FAQ - Cleaning optical windows, insulators and chamber interiors

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tligon
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by tligon »

Carbon deposits would certainly explain the lack of effectiveness of HCl on my window.

I have a stainless steel "ratwire" screen between my fusor grids and the viewport, just the thing to keep stainless steel rats at bay, and with the hope of intercepting electron beams before they damage the glass. The screen is about 1 cm off the glass. I notice it leaves a sharp shadow pattern consistent with something coming from the central focus point.

Ions should not do this. We know electrons will. Possibly the glass "charges up" in this pattern and attracts various ions, be they metals, pump oil molecular fragments, etc. Carbon or hydrocarbon fragments would probably attract to the charged-up portions as in Xerography. Fast neutrals kicked by the main ion stream could also contribute.

Since I do not presently have a trap between my pump and vacuum chamber, backstreaming oil vapor could very likely be the culprit. While at EMC2 I operated the unit with a MicroMaze trap, and thinking back on it the deposits seem worse now than when I was there.
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Doug Coulter
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by Doug Coulter »

I have used, and I believe posted prior to this, all 3 main methods. I think anyone who actually runs a fusor for many hours runs into these things.

I do use a sacrificial layer in front of my viewports. Sometimes it's just a sheet of mica stuck on with a tiny bit of apeizon wax, sometimes another full-thickness piece of pyrex. I'd rather have to toss a ten buck piece of that than replace the window in my viewport/door at vacuum supplier prices -- they use odd sizes that are hard to find elsewhere.

More often than not, plain old HCL will do the trick on many things. For ceramic, I follow with a rinse in something basic (ammonia or NaOH solution) then a final set of rinses with Di water/alcohol in an ultrasonic cleaner, but for the ceramics, the acid doesn't always do. I use both tungsten and titanium grids here with sometimes some exposed Cu or Al for the HV stalk. The shadow method is good -- prevention is worth more than cure to the extent it works.

Particularly if the stuff doing the shadowing is disposable or arranged in such a way that it doesn't hurt things if it becomes conductive. I have had mica fail horribly in this case because once it got somewhat conductive, it attracted enough ions to heat up and thermally decompose. Gives a pretty light show while it dies and takes the fusor down -- you'll be getting inside again to fix that before you can run again. Mica just doesn't work well in that place, but seems fine in front of a viewport.

Sandblasting works well on ceramics, especially with the little gun like Steven pictured. It's really cheap, the whole kit is on the order of 10 bucks at Harbor freight and some very cheap compressors meant for airbrush painting will do in this case -- you don't need 120 psi shop air or even a portable tank you fill at the gas station, and the media that comes with the kit works, as does other sandblasting media. You may want to avoid silicon carbide though -- conductive. Since I do have a shop compressor, I never considered that as a problem though, and they are getting cheap for the crummy and noisy ones anyway. If you do sandblasting -- do it outdoors, as it can really make a mess in the shop...you will have sand down your back, and regular safety glasses may not keep it out of your eyes, either, as the stuff bounces around everywhere. I use a cheap hood when doing that.

I too have had interesting problems with browned glass (but interestingly not with sapphire, which is getting hit enough to glow a pretty blue during runs). Luckily it was the 3/8" thick sacrificial piece of pyrex from McMaster -- cheap, and they sell it in round pieces the right sizes. When nothing seemed to work to remove it, I broke it to see how deep the brown went and it was virtually all the way through the glass (which had no doubt been placed in there either way facing in at one time or another), so I am not sure which theory is correct there -- X rays could be part of it too, as well as things driven deeply into the glass, but I've not run energies here that wold drive things that deep. I bet someone here who does ion implant could chime in with a number for expected penetration depth vs the few tens of Kv we tend to run -- making the assumption that any ions actually reach that going *away* from the grid (reaction products might, and they can be plenty hot). To me it would seem more reasonable that it is electrons because of other things we see -- as in the glass lighting up white under their bombardment and a faraday probe saying that it's electrons coming that way (or negative ions?).

Warm ammonium bifluoride solution works pretty well on things other than glass....and if you are desperate, even glass, which you follow with cerium oxide and elbow grease to un-frost it again. This is pretty tame stuff compared to real HF. I happened to have a quantity of it provided by my electroplating supplier (Caswell Plating) as a pickle/cleaning solution mix.

