Ion source working, 49 bubs / 3 min!

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Carl Willis
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Ion source working, 49 bubs / 3 min!

Post by Carl Willis » Sun Dec 03, 2006 8:32 am

My magnetron ion source is improved and back in business after a hiatus due to a meltdown of the first one. Operation is much smoother and highly controllable with it. I am still using a coaxial configuration, but this time I am simply using the 1/4" SS tube on the CeramTec feedthrough as the magnetron anode. Gas is still delivered through this tube, although I think the pressure difference between gas at the tube outlet and that in the middle of the fusor is insignificant. I do know that at higher currents, the magnetron anode heats up to a reddish color that can be seen reflected in the chamber due to electron bombardment. I will have to have some more experience before I decide if I need a thicker tube or a water-cooling loop on the source, which overall does get quite warm. The discharge produced while the maggie is the primary source of ions lacks the beautiful symmetry of "star mode." Instead, the glow is predominantly along the diameter defined by the maggie anode. Not too surprising. But this discharge is a hot neutron producer, see numbers below.

Furthermore, Jr. has a new grid, this time a 2.25" OD grid, 5 loops of 26-mil tungsten wire. Construction was by the Rosenstiel technique with one modification (a little clip of stainless wire at the "north pole" of the grid to hold it in alignment). The new grid was not necessary at the power levels involved so far, where even the former 1.5" stainless grid did not reach red heat. However, I had the chamber open to clean up slag and chips from the last ion source casualty, and figured I'd go ahead with the mod.

The title gives the neutron yield this evening: 49 bubs / 3 minutes is about 300,000 n/s with Carl's Jr. This was accomplished at 50 kV / 4.5 mA. I don't yet know the magnetron current (have to install a precision dropping resistor in the power supply return lead). But as I said, the ion source is an excellent controlling mechanism. I do know that the anode glows a reddish color due to electron heating, and I can see reflections on the inside of the chamber.

Pics attached; closeup of ion source showing NdFeB magnet "belt" (made on a strip of Al flashing, magnets held down with Al duct tape). This belt wraps around the QF half-nipple to provide an axial field. The feedthrough is held by its long weld-lip in a properly-sized quick-connect. One advantage of this technique is that the position of the anode tip can be varied easily without opening the vacuum. In the last photo is the water-cooling pump thrown in for good measure. I have no specs on it since it is a surplus pump.

-Carl
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Carl Willis
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wayne
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Re: Ion source working, 49 bubs / 3 min!

Post by wayne » Sun Dec 03, 2006 12:28 pm

Carl-Great job.

What is the copper tube on the left?
Could I get more information on the magnetron source?
With the cap. manometer at 45 deg. is this a problem?
What is the operating pressure?

wayne

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Re: Ion source working, 49 bubs / 3 min!

Post by Starfire » Sun Dec 03, 2006 12:45 pm

Carl, be careful with the magnetron - if the load and wave guide are not correctly matched it will radiate and proximity to it can result in severe eye damage and cataracts

- THIS WILL OCCUR AT VERY LOW RF RADIATION LEAKAGE LEVELS -

A general warning to any experimenter with Magnetrons. The eye's are particularlly suseptable to permanent damage by RF at the 2.4Ghz of microwave oven magnetrons and it takes only one mistake. It ain't worth it to go blind if you don't know what you are doing - we need a general FAQ on this before folk start to play.

Permanent damage can occur in less than a second of exposure - if your eyes feel rusty when working with an active Magnetron - it may be too late.

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Re: Ion source working, 49 bubs / 3 min!

Post by UG! » Sun Dec 03, 2006 7:10 pm

i second Johns call for a magnetron or general RF FAQ. i have been considering investagating RF ion generation and high power ion resonance in the fusor, but need more information on keeping all that RF where its suppost to be, instead of in my eyes, turbo pump, computer systems and instrumentation etc.

also great work Carl! :)

Oliver

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Carl Willis
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Re: Ion source working, 49 bubs / 3 min!

Post by Carl Willis » Sun Dec 03, 2006 8:29 pm

I'll try to briefly explain the ion source a bit more.

First of all, this is not a cavity magnetron that generates or uses RF in any way. It is a DC magnetron ion source. That is what this kind of crossed-field device is called. Cavity magnetron oscillators are a subset of this general category of magnetron devices and are not what I am using.

A magnetron ion source works by trapping electrons in E and B fields at right angles to each other, causing them to travel in long helical paths and hence be more effective at ionizing the background gas. It's a very simple concept.

