Fusor Update - Liam David

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Liam David
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Fusor Update - Liam David

Post by Liam David » Sat Jul 25, 2020 12:50 am

I've been working steadily on my fusor for the past several months and have a few noteworthy updates to share.

DAQ / Control System
I made a new control box that has hardwired connections for voltage, current, and MFC deuterium flow. The signals are passed through 100ft of 15-core, stranded, shielded wire running alongside a fiber optic cable. Fiber to ethernet converters on either end connect to a computer and an Arduino Ethernet shield to allow for data collection. The Arduino drives four ADS1115 16-bit ADC modules that monitor pressure and neutron production rate, and soon will also measure deuterium flow, and several other variables. My Spellman power supplies are also connected over ethernet for voltage and current monitoring. Values are periodically sent through the fiber and a custom Matlab GUI parses, displays, and saves the values. There are still several bugs to work out, most notably that the Arduino is EMI-sensitive. I also replaced the needle valve with a 15sccm MFC which gives superior (and remote!) pressure control with the turbo throttled. The voltage, current, and deuterium flow are hard-wire-controlled through the cable and a control box. I can now comfortably sit up to 100ft away from the device, should I desire.

GUI 3 July 20 2020.png
Version 3 of the data acquisition system

Don't look, it's pretty ugly...

15-conductor and fiber umbilical cable, 100ft


Control box

Power Supply
I obtained a second Spellman DXM70N600 power supply off Ebay in working order. With the help of some engineers from Spellman, I replaced the DXM firmware with SLM firmware. This allows the supplies to regulate against voltage and current setpoints rather than shutting off upon exceeding them. They work in parallel as a simple master-slave system with both voltage programming lines connected in parallel and the master current monitor driving the slave current programming line. The setpoints are controlled through the aforementioned 100ft cable. I 3D printed custom connectors for both supplies and potted them with RTV silicone. The outsides of the connectors accept o-rings and are packed with silicone grease to prevent arcing. So far I've had no issues at >50kV. (Edit: One connector arced along the length of the cable and now has to be replaced. Voids in the silicone are to blame.) The HV cable is standard RG-58U with the shield pulled ~8" back from the ends. Each supply connects to separate 200k resistors and 100kV diode strings before joining at the HV feedthrough. This configuration allows me to run the system at up to 16mA, but I usually keep it below ~8mA to reduce heating. I only push it higher for short runs measured in the tens of seconds. The grid and chamber get very hot very quickly at high currents which cause the neutron production rate to plummet (wall/grid loading, I presume).

Not proud of the floating HV components, but it works ok

3D printed connector

I made a couple changes to the vacuum end of the HV feethrough. The ceramic washer that was designed to shield the quartz from direct plasma bombardment has sputtered full of material and now arcs profusely at anything above 25kV. I retracted the quartz and added an alumina stalk for the last few cm. More arcing then forced me to forego all ceramic and leave the stalk bare. Like I mentioned in the dedicated design thread, this new version works flawlessly.

New feedthrough

Gamma Spectroscopy
I made a 3" NaI(Tl) detector using a Photonis PMT and crystal from Russia. The housing is 3D printed PLA. I use an old Ludlum Model 12 set to 1100V as the HV supply and for simple count-rate measurements. Background is ~20kCPM. My poor-man's MCA, if you ignore the $500 scope (Siglent SDS1104X-E), is as follows: I tapped into the the model 12 circuit directly behind the HV capacitor and passed the signal through the housing with a BNC connector. This connects to an oscilloscope which then interfaces with a computer via Ethernet. With the scope set to 100-200ms/div, a Matlab script queries the scope for the waveform data. The 14Mpts worth of data are then filtered in-software, peaks are located and binned, and then a histogram is displayed. I get about 7% FWHM resolution for 137Cs and can detect x-rays down to ~15kV before the noise floor interferes. It easily picks up x-rays from the fusor, and I've confirmed the calibration using 241Am, 176Lu, and background K40.

gamma spectrum.png
Gamma spectrum with oscilloscope MCA

Custom 85x60mm NaI(Tl) probe

I picked up a CD V-742 dosimeter from a Hamfest a while back and have been placing it at the fusor viewport behind the lead shielding. It read zero when I got it but is now at 60 roentgen. Yikes. Keep those shields up.

