50 kV Zener Array

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Steven Sesselmann
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50 kV Zener Array

Post by Steven Sesselmann »

One of the critical problems I faced with my last experiment was electron suppression. When we accelerate ions through a negative potential there are always electrons wanting to take a shortcut to ground, and you can trust they will always find a way. The most likely escape is straight up through the your accelerator tube or chamber, which is a major cause of current loss and dangerous x-rays.

X-rays or bremsstrahlung (german for breaking radiation) is generated when the fast electrons hit the chamber wall or a stainless steel flange at ground potential and come to a sudden stop, their kinetic energy has to go somewhere, so it comes out as x-rays.

There are a couple of ways to prevent this. In a linear accelerator like the ones I build we can add a suppressor ring close to the cathode which is negatively biased with respect to the cathode, positioned and biased correctly it can prevent electrons flying up the tube. This is normally achieved by connecting the negative HV to the suppressor ring and then connecting the cathode to the suppressor ring with Zener diodes of around 500-1000V.

This configuration was not going to work for my next experiment, because I want to attempt to measure the fusion rate by measuring the current of the fusion ions with enough kinetic energy to reach ground. To do this I need a) to prevent electrons going back up the tube b) channel them to ground via a conductor. So my plan at the moment is to isolate the cathode (reaction chamber) from the last dynode (D9) and connect the cathode to ground via a 50 KV Zener array and a device to measure the current through the zener array. HV will be connected to D9, this will accelerate ions into the reaction chamber where more ions will be created and fuse, the fast fusion ions will fly up the tube to ground, and the electrons will go via the zener array to ground thereby closing the circuit.

The zener array construction is simple enough, I have made up circuit boards to hold arrays of 200V zener diodes, the image below shows one unit (25 kV).

I intend to build these into a solid polypropylene container with a series of connectors so I can switch the zener voltage in 5 kV steps.

The objective is then to accelerate ions into the cathode at say 51 kV but have the cathode limited to 50 kV so the accelerator ring is always 500-1000V above the reaction chamber. Hope that makes sense.

Steven
IMG_0169.JPG
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http://www.gammaspectacular.com - Gamma Spectrometry Systems
https://www.researchgate.net/profile/Steven_Sesselmann - Various papers and patents on RG
prestonbarrows
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Re: 50 kV Zener Array

Post by prestonbarrows »

I am not familiar with your setup, but in general there will be two main sources of electrons in a beamline which need to be suppressed.

Firstly, ions striking metal surfaces will emit secondary electrons. Ideally, the target is the main place this happens. However, expect to get some beam scraping on you accelerator before it reaches the target, especially if you are rolling your own ion optics. Secondaries from surfaces are born with only a few eV of energy and are simple to suppress with only ~10-100 V in the proper geometry. Since the target is usually at ground and there is ideally no current draw, pretty much any old power supply will work here. A few small permanent magnets behind the target can also work here.

Secondly, you will never get perfect vacuum and always end up with some finite ionization of background neutrals in the beamline. Free electrons from beam ionization will be born with some fraction of the energy from the incident ion. More importantly, they are born in the high electric field of the accelerator and will travel 'backwards' up the beam. Those born near the target will gain nearly the full accelerator voltage while those born near the ion source will have a smaller potential drop to travel through. Typically, more are born near the ion source where the pressure is highest. Some electron suppression between an accelerator stack and ion source is usually a good idea. These are more difficult to deal with because the suppression voltage needs to be on the order of a few kV and tend to draw more significant currents. Additionally, this suppression is usually floating up at high voltage making it practically difficult. The big boys will often use a bending magnet to prevent any backstreaming electrons from the accelerator from reaching the source while also filtering any impurities from the beam.

Using zeners is a pretty clever way to go without needing to float supplies. I would be concerned about arcing transients blowing them out if you don't have any series resistance though.

If you are using electrostatic suppression, make sure the electrode has a length/diameter ratio large enough that you actually get a field inversion along the center axis. The potential will 'sag' further from the electrode and it is possible to have the local electric field never actually flip direction along the axis even though you pass through an oppositely biased electrode.
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Steven Sesselmann
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Re: 50 kV Zener Array

Post by Steven Sesselmann »

prestonbarrows wrote:I am not familiar with your setup, but in general there will be two main sources of electrons in a beamline which need to be suppressed. Firstly, ions striking metal surfaces will emit secondary electrons. Ideally, the target is the main place this happens. However, expect to get some beam scraping on you accelerator before it reaches the target, especially if you are rolling your own ion optics. Secondaries from surfaces are born with only a few eV of energy and are simple to suppress with only ~10-100 V in the proper geometry. Since the target is usually at ground and there is ideally no current draw, pretty much any old power supply will work here. A few small permanent magnets behind the target can also work here.
Yes normally that is correct, secondary electrons from target and incidental strikes with neutrals, in my design there is one more factor, the neutral deuterium gas will be admitted directly into the hollow cathode, where it will ionise at cathode potential, these electrons need to go somewhere...
prestonbarrows wrote: Using zeners is a pretty clever way to go without needing to float supplies. I would be concerned about arcing transients blowing them out if you don't have any series resistance though.
If you are using electrostatic suppression, make sure the electrode has a length/diameter ratio large enough that you actually get a field inversion along the center axis. The potential will 'sag' further from the electrode and it is possible to have the local electric field never actually flip direction along the axis even though you pass through an oppositely biased electrode.
So as I will have the 9th dynode set to a lower bias than the zener array, electrons will be forced to take the zener route to ground rather than flying up the tube.