One thing that may lend some credence to the carbon theory is that I had a lot more trouble when the fusor was pretty new, and presumably had more contamination in it. However, for the last fairly long time, I've been using high grade graphite (McMaster) for endcaps on my cylindrical grids to hold the tungsten rods and not noticed any big increase in deposits, even though I can see some degradation on the graphite. Basically it started off with a mirror polish, and got more like flat black over time, a somewhat roughened surface. Nothing else seemed specially affected at least so far. The McMaster graphite rod stock is really good stuff -- not carbon, but really graphite, no binders to outgas (or mess up lathe tools), and of course it easily takes incandescent heat in fusor conditions when you put in too much power.

My policy, which seems to be working out, is to prevent deposition on sensitive stuff as far as possible,
following the old saw, an ounce of prevention....is worth a lot of effort in a cure. I don't worry about the tank walls at all. If I sputter some Ti or W onto them, it only seems to help, though I'd avoid getting too much W on there as it will then make X rays more efficiently under electron bombardment than Ti or stainless steel elements. Not an issue here, we bit the bullet and totally shielded our fusor with thick lead sheet. A lot of work, but no worries now.

A lot can be solved by getting the geometry right so the things that get coated don't matter, but that takes significant skull sweat and a few tries, usually. My new HV feedthrough design accomplishes this pretty well. This: http://www.coultersmithing.com/AuxCP/FT.html describes that, and a couple samples of this have run hundreds of hours without any problems or noticeable degradation. The very tip of the quartz tube gets a little deposition on it, that's all, and it doesn't affect how well it works at all, it seems. Key are the dimensions near the end that sticks into the tank -- and the aluminum design pictured there is the best so far -- note the stalk is turned down thinner near the end where the action is -- that turns out to solve a lot of things. Small changes there make large differences.

If that fails, harbor freight sells some nifty small diamond hole saws that easily make clean holes in glass or quartz (not so good on alumina sheet) using the normal procedures -- slow drill press, water lube in a dam made of modeling clay, and patience - slow rotation and light pressure gets it done. You can put them over the HV stalk and they catch the stuff before it gets on the harder to clean ceramic, and no compunction about either putting them into nasty chemicals to clean or simply replace them.

www.quartz.com is where I get my quartz, good people to deal with, but quartz isn't cheap no matter where you get it -- I generally buy in bulk to get the discount. I sometimes resort to McMaster for small round quartz windows even though they tend to be a little pricey there, still not too bad when you only need one or two small ones.

I find it interesting that you can pretty much tell who really runs their gear by the problems they run into, and if they don't see this (or some other issues) -- they must not be running very much!

I am wondering where the carbon might be coming from, assuming Chris is correct, as I only generally see the usual small amounts of CO and CO2 on the mass spec. Could it be impurities in the normal CP grades of D that people are using? There is always a tiny bit in the glass already, but I'd think that it may not be the first thing to be reduced by electron bombardment. I do know that I am often making enough X rays to do the "color center" thing in glass and brown it that way. I am finding that really pure quartz (as obtained at quartz.com, not vycor but the good stuff) gets reduced *or* browned a lot less easily than most ceramics or glass, for whatever reason - I am finding a lot of good uses for that material around here. It is one heck of a lot trickier to do glass work on than pyrex, though. The score and snap technique does NOT work with quartz, and the melting point and boiling points are fairly close together - it takes good tools and some practice to work with even once you get good at doing the same things with pyrex, but drilled holes are easy in either case. To cut tubing I pretty much have to use a thin diamond wheel in a toolpost grinder on the lathe to get good results every time. As usual, use water lube and go slow and it works fine.
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Richard Hester
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by Richard Hester »

Huh - the "brown all the way through" may be some sort of solarization, I've seen it happen in CaF2 exposed to EUV (193 nM) as well - in that case, the solarization was in the form of deep blue inclusions in the window. If it's solarization, annealing the glass may fix it. It'd be interesting to try it on a lark if you were inclined to otherwise throw the glass away.
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by tligon »

More than once I have been tempted to use a sacrificial glass round to protect the viewport (my 8" CF viewport, intended for audiences, costs nearly $700). However, I never got around tuit.

You spurred me to go on-line at MMC and order one. A borosilicate round tuit of 6" diameter is about $29.
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Doug Coulter
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by Doug Coulter »

Thanks for the input, Tom and Richard, and yes I will try annealing the stuff and see if that helps, just for grins -- I have a metallurgical heat-treat oven that ought to do fine for that. I paid a bit more than that for my window/door from Lesker, but find that it was a great investment and way-way worth it. Especially for diagnosing little arc problems, too much heat and suchlike. And I'm real glad the cheap sacrificial piece guarding it seems to be working fine after hundreds of hours of operation -- I'd hate to have to buy that glass piece and new gaskets retail from Lesker. Sure is nice to have a big door to get into the tank through and not have to buy copper gaskets each time. Makes it far easier to try new things and make progress quicker.

