Langmuir Probe Creation and Design

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bk8509a
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Langmuir Probe Creation and Design

Post by bk8509a »

In advance: This is going to be a long post, but pretty much a intro on how to build a Langmuir probe. As I do the rest, it will follow.

I browsed around on the site to see if anyone here has produced a Langmuir probe. There were a few 'don't try it wont works' and thats about it. To my knowledge, I don't think anyone has done this yet here. Its simple and cheap, and that's what we are all about right?

I started reading a few papers on the theory of Langmuir probes. Then I figured that I wanted instantaneous measurement, so I got a paper " Sin-Li Chen and T. Sekiguchi, J. Applied Phys. 36, 2363 (1965)" which is on the Wikipedia site for Langmuir probes. For anyone interested in doing this, start there.

To the meat:
The design is based around a 4 BNC feed through which is attached to a 6" to 2.75" zero length reducer flange. Each BNC tip has a beryllium copper wire grabber on it. Three of these be attached to the three wires from my three probe tips.

The probe itself is made out of .187" diameter alumina tube that is 5" long. The tube has 4 bores in it which are each .047" in diameter. Down three of the bores are .047" diameter pure tungsten welding rods. They are attached to the butt end of the alumina with high vacuum epoxy.

The probe is attached to the zero length reducing flange via a home made aluminum shield which will protect the connecting wires and also the exposed conductors from creating a false reading. The aluminum housing holds the probe with a set screw and is attached to the zero length reducing flange via holes we've tapped into the flange.

The design of the probe allows a melted/cracked/destroyed probe tip to be unscrewed and replaced for around 10 dollars.

I hope that the probe will be in by today and taking measurements of the ebeam in the end of jet mode next week. I will post my data from those runs.

If you are interested, don't hesitate to ask. Criticism is also encouraged, as that is how we learn.
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Chris Bradley
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Re: Langmuir Probe Creation and Design

Post by Chris Bradley »

I had rather presumed some had tried this, but that there's not much to measure in a fusor.

I have tried out similarly with my ion magnetron "crimson doughnuts" and I have obtained some degree of consistency in the results, though I think with any experiments like this there is the question of whether Langmuir probe theory applies. For mine, possible non-neutrality and the magnetic field are going to throw up plenty of question marks over any results. In a fusor I don't really see what you would be measuring, maybe the central plasmoid or the general distribution of the background. The beams themselves are all [likely to be, in any simple analysis] rather tenuous to say the least, and I can't see accelerating electrons already headed for the shell (/ions towards the centre) to be hanging around a Langmuir probe for long enough to slow them up and measure a saturation current.

But we'll see!! Experimentation is experimentation, after all. Sometimes the theory just has to tag along *behind* the experiment to explain what is observed. This is true of plasma science more than most! We look forward to seeing your results!


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Re: Langmuir Probe Creation and Design

Post by Chris Bradley »

I'm currently experimenting with ways to make robust Langmuir probes, myself, and I have a choice of tip materials. I was tending to stick with stainless steel as it has good all round properties at an insignificant price. I have pure tungsten (not welding rods) and was considering using this, but I'd have thought it more likely to create an oxide layer on the tip, compromising the results? Or would a low-sputtering option like aluminium be preferred, which surely will form an oxide layer but that will be 'blasted off' quickly?

I'm gonna stick with SS, unless anyone has good, knowledgeable advice on this.
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Re: Langmuir Probe Creation and Design

Post by bk8509a »

Wouldn't the oxide layer immediately get sputtered off? All the literature I've read has the probe with a Tungsten tip.
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Re: Langmuir Probe Creation and Design

Post by DaveC »

You should have some interesting experiences with your measurements.

My only thoughts on the process are first one needs to keep the mass (physical size and area) small if you are going to poke the probe into the ray.

More important to the answer you may find, is what voltage you allow the sensor to reach.. If the probe is a ground potential, (same as the shell) then it would more less strongly affect the electric field geometries depending on its position.

If the probe is at other potential and polarities, ( plus attracts electrons, and negative will collect ions.) it will collect currents of whatever is flowing. I have seen rather small probe voltages cause quite large changes in pickup current.

[Edit: Above to replace electrons with ions.]

You can of course, use this to construct an "energy spectrum" of the particles in the rays.

Since the rays are glowing, one would expect ions and electrons. So appropriate voltages on the probe should collect current of the appropriate polarities.

