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Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Patrick wrote ...
.Id test plastic diodes as carl says, long before I pay big $ for a few glass ones. You dont even know if the outgassing problem exists yet.
I'm pretty confident that at the high vacuum that this will have to run at that any plastic will outgass. I've learned enough about vacuum systems to know that this will be further compounded by the fact that the capacitor plates will also be exposed to this, and that that this will further frustrate attemps to remove the products of outgassing (mean free paths, etc.)
Basically, it is folly to put anything into this system that will outgass at all.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Proud Mary wrote ...
X100UFG 10kV trr 100 ns glass
Z150UFG 15kV 300mA trr 100 ns glass
They won't be cheap.
Yep. Those are from the VMI range that I found. RS is offering them for between £5 and £10 each, slightly cheaper for bulk orders.
Patrick wrote ...
I dont think outgassing at the sub-single micron ultimate vacuum level will effect dielectric strength, and cantelectrons fly through a vacuum given a high enought V? IE, a CRT or VT?
As I understand it, CRT's, VT's etc require a hot cathode. Vacuum capacitors wouldn't work if the electrons just 'flew' from one plate to the other. Any molecules present due to outgassing will be ionised. with any luck they will then become embedded in the capacitor plates (probably), but this stands a much greater chance of working if outgassing is eliminated completely in the first place.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Ash Small wrote ...
As I understand it, CRT's, VT's etc require a hot cathode. Vacuum capacitors wouldn't work if the electrons just 'flew' from one plate to the other. Any molecules present due to outgassing will be ionised. with any luck they will then become embedded in the capacitor plates (probably), but this stands a much greater chance of working if outgassing is eliminated completely in the first place.
Electron emission into a vacuum also occurs by field emission - wave-mechanical tunneling - where the charge density is high enough. This is how 'cold cathode' X-rays tubes produce their electrons.
Field emission does occur in vacuum capacitors and limits their maximum working voltage.
Unwanted field emission effects can be reduced by thoughtful design of electrode geometry, careful selection of metal alloys, and so on.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Proud Mary wrote ...
. Electron emission into a vacuum also occurs by field emission - wave-mechanical tunneling - where the charge density is high enough. This is how 'cold cathode' X-rays tubes produce their electrons.
Field emission does occur in vacuum capacitors and limits their maximum working voltage.
Unwanted field emission effects can be reduced by thoughtful design of electrode geometry, careful selection of metal alloys, and so on.
I'm currently reading up on Fowler-Nordheim tunneling. I'll reply tomorrow.
Registered Member #2838
Joined: Fri Apr 30 2010, 07:55PM
Location: tehachapi, CA
Posts: 333
I still have some glass selenium diodes that I accidentally got a lo g time back. Was trying to sell/trade them way back. No idea of the ratings but definately high voltage. If you are I treated send me a PM. Check this old post for a pic.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Patrick wrote ...
Proud Mary wrote ...
Unwanted field emission effects can be reduced by thoughtful design of electrode geometry, careful selection of metal alloys, and so on.
Reduced but not eliminated, I presume?
Field emission into a vacuum which produces measurable current at an anode has been reported below 20V using specially prepared 'emitter tips.'
Adler, E.A. Bardai, Z. Forman, R. Goebel, D.M. Longo, R.T. Sokolich, M. Demonstration of low voltage field emissionElectron Devices, IEEE Transactions on Vol: 38 Issue 10, pp.2304 - 2308, Oct 1991
The sharpest of the latest sharp emitters taper down to a single atom at the tip.
So far as I understand - which is not very much - given the right circumstances, field emission into a vacuum can occur at any PD exceeding the work function of the emitter, which is highly dependent on surface effect. Different crystalline forms of the same chemical element will have different work functions, and all expectations go out of the window at the nano level. Carbon nanotubes are now used for cathode surfacing in sub-miniature X-ray sources, and there are even UK amateurs exploring this new technology.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Patrick wrote ...
Proud Mary wrote ...
Unwanted field emission effects can be reduced by thoughtful design of electrode geometry, careful selection of metal alloys, and so on.
Reduced but not eliminated, I presume?
Initial reading suggests that, for cathode temperatures below ~1,500°C, a charge of ~1GV/m is required for field emission in a vacuum.
This would also appear to conform (approximately) to the concepts involved in the Siemens design mentioned here:
EDIT: This doesn't apply for sharp, pointed cathodes, as PM has pointed out above.
Oidium45 wrote ...
I still have some glass selenium diodes that I accidentally got a lo g time back. Was trying to sell/trade them way back. No idea of the ratings but definately high voltage. If you are I treated send me a PM. Check this old post for a pic.
I will try to post part numbers when I get home.
I think they will be too big for this. I think I will also require at least a hundred 10kV diodes. I'm planning a (nominal) 13 stage multiplier fed with 50kV. I also know absolutely nothing about the speed ratings of selenium diodes.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Ash Small wrote ...
Patrick wrote ...
Proud Mary wrote ...
Unwanted field emission effects can be reduced by thoughtful design of electrode geometry, careful selection of metal alloys, and so on.
Reduced but not eliminated, I presume?
Initial reading suggests that, for cathode temperatures below ~1,500°C, a charge of ~1GV/m is required for field emission in a vacuum.
This would also appear to conform to the concepts involved in the Siemens design mentioned here:
EDIT: This doesn't apply for sharp pointed cathodes, as PM has pointed out above.
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 classi fied 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 eff ects can produce emitting sites commonly called 'emitters,' which can be identi fied 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
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Looking for some GLASS high voltage fast diodes.
