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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Carl Willis
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The "wire" ion source

Post by Carl Willis »

In my job (Qynergy Corp., Albuquerque, NM), I am currently responsible for an effort to develop small, ultra-compact, piezoelectrically-powered neutron generators. One of my side goals in this project is to explore a variety of concepts in simple ion source technology that might turn out to have advantages in tightly-constrained, sealed-tube geometries. So here's a simple, hobby-accessible ion source that gets very scant mention in the literature but offers some interesting possibilities. It's called a "wire ion source." Like the fusor, it's an electrostatic trap designed to facilitate a self-sustaining discharge at low pressures. I am eager to try one in my linear beam-target neutron generator project at home, but won't get to do that for some time as I am currently working on a major upgrade to the existing RF ion source on that.

Construction and operation are straightforward. A grounded metal tube (1" OD) coaxially surrounds a very thin (0.07 mm) tungsten wire at high positive potential. (This is incidentally the same wire I used to make my version of Tim Raney's alpha spark detector a while ago.) I use $30 braze-able terminals from Ceramaseal to insulate the ends of the wire. The wire is spot-welded onto a plug at one end, and tied around a very small stainless tensioning spring at the other. So with the wire at a high positive voltage, a logarithmic potential well is thus established around it, and electrons that happen to be liberated within the tube's interior fall toward the wire. However, since the wire is so thin, most electrons will have enough angular momentum in the trap to avoid it and orbit instead. So that's the radial confinement scheme. Significant confinement in the longitudinal direction can be achieved with planar end walls having small-diameter holes for the wire to penetrate. The smaller the hole, the fewer electron orbits can leak out along the wire. The electrons' extended paths are reminiscent of those found in microwave ECR sources and in Penning sources and serve the same end: lots of ionizing collisions with the spare background gas and a stable discharge at very low pressures. Unlike those other sources, however, no magnets are involved--just an electric field (no RF required either).

I'll share performance details as I get them. I built a beamline with a "Colutron" Wien-type velocity filter to look at ion species fractions, and I have a Faraday cup to look at total current. Right now though, no data, but I do have some eye candy and a couple preliminary numbers. In the eye candy, the first photo shows operation at about 400 V / and 0.2 mA at a pressure of 6.9 mtorr in ordinary research-purity hydrogen. Nice glow discharge around the wire, and the faintest hint of heating. In the second photo, I'm up at 2 kV / 0.7 mA / 3.4 mtorr and the wire is red-hot, dominating the visible plasma. Following are some photos of assembly.

-Carl
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Doug Coulter
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Re: The "wire" ion source

Post by Doug Coulter »

A thing of true beauty, Carl! No mag field! Totally cool! This is getting down to the pressure range I'd like to have on my own linac (in build now), but a lot simpler than the microwave source I was planning to use here - there's still time to do this instead if the performance is suitable, at say e-5 or e-6 mbar or thereabouts. I'm looking for less than a milliamp of beam (but more is always good), with singly ionized D, for my project.

Do you suppose that having the cutouts in the outer shell help with getting electrons to often miss the wire? No way those cuts are "perfect" so you'd have a bit of imperfectly circular symmetry, right?
I'm supposing this might work even better with the 1 mil wire I have here (rather than the 2.7 mil you're specifying).

At any rate, thanks for the eye-candy - great stuff!
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Re: The "wire" ion source

Post by George Schmermund »

Carl - Very nifty!
Anything obvious in high vacuum is probably wrong.
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Chris Bradley
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Re: The "wire" ion source

Post by Chris Bradley »

Hi Carl,

When I introduced my experiment last year, you might recall you observantly asked about 4 parallel wires that were shown in the photo forming the central electrode in my device.

As my device aims to have the anode at the centre (to accelerate ions away), part of the intention was to therefore try generating ions by positive coronal discharge in the way you are doing, because the electric field in my device is in 'the correct sense' that corona ions would be accelerated away, leaving the electrons to 'fester' around the anode and not waste any accelerator power.

It has worked very well for me! My experiences may differ from yours because I am operating in a magnetic field, but in my experiment I have found that I can push the diameter of the wires up to around 1mm before I lose the effect. Having the anode as a multiple-element is also a means to improve ion production rate, with lower current heating in each.

The main problem for my experiment at hand is, actually, that the method can be *too* successful at generating ions! It means there can be too high a conductance at higher pressures, so for me to be able to achieve high acceleration potentials (at the units-mA current levels I am rigged up for) I generally have to pull the chamber down to around a half micron.
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Re: The "wire" ion source

Post by Steven Sesselmann »

Carl,

Thanks for posting..., I love the simplicity of the design.

Steven
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Re: The "wire" ion source

Post by Tidbit77 »

Very, very nice ion source. I'm trying to decide what kind of source to put on my next reactor, and I've been debating between an anode layer or a DC magnetron source. Currently I'm leaning towards an anode layer source due to it's efficiency, although it's slightly more complex.

