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Neutron Club Application - Erik Smith

Posted: Fri Apr 27, 2018 11:50 pm
by Erik smith
Over the past few months, my fusor has been sitting in the basement of the Nuclear Engineering lab at the University of Illinois. After several months of fiddling with the chamber, I finally decided to go for a fusion run.

Neutron Run:
Pressure: 52mtorr
Voltage: 30kV
Current: 4.7mA
Run Duration: ~2 minutes
Count Rate: 11.5CPM
Background Count (w/ moderator): 3CPM
Background Count (w/o moderator): 1.4 CPM
Detector distance from Grid: 35cm
Fusor_9 Plasma.JPG
The Stainless Steel Grid at 30kV (52mtorr)

Vacuum Chamber
The vacuum chamber used is the very simple, and popular 2.75" conflat 4 way cross. In the past, my chamber has had some serious outgassing problems: the pressure would start increasing dramatically as soon as I struck a plasma. I tried cleaning it with acetone, IPA, ethanol, mineral spirits, and methanol to remove any contaminants inside, but nothing did the trick. To combat this, I wrapped the chamber in heating wire and aluminum foil, then baked it several times over the course of 3 weeks at 120C. After this long ordeal, the chamber still does outgass, but not as severely as it did 2 months ago. I also has problems striking a plasma with Deuterium gas at pressures below 50mTorr.
Fusor_6 Main Chamber with Heating Wire.JPG
Main Chamber Wrapped in Heating wire
Fusor_8 Plasma.JPG

Vacuum System
My rouging pump consists of an Alcatel 2002 BB. While it does have a low pumping speed, its enough to pull my chamber down to 15mTorr in under 5 minutes. My secondary high vacuum pump consists of an Alcatel diffusion pump pulled out of an ASM-10 Helium leak detector. I usually turn this pump on arond 20 minutes after my rouging pump bottoms out. The diff pump takes around 30 minutes to heat up and pulls the chamber down to around 5.5mTorr. Pressure is measured with a Mastercool thermocouple gauge. Previous testing on other chambers showed that my gauge typically bottoms out at around 5mtorr, I plan to replace this with a pirani or convection gauge.
Fusor_1 Vacuum Pump.JPG
Rouging Pump
Fusor_2 Diffusion Pump.JPG
Diffusion Pump with Cooling Fan

High Voltage Power Supply
I used a Hipotronics-30B 30kV 5mA supply to power the fusor. While its current output is low its still enough to produce some measurable fusion events. In the future I plan to upgrade this to a Hipotronics-30C, which can provide 30kV at 10mA. Voltage is measured with a Fluke 80K-40 probe. connected directly to my 30kV high voltage feedthrough. I measure the current by grounding the positive output of the supply through a 10 ohm resistor and then measuring the voltage across the resistor.
Fusor_3 Hipotronics 30B Power Supply.JPG

Deuterium Gas System
I used a Deuterium tank sitting at 900 PSI. The gas i stored in a small section of 3/8" stainless steel tubing. For controlling gas flow, I have a high flow needle valve connected to a fine leak valve. While this setup is very sloppy, it works, and with some fine tuning I can comfortably flow gas into my chamber.
Fusor_5 Deuterium Tank and Needle Valve.JPG
Deuterium Tank and High Flow Needle Valve
Fusor_4 Fine Leak Valve.JPG
Fine Leak Valve

Neutron Detection
Neutrons are measured with a LND 252 Helium-3 neutron detector. This tube is powered by a ludlum 2200 Scaler. These were both sent to Ludlum for calibration around 2 months ago. The tube was also tested with a 480mCi Pu-Be neutron source at my University.
Fusor_9 Neutron Detector in Moderator.JPG
Detector in its Moderator
Neutron Detector with Plutonium-Beryllium Neutron Source.JPG
Detector Tested with Plutonium-Beryllium Neutron Source

The fusion run was done by pumping my chamber down to ~6mtorr with my diffusion and rouging pump. After letting it settle and stabilize for 30 minutes, I pressurized the deuterium line to 16 PSI. I then made sure my throttle valve was opened all of the way before I started adding the gas. Once this was done, I slowly opened the fine leak valve. This caused the pressure in the chamber to increase dramatically. I then opened the throttle valve slightly and adjusted the leak valve. It took me around 5 minutes of fiddling with the throttle and fine leak valve to get a pressure of 20mTorr in the chamber. However, this pressure was not high enough to strike a plasma at 30kV. Trial and error revealed I had to go all the way up to 52mtorr before I even got a plasma discharge. These poor conditions were probably due to my minimal experience with operating and flowing gas into my chamber. Once I play with it more, i'm sure the conditions will improve.

After I achieved a plasma at 52mTorr, I slowly raised the voltage to -30kV. At this voltage I observed my Ludlum scaler increase in counts. To discriminate against noise, I pulled the tube out of its moderator and the counts stopped immediately. In the 2 minute run I managed to get around 23 counts on my scaler (11.5 CPM). Adjusting for background (3CPM) this means I got around 8.5 CPM of neutrons from my little fusor.
Fusor_10 Reactor Setup.JPG
The Complete Setup
fusor_10 Neutron Results on Ludlum Scaler.JPG
23 Counts Picked up by Ludlum 2200

Feel free to ask any questions
-Erik Smith

Re: Neutron Club Application - Erik Smith

Posted: Sun Apr 29, 2018 10:47 pm
by Richard Hull
While the counting statics are not great the gear and technique are unusually good. Therefore, you are in the Neutron Club.

I totally forgot about the old hippotronics linear oil cased smaller supplies. I have several from about 10kv to 25kv and most of these were in the 5ma range. While ancient, they are linear and will work great on a variac. They are not fused, to my knowledge, and like many linear supplies they will take it on the chin if a bit over currented without failing until you burn something out if left heavily loaded for too long.

The real tell the is in moderator and out of moderator count, even if it is not a usable statistic to a critical scientist on a single pass measurement.

Nice work

Richard Hull

Re: Neutron Club Application - Erik Smith

Posted: Thu May 24, 2018 4:27 pm
by Ryan Copeland
Hi Erik, I'm building a similar chamber and was wondering where you got your HV feedthrough.
Nice job!

Re: Neutron Club Application - Erik Smith

Posted: Fri May 25, 2018 1:57 am
by Erik smith

I have gotten three 30kV high voltage feedthrough from eBay, all under $90 each.

Re: Neutron Club Application - Erik Smith

Posted: Fri May 25, 2018 12:06 pm
by Dennis P Brown
Congratulations on being accepted; while your count is extremely low for the pressure and current, that may be due to your neutron detector (I do have questions on that)and/or your electrode design. Maybe that rather limited electrode is causing the lower flux readings - your pressure should, I'd think, have made up for that. So maybe you should improve your electrode design with a bit more effort and that will increase your neutron flux?

I am confused by this:

Count Rate: 11.5CPM
Background Count (w/ moderator): 3CPM
Background Count (w/o moderator): 1.4 CPM

How do you get a count rate of 11.5/min (I assume with a mod) but then you say 3 counts/m with a mod and 1.5 counts/m without? Are you saying with the fusor off you get those rather different results? If so, having/removing a mod should make no difference without the fusor operating on the detector system - that would indicate your detector is rather unstable and your results could have a significant noise issue. So, what count rate did you get with the fusor running and the detector running/reading but no mod installed?

Re: Neutron Club Application - Erik Smith

Posted: Fri May 25, 2018 4:46 pm
by Ryan Copeland
Thank you for the info Erik. Would you happen to have one that you do not need, or could you point me towards the eBay seller please?