Here, viewports and bellows and feedthroughs are the main sources of X rays escaping, and to solve the viewport issues we found a very thick piece of PbO glass from a radiology dept that still allows good stereo (human) vision and also very nice high resolution photography without getting a chest X ray in the process. I just built a frame for the 18" by 24" piece so we can set it in front of the door when we are running, but remove it to get the door open other times. We tried the webcam thing and basically rejected it as those are too low quality (rez and color rendition both) to tell much about what is going on in there. So far we're not seeing very significant leakage through the thick CF flanges themselves -- that's a lot of SS to go through for all but the very high energy (and also fairly rare) gammas from the reaction itself. But they obviously get through bellows and ceramics like they weren't even there.

BTW, before we got a little more careful, and somewhat smarter, we were getting enough X rays through the window (with the thick sacrificial pyrex behind it) to actually mess up pixels in a high quality Kodak CCD camera. After looking at that picture with the random white dots, I got religion real quick-like.

To solve the other X ray escape problems we've been able to make articulated lead shields for the bellows (wobble sticks are way cool!) and low volt feedthroughs (probes and thermocouples), but for the high voltage one the only thing we've managed is to have that "leak" pointed away from the experimenter -- in my case out through the wall of the lab, which is also safer as it makes it harder to get too close to the HV as well.
As I live in the far boonies, there's no risk to anything but the deer and the lawn getting hit outside my walls. I'd worry if I still lived in an apartment, though.

Doing up all that lead sheet for a complex shape tank was truly a pain (several days full time), but worth it, as now all our counters stay quiet during a run, and I'm pretty sure I'm not frying my wife as she watches TV upstairs -- we actually did check that one with a very sensitive scintillator. But it was one heck of a lot of cutting complex shapes and soldering overlapping joints to get there on my tank, shown before that process on my homepage.
www.coultersmithing.com for those who haven't seen it. It's pretty ugly now to all but the eyes of those who understand the radiation issues. We run over 50kv sometimes so the SS tank is way not-enough.
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Starfire
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by Starfire »

Doug - X-ray is easy to deal with - use a mirror at 45* and a lead shield alongside the camera which should be at right angle to the view port.


BTW Thanks to Richard for another useul FAQ's.
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Doug Coulter
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by Doug Coulter »

We did that, but we didn't like the results too well -- the mirror tended to jiggle, the field of view wasn't so good (just having to tilt it 45 deg makes it not so good if you need a view at an angle) with what we could lash up, and it was just a pain because of random other light scattering off the mirror and so forth. The lead glass is the greatest stuff since sliced bread, really. You can put it up against the viewport and look in from all angles and suchlike, which with a mirror usually means having to adjust that -- in other words, putting parts of you into the fierce field to do so. I am not a scaredy cat, and I believe in hormesis, but there's a limit....
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tligon
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by tligon »

I ordered the 6" round on a whim, without checking the diameter of my chamber. My actual working diameter is 5.9 inches.

Not a problem for a guy with a Moto-Tool and a diamond wheel, although dust control was a non-trivial problem. You really don't wanna be breathing finely powdered glass. I looked like a bandit wearing a damp flannel bandana and full face shield.

DP-II is now back together with new grids, and the extra piece of 1/4" thick borosilicate glass outside my front Faraday screen but inside the front viewport. As soon as I got to low pressure and high voltage, circa 8 kV, the glass started to flouresce a yellow-green, in patches. I'm in the habit of manipulating the e-beam that characteristically comes from the Poissor with a magnet. The magnet also shifted the flourescent glow pattern. The pattern shows sharp shadows of both the Faraday shield and the outer grid wires. I conclude it is probably electron-driven, a classic "cathode ray" such as Thomson demonstrated, a flood of electrons that can be masked to cast a shadow.

Without deflection, the old grids tended to shoot an e-beam at an edge of the viewport, and I worried I might shatter it one day with the public gawking at it. I feel better now that the new glass will protect the viewport.
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by Jerry Biehler »

Dont go at it with a dremel. Too easy to create a hot spot and shatter. Use a slow running 6" Silicon Carbide wheel wet. There are special coolant mixes used for glass that will help. They sell the stuff at stained glass shops.
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by tligon »

The glass chipped a little on the edges when I got carried away, but nothing worse. Borosilicate is low expansion and much more tolerant than the ordinary variety.

