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

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

Post by Andrew Seltzman »

Some new upgrades:

Deuterium supply and control:
Got a better quality lecture bottle regulator on ebay
SAM_4520a.jpg
Testing regulator with argon
SAM_4523a.jpg
A piezoelectric valve is being outfitted to control pressure in the fusor.
SAM_4431a.jpg
Converting the valve from swagelok tube to VCR inlet and outlet ports
SAM_4430a.jpg
Deuterium/dry air admit(for venting the core to atmosphere) manifold to attach to the back of the conflat cube, used swagelok SS-4H bellows sealed valves w/ 1E-9 TL/s leak rate
SAM_4513a.jpg
Bakeout system, to drive water/contaminants off the chamber walls without running plasma(reduces ware on grid / metal deposition from using ion bombardment to heat the chamber walls)

20W stick on heater
SAM_4519a.jpg
8x heaters(160w total) attached to the core
SAM_4518a.jpg
Heaters connected to fusor 24v power supply
SAM_4516a.jpg
Bake out of core after up to dry air to install deuterium manifold, bakeout had to be stopped to prevent damage to the magnets shielding the viewports from ion beam damage(80c max), core reached 90-100C, though cooler out on the nipples
bakeout-130c-annotated.jpg
bakeout-130c-annotated.jpg
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Latest test runs:

Pump down time after the bakeout has decreased to 10min to get from 500mTorr to 3e-4 torr

A second lead glass shield on the topside viewport has been added allowing pictures of the upper segment of the grid

Operation at 40kV, 2.2e-4 Torr
SAM_4528.JPG
Interestingly enough it seems like only the bottom viewport fluoresces under ion bombardment(residual ones not deflected by the shield magnet), not the top one. Also there is in interesting fluorescing pattern on the top of the boron nitride insulator
SAM_4528b.JPG
It looks like secondary electrons from the grid are hitting the top of the insulator causing the fluorescence, however there is a dark area between the grid rings, apparently where there are no electrons hitting. The incoming ions hitting the outer surface of the grid are generating secondary electrons streaming off the grid, however the electric field directs them radially outward causing the dark area between rings.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Operation in the 9e-5 to 1e-4 torr vacuum range
SAM_4534c.jpg
Video of the plasma during voltage ramp up
https://www.youtube.com/watch?v=4pOdWpW ... e=youtu.be

You can see the structure of the ion beam coming from the opposing sources, compared to a picture of the ion beam from the injector in a test bed with no grid potential, particularly on the lower left beam
SAM_3244a.jpg
The cylindrical beam is being focused by the grid potential into a conical beam. I think it might be useful to put some focusing directly on the injector to get a tighter beam at the focal point.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

First fusion test runs with the new system

System setup with piezo control valve and deuterium tank(an older one with about 90psi left in it) and regulator)
SAM_4536.JPG
Deuterium tank and regulator
SAM_4538.JPG
Piezo control valve, takes a 0-100V signal, accurately admits gas to the fusor allowing operation in the 3e-4 to >10mTorr range
SAM_4539.JPG
Valve control circuit, 90v boost converter and potentiometer to control valve voltage
SAM_4537.JPG
Gas inlet manifold on back of cube
SAM_4553.JPG
Pressure during fusion test run
fusion-run.jpg
Star view, bottom, ~6mTorr
SAM_4542.JPG
Star view top, ~6mTorr
SAM_4546.JPG
Videos of test run
https://youtu.be/sZnh9uPCKvU
https://youtu.be/3hJ6Pn14SL4
https://youtu.be/dqJbiwHoiNk


Some counts detected on a BF3 type neutron detector, no exact calculation of neutron flux yet
Operation in the 3e-4 torr to 8mTorr range
40kV on the grid, ~5-8mA current
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Steven Sesselmann »

Andrew,

Beautiful engineering as always on your projects.

Did you have the ion sources running during this run?

I am really interested in your ion sources, is there a post describing these in more detail?

Steven
http://www.gammaspectacular.com - Gamma Spectrometry Systems
https://www.researchgate.net/profile/Steven_Sesselmann - Various papers and patents on RG
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Yes, all 4 ion sources were on. you can see the distinct difference in a gunned fusor, in that the star mode only has 4 rays(for an 3 ring grid with 8 openings), with no visible rays coming out of the segments of the grid where there are no ion sources feeding in ions.

