The "wire" ion source

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
User avatar
Carl Willis
Posts: 2841
Joined: Thu Jul 26, 2001 11:33 pm
Real name: Carl Willis
Location: Albuquerque, New Mexico, USA
Contact:

The "wire" ion source

Post by Carl Willis » Thu Nov 03, 2011 11:19 pm

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
Attachments
P1030067_s.JPG
P1030067_s.JPG (174.88 KiB) Viewed 3394 times
wis_2_s.png
wis_2_s.png (124.32 KiB) Viewed 3394 times
wire_source_1_s.JPG
wire_source_1_s.JPG (374.08 KiB) Viewed 3394 times
wire_source_3_s.JPG
wire_source_3_s.JPG (181.52 KiB) Viewed 3394 times
P1030072_s.JPG
P1030072_s.JPG (213.77 KiB) Viewed 3394 times
P1030061_s.JPG
P1030061_s.JPG (133.09 KiB) Viewed 3394 times
Carl Willis
http://carlwillis.wordpress.com/
TEL: +1-505-412-3277

User avatar
Doug Coulter
Posts: 1312
Joined: Sun May 27, 2007 7:18 pm
Real name: Doug Coulter
Location: Floyd, VA, USA
Contact:

Re: The "wire" ion source

Post by Doug Coulter » Thu Nov 03, 2011 11:48 pm

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!
Why guess when you can know? Measure!

George Schmermund
Posts: 262
Joined: Wed Aug 16, 2006 12:51 am
Real name: George Schmermund
Location: Carlsbad, CA

Re: The "wire" ion source

Post by George Schmermund » Fri Nov 04, 2011 12:09 am

Carl - Very nifty!
Anything obvious in high vacuum is probably wrong.

User avatar
Chris Bradley
Posts: 2931
Joined: Fri May 02, 2008 11:05 am
Real name:

Re: The "wire" ion source

Post by Chris Bradley » Fri Nov 04, 2011 12:37 am

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.
Attachments
P7160499_s.jpg
P7160499_s.jpg (128.79 KiB) Viewed 3394 times

User avatar
Steven Sesselmann
Posts: 2108
Joined: Thu Aug 11, 2005 1:50 am
Real name: Steven Sesselmann
Location: Sydney - Australia
Contact:

Re: The "wire" ion source

Post by Steven Sesselmann » Fri Nov 04, 2011 1:51 am

Carl,

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

Steven
http://www.gammaspectacular.com - Gamma Spectrometry Systems
https://www.researchgate.net/profile/Steven_Sesselmann - Various papers and patents on RG

Tidbit77
Posts: 65
Joined: Mon Feb 22, 2010 3:02 am
Real name:
Contact:

Re: The "wire" ion source

Post by Tidbit77 » Fri Nov 04, 2011 2:42 am

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

User avatar
Carl Willis
Posts: 2841
Joined: Thu Jul 26, 2001 11:33 pm
Real name: Carl Willis
Location: Albuquerque, New Mexico, USA
Contact:

Re: The "wire" ion source

Post by Carl Willis » Fri Nov 04, 2011 5:53 am

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
Carl Willis
http://carlwillis.wordpress.com/
TEL: +1-505-412-3277

User avatar
Carl Willis
Posts: 2841
Joined: Thu Jul 26, 2001 11:33 pm
Real name: Carl Willis
Location: Albuquerque, New Mexico, USA
Contact:

Re: The "wire" ion source

Post by Carl Willis » Fri Nov 04, 2011 6:15 am

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
Carl Willis
http://carlwillis.wordpress.com/
TEL: +1-505-412-3277

User avatar
Carl Willis
Posts: 2841
Joined: Thu Jul 26, 2001 11:33 pm
Real name: Carl Willis
Location: Albuquerque, New Mexico, USA
Contact:

Re: The "wire" ion source

Post by Carl Willis » Fri Nov 04, 2011 6:26 am

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
Carl Willis
http://carlwillis.wordpress.com/
TEL: +1-505-412-3277

User avatar
Carl Willis
Posts: 2841
Joined: Thu Jul 26, 2001 11:33 pm
Real name: Carl Willis
Location: Albuquerque, New Mexico, USA
Contact:

Re: The "wire" ion source (DATA)

Post by Carl Willis » Wed Nov 23, 2011 4:41 am

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
Attachments
composite_s.JPG
composite_s.JPG (346.64 KiB) Viewed 3394 times
VI_graph.jpg
VI_graph.jpg (308.73 KiB) Viewed 3394 times
PV_graph.jpg
PV_graph.jpg (148.62 KiB) Viewed 3394 times
Carl Willis
http://carlwillis.wordpress.com/
TEL: +1-505-412-3277

Post Reply