Vacuum Levels and Plasma Extinguishment

[Dave Xanatos]

Hi,

Referencing the image attached, I am using a pair of Edwards Wide Range Gauges, for which the voltage-output to pressure-measured graph is provided in their manual. The output stabilizes at 6.36 volts after about 10 minutes (90% of the vacuum is achieved within the first minute or two).

According to the Edwards WRG chart, 6.36 volts corresponds to 3.5 microns, or 3.5 miliTorr. Am I reading that correctly? (Blue cross-hairs on graph).

In the upper right corner of the graph I have superimposed Paschen's curves for voltage vs. pressure for various gasses, and scaled the graphic so their pressure axis matches the pressure axis of the WRG Output chart.

The behavior of the plasma, which extinguishes at right around 7.8 Volts, seems to match the expected data in Paschen's Curves.

My basic question is: Am I reading all of this correctly, given the measured voltages on my two WRGs? Does my placement of the Paschen Curve graph appear to be proper with regards to the WRG graph?

Just verifying things I have been assuming for a while now without confirmation... I want to be sure I'm getting and using good numbers before I continue further down this line.

Thanks,

Dave

[Steven Sesselmann]

Dave,

If I am reading the chart correctly 1 micron should be 1E-6 so according to the chart the range 3-4 micron should be arond 4V from the WRG.

Your cross hair indicates a pressure somewhere around 0.005 Torr.

Steven

[Dave Xanatos]

So I think my understanding of microns vs. militorr is incorrect. 0.005 Torr is 5 miliTorr. Zooming in on the graph to position the crosshairs, it's less than .005 Torr, reading the divisions I get .0035 Torr.

As for Microns vs. miliTorr - 1 micron would then be 0.001 miliTorr, correct?

Thanks.

[Steven Sesselmann]

Dave,

Maybe I was reading the chart wrong, it's confusing, I think you are right, the Torr line terminates at the top of the chart at around 760 Torr which is 1 Atm, that would make the 10E-3 1 millitorr.

Unusual to see gauge charts such large range.

Steven

[Dave Xanatos]

The Edwards Wide Range Gauges combine two sensors to provide the range. I like them a lot for that reason.

So am I correct that, given the graph and output voltage, I appear to have a 3.5 mTorr vacuum level?

Thanks!

[Jerry Biehler]

Don't use the graph, the equation to convert voltage to pressure is right in the manual, use that.

[Rex Allers]

Dave,

Looking at your chart where you placed the Paschen curves on your pressure chart caused me to think again about something I've never understood. I still haven't set up a fusor, myself, not even a demo one. Anyway let me try to say what I think regarding your graphics and then add my own question about Paschen curves and Fusors.

I think you are using the gauge's voltage to pressure graph correctly but just a little inaccurately. After Jerry's reply I searched and found a manual for one of these gauges. As Jerry mentioned there are formulas for each of these lines. The equation for Torr is:
P = 10^(1.5 * V - 12.125)

So for your measurement of 6.36 V the pressure result is 2.6E-3 Torr. So your reading of 3.5E-3 Torr is a little off. I think I see part of the problem. It looks like you added tick marks to the bottom scale, but the ticks are linear and the scale is logarithmic so not helpful for reading a value. I measured the fraction of the decade where your vertical line is and converted it to log. I get about 2.4E-3 T, so very close. I think accurately drawn, the line should be slightly more to the right.

So I think the real pressure value should be 2.6E-3 Torr. This also assumes the gauge is properly calibrated. I skimmed that there were some procedures for that in the manual.

Now on to the Paschen Curves. It looks like you scaled the inserted paschen chart so the bottom scale decades match the gauge pressure decades. That looks to be done right, but there is a problem. You treated the bottom scale of the Paschen chart as if it was pressure, but it isn't. It is pd or pressure times distance, where the distance is the spacing of the electrodes. Let's say we have an 8" diameter chamber and a grid 1" in diameter. The radial distance between the grid and the shell is about 3.5" or a little less than 9 cm. The units on the bottom of the Paschen chart are [Torr cm]. The distance is fixed, so to map the Paschen chart on the pressure chart, it should be shifted almost a decade to the left.

