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Greg Courville Neutron Club entry Feb 2016

Posted: Sun Feb 14, 2016 3:03 am
by Greg Courville
Hello all,

After an awfully long time away, here is a fusion report.

I started building a fusor some time in 2006 and didn't end up getting around to trying for fusion before running off to university (and subsequently taking a job in a research lab). I left behind a machine that was more-or-less fusion-ready, but with an overengineered control and data-acquisition system that was only half done (picture stacks of home-etched PCB's partially assembled). In March 2015 I managed to set aside a few days to come back, dust everything off, patch in some rough-and-ready instrumentation and do some deuterium runs with the neutron detector going. I got some suggestive pulses out of the neutron detector, but in my rush I didn't have the presence of mind to do any running with the moderator removed from the neutron detector. I never got around to really going over the data until just recently when I had already started pulling parts off to sell and even sold and delivered my deuterium bottle to its new home.

To make a long story short, I ended up borrowing back the deuterium, bolting the vacuum system back together, and taking another shot at it.

There were some setbacks. Within a day of closing the system back up, I had managed to blow my HV supply. A couple of the zip ties holding things together in the HV tank had failed after around a decade sitting under oil, causing things to shift around, and eventually one of the HV AC leads arced over to the side of the filter capacitor, carbonized my rubber supplementary insulation and formed a persistent HV short. Thinking I had blown a secondary, I was fully prepared to pull the transformer and saw off the bad one, but fortunately the actual problem was fairly obvious once the lid was off. I got rid of the cap, tossed the burnt rubber pieces and cleaned up the sooty stuff with a pipette, and the HV supply was back in operation.
I also discovered a couple vacuum leaks which prompted an early morning run to the grocery store for some emergency nail polish.
Finally, as I started with the trial runs, I found that the system would start becoming increasingly unstable within a couple minutes of operation at only 100W or so. Neutron counts would drop off, crackling and sparking would crop up and keep getting worse, and eventually the plasma would drop out. Grinding some stray epoxy off the grid stalk insulator, sealing up the aforementioned leaks and increasing gas flow seemed to make a substantial improvement.

My main chamber is a 5-way Conflat junction with 8"CFF main flanges, with a blind reducer on top and a DIY viewport assembly on the bottom. The HV feedthrough was made by soldering a military surplus BeO transformer feedthrough onto a 2.75"CFF flange. My center grid is made up of 3 ~1" dia. hoops of 0.020" tungsten wire.

The pumping system consists of a monster Welch 3012 belt-drive (!) turbomolecular pump backed by a Welch 1402 rotary vane pump. Total pump overkill. The TMP motor was run off a variable-frequency drive, generally run at a fraction of its nameplate speed. The Chamber is connected to the pump via a long SS bellows hose with a right-angle valve and a small butterfly valve in between.

Pressure measurement was done with an MKS 325 pirani gauge. Note that all pressure values here are with respect to the N2 curve supplied by MKS, not corrected in any way for deuterium.
Edit: Pirani, not micropirani.
Edit: The Stanford Research Systems app note "Gas Correction Curves for PG105 Readings" gives an N2->D2 correction factor of 0.79 for thermal conductivity gauges in the molecular flow regime, suggesting an operation pressure of about 18 microns for these runs (discounting water vapor, hot epoxy byproducts, etc.).

Deuterium injection was controlled by a pulsed solenoid valve. Since last time I added an actual PID loop to the pressure control program and this made for vastly more stable operation. I used a manual butterfly valve on the pump port of the chamber to control the relative rate of gas flow while the PID routine maintained the pressure. Varying the TMP speed was also found to be effective at controlling the pumping rate, but obviously the valve offers vastly quicker response.

