Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA-9mA

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Steven Sesselmann
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Steven Sesselmann »

Andrew,

Great engineering and incremental improvements and it's obviously working. Your fusion rate and quotient is improving with every iteration, you are now sitting somewhere between Richard Hull and Jon Rosenstiel on the list, despite running at lower voltages then Jon. I shall hold off putting name on the list until you report back again after a 70 kV run.

Steven
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Best run to date is 1.4e6 n/s, at a slightly increased pressure of 7.8mtorr, 40kV and 8.8mA. 1e6n/s is now easily achievable with 1.4e6 n/s possible with some tweaking. Best to add this Q value to the list for now, it might be a while before the system is capable of 70kV operation.

The molybdenum cap under the grid is also heating up during the runs, getting into the dull red range. I might switch to a stainless cap and braze it onto the grid tubes for better heat conduction to the coolant.
SAM_5073_crop.jpg
Has anyone noticed how the bubbles seem to be clustered towards the center of the dosimeter? Is anyone else seeing this of their BTI?
SAM_5091_count.jpg
rate.jpg
Here is a list of Q values for university fusors, mine is currently at 4e6 n/s/kw
UW-Q.jpg
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull »

Keep pushing that pressure!! Old hands who have been here for years, and still working with their fusors, will tell you.......Once you have a fusor working over 35+kv, GO FOR PRESSURE!! My very best runs are always over 10 microns and my best run, ever, of 1.58 e6 neutrons was at 42 kv applied and 15 microns! Right after doing fusion, the secret is to run and run and run and learn with each run. Learn operational technique!

We are working on a ragged edge, bull-heading fusion in a very narrow zone not found or to be had casually for the finest results. Even the finest constructed systems from the best materials have to be considered crude based on our modus operandi in doing fusion. In the end, the science allows it to happen, but operational artifice makes it happen. Improved artifice makes it happen better.

Newbs who come, see, marginally do and then leave, never develop the artifice for any number of reasons. Yes, the fusor, as we typically configure it, can only go so far, but pushing it becomes an art and craft.

I have trumpeted this for years here in numerous postings. It is good to see someone pushing their individual envelope in continued work with a good fusor system.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Steven Sesselmann »

Interesting, I just looked up some information on the SIGFE reactor and I never realised how close the construction is to my current FICS design, as in FICS the cathode is semi enclosed with ions entering through holes, forming almost a hollow cathode, it also has electron suppression. SIGFE and Hirsch design clearly stand above the rest in terms of efficiency.

The only critical mistake they made IMHO is to accelerate the ions into the cathode, obviously they believed ions need speed to overcome a coulomb force and fuse, quite to the contrary I think the ions need to stand still before they can fuse, but time will show.

Steven
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Latest run updates:
2.4E6 n/s at 39.8kV, 17.68mA, 9.24mTorr
SAM_5106.JPG
SAM_5109_count.jpg
SAM_5110_count.jpg
neutron-flux.jpg
This higher current run was made possible by paralleling 2 spellman PTV power supplies to double the output current
SAM_5114.JPG
A new plastic insulator was machined that allows 2 HV cables to be plugged into the grid via a splitter adapter. This insulator will also allow use of the 70kV supply that has a larger cable.
SAM_5101.JPG
SAM_5095.JPG
SAM_5111.JPG
A control connector is then made to allow the master supply's current monitor line to drive the slave supply's current program line
SAM_5113.JPG
Both supply's are then run in constant current mode at slightly below the voltage setpoint

There are definitely heating issues on the moly cap, with it now becoming bright red. The macor is still holding up very nicely with no discernible outgassing, even when in contact with the red hot moly cap. I'll probably have to switch to a stainless cap brazed to the grid structure if I want to continue at these current levels.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Steven Sesselmann »

Andrew,

You are really cooking now, heading towards the 5 meganeutron mark, I trust you are keeping a safe distance.

Even at this emission rate the efficiency has remainded roughly the same.

