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 Steven Sesselmann »

Nice work, so you basically made your own pressure controller from a vacuum gauge and a piezo pressure valve. I guess this would be not so different from using a ready made MKS pressure controller with a built in baratron gauge. My only concern about using a pressure controller is accidental loss of gas if for some reason a gate valve is opened and the pressure drops rapidly, causing the gas to vent too fast.

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

Post by Andrew Seltzman »

By limiting the maximum valve voltage, the maximum flow rate can be limited, preventing a runaway in case of control system failure.

The maximum conductance of my vacuum system is 0.3L/s, with only an open and closed valve
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull »

Fabulous coup on your pressure controller, Andrew. A real win on that one.

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

Post by Andrew Seltzman »

The ludlum model 12 counter has been modified to provide a 0-10v output based on meter movement to allow the power supply control system to feed back on the neutron signal and provide a constant neutron flux

A boost converter provides +12.5v to a rail-rail opamp
SAM_4802a.jpg
Amplifier board converts 0-1.25v signal to drive meter to 0-10v signal on output
SAM_4803a.jpg
Boost converter and amplifier installed
SAM_4801a.jpg
Voltage and current control system construction in progress
SAM_4808a.jpg
Right now it can display power supply voltage and current and control power supply voltage. The final version will have a 2 channel PID controller for current and voltage control. The system will either directly set output voltage, or control voltage using the neutron flux for feedback. For current control, it will look at the power supply current monitor output and adjust ion source current to track a specific grid current. This will allow the system to accurately track a given voltage/current/pressure operating point with minimal operator tuning.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull »

Wow! Shooting for a constant neutron source in a fusor. Lots of feedback loops here. I hope they don't go "slinky" on you. Fine tuning such systems can range from moderately simple to near impossible. Damping system wide swings can be the real win on such efforts. Good luck.

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

Post by Steven Sesselmann »

Andrew,

Nice work and AFAIK a first here on fuser.net, I echo Richards concerns, you have quite a few loops happening there, so I guess you need some kind of hysteresis to compensate for delays. If anyone can do it, I am sure you are the one.

Looking forward to your next run..

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

Post by Andrew Seltzman »

I have the control system successfully holding a constant neutron flux based on feedback from the neutron detector. At the lower rates that the fusor is producing the integrator time constant has to be fairly long to prevent the system from oscillating. For these tests the ion injectors are just holding at constant emission without feedback control from current since that part has not been built yet.

Video of operation
https://youtu.be/BiEOAbmksow

Circuit schematic
SAM_4810a.jpg
Control system close up
SAM_4815a.jpg
Control system and neutron detector
SAM_4813a.jpg
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull »

This appears to be a first in electronic fusor control that appears to be rather successful. Do you have a current fusion level at which the system seems stable over about 20 minutes or longer? What is your longest stable, controlled run? Just curious.

Really a super effort.

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

Post by Andrew Seltzman »

Based on comparison between the old BTI(it finally did fail) I had and the BF3 detector, I would estimate neutron rate in the 2e5 n/s range. More fine control will be possible with higher neutron rates as the effect of a single count becomes less. As of now, the control system is running with mostly integral feedback with a very long time constant to prevent oscillations. If the neutron rate was higher, the time constant could be decreased.

At high proportional gain or lower integral time constant the system does start to "slinky" rather noticeably, but with low Kp and long Ki, it runs stably.

The longest run would be in the 2-3min range. I'm still having problems with out gassing of the boron nitride grid insulator and possibly the ZnSe viewports as things heat up which limits run time as the pressure controller reduces gas flow to maintain constant pressure until no deuterium is being admitted to the chamber. Removing the ZnSe ports and upgrading the grid insulator design is on the to-do list.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Dennis P Brown »

I am a bit concerned by the ZnSe window - powerful electron bombardment could release Zn into your system and cause long term contamination - Zn, if it does deposit will re-vaporize again and again - not good for electronic sensors, I'd think. I assume you need that ZnSe window as an IR clear window but if you aren't using it at the moment for that purpose I'd suggest replacing it with a glass window for now. Why keep an unknown that might be causing issues until you have the system fully operating in the manner you want and then installing the ZnSe window? Just a thought.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

That is one of the things I worried about with the ZnSe viewports. There are defector magnets to prevent the electron beam from hitting the surface, but it doesn't really deflect the D- ion jets.

