Bigger Feedthrough for my fusor.

How to get to vacuum, what the classes are, and what is needed for what job.

Bigger Feedthrough for my fusor.

Postby Doug Coulter » Sun Mar 03, 2013 6:21 pm

Well, this is an ongoing issue. With the 1" OD original design, the failure rate was too high for my taste. I have noticed that getting it really hot doesn't help, and being sloppy about super-cleaning all the glass/quartz parts is a fairly big deal. But I want to not only use all the SL2KW (50kv/40ma) supply can crank out, but go to a newer stack that can make more voltage - 80-125 or thereabouts. I am hoping I won't have to encase the "in-air" parts in oil as Carl W cleverly did, as that would reduce the adjustability and convenience of the thing a good bit. Pursuant to that goal, I've redesigned the thing to go to 1.5" OD, and with a very radiused end that is also the end connector for the large cartridge type ballast resistor I use (50k/225w, forced air cooled). Further, I've bought some much thicker wall PVC to wrap around it all and direct the cooling air, inside the 7" PVC outer piece that is copper covered (for my safety and EMI reasons).

So, here are some shots and movies of the build. I'll of course report further once this is "in action" as to how it works (or fails). This is a very brutal envirronment for a feedthrough - charge accumulation (we have a capacitor in every piece of glass or quartz whether we like it or not), chemical reduction to conductive stuff - silicon etc, and sputtering of conductive stuff onto the FT. So this hues true to the original FT design idea - I can't make it so it can't break - so I make it so it's easy and cheap to fix.
Yes, pure BN (not the stuff with a binder) or pure alumina would not be reduced, and have some other advantages. But from experience, regular alumina (AD-84) is not pure enough by far - the binders reduce, and pure stuff is so expensive and hard to machine - as well as not gas tight to an O ring seal - it's not worth it. And when I asked Accuratus about pure BN (comes in several grades and of course we'd want the non-hygroscopic, no-binder version) - we're talking multiple thousands of dollars. What if it fails? That's a real big hit to the wallet. We might have to go there - the other advantages, like huge thermal conductivity - are compelling, but that price... :roll:
Pieces before adodizing the metal. Haven't cut the quartz inserts yet, the outer tube is Pyrex for the nice automatic voltage gradient you get due to its mild conductivity.

Nicely polished with the big coupler. You can also see the lathe tool I made to cut O ring grooves. Just a chunk of A2 drill rod ground up with a support for my tool holder. I run this slowly and delicately of course, and have to resharpen it after every job, but since I don't have to do this much, that's no big deal. Also shown in the little jar is "Doug's magic vacuum lube" which is about 85% beeswax mixed with a little Diffoil-20 to make it spreadable at room or warm temps. You can't put this together without this. After it's heated a few times, it all disappears out of the system according to the mass spectrometer.

I've found through experience that Al makes about the best conductor. It's good and heat conductive (well...long and skinny it's not perfect like a heat pipe, and we might go there later) but the real deal is - after anodizing it, it just doesn't arc or draw current in the exposed plasma anywhere like copper does. My longest lived example of this (still living) has a zillion hours on it and little to no sputtering to the insulators, it just works. I've even heated the end of that one to very close to the melting point of Al, and it didn't even sieze threads on the grid that was on it, or show any damage. Aluminum oxide, the eventual result of anodizing and sealing, is impervious to reduction by hot hydrogen, and somewhat self-healing it seems. Just an FYI - for whatever reason, this works better than all other things tried, and there have been a few.
Anodizing need not be hard. I use one part battery acid (H2SO4) to 3 parts water (distilled) for the solution. Do a little work on a junk piece, as the solution works better once "aged" with a little aluminum sulfate in it. You can use Al flashing for the cathode, or lead as I show here, and I use roughly 7 amps per sq foot. If you do this with a constant current capable supply, you go for peak anodic resistance - the voltage rises, then drops - you stop right then to get the maximum thickness. This proceedure is somewhat complicated by the fact that as the solution heats up - it has a huge tempco and will make the voltage start to drop before you get there...try 45-60 minutes for ideal results, then seal by dropping the piece in already-boiling (de-oxygenated thereby) water for half an hour to seal the anodizing (close up all the honeycomb pores).
I did this with all the metal pieces after polishing them to mirror smoothness. Anodizing makes the surface look hazy,'s worth it. It still seals to O rings fine.
I just used a tilted tray to avoid having to mix up too much electrolyte, and a piece of lead foil as the cathode. If you use Al for the cathode, you have to remove it when not in use - it will dissolve over time.

