Question Regarding HV DC PSU

[gamempire]

Hi All, first time posting here, and I'm sure it will be the first of many posts.

Just a quick question regarding a power supply for a fusor, as I am in the process of collecting parts.

I've done alot of reading and study on the subject, and was under the impression that a positive polarity (negative grounded) supply was required. From my initial understanding of the F-H and H-M designs, I thought the positive supply was connected to the outer grid, and the negative supply was grounded. I assume then that the gas entering the chamber would be ionized by the outer positively charged grid, and then attracted to the grounded center grid, where the ions would then smash into neutral deuterium nuclei and then fuse.

I seem to have misunderstood the theory of operation somewhere, and have been told I need a negative polarized/biased (positive grounded) supply to attach to the center sphere, while the outer sphere is grounded.

Can someone maybe explain to me how the negatively charged inner grid ionizes the gas then?

Also, I can get a -25kV supply, but it only outputs at about 20mA. Is this enough to achieve fusion?

Thanks,

Josh

[Carl Willis]

Hi Josh,

The typical amateur fusor uses a grounded vacuum chamber as the anode and a high-voltage (negative) cathode grid. It is possible to use an additional outer grid at +HV, though this is seldom done because of the added complexity and seems to have few advantages.

The mode by which a hollow-cathode discharge is established is by ion recirculation. Radial electric fields confine ions to radial paths, and impart high kinetic energies to the ions toward the center of the fusor because the cathode is at a high negative potential. Since the cathode is largely transparent, the ions pass through it and continue their trajectories in the radial field. Along these long path lengths, ions participate in collisions with neutral gas, resulting in many electrons that are accelerated toward the anode by the fields. The electrons produce additional electrons and ions, the latter are caught in the hollow-cathode trap, causing more ionization, and the self-sustaining discharge continues thusly.

A 25 kV / 20 mA supply is sufficient to produce maybe 10,000 - 20,000 neutrons / sec. in a deuterium fusor Detectable, but not impressive. Higher voltages are recommended. The current is probably adequate. Note that anything is sufficient to achieve fusion in the DD reaction--it is a spontaneous and exothermic nuclear reaction. It just happens much faster with higher energies. Quantum-mechanical tunneling through the Coulomb barrier is the theoretical explanation for how the two D nuclei "fuse."

-Carl

[gamempire]

Hi Carl,

Thanks for the quick reply. Just finished an electromagnetics midterm, so my brain is a little shot at the moment.

Can you do me a favor and take a look at this Glassman spec sheet. Its the supply I'm considering using. http://www.glassmanhv.com/PDFs/EW_Series.pdf

I'd be getting a EW25N24, which is a -25kV 24mA supply. I can also get my hands on a -60kV supply at 16mA, however, the xray and neutron radiation is something I can't really shield against right now (especially since the xrays will start passing through the SS chamber around 45kV). I'd rather take it a step at a time, and first achieve fusion, and then make something impressive.

Thanks again for the help.

-Josh

[Richard Hull]

Much of the neutron number game is obviously reltaed to voltage. See my recent post and subsequent images on just such data.

viewtopic.php?f=6&t=2806#p12316

the detector is as important as the fusor itself. If you limit yourself to 20-25kv, the simple BF3 detector will be OK if hooked to a counter, but a lot of collection time will be needed to run the fusion out of the noise. (especially below 20kv)

A large HE3 detector can sniff out much, much lower fusion levels. (10-12KV)

For a real easy detect you will need to boost up the cross sectional curve to 30 KV and above. My large He3 detector will roar at 32kv @ 10 ma and 15 microns of D2 pressure. 10,000 or more counts each minute.

At 50kv or more the fusor becomes a serious health hazard from X-rays. Also, the neutron numbers are in the millions per second from the device. Insulation issues, cooling issues and radiation issues limit this super performance range to only the bigger amateur players with deeper pockets and, or, shallow pocketed expert scroungers.

Above 60kv it is not a question of verve, but of nerve in amateur fusion.

Richard Hull

[Carl Willis]

Hi Josh,

The Glassman EW supplies look good enough for fusor purposes. The most significant detractor might be the price (if you buy the EW25N24 new, I'd guess you're looking at ~ $5000). An attractive feature of this particular series is that they are claimed to be air-insulated. This presumably means that all parts are unpotted and accessible if you want to do mods or repairs. I don't figure how they can put an air-insulated 25 kV supply in a 3.5" chassis, however, so it might pay to call the factory for clarification.