I have removed the ZnSe viewports and still have the problem. It's most likely the BN on the grid insulator, the binder has a tendency to soak up some moisture.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

The ZnSe windows have been removed and the damage analyzed. Both top and bottom windows showed deposition from sputtering considerably more visible then that on the glass viewports.
SAM_4818a.jpg
SAM_4817a.jpg
SAM_4863a.jpg
This is likely due to the deposited material interfering with the BBAR(broad band antireflective) dielectric coating on the window causing considerably more change in transmitted light. Examination of the window surface under a microscope showed pitting of the BBAR coating where the D- beam was hitting the window.
top-lens-d-ng-beam-centera.jpg
Subsequent cleaning of the window was able to remove most of the BAR coating(and the deposition with it). Post cleaning the window is transparent again.
top-lens-d-ng-beam-center-cleana.jpg
None of the pitting or damage extends into the ZnSe material, it appears only the BBAR coating is strongly affected by D- bombardment, however to err on the side of safety, the ZnSe windows were not re-installed to prevent any potential sputtering of zinc from the now unprotected window surface into the vacuum system.

The viton o-ring sealed glass disk viewports were replaced with AR coated vacuum rated viewports.
SAM_4859a.jpg
Due to the previously observed interaction of the sputtered material and D- beam with AR dielectric coatings an internal shield system was designed to prevent exposure of the viewport internal surface with plasma or sputtered material.
SAM_4852a.jpg
A stainless adapter holds a 1.25" diameter 1/8" thick glass disk inside the vacuum system in front of the the viewport. The glass disk is held in place with a internal retaining ring. No groove in machined in the adapter, the friction force of the ring is sufficient to securely hold the disk in place.
SAM_4857a.jpg
SAM_4854a.jpg
The adapter allows the glass disk to hang off of the free space inside the copper o-ring between the two conflat flanges.
SAM_4858a.jpg
A pump out duct in the side of the adapter vents the volume between the shield and the viewport.
SAM_4853a.jpg
The small distance between the shield and viewport, and large aperture of the shield prevents any restriction in field of view.
SAM_4883a.jpg
The lead glass x-ray shield is held in place with an o-ring but a plastic holder will be machined to securely hold it in pace.
SAM_4884a.jpg
Dimensions of the shield holder are as follows
Glass disk is McMaster 8477K28
Retaining ring is McMaster 92479A790
SAM_4864a.jpg
Additional upgrades included the design and installation of a faraday cup to measure D- beam current reaching the end of the CF nipples on the vacuum chamber
SAM_4843a.jpg
SAM_4848a.jpg
SAM_4845a.jpg
SAM_4846a.jpg
The collector plate is recessed inside a secondary electron suppressor shell. The shell is also isolated from chamber ground so that both the collector and shell can be biased for ion energy analysis.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Robinson »

Outstanding work as usual Andrew!
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Robinson »

Ha... Sounds weird saying that. Feels like I'm patting myself on the back lol.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull »

This is the kind of advanced work that we would hope to see more of here. Again, thanks for the continued sharing.

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

Post by Andrew Seltzman »

Some power supply upgrades are coming to the fusor allowing increased voltage operation. Earlier this year I got a second spellman PTV power supply on ebay for $200 with the intent of paralleling the outputs for increased current
http://www.ebay.com/itm/Spellman-Lunar- ... 7675.l2557
Unfortunately the seller had one positive and one negative polarity and sent me the positive polarity one, which had to be returned. Kuba got the other one that had the negative output.

Fortunately Bern spotted a spellman x-ray supply on e-bay which I managed to win. It's a -70kV, 8.56mA spellman power supply(DXM70N600X3547)
http://www.ebay.com/itm/Spellman-Power- ... 7675.l2557
The manuals are listed here
http://www.spellmanhv.com/DXM
http://www.spellmanhv.com/~/media/Files ... XMMAN.ashx

It has digital control of the output(serial, usb, and ethernet)
s-l1600.jpg
though it does use an unusual connector, a Claymount Mini Federal Standard X-ray connector, identified as a model CA11
http://www.claymount.com/en/products/hi ... /connector

I managed to find some short cutoff cables for testing of the supply
http://www.ebay.com/itm/Claymount-Xray- ... xyMZ5RnAlY
But I am looking for the screw ring for the connectors and longer cables that I can adapt to my system if anyone knows where to get them.

