Condition: Thinest Atmosphere Possible

7 08 2011

All photos.

My goal today was to find the thinest atmosphere that still creates a plasma.

The oscilloscope attached to the langmuir probe reveals this behavior transition around 8 millitorr:

Above 8 millitorr the plasma is relatively stable. As the pressure drops below 8 millitorr, the behavior transitions to extreme spikes in floating potential. The electron gun is at 25KV, current limited to 1mA.

This transition is not apparent from watching the plasma or power supply. New information!

As the pressure continues to drop, the spikes become less frequent and more extreme.

Until eventually I heard a rather loud pop…. which blew out the trigger circuit on my coil power supply!

I’ll have to repaire the coil power supply now. I must point out the bleed resistors did their job and slowly discharged the coil capacitors. Crucial safety feature.

So we learned something and I broke something. Good day.

UPDATE: A continuity test of the coil failed. The coil is damaged. The coil power supply is fine

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12 responses

7 08 2011
yonemoto

Presumably the dielectric of air is not quite the dielectric of hydrogen/deuterium so do you forsee repeating the experiment with plasmas of those gases?

8 08 2011
Mark Suppes

I have deuterium on hand, so eventually yes.

7 08 2011
Raymond Rogers

Do you have an electronic record of the readings?

Ray

8 08 2011
Mark Suppes

Only the oscilloscope captures shown.

8 08 2011
Raymond Rogers

Ah well; it just looks like some pictures from my study of Chaos Theory, Including the first quasi-period of three. The theory of Chaos (as presently formulated) is not chaotic; but follows certain patterns.
Ray

8 08 2011
chrismb

Mark, throwing ideas into the pool; looks like you might be generating a local postive corona around your passive probe, which (to the surrounding negatively charged plasma) is looking like a +HV spike to the surrounding electron-stimulated region. Positive coronas work by pulsing, just like this. First you split up the local gas into ions and electrons, the ions push off, the electric field collapses, leaving negative ions behind which then re-ionise the surrounding, then ‘repeat step 1’. This usually runs around unit MHz range, just like your scope trace. I’ll see if I can post in a reference to a paper on positive corona so you can see what I mean.

Try appying a negative voltage to the probe and if you find the pulses diminuish and disappear, then I would personally judge [from my own experiments] you are probably showing your local plasma potential to be a few 100 V to a kV or so (which’d be quite cool, actually, because it takes a bit of doing to drive a plasma to that kind of potential). I expect if you dial up -500V on your probe, it’d all disappear. (Couple your scope capacitively to see this, if you’ve not already done so.) If that’s so, then perhaps the plasma took your coils up to a few hundred volts and fed back to the supply. Maybe sitck a TVS/MOV or two between coils and ground to stop that risk again (and also to reduce risks of high voltages from inductive effects when you are firing).

There’s my guess work/arm-chair theorising on this one!

Is your scope set on AC coupling, or have you already isolated it capacitively?

8 08 2011
Mark Suppes

For this run I had the oscilloscope set to DC coupling.

8 08 2011
chrismb

Sorry if I missed your setup decription elsewhere. Do you mean it is straight off the probe into the oscilloscope, or via some resistance splitting, or other?

8 08 2011
Mark Suppes

The probe is directly connected to the langmuir probe, and the channel is set to DC coupling.

8 08 2011
chrismb

You’re braver than I am to hook a scope straight to an electrode in a plasma!!

I’m not entirely sure what you are measuring, then, because [I presume] you’re measuring something in the chamber due to charging up of your probe, but that would likely be a high voltage and very low current, so how is the impedance and capacitance of your scope input affecting what you are measuring?

8 08 2011
Mark Suppes

I’m using a HV probe for this. Good up to 2.5KV

8 08 2011
chrismb

Sorry, I misunderstood. That makes more sense!! What is the attenuation ratio, and do you know its capacitance, and/or rated rise time or bandwidth?

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