Multipactor

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
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pfostini
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Multipactor

Post by pfostini »

Thanks Hal for the drawings. I had a little fun today and made a small multipactor gun and a linear test bed suggested by Richard Hull.

The bed is simply a vacuum tube with the gun at one end. a quartz plate can be seen inside to act as a screen.

The multipactor was very similar to what was in the drawing for except for a few items.The o-rings are still around the accelerator sleeve but I used 8 glass rods to hold the dynode in place. I made the dynode out of nickle. The anode was made of tungsten. It is held in place by a weld on one end and a small hole on the other as a locating key. I made the housing out of Stainless and the ends were stainless washers I had that fit perfectly. As soon as I run some tests I will post them.

The gun is just torr sealed to the end of the test chamber. I also have some uranium glass inside to check for other goodies flying around in there.
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hellblazer
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Re: Multipactor

Post by hellblazer »

<heh> This is so cool.

The patent mentions using Aluminum as the dynode material, due to secondary emission characteristics. Looking in my CRC, it seems that nickel isn't too shabby. <heh>

It'll be interesting to see if this gun works without a starting filament. The patent mentions that, and I was trying to see how to work one in simply. My fingers are crossed.

Just some questions from the curious: How did you make your dynode? Are the points on the inward triangles nice and sharp, or did you build it by bending sheet metal?

Again, this is so amazing to see in actual physical form! Waiting with a worm on my tongue for the results of the tests...
pfostini
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Re: Multipactor

Post by pfostini »

I tried a first run. Do Not use o-rings. The instant you get an arc they carbonize and its time for a rebuid.I am glad I used glass on the dynode side.

I simply used a needle nose plier sideways and formed the dynode. Then crushed it to make sharp turns , then expanded it. I don't have an rf source tied in yet but I am just getting the feel.

I put some glass tubing aroung the accelerator section, now for a try after the torr seal dries.
hellblazer
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Re: Multipactor

Post by hellblazer »

sharp points might be necessary to increase the field emission for self starting... Don't really know...

Sorry 'bout the ORings... Man, I bet that smelled.
pfostini
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Re: Multipactor

Post by pfostini »

I think you are right on the money. It takes a higher pressure to start and then I had to bring the pressure down slowly. I have been playing with it a bit more today.
hellblazer
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Re: Multipactor

Post by hellblazer »

What is the potential you're using in the multipactor? Also, what current is being drawn. Are you putting a 2 Mhz signal across the dynode and anode? The frequency might need to be fine tuned. Also, for nickel, I think you need at least 550 V to produce secondary electrons. For aluminum, it's around 300. I'm a little bit fuzzy still on secondary electron emission, so I'm really whistling in the wind on this...

As to power requirements, I would hope that if things are working well, they should be relatively small - i.e. lots of electrons from secondary emission as well as low losses due to the dynode and anode design of the multipactor.

If there's a lot of losses and there's no secondary electron emission, and no RF, then the power requirements could be pretty big. But I would think if you're drawing 500 watts through the multipactor supply, either something is wrong, or the design just isn't that efficient. But then again... As usual, standard disclaimers apply.
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