voltage, current, pressure and temperature

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gary_f
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voltage, current, pressure and temperature

Post by gary_f » Sun Aug 24, 2008 5:04 pm

Since my last post a few months ago as a "noobie", I've assembled a demo device and spent some time exploring the relationship between voltage, current, pressure, and "glow". (This is an addictive pastime!). I'm puzzled about what I see and I'm not sure whether it's common to all fusors or to something about my particular construction.

I typically pump down to a pressure where I can turn up to voltage (with no current) to about 20KV. There's a an abrupt threshold where I get bright plasma within the inner grid, but the pressure starts to climb and so does the current. Within seconds, my power consumption goes from 20 watts to 450 watts, and I have to turn the voltage down to prevent overheating of various things.

It seems like there's a hysteresis loop here that prevents steady state operation. It seems like the plasma glow raises the temperature of what little gas is in the chamber, this drives the pressure up, which causes runaway current. If I turn the voltage down quickly, the pump catches up, and I have to start raising the voltage again to keep the plasma lit. I can't find a steady state.

Any thoughts or help?

Gary

AFW
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Re: voltage, current, pressure and temperature

Post by AFW » Sun Aug 24, 2008 9:09 pm

Could the initial discharge be causing outgassing by heating up various components??
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MarkS
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Re: voltage, current, pressure and temperature

Post by MarkS » Mon Aug 25, 2008 2:40 am

The plasma heats up the walls of the vessel, and other various things. This frees all the gunk stuck into the walls, especially H2O. This, like you said, creates and abrupt rise in pressure, causing run away current. What you want to do is use the plasma to glow clean the inside of the chamber. To prevent run away current bring the chamber pressure down to where 2000 volts can ignite the plasma. Keep the pressure roughly stable by throttling the foreline back and forth. Run the current as high as possible (within limits of the supply and grid heating). This will clean the chamber, while keeping the surging of the power supply down. From there take is slow and raise the voltage as the pressure drops. I would say its ill advised to raise the voltage to 20kV before igniting plasma.

-Mark Siegel

gary_f
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Re: voltage, current, pressure and temperature

Post by gary_f » Mon Aug 25, 2008 4:43 am

That's the advice I was looking for, thanks. I'll try glow cleaning for awhile. A few weeks ago, I was working with a smaller chamber and had a 2 stage metering valve I used to control the pressure (and I didn't have this trouble). When I went to a larger chamber, I left out the metering valve, thinking it was unnecessary if I just ran the pump for an hour or so. I guess you need the heat to fully dry it out.

But a larger question here is, according to the ideal gas laws pressure is proportional to temperature for a fixed volume. When the gas ionizes, it suddenly goes from room temperature to millions of degrees! I'm sure it's no longer an ideal gas, but wouldn't you think that this by itself would cause the pressure to go up?

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Chris Bradley
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Re: voltage, current, pressure and temperature

Post by Chris Bradley » Mon Aug 25, 2008 9:13 am

Only a very small fraction of the gas ionises when a glow discharge initiates. Temperature is the ensemble average of the energy of the ions and the still-neutral gas, so there is no sudden ramp-up of temperature/pressure.

However, as already accurately discussed, outgassing will kick off. Once you form ions/electrons, they will then be accelerated to much higher energies than just their ionisation energies by the electric field. They'll then wham into the surfaces, liberating all sorts of materials through ablation/sputtering processes. Hence, you'll get a ramp-up in pressure once you initiate a breakdown gas discharge.

It is true that an individual ion may have an energy state 'equivalent to' millions of degrees. In fact, an individual ion being accelerated in a fusor with 90kV drive will have 'an equivalent temperature' of a billion degrees!! But there is no way the insides of a fusor gets to this 'temperature'. It is only a tiny, tiny population of fast, 'hot' particles that have this energy, and it is these that do the fusing.