My ion source has only three parts: the inner electrode (the anode); the outer electrode (the cathode, a QF-25 half-nipple that is part of the chamber); and a magnet that produces an axial field in the QF-25 half-nipple. The magnet is made of a bunch of little neodymium magnets I picked up cheap on eBay. They must be held close to one another with their north poles all at one end and their south poles all at the other, so naturally they want to fly apart with great force. Aluminum duct tape is used to ensure that they don't, and the whole set of magnets is wrapped around the QF-25 like a belt. Now the same thing could be accomplished by putting a single ring magnet like those in microwave ovens around the QF-25. However, you'd have to put the magnet on before welding the QF-25 nipple into the chamber, and it would never be able to come off afterward. The belt idea allows the magnet to be moved, changed, etc. So the feedthrough conductor is the anode, the QF-25 is the cathode, and there's a magnetic field acting on the gap between them to trap electrons that otherwise would just sail radially from one to the other.

Did I do a good enough job explaining this?

Oh-Wayne, the "copper tube on the right" is the deuterium feed. It's got a metering valve, then you can see where it enters a tee and meets the valve for the vacuum clean-out line. Above this tee is a glass insulator held in little quick-connects that allows gas to flow through but isolates the ion source HV. After this is the stainless capillary, which is brazed into the ion source feedthrough.

-Carl
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Jon Rosenstiel
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Re: Ion source working, 49 bubs / 3 min!

Post by Jon Rosenstiel » Sun Dec 03, 2006 9:24 pm

Carl- I’ve been thinking of adding a magnetron ion source to my fusor, but so far have lacked the knowledge on how to proceed. Your explanation helped tremendously, thanks.

I do have a couple of questions though…
1. Does the gas exit through the end of the anode?
2. How far does the anode extend into the fitting in relation to the magnets?
3. Power requirements of the ion source?
4. The N & S poles must be at the ends of the magnets?

Jon Rosenstiel

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Re: Ion source working, 49 bubs / 3 min!

Post by Starfire » Sun Dec 03, 2006 10:24 pm

Yes - a great job Carl - I stand corrected - I had assumed it was a MO cavity Magnetron - the caution will not be wasted

I like the belt idea - quite inovative - what is the anode voltage?
What beam current are you getting?
***** I should read the threads more or make a bigger effort to understand what I read :)), sorry Carl, I just researched back to your explaination =

viewtopic.php?f=6&t=2620#p12130

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Carl Willis
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Re: Ion source working, 49 bubs / 3 min!

Post by Carl Willis » Mon Dec 04, 2006 2:18 am

Hi Jon,

1. Right now the gas does exit through the end of the anode. I've thought of plugging it and putting holes around the circumference so that the gas will be closer to where I think the discharge is, but for all I know the discharge is actually at the end of the anode so I have refrained from this mod.

2. The end of the anode is flush (+/- 1 mm maybe) with the inner surface of the fusor, just 1/8" further from the bottom of the magnet.

3. Right now I am using a variac-controlled Plastic Capacitors 15 kV / 5 mA power pack. However, I think it has been drawing more than 5 mA and hence operating beyond the design envelope of this unit, so I plan to move up to a 10 kV / 10 mA Universal Voltronics power pack. On the other hand, the highest voltage I have used is probably ballpark 5 kV. I'm sorry I don't have more precision. Good measurements are on their way.

4. The N and S poles are at the ends of the magnets (i.e. the ends of the cylinder they form around the QF).

It turns out I am not the first to try using a magnetron ion source on a fusor. There is an excellent paper by Takamatsu, Masuda, et al., "Intertial electrostatic confinement fusion device with an ion source using a magnetron discharge," Nucl. Fusion 46 142-148 (2002). Very nice writeup and data showing neutron yield with and without the ion source.

-Carl
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Re: Ion source working, 49 bubs / 3 min!

Post by Jon Rosenstiel » Mon Dec 04, 2006 3:13 am

Carl-
It was a paper titled "Magnetron Discharge Characteristics for the Improvement of IEC Performance" by Mizutani, Masuda, et al., which got me interested in trying this.

Thanks for the info and the tip on the paper; I'll have to track it down.

Jon Rosenstiel

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Carl Willis
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Re: Ion source working, 49 bubs / 3 min!

Post by Carl Willis » Mon Dec 04, 2006 3:39 am

*oops. The Takamatsu citation should be 2006 (the rest is correct).

I posted below a fair-use excerpt from the Takamatsu paper (for those without the luxury of a library account, can be found here: http://www.iop.org/EJ/article/0029-5515 ... _1_016.pdf)
that basically illustrates their magnetron concept and the resulting discharge. In their case they put the magnets in the center electrode, and they prefer to make the center electrode the cathode instead of the anode. Principle is the same as mine. I will have to see what happens when I change polarity.

-Carl
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