The "wire" ion source

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
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Richard Hull
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Re: The "wire" ion source (DATA)

Post by Richard Hull » Wed Nov 23, 2011 4:28 pm

Nice work Carl! Thanks for the report

I am just about to dismantle the fusor IV setup and I just might try the long wondered at W needle ionizers I have mused at for so long.

6 to 8 - .020" diameter W needle electrodes of about .5-inch length affixed to and about the interior shell.

I am sure the pressure voltage regime will be bizarre and all the normal operational skills are totally out of the window. Will fusion numbers alter? I guess we will see.

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Re: The "wire" ion source (DATA)

Post by Richard Hester » Wed Nov 23, 2011 10:32 pm

Four of these wire sources (long) arranged around a cylindrical inner grid might be interesting. I have appropriately sized tungsten wire. I envision 4 pieces of SS tubing opened out similar to the first pictures. but with a much longer wire. This is far easier than another scheme I was considering using heated tungsten filaments and a big solenoid to persuade the electrons to linger about and make ions.

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Re: The "wire" ion source - link

Post by Doug Coulter » Fri Nov 25, 2011 3:46 pm

Thanks for this, Carl, these look really interesting and worthwhile.

You can get the papers here:
http://www.coultersmithing.com/forums/v ... f=28&t=520
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Re: The "wire" ion source

Post by DaveC » Wed Nov 30, 2011 12:18 am

Just caught up with this thread. Very nice work Carl.

The Wire-Ion Plasma source (WIP gun) is a work horse method to get lots of electrons or ions fairly simply. Some good papers have been published by Hughes in the 60's and 70's.

The ion generating voltage is determined from the Paschen Curve for the Pressure-Distance value of the ionizer geometry and gas pressure. With the grounded screen, the exiting ions are then under the influence of the cathode (negative) field exclusively. And conversely, the ionizer is more or less free of any cathode potential influences.

With a current limited supply feeding the ionizer wire, you can measure ionizing voltage versus pressure and plot the "real" paschen curve for your particular configuration.

A WIP gun built at work a few years back had dual "several" by about 20 inch beams of He ions. It also used a fine tungsten wires such as the one you show. It was a pretty robust source. We used the configuration to make a secondary emission electron gun. Electron operating parameters were generally above 100 kV at around 100 mA.

Pressures were nominally in the -3 to -4 Torr range.

One down side is sputtering of the ground screen by the fraction of ions not exiting the source. Interestingly, cathode sputtering was not an issue. The high field probably causing all sputtered metal atoms to return to home quickly.

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Re: The "wire" ion source - link

Post by Dustin » Wed Nov 30, 2011 4:18 am

Nice work Carl.
Do you think it would be possible to increase efficiency by an irregular,
perhaps octagon outer wall?
This may reduce current consumption/ion current.
Steve.
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Re: The "wire" ion source - link

Post by DaveC » Wed Nov 30, 2011 10:08 pm

The field around the ionizing wire controls the emission.

The shell geometry figures only in a minor way, as long as we aren't talking about sharp edges, points and such. So the easiest outer container shape will work.

Ion generation is by avalanche multiplication outward from the wire. The generalities of inception voltage are indicated by the Paschen curve for an electrode system at the pressure "p" being used, with a wire screen distance used for the "d" of the "pd" parameter.

While not exact, since most Paschen data is for either sphere-sphere or planar electrode systems, these data are usually close enough to be "exact" for physics work.

As suggested above in my post yesterday ( which got into the wrong part of the thread, sorry about that) you can plot your paschen curve for your particular electrode system with some V-I measurements of the inception points of discharge.

Dave Cooper

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Re: The "wire" ion source - link

Post by Chris Bradley » Wed Nov 30, 2011 10:56 pm

Positive coronal discharges (that is to say, *if* this device follows the same principles) are much more complicated than just an avalanche discharge. It would be interesting to establish if there is a high frequency cycle with this, as there is with positive corona discharges, in which the region around the positive central electrode repeatedly sheds its ions then the electrons left behind act to re-ionise the gas again and the cycle repeats.

That is not to say that the 'principles' of the initial Paschen breakdown do not apply, but that the 'd' in the 'pd' term may be defined rather by the geometry of what amounts to a 'virtual plasma electrode' around the wire, rather than the physical wire.

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Re: The "wire" ion source - link

Post by Carl Willis » Wed Nov 30, 2011 11:25 pm

In response to Dave and Steven and Chris:

Most of the attested previous uses of these kinds of discharges are in secondary-emission electron guns of the type Dave was describing, high-power pulsed electron sources and lasers. In fact, the pair of Gueroult papers referenced above--and particularly the one from this year in Plasma Sources Sci. Technology--are the first occasion in my perusal of the literature where interest was evoked in this device as a high-quality source of ions specifically. It has a very convenient geometry and simple ruggedness that suggests reliability in sealed neutron generators, hence my attention.

Published Paschen curves aren't predictive in this situation where anode radius is much smaller than electron mean free path. They also don't capture the unique nuance of the normal-to-obstructed mode transition that is critical to the behavior of this device. As seen in my previous post, in the earlier Gueroult paper, and the attached figure below from Druyvesteyn and Penning (in their seminal paper in Rev. Modern Physics 12 (1940), p 87), there is a p-V curve at constant current that does have generally the same shape as a usual Paschen curve, and undoubtedly for the same reasons. But a number of assumptions in Paschen's Law aren't valid here as the Penning paper makes clear. The discharge behaves dramatically differently depending on electrode polarity, and in order to reap the low-V, low-p benefits that interest someone in a neutron generator, the wire MUST be positive. In the obstructed mode, the cathode diameter arguably plays an insignificant role, but as Gueroult shows in his earlier paper, this diameter matters in the diffuse mode where the "Child-Langmuir limiting current is a function of the interelectrode gap dR" and so forth.

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Re: The "wire" ion source - link

Post by DaveC » Wed Nov 30, 2011 11:27 pm

Chris - no real argument, in general with that. The mechanism IS more complex, if rf is being used.

We used a single 0.25 mm diameter ionizer wire (Tungsten), with a ballasting resistor, and a variable DC supply that had current and voltage control modes.
Changing the ionizer -screen distance affects the combination of voltage and pressure needed to initiate ionization.

At a low enough pressure (gas density) there will be little or no avalanching, hence no need for the ballast resistor, yet ions will still be produced.

Running the ionizer on AC usually doesn't hurt anything, but one half of the cycle may not produce ions, depending on gas pressure and geometry, again.

The obvious AC working example is, of course, the neon sign, or fluorescent lamp.

Dave Cooper

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Re: The "wire" ion source - link

Post by Chris Bradley » Wed Nov 30, 2011 11:51 pm

> The mechanism IS more complex, if rf is being used.
Dave,

An 'RF' (typically reported as units MHz) oscillation is set up in a positive corona discharge when a DC current source is applied. It is a naturally occuring frequency, according to the way the ions are shed from the region around the central electrode. Positive corona discharges are reported as dis-continuous DC, they pulse.

In my experiment, I believe* I have detected such resonant effects but at slower than the MHz range reported in papers on the subject (but that do not have a magnetic field, unlike mine). *[I'll have to re-visit this to rule out other mechanisms or background interferences.]

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