You seem to have placed the Paschen chart vertically so it is in the proximity of the diagonal gauge line. Of course it doesn't matter where it is placed vertically since the vertical scale for the gauge is the gauge voltage output, while the vertical Paschen scale the the high voltage between the electrodes.

In your post you said, "The behavior of the plasma, which extinguishes at right around 7.8 Volts, seems to match the expected data in Paschen's Curves." It took me a while to figure out what that meant. Then I noticed that 7.8V from the gauge represents a pressure which is in the vicinity of the paschen curves. I guess that's what you mean but that seems like a coincidence to my understanding of the Paschen curves. Then again, the implications of what I think I understand don't really make sense to me either.

As I understand these curves, a particular operating pressure and electrode distance gives a pd point. This will form a vertical line on the Paschen chart. If the vertical line that represents this value intersects with the curve, the vertical value of this point represents the minimum value of the HV needed to start a discharge. This seems to say you can have a discharge if the (pd, V) operating point is on or above the curve line. (Above also is to the right of the more vertical portion of the curve.) This would also imply that if the operating point is below (or to the left of) the curve, there would be no discharge.

*My problem / question*
Reading Richard's guide to operating a fusor, it is my understanding that the typical operating pressure, with D2 flowing for fusion, should be around 6-8 microns (or milliTorr). For an 8" fusor, this would imply a pd of around 7E-2 Torr cm. Looking at a typical chart of Paschen curves on the wiki page for "Paschen's Law", this value is way to the left of whole chart. This seems to imply no plasma.

Have I gotten something very wrong in my understanding of Paschen curves or my numerical estimations?

[Rich Feldman]

Rex beat me to it, about reading the logarithmic horizontal scale. Must be another night owl or very early bird.

[Dave Xanatos]

I love this place!

Hadn't really considered the logarithmic scale issue when I added in the tickmarks - excellent catch, thank you!

I had forgotten about the formula and looked that up last night following Jerry's post, so was happy to see in the post from Rex that I calculated it correctly to find 2.6 mTorr from my voltage of 6.36v on the gauge.

My only calibrations thus far have been to set atmosphere at... atmosphere. Both gauges seem to agree on voltage outputs within a few miliVolts, so I am accepting of their calibrations for the purposes of my fusor at this point.

It may be purely coincidental, as was mentioned, that my voltage output of 7.8 at plasma extinguishment roughly matches up with where the curves would show extinguishment - but it is an interesting coincidence! While I was aware that the vertical scale would be arbitrary on my superimposed mashup, I was not thinking of the distance component on the horizontal scale, and treated it as pressure only. That said, though, happenstance can at times be fortunate, and the behavior of the plasma at the gauge output voltages covered by the Curves Graph seems remarkably consistent with those curves!

And thanks, Rich, for the really nice logarithmic graph which I will keep as my quick reference graph from now on, much appreciated.

Thanks very much everybody for helping clear away some of the misunderstandings or misuses of knowledge here. The electronics side is my familiar neighborhood.... the vacuum side is still mysterious and full of discoveries for me!

Dave

[Rich Feldman]

Dave,
Apparently you have a computer. If you don't have a spreadsheet calculator program, you need to get one.

Here is a lookup table based on the same formula.
It has all the resolution needed here, and demonstrates three styles of stating pressure numbers.

w_r_table.JPG (38.64 KiB) Viewed 5855 times

The table and the semilogarithmic chart were drawn using MS Excel. Many days I use that program more than the Internet browser. You can do the same tasks using free, share, open-source type programs.

[soapbox]
Proper capitalization of units is often confusing.

The names of all scientific units of measurement are ordinary nouns. In English they are capitalized only when they begin a sentence, appear in a title, etc. Multiplier prefixes like milli, kilo, and mega don't affect the capitalization. Millihenry, kilovolt, kilopascal. Proper English has no place for capital letters in the middle of words, except perhaps for a recent fad in the names of companies and products, and of course in many non-Anglo proper names.

In SI, and more broadly, each unit comes with an official symbol. Capital letters in symbols generally mean the unit name comes from a person's name. mH, kV, kPa, ms, mS. The last example symbol stands for millisiemens, the conductance of a 1 kΩ resistor.