If you wish to poke around in close to the grid, the probe will need to be biased. Some times it may be that probe bias voltage can be controlled to give zero current pickup, thus identifying the exact voltage in the ray at that point. A closed loop controlled low power voltage multiplier is probably the simplest way to go there.

It will be interesting to see your results.


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Re: Langmuir Probe Creation and Design

Post by Richard Hull »

This is an ancient endeavor with the fusor and goes back to the original ITT effort in 1960-68.

The spherical focus nature of the device sort of obviates this approach in crticial data gathering as the introduction of a probe distorts that which is being measured. Other methods of probing and data collection in zones of the fusor have been cleverly tackled by Miley's group, Urbanna and others at Univ. of Wisconsin.

A check of the literature and search of the past URL links on this material will speak to these issues.

Richard Hull
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Re: Langmuir Probe Creation and Design

Post by Hector »

I tend to agree with Richard that the probe will interfere with the fusor plasma process and thus give less than true measurement of the plasma environment. This is why you never see the University teams used them on fusors.

Don't get me wrong, you never really know until you try, but you might want to consider using Laser Induced Fluorescence (LIF) instead, because it's far less disruptive to the plasma process of the fusor than any other probing method.

I have a Thesis done by a researcher in the 90's that used this method with good success to map the potential wells inside a fusor. I believe Dr. Millie and team at Wisconsin have done the same.

Just a suggestions, like I said you never really know until you try.

Good luck.
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Re: Langmuir Probe Creation and Design

Post by bk8509a »

My first series of data is in. I've taken measurements with a probe an inch outside of my outer, grounded grid. I wasn't sure of what voltages I was going to find or if the probe was going to get too hot, so I put it in the safest place in the fusor. The measurements are interesting, to say the least.

The most surprising thing for me is that electron temperature goes down as the voltage increases, until the x rays start hitting the tungsten and creating a voltage that way. As of now, I believe that the temperature decreases due to the fact that the probe is outside of the grounded grid, but i'm really not sure why this is happening. I have to do a few more tests. For all I know, the probe is not set up properly, or I'm not following the theory from the triple probe 100%, but I'm almost sure its working due to the x-ray excitation of electrons at 10kV (and the fact that its a super simple system).

Another disheartening problem is that as I increase "V d3", the curve should stay the same. It doesn't, which I'm blaming to the non-uniformity of the electrons coming out of the center of the fusor. As the Vd3 term goes up, i'm guessing the probe can pick up more electrons gaining a better average. I'm suspecting that the curve will converge around Vd3=50 V as it will be able to pull a nice average set of electrons.

A few other weird points. Check out that little bump? Where the hell is that coming from?! It happens around 17 mTorr (5KV) each time. That's right when my e beam jet is fading.

Thoughts as right now: the electron temperature increases RAPIDLY in the lower voltages, which I thought it would, but then it declines? WHY? Remember, this is not in the poissor, but outside of the grounded outer grid.

All theories accepted.

BTW, the triple probe paper I went off was

Chen, Sin-Li, and T. Sekiguchi. ``Instantaneous Direct-Display System of Plasma Parameters by Means of Triple Probe.'' Journal of Applied Physics 36.8 (1965): 2363-375. Print.

Would post it, but its too big.
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Re: Langmuir Probe Creation and Design

Post by Dustinit »

The clue lies in your last plot.
The voltage on the probe determines what range of energy electrons can have that it will detect. As the voltage on your probe is quite low it can only detect low energy electrons. Fast electrons will whizz past without noticing the small field of your probe. As your fusor volts increase so does the average energy of the electrons so you detect less. It only starts to increase again because electrons hit your outer grid releasing lower energy secondary electrons which your probe will detect. In order to measure the higher energy electrons you have to increase the voltage on your probe.
Your probe will only detect electrons of lower energy than the probe voltage.
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Chris Bradley
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Re: Langmuir Probe Creation and Design

Post by Chris Bradley »

I'd be interested to see the raw data of your probe measurements. Can you post up a typical set of measurements from your runs (in linear(current) and ln(current))? Did you get to a steady ion saturation current?
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Re: Langmuir Probe Creation and Design

Post by bk8509a »

Chris, the probe is a triple probe and only needs current if you want to determine electron density. I have no electron saturation current, ion saturation current, or any graphs that I need pick data off of. I get a straight number out, every half second.

I'm using equation 16 of the paper to do all my calcs, which is non-solvable with algebraic methods. I end up making LabView do Newton–Raphson on it.

An average data run is attached in Excel.

The article i'm using for my triple probe is attached. I couldn't post the pdf (1.3 MB)....

