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Looking for some GLASS high voltage fast diodes.

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Ash Small

Mon Sept 05 2011, 12:07PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Proud Mary wrote ...

.
An atomically sharp emitter might, of course, appear perfectly smooth and flat to the eye, a matter of concern in linear accelerators:

"In practice, no metal surface in an accelerator cavity is perfectly flat and clean.
As a result, there are large variations in the microscopic surface field which
lead to so-called enhanced field emission (EFE). Considerable progress has been
made in the past few years in our understanding of the various contributions
to EFE. These can be classified into various categories:

(a) Metallic surface roughness due to imperfect machining, scratches, microprotrusions,
"tip-on-tip" protrusions
(b) Metallic dust, microparticles
(c) Grain boundaries
(d) Molten craters after breakdown
(e) Dielectric impurities and layers
(f) Absorbed gas
(g) Metal-insulator-vacuum (MIV) or metal-insulator-metal (MIM) layers.

All of these effects can produce emitting sites commonly called 'emitters,'
which can be identified by various surface physics diagnostic techniques."

Source:
Wang JW, Loew GA Field Emission and RF Breakdown in High-Gradient Room-Temperature Linac Structures SLAC-PUB-7684 October 1997

Pretty much everything inside the vacuum system, especially the capacitor plates, will be electro-polished.

I will probably use stainless steel for the capacitor plates, as aluminium can be 'problematic' at high vacuum, and I already have the facilities to electro-polish stainless (although electro-polishing aluminium is fairly straightforward).

We're talking milliamps of current, so using stainless shouldn't be a problem.

Eliminating outgassing as described above should avoid problems related to absorbed gas.

I have some 'special cement' (I'll post details when I dig it out) for insulation within high vacuum systems (for mounting/insulating the capacitor plates, 50kV feedthrough (although the 'fusor crowd' use modified unsuppressed automotive spark plugs for their 50kV feedthroughs), etc.)

I'm anticipating that the diodes themselves will be the most problematic, as far as possible sources of emission are concerned.

I obviously need to construct the 50kV supply before I can start running tests.

Maybe I should read up on sputtering of Sn, as the diode leads are (usually) tin plated copper (copper is 'horrible' as far as sputtering problems are concerned). If this could cause problems maybe I could electro-plate another, sputtering resistant, metal on top? (Maybe strip off the tin and plate the copper leads with nickel, I'll have to read up on this)

EDIT: It looks like Sn doesn't sputter very easily

jpsmith123

Mon Sept 05 2011, 04:35PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

Maybe in your case (1 MV or less) it's not worth the hassle, technical risk and expense of trying to put the multiplier inside the vacuum system.

Maybe something more conventional, like what's described in the following link for example, would be a better approach:

Sulaiman

Mon Sept 05 2011, 05:28PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

I have used BY84xx diodes,
4kV to 24kV, 20mA to 3mA, 100ns glass, e.g.

Ash Small

Mon Sept 05 2011, 06:53PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

jpsmith123 wrote ...

Maybe in your case (1 MV or less) it's not worth the hassle, technical risk and expense of trying to put the multiplier inside the vacuum system.

The main reason for putting the multiplier inside the vacuum system is due to the problems associated with a ~650kV HV feedthrough. While these voltages have been achieved by some of the coilers here, trying to feed this into a vacuum system presents some serious problems. Also, the biggest multipliers I've seen here (Daniel's, I think) are only around 200kV.

jpsmith123 wrote ...

Maybe something more conventional, like what's described in the following link for example, would be a better approach:

I've looked at a number of 'amateur' designs over the past couple of years. That one you've linked to is pretty typical of a lot of the smaller ones. 10^-6 is about what you'd expect with plastic in the system, and you won't get much current from a VDG.

My proposed design is all stainless and ceramic, apart from the diodes.

As I said in another thread, I'd realised I needed some form of vacuum capacitor. That Siemens design you posted just gave me a few ideas.

(FWIW, I used to have a stainless fabrication business, I still have my own electro-polishing facility, and I've worked on some of the biggest accelerators in Britain, as well as in other areas of the vacuum/ion beam/plasma industry, so I've a pretty good idea of what works and what doesn't. I'm still open to suggestions, though. (Other forums have told me that 650kV in a vacuum chamber is impossible for an amateur, but the criticism here at 4HV is a lot more positive, and helpful.))

Sulaiman wrote ...

I have used BY84xx diodes,
4kV to 24kV, 20mA to 3mA, 100ns glass, e.g.

It looks like those are no longer made, and out of stock, but thanks anyway. It's beginning to look like I'll have to fork out Â£500 to RS.