My grid system is in the process of being upgraded to prevent arcing from the tip of the ceramic insulator and prevent outgassing by replacing the boron nitride with macor and alumina. With the grid removed the fusor will easily pump down into the 1.5E-5torr rang and into the high E-6 range with help from the bakeout heaters and ion sources to desorb water from the walls, so the grid BN ceramic was definitely causing the vacuum problem. With the bakeout heaters on and the fusor shell in the 80-90C range, the vacuum will hold in the mid E-5torr range, so it should still have 99% deuterium purity when operating in the mid E-3torr range. Leakage through the few remaining viton o-rings still lets in some water, but the vacuum is getting progressively better.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

The old grid was modified to remove all boron nitride components that were causing vacuum problems due to their porosity and water retention.
Deposition on old boron nitride grid insulator
SAM_4889a.jpg
Arcing damage to top of old grid insulator cracked the quartz tube
Still0003a.jpg
Still0008a.jpg
Still0012a.jpg
New grid system with macor deposition shield, alumina insulator stalk, and molybdenum field control ball
SAM_4900a.jpg
molybdenum field control ball fits over the tip of the alumina stalk preventing any sharp edge from being exposed to the plasma like on the last grid design. Sputtered metal deposition will eventually make the lip of the insulator conductive leading to arcing. The molybdenum ball fits like a cap over the tip of the ceramic rod presenting only smooth surfaces to the plasma.
SAM_4899a.jpg
The new macor bushing adapts the quartz vacuum feed through to the alumina rod and includes a deposition shield, the bushing has a machined in pump out duct to vent the inside of the grid assembly to prevent a trapped volume
SAM_4898a.jpg
Design of bushing and molybdenum cap
SAM_4908a.jpg
SAM_4909a.jpg
Operation of new insulator
SAM_4907a.jpg
Unfortunately the new design arcs at 25-30kV at 10mTorr at the base where the alumina rod meets the macor bushing. This only occurs when exposed to plasma, in a neutral gas at 10mTorr with the ion sources off to prevent any plasma breakdown the insulator will hold off 40kV. This design needs to have additional bushings outside the alumina stalk like on the boron nitride grid. These will be machined out of macor and added to the system.
grid-rev4-down2a.jpg
The vacuum performance of this grid is perfect with no outgassing or water retention. The system will easily hit the mid 5E-5 torr with the grid installed, macor is definitely the way to go.

Preliminary tests are being conducted on the new Spellman DXM supply. It turns out the supply is brand new, with no logged hours on the HV on hours counter. The supply can be controlled and monitored over an ethernet connection, or with analog inputs.
SAM_4911a.jpg
Matlab program generating a 5kV ramp waveform of the voltage output
spellman_kV_rampa.jpg
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by John Futter »

Andrew
I have had various results with the ethernet control program as supplied by Spellman with our 6kW 140kV unit
it disappeared from our intranet when we needed to shut it down urgently due to excessive x-ray production in our experiment ie counters went over 300mSv.
So we now have a kill switch wired through the interlock connector.
Not sure why you are having so much trouble with the BN insulator.
As for o-rings some of our equip has nearly a 100 meters of viton o-ring and it sits at around 2x 10^-8 millibar base pressure with a single 300L/S turbo, the RGA does not show appreciable water signal we see a weak air signal N2 O2 H2 Co2 and heavier hydrocarbons probably fingerprints and grease ,backing pump oil etc.
Pyrex makes very good shields as it is slightly conductive and defines the "E" field with a nice gradient.
I do not like MACOR for this as it is too insulative and the field can concentrate in the wrong places IMHO
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

John,

I definitely plan to have a hardware interlock/kill switch on the final setup.
The problems with the BN insulator were mainly due to it having 7% porosity and a boric oxide binder, which is hygroscopic. It soaks up water vapor and then released it when it starts to get warm.

I'm trying to avoid any non-quartz glasses like pyres since they benin to get conductive when heated by direct ion bombardment. This is especially a problem at the lip next to the grid. The quarts has much less problems and can tolerate much higher temperatures, but still needs to be shielded from direct bombardment at the top lip. It's also kind of brittle and can fracture during an arc. Alumina will not fracture like quartz, but has much lower dielectric breakdown strength.