On short notice I had to go with what I had.

Another option, something I used at EMC2, is a diamond tile-cutting saw. These are amazingly cheap, a small table saw with a 4" diamond wheel dipping into a water bath. Ours cost about $95, and would easily cut alumina tubing.
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by Richard Hull »

I have used one of these cheapo diamond table saws for tile to slab and shape U rock and other small mineral specimens as well as certain other ceramics.

It's wet and messy and you need to be in a swim suit to use it, but it works great outdoors.

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Doug Coulter
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by Doug Coulter »

I've had OK luck with borosilicate going after it pretty hard, even using a belt sander with 60 grit.

Of course it's nicer with more refined tools, and if you can, use water as a lube/coolant, it really helps amazingly.

Yes, breathing powdered glass is how you get silicosis -- don't. It seems inert stuff is worse for your lungs than even cigarette smoke -- at least your body knows how to "eat" that latter, but stuff that just stays in there and irritates, like this or like asbestos is to be really avoided. I always use water to keep that dust out of the air and out of my lungs.

But of course, next time just get the right size -- I ran into that one too -- the SS pipe with weld bead tends to be about 5.9" inside.
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by tligon »

I tried the belt sander with a fresh belt. Half an hour of work reduced the diameter about 0.005".

The already used diamond wheel took off 0.050" in about 5 minutes. I was amazed at the difference.

I used the belt sander to put a smoother finish on the result.
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by Doug Coulter »

Yeah, I use it mainly on thinner stuff -- a good belt will eat 1/4" tubing almost as fast as you can push it in there, or a thin (like 1/16") window. The thick stuff....probably mostly just strips the abrasive off the belt.

I use diamond wheels and hole saws extensively here in glass and quartz working -- no question that's the way to go. I built what amounts to a lathe mountable dremel tool to hold the wheels for cutting tubing, and it's the cat's meow for that, I just use an eyedropper to drip water on it and keep the dust down. It wouldn't occur to me to try the table saw or angle grinder versions -- too fast and too brutal -- and I don't like having to wear a suit.

I use the hole saws in the milling machine to hole even thin wall quartz tubing, it works fine, and saves work making Tees and so on. You use modeling clay to make a little dam to hold some water, which also cuts down chipping around the hole, and just be patient -- it doesn't actually take all that long to get through and does a nice job. The reason to use the mill is it will go nice and slow (80 rpm), and has less wobble in the quill than any drill press I've had.

I use cerium oxide for polishing when I bother to do that (Caswell plating sells it online), make a slurry on a cloth wheel for example, chucked into a hand drill.

I did that to make some old CRT lead glass usable for a viewport cover to keep X rays down, the inside side wasn't smooth enough without that step. That was a decent stopgap until we found some real good lead glass from a radiology dept that really stops them cold and is clear (but must be almost all PbO, that is one heavy chunk of glass).

With that, and the lead dressing on the fusor, I can now stand right there and observe and take high rez pictures with the detectors sitting at background levels (except for neutrons). Really makes it easier to learn and make progress on these -- worth doing. The webcam approach just wasn't getting it done for us here, and it's hard to move a wobble stick from across the room anyway.
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by tligon »

Some people like wet and messy.

I recall a bit of a learning curve. Stand to the side, wear old clothes, and only do it in the late evening when there is nobody to give you a hard time about being all covered in speckles.

And do your own laundry.

I got diamond wheels for the Moto-Tool to cut alumina tubing. What resulted was worn out diamond wheels. Dry-cutting alumina peels the diamonds right off the wheel. But the wetted tile-cutting diamond wheels cut right thru alumina with no apparent wear to the diamond coating in the time I used it. The cuts were beautiful.

The neat thing about a tile-cutting saw is my wife will think I bought it for her projects! She is thinking about some tile in the kitchen and has a gemstone project in which it would be handy.
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by morgoth31 »

ok in my vacume coating experiance we used mostly green scratch pads and iso propyl it takes elbow grease but let me tell you iso p evaps so easy you dont have outgassing problems.

for the glass protection

some systems used metallic mesh grids that are electrically connected to the chamber it makes it much cheaper. some used thin quartz glass as a sacraficial glass.
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by Doug Coulter »

It seems most solvents work fine unless they combine with something in the tank, like Viton O rings not liking acetone (or, liking it all too well). The water in most alcohols is what's going to take the longest to finish outgassing, but even that usually isn't so bad. I've used NaOH/Water to clean off aluminum that got loose from the evap setup.

