Electron gun- what's wrong here?

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David Kunkle
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Electron gun- what's wrong here?

Post by David Kunkle »

Trying to construct a simple electron gun. More like an electron shotgun- I don't need or want any collimation. Tried it last night for the first time and - nothing. Hope the pics are fairly obvious as to construction. That's the best my camera can do up close. The filament/cathode is run off a 12V DC source (12V, 6W filament). The copper wire ring/anode is run off a separate 12V DC, 1A source through a transformer to a 320V output. I have a few different ideas what's wrong, but hopefully it's obvious to the forum.
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Re: Electron gun- what's wrong here?

Post by Richard Hull »

Draw your circuit! How were you detecting that it did not work? Was the high voltage AC or DC and involved in a closed circuit? Was there a milli or microammeter involved to see if current flowed? If it is a gun, where is the target. At best, with a suitable vacuum, you have a diode and not a gun.
You have supplied us with no real info.

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Re: Electron gun- what's wrong here?

Post by Dennis P Brown »

If you are just "boiling" off electrons (12 volts) you need a focusing (nice but not critical) and accelerating field (essential) for the electrons or the gun to really work because the electrons can not just go anywhere while under vacuum! The idea for an "electron gun" is to have high enough voltage gradient so that the electrons are forced to travel through the massively insulating vacuum to the ground along a path you desire. Sorry but 12 volts can't possibly do that.
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Re: Electron gun- what's wrong here?

Post by John Futter »

David
your anode potential is not large
lots of things are going to be needed to know.
how hot is the filament in your case there will be no electron current until your filament is glowing near white hot ie like a torch bulb or brighter
anything less than this will not work.
I use standard 6 volt torch globes @ work as electron flood to eliminate charging on non conductive sputter targets.
I need the filament hotter than a normal torch globe and 35 volts anode to cathode to get 3mA of emitted electrons. And yes the filament only lasts a couple of hours
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Re: Electron gun- what's wrong here?

Post by Steven Sesselmann »

I agree, a schematic or a diagram would be most useful here.

Anyway, you can ground the copper ring, and float your low voltage supply at around -1000 volts, when the element is heated electrons will boil off and head for the copper ring, and in the process you will be making some low potential ions that head for the grid. This is a bit like having a second grid, check out Doug Coulters fusor.

Make sure you float the source safely and put a high ohm ballast on it just in case.

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Re: Electron gun- what's wrong here?

Post by David Kunkle »

sch 001.JPG
Sorry, I can see now I should've provided more detail- thought maybe something simple like anode too close to filament. At any rate, this was supposed to be modeled after the e-gun used on the Prometheus site: https://prometheusfusionperfection.com/2012/06/

He used 500V, so I thought the 12V in/ 320V out supply I have would be in the ballpark. My one 12VDC PS is actually variable to 120V. I accidentally hooked up the 12V input/320V output PS to the variable PS when it was still set to 100V instead of 12V and blew up a capacitor rated at 16V. Didn't hurt anything else and still works fine after replacing the cap. So I figure worst case, I can boost the output on the 320V PS to close to 450-500V for the short times that I need it?

I have been using a 120V/40W filament as a passive emitter with an anode in the center of the chamber at 10-15kV, and 10-6 Torr range. Distance between the two is about 6". With that setup, the HV has no trouble pulling e-'s off the filament in the mA range. The HV PS goes to 110mA and easily gets pegged and shuts itself down.

Building the gun to make an active emitter to make a controlled flow to the chamber's center anode. The gun filament is instead 10" from the chamber's central anode and will not passively flow any e-'s to the central anode at that distance. Therefore, the above attempt at an e-gun. Detection was supposed to be reading in the mA range on the main HV PS.
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Re: Electron gun- what's wrong here?

Post by Steven Sesselmann »

David,

My suggestion for the simple ion source is to use a nipple and a feedtrough with two wires. You will also need a small AC isolation transformer and a source of DC high voltage. A cheap DC voltage source can be made from a simple inverter and two rectifier diodes. http://www.digikey.ca/product-detail/en ... -ND/652722

Connect the high voltage with positive to ground and negative to the filament loop through a 1M ohm load resistor and adjust the brightness of the filament with a variac on the isolation transformer.

For best results the gas feed should go through the flange so the gas flows over the filament.

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Re: Electron gun- what's wrong here?