However, I really like the simplicity of the the source, and I imagine it could be a viable source. I'm fairly curious as to how it compares to the DC magnetron you built, especially in terms of reliability/ease of operation/"finnicky-ness".

I can't wait to see some data from this thing!

-Will
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Re: The "wire" ion source

Post by Carl Willis »

Hi Doug,

The cutout allows me to extract a beam that can be focused and transported in a beamline for analysis, while maintaining the mechanical rigidity of the piece of pipe. I didn't intend, nor do I suspect, that it has any noteworthy impact on the source's behavior relative to ideal symmetry. As you can see, I put a piece of nickel mesh over the cutout so as to maintain (to a fair approximation) the radial symmetry of the trapping field.

A thinner wire can be expected to start at a lower voltage and produce somewhat higher current at a given voltage, although I have no model right now to predict the that trend quantitatively. Wire thickness is important because of durability and current handling too, with smaller wires being more fragile in that respect.

Right now I am most interested in learning how the wire discharge's properties scale with pressure, as this is key to being able to design this kind of source and package it in a cylindrical sealed neutron generator (for example).

-Carl
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Re: The "wire" ion source

Post by Carl Willis »

Hi Chris,

I would classify your device as an inverted magnetron. I agree that its trapping action shares some features with the simple wire, and expect an analogous dependence on wire diameter. By virtue of the magnetic field, it's certainly a more effective trap than the simple wire. Where the magnet is tolerable, this approach has a lot of possibilities.

-Carl
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Re: The "wire" ion source

Post by Carl Willis »

Hi Will,

Nothing's likely to beat the DC magnetron for sheer simplicity, and that's always a virtue. I don't know if I'll ever be recommending the single-wire source for fusors; its geometry is considerably more adapted to cylindrical beam-on-target devices, and that's my impetus for studying it. However, an embodiment with multiple, electrically-distant wires disposed around the perimeter of a cylindrical fusor (with conventional coaxial cylindrical grid at high negative potential) might be a winner. Such a configuration would be trapping electrons near the wall to create ions there, and trapping ions toward the center where they hopefully fuse of course. So the wire source is just another concept to be aware of when thinking about improvements. I'll try to oblige quickly with data.

-Carl
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Re: The "wire" ion source (DATA)

Post by Carl Willis »

Here are some observations and data from operating my wire ion source in hydrogen.

First of all, the wire discharge has two distinct modes: a "diffuse" glow discharge that prevails at higher pressures (above about 15 mtorr) and fills a large part of the cylindrical volume with visible plasma, and a "constricted" discharge at the lowest pressures associated with intense heating of the anode by electron bombardment and a glow around the wire of very limited radial extent. The photos shown previously are of the latter mode of operation. In the first pic below, you can see both modes side by side (left is constricted, right is diffuse). The photos were taken within about three seconds of each other at a fixed current of 0.5 mA and a pressure of 16 +/- 1 mtorr, right as the transition occurred. Transition is always sudden and dramatic, but predictable. My use of the descriptors "diffuse" and "constricted" follows that of R. Gueroult in the following wonderfully-explanatory paper from last year:

R Gueroult et al. J. Phys. D: Appl. Phys. 43 (2010) pp. 365204-365214

This document is unfortunately too large to post here, but I will email to anyone interested. It can also be downloaded by anyone having access to a corporate or academic subscription.

The second image shows voltage and current in the constricted discharge for a number of fixed pressures. One interesting feature of these relationships is the negative-resistance characteristic at voltages beyond where the current peaks at about 5 kV. Those familiar with this kind of negative resistance are probably aware of applications in amplifiers, oscillators, and so forth. What is responsible for this effect? The Gueroult paper doesn't explicitly discuss this, but an explanation is to be found in their Fig. 10 (plotting simulated electron energy distributions against the reduced ionization frequency). Basically, ionization cross-section for the background gas goes through a maximum as energy rises, after which it falls. As anode potential rises, so does mean electron energy in the discharge. There's an anode potential where the value of N*INT(sigma[E]*v[E]*f[E]*dE), which is the total rate of ionization, is maximized, and above which it falls as the energetic electrons have a limited ability to "reproduce." So that's kind of interesting.

The final graph shows pressure and voltage at a selected current of 0.33 mA. I have such curves extending from 0.2 mA all the way up to 1.3 mA, which is the maximum current the wire will sustain in the constricted mode before burning out. (It's simple to replace, but annoying to vent and pump the vacuum system every time the source has to be retrieved for rewiring). In the diffuse mode, the discharge currents can be much higher without endangering the wire. Anyway, What we see is an abrupt ~150V transition from the steep PV curve in the constricted mode, to a flatter curve in the diffuse mode. The Gueroult paper goes into excellent detail explaining the theoretical underpinnings of this transition, and also illustrates results from a nicely-predictive PIC simulation they wrote to model this kind of discharge (in helium).

More later...