IMG_20200516_120918.jpg (1.14 MiB) Viewed 2608 times

Breaking the Mega Mark
I am reasonably confident that I hit 1e6 n/s the other day while pulsing the system at higher currents. Since the system heats up very quickly and neutron rates drop, I was limited to ~5s runs. I verified that my neutron detector, which is simply a SNM-17 tube surrounded by paraffin coupled to a Ludlum Model 12, is immune to fusor noise using the standard moderator-removal test. I waited for the needle to stabilize for a few seconds before recording the CPM. The detector axis is just 13.5cm from the poissor, so given the anisotropy from beam-target fusion and the neutron-emission resulting from a single-beam grid, the results should be taken with a grain of salt. The three most impressive runs are as follows:

28kV, 12mA, 13.3mtorr, 100kCPM, 3.4e5n/s, Q=1.13E-09
37kV, 14mA, 17.0mtorr, 200kCPM, 6.7e5n/s, Q=1.49E-09
37kV, 11mA, 14.3mtorr, 300kCPM, 1.0e6n/s, Q=2.86E-09
33kV, 11mA, 17.0mtorr, 350kCPM, 1.2e6n/s, Q=3.66E-09
43kV, 15mA, 17.0mtorr, 300kCPM, 1.0e6n/s, Q=1.84E-09

I found that the neutron production rate was heavily dependent on grid temperature, which is why the 43kV and 37kV runs were less successful. I did not give the system adequate time to cool. Also, the rotation of the grid and symmetry with respect to the 6-way conflat arms was critical. I did my best to align the grid holes with the chamber axes. On these runs only a single bright beam formed along the long chamber axis (extra conflat crosses, while the other axis has blanks), although a very faint orthogonal beam is visible.

Single beam

I realize that the voltages are rather low for this kind of output, and perhaps the higher currents compensate, but I have not found any sources of noise. I was able to achieve rates between 3e5 n/s and 1e6n/s for two days until an unrelated issue forced me to shut down and open the chamber. Due to the semi-repeatability, I doubt it's simply Poisson noise, and detector pileup isn't an issue at these count rates. I should have moved the detector and verified an inverse-square relation, but such is high voltage safety and hindsight.

Chamber Reconfiguration
My 6-way chamber was nestled between other conflat components which made shielding difficult and the entire thing difficult to access. I reconfigured the vacuum system, put the chamber above the frame, and subset the rest of the parts below. Now 5 of the 6 sides are available for the feedthrough and viewport, leaving 3 blanked-off for whatever comes next.

Chamber above and vacuum system below, HV feedthrough off for maintenance

I took the opportunity to replace the viewport and sacrificial glass. The latter is chipped and sputtered brown, and since I previously used the viewport without protection, it has degraded as well. Since I didn't properly mount the sacrificial glass, it rested on the bottom of the tube and led to the following:

Offset section of glass protected from sputtering

Chipped sacrificial glass

The new viewport is better protected by a new glass disc, which is now held in place using expandable metal sleeving.

New viewport

Indium Activation
After reconfiguring the chamber, I activated a piece of indium foil. It was placed between two paraffin blocks at one of the end caps and irradiated for 1-2 minutes at 1e5-3e5 n/s. Count rates on a 2" pancake were between 180 and 300 CPM and quickly decayed to background. I haven't yet taken a gamma spectrum nor irradiated the foil for longer due to chamber heating. The chamber reaches well over 100C after just a couple minutes, and I prefer to keep my viewport intact.

Indium foil

Some miscellaneous pictures:

Neutron detector. Wires bridge analog meter for neutron reading by DAQ.

Pressure gauge for monitoring remaining deuterium

Feedthrough broken by my idiocy, not heat or HV.

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Liam David
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Re: Fusor Update - Liam David

Post by Liam David » Sat Jul 25, 2020 12:55 am

Solenoid valve to protect turbos in case of power failure

And some plasma pictures:
droidcam-20200120-153339.jpg (13.3 KiB) Viewed 2605 times

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Mark Rowley
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Re: Fusor Update - Liam David

Post by Mark Rowley » Sat Jul 25, 2020 1:47 am

Wow! Excellent report Liam.
The remote operating arrangement is quite something and almost unheard of.

I have also noticed the same drop in numbers as the chamber heats up. Active cooling is something I’ve been toying around with. Forced air seems to help but chilled water in a copper tubing “jacket” may be the best bet.

Congrats on the mega mark and indium activation!

Mark Rowley

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Richard Hull
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Re: Fusor Update - Liam David

Post by Richard Hull » Sat Jul 25, 2020 3:04 am

Great work and a great report. Very detailed.

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.

John Futter
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Re: Fusor Update - Liam David

Post by John Futter » Sat Jul 25, 2020 6:43 am

looking at your droidcam photo i see the hallmark of radiation exciting the camera plenty of white / colored pixels
remote is good when you see that amount of radiation interaction--you do not want a suntan from this process
keep up the good work

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Liam David
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Re: Fusor Update - Liam David

Post by Liam David » Sat Jul 25, 2020 4:18 pm

Thanks for all the compliments!

There is a fan blowing directly onto the chamber which helps by a few degrees, but it's far from enough. It cools stuff down between runs more than anything. I've started thinking about water cooling as well, perhaps some 1/8" copper or aluminum tubing around the arms. There's just so little space between the flanges, especially with the nuts and bolt stick-out. Some 2.75" crosses like the one Jim Kovalchick uses have longer arms which would be nice.