The neutron detector consisted of an ancient N. Wood BF3 tube, 5/8" OD, 3.5" active length, operated at 1600V, inside a 139mm OD HDPE moderator, oriented vertically, placed about 185mm from the central axis of the chamber. The neutron detector was mounted on a fixture to allow it to be repeatably positioned w.r.t. the chamber, so that the moderator could be removed and reinstalled between runs. A cardboard jig and markings on the chamber helped to ensure accurate positioning (see photo below).
Cardboard jig for neutron detector positioning
The high voltage power supply was built from an old dental X-ray head. I kept the original housing as a tank, ripped out everything but the transformer and used the gained space to build in a rectifier. The transformer is ballasted with a choke made from an old MOT -- secondary removed, welds ground out and core gapped with plastic cut from a credit card. Control is via a run-of-the-mill variac. Input power was monitored with a Kill-A-Watt.

Pulses from the neutron detector amplifier were fed straight to an oscilloscope which was used for counting. My software downloaded the waveform after each trigger, allowing after-the-fact filtering and analysis. A second oscilloscope was used to monitor the HV probe and pressure gauge. The 8-bit vertical resolution on a digital scope isn't ideal for precision voltage measurement, although this is helped by heavily oversampling. The analog frontend on an oscilloscope is incredibly handy for makeshift data acquisition setups even if the sampling resolution isn't ideal.
Edit: The HV measurement was done by taking waveforms off the HV divider, taking an RMS average in software, and multiplying by a conversion/calibration factor. The setup was calibrated against a multimeter with a Fluke HV probe. The data logging software was set up to also occasionally save the raw waveforms themselves for later inspection, but this failed due to an implementation error.

Here is the whole works. Please excuse the background -- this is my bedroom from high school, and hasn't changed a whole lot since I left.
Fusor cart and "control station"
Fusor detail
Data acquisition and gas control were handled by a whole bunch of Python code.
Screenshot during a run
After some experimentation I settled on a pressure setpoint of 23 microns (*indicated* pressure -- see above note about pressure readings). I ran with the TMP at about 20% speed and the butterfly valve open to about 45 degrees. I had tried a lower-flow configuration with the the butterfly valve mostly shut, and although it was easier to get rock-solid pressure, performance seemed to suffer due to outgassing, etc. when things got hot.

I did a series of 8 5-minute "burns" (HV on, discharge going) separated by 5-minute "rests" (HV off). The third, fourth, seventh and eighth runs were done with the moderator removed from the neutron detector. The first four burns were done at a nominal power level of 80W, and the last 4 at 160W. The variac was set to produce the specified power draw at the beginning of each set of runs, and the same setting used for subsequent runs. The power draw generally started out around the nominal value upon ignition, and gradually dropped by about 1/4 over the course of a burn.

Star photos:
Looking in through the viewport via a pocket mirror hot-glued to the flange
Action close-up
Pulse events were filtered by hand after-the-fact: every single event was plotted out, and obvious false triggers, EMI events, etc. were marked and discarded. This was done "blindly", i.e. without looking at the event timestamp or any other data channels. This was followed by a fixed 0.75V peak height cutoff in software. Obviously this sort of scheme would make no sense at any reasonably high count rates, but it worked out fine for my non-optimized fusor and tiny detector tube, and was the easiest solution to set up on the data-acquisition side of things.

Here's a nice plot:
Summary plot
The numbers:

Code: Select all

Region  0 ( blue):   38 counts ->  7.6 counts/min
Region  1 (green):    1 counts ->  0.2 counts/min
Region  2 ( blue):   52 counts -> 10.4 counts/min
Region  3 (green):    0 counts ->  0.0 counts/min
Region  4 (  red):    0 counts ->  0.0 counts/min
Region  5 ( gray):    0 counts ->  0.0 counts/min
Region  6 (  red):    0 counts ->  0.0 counts/min
Region  7 ( gray):    0 counts ->  0.0 counts/min
Region  8 ( blue):   95 counts -> 19.0 counts/min
Region  9 (green):    1 counts ->  0.2 counts/min
Region 10 ( blue):  106 counts -> 21.2 counts/min
Region 11 (green):    0 counts ->  0.0 counts/min
Region 12 (  red):    2 counts ->  0.4 counts/min
Region 13 ( gray):    0 counts ->  0.0 counts/min
Region 14 (  red):    0 counts ->  0.0 counts/min
Region 15 ( gray):    0 counts ->  0.0 counts/min
Finally, here is the requisite selfie:

Re: Greg Courville Neutron Club entry Feb 2016

Posted: Sun Feb 14, 2016 4:00 pm
by Rich Feldman
Greg, you've made a believer out of me.

Applause for your demonstration of scientific method, switching both the power and the moderator for multiple runs. That didn't take so long, did it? :-) Too many people post claims as soon as they see a faint positive signal from a detector with Neutron somewhere in its name.

Applause for getting useful count information from equipment at hand -- BF3 tube, bias supply, and digital 'scope. Pulse discrimination by eye is approved in my book. You set a good example by making it a blind test, not knowing at the time which pulses came from burns with neutrons expected. It could have done the job even if the signal were much less than overwhelming. It would be interesting to see how many pulse-like events you dismissed.

Thanks for a generously worded and illustrated story. I bet 'most everyone here will learn something new from it. Nylon seems to generally get an A in the oils section of chemical compatibility tables. Apparently that's not good enough for highly stressed zip ties in HV tanks. (The bobbins in my dental transformer look like nylon.)

And thanks for not flouting the selfie rule. Want a job? Oh, I see you have one already.

Rich Feldman

[edit] It would be nice to see actual voltage and/or current measurements, to complement your HVPS AC input power readings. Want me to bring over an HV voltage attenuator probe? And how about a SWAG at deuterium pressure to make your Pirani gauge read 23 microns when calibrated for N2? I think those details aren't show stoppers for neutron club entry, given some deficiencies others have gotten away with. But what about when RH opens the n club Hall of Fame? :-)

Re: Greg Courville Neutron Club entry Feb 2016

Posted: Sun Feb 14, 2016 9:23 pm
by Greg Courville
Thanks, Rich!

I was surprised by the zip tie failures as well, but these were novelty-grade neon-colored zip ties from the hardware store, not mil-spec, so I wouldn't be surprised if the makeup was something other than pure high-quality nylon resin.

I read up a little on correction curves for thermal gauges, and it looks like for conduction-type gauges operating at these pressures, you can convert by simple multiplication and the correction factors are more-or-less universal. I found a document from Stanford Research Systems giving a correction factor of 0.79 for N2 to D2, which suggests an operation pressure somewhere around 18 microns for these runs.

As for the HV readings, those are in fact RMS measurements recorded off a voltage divider right behind the HV feedthrough. I'll edit the writeup a bit to clarify.
It certainly would have been possible to add a low-side current shunt (between the secondaries and ground), but it would have required some pretty substantial rework in the HV tank (and I was already out of data acquisition channels). I figured a manually recorded input power measurement was better than nothing.

I'll be off working on a very large physics machine in Germany through some time in Spring, but there's nothing concrete on my schedule after that, job-wise...

I'll crank out some more event summary data in a bit.


Re: Greg Courville Neutron Club entry Feb 2016

Posted: Sun Feb 14, 2016 9:28 pm
by Greg Courville
Here's some more about those detector pulses. One thing that stands out is that garbage pulses get a lot worse at higher power levels. Most of them had a fairly distinctive shape to them and many were associated with an audible crackle or similar event.