Torr stands in a gray area. Used as a unit name, it should be all lowercase: torr, millitorr.
But it has no symbol with fewer than four letters.
Some editors and instrument makers use Torr and mTorr in places where a symbol could appear.
It's debatable. http://www.eoearth.org/view/article/155345/ says:
There is no consensus in the technical literature about whether the name/unit torr should begin with a capitol[sic] letter - as in "Torr" or simply "torr". Both the United Kingdom's National Physical Laboratory (see Pressure Units) and New Zealand's Measurement Standards Laboratory (see Barometric Pressure Units) use "torr" as the name and as the symbol. An extensive search of the website of the U.S. National Institute of Standards and Technology (NIST) found no such clear-cut definitions. Therefore, this table uses "torr" as both the name and the symbol.
[/soapbox]

[John Futter]

Torr should be a used with a Capital T after Torricelli the father of Vacuum and inventor of the Barometer
one of his famous sayings
"horrors et vacuui" after a vacuum implosion killed one of his servants

[Rich Feldman]

With utmost respect for John F and Mr. Torricelli, and the unfortunate 17th century technician, I beg to differ.

When we write about newtons and pascals, hertzes, volts and amperes, farads and henries, etc., we don't use capitals. Except when the names are replaced by symbols: N, kPa, GHz, V, A, F, nH. Vacuum is great, but why should its traditional unit be treated differently?

John and I stand on the same ground, figuratively, that Torr is acceptable.
In fact, the regular spelling of Torr the symbol needs a capital T. That makes mTorr also OK.

I still maintain that torr and millitorr are OK, mtorr should be regarded as wrong, and milliTorr is abominable.

[Dave Xanatos]

Dave,
Apparently you have a computer. If you don't have a spreadsheet calculator program, you need to get one.

...

I tend to script things manually... below is a script that outputs the pressure for every 0.01V increment... Copy it, save it as something.htm and open it with a browser.

Code: Select all

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>Untitled Document</title>
</head>

<body>

<table width=300 bgcolor="#cccccc" cellpadding=4 cellspacing=1>
<tr><td width=100 bgcolor="#ffeedd"><b>Voltage</b></td><td width=200 bgcolor="#ffeedd"><b>Vacuum (Torr)</b></td>
<script>


for (v = 800; v > 500; v--) {
	P = ((1.5 * (v/100)) - 12.125)
	x = Math.pow(10,P)
	x = Math.round(x*10000000)/10000000
	document.write("<tr><td bgcolor='#ffffff'>" + (v/100).toFixed(2) + "</td><td bgcolor='#ffffff'>" + x + "</td></tr>")
}

</script>
</table>

</body>
</html>

[soapbox commentary]

To each, their own

For me: mA, mV, mTorr, mH, GHz... I'm weird... it's a respect thing for me to try to remember to capitalize the letter of things named after people who spent their life working to leave behind the building blocks of our knowledge. Hertz, Pascal, Volta, Ampere, Henry, Torricelli, Faraday, Newton, Tesla... these are the giants upon whose shoulders we stand to gain a higher view than they could in their time. I like to remember that behind every mA, GHz, mTorr, there stands a man whose life work allowed us to know what that was, exactly.

[/soapbox commentary]

[Rich Feldman]

Wow, Dave, you just helped me to do my first ever HTML script work.
Thanks!
Your program seems too small & clear to be hiding malware, and it worked here on the first try:

Then I successfully tweaked it to reduce the X and Y resolution. Went online & learned how to insert a comment in the source code. The result:

If you composed the program in something fancier than a plain text editor, I'd love to see how it looks in the UI.

[Dave Xanatos]

Happy to help! I designed websites for nearly 20 years... did a lot of custom scripting. I just used a text editor for that... looked just like it does in your editor.

Happy Scripting!

[Rex Allers]

When I posted a reply to Dave on Tue Apr 26, 2016 11:50 am, I also asked a question that nobody commented on.

Basically, what I understand as normal operating pressure for a fusor seems to be below any part of Paschen curves that I have seen. I was hoping that someone would tell me I got something wrong or explain how Paschen curves and fusor operating points relate.

For easy reference, here is a copy of what I posted earlier...