Don't forget to consider the savings inherent in building something yourself or in using a surplus x-ray unit. The markup on lab HV supplies is astronomical because the companies involved are foraging for lush scientific grant money.

-Carl

[Wilfried Heil]

Hello Josh,

welcome and good luck in scrounging.

If I had the choice I'd get the -60KV supply. Even if you run it at just 100W, you will have plenty of headroom for higher fusion levels, which depend much more on voltage than on current or raw input power. You will need a geiger counter or ionisation chamber to monitor x-ray levels anyway, so you can limit the voltage before these emissions become excessive.

Not all HV supplies are air insulated, but many from Glassman are build in this way.

Here's how such supplies are constructed, since I had to reverse one:
>viewtopic.php?f=11&t=4622#p27460
Tests of the voltage dependence of fusion rates:
>viewtopic.php?f=14&t=6733#p42416
You can search the HV forum for more, just about everyone with a fusor has done similar tests. At least this gives you a rule of thumb on what you can expect in this range of voltage and power levels.

Wilfried

[gamempire]

Again, thank you all for the input.

Richard - Trust me, the detection requirement has been thrown on me plenty, I know its very important. I'm going to be using a BF3 tube from Nwood labs to do the neutron detection (still working out the actual counter). My professors have also asked me to do some optical spectroscopy of the plasma, first with my proof of principle design which is just going to be a glow discharge plasma and then with the D-D plasma fusion reaction.

Carl - I know the price on the glassman supplies. I'm hoping to get a Glassman or Spellman or something along the likes off of ebay, but I do have a considerable bit of funding to work with. I've scrounged on alot of parts for collecting so far, but the power supply is not something I wanted to skimp out on.

Also, I have a 15kV 30mA neon sign transformer, but my professors want me to stick with a pre-built HV supply for safety issues.

Wielfried - Thanks for the reply, as I said before, I don't plan to go over 20 to 30kV. I'll read through your efficiency tests tonight, but just a quick question...Did you actually achieve fusion at 20kV at such low current? I understand that fusion levels depend heavily on voltage, but I still thought a reasonable amount of current is required. I can very easily get a -30kV supply with 5mA, as there is one that is not in use in the thin film lab on campus....but is that enough?

-Josh

[Wilfried Heil]

You can get detectable fusion at 15-20 kV and below. With 20 kV and 5 mA current (100 W) we measured 5000 n/s, which isn't much fun at all, but easy to detect with the BTI bubble dosimeters.

Like Carl has mentioned above, some fusion will take place at any voltage or current. The question is just how much you need and for what you want to use it. If you intend to do activation experiments, you will be happier with higher neutron rates in the 100 000-200 000 n/s range. The PSU with 30 kV at 5 mA should give you about 50 000 n/s with a reasonably clean chamber, the efficiency increasing quickly with higher voltage. There is a limit on the amount of power that you can feed into a given fusor, because the grid will have to dissipate a significant portion of it without thermal runaway or melting.

Have a look at Jon Rosenstiels neutron activation experiments, which should give you an overview of what materials can be used and the neutron rates needed.

For what purpose do you intend to build a fusor, do you have specific experiments in mind?

[gamempire]

Thanks for the further info Wilfried.

As to the question of why I intend to build a fusor? Well, I'm an electrical engineering student at the George Washington University. We're required to complete a 3 semester long senior design project/course. Most students get away with designing and building very simple projects. For instance, almost every year there is a EE or Biomedical Engineering student who builds an electronic stethoscope. Simple and easy. Not my cup of tea.

I wanted to challenge myself however. Fusion's interested me every since middle school, and I've been lurking around the fusor.net forums for a year or so now (as you can see, I've only recently begun posting =P), just reading up on alot of material.

The professor who runs the senior design program as well as my mentor worked in plasma physics for many years (my mentor at NRL) , so when they saw my proposal, they were incredibly interested in me pursuing the project. I'm sure future experiments for the fusor will come about as progression is made, but right now, its just the principle of achieving fusion (neutron counting and such) and doing a little optical spectroscopy that is the reason behind the project.

Also, as part of the project, we have to complete a preliminary design report, a complete design report, and a final design report over the course of all three semester. Its really meant to be a "real world simulation" in engineering technical writing, and eventually, I'd like to provide my final report to the community as a true manual to building a fusor.

I'd like to thank everyone again for all the quick replies and help you've provided. This website is definitely a great resource to the amateur fusion community. I can't wait till I start having questions on the gas handling system! =P