Code: Select all

**Event disposition summary**
Region  0:   4 garbage,   7 below thresh,  38 accepted
Region  1:   0 garbage,   2 below thresh,   1 accepted
Region  2:   0 garbage,   3 below thresh,  52 accepted
Region  3:   0 garbage,   2 below thresh,   0 accepted
Region  4:   1 garbage,   4 below thresh,   0 accepted
Region  5:   0 garbage,   0 below thresh,   0 accepted
Region  6:   1 garbage,   3 below thresh,   0 accepted
Region  7:   0 garbage,   6 below thresh,   0 accepted
Region  8:   7 garbage,   4 below thresh,  95 accepted
Region  9:   0 garbage,   1 below thresh,   1 accepted
Region 10:  44 garbage,   8 below thresh, 106 accepted
Region 11:   0 garbage,   3 below thresh,   0 accepted
Region 12:  13 garbage,   8 below thresh,   2 accepted
Region 13:   0 garbage,   0 below thresh,   0 accepted
Region 14:  26 garbage,   2 below thresh,   0 accepted
Region 15:   0 garbage,   8 below thresh,   0 accepted
And here are some example traces:
Example of an "accepted" event
Example of a "below threshold" event
Example of a "garbage" event associated with HV instability

Re: Greg Courville Neutron Club entry Feb 2016

Posted: Mon Feb 15, 2016 2:02 am
by Steven Sesselmann

Nice report.. in regards to your nancy wood detector you could try adding a simple RC filter and reading it with your PC sound card using PRA software.

PRA does the pulse shape filtering on the fly and logs your count rate over time.


Re: Greg Courville Neutron Club entry Feb 2016

Posted: Mon Feb 15, 2016 5:40 pm
by Rich Feldman
Greg Courville wrote:.As for the HV readings, those are in fact RMS measurements recorded off a voltage divider right behind the HV feedthrough. I'll edit the writeup a bit to clarify.
The only place I saw a kV value is a chart in your screen capture (yay for cygwin!)
which appears to settle down around -16 kV.

If that's really RMS, with the minus sign added, then the amplitude exceeds 22 kV.
If it's the average of a rectified sine, then peaks are -25 kV. That ought to appease some doubters.

To indicate 16 kV RMS on a true-RMS voltmeter, whose AC ranges are AC-coupled (like my Fluke 179), peak voltage must be -52 kV. Not consistent with a dental XRT and 2-diode rectifier, IMHO.

Re: Greg Courville Neutron Club entry Feb 2016

Posted: Mon Feb 15, 2016 7:42 pm
by Richard Hull
Greg, Welcome as the newest member of the neutron club! Your name has been added.

I appreciate the detailed report with data. If there ever was such a thing as an "Image of fusion" Your photo of the glowing grid and star mode is it. Such images do not prove fusion, but boost the confidence that a report which follows will probably be a fusion win, provided the data set and safegaurds against being fooled are in place.

You have earned your place in the neutron club via good science.

Richard Hul

Re: Greg Courville Neutron Club entry Feb 2016

Posted: Mon Feb 15, 2016 8:47 pm
by Greg Courville
Rich Feldman wrote: The only place I saw a kV value is a chart in your screen capture (yay for cygwin!)
which appears to settle down around -16 kV.
HV is in the third subplot from the top (the one with the color-coded time windows drawn over it).
Rich Feldman wrote: If that's really RMS, with the minus sign added, then the amplitude exceeds 22 kV.
If it's the average of a rectified sine, then peaks are -25 kV. That ought to appease some doubters.

To indicate 16 kV RMS on a true-RMS voltmeter, whose AC ranges are AC-coupled (like my Fluke 179), peak voltage must be -52 kV. Not consistent with a dental XRT and 2-diode rectifier, IMHO.
To clarify, the HV measurements were made by recording waveforms directly off the HV divider, then taking the RMS and multiplying by a calibration factor in software. On a whim I set up the data-logging program to occasionally save the raw waveform as well (in anticipation of this sort of discussion), but it turns out I fudged the implementation and didn't end up with any data. However, I do have some photos which happen to include the scope screen.
This setup was calibrated against a multimeter with a Fluke HV probe, before the arcing incident and removal of the filter cap from the HV supply (so there was a nice flat voltage for the multimeter to read off).

