Fusor Forums

If you're like me, then in the course of vacuum work you come across the fact that glow discharges and particle beams easily heat things up... too hot. Hot enough to demagnetise, to outgas or oxidize, to melt, to enter thermionic runaway, or to destroy plastics or rubber seals.

The solution of course is liquid cooling. But this has some problems. As every liquid-cooled gaming PC afficionado knows, mixing copper and aluminum in the same system, even if they are not electrically connected, is just asking for a mess of galvanic corrosion that will pit and pinhole the aluminum pretty rapidly. And this applies more generally.

Also, if you're using water with additives for coolant, you can't have something you're cooling at other than ground potential. The coolant will conduct.


The solution to this is to use dielectric coolant, and that has... problems.

The best-known dielectric coolants seem to be "Fluorinert" and various other fluorocarbon or PFPE-based liquids. These are expensive and probably not great for the enviroment or health. Most ordinary hydrocarbons are far too flammable, as well as being polluters if they evaporate and tending to destroy plastics.


What are my options?

Fluorinert is best. It has no health issues at all, unless you burn it (don't).

https://www.ebay.com/itm/3M-Fluorinert- ... SwpOVdhGs6
https://www.ebay.com/itm/3M-Fluorinert- ... SwsR9c73pt

The idea of cooling in the fusor goes way back on this site. From cooling a grid made of hypo needle tubing with flowing coolant, to cooling the fusor body when Carl Willis brazed1/4" copper piping to his hemispheres and pumped ice water through them.

Cooling the grid seems the most difficult. Cooling the chamber walls might just assist in allowing more D2 wall loading. General plasma streamlines require steering gear or potential shielding screens over sensitive regions where such beams might hit sensitive items which might out-gas or be damaged within the vacuum chamber, itself. As noted, cooling within a vacuum and among HV components within can be daunting.

Fluorinert has been mentioned many times here.

Richard Hull

Interesting.

Flouorinert has a fairly low vapor pressure. Do you need to make a completely closed system?

My vacuum chamber does have a water jacket (or liquid jacket if you prefer) I never got around to hooking it up yet. I believe there are some archived posts here with someone cooling the grid with Flouorinert. Because of the expense, the Flouroinert loop goes to a water cooled heat exchanger close to the grid feedthrough.

Professional systems like accellerator mass spectrometers use deodorised kerosine to cool the ion source
I use ordinary kerosine to cool my ion sources at work ie 500 - 1500 watts of heat input tends to destroy the ion source magnets if the cooling stops.
Kero runs closed loop into a heat exchanger that is water cooled. Ion source has a copper backing plate that the kero flows through
and I use a petrol pump as used at service stations to circulate but you could use a high flow rate car type fuel pump


Cheap as chips

By "completely closed system" I mean one that's all sealed up, not just a closed loop.

Here is the documentation on the fluorinert cooling of my grid:
http://www.rtftechnologies.org/physics/ ... -index.htm

Fluorinert does evaporate slowly (much slower than ethanol), so a mostly closed system is recommended. A simple plastic cap or over pressure vent valve on the reservoir will do. It doesn't have to be a hermetic seal, but it shouldn't be an open reservoir either.

Behlke (Behlke.com) uses a sealed closed loop fluorinated fluid system to cool and insulate their HV pulse switches. They use a pump which is sold for cooling computer CPU’s. There are a large number of different pumps sold for this purpose largely to the high performance gaming community. This would be a source of reasonably priced cooling components.

Two angles here.
1) Water is used as a HV dielectric in some pulse forming networks, where its high dielectric constant is favorable for electrode geometry reasons.
But even if deionized to double-digit resistivity (in megohm-cm), which is hard to keep that way, the self-discharge time of a water capacitor is on the order of a millisecond (IIRC).
Edit: RC for 1 meter cube is (8.85 X 80.1 = 709) pF X 0.18 megohm = 128 us (at 20 degrees C). The bottom line value is the same for any geometry.

2) How about long, thin tubing to make the electrical resistance tolerable in a water cooling connection? Trading off water resistivity, pressure drop, and temperature rise. Hot water is less viscous and less electrically resistive than cold. I think the electrical leakage current will galvanically eat at HV-positive pipes and not at those which are HV-negative.