The ion source designs are
viewtopic.php?f=12&t=5011
and
http://www.rtftechnologies.org/physics/ ... source.htm
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Steven Sesselmann »

Andrew,

Thanks for the links, it seems my memory is not so good, I was active in your ion source thread back in 2009, phew time flies when one is doing fusion...

If your ion source can operate with negative bias, I think it would be a worth while experiment to do a fusion run with a) Positive bias and b) Negative bias, if I am not mistaken you should see more fusion with a negatively biased ion source. As you are only running a few kV the central grid will still be able to extract the ions.

Read my recent post to understand my reasoning (viewtopic.php?f=50&t=10443)

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

Post by Andrew Seltzman »

Neutron output is estimated to be in the 25k-30k per sec range based on an ~500sec run at about 8mTorr, 40kv, 8ma, 8.9cm radius from BTI to center, both a BTI bubble dosimeter(pretty old, won't recompress, 8 bubbles present before run started, ~29 after run). Flux estimate from BTI matches estimate from BF3 neutron detector
SAM_4559.JPG
the neutron rate drops off pretty rapidly as things heat up and start to outgas, requiring reduction in D2 flow and thus reducing plasma purity. I might need to redesign the grid to remove the BN ceramic which I suspect may be the main source of temperature driving outgassing.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

I've been getting my bubble dosimeter to re-compress by inserting a pencil eraser in between the screw on tap and the rubber membrane(still looking o buy a new one). This one was purchased in 2009 and is still working with some effort to get it to re-compress. The only thing that i notice is the bubbles are smaller then when it was new.


Best neutron numbers to date 2.1E5 n/s at 8mTorr, 40kV, 8.9ma, 4 ion injectors at 700v anode voltage
24 bubbles over a 100sec run
SAM_4568a.jpg
Calculations
http://www.gammaspectacular.com/fusion_calculator.html
fusion11-21-2015.jpg
Something is still heading up and outgassing which requires the reduction of D2 flow to maintain pressure and kills the neutron numbers after about 100sec.
Best vacuum to data 5e-5 torr, 96mTorr rise after 5 days, the bake out heaters really seem to help keep the system clean

I also swapped the grid cooling pump with one with lower displacement, allowing the pump motor to provide more flow without hitting the torque(current) limit on the pump. Flow through the grid is now at most 200ml/min

On the to do list:
Replace NdFeB magnets in the ion sources with SmCo for higher temperature operation and comparability with deuterium without disintegrating like NdFeB magnets eventually do.
Replace hhe viewport on the vacuum hub with one that uses a cu o-ring
Re-design grid insulator to vent virtual leak from body and use a non-outgassing material(0 porosity ceramic)
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull »

Fabulous image and fabulous results. It just gets better and better as operaational experience grows. It is always true and the key to real fusor operation. For the simple amateur fusor there is more that science and engineering, there is operational artifice required due solely to the simplicity of the device and the fact that we are working on a "razors edge" located at a fine point in the gas/electric, ionization routine, at rather high pressures. It all comes together solely due to operational control excellence found through experience.

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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 Daniel Firth »

I don't have much to add... just wanted to say it's been interesting watching your progress in this thread.

I think this got buried in a thread about my construction, but I based my grid off of yours. It's not water cooled, but I used your bending method with the slotted pipe.

Also, my BTI bubble detector was done in about a year. I got it in 2012, and now it has many gigantic bubbles. Do you keep yours stored compressed, or uncompressed?
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Bob Reite »

If you got a full year out of your bubble detector, you did well. Mine was gone in 9 months. They are supposed to be stored compressed in the factory supplied tube between 15-20 degrees C. I was shocked when I took mine out of storage and found large bubbles in it, even though it was stored compressed. That seems to be the failure mode. The bubbles from the last measurement "grow" even while under compression.
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

I think there might be a lot of variation based on how well the dosimeter seals when assembled.

They used to manufacture the dosimeters with an anodized aluminum cap and compression system, but then switched out to a plastic one which seams to seal a lot better, probably since the epoxy that they fill the threads with bonds well to the plastic. The first one that I had was made with the aluminum handle and always had a "fishy" smell inside the tube. It lasted about 1year range before it was leaking and would not recompressible. By 2 years it had huge bubbles as well
viewtopic.php?f=13&t=6121&p=34565#p34565

The second one that I bought had the plastic handle and it looked very well epoxied in place. It never had any hint of a "fishy" smell in the storage tube. I got it in 2009 and it would recompress until 2012, with the built in compression chamber, and after that it would still recompress by placing a mechanical pencil eraser in the compression chamber before tightening it down to increase compression.