Here is a mockup without the telescoping quartz inner insulating tubing. This shows the big Lesker tubing coupler, the metal pieces and the custom plastic clamp I made to prevent the thing from creeping into the vacuum tank - it's a problem even with 3/4 and 1" ones, so I assume this is going to be even worse. I didn't have PVC of the right size (at least not without a change of lathe chuck to chuck the too-large stuff I have) to do this, which is what I prefer. Instead, I cut out a round from HDPE with a hole saw, turned it to exact OD and bored it to exact ID dimensions, then added 3 setscrews with inner tube rubber on the glass side so the screws wouldn't shatter the glass with point-force. This is because, hey, HDPE is slippery - the opposite of what's wanted. At least it's a great electrical insulator. This piece also serves to block airflow from that end of the 3" ID PVC clear pipe we got to help insulate this whole mess, so the air I inject into there with a regenerative blower will flow out, along the Al end, along the ballast resistor, and out the back efficiently.
Mockup without the quartz in there

I decided to use quartz as inner insulator. For a lot of reasons. Pyrex is ideal for the outer part as you can get it in inch sizes (quartz is all mm) that fit, and its slight conductivity has the same effect as corona rings/resistors in equalizing the voltage drop along it - but without all that big junk and wasted heat. But pyrex has a high D (around 8) and is not ideal as an insulator. So quartz, with a lower D, higher Q, and better standoff volts/inch is the bulk of the insulation.

Here's some vids on how I do the quartz, and another showing of the mockup. ... jA&index=3
Cutting off the chipped end - cats are dangerous to quartz if you just lean it on the wall(!).

Finishing the cut on one of the pieces: ... jA&index=2

And the result: ... jA&index=1

All of this will be "super cleaned" before final assembly - I've found that even a trace of skin oil makes for trouble. There is a bit of carbon stuck inside the quartz from flame annealing and soot coating, but I'm thinking that will make it work better, actually. The main failure is charge concentration at a spot - which then makes an arc through. A little conductivity along the length of each piece should (in my theory) help equalize that and keep "hot spots" down a bit. We'll find out in use, of course, and I'll report how it works out. This is somewhat making a virtue out of a bug - that carbon is darn hard to get off even with abrasion and ultrasonic cleaner.

Hopefully, I'll get all this installed and fired up in a week or so - there is a lot to do as ancillary work for this teardown. We plan to also wind some water-cooling pipe around the sidearm, insulated so we can also use it to make an axial magnetic field of a few hundred gauss, and a couple "ring electrodes" off each end of the grid to play with making the electrons dance around as I think we want to do. More on that perhaps in the theory section here, but the basic idea is to make the electrons take a much longer path from the grid to the tank sides - getting more ions per electron, and maybe make them oscillate axially near the sidearm walls for the same reason with some RF on the ring electrodes, same idea. This is one case there the hugely larger e/m ratio of electrons vs ions is on our side - so I'm going to try and take advantage of that. The tiny fields that will really affect electrons won't be noticed much by the ions, which are lumbering giants by comparison, particularly if we are talking the effects of RF fields. The amount that will move electrons back and forth 6" or so at some frequency won't make the ions dither more than a tiny amount. Ditto with the magnetic field. That will be its own post, of course.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
User avatar
Doug Coulter
Posts: 3023
Joined: Wed Jul 14, 2010 7:05 pm
Location: Floyd county, VA, USA

Re: Bigger Feedthrough for my fusor.

Postby Doug Coulter » Mon Mar 04, 2013 10:08 pm

Was it Mike Tyson, not otherwise known for pithy sayings, who said "everyone goes into a fight with a plan...then you get hit"?
(as always, click on the picture to see a bigger version)

One of those days. So, I assemble the thing, all super cleaned, then find out it won't quite fit through the hole in the flange - it looked like it would, but the hole was .05" too small, Sigh. So...the mill and boring bar just paid for themselves in one operation (guess how much that fancy adapter flange costs?). A picture is worth...
Only had to re-grind the cutter 3 times, this stuff is tough.

