RE: vacuum tube construction. (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Mon, 13 Aug 2007 14:21:12 -0700
From: Jim Lux <jimlux@xxxxxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: RE: vacuum tube construction. (fwd)

At 12:28 PM 8/13/2007, you wrote:

>---------- Forwarded message ----------
>Date: Mon, 13 Aug 2007 12:13:06 -0700
>From: Frank <fxrays@xxxxxxxxxx>
>To: Tesla list <tesla@xxxxxxxxxx>
>Subject: RE: vacuum tube construction. (fwd)
>
>All this tube is is a simple Geissler tube.
>The vacuum levels are not high enough to ever generate X rays.

Not entirely true.. More if the mean free path is greater than the 
distance from electron source to electron target, and the voltage is 
high enough, you can get x-rays.

There's also soft x-ray sources using fast rising edges that I think 
can tolerate higher pressures.  An example might be a z-pinch device


 >You need a hard vacuum and a diffusion or turbine pump along with a
>scavenging method to pump the tube down hard enough to be able to
>generate X rays.

Or a sorption pump (you only have to pump once, and then you getter it).


>   Tubes that can be taken apart for cleaning will not
>have seals good enough to hold an X ray vacuum.

Not precisely true.. There are continuously pumped tubes that are 
dismantleable.




>Large mercury arc rectifiers will work as a switch with some
>additional circuitry but are not available anymore. Those were used
>on some of the early wireless transmitters.

They were used far more recenly than that in high voltage DC converters.

As recently as 2005, they completed an upgrade of the Sylmar 
Converter station at the southern end of the Pacific HVDC Intertie , 
replacing the mercury arc valves with thyristors.

They normally serviced the mercury arc valves on a 7 year cycle, 
however the 1994 Northridge earthquake interrupted the service cycle, 
and encouraged replacement with more modern thyristors.

Interestingly, the mercury arc based converters work just fine, it's 
just that they require a larger maintenance crew and require more 
servicing, so they have an availability of 92% as opposed to 98% for 
thyristors.  There's also the environmental hazard of a system with 
many pounds of mercury.


>Dosimeters are a device to measure radiation over time, basically a
>charged capacitor that is discharged by radiation. Usually they are
>calibrated in REMS and if they ever register, the amount of radiation
>is enormous and you better run!
>
>A CD Geiger counter, Model CDV 700-XX is