EEVblog Electronics Community Forum

General => General Chat => Topic started by: daqq on September 24, 2017, 12:29:35 pm

Title: Photoelectric cathode for low power vacuum tube?
Post by: daqq on September 24, 2017, 12:29:35 pm
Hi guys,

I've been thinking - the point of the filament heater is to enable the release electrons from the cathode. Would it be possible to use a cathode that uses photoelectric release of electrons? Basically instead of heating a bit of metal to hundreds of deg.C, you could shine an LED (or other light source) on to a coated cathode and use such a tube for the same purposes you'd use a thermionic one, if obviously at much lower powers.

It would draw much less power, start up instantly, have a longer life span. The downsides are obvious - much less power.

Basically I've been thinking about how to make a very low power x ray source that could be used inside of a detector, and this came up as an idea - no filament, just the accelerating voltage, which would draw pretty much no current.

I know there are phototubes, PMTs, etc. but they are used to actually measure light and not use the photoelectric electrons to do signal amplification and other tube functions.

So, could such a tube work?

Thanks ,

David
Title: Re: Photoelectric cathode for low power vacuum tube?
Post by: T3sl4co1l on September 24, 2017, 06:07:49 pm
It's not a bad idea for the x-ray tube.  But, keep ions well away from the cathode.

That, and the piss poor current density, are the limiting factors in general use.  I suspect you wouldn't get better efficiency than a coated filament (battery powered, 1xxx series) type, except in broad daylight perhaps.

I suppose you'd make the grid(s) as normal, and the plate as a mesh?  Still not going to be much light reaching the cathode...

Tim
Title: Re: Photoelectric cathode for low power vacuum tube?
Post by: NiHaoMike on September 24, 2017, 06:26:00 pm
Field emission would probably be a better choice.
https://en.wikipedia.org/wiki/Field_electron_emission#Large-area_field_emission_sources:_vacuum_nanoelectronics
Title: Re: Photoelectric cathode for low power vacuum tube?
Post by: daqq on September 24, 2017, 06:51:54 pm
Huh, did not know about the field emission technique! Looks very nice!

Note: I'm not actually building this, I'm just thinking about stuff at random.
Title: Re: Photoelectric cathode for low power vacuum tube?
Post by: Kleinstein on September 24, 2017, 07:10:32 pm
X-ray tubes usually use quite some power - more like kW instead of mW. So who case about some 100 mW for a filament. A powerful LED / laser also needs some power. For efficient xray generation one usually need's rather high voltage.
Title: Re: Photoelectric cathode for low power vacuum tube?
Post by: Gyro on September 24, 2017, 07:17:33 pm
Minature Cold cathode X-Ray tubes were used in the Soviet era...

http://www.ebay.co.uk/itm/Miniature-X-Ray-tube-for-x-ray-photography-and-research-Cold-cathode-Rare-/252679396257?hash=item3ad4dd9ba1:g:UwIAAOSwd4tTtb1j (http://www.ebay.co.uk/itm/Miniature-X-Ray-tube-for-x-ray-photography-and-research-Cold-cathode-Rare-/252679396257?hash=item3ad4dd9ba1:g:UwIAAOSwd4tTtb1j)