Electron gun- what's wrong here?

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David Kunkle
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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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Re: Electron gun- what's wrong here?

Post by Steven Sesselmann »

David,

Plenty on ebay if you dont mind waiting for them, just search for "8kv rectifier diode"

http://www.ebay.com/itm/20-x-30mA-8kV-1 ... SwcF9UXGpR

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Re: Electron gun- what's wrong here?

Post by Rex Allers »

Steven and Dave,

I've had good results with that ebay seller 'high-voltage-hv' too. I've bought a number of hv diodes, those blue ceramic hv caps, and some hv wire from them. No complaints so far. Check out their 'store' or 'see other items' on ebay.

Outside of ebay, the site hvstuff.com seems pretty good too.
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Re: Electron gun- what's wrong here?

Post by Jeroen Vriesman »

I don't really get the idea of a high negative bias on the filament.

Indeed, it would accelerate the electrons more, but imho this might be the problem, not the solution, and the negative power supply would just be used for running current through the diode (filament + anode ring = diode).

If you take a look at the field strength between the filament and the anode ring, and compare that with the field strength between the anode ring and the final target:

if the distance between the filament and the anode ring is about 4mm (estimated from the photo), the field strength will be 320/4E-3 = 80 kV/m.
That means, if you want any substantial electron yield, the field strength between the anode ring and the target must be stronger than 80 kV/m near the center of the ring!

What I would do:

increase the distance between the filament and the anode ring a bit, give the target a high positive potential (e.g. 10 kV), make the voltage on the anode ring variable.

Now start with 0V (with respect to the filament) on the anode ring, measure current on the target, you should be able to make the current on the target zero by making the voltage on the ring a bit negative (a few volts), just like a triode.

Then slowly increase the voltage on the ring, a current on the target will flow, you will see that there is a maximum met the voltage on the ring is a bit positive, but when it gets too positive all electrons will go directly from the filament to the ring and not to the target.
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Re: Electron gun- what's wrong here?

Post by Steven Sesselmann »

Jeroen Vriesman wrote:I don't really get the idea of a high negative bias on the filament.

Indeed, it would accelerate the electrons more, but imho this might be the problem, not the solution, and the negative power supply would just be used for running current through the diode (filament + anode ring = diode).!
.
Jeroen, the idea behind a negatively biased ion source is to accelerate the electron more which means accelerating the positive ion less, Preferably we want the deuterium nucleus to stand still, and we can in fact achieve that by ionising it at -62 kV, but the world of fusioneers still believe in building atom smashers to overcome the imaginary Coulomb force, which is why they are still trying after more than 60 years.

Unfortunately the little CCFL inverter won't be able to deliver much current, so the negative bias will fall off as the current goes up, I used a rectified MOT in my ion sources, but that becomes a bit more dangerous.

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Re: Electron gun- what's wrong here?

Post by Jeroen Vriesman »

Hi Steven,

I just tried to answer the original question about an electron source, not the ion source related stuff.

The filament-anode ring-target setup is comparable to a ordinary triode (cathode-grid-anode) with well-known behaviour.
Making the grid in a triode positive a little bit gives more anode current, but make it too positive and the current will only flow between the cathode and the grid (and it will burn the grid in a regular triode).

The absolute potential of the entire setup doesn't make any difference, and having a large potential difference between the filament and the anode ring just draws maximum current between them (limited by emission, space charge or supply current) . Having the filament and the anode ring both biased with an negative potential is the same as have a higher positive potential on the target, but with the side effect that the chamber walls are positive with respect to the electron source, this will pull the electrons to the walls of the chamber.

I've seen these effects with the emission measurements of my thoriated tungsten wire activation experiments, something with the coulomb force I guess.
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Re: Electron gun- what's wrong here?

Post by Steven Sesselmann »

Jeroen,

Yes, it will behave like a triode, however it is not irrelevant at what potential an atom becomes ionised, and in a fusor you will get more fusion when you create low potential ions. The way to do this is exactly as David is doing, one could go a step further and lower both the ring and the filament together, making sure there is enough differential to generate a stream of electrons.

As for the Coulomb force and the gravitational force for that matter, have you ever seen either?

