3W UV Led´s can replace hot cathode by photoemission in ion gun design ?

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
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gabrielArgentina
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3W UV Led´s can replace hot cathode by photoemission in ion gun design ?

Post by gabrielArgentina » Wed Jan 14, 2009 7:35 pm

3 W at 400 nmts is a lot of power!...if is "output power"...!!!.
But the real things is that you can make ions or electrons from the outside of ion chambers
trough a quartz bar for guide the UV radiation...no isolation transformer for filament..and better yet ..not filament at all!.
Questions arises ..like ..can the neutral atoms be ionized directly by radiation ?..the answer is yes..but in what practical amount.?.
Can a simple photo cathode (example here please) be an efficient and practical electron emitters for make ions by collisions ?.
The advantage ,in case it be possible , are several..like high frequency pulsed ions gun...you don't need complex high voltage/speed circuit...only switch on off the led at ground level .
Whats the forum opinions and suggestions and or previous experience in this matter???.
Gabriel.

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Richard Hull
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Re: 3W UV Led´s can replace hot cathode by photoemission in ion gun design ?

Post by Richard Hull » Wed Jan 14, 2009 9:31 pm

It is all about ion current and you note that in a way.

Photo ion currents are ultra low. Vacuum UV of much shorter wavelegth would be better. If this 3 Watt LED will make a significant ozone smell in free air around its package you are only just barely getting good. If it won't, then just forget it.

We make undirected ion blasts over the entire fusor volume in the multi-milliamp range with just our old design. To get a photo current of just a single milliamp using UV light of short wavelength would deplete or damage a classic photo cathode in short order.

There are possibilities however, should someone want to go with this avenue of reasearch in a fusor. Vacuum UV due to multiple intense short arcs in a fusor at the shell would do a far better job assuming one is not efficiency conscious.

I could see a bunch of micro series gaps cinched up in an equatorial ring in a common fusor located just at the wall or shell. The gaps need only be tiny, (~1mm) but have an amp in the arc against .020" W electrodes.

You may have started a lot of wheels turning in the "doer's" brains here, even if the UV LED thought is a wash. Thanks.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

gabrielArgentina
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Re: 3W UV Led´s can replace hot cathode by photoemission in ion gun design ?

Post by gabrielArgentina » Thu Jan 15, 2009 1:51 am

Richard wrote<To get a photo current of just a single milliamp using UV light of short wavelength would deplete or damage a classic photo cathode in short order.>.

Richard you are right in classic photocahtode , however in my mind I see an small all metallic surface emitting electrons..
I do not yet the research of stainless steel,cooper,gold,silver,etc ....efficiency as photoelectron emitter @ 390nm ,Any add or proper links will be welcome!!.
My need is in the order of 10 to 20 microAmps only!..
Yes ..in the free air , this kind of device (200mw real output) make a lot of ozone !.
Gabriel.

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Frank Sanns
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Re: 3W UV Led´s can replace hot cathode by photoemission in ion gun design ?

Post by Frank Sanns » Thu Jan 15, 2009 2:28 am

If you are referencing the photoelectric effect then you will not produce any significant ions with UV light illuminating a metal surface. In order to kick an electron out of a metal surface, energy needs to be put in to knock the electron out. This is the work function. These are single eV values. The metal can be biased to this level so any photon above a threshold will kick out an electron but still not many can escape. Those going in any direction other than straight out of the metal AND from the very surface of the metal will still not emerge as they will be trapped within the metal. It is another bleak game of statistics and losses.

If you are hoping for direct ion formation of ions then this is even bleaker and actually impossible. Chemical bonds are of lower energy than to remove an electron from deuterium. Oxygen (O2) takes just under 3 eV to break the bond and have the chance that ozone (O3) can form. This corresponds to UV light. No amount of visible light can do this task but as the energy of the photon increases then it will happen at the ~3 eV threshold. To remove an electron takes 13.6 eV which is in the low x-ray energy range. This too is a very ineffiecient process.

Nice idea but the physical laws have heavily stacked the deck against you.

Frank Sanns

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Re: 3W UV Led´s can replace hot cathode by photoemission in ion gun design ?

Post by DaveC » Thu May 07, 2009 6:52 am

Gabriel -

A UV stimulated photocathode was used in one of the early free electron lasers, that I got to see at a nearby Aerospace contractor's facility. A free electron laser wiggles an intense, high energy electron beam magnetically, causing axial emission of EM radiation, which is then coupled into the electron beam to phase lock it.

Everything about that device was over the top. But the photo-cathode was particularly interesting. It was made of Lanthanum HexaBoride.. LaB6, and illuminated by a 3 MW (that's megawatts) sub-nano second long UV laser pulse. Can't remember the UV laser details, but I think it was a frequency quadrupled IR beast.

I recall asking whether the photocathode actually had enough electrons to supply the total current, and was told about "bleaching" the photocathode by such intense illumination. This in fact, limited their pulse rep rate. The cathode had to recharge so to speak.

The electron beam that was to be "wiggled", was 0.5 mm in diameter, 20 amp average current, and was bunched to make packets of electrons a few mm long. Quite an impressive operation. It was expected to produce about 1 kW of fairly longwave IR laser illumination.

What got my attention also, was the generous supply of lead bricks that were around. There were lots of xrays put out by this animal.

So, yes you can make a UV photocathode... but, going along with what Frank has mentioned, it takes quite a bit of UV to get a useful current. The process isn't too efficient.

Dave Cooper

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