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

[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

[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

[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.

[Roberto Ferrari]

Thanks Andrew, we wait for future reports.

[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

On the back panel, valve output connection, power input, error voltage monitor, external pressure setpoint, pressure input from gauge

Final circuit values

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

Tuning between variable pressures

Holding constant pressure

step response, achieves final value in about 8 seconds

The pressure control system holds pressure very stability, now onto the automatic control system for the power supply and ion injectors.

[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.

Steven

[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

[Richard Hull]

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

Richard Hull

[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

Amplifier board converts 0-1.25v signal to drive meter to 0-10v signal on output

Boost converter and amplifier installed

Voltage and current control system construction in progress

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.

[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.

Richard Hull

[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..

Steven

[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

Control system close up

Control system and neutron detector

[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.

Richard Hull

[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.

[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.

[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.

[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.

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.

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.

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.

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.

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.

The adapter allows the glass disk to hang off of the free space inside the copper o-ring between the two conflat flanges.

A pump out duct in the side of the adapter vents the volume between the shield and the viewport.

The small distance between the shield and viewport, and large aperture of the shield prevents any restriction in field of view.

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.

Dimensions of the shield holder are as follows
Glass disk is McMaster 8477K28
Retaining ring is McMaster 92479A790

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

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.

[Andrew Robinson]

Outstanding work as usual Andrew!

[Andrew Robinson]

Ha... Sounds weird saying that. Feels like I'm patting myself on the back lol.

[Richard Hull]

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

Richard Hull

[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)

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.

[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

Arcing damage to top of old grid insulator cracked the quartz tube

New grid system with macor deposition shield, alumina insulator stalk, and molybdenum field control ball

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.

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

Design of bushing and molybdenum cap

Operation of new insulator

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.

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.

Matlab program generating a 5kV ramp waveform of the voltage output

[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

[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...?

[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!