I always stored both in the tube, compressed. It's possible there is a lot of variation in life due to problems with the sealing system.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Here is an excellent publication on leak rates for viton/kalrez/teflon o-rings for UHV systems(atom optics traps), the vacuum tests start on the 3rd page of the publication
viewport-oring-leak-rate.pdf
(440.42 KiB) Downloaded 701 times
And a reference of various vacuum compatible ceramics, I'm thinking of making part of the grid insulator out of macor, or LAVA, does anyone have any experience using these in a fusor under ion/electron bombardment?
http://www.technicalproductsinc.com/ceramics.html
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

All 4 ion sources have been cleaned and rebuilt and baked out at 200C before instalation. They now use the angled anode rings and SmCo magnets instead of NdFeB magnets for compatibility with Deuterium with out disintegrating, and to allow high temperature operation(up to 300C) without loss of strength. I decided to go with the original style pole piece for now since it provided a near parallel beam with the angled pole piece but may switch over to the other style later. The SmCo magnets have lower field strength though and the ion sources require higher voltage to operate and produce a less intense beam which is fine since they are still significantly over powered for the fusor.

Ion sources before cleaning
SAM_4686a.jpg
In addition, one of the glass disk viewports was replaced with a conflat viewport eliminating one more viton o-ring (5 removed in this modification)
SAM_4691a.jpg
They now use copper o-rings instead of viton and have pump out ducts to avoid virtual leaks.
SAM_4692a.jpg
SAM_4689a.jpg
One of the swagelok valves was outgassing something when it was getting hot and was also replaced, and a 0.5um filter was added to the dry air admit line for venting the fusor to atmosphere for servicing
SAM_4694a.jpg
Some burning on the inside coating of the ZnSe viewports was observed when running with deuterium(doesn't happen with air). There seems to be some charged particle coming from the grid that is not readily deflected by the magnetic beam deflectors. I'm assuming it's a negative deuterium ion
SAM_4695a.jpg
I've doubled the field strength on the beam deflectors by adding a second magnet but it doesn't help much. I may have to remove the ZnSe ports for deuterium runs or re-design them or the grid to deflect the ion beam away from that area.
SAM_4697a.jpg
A feedback control circuit has been built to regulate fusor pressure by feeding back from the vacuum gauge analog output, It will stabiily control pressure from the 1e-4torr range to the 10s of mTorr. This considerably reduces workload during fusion runs and increases neutron output by tracking the optimal pressure. It's a very simple circuit.

Pressure controller circuit
SAM_4698a.jpg
An NP10 boost converter similar to this, or other boost converters used to drive nixie tubes:
http://www.ebay.com/itm/SHV5-5Vin-400Vo ... SwwE5WVDQB
provides 87V from the 24V supply, then an LR12 adjustable regulator steps the voltage down to 70V to feed the positive side of an OPA445 high voltage op amp which is configured as a gain 21 non-inverting amplifier. A recom RS3-1212D DC-DC converter provides -12v to the OPA445 and an AD826 that is used as an input differential amplifier. The vacuum gauge output(quattro 999, 0-10v, logarithmic to pressure) is connected to the negative side, while a potentiometer supplying 0-12v is connected to the positive side. The valve will now accurately control fusor pressure. There is still some drift and overshoot since it's only a proportional feedback controller, but the final version will be a full PID with valve dither ontop of the analog signal to improve performance.
12-5-2015-pressure-control-close.jpg
The fusor will now pump down to the low E-4 range in about 5-10 minutes and max out at about 5e-5 torr after several hours.
12-3-2015-pumpdown2.jpg
The cleaning of the ion injectors and removal of 5 viton o-rings seems to have really helped, though the pressure still climbs when running as the grid insulator heats up. It's made of boron nitride with a boric oxide binder which is fairly hydroscopic and will soak up water every time it sees atmospheric air. It will have to be replaced with a combination of macor/alumina/fused quartz.