The x-rays get pretty fierce at >45kV and start to bounce around the lead shields I have in place. Rest assured the camera is followed by lead so I won't get a tan. There's a lead sheet cube with an open top around the main chamber and then a large 2'x4' sheet between me and the whole setup (when I'm operating it in the same room). Unfortunately at these voltages x-rays are reflecting off all kinds of surfaces, so while line-of-sight rays are stopped, I occasionally measure ~1kcpm on a 2" pancake by the operator station. I keep runs at these voltages short, and really need to start taking advantage of the long cable for remote operation.

One thing I can attest to about smaller chambers is the need for symmetry. I was repeatably able to hit the mega mark in the previous configuration, but just a few minor changes in chamber setup, grid positioning, and such have limited me to ~3e5 even at 55kV and 8mA. I've started messing with single-axis grids (made from shaft collars since I have neither a lathe nor a welder) but haven't yet exceeded 3e5. Instead of one beam, two form in an X shape in line with the stalk, like in this picture:


The pattern on the grid tells the same story.


There are no magnets or protrusions in the chamber--it is simply a blanked-off cross. Changing the grid positioning didn't help nor did careful alignment with the center-line. I'm going to grind down the hexagonal adapter to 1/4" OD like the rest of the stalk to see if that helps. Kind of at a loss about what else it could be. Hopefully I'll get back to the mega mark soon. With a limiting voltage of 70kV and based on Jon's cube fusor work, I should be able to hit at least 5e6 with careful design.

Liam David

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Jim Kovalchick
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Re: Fusor Update - Liam David

Post by Jim Kovalchick » Sat Jul 25, 2020 6:18 pm

Your work is inspiring. Well done.

My fusor really hits a wall when it heats up. I think the ideal solution would be a water jacket as opposed to coils because the coil contact with the chamber can only be so good. I am not sure yet how I would do a jacket yet but water directly on the chamber shell would be great.

Jim K

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Liam David
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Re: Fusor Update - Liam David

Post by Liam David » Mon Jul 27, 2020 7:38 pm

Yeah, I don't think coils will cut it without exceptional contact and high flow rates. I agree a jacket would be ideal, just not sure how to implement that at the moment.

I had one of my 3D printed CA11 connectors (for the Spellman supples) fail a few weeks ago. I was also asked to provide more info about it's construction, so here's some more details.

The connector pictured in my post has been working flawlessly and has withstood 60kV at length. It is 3D printed from PLA with about 20% infill at 0.4mm layer height and potted in neutral cure 737 RTV silicone. Credit where it is due, I was given the 3D model by Chad Ramey and the original author is Peter Schmelcher as discussed in the DXM70N600 thread. Referencing the model (attached below), I used the nose, body, and nut components and discarded the taper parts since they serve no purpose with a small diameter cable. The HV cable is from HVStuff but is no longer sold there. I found it on AliExpress as "150kv 8mm electrostatic spray gun cable" but haven't purchased any more. I kept the insulation on the potted section. The connection between supply and cable is made with standard 2mm bullet connectors pressed into the nose component. O-rings (McMaster #9396K428) sit in grooves on the outside of the connector and the remaining space is packed with silicone vacuum grease. Now I can't remove the connector from the supply due to suction...but it works so no need.

The second connector was identical to the first except I used RG213U coax for the HV cable. I actually potted two cables in the connector to give me access to the floating filament, but I didn't use it. It unfortunately failed at ~55kV by tracking along the cable surface and then arcing to the case. Notably, it did not arc through the connector. I think a trapped air bubble was responsible, but the autopsy wasn't particularly enlightening. I'll make another connector soon with RG213U for the cable, but will ignore the floating filament and use just one cable.

Connector after failure

No voids found in silicone

Failure mode

Silicone used

Hope that helps.

Liam David
(5 MiB) Downloaded 53 times

John Futter
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Re: Fusor Update - Liam David

Post by John Futter » Tue Jul 28, 2020 6:50 am

Liam from what i see of your failure the earthy end is far too close to your hot end
extend your printed bit alot further ie x2 and put plenty of silicon stopcock grease over the outside of the cable inner to stop air gaps there allowing corona hence conduction to occur that then carbonises the polyethylene surface

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Re: Fusor Update - Liam David

Post by Peter Schmelcher » Tue Jul 28, 2020 7:49 pm

Nice work/report.

Some thoughts on the connector.

The commercial CA11 connector had vents and far fewer seals so you could nick the O-rings on the inside to create a vent but alternate the vent positions so they do not line up. If a flash over ever happens it should track and damage the surface of the printed connector.

Consider using a copper plumbing cap as a faraday shield near the pins to control the electric field at the cable end. Drill an interference fit hole(s) and slip the insulated conductor through it. Then electrically connect it to one or more of the banana pins. I would trim away most of the cap to make soldering the pins easier. Might as well polish out any scratches in the copper to reduce field enhancement. FYI, I have not simulated the field improvement just intuition.

I also believe polypropylene and other plastics need to be surface oxidized using a flame for chemical bonding of inks and??? For all I know these days you might find a liquid that does the same thing.

Like your HV vacuum feedthrough.

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