Though it will mess up my pix a little, I plan to try that screen deal soon. The reason is I just found it a good idea to replace the 3/8" thick sacrificial piece of pyrex in front of my viewport. Turning brown was one thing (probably electron bombardment) but it was developing a series of circular craze/cracks, kind of like a bullseye target, and no way would I like to see that fall apart in my turbo system (which is well screened and baffled, but...).

I do have to wonder if putting the grounded screen on there would simply attract electrons even more, most of which would get through vs a mostly floating piece of pyrex (which IS slightly conductive)...I suspect most of my browning isn't X rays, because pyrex doesn't really do much to cut them down, and the real window behind it is staying nicely clear. Tons of X rays come through both, so between them and me sits a very thick piece of PbO glass we scrounged from a hospital -- now that *really* stops them, where the two thick pieces of pyrex only cuts them about in half. That radiological room glass is very highly recommended -- you can then sit close and watch with both eyes, which beats a webcam all to pieces. My survey meter sits right at the normal background while running now -- the rest of the tank is covered in fairly thick lead so only the rare gamma from the fusion reaction itself gets through that. (there is a 1:10000 chance of a DD->He + y reaction that puts out one heck of a hot gamma ray) Not many of those happen, but the shielding you'd need would make your floor into a sinkhole if you wanted to stop those.
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by morgoth31 »

one of the big things is dont use your pharmacy grade isopropyl need to use the scientific grade this contains far less water. and remember to keep it closed at all times use it then close it. some high end vacume ppl i know have even gone as far as doing temprature contrled distilling to achive low water. but they were making very low torr systems (company i worked for was called UHV which is ultra high vacume) some of those guys were making 10-7 torr chambers space is only 10-3 lol.

you can also do bake out if you have real bad problems. a thermal blanket raises the temp while under vacume but this is only used for very high vac levels which these systems dont use.

alot of the dep your seeing might be from the ablation caused by the plasma itself this tends to be a brown funk that covers everything. the screen that i was talking about acts like the outside of the chamber and gives it something to accrue on instead of the glass. tho the glass normally isnt being a plating medium the atomic particles in that chamber will sometimes lay on even the insulators. (tho tends to be a small precentage compared areas that are conductive and in contact with the chamber) tho the glass can become electrostaticlly charged and can accrue at a higher rate.

i am not a SME (subject matter expert) but i have a working knowledge from working with gurus before. my knowledge is infamatesmal compared with thiers.

im sure if you look at your ball under a escope before and after you would see some ablation is happening. and charged particles from the reaction is prolly ablating your chamber while redeping the material to it.
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by morgoth31 »

one more thing the gaskets you should just use the vacume grease its high viscosity and will actually help your seal not alot tho just a bit to wet them.

you can also use the grease to help leaky areas from the outside by buttering some over the problem areas.
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Re: FAQ - Cleaning optical windows, insulators and chamber interiors

Post by Doug Coulter »

Baking out helps, period, and at ALL vacuum levels. Because what is stuck to all the surfaces isn't the stuff you want in there -- air and water. You don't need to go to real high temperatures to greatly speed the removal of this contamination, and as the Pfeiffer guy once told me - just about anything that puts in energy helps. Might as well blast all that junk off the tank walls (and it represents a lot -- calculate what a monolayer works out to if it were all released -- it's a good part of STP!). So baking in some form makes the pressure go up for awhile, which helps the pump take it all out of there quicker.

I know Richard has mentioned he will often run a glow discharge during pumpdown -- I've tried that and yep, it helps a *lot* to get to purity fastest -- I will even deliberately slow pumpdown by letting in some D during this process and that works better yet (for speed from STP to running conditions, not economy of D).

I use internal quartz heaters (basically big halogen lamps meant for this -- the normal ones will melt themselves in vacuum, or the connections to them) the the difference in gas evolution depending on whether I run them half power (not much UV) and full power (enough short UV to disassociate water molecules) is substantial, long before the tank walls themselves get warm -- you see a more or less instant response to this on gages and a mass spectrometer, with the former all the way "up" to vacuum (pressure?) merely factor 10 better than running conditions. (mass specs don't work well above e-5 mbar or so so at high pressure it's turned off) Which means if that's the best vacuum you can get, you have 10% *at least* contamination in your system, while experiments here show that even 0.1% of some things definately has a large effect. Look up "Penning mixture" for a bit more on that one. This is not the best link, but it gets to the basic idea:

http://en.wikipedia.org/wiki/Penning_mixture
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