Of course, it cut a slightly tapered hole, so I still had to dremel it some. Then the 1.5 rated ID gasket wasn't either, and there was enough slop so that even after I diddled it a bit (sanding wheel in the dremel), I had to assemble in the old Volkswagen fashion - start everything before tightening anything. But I finally made it fit, while just leaving enough knife edge on the flange to seal (I hope, we've not tested in vacuum yet).
It fits now. Barely, and since neither the hole or the pyrex are perfectly round...only rotates a little.

So, I think I'm getting there. Then oops - the finest chinese rubber innertube material proved not to be proof against a bit of pressure, and the setscrew jumped right through it and broke the glass.
Don't get crazy on the torque...

So, I doubled up on the rubber, and ground the ends of the setscrews flat - they had that cone shape on the end that concentrates force too much for this. Nice to have that little punch and die set - makes punching out rubber washers a piece of cake, perfect every time (but not better than the source material).
So, I cut a new piece of glass, chamfer one end inside (can't start the O rings without that), flame polish it, and made it about an inch longer - the inner rod was on the long side anyway - so it was cut that, or lengthen this. Note in the pic here - the inner O ring is really crushed in there - big contact patch. The back one is just to keep things located and kind of straight - not for sealing, though it probably would.
O-ring detail

Note the cool attachment scheme for the 50k 225 watt cartridge resistor I use for ballast - fits right into the end of the aluminum, held by setscrews, no sharp edges to start streamers from. Or so I hope.

I used to use a 2" ID pvc pipe over the whole mess, and push air into it to cool things - in fact, that's why that resistor end (look, it fits perfectly into 1" copper pipe fittings!) has 3 rods glued to it - to center it in that. It's also why the old FT had a piece of foam rubber to seal the tank end so air would be forced down the tube over the things that get hot - mostly the resistor, but also the FT end. This time, I made the glass-anti-creep clamp just the right size, and won't need that. The PVC I had used (sched 40) was on the thin side to prevent arcs to the outer 8" ID pipe that has a grounded screen over it - so this time we went all Texas on it, bought the right pipe, 3" sched 80 (5/16" walls!) despite an 8' piece costing $157 - the rest will come in handy I'm sure at some point.
Nice new inner insulator - held to the flange by setscrews so the rest doesn't have to hold its weight.

Just to show how much of an improvement I expect here, here are the old and new side by side - I should be able to handle a lot more kV with this new one. Hope I don't melt the inner rod, but if I do, we'll make a composite with just the exposed end anodized AL, and the long part out of heat pipe to keep it cooler. It's a bit long and skinny as is.

Side by side - new lamps for old.

Enough for one day, it's way past normal miller time. Now I will also make a new glass/metal seal for the side tube on the flange for two conductors. The plan is to put two rings at the od of the sidearm, one behind, one ahead of the grid center, and put positive voltage and perhaps some push pull RF on them to shake the electrons around a little - we can't get rid of them, they are the main source of waste - so might as well get some better use out of them to make ions and let me run lower pressure (which gives higher Q, as previous parametric sweeps have shown) and probably not need an external ion source, at least not once it's lit off.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
User avatar
Doug Coulter
Posts: 3023
Joined: Wed Jul 14, 2010 7:05 pm
Location: Floyd county, VA, USA

Re: Bigger Feedthrough for my fusor.

Postby Doug Coulter » Wed May 22, 2013 1:21 pm

Well, as we often find in this empirical world, things don't always work out as planned, and this had a fairly major "duh" associated with it. When you try to stack a bunch of fairly loose fitting insulators in a vaccum (D=1.0) made of quartz with a higher D, all the voltage just appears between the "insulators" since it takes less energy to charge a D=1 gap than a thin quartz layer - then it arcs through the quartz. Sigh.

So, what we did was buy, at some financial risk, a piece of boron nitride rod, 1' by 1" diameter, to replace the quartz inside. The outer glass had already taken some damage in just a couple of runs and had to be cut off about half an inch and re polished as well. This looks as though it's the right move, but as usual there's a but.

