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First Light and Plasma Photos

Posted: Sat Aug 04, 2012 11:03 pm
by JakeJHecla
Hello all- I've been a member lurking here for a year or so, though I haven't been posting much. I'm a high school student located in Seattle, Washington with a love for amateur science. I've undertaken a fusor project as a way to experiment with the basics of electrical engineering and high energy physics.
The apparatus I've pieced together is basic at best, though I believe it will eventually be capable of fusion should I put in the time and effort. I have an extremely limited budget (I insist on funding this myself), and I've taken a low cost approach while maintaining a reasonable level of safety. Almost all of the equipment I purchased came from ebay, and was in non-functioning condition when I received it. This was intended to save money, but also gave me a better understanding of the system.
The unit's power is supplied by a 12A "Select-a-volt" variac leading to a12kV 30ma NST with a single stage CW multiplier (5 oven caps in series per side) under oil. Roughly 750Kohm is on the output as ballast. The HV enters the chamber through a modified AC44XLS spark plug welded into the top 6” CF flange. Vacuum work is handled by a Welch 1402 and a CVC VMF-21 diff pump. The chamber itself is a SS 6"x18" cylinder with a 2.75" CF quartz viewport (a gift from Robert Tubbs). While the bottom flange seal does have a leak (1 micron/3min), the entire vacuum setup has performed solidly. On the roughing pump alone, I can get the system down to 20mTorr, and with the diffusion pump going, I can exceed the limits of my TC gauge. At the moment, the only issue I'm encountering is minor arcing on the outside of the feedthrough, which will be solved by potting it in oil.
My next steps are adding a second vacuum gauge and building a heavy water electrolysis rig to push this thing into fusion territory. Many thanks to everyone who has helped me along the way, especially Carl Greninger, David Housley, Robert Tubbs, Carl Willis (the PMT you gave me is almost running!) and Richard Hull (for the FAQs)

1) 40mTorr (~3kv)
2) 35mTorr(~3.5kv)
3) 25mTorr (~5kv)
4) 20mTorr (~8kv)
5) 15mTorr(~10kv)
6) 10mTorr (~18kv)
7)Photo of the setup with temporary lead shielding (4x 60lb lead ingots from the USS Menhaden)

Re: First Light and Plasma Photos

Posted: Sun Aug 05, 2012 7:09 am
by Carl Willis
Hi Jake,

That's progress all right! Welcome to the plasma club. I think you'll find that it just gets more enjoyable from here as you push toward fusion.



Re: First Light and Plasma Photos

Posted: Sun Aug 05, 2012 10:12 am
by Chris Bradley
Hello Jake, nice work and a healthy thickness of shielding too, to keep behind!

I just wanted to ask on your voltage source/measurement: When you say 'a single stage CW' I presume you mean it is configured as a Greinacher voltage doubler, or is it as a doubling bridge? (If it was a CW/Greinacher, then I presume you're running a high frequency NST, as the caps at 50/60Hz would be quite large for HV?)

Presumably you would be doubling a half of the NST's rated peak volts, which'd be 17kV unloaded. I'm not doubting it'd stretch to a light load at 18kV (especially if your variac was set to put out over 100% input volts) but I was just wondering how you measured this to confirm it, and what sort of current you were getting at those volts. Thanks.

Re: First Light and Plasma Photos

Posted: Sun Aug 05, 2012 5:12 pm
by Dennis P Brown
Wow- if your system leaks at 1 micron per three minutes, that is outstanding - I'm lucky for my accelerator to 'only' leak 30 microns per minute. Of course, I have a lot of feedthru's in the accelerator tube and connectors.

I am sure your DP is getting your system in the low 10^-5 torr (could easily make low 10^-6 torr with a cold trap.) I can bottom out to 1.2* 10^-5 torr with my leak rate even just using my DP which does not have a trap (I mostly use my turbo, through.)

Good idea to shield - without a Geiger counter and dose monitors, you are doing both the smart and safe thing.

For a low cost 'leak' valve, do search on the very cleaver idea that someone here did - they put a small metal rod in a small feed tube to create a control leak capillary tube which they fed using a valve.

Congratulation on a job so well done - the pictures were very nice.

Re: First Light and Plasma Photos

Posted: Sun Aug 05, 2012 8:20 pm
by JakeJHecla
Chris, I've attached a basic schematic of the multiplier setup. It's bad enough to make any EE cringe, but I think it gets the idea across. You may be right that the output voltage is lower than I originally thought. The panel meter I was using started displaying ridiculous readings yesterday, and I'm in the process of switching to a 100microamp analog meter at the moment. When I got the 18kV reading during the plasma run I posted, the current I observed was roughly 5ma.

Re: First Light and Plasma Photos

Posted: Sun Aug 05, 2012 9:40 pm
by Chris Bradley
Are you sure this is an accurate schematic that represents your circuit, Jake? In particular, is this actually how you set up your diodes? How did you measure current?