Post by Jerry Biehler »

Use a good isolated filament transformer with a small variac to control the filament emission. Then tie negative to the filament and positive to the chamber ground. If you want to use that that ring as an extractor add a separate variable power supply for that.
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Re: Electron gun- what's wrong here?

Post by Dennis P Brown »

Without any type of focusing your electron beam will not be a beam at all but a very diffuse cloud and the majority of the "boiled off" electrons will be absorbed by your acceleration ring. What small amount of electrons that do accelerate past the ring will rapidly spread out. Installing an Einzel lens to focus the electrons would be simple to install and significantly increase the beam current delivered to your desired area in the vacuum chamber - especially if the target isn't practically on your emitter.
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Re: Electron gun- what's wrong here?

Post by Steven Sesselmann »

Dennis P Brown wrote:Without any type of focusing your electron beam will not be a beam at all but a very diffuse cloud and the majority of the "boiled off" electrons will be absorbed by your acceleration ring. What small amount of electrons that do accelerate past the ring will rapidly spread out. Installing an Einzel lens to focus the electrons would be simple to install and significantly increase the beam current delivered to your desired area in the vacuum chamber - especially if the target isn't practically on your emitter.
Dennis,
I think all David wants is more ions too keep his fusor alight, a diffuse cloud of electrons will ionise the gas just fine, no need for a focused beam here.

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Re: Electron gun- what's wrong here?

Post by David Kunkle »

Thanks for all the info and suggestions guys. Filament should be hot enough-haven't had problems getting e-'s to flow anymore- some experience with that now. Usually I melt them if anything.
I've ordered the inverter Steven recommended and a 1M ohm resistor. Found an isolation transformer on ebay. I plan on using a couple extra diodes laying around from microwave ovens.

Steven's suggestion for adding a gas feed makes it all look an awful lot like his patent. Not worried about patent infringement? ;)

Doesn't appear that I need to hack up what I already built. There does seem to be a difference of opinion whether the extractor ring will do anything useful or just cause problems. Looks like I can simply rewire things and try it several ways. Worst case, cut it off and get it out of the way. I am aiming for a diffuse cloud of e-'s.

Thanks everyone.
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Re: Electron gun- what's wrong here?

Post by Steven Sesselmann »

David,

Let it be known that I am totally over patents, the patent process which was originally introduced by governments to encourage inventors by giving them a 20 year monopoly, has been hijacked by lawyers and large corporations who can afford to pay them. My patent applications have all lapsed and serve no other purpose than to put a name to the idea. When it comes to the fusion business a 20 year patent is totally useless, it's a lifetime project.

One note on the ion gun I suggested, It gets very hot, in fact the ones I made had cooling ribs and a tungsten ring around the filament.

Steven

These are the two I made for the late forum member John Hendron "starfire", John was an electrical engineer and a great source of inspiration for me in those early days. The ion gun project was a joint project between me and John and we became great friends even though I never actually met him in person.
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Re: Electron gun- what's wrong here?

Post by David Kunkle »

I thought I'd read previous posts where you didn't care for or think much of patents anymore. Didn't quite realize it's been almost 10 years and that it only went as far as an application.

Always wondered why you went to all the trouble with machining that large nipple and cooling fins. Now I know- also why you use something more robust like the magnetron filament. Right now I just need the e- source, but if I get to the point where I need an ion source also, I wonder how long a light bulb filament will last if it gets so hot. Maybe at least turn down the power to the filament as the whole thing heats up to avoid burning it out- or convert to a magnetron filament.
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Re: Electron gun- what's wrong here?

Post by David Kunkle »

I have the inverter that Steven recommended in his hyperlink above. 2 things though:

1) Didn't exactly come with an instruction sheet. Need to check my wiring with you guys. In the photo, the 2 wires at the top are 5V DC input with the left labeled GND- using that as -/ground wire. The bottom 3 wires are output. The 2 on the lower left labeled out1 and out2 being used as 900V positive output to the 2 diodes. And the lower right wire is labeled GND- using as ground for the 900V. Is this close to correct?

2) The 2kv diodes are out of microwaves. When I hook them to my DMM, I get no reading either way on the diode setting and OL either way when checking their resistance. At least one of them came from a brand new unit. Are they both bad or does it have something to do with their voltage rating and the DMM? Wanted to check before I order new 1kv diodes off ebay.

Thanks.
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Re: Electron gun- what's wrong here?

