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LAN_403

attune

Fri Aug 18 2006, 11:42PM

Registered Member #259 Joined: Sun Feb 26 2006, 05:57PM
Location: Friday Harbor, Wa
Posts: 18

I have recently aquired a vacuum system that can pump down a decent volume to 10-5 torr. I am also building a 100kV C-W multiplier. The idea of combining the two is exciting. I have seen various websites with vacuum tubes made of insulating material accelerating electrons with high voltage supplies. 100keV electrons are obviously cool and useful for many things. What would be the minimum tube length that allows for the electrons to be accelerated to the full velocity? The kinematic equations are basic, but what is the force on the electron as it travels through the tube? Is seems that it would change with distance. Any help appreciated!

Dr. Slack

Sat Aug 19 2006, 07:38AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Rather than worry about integrating force and suchlike, go for the easy route and consider energy. Once the electron has fallen through 100kV, it will have an energy of 100keV. I think this is still non-relativistic speed, so if you want the velocity from this just invert E=1/2mv2 (in consistent units of course), where m is the electron rest mass.

The minimum tube length is that which will withstand the voltage difference without breaking down. A hard vacuum is very strong, but you might find that 5torr will support long discharge columns, do some research into gas discharges.

One thing 100keV electrons are very useful for is making Xrays when they hit heavy atoms in the anode. You might want to consider shielding, distance or only short run times to protect yourself from exposure

<edit> duh, idiot me, I wondered why I thought 5torr wasn't the good vacuum you asserted, but 10^-5 is rather better </edit>

Sulaiman

Sat Aug 19 2006, 09:25AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

As above, the minimum distance is that required to prevent flashover.

The maximum distance is determined by 'mean free path'
which for air at room temperature is approximately 5m @ 10-5 torr
(500mm @ 10-4 torr etc.)
so unlikely to be a problem at your working pressure.

100 keV electrons will not penetrate very far, about 0.1mm water/skin
so not very useful as an electron beam/death ray.

(you'd better check my figures, 25 year old memories here)

Also, as above, consider accidental x-ray generation hazard.

Tesladownunder

Sat Aug 19 2006, 11:12AM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

Sulaiman wrote ...

.... 100 keV electrons will not penetrate very far, about 0.1mm water/skin
so not very useful as an electron beam/death ray.
(you'd better check my figures, 25 year old memories here)..

As I understand, modern x-ray machines use perhaps 125 keV for "big" people so it certainly gets through more than 0.1mm. Also consider the x-ray aperture as a lot of simple glass blowing is lead glass and opaque to x-rays. Use soda glass instead perhaps.
When making glassware more important is surface tracking which at 3 times a room air spark distance will be 15 inches minimum that needs to be allowed for on the outside.

The beam distance in an x-ray tube with a pretty good vacuum is only an inch on the outside and all oil enclosed.

Peter

Sulaiman

Sat Aug 19 2006, 01:20PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I meant the actual electron beam, not x-rays generated by the beam.

Bored Chemist

Sat Aug 19 2006, 03:07PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

Any decent current at 100KV will produce a nasty set of Xrays. You should probably think about shielding before you do anything else.

attune

Sat Aug 19 2006, 03:45PM

Registered Member #259 Joined: Sun Feb 26 2006, 05:57PM
Location: Friday Harbor, Wa
Posts: 18

15 inches to prevent the skin effect? I will to some testing of materials to find out just how small I can make it. As for shielding and exposure time, no kidding! I will be careful. Immersing a smaller tube in an oil was mentioned. Pyrex in Mineral Oil?

IamSmooth

Sat Aug 19 2006, 08:07PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

Just to give a point of reference, in the radiology suite they use about 70-85KV routinely for flouroscopy on the average-sized individual. 100KV will certainly be a problem and I would avoid the exposure. If you really want to do this I suggest you get a radiation badge and monitor your exposure. You don't want to delude yourself that you are protected only to find out in 10 years as you look at a picture of your cool electron tube that you are dying from lymphoma.

Carbon_Rod

Sat Aug 19 2006, 09:25PM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

NeilThomas and BC pointed out this can be a very unhealthy apparatus to be around -- and I fully agree.

Even multi-stage accelerators will have numerous shielding precautions and in fact can prove just as dangerous if one happened to wander into the targeting chamber during operation.

If you have to ask how itâ€™s done then you are unlikely knowledgeable enough yet to safely handle the physics involved. Be patient in your studies as the nature of the universe is unlikely going to abruptly change anytime soon.

You get an A for effort though.
Cheers,

attune

Sat Aug 19 2006, 09:59PM

Registered Member #259 Joined: Sun Feb 26 2006, 05:57PM
Location: Friday Harbor, Wa
Posts: 18

Peter Bradly at Houghton college has a very good paper on the constuction of a similar accelerator, though of twice the voltage. He did a careful measurement of the x-ray and gamma radiation, and found that even right next to the fully energized device it would take 10 hours to reach the recommended yearly exposure. As for my own device, I will be purchasing a reliable geiger counter and will be working with an ex-nuclear engineer. Radmax Lead sheilding is cheap, and will cover the device with the greatest practical thickness. Distance will be kept to a maximum, with remote control unites, and exposure time a minimum, with only short pulses of power supply. Sound good?

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