The macor is something new I'm trying due to it's machanability, I hope to make something similar to the BN insulator but with macor shields, an alumina core, and a molybdenum field control ball at the top to prevent any ion bombardment if the insulator lip. I really think the re-entrant molybdenum ball will greatly increase grid insulator life by preventing conductive deposition on the insulator lip by the grid from initiating field emission discharges.

Do you have any experience using macor under intense ion bombardment, like near the focal point of a fusor? Does it begin to melt/degrade/outgass etc...?
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Robinson »

Great to see you're still making progress on this design. We're running into some of our own operational challenges with our liquid feed through. Always great to see the challenges you are running into. It has helped us immensely in the past to avoid some costly mistakes. Thanks Andrew!
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Latest upgrades to the mark 3:

A complete rebuild of the grid cooling system has been completed.
The old system was removed
SAM_4966a.jpg
and replaced with an upgraded version consisting of a compact thermometric heat exchanger designed around an all in one waterblock pump combo (swiftech apogeedrive 2) http://www.swiftech.com/ApogeeDrive2.aspx
SAM_4976a.jpg
as well as a new radiator and low noise fan
SAM_4975a.jpg
An acetal block is machined to hold a second copper waterblock that fluorinert flows through
SAM_4989a.jpg
SAM_4980a.jpg
A thermoelectric cooler transfers heat from the fluorinert to the water cooling loop
SAM_4999a.jpg
A CPU retention plate is outfitted with extension standoffs
SAM_4988a.jpg
Allowing the apogeedrive cooler to clamp down to the thermoelectric cooler and fluorinert waterblock
SAM_5002a.jpg
Dimensions of the acetal block
SAM_4994a.jpg
SAM_4995a.jpg
The newer system is considerably more compact
SAM_5003a.jpg
A stainless reservoir stores fluorinert for the cooling loop
SAM_5039a.jpg
SAM_5040a.jpg
Complete system mounted in reactor frame
SAM_5044a.jpg
SAM_5046a.jpg
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Upgrades to the grid system have also been completed

The arcing problem on old grid design was traced back to the area between the alumina insulator and the macor bushing
SAM_5024a.jpg
SAM_5025a.jpg
This was only a problem when exposed to plasma bombardment. In a vacuum it would hold off 40kV, but arc at 25kV under plasma bombardment

A new design with a macor insulating cap and new bushing with a overlapping interface that prevents any scattered plasma from forming a conductive path
SAM_5011a.jpg
SAM_5019a.jpg
SAM_5021a.jpg
The new version will operate without arcing at 40kV in a plasma and will not produce any outgassing(unlike the boron nitride version) allowing continuous operation of the fusor
SAM_5054a.jpg
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Robinson »

As always, fantastic work Andrew! Both myself and my team always look forward to updates on your grid design and have been following them with great interest.

You have caught my particular interest with Macor. Will need to read up on this ceramic. Thanks!
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman »

Latest operation data from the new grid and cooling system:

The new macor insulated grid allows significantly better vacuum performance due to it's zero porosity and lack of binders. There is no discernible outgassing as the ceramics heat up, allowing continuous operation.

Latest operation at 7.2mTorr, 40kV, and 8.8mA
SAM_5051a.jpg
Generated 70 bubbles (30bub/mrem) in 60 sec at 3.5"(8.89cm) from the center
SAM_5068_counta.jpg
SAM_5067_counta.jpg
Current best neutron output is 1.08E6n/s
fusion-6-26-2016-7,2mtorra.jpg
Next on the to do list is connect the second PTV supply in parallel to run at 40kV, 17mA, and hook up the new 70kV, 8.5mA spellman supply for higher output.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull »

Fabulous report and great work across the board. Few folks who have done fusion here have hit the mega mark. Probably as few as 10%. You kinda' have to stick with it once you have done fusion to get numbers up. You system is clean and very well executed. You can now easily do activation on a significant scale, far from any need for satisitcal back up.

You might try to calibrate a fixed silver activation set up against your BTI bubble detector, if that is your only neutron indicator. When the bub dosimeter craps out, you can rely on the silver activation as a crude but calibrated indication of future performance. (assuming you don't want to blow the coin needed to replace the BTI when it goes south.)

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Fusion is the energy of the future....and it always will be
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