Post by Dennis P Brown »

What you said about wiring the unit agrees with my nearly identical unit. And no, high voltage diodes do not work with std checkers but should work with that PS. You will likely need a voltage divider since 900 volts tends to be at the upper limit of most meters (and will likely fry the meter.) Check the FAQ's on how to check HV diodes; I put such a unit together and it works. Now, with my HV x-formers (NST) and polarized HV meter I just test the diodes the old fashion way (explosion any one? Yes, I wear safety glass and assume the diodes will blow.)

Aside: the picture looks like you wired both diodes together at one end? Not sure that is a good idea. Don't remember if those two outputs on the converter PS are supposed to be connected. I never did that with mine. These units produce no significant current so don't see the point of wiring the two outputs together anyway. Also, there is a reason these units are cheap - they will take little abuse and fry easily.

If the diodes are wired together, undo them and try measuring the voltage again.
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Re: Electron gun- what's wrong here?

Post by David Kunkle »

Good to know about the diodes. I'll assume they're OK for now unless I can't get the right output from the whole contraption after I get the wiring correct.

As a matter of fact, the negative ends of both diodes are wired together, then the resistor would come after that. I was thinking the 2 outputs were out of phase AC, and the diodes would convert that to constant DC. Of course, now that maybe doesn't sound right in my head. :)

If that's wrong, where do the 2 diodes go?
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Re: Electron gun- what's wrong here?

Post by Steven Sesselmann »

David,

The USB ground crosses over to the ground pin on the HV side and the two HV pins are connected together.

If you use fast switching high voltage diodes you can get away with two diodes.

After that you just put in a 3KV capacitor and a load resistor and if you like another capacitor across after the load resistor as well.

I hope it's needless to say, "DANGER HIGH VOLTAGE" these little inverters look harmless but can bite you.

PS: When you rectify 900V AC the output potential increases to around 1400V DC

Steven
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Re: Electron gun- what's wrong here?

Post by Rex Allers »

David, what you are doing does not look reasonable to me. I didn't look back in the thread to figure out what you want to drive with this and I didn't find a link where Steven mentioned this inverter, but from the pic, it looks exactly like one I have, made by TDK.

These inverters were made to drive CCFL lighting. For reference about what's in them you can look for the topic "CCFL inverter" on Wikipedia.

I think you have it right about how you connected the 5V DC input. IF you make the 5 V adjustable (say 2 to 5 V) you can control the output voltage somewhat.

The two outputs you have connected are the part that is not reasonable. The transformer has one high-voltage secondary winding. The two blue things on the HV-end of the inverter board are caps (capacitors). One end of each cap is connected to one of the two output pins. The other end of the caps is tied together and connected to the x-former hv output. The caps are there to limit the drive to CCFL lights. In this case the board was designed to drive two lights but they both come from the same secondary wire of the transformer.

So for the best use as a generic HV supply, you don't want these caps in series with the output. You'll just be limiting the power you can get from the supply. You also only want one output since the two come from the same place anyway. You can solder a wire to the other end of one of the caps as your output, or you could jumper across one cap and use that one output pin. Or you could remove the caps and re-purpose them as filters on the other side of your diode rectifier. (With the caps removed, you'd need to jumper the HV across to one of the output pins or solder an output wire to the inside end of where one of the caps was removed.)

These inverters are switching supplies that run at 10's of kHz through the transformer. To rectify the HV efficiently you need fast diodes. I think most microwave oven diodes are intended for 50 or 60 Hz so they are not fast. Probably not a good fit for this application. The key diode spec is reverse recovery time. I think diodes for this kind of frequency would typically be about 80 to 100 nS trr.

One last thing. Most of these small inverters have low end of the secondary (output ground on this board) tied to the input ground. That should be fine, but I think on this one it is easy to isolate the two grounds so that the secondary is not tied to the input ground. If you look, I think you will see a GND jumper running very close to one corner of the transformer on the top of the board. Removing this will float the secondary side from the input ground.

I hope all that made sense.
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Re: Electron gun- what's wrong here?

Post by Rex Allers »

I see Steven also gave a post about using this inverter, complete with schematic.

Since, in my last post, I talked about removing or bypassing the caps in the output of the inverter, I thought I would comment about Steven's circuit. His configuration, with two diode strings, is a voltage doubler. In this case the two caps on the inverter are put in parallel as the charge storage for the first stage of the doubler. That's a valid approach and doesn't require any hacking on the inverter board.