I bet the main advantage of Fluorinert over refined hydrocarbons is its non-flammability.
As with PCB's in capacitors and transformers, there's an obvious & immediate benefit because it reduces the risk of fires.
As with all liquids, you need to pay attention to compatibility of wetted materials like elastomeric seals.

p.s. Regarding toxicity, ozone depletion, and global warming potential, the numbers for propane and butane are many times lower than even the latest nonflammable refrigerants.
I bet the same applies to the higher-boiling-point nonaqueous coolants under discussion in this thread.

Andrew, I'm curious about the details of the not-high-voltage portion of the fluorinert loop. It looks like you used a lot of swagelok - including teflon swagelok which I've never encountered before.

Obviously in many cases (especially if using PC liquid cooling equipment) there's going to be plastics - which of these are usable?


The other side of things is that I'm actually more worried about corrosion than shorting out HV. It looks like there are some... Interesting coolants in the PC world as well. (NOT the ones that are bright green and UV-fluorescent for aesthetics).

Please excuse one answer to a question addressed to somebody else.
Some of my co-workers worked on a Fluorinert-cooled instrument during years when I worked elsewhere, and I will ask about it on Monday. Meanwhile, LMGTFY. Found a lot here:
https://detector-cooling.web.cern.ch/de ... rinert.pdf
and
https://www.besttechnologyinc.com/preci ... -transfer/
including

I think it’s generally agreed that the fluorinert fluids are environmentally unfriendly, but they are used in such small amounts that it is also generally agreed that they are not a significant problem. I think their astronomical cost will insure that they are never used in sufficient quantity to constitute a significant environmental risk.

Teflon and nylon tube are both good, as are viton o-rings. You can get teflon and nylon ferules and fittings from swagelok.

We can not import Fluorinert into New Zealand as it is a super green house gas nasty
NZ is about to ban SF6 as well alot of switchgear to swap out

From the interweb
"Global Warming Potential
Fluorinert perfluorotributylamine absorbs infra-red (IR) wavelengths readily and has a long atmospheric lifetime. As such, it has a very high global warming potential ("GWP") of 9,000, and it should be used in closed systems only and carefully managed to minimize emissions." ie no vents to atmosphere at all

therefore no amateur sytems should use this!!!!!

I will mention that propane is an excellent heat absober (better than water) and works great as a coolant; while not as hazardous as hydrogen, it is explosive in air (in the right mixture ratio's) so it must be used with extreme care and no leaks can occur. I have built a number of refrigerator systems using this substance. More importantly, it is an excellent electrical insulator. If one is comfortable using this very flammable gas it is easy to liquify and use as a coolant just know its extreme dangers.

Last year I used unpressurized liquid propane as a freezing bath, outdoors in a well ventilated area, with excellent success.
Started with a thermos full, drawn from a BBQ tank so old it still has a POL valve. Glad to be with Dennis on a safety thing!

Am reminded of one classic HV high bandwidth scope probe: the Tektronix p6015.
They came with freon insulation (dichlorotetrafluroethane), as vapor with some visible liquid so you could tell when it needed to be refilled.
https://www.eevblog.com/forum/testgear/ ... ke-or-buy/
Amateurs have used butane as a replacement dielectric, with similar pressures & HV breakdown voltage.
https://yarchive.net/car/high_voltage_probe.html

Aside from ozone and GWP issues, simple hydrocarbons are less toxic than CFC's and HCFC's and fluorocarbons.
Many people think of freons as nontoxic (with asphyxiation risks), but then why do teens huff Dust-Off when they want to get a buzz?
Look at the Threshold Limit Values in occupational safety tables; tolerable concentrations in air are higher for propane and butane.
Same applies to the IDLH (Immediately Dangerous to Life or Health) concentrations, when not bounded by Lower Explosive Limit.
[edit] Looks like NIOSH has revised them down to generally 10% of LEL, when the poisonous aspect doesn't get you first. https://www.cdc.gov/niosh/idlh/default.html