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

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

Post by Andrew Seltzman » Sun Sep 20, 2015 7:22 am

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 » Mon Sep 21, 2015 2:57 am

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
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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 » Mon Sep 21, 2015 9:53 am

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 » Tue Sep 22, 2015 3:59 am

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 » Tue Sep 22, 2015 9:34 am

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

Post by Andrew Seltzman » Tue Sep 22, 2015 3:48 pm

I have a planed upgrade to measure the current collected into each hemisphere. Both hemispheres will be grounded, but the collected current will be measured separately. I'll also be operating in the sub-micron range.

I'll look into the gauge and turbo
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman » Thu Sep 24, 2015 2:00 am

A trap has been added to the turbo station.
https://www.youtube.com/watch?v=4F4qzPbcFiA

I'll need a few more adapters to plumb it to the turbo pump, but the station should be up and running in a week or so.
Backing pump with vent valve removed
SAM_4436.JPG
Stainless seive trap
SAM_4437.JPG
Trap mounted on pump
SAM_4439.JPG
Bakeout heater for the turbo pump
SAM_4440.JPG
I also have bought a tank of deuterium for use some time in the future
SAM_4438.JPG
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman » Fri Sep 25, 2015 8:23 am

Gas drier purchased(for dry air venting). This is the one with the built in 40um outlet filter
SAM_4441a.jpg
Gate valve purchased(for higher conductance)
SAM_4442a.jpg
In other news, the turbo and gauge have both been confirmed good, the problem is purely based on conductance and the viton gas permeability. Today I was installing a replacement vacuum gauge fresh from the manufacturer on one of our turbo stations for my PhD work and used the opportunity to test the hot cathode ion gauge as well. The gauge was compared to a known good/calibrated vacuum gauge on a similar known good turbo station with a low conductance valve and the readings matched perfectly. The pump down times were also equivalent, reaching 7.5e-5 torr in several hours
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The gauge was then attached to a separate, known good turbo station, with higher conductance hardware and a second known good gauge.

The gauge reached about 2e-5 torr in 1.5 hours, with the gauge nearer to the turbo inlet was considerably lower in pressure
b.jpg
pumpdown4.jpg
The gauge was then attached directly to the turbo inlet, resulting in a pump down to 2e-6torr in a few minutes
a.jpg
It's shocking how much pressure difference a single very short NW16 adapter can make, compared to an NW40 directly coupled to the turbo inlet. The difference in pump down rate and final pressure is enormous. At UW we use these turbo stations to pump out diagnostics and probes before opening the high vacuum valves to madison symmetric torus, but we do the pump downs through 4 feet of 0.25" ID plastic hose. We might as well not even have a turbo on the pumpdown station at that point. We let them pump down over night, but all of our de-mountable seals are viton as well, we probably are not even getting much past the mtorr range at that point. Our continued use of the stations in this manner has apparently given me a skewed sense of system performance expectations when using low conductance tubing with viton o-rings.
Andrew Seltzman
www.rtftechnologies.org

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

Post by Jerry Biehler » Fri Sep 25, 2015 8:20 pm

I did the calculations when I was connecting my big turbo with an ISO-250 (10") fitting to my chamber with a 6" ASA and it was amazing how much conductance you loose. If I recall I lost over 500l/s and that was with about 6" length of 6" passthrough. Still it did not turn out to be bad. I can get down the low -7, high -8 without too much trouble. I can get down to -5 within just a couple minutes of firing up the turbo, it still pulls down even when it is not at full speed. If I remember it takes about 5-6 minutes to get up to speed (27000 rpm).

Moral of the story, mount your high vacuum pump as close to the main chamber as you can. Thats why you see semiconductor tools with the turbo mounted right to the chamber. The portable pump units are nice but you really cant constrict the line down any if you want to see good vacuum at the other end.

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

Post by Richard Hull » Fri Sep 25, 2015 10:11 pm

We've been yappin' about conductance for years in this forum. The rule is use the largest diameter plumbing up to the diameter of the smallest throat and jam alll vacuum components together as tightly packed as you can within the limits of easy operation and disassembly.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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