-----
As I understand these curves, a particular operating pressure and electrode distance gives a pd point. This will form a vertical line on the Paschen chart. If the vertical line that represents this value intersects with the curve, the vertical value of this point represents the minimum value of the HV needed to start a discharge. This seems to say you can have a discharge if the (pd, V) operating point is on or above the curve line. (Above also is to the right of the more vertical portion of the curve.) This would also imply that if the operating point is below (or to the left of) the curve, there would be no discharge.

*My problem / question*
Reading Richard's guide to operating a fusor, it is my understanding that the typical operating pressure, with D2 flowing for fusion, should be around 6-8 microns (or mTorr). For an 8" fusor, this would imply a pd of around 7E-2 Torr cm. Looking at a typical chart of Paschen curves on the wiki page for "Paschen's Law", this value is way to the left of whole chart. This seems to imply no plasma.

Have I gotten something very wrong in my understanding of Paschen curves or my numerical estimations?

[Richard Hull]

The pressure it definitely and always preciesly what it is when the fusor is fusing. It can fuse over a very wide range, as I have noted. No one fuses at much over 25 genuine microns and only a tiny amount fusion is registered just below extinction. In short, if you are having problems, you are having problems because you or your gear is doing something wrong.

An old time fusioneer can be in the presence of a fusor that is misbehaving and immediately know what the issue is. Newbies are famous for not fully expressing things to the enth degree and typically have poor to non-useful supplies, miscal'd instruments, terrible neutron detction gear, etc.

A willing, would-be fusioneer might spend several weeks or months figuring out the sweet spot for their particluar fusor. VCU and the fusor team, here, had a terrible time solely due to a super professional $4500.00 high voltage supply that was current limited at 10ma. Current limiting, in a fusor supply, is like catching a bullet in the foot. Millisecond-plus, un-observable current excursions are extremely common in an amateur fusor. They absolutely doom a current limited HV supply to cutting off everytime you think you are close.

A full 9 ma at a "useful voltage" will fuse anywhere...anytime, in any fusor if the pressure is right and the chamber conditioned. All of this assumes a very sensitive and efficient neutron counter is at hand and the system can operate over a full ten minutes at this level. A stumbling newbie becomes a confident fusioneer only when all the foregoing are stable and under full control. It is an art and not a calibrated science. The fusor is far too crude, too simple and working in a ragged edged "no man's land" of pressure voltage and current within a leaking gas, "vacuum like" environment.

I spent one full year with two demo fusors, I & II, (1997-1998), learning all about what lights plasma in vacuuo and what doesn't, at what voltages and what pressures success was to be had. I didn't care about the Paschen curve or its failure to meet any sort of theoretical or empirical specifications related to my efforts. I learned by doing....constant doing. it's all about the journey and not the arrival.

Richard Hull

[Rex Allers]

Richard, thanks for another great discussion of the realities of things that might matter. I think I was aware of the stuff you just described from reading several earlier posts. Unless anyone else adds anything enlightening, I guess I'll go back to not worrying about why Paschen curves don't seem to make sense to me at pressure and voltage points where I think a fusor typically runs.

If I eventually plod my way to actually assembling stuff and trying it, I was planning to spend a bunch of time with air, and then possibly hydrogen, before I start wasting any precious deuterium.

[Richard Hull]

In the doing there are no secrets, just discoveries that lock into your gut far more intensely than a thousand words on a written page. Do not think I abhor reading. Quite the contrary. I find reading is the very pathway to doing things right and staying out of major trouble and danger in this business. Reading tells you what others have done and discovered. Based on reading you need to be doing and discovering along those lines for yourself.

Von Engels book "Ionized Gases" was my early guide. He was wise enough and experienced enough that after giving valuable equations and data related to ionized gases would quickly note that the equations only held for simple, rigidly controlled and narrow range conditions. In short, they were idealized. Very few, if any, amateurs on a limited budget, can support or hope to maintain rigidly conrolled conditions where curves and computations will prove spot on. This is especially true in the finely tuned, "bleeding edge" conditions where the fusor actually does fusion. You pay for its abject simplicity with having to have 5 arms and two brains all working at one time to find the sweet spot. Fortunately, once found, you OWN the klnowledge to bring to heel any fusor in future and from that point on, only one brain and a pair of arms suffice just fine.

Richard Hull