I would be most surprised if you said yes, because I have never heard of anyone observing any of the four fundamental forces we are being taught about, the fact is we observe constant or accelerating velocity and so as to not look like idiots, the so called educated people gave it a fancy name like "Coulomb Force", "Gravity" etc. and those who were not so smart bought it.

Unicorns that's all they are, and the sooner you spot them the sooner they gallop away..

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Re: Electron gun- what's wrong here?

Post by David Kunkle »

Jeroen Vriesman wrote: Having the filament and the anode ring both biased with an negative potential is the same as have a higher positive potential on the target, but with the side effect that the chamber walls are positive with respect to the electron source, this will pull the electrons to the walls of the chamber.
I understand that the chamber walls and nipple the electron source is contained in would be positive w/ respect to the negatively biased source. But with Gauss' Law, wouldn't there be zero E field inside the nipple and the spherical chamber to pull the electrons to the walls? Or is it really just a matter of the negative fields pushing the electrons away, and some of the electrons just happen to find their way to the walls which then simply go to ground?
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Re: Electron gun- what's wrong here?

Post by Steven Sesselmann »

Having the filament and the anode ring both biased with an negative potential is the same as have a higher positive potential on the target
This statement which is widely believed to be true is a false statement. I think I have a reasonable argument for absolute potential and for the statement above to hold true, potential would have to be scalar. A particles velocity appears as a function of it's potential with respect to the observer, so yes Schroedinger was right, it does depend on who is looking.

Steven

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Re: Electron gun- what's wrong here?

Post by Jeroen Vriesman »

Hi david,
But with Gauss' Law, wouldn't there be zero E field inside the nipple and the spherical chamber to pull the electrons to the walls?
Yes, but only if there are no objects in the chamber with some other potential. If you have something inside your chamber at a potential U, and the chamber is grounded, the electric field is about U/d (the shape of the field can be much more complicated, U/d is just an indication) where d is the distance between the object and the wall.

In my experiments involving emission I have to make the filament a bit positive to avoid all the emission current running between the filament and the chamber walls.
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Re: Electron gun- what's wrong here?

Post by Jeroen Vriesman »

Hi Steven,

in your reference you seem to state that potential is limited because relative velocity is limited, however, the energy associated with relative velocity is not limited, it goes to infinity near the speed of light, so the potential is not limited. You are using the special theory of relativity in one argument, but you skip it for the conclusion.

There is some truth in force being an illusion, Newton already knew that his theory of gravity was problematic, it introduced a "mysterious force working at a distance".
However, the problem is not physical, it's a psychological problem, the idea that some kind of "contact" is needed for a "force" is just a result of our size and the way humans experience the world. All "forces" work at a distance, when we push things around there is not a single molecule actually touching another molecule.

What's happening is that the phenomenon we call "force" is just the first derivative of the total energy of the system, we just exchange forms of energy, gravitational, electrical, thermodynamic energy etc.
When we push against a wall it just means that we do not shift enough energy from one place to another to push our hands through the wall, "contact" just means "not having enough energy to change the state of the matter involved".

This insight leads to some very useful mechanical methods, called the Hamiltonian and Lagrangian, https://en.wikipedia.org/wiki/Hamiltonian_mechanics

So there is just the potential of the system. Thermodynamics show us that states which can be solely described by information theory still represent an energy, so the whole thing can be modelled as information only. Attempts at modelling gravity that way have recently been made by Verlinde https://en.wikipedia.org/wiki/Entropic_gravity

So indeed, "force" can be considered less fundamental than potential, but potential is not limited by the speed of light, since the energy needed to reach the speed of light goes to infinity.

In physics the EM theory is considered a "gauge" theory, the potential of the system as a whole can be varied without changes in behaviour.
Only by assuming the entire universe does not have a net charge one could define the absolute zero potential. (the idea that the potential at infinity is 0)

That force (and even the derived concept of a "field") are less fundamental than potential is experimentally proved by the Aharonov-Bohm experiment https://en.wikipedia.org/wiki/Aharonov% ... ohm_effect, a wonderful experiment!

As for your "second case for absolute potential"... seems to me a case of "not even wrong". Choosing the proton is arbitrary (if you take a charmed lambda or sigma particle you get different "absolute maximum" potential), equating the equivalent energy to surface potential, stating that a collection of protons has the same surface potential as a single proton (without even defining the "surface" of the charge collection)? Strange stuff.
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