A fusor is a much much 'cooler' device than a 'thermonuclear' magnetic device like JET/ITER, but conversely it has ions that are much MORE energetic than the thermalised plasma in those devices, it just doesn't have many of them!

best regards,

Chris MB.

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Re: voltage, current, pressure and temperature

Post by Fusion » Mon Aug 25, 2008 2:13 pm

You could install serially an RC and discharge the capacitor using spark gap or rotating switch, then you limit current to 20kV/resistance and also the output power.
Another way to limit output power is using a lamp serially with the high voltage transformer at 110 or 220V input

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Richard Hull
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Re: voltage, current, pressure and temperature

Post by Richard Hull » Mon Aug 25, 2008 5:49 pm

I could tell you had the full experience. The long and short of it is that your vacuum is good and you are ready to do fusion.

As you admit deuterium your will raise the pressure and smooth glow will occur at a lower voltage.

The art is in running the device long enough with deuterium pressures of between 8 and 16 microns. and working the voltage up to 20-30 kv at a stable pressure within these limits where the current is moderate. It is all art and science mixed.

Explanation of theory and such is just great, but you have to be there doing it before genuine appreciation for the gas laws and gotcha's of ionization at "the edge" lodges into the inner fabric of the psyche. From this point on you own a knowledge only glimpsed at by the theorizers.

To get a non-fusing fusor to high voltages that are stable, you need to slightly squeeze off the pump and start admitting air or some other gas, just as in the case of deuterium.

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.

DaveC
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Re: voltage, current, pressure and temperature

Post by DaveC » Mon Aug 25, 2008 10:00 pm

Gary -

At pressures in the micron range, you need a ballast resistance in cathode circuit, to limit the input current. When the gas density is high enough, ion-electron multiplication is a major factor, which results in a small current growing exponentially in magnitude across the interelectrode space.

If I understand your description of the plasma conditions correctly, this is what you are experiencing. The ballast resistor will solve this while the system cleans up.

Assume an actual anode cathode voltage during glow discharge conditions of a few hundred volts... we often use about 300 - 400. The ballast throttles the current back by dropping all the rest of the supply potential down to the low voltage. It will dissipate virtually all of the input energy and get hot. A fan is good here.

Once the system is clean, it will stabilize and the base pressure will drop. Then when you add Deuterium, the pressure will climb only slightly and stabilize again.

If you had an RGA (Residual Gas Analyzer) on the system, you would see that under the "stable" conditions, most of the water has been driven off. But it doesn't happen in 5 minutes...takes a few hours, minimum. And everytime the system is vented, unless you do it with dry nitrogen (or Argon). You will have some conditioning to do again.

If it is actually opened to air... you have the whole process to repeat.

Dave Cooper

gary_f
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Re: voltage, current, pressure and temperature

Post by gary_f » Mon Aug 25, 2008 11:43 pm

Thanks everyone! It's pretty clear now that I have more cleaning to do. I also see that nudging up from below is better than forcing from the top. As Richard put it, a feeling for conditions at "the edge" is starting to sink in.

Dave, I'm using a ballast resistor, but I should probably increase the ohms.

BTW, I've found this little wattmeter, sold by Harbor Freight ($29), to be more useful than separately watching current and voltage. I just plug my Variac into it.

http://www.harborfreight.com/cpi/ctaf/d ... mber=93519

gary_f
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Re: voltage, current, pressure and temperature

Post by gary_f » Mon Aug 25, 2008 11:46 pm

I posted this out of order above, so I'll repost.

Thanks everyone! It's pretty clear now that I have more cleaning to do. I also see that nudging up from below is better than forcing from the top. As Richard put it, a feeling for conditions at "the edge" is starting to sink in.

Dave, I'm using a ballast resistor, but I should probably increase the ohms.

BTW, I've found this little wattmeter, sold by Harbor Freight ($29), to be more useful than separately watching current and voltage. I just plug my Variac into it.

http://www.harborfreight.com/cpi/ctaf/d ... mber=93519

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