Fusion rate is now at 3e5 n/s
12-5-2015-calculations.jpg
Star mode at ~3mTorr during fusion fun
SAM_4704a.jpg
Last edited by Andrew Seltzman on Sun Dec 06, 2015 5:19 pm, edited 1 time in total.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Steven Sesselmann »

More nice work Andrew, efficiency is improving you are now in the E-9 range and it should get better with use. If you can get unto the -65kV range I thing the numbers will really shoot up.

If I understood correctly you are using feedback to control a vacuum valve, another possibility is set the vacuum valve and use a combined pressure and mass flow controller, to regulate the pressure with the gas inflow. Something like an MKS 649 or similar.

I look forward to see the more run data.

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

Post by Andrew Seltzman »

Actually it controls the inlet gas flow by controlling the voltage on the piezo valve.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull »

Getting "betterer and betterer"!
Good work

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

Post by Andrew Seltzman »

The gas control system has meed modified to include a full PID feedback system and in instrumentation amplifier to buffer the input.
SAM_4780a.jpg
This results in better long term pressure stability and lower overshoot
12-18-2015-PI-feedback.jpg
step-responce-log.jpg
A dual layer circuit board has been designed in expressPCB for the final configuration of the control system
SAM_4790a.jpg
gas-pid.jpg
The circuit board holds 2 independent PID control circuits, for the gas valve control only one will be used, the version with both in use will be used to control the HV power supply and in injector current. The one connected to the power supply will compare the grid current to a setpoint and adjust the ion sources accordingly to allow operation at any pressure/voltage/current point with all 3 adjustable independently.

The ion source control will use this circuit to control the input voltage to a set of emco F40 proportional HV supplies driving the ion sources:
http://electronicdesign.com/power/simpl ... ers-output

And assembly is in progress
SAM_4781a.jpg
SAM_4782a.jpg
The boards will be housed in Hammond 1590BB boxes, one for the power supply/ion source control, and the other for pressure control
SAM_4783a.jpg
SAM_4789a.jpg
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Roberto Ferrari »

Hi Andrew

Your comment about the SeZn screen and its damage when running on D2 is consistent with the reductive power of hydrogen.
Please comment on any chemical alterations.
Congratulations for your excellent work!
Roberto
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by DanielSandner »

Hello,
there is something very effective against long pumping times with a Turbo.
A Turbo can pump heavy molecules like hydrocarbon very fast in contrast to light molecules like D or He.
So if you can i would try to fill the chamber with an heavy gas and 1% D. The Turbo will remove the heavy molecules much faster and after 1-2 hours pumping you should have only Deuterium at a very low pressure.
kind regards, Daniel
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by John Futter »

Daniel
your post seems to be arm chair posteuring.
most pumping time on vacuum chambers is getting rid of gas and water molecules that have stuck to internal surfaces
with water vapour being one of the hardest to remove as the molecule is polar and sticks to the chamber walls with great tenacity.
Just backfilling the chamber with a dry inert gas ie argon will save hours of pumping when restarting a vacuum system
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Roberto,

I'm not sure about any chemical changes to the lenses, It looks like beam burn in there the D- beams hit the window surface. There is some evidence of the same thing happening to the pyrex glass viewports as well, though to a lesser extent. I plan to remove the ZnSe ports and may replace them with germanium viewports at a later time to see if that makes a difference.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Roberto Ferrari »

Thanks Andrew, we wait for future reports.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Final construction and tuning of the PID gas control system is complete

The control system is now in a complete box,
The controls in the front are(from left to right), power LED and switch, valve purge button and internal/external pressure setpoint reference, Direct output voltage control pot and direct/feedback control switch, internal pressure setpoint pot, process variable and output voltage displays
SAM_4793a.jpg
On the back panel, valve output connection, power input, error voltage monitor, external pressure setpoint, pressure input from gauge
SAM_4794a.jpg
Final circuit values
SAM_4795a.jpg
The final circuit is a full PID controller with output dither, there is a 6v, 90Hz triangle wave superimposed on the output to improve accuracy
valve-dither.jpg
Tuning between variable pressures
PID-final-tune.jpg
Holding constant pressure
PID-holding-presure.jpg
step response, achieves final value in about 8 seconds
PID-step-resp.jpg
The pressure control system holds pressure very stability, now onto the automatic control system for the power supply and ion injectors.
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