Steven also mentions using fast diodes, but the 1N4007's in his schematic aren't particularly fast; about 2 uS trr, I think. For this kind of application I ordered some surface mount BYG23M diodes from Mouser, a while back. They are spec'ed at 1 KV, 1.5A, 75 nS trr. Overkill on the current for this but pretty cheap, if I remember right. Maybe others have recommended part numbers to look for.
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Re: Electron gun- what's wrong here?

Post by David Kunkle »

Steven Sesselmann wrote:David,

The USB ground crosses over to the ground pin on the HV side and the two HV pins are connected together.
If you use fast switching high voltage diodes you can get away with two diodes.
After that you just put in a 3KV capacitor and a load resistor and if you like another capacitor across after the load resistor as well.
As far as the diagram goes- thank you, thank you. Now I know what the heck I'm supposed to be putting together. No electrical engineer here, but I can read that perfectly well!

As far as a 3kV cap, what farad rating should I look for?
Another cap across the load resistor will further smooth out the ripple?

Thanks.
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Re: Electron gun- what's wrong here?

Post by David Kunkle »

Rex Allers wrote:Since, in my last post, I talked about removing or bypassing the caps in the output of the inverter, I thought I would comment about Steven's circuit. His configuration, with two diode strings, is a voltage doubler. In this case the two caps on the inverter are put in parallel as the charge storage for the first stage of the doubler. That's a valid approach and doesn't require any hacking on the inverter board.

Steven also mentions using fast diodes, but the 1N4007's in his schematic aren't particularly fast; about 2 uS trr, I think. For this kind of application I ordered some surface mount BYG23M diodes from Mouser, a while back. They are spec'ed at 1 KV, 1.5A, 75 nS trr. Overkill on the current for this but pretty cheap, if I remember right. Maybe others have recommended part numbers to look for.
Rex,

The 5V will be coming from an adjustable supply. Be interesting to see how much I can vary the output. Good tip.

I have found 1KV, 1A fast switch diodes on ebay with trr of 500ns. 4 for about $15 incl. shipping.

You're right about the grounds being connected. I found the jumper between the 2 grounds. What would be the advantage to disconnecting these?

Thanks.
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Re: Electron gun- what's wrong here?

Post by Steven Sesselmann »

David,

The capacitor should be around 10 nF and yes another one will smooth the ripple, but for your purpose ripple is not an issue.

One more thing I should have mentioned above is that you need to turn the diodes the other way, because you obviously want negative bias on your filament.

These inverters are not going to supply much current, but since you are heating the filament you just need some negative bias on the filament to make it become an electron emitter, this rather weak electron emitter should generate enough ions to keep the fusor lit up at low pressure.

Good suggestion from Rex to put a potentiometer on the input, so you can control the bias on the filament.

Steven
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Re: Electron gun- what's wrong here?

Post by David Kunkle »

So you can make it + or - bias just by turning the diodes around?

According to this: http://dkc3.digikey.com/PDF/CA2011/P2792.pdf , pink column, digi-key part # 445-1621-ND , the output is about 10mA if both outputs are wired together?

On that same pdf, same page, I see the largest (V-wise) inverter outputs 2100V. If my 900V inverter will hit 1400V, would that 2100V inverter wind up putting out about 3200V if I needed more electrons or would it just run out of mA being used up through beam current?

Thanks again.
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Re: Electron gun- what's wrong here?

Post by Rex Allers »

David, there's no reason in this application to float the HV ground. I just mentioned it to be complete. I think I had a reason once, but I can't remember why now. Maybe monitoring current out.

Those diodes you mentioned seem very expensive. Also, 500 nS isn't real fast as "fast diodes" go.

You can do much better buying new direct from Mouser.com, probably Digikey too but I didn't check there. I looked at mouser for those BYG23M that I bought earlier. I bought 100 when I ordered. Looks like the price for 100 now would be $16.90. Those are smd package. A standard fast axial diode is UF4007; 100 of them is less, $10.20 for the first Fairchild listing.

Both of these diodes are 1KV, 1A, 75nS. You can order smaller quantities but the price goes up a little. Still way better than the ones you found. If you have time to kill, you could try the parametric search at mouser or digikey. There may be something similar with 2KV rating or etc.

Edit: I forgot to mention, yes, you can switch all the rectifier diodes around to change the polarity of the output.
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Re: Electron gun- what's wrong here?

Post by David Kunkle »

Just got the proper diodes from Mouser. Got 6 for 45 cents each- then there's the $8 for shipping.
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