - it is the turbo or gauge system is my guess.

If a university physics department is near by, call them and talk with one of their physicist - I am sure they will be able to let you test your complete high vac gauge system on one of their high vac systems.
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