I switched to RMS (versus simply taking the mean) after taking the capacitor out of my HV supply, expecting to see something resembling a rectified sine wave. However, it turns out there was still enough capacitance elsewhere to pretty effectively filter the output when there was no load. Interestingly, the fusor acts almost like a big shunt regulator, resulting in a nearly flat line for the majority of the AC cycle when the discharge is going.

Even more interesting was that the forward voltage was very sensitive to pressure and the HV plot actually gave a clearer view of short-term pressure trends than the pressure gauge! This can be seen in the screenshot I posted. Note that the magnitude of those pressure oscillations is on the order of a few tenths of a micron.

Edit: Grabbed this off the scope screen in one of the photos:
Example of HV waveform with discharge active
ripple.jpg (17.55 KiB) Viewed 6618 times

Re: Greg Courville Neutron Club entry Feb 2016

Posted: Tue Feb 16, 2016 11:32 am
by Jim Kovalchick
Congrats Greg. I love your presentation with it's thoroughness. I can tell you are having fun too.

Re: Greg Courville Neutron Club entry Feb 2016

Posted: Mon Feb 22, 2016 12:10 am
by Doug Coulter
Congrats! Nice work! You'd be welcome in my lab anytime.

Having been about where you are - I'll warn you that a little down the road (should you continue) with higher voltages and a need for lower pressure so as to limit current to something not-insane, you're going to need something like a current limited (and sensing) power supply, and control your gas via the difference between desired and actual current on the main grid at the voltage setpoint.

So far, no gas gage we can discover has the required resolution at the range we are running now - the fusor itself is the best measuring tool at that point.
(One difference might be that we are running pure filtered DC in our demo mode)

Given adequate gas control (so we don't run out of power or melt things), we see a super steep curve of increased neutrons with main HV potential. We get detectability in a nearby He3 around 16kv or thereabouts, and into the several millions neutrons/second at 50kv - it's a very fast rise curve that looks a lot like the square of (voltage - threshold) - or maybe a higher power, haven't curve-fitted it yet. You should be able to see some correlation between your instantaneous HV potential and neutron production if you have a lot of ripple on your supply. We did here, and it's a good thing to have audio from the detectors to use the human ability to notice things like that.

I also use a solenoid valve for gas inlet, and another for outlet, with a PID loop controlling an ion source that tries to keep the main grid current "in range", and only resorts to moving gas when it can't do that. My gage shows basically no change from "way too much gas for -53kv" to "it won't light off at all even with an ion source, and even the ion source won't light".

It's normal to see some of those "glitches" you show in most gas tube outputs (even in the rare case of no EMI). They all respond somewhat to high energy gammas such as are given off during neutron capture in things, or simply decay (as well as the usual cosmic ray junk). Here we usually just set a threshold (we are using a hornyak for our main detector, but also use B10, and He3 as well as silver activation and if they don't all agree - we find out why and fix it). Remember, that even if you remove intentional moderator, unless you remove walls and the air itself, you've still got some moderation and reflection going on...the old joke in high energy neutron physics is "first, remove all the air from the lab". And hey - super high energy gammas don't come from nowhere - other than cosmic rays, they are coming from "something" in your lab. If you did your homework at all re voltage and normal X ray shielding, that something almost has to be neutron capture or decay. I suggest a nice pancake geiger somewhere in your setup for your own safety. When it starts counting fast even inside a box of 4" thick lead'll easily guess why(!).

I'm in the process of documenting our "stuff" as well (as development continues on it) - we got successful enough to require remote control as it's no longer safe to be in the room with that much flux.
We are using a bunch of Raspberry pi's, arduinos, teensies, and of course, plain old PCs for this. I'll report and of course, share any code and schematics when I get that moved along. We are finding that up close to the fusor, things like arduinos (for hard real time) and pi's are a lot harder to crash than most PCs, FWIW.