The but is that the BN doesn't fill the glass either - I wanted to try this material for hot D resistance, but the right size would have cost double and had to be turned to fit inside the glass tightly. And, as it turns out, I got the "wrong" grade, or so we think now - the right grade and the right size costs even more (over $1k) - but it looks like the answer. As I broke the good Ti/W grid taking this apart, for these tests I reused our carbon ended grid. Turns out that the water coming off the hygroscopic BN grade we got (and who knows what else - binder and BN dust perhaps?) is a BIG problem. We're not sure if the double expensive next grade up (calcium borate binder vs boron oxide) will do it - and we know we cannot afford the super grade (no binder at all) rods, nor would we be able to machine those - some of the BN is converted to cubic BN in the process of making the super grade, and that's harder than way I can turn and bore that.

Anyway, in the interest of being informative, here's some movies of it running as is, showing some problems. Whatever it is the BN exhales on heating - it makes the grid base spark. No sparks to the glass (for once, and finally, that problem looks like it's solved with the BN sticking out about an inch past the glass), but those sparks short the supply out, and make other troubles - water, BN, binder etc pollute the deuterium, and the shorts are, well, shorts, which kinda puts a lot of stress on things.

Like Edison said - we now know one more thing that doesn't really work. However, this is quite promising - looks like now the FT itself can live "forever" compared to all previous attempts for one thing, and that's MAJOR - it's been a problem the entire project. Also, for whatever reason, we broke 1m neuts/second with only 35kv in - a record for us, though we can usually run 53kv and get over 2m/sec, we've never hit a million at such a low voltage (disregarding that there might be some kickback effects in the ballast we are not able to measure that might produce temporary high voltages).


This last one is kind of strange, and sadly, I didn't capture it too well. There's a .05" or so gap between the OD of the BN and the ID of the glass. At low enough pressures, it seems paschen's law allows for a lengthwise discharge - looks like a neon tube kind of thing, that shorts the supply. In this case the electricity takes the *longest* possible path, from the grounded shell of the tank and down the length of the FT, even though the potential at both ends is the same. You can't make this stuff up!

It's a lot easier to see at night and at the back end of the feedthrough, but at these voltages, that means some fairly serious X ray exposure to the cameraman, which I didn't want to take just now - someday I might set it up on a tripod and attempt to capture a nicer picture of it while I'm safely on the other side of the lead shielding.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
User avatar
Doug Coulter
Posts: 3023
Joined: Wed Jul 14, 2010 7:05 pm
Location: Floyd county, VA, USA

Re: Bigger Feedthrough for my fusor.

Postby Doug Coulter » Wed May 22, 2013 2:10 pm

OK, I went ahead and "took one for the team" and got myself about a chest X ray doing it - but here's a better look at that strange longitudinal discharge from the back (unshielded) end of the feed through.

Of course, I was running data aq for these runs. Here's a couple screenshots - I didn't bother taking a log file since, well, things are a bit messed up as of now and these aren't record runs - except that at fairly low voltage inputs we've broken 1 m neuts/sec a couple of times - at under 35kv input(!). Interesting. Note these aren't calibrated, they're the raw a/d and counter outputs, but I know what the voltages etc were as I was watching the meters on the supply. I had it running 20ma on peaks, which is on the high side (even for best Q, normally) but the point here was to heat things up for better bakeout and better performance later on.
First run (corresponds to first movie above)

Second run during which I took the pic of the long arc

I should add that the solar system upgrade is really helping here - I am right now running 2 large computers with two large (one 36") monitors, a vent fan, air conditioning, the forepump full blast - and the fusor power supplies, and I'm not using it all! Now we're cookin! So we have now as far as I know, the only fusion research going on with solar power - no generators, no power company power required. That's gotta be pretty cool (well at least the A/C)!
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
User avatar
Doug Coulter
Posts: 3023
Joined: Wed Jul 14, 2010 7:05 pm
Location: Floyd county, VA, USA

Return to Vacuum Technique

Who is online

Users browsing this forum: No registered users and 1 guest