The theory is completely different that regular Langmuir probe theory.
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Chris Bradley
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Re: Langmuir Probe Creation and Design

Post by Chris Bradley »

Thanks for posting up the paper, though my computer was struggling with those png images. (png?!) Are there any more pages?

I converted to pdf so I can view it... maybe it'll also be useful for others in pdf... and will study on it further, later..

view.php?bn=fusor_files&bn=fusor_files&key=1272034039 (BROKEN LINK)
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Re: Langmuir Probe Creation and Design

Post by bk8509a »

Chris,

The whole paper is on the files forum now.
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Re: Langmuir Probe Creation and Design

Post by Doug Coulter »

I've done a very tiny amount of probing myself, nothing so pretty yet though.

I would suggest a couple possible improvements though (though I may be talking out of the wrong orifice since I've not duped what you did ;~)

One is that I have some of that 4 bore ceramic, and it's electrically pretty leaky, particularly when warm. This will affect a number of things, including the electric field far from the tip -- which may distort your measurements a good bit. Quartz is many orders of magnitude better for this kind of thing -- but I doubt you find it in a 4 bore tube, so it'd have to be larger...dunno which effect would be most important here. I'm thinking more of conduction to the plasma outside the main tube along the sides, not so much wire to wire leakage, but both would be fairly high once warm for the tiny signals involved.

Next, I'd use something other than tungsten -- something with a higher work function that won't have big errors due to secondary emissions. Ti perhaps -- it's cheap, anyway (McMaster of course).

Third, nothing beats being able to move the thing while it's working. A wobble stick or a push-pull twist adapter with this on the end would teach you a ton in very little time. It's just not the same having to tear down the system to put it in a new place -- the conditions won't otherwise be the same, and you might actually be able to see any leakage field of this distorting the visible plasma, which itself will tell you a lot diagnostically. As in, is my probe changing the thing I'm trying to measure too much?

I am lucky to have both a wobble stick and a push-pull twist for 2.75 CF here and they've sure taught me a lot -- fast. Even with a dumb pinhole camera for charged particles and X rays there were some surprising results as the thing looked from different angles, which I am still digesting, but it is definitely nicer to have things to ponder than not to have a clue!

For probe kinds of things, I just run the wires down the motion stick to another feedthrough for the signals....in my case I can do this "far from the action" as my fusor is a cylinder running in a 6" ID sidearm off a 14" diameter main tank, which helps for things like that. Plenty of room for other junk in the main tank that doesn't affect the main fusor operation much if at all.

Just my $.02.
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Carl Willis
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Re: Langmuir Probe Creation and Design

Post by Carl Willis »

Hi Brian,

This is a very good experiment. I haven't had time to digest it fully, but it's a well-documented effort that adds a new technique to the community repertoire, and the results appear to be very consistent between runs. Consistency is a challenge with fusors. As to what it means, or might mean, I have to consider later if I might have any observations on that front. In the meantime, I appreciate your contribution here and keep up the good work.

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Re: Langmuir Probe Creation and Design

Post by DaveC »

Hi Brian -

Very interesting results, to say the least. Also thanks for posting the paper.

Do you have a schematic of your measurement setup? One question the hopped off the page of the paper, was spawned by the phrase " floating" , which I loosely translate as meaning "isolated", as in not connected to anything at ground potential.

While an easy concept in theory, my own experience has been that this can be hard to accomplish experimentally. With electrodes surrounded by plasma, it seems hard at first glance to be able to even say the apparatus could be floating.

Experimental process here is very important.

Nonetheless, the very consistency of your data, suggests something real is going on.

I'd be delighted to see how you were able to do this. It's a very nice technique.

Best regards,

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Chris Bradley
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Re: Langmuir Probe Creation and Design

Post by Chris Bradley »

Thanks for uploading the whole paper. Not read it in depth yet, but my main question(s) arising so far is the envelope of applicability of this technique. I wasn't quite sure if I read it right but it seemed that there was an argument given over why the probes should be close together, but does this suggest, perhaps, that there is a limited range of applicable plasma parameters for the technique?

In the case of the fusor, in the position you are holding these things, I'd imagine the charge density to be sufficiently low such that the size of the probe sheaths are comparable with the probe separation. In fact, they might be very much of the same order, such that your measurements are actually some function of sheath thickness up until the overall charge density increases [due to more gross input power] quicker than the electron temperature increases, and the sheath thickness might then decrease.
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Re: Langmuir Probe Creation and Design

Post by bk8509a »

In advance, sorry for the long post. I will try to comment on all the great advice and then I'll talk to you about the newest problem that has come up.