There will be a 10K Ohm resistor across the final link to ground preventing surges, this will also function as the load resistor for my current measurement.

I will then be able to compare current in vs. current out, to measure the net current consumption.

Steven
http://www.gammaspectacular.com - Gamma Spectrometry Systems
https://www.researchgate.net/profile/Steven_Sesselmann - Various papers and patents on RG
John Futter
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Re: 50 kV Zener Array

Post by John Futter »

Preston
Secondary electron suppression is dependent on acceleration voltage and target type.
From my humble background you will need more than -100 volts to suppress at typical fusor levels.
typically @ work I use 400 volts up to 40kV accel after that I use -1000 volts or more self generated by transil diodes as these have extremely low leakage below break over < 1 ^ -11 Amps
Steven is aware of my work and I think he is using what he saw when visiting a couple of times.

see some of my previous posts
prestonbarrows
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Re: 50 kV Zener Array

Post by prestonbarrows »

John,

Secondary electrons from surface impact of light ions will have a very low energy, on the order of 10's of eV. A hydrogen or deuterium beam on the order of 10's to 100's of keV into a solid target will produce almost all of its secondary electrons with energies below 50eV with a few at a maximum of ~500eV. ~200V is pretty standard for suppression. Of course it does not hurt to run the suppression at a higher voltage if you don't have any near by ion sources which would draw a significant current.

Here are some good references:
-D. Hasselkamp et al., “Particle Induced Electron Emission 2”, Springer-Verlag (1992)
-E. Ebrahimibasabi , "Design and simulation of a new Faraday cup for ES-200 electrostatic accelerator" https://accelconf.web.cern.ch/accelconf ... ppd061.pdf

Any references for Faraday cups will generally give good information about suppression of solid targets.

You can test this for yourself in your specific setup by sweeping the suppression voltage and recording the current on your solid target. For a positive incident ion beam, secondary electrons create an erroneously high current reading; ramping the suppression voltage up from zero will reduce your measured beam target current until all the secondary electrons are suppressed, then plateau at the actual ion beam current when the applied suppression voltage surpasses the energy of the secondaries.

The thing that is actually important is the electric potential on the beam axis rather than voltage applied to the suppression electrode. If you have a poor suppression electrode geometry, it will require a much higher applied voltage to invert the potential along the axis and reflect the electrons. The paper linked above has a good image of the simulated potential and how it sags toward the axis. The hardest secondaries to deal with are the high-energy tail that happen to be oriented towards that droop along the axis.

As I mentioned before, secondaries in the presence of an accelerator field will have much higher energies and require suppression voltages more on the order of kVs (depending on the accelerator voltage/geometry).
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Re: 50 kV Zener Array

Post by John Futter »

Preston
Keep reading
I'm only giving advice from the school of hard knocks.
This is where I started from reading and found I had to up the suppression
400 volts up to to 50kV and more thereafter as used in our implanters.
All of this measured against total charge (ions) implanted, then measured with RBS, if the total charge reduces back off the suppression a bit.
This backed up by serious reduction in x-ray flux 10 ^ 4 when you get it right in the case of my experimental 120kV implanter
a very old pic of it without the secondary suppression here
http://www.coultersmithing.com/forums/d ... &mode=view
the thing with the red light on in the rack is the radiation monitor you will see that it is nearly fullscale and this is outside the 2 feet thick of concrete filled cinder blocks that are my radiation shield.
The Rad reaching the monitor is from the x-rays being reflected down from the roof above ( no shileding going up) and the high Z of the roof is reflecting more than I like
Now it does not throw up the red light @ 120kV 15mA beam current
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Steven Sesselmann
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Re: 50 kV Zener Array

Post by Steven Sesselmann »

50 kV Zener Diode
50 kV Zener Diode
So I finally got around to finish off the 50 KV Zener, after looking for a housing for some time I settled for a 65 mm PVC pipe. At first I wasn't sure how it would hold up to transformer oil, so I did a test and pored a bit of oil into a cap and let it sit for a couple of weeks, and it seemed to have no effect on the PVC.

The construction is pretty simple, there are two PCB arrays of 125 200V zeners in series, which fit snugly into the pipe. I then machined two stainless steel bolts to fit standard 4 mm banana plugs and fitted them into the pipe with rubber O'Rings.

When I install this diode it will have a 1 MOhm load resistor to ground, and I will log the backflow voltage (if any) across the resistor ( 1µA = 1V). If it turns out that voltage fluctuations are too high I can just drop the value of the external resistor.

This component is a part for the FICS Experiment documented here: viewtopic.php?f=6&t=10479

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