-Carl
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Re: The "wire" ion source (DATA)

Post by Richard Hull »

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 »

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 »

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 »

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 »

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.
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Re: The "wire" ion source - link

Post by DaveC »

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.

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Re: The "wire" ion source - link

Post by Chris Bradley »

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 »

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.

-Carl
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Re: The "wire" ion source - link

Post by DaveC »

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.

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Re: The "wire" ion source - link

Post by Chris Bradley »

> 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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Re: The "wire" ion source - link

Post by DaveC »

Good comments, Carl.

The additional information adds to fill in the picture. Do you have the pressure in the Druyvesteyn-Penning paper? The tests I ran were able to move out to right some distance past the Paschen minimum, until the DC supply ran out of voltage... (around 2kv).

The point regrding curve shapes, that you make is most important. These classic curves come with several important assumptions, that often are not realized in actual systems.

Our ionizer geometry was roughly cylinder-square. The grounded screen had a several mm open mesh, and it and the solid chamber walls were several cm away from the fine tungsten wire. So this was only somewhat like a coaxial cylinder setup, with a D/d ratio of maybe 100.

The ions travelled about 0.25 meter to the main electron beam channel, entering at about 45 deg angle, where the strong local cathode field drew them in on axis.

I was able to model (approximately) the paths of both ions and electrons using Simion 7.xx treating both as non interacting. (I also tried unsuccessfully to use Vector Fields for a more refined analysis, but the mesher choked repeatedly on the relatively complicated geometry of the ionizer screen. Simion's transparent screen is a very, very useful feature. David Dahl knew his stuff, when putting this software together.

One interesting result I remember getting was from an attempt to measure incoming ion energy. By adjusting the bias on a collector plate in the electron channel (cathode off) until current went to below 1 nA, a rough ion energy spectrum was developed. I'd have to go dig up the test results, now, but I recall ion energies (He) were pretty large, perhaps up to half the voltages in the ionizer.

This suggested the ions at 10-4 Torr pressures were fairly long lived, and could travel significant distances ( and around corners). This probably is not what happens at the higher pressures in the fusor. Again, as you say, it's mean free path.

We always ran with a positive wire ionizer, because the ions were for the SEE (secondary electron emission) gun application.

Our pressures had to be quite low, in order that main e gun did not itself go into glow discharge... (which did happen from time to time).


So much for the anecdotes....

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Re: The "wire" ion source - link

Post by Chris Bradley »

In the limited sample diameters of electrodes I have tried out, there also seems to be a 'cut-off' point when the electrode diameter gets bigger than some value between 1 to 2.5 mm. Up to then, electrode diameter appears to play only a very small part in the voltage (600 to 900V - with respect to an intermediate electrode positioned at the ends of the volume) at which the thing lights up. I moved to 0.2mm wire (from the picture I show above, with 1mm diameter electrodes) and was surprised to find that it made very little difference to the voltage at which a corona was initiated. At ~2.5mm diameter, it just wouldn't light up at all, at seemingly any voltage (at least, below 10kV).

My preliminary conclusion is that you need a certain voltage gradient to get the process underway, but if the diameter gets too large then the voltage gradient does not drop off (with radius) quick enough, such that ions are repelled from the region before they have a chance to sustain a discharge through further ionisations. This is just a 'working hypothesis'.
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Re: The "wire" ion source - link

Post by DaveC »

Chris-

Quite likely you moved to a value on the Paschen curve whose breakdown voltage (or field) exceeded yoursupply capability for the pd product of the gas you were using.

Away from the Paschen minimum, the voltages climb with either a higher or lower pressure for a given electrode geometry.

The natural oscillations that I've measured in long ( 1meter long) plasma discharges were in the 60 kHz range. They occurred towards the low end of an unassisted mechanical pump's pressure, (guessing now at perhaps a few tens of microns maximum).

They coincided with the appearance of plasma bunchings that were approximately spherical. I've called them "ion acoustic" oscillations in past references, simply because the plasma bunches coincided with the nodes of the measured oscillation acoustic frequencies. I used a shielded piezo-electric membrane to detect the acoustical) signal.

I need to set up this apparatus once again, and this time take and post photos along with scope traces of the acoustical signal. It was an interesting experiment.... sometime soon, I hope, when the shop-rebuild project plateaus.

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Re: The "wire" ion source - link

Post by Dustin »

Sorry Dave, I think I have been unclear,
In very low pressures ie:the mean free path being greater than the device diameter,
I was hoping to illustrate that an irregular outer shell may extend electron oscillation lifetime.
As a circular outer shell, all fields focus the electrons on the wire, so an electron has a relatively high probability of hitting the wire.
By changing the shell shape, you can reduce this probability by defocussing and promoting eliptical orbits.
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Re: The "wire" ion source

Post by ab0032 »

Would one also get a positive corona discharge if the center wire was at zero potential or perhaps even negative and the outside more negative, if one wanted ions that start their existence at a certain negative potential?

Could one use a grid or mesh on the outside if one is not interested in sending the ions only in one mayor direction?
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