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

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

Post by DanielSandner » Mon Jan 11, 2016 4:45 pm

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 » Mon Jan 11, 2016 5:30 pm

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 » Tue Jan 12, 2016 7:19 pm

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 » Tue Jan 12, 2016 9:01 pm

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 » Sun Jan 17, 2016 4:56 pm

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

Post by Steven Sesselmann » Sun Jan 17, 2016 10:28 pm

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
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 » Mon Jan 18, 2016 2:42 am

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

Post by Richard Hull » Mon Jan 18, 2016 6:12 am

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

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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

Post by Andrew Seltzman » Wed Jan 27, 2016 2:46 am

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

Post by Richard Hull » Wed Jan 27, 2016 4:06 pm

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
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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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