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cons and pros/ my vacuum system

Posted: Fri Feb 08, 2013 6:13 am
by electron
Hello everyone,

I having been busy designing my vacuum system and I was hoping someone could point out the pros and cons .
If you notice below, on the left hand side of the included picture, my choice of vacuum chamber is
a clamshell flanged spherical chamber from wiederlabs.
The cost would be around $800.
I then want to attach the above chamber port to a nipple reducer (picture below) which will follow on to the diffusion pump.
On the other port a high voltage feed through.

The other starting path will be a much larger uhv chamber.
The cost would be around $500 to $1000.

The types of questions that are running through my mind at the moment, besides price, would the first set up work? Also, for a first attempt, how well have I designed the system so far?
The second most important question for me, regarding vacuum pump down times, is will a smaller chamber be a better choice than a larger chamber ?

Any opinions, links or help will be greatly appreciated.

Re: cons and pros/ my vacuum system

Posted: Fri Feb 08, 2013 5:35 pm
by Carl Willis
Hi James,

You have identified some viable, very standard vacuum chambers. Either chamber can be successful.

You might want to read about other community members' experience with the vendor for the spherical chamber in the forum archives. Some people have found his business practices and workmanship to be frustrating, while others point out that his prices are a bargain.

A complete fusor vacuum system will include a roughing pump, a high vacuum pump, a foreline valve, and a high-vacuum throttle and shutoff valve, as well as plumbing. Some more versatile systems have a roughing valve that bypasses the high-vac pump. Flange adapters may be required; however, you will save a lot of money and hassle by avoiding them in the design. Pick a standard flange system for your high-vac components and another for your low-vac components. 2-3/4" CF and KF16, for example. In the thumbnails you have posted, note that the diff pump has an ASA flange, the chamber uses 2-3/4" CF, and the CF adapter is probably 2-3/4" to 4.5" CF. You would still need an ASA-to-CF adapter. You'd be smart to pick a 2-3/4" CF throttle valve.

I have one general guideline for the diff pump: keep it small. 4" and larger diff pumps are often sold at what looks like a bargain, but the big pumps have big oil reservoirs (hundreds of dollars to fill with a suitable fluid), high electrical draw (kWs, likely at 240V), and also consume water for cooling. When you consider what the high-vacuum pump actually does in a fusor, you will note that ultimate vacuum and speed criteria are very modest. The smallest available diff pumps are best for this job.

-Carl

Re: cons and pros/ my vacuum system

Posted: Sat Feb 09, 2013 2:53 am
by electron
Carl, I really don't know much about vacuum line accessories and what they do.

You mentioned the throttle valve in your previous post and I am very glad you did.
After a while I started to read about upstream and downstream pressure control, and although I do get ahead of myself, it looks like the throttle control valve plays a vital role in balancing and sustaining fusion.
Thanks for the cost saving tip too, I think in my situation buying a nice small diffusion pump will pay off in the end.

Before I go I wanted to ask, how many people were controlling there fusion devices with manual controls. Was anyone attempting or had achieved computer control via electrical signals and sensors.

vague but just curious.

Re: cons and pros/ my vacuum system

Posted: Mon Feb 11, 2013 4:04 pm
by Richard Hull
Very few here control via computer. Maybe one or two at most. Virtually all fusioneers use only manual controls. It is more a matter of expense in obtaining the electrical valves and other related interfaces. The programming should be straight forward, however, once the programmer has been at manual controls for some time at his fusor. Proper programming would demand an extant knowledge base to be successful.

Richard Hull

Re: cons and pros/ my vacuum system

Posted: Mon Feb 11, 2013 7:55 pm
by Tyler Christensen
I had my fusor running on digital controls for a short time (with the exception of current since I didn't get around to digitizing my HV supply). I gave the program a voltage and it controlled the valves to maintain a pressure that could sustain that voltage at the HV current of user control.

Unfortunately my MFC broke shortly thereafter so I didn't run it a lot or have a chance to get 100% hands free operation.

If you have any programming experience, the programming is trivial once you get a data acquisition system (assuming you have thorough knowledge of the operation of a fusor already as Richard mentioned. It'd be hard to write the software if you've never run a fusor before). I monitored voltage, current, pressure, and neutron rate into the software, and controlled the chamber pressure. Even with a fresh run in a "dirty" chamber, the software could keep it extremely stable while manual control would be almost impossible to keep 50kv stable without first doing lots of vacuum cleaning.

Tyler

Re: cons and pros/ my vacuum system

Posted: Mon Feb 11, 2013 9:05 pm
by Richard Hull
Thanks Tyler for your input. I had forgotten the few who have attempted this effort. Ostensibly, this is the ideal situation and with enough bucks in hand to get the good materials needed, it should present few problems to the competent fusioneer.

Richard Hull

Re: cons and pros/ my vacuum system

Posted: Mon Feb 11, 2013 9:31 pm
by Chris Bradley
james brown wrote:
> Before I go I wanted to ask, how many people were controlling there fusion devices with manual controls. Was anyone attempting or had achieved computer control via electrical signals and sensors.
I have some electrically operated valves that I have done automatic control with, if that is the objective. Not quite 'computer controlled' but essentially 'digital' nontheless:

The method I used was to pulse the valves with a 555 circuit, in a 'PWM' configuration. The circuit is controlled by a comparator. The 555 is adjusted to deliver a steady stream of pulses, with user controlled 'on' and 'off' intervals. It drives a small fet that in turn drives a big darlington TO-3 that is robust to inductive transients driving the valves.

There is a comparator that is configured to interrupt the signal to the fet if an input voltage exceeds a set point.

The set point can be compared with several different sorts of input; an output from an active pressure gauge, a voltage reading, a current reading, a scaler, or any other measurement that can be taken from the apparatus and inputted into the comparator as a voltage proxy.

Taking voltage as the input is a particularly effective control to sustain an unstably lit plasma, when used with a 'soft' unregulated supply. As the volts go up beyond the set-point the valves don't open to all of the pulses, and the pressure drops so that the plasma re-lights, and as the volts go down so the valves open on every pulse and the pressure drops. This allows for very fine control to push very high volts/very low current applications where manual control of such a delicately lit plasma is essentially impossible.

The system worked great, but the constant clicking operation of the valves got very annoying!!! I rarely used the set-up in the end (but I am in the process of boxing it all up now).