Steven
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Runs are kept short for safety(and thermal issues)
x-ray rate at the viewport surface is 10-15mR/h through the lead glass shield
neutron rate at the surface of the vacuum vessel is 140mR/h at 2.4E6n/s
Runs are 30s with the two parallel supplies, about 60s with the single supply, and I only run long enough to get good counting statistics from the BTI dosimeter, so about 2-3mR of neutrons at the surface of the fusor(~3.5" from focal point) per run. I'm usually about 1-2ft from it so it drops off a lot due to inverse square law.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Steven Sesselmann »

Andrew,

I know only too well how fast the minutes pass when one is fiddling with the knobs on a fusor, big difference between 30 seconds and 5 minutes. At those emission rates I would want to be further away than 2 feet.

My new fusor may not achieve 5x10^6 neutrons like yours, in any case I have built it for remote operation with a 10 metre fibre optic control cable extending to my laundry behind a 2 foot sand stone wall.

Steven
http://www.gammaspectacular.com - Gamma Spectrometry Systems
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Agreed, adding more shielding is next on the to do list. In this case there isn't really any way to push the runs any longer, the difference between 30s and 40s of operation is the moly cap not glowing and glowing bright red, it's a pretty reliable timer.

More automation is also in progress to allow remote operation, I bought a labjack T7
https://labjack.com/products/t7
to monitor and control the fusor. It has an ethernet interface and numerous digital and analog outputs for control.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull »

Great Work! I guess the best of all current operating fusors!!! Very, very clean system and star. Those ion sources are making a real impact. Plus ultra all the way.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

I've tracked down the source of x-rays coming from the fusor. I had initially thought the 15mR/h was some of the higher energy x-rays coming through the lead glass, or stainless shell, however it turned out to be coming from the viewport near the vacuum gauge. Some of the secondary electrons from the grid were getting up into the conflat cube and generating x-rays. A lead foil cap over that viewport has now eliminated any measurable(on a ludlum 17 ion chamber, minimum resolvable rate ~0.5mrem/h) x-ray flux from the fusor. I'll still need to add lead shielding around the vacuum chamber when I start running at 70kV, but for now the x-ray issue is solved.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull »

The shell starts to go transparent on the standard .060 inch thick SS shell at about 35kv applied. At the shell, working at 40kv, using a victoreen ion chamber, I get about 10mr/hr. It will only go up from there. Any window or view port will be much higher at lower voltages as the lower energy x-rays produced will pass through the glass as electrons strike the shell on the opposite side. This is much the same as old x-ray tubes.

The lead cap trick over the view port was used by Joe Zambelli in his 2001 fusor. I avoid it by pointing my view port straight down into the concrete floor through the video camera. View ports are the most dangerous emitters of radiation in the amateur fusor and all that danger is x-radiation. Neutrons from fusion are of no real consideration for the average successful amateur fusioneer as few will use more than 40kv applied.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

I recently scored an URSA multichannel analyzer on ebay and am hooking it up to do neutron activation analysis
SAM_5159.JPG
The system was calibrated with a Cs137 check source
SAM_5161.JPG
Cs137.jpg
And an Am241 source(smoke detector)
Am241.jpg
All of which showed distinct peaks.

Spectra was also taken of the x-ray emission of the fusor
At 25kv (with the lead shielded cap off)
fusor-25kv.jpg
At 30kV (with the lead shielded cap on)
fusor-30kv.jpg
At 40kV (with the lead shielded cap on)
fusor-40kv.jpg
It's interesting to see how the 30-40keV x-rays are just starting to get through the 1/8" stainless shell, while the 25keV ones are blocked entirely unless the lead cap is removed.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull »

Great work Andrew! I have noted this above and for some years. Your work puts a real fine point on this. Thanks for the excellent report.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Additional progress is being made on the liquid cooled grid to address the heating of the field control cap that covers the lip of the ceramic by brazing it to the cooling lines on the grid. Cusil brazing alloy (72% silver and 28% copper eutectic) was used in the brazing with a boron modified flux(Superior No. 601B/3411)
SAM_5190.JPG
The allow has reasonable wetting properties on stainless and only contains vacuum compatible materials(no zinc or cadmium)

A test braze was completed on an older grid, shown after sandblasting and some cleaning
SAM_5191.JPG
SAM_5192.JPG
SAM_5194.JPG
The braze seems structurally and thermally sound and generally wetted ok on the stainless, though the gap may need some tuning to get good flow down into the area between the cooling lines and the cap.