Proud Mary

Mon Sept 05 2011, 07:45PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Ash Small wrote ...

10^-6 is about what you'd expect with plastic in the system

Edwards fluoroelastomer seals are good to 8 x 10E-10 torr. PTFE is frequently used in high vacuum systems.

jpsmith123

Mon Sept 05 2011, 08:17PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

Well if the tube (and corresponding multiplier parts or whatever) are electrostatically graded from the ground end up to the HV end, and the HV end is shielded with some kind of spheriod or toroid or something, then I don't see where you would have a problem. In this case the ion or electron source would be mounted inside the shield and adjusted to expose it to whatever electric field you want to extract the ions or electrons from the source.

Ash Small

Mon Sept 05 2011, 08:51PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Proud Mary wrote ...

Ash Small wrote ...

10^-6 is about what you'd expect with plastic in the system

Edwards fluoroelastomer seals are good to 8 x 10E-10 torr. PTFE is frequently used in high vacuum systems.

Yes, and yes.....I have a large number of different types of seal in my 'vacuum parts bin', but I'll only use re-usable seals on frequently opened flanges. PTFE still absorbs and releases gasses, etc. but is better than most plastics. One of the issues here is 'what will happen to it when subjected to protons and other ions, etc at energies of up to 650keV?' or whatever energies I manage to achieve. The last thing I want is excess hydrogen (or other organic molecules) contaminating the system. Copper seals will hold a 10^-12 vacuum, silver plated copper performs even better, especially if the system is to undergo long periods of 'bake-out' at high (ish) temperatures. I think the 10^-10 figure you mention is a 'best case scenario'. If I have access to better equipment it's a bit silly not to use it, especially in a project as ambitious as this.

Nigel Harris, former manager of the Edwards Vacuum Training Centre, says in his book, 'Modern Vacuum Practice' that 'The main requirement for UHV joints is that they have low outgassing rate and leak-tightness to less than 10^-10mbar l s^-1. In general, therefore, metal gasket materials are preferred'.

BTW, I was 'very' surprised to see that on the ISIS 800MeV proton accelerator at the Rutherford Appleton Laboratory they used a viton seal on the ~Â£25,000 several kilogram oxygen free copper tube that forms the end of the the accelerator where the beam enters the target station and hits the 'first' carbon target that produces pions which then decay to muons, before it hits the 'second' tantalum plated tungsten target that spallates neutrons. Presumably this is because this is one of two seals designed to be opened by the four robotic arms that will carry out maintenance, etc. for the next 20 years.

End of one of the arms:

Proud Mary

Mon Sept 05 2011, 09:17PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Teflon-FEP stood up very well to 1 MeV electron and 800 keV proton bombardment in tests described in this NASA Technical Memorandum TM X-1905:

Ash Small

Mon Sept 05 2011, 09:54PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

jpsmith123 wrote ...

Well if the tube (and corresponding multiplier parts or whatever) are electrostatically graded from the ground end up to the HV end, and the HV end is shielded with some kind of spheriod or toroid or something, then I don't see where you would have a problem. In this case the ion or electron source would be mounted inside the shield and adjusted to expose it to whatever electric field you want to extract the ions or electrons from the source.

Would you mind expanding on this a bit, JP?

I'm still not sure I want to be 'playing' with 650kV outside of a grounded chamber.

Proud Mary wrote ...

Teflon-FEP stood up very well to 1 MeV electron and 800 keV proton bombardment in tests described in this NASA Technical Memorandum TM X-1905:

Another very interesting paper, PM. How do you manage to find them all so quickly?

I've not read it all yet, but first impressions are that one amp for one second (one coulomb) is ~8x10^17 (I think). The paper refers to ~~10^15. also, Teflon FEP isn't PTFE (poly-tetra-fluoro-ethylene), it is a Teflon fluoro-ethelene-propylene co-polymer, but I appreciate that it is probably similar.

jpsmith123

Tue Sept 06 2011, 02:48AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

Here are some "pictures". The sphere on the left (the HV terminal with source inside) is charged to 600 kv with respect to the grounded plate on the right. The intermediate toroid electrodes grade the field uniformly.

And here's a cutaway view so you can better see the source (in this case electron source).

And here's the electric field:

And here's a close-up view of the field near the electron source. You can see how the field only penetrates a short distance into the inside of the sphere.

And for a different representation of the field, here are the potential isolines.

And here's a simulated 10 ma electron beam traversing the accelerator.

1315277246 1321 FT123704 Simulated Beam 10 Ma Fixed Emission

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