Archived - Electronbeam in air

[Rapp Instruments]

Hi,
I made a tube with a aluminium folie window (lenard tube) to see a moderate energy (60 keV) e-beam escaping to air. The folie window is about 7 micrometer thick and has a diameter of 1.5 mm. The cathode is made from a old lamp, the glas bulb was opened and runs with a heating voltage of 6 V. The cathode is high (-60kV) the window is used as anode at ground. The electrons in the air are diffussed after a short distance and have a range of 2 or 3 centimeters. If I place crystalls in the beam they are fluorescentig nicely. Fluoritcrystalls show a blue fluorescence and a ruby rod show the typical dark red radiation. Because of a lot of x-rays generated the pictures where taken through a lead glas shield.
Thomas

[Scott Fusare]

Thomas,

Beautiful work! I have never before seen photographs of a Lenard tube in operation. Thanks for sharing.

BTW - What pressure are you operating at? I assume it to be quite low, <10E-5 Torr?

Scott

[Carl Willis]

Hi Thomas,

Again, a very nice piece of work from your busy lab. Do you have any photos of the whole assembly? Have you tried taking any photos of the beam with in the presence of a magnet? Any idea what kind of electron current is being emitted?

Thanks,
Carl

[adrian.f.h]

That's an amazing effort.

A lanard-tube is a Project I'm already working on but first I need to get my next rotary pump on line. So you used a 60kV PSU? What kind of? What's the pressure?
I'm asking because I don't have a 60kV transformer yet (I plan to build some sort of high-power ZVS-driver and a selfwound transformer one day) and was wondering if also a marx- or Van de Graaff-generator is suitable.

I'm busy with some Lasers and a view BEAM-bots.

[Rapp Instruments]

Hi
the pirani meter shows zero, which means a pressure lower than 10-3 mbar. I think it's from 10-5 to 10-4 mbar, anyway low enough that there is no glow discharge with highest voltage.
The assembly is rather simple, just a NW 40 tube made from plastic material. To supress surface discharge I fixed a PVC shield with hot melt. The cathode is mounted on a feedthrough. The filament ist surounded by a aluminium tube working as a wehnelt cylinder.
Meantime I got a problem with the filament-transformer,so I had to change the the circuit. The cathode is grounded now and the window is on high positive voltage. For this I hat to rewire my homemade high voltage supply for positive output. The supply is of common construction, inverter, ferrit transformer and multiplier chain. With no load it reach about 120 kV.
The emission current is arround 80 µA, but I think only a very small part of the beam escape to open air. I fixed a ring magnet in front of the window, and the beam is compressed a little bit, but I think the field is to small I have to look for a stronger magnet

[DaveC]

One of the companies I've worked for, made sealed e-beam tubes, with 3um thick Si- SiN3 windows. Their best could be stably operated above 80 kV. The maximum permissible beam current depends on the voltage, higher giving greater window transparency. Above a fairly low wattage output, the windows will need active cooling to avoid melting, and failure.

For a truly impressive photo, try having the tube emit into a He atmosphere, at stp. Depending on the voltage driving the tube, the beam may travel up to 8 inches! Makes quite a show.

We always operated with the cathode at (-kV) and the window at ground potential. Not only a safety issue, but the ions produced at the window when at HV, create all sorts of fun breakdowns, and often take out the window, too.

To answer Carl's question, the external beam is deflectable by modest magnetic fields.... a few hundred gauss or so. The deflection tends to resemble bending a paint brush. Since the electrons are not only diverging at about a +/- 45 degrees from the axis, but are travelling at a variety of energies after they emerge from the window, their radii of curvature will have a wide range of values.

Also the xrays, produced are substantial so a leaded glass shield is a very good safety feature.

Very nice work Tom.... and great pictures...

Dave Cooper.

[Richard Hull]

Wonderful work and well presented, as usual. Archived for its illustrative nature.

Richard Hull

[jlangridge]

Rapp:

Very nice work!

Did you make the plastic T tube with NW40 flanges (plastic T with NW40's epoxied on?) or did you purchase this item? I am in need of a plastic T section with NW40's and any details on this item would be appreciated.

Thank you!

John Langridge

[Wilfried Heil]

Nice work and as usual, great improvisation! I'm also curious how much current you are emitting into the beam.

Below is an article on an e-gun (Navy funded) by J.Ziegler, one of the makers of SRIM.
>http://www.srim.org/e-Zapper/2005%20e-Z ... Ziegle.doc

[Rapp Instruments]

Hi John,
this plastic tubings with KF-flanges are made by Alcatel. They are made in all sizes from NW 16 to NW 60. I found some pieces a ebay.

Hi Wilfried,
the total emission current of the filament was about 100 µA, but only a very small fraction escapes through the 1,5 mm diameter hole.

Thomas

[jlangridge]

Thanks Thomas! I will check with Alcatel and keep a close watch on E-bay!

As a follow-up question to your response to Wilfried regarding the beam current, how much heating of the anode and the foil do you detect when the beam is on? It must be significant given that a large number of particles are terminating in the anode and not passing through the thin window.

Thanks again!

John Langridge