Additionally, the neutron detection system is being upgraded to use He3 detectors instead of the older BF3 detectors that were previously in use. The new detector is about 7x more sensitive then the BF3 tube on the system. This should allow the time constant on the constant neutron flux control system to be reduced for tighter regulation while still remaining stable.
SAM_5195.JPG
SAM_5196.JPG
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Steven Sesselmann »

Andrew,

It's really good to see you make these small incremental improvements it means the technology is being refined and becoming more mature.

By the way I have one of those LND He3 tubes, and I'm interested in knowing where you got the connector. Does it come like that with the threads or is it something you have custom made?

Steven
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

The connector is a TNC bulkhead mount adapter to clamp on coax, the clamp on section has the same 7/16-28 as the TNC and the LND tube. the other fitting is an Amphenol 79675 Adapter TNC to BNC adapter, a small spring connects the pin on the end of the He3 tube to the center pin on the other adapter
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Pictures of the molybdenum cap glowing under ion bombardment after about 40sec of operation at 17mA
SAM_5200.JPG
Pictures of the molybdenum cap glowing under ion bombardment after about 40sec of operation at 17mA
SAM_5201.JPG
The old grid was removed and inspected prior to the upgrade to the version with the brazed cap. The macor ceramic has held up extremely well with no signs of thermal or arcing damage.
SAM_5210.JPG
SAM_5211.JPG
A cooled grid with a brazed field control cap has been installed and is currently being tested
SAM_5217.JPG
SAM_5218.JPG
The new grid was tested for several minutes at ~2.5e6n/s with no discernible heating of the field control cap
SAM_5220.JPG
The current limit for operation time is now the heating of the vacuum vessel which will require some additional fans to keep cool .
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull »

I have never really worried about the shell heating which tends to reach about 200 deg F after 1.5 hours of running. (electron bombardment and neutral crash area).

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
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Mine seems to have localized spot heating near the ion sources. I think the fraction of the deuterium beams that is neutralized or charge exchanged at the focal point is being accelerated outward and striking the shell near the injector ports on the opposite side.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Steven Sesselmann »

Andrew,

Did I understand this correctly, the cap was initially not brazed to the cooled grid and so became heated from the ion bombardment, so you have now brazed it to the grid so it now stays cool?

Steven
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

That's correct. The previous version had a molybdenum cap for shielding the lip of the ceramic. The cooling lines just stuck through, making electrical contact, but without making good thermal contact. The new version has a brazed stainless cap.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull »

Thanks for the info. My shell has no ion sources pointing at one another across the sphere. Those beams on delecate parts in the chamber would be a bad thing.
So much for taking the time to critically align stuff during assembly.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

The beams seem very well aligned, the fact that they are hitting near the ion injector ports is due to the electrostatic lensing effect from the grid. The grid consists of 3 rings, and while it resembles a spherical source at a distance, nearer to the grid the discrete effects of the grid rings are noticeable. An ion beam fired directly radially inward at +-45 degrees to the midplane of the vacuum vessel will pass through the grid and bend towards the midplane. When exiting the grid it will be at an angle nearer to horizontal and will strike the shell about halfway between the midplane and the opposing injector port.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

I've been working on converting my older CAD files on the fusor into autodesk inventor, here's what I have so far
core1high.jpg
Cross sectional view
core3high.jpg
Grid Detail
grid.jpg
Grid insulator box detail showing cooling lines
grid_box.jpg
Detail of machined macor grid insulator
grid_tip.jpg
grid_tip2.jpg
grid2.jpg
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