In regards the two input lines from the left, is one from the grounded centre-tap of a 50/60Hz NST?

Sorry for the questions, but its good to understand the origin of data quoted with your work.

Re: First Light and Plasma Photos

Posted: Sun Aug 05, 2012 9:58 pm
by JakeJHecla
Here's a better representation. Please excuse the sideways rotation

Re: First Light and Plasma Photos

Posted: Mon Aug 06, 2012 1:22 pm
by Chris Bradley
JakeJHecla wrote:
> Here's a better representation. Please excuse the sideways rotation

Hi Jake,

OK, so the diodes look 'right' now, but if this is a correct representation and you are showing that this arrangement is driven by the two outputs of an NST (at 180 deg out of phase) then this isn't a conventional CW stage but will act somewhat like a voltage quadrupler.

If we think through an AC cycle, let's say it is unloaded and say the top output hits +5kV peak (for the sake of an easy number to work with): At this point, the caps on the top line will charge up to -10kV, as one side is connected to this +5kV and the other discharges to the -5kV (the other NST output) through the diodes.

Now the phase changes and the top output goes to -5kV and the lower to +5 kV: The caps on the top now have -5kV on one side and so the other side of the string are now pulled to -15kV (because they have -10kV across them already). The caps on the bottom now have +5kV on one side, and the other end therefore discharge through the other diodes into the -15kV end of the upper caps. So the lower caps end up having -20kV across them.

Of course, this won't happen in one cycle because as the lower caps discharge into the upper ones then there will be an equalisation of the volts for a while. But if the circuit were unloaded, then over time you'd see the caps build up these potential differences, resulting in a doubling of the full pk-pk output of the NST (quadrupling of the peak) across the lower caps (and doubling of the peak on the top ones).

However, the reason I think you are getting odd voltage measurements is because as you head towards an equilibrium voltage on the lower caps, so these will carry through the AC output of that lower line. They are not holding a steady charge wrt ground, but are bouncing up and down along with the other NST output. As a result you would be seeing the output follow the lower voltage output, by a difference determined by the charge on the lower caps. So it looks to me that [in the unloaded case] you will end up with an ac output that has the same RMS as the NST's output but is DC displaced by 4 times the peak voltage; viz, in the above case an AC waveform pk-pk between -15kV and -25kV.

In practice, as you load up the output, the DC offset will be pulled down, because that charge/recharge process will be having current drawn off the caps mid-cycle, so they won't get up to the full doubled/quadrupled unloaded voltages. This effect will be quite strong with just 60Hz driving it. At some point, the load may be sufficient that the lower magnitude range of the AC cycle may begin to reach zero and you will end up with a waveform that crosses zero and applies a +ve voltage to the fusor. This will happen if the lower caps end up with less volts across them that is the NST peak voltage.

I'm pretty sure this'll be what happens, but feel free to run a spice simulation and correct me if I have confused myself. That being said, this arrangement certainly should allow you to hit high voltage peaks during the AC cycle with quite a minimum of components, so long as the caps hold up enough charge under load. The high peak (the RMS plus your 4xpk DC offset) might explain why there is a tendency for some arcing - there is a possibility that you may be actually hitting higher volts than you're reading.

If you find you have one more lot of diodes and caps available, then you can put diodes before and in series with the 750k resistor, and then caps from that new junction to ground. I think this will then amount to a voltage 'pentupler'?

Re: First Light and Plasma Photos

Posted: Mon Aug 06, 2012 1:54 pm
by Richard Hull
Nice work Jake! I have added you to the Plasma Club. I think a simple full wave rectifier setup like I present in one of the FAQs might serve better from a DC voltage and safety standpoint. With a 15kv neon sign transformer, the unladed output would be about 11-12kv if you pushed the variac to its 135 - 140 volt output. It is best to use 2 microwave oven diodes, though stacking lower voltage diodes is OK.

Chris has already pointed out the issues with the circuit you presented.

Good luck in forging ahead in you efforts towards fusion.

Richard Hull

Re: First Light and Plasma Photos

Posted: Mon Aug 06, 2012 5:46 pm
by Chris Bradley
Richard Hull wrote:
> It is best to use 2 microwave oven diodes, though stacking lower voltage diodes is OK.

To bear in mind;- in the particular circuit of Jake's, the diode [string] nearest the transformer will need to be capable of sustaining 4 times the peak voltage output of the transformer, as the upper caps may charge to double the peak, and then when the upper output is -pk and the lower one +pk, so that tots up to 4 x pk across the diodes.

The other diode string also needs 4x pk rating: The lower caps should charge to x4 pk (that probably means they'll be pushed very hard if they are 4xMO caps), so when the upper transformer output is +pk then the upper end of that diode string will be at -pk (as it's 2 x -pk lower that the other end of the caps, which are at +pk) and when the other transformer output is -pk then that far end of the diode string will be at -5pk, also tots up to x4 pk across the furthest diode string.

..Easy to check all these things with a Spice model ...