Dustin,

I think you might be on to something about how I can't detect the whizzing by electrons. I have new data to refute that though.

Doug,

I looked for Ti rods on McMaster, the smallest they get is .062, which is .02 too big. I could use wire and straighten it, or try to find rods in a tinier diameter.

I've also looked into controlled movement of the whole device. I bought a 3'' bellows last week that I will eventually attach. These are just the fixed intro runs.

Carl,

The more I look at this data, I start to think, "It consistently means something, something that I have no clue about". I appreciate your praise of the experiment, as this is my precursor to grad school at Wisconsin Madison. Hopefully I can thrive there.

Dave,

I'm going to have my senior thesis on this website soon which will give diagrams, schematics, and other facts such as construction and experimental procedure on here. These are all important, I agree.

Also, in plasma physics, "floating plasma potential" means just hanging out in the plasma and taking on any potential that the plasma takes on. You cant get a true floating "ground" in plasma.

Chris,

Applicability is a huge concern of mine, as you will see from my next few pieces of data. Also, I'm doing the calcs on the Debye length and finding out that the density would have to be extremely low in these areas, with these temperatures, in order to have the sheaths not cross. You make a valid point.

NEW DATA:

Alright, so yesterday I decided that it would be a good idea to test to higher voltages of Vd3, to see if the temperature converged. This meant I had to use a different power supply than I used for the first couple of measurements. I got some results that are making me question certain parts of the experiment.

For the first series of measurements I used a Instek GPS-3030DD:

http://www.metrictest.com/product_info. ... -3030DD(N)

to apply Vd3. For the second series I used a OLD (but functional and accurate) tube power supply the Kepco ABC 200M:

http://www.photomachining.com/inventory ... 3102-L.JPG

The good thing is that the data taken with the GPS-3030DD was consistent with previous trials. But when the same Vd3 was applied to the Kepco 200M, the graph was similar, but not the same! Which power supply is telling the truth? What could be going on? To be honest, I feel that the tube is correct. The reason for this is I expect a RAPID increase in electron temperature as voltage goes up, the Kepco gives me this. I'm assuming those bumps in the beginning of all the graphs are just due to interference with the grounded grid.

There is a wild discrepancy between the two power supplies here (See picture 1). I know some of you are experts on this, so I expect some insight about this.

The new power supply made some new curves, all which are consistent (See Pics 1-5).

The comparison of these curves is shown in picture 6. The problem is, the more I increase Vd3 the higher the electron temperature goes. My high voltage supply should be making the electrons get fast, not the probe's voltage!

ANALYZING THE DATA:

As you can see in pictures 6, the curves maintain the bumps that were shown with the old power supply, but they have a high tail end. The tail end makes me think that the Kepco Power supply is giving an accurate reading. These electrons should be zooming around at -20000 V.

CONCLUSIONS ABOUT THE PROBE THUSFAR:

I believe its working, but there is some underlying error with the equipment such as the power supply. How do I know this? See picture 8 and 9. The curves are exponential and on track with the only calibration curve in the Chen Paper. Its definitely working.

PROBLEMS TO BE SOLVED:
-Figure out which power supply is working correctly.
-Firgure out why there is a discrepancy between power supplies.
-Understand why the bumps appear in all taken data (outer grid lensing?)
-Determine why the electron temperature goes up as Vd3 goes up in all taken data.

NEXT STEP:

I'm sticking this probe in the center of the fusor next. The plasma should be more uniform, allowing me to make some deep conclusions about whats going on outside of the outer grid, and if my probe is working.

Sorry for the long post, it helps me figure out whats going wrong/right, and helps you help me.

-BK
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Doug Coulter
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Re: Langmuir Probe Creation and Design

Post by Doug Coulter »

FYI, McMaster calls it "wire" below that size, and they go down to .004". I use the .032" stuff to make grids sometimes, and some times also the 20 mil thick plate for endcaps on the cylinder grids. I use the pure (not deliberately alloyed) grades always for things like this, they sputter very little and live long.

McMaster is a little strange that way -- for large clear mica sheets, you don't search for mica, you look for "india glass" instead....this kind of thing abounds there.

Whether it's a big effect or not for this (it has been here for probes) the alumina 4 core they have has roughly 10e4 times the leakage current as quartz does at a given temperature. That's a big one. It's even worse than 7740 (or K-33) pyrex glass, which is the most conductive glass in common use.

Ceramic has shown me enough leakage to wonder about an in-tank preamp on homebrew ion gages, it's pretty bad even when there's a lot thicker piece and a longer leakage path. Then I got into quartz...no problems now. The tiny multi bore ceramics are sold for thermocouple uses, where a fairly large amount of conductivity doesn't make any difference. I am using that for type C thermocouples with no problems, for example.

Very small quartz tubing, single core, is available at www.quartz.com/quartz.html -- down to .5mm ID by 1 mm OD, though you probably would want something a bit more thickwall for better insulation/standoff voltages. At those sizes, a bundle would fit in a larger piece of tubing which would still be pretty small. I like this outfit, and anybody who wants to go a group purchase from them can contact me, as I need to build an order to get to their discount threshold. I'm using it primarily for feedthroughs (tubing coupler or just glue through a hole with Hysol), as it has better performance for fusors than the commercial ones -- by far, and it's not just conductivity for that -- its comparative less trouble of being reduced by hot H, low dielectric constant, and low RF losses all at once that make it superior to the ceramics.

Keep those results coming! I get bored with this place when we don't do real experiments.

I'm pondering the paper and your results now, as they might dovetail with some of mine nicely.

But I'll wait to understand before comment (at least this once ;~).
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Re: Langmuir Probe Creation and Design

Post by Chris Bradley »

Brian Kelleher wrote:
> Alright, so yesterday I decided that it would be a good idea to test to higher voltages of Vd3, to see if the temperature converged. This meant I had to use a different power supply than I used for the first couple of measurements. I got some results that are making me question certain parts of the experiment.

I can imagine a host of issue that might lead to such effects; switch mode power supply effects, internal impedance, current feedback into the power supply from the plasma [current seepage through the earth connections] etc..

Unless there are issues I'm not seeing here, I would suggest as an initial check you stick a very large capacitor across the supply outputs and measure V across the cap with a trusted volt meter.

Not sure I agree with you about the Kepco supply. Remember you are likely measuring the state of the background medium, not the state of the beams. As far as I would presume, that'd mean you're only gonna measure a real think 'slice' of energy, so to speak, off the top of the fast electrons hammering their way outwards. I would tend to trust the linearity of results from your Instek, which generally extrapolate back to 0eV=0V drive (a know point!) from the higher values. Looks to me that, at that physical location, your readings from the higher drive potentials is showing a [background electron energy]:[beam electron energy] of around 1:1000 (given that the electrons where you are measuring is likely around a half-way up the drive potential).
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Re: Langmuir Probe Creation and Design

Post by Mike Beauford »

Hi Brian,

My bet is it's the impedance of the power supplies that is causing the curves to be different. I'm wondering if there is a way to massage the data to remove any type of impedance mis-match?

My 2 cents!

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Re: Langmuir Probe Creation and Design

Post by Richard Hull »

I'll go along with Brian on the impedance issue. Like Chris noted, put a large external capacitor across the supply output and do the work. I am with you that the old Kepco might be the correct supply to use here.

Richard Hull
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Re: Langmuir Probe Creation and Design

Post by bk8509a »

Center probing. I'll tell you guys how it goes in a few days.
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Re: Langmuir Probe Creation and Design

Post by DaveC »

Brian -

My question was not so much about what "floating" or "isolated" meant , but how you will or are making this measurement. In other words, How are YOU isolating your power supplies? (That's was behind the schematic request.)

Most low voltage power supplies will bring into the circuit the internal impedances to ground of the supplies. At low plasma potentials, this is rarely a problem. But at multi-kilovolt potentials, a low voltage power supply's insulation resistance will be broken down.

At times this issue is circumvented by using batteries in some sort of insulating housing, long range potential adjustments with plastic rods, voltmeters and ammeters surrounded by corona shields and possibly suspended by monofilament fishing lines, or supported on plasctic cups, ... you know the "good" experimental procedures that Feynmann seemed to appreciate most. (See his comments about experimental work at Princeton vs MIT in "Surely You're Joking Mr Feynmann")

Regarding your latest adventure with measurements at the center of fusor, these will bring the isolation issue into sharp focus, since the ion potentials would be expected to approach HV supply voltages.

So be careful about floating low voltage supplies here, you may breakdown the isolating resistance. A good HV isolating transformer (see one of the earlier threads) would work well to preserve the hardware.

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Re: Langmuir Probe Creation and Design

Post by bk8509a »

I will have a bigger post on this later. I'm writing my senior thesis right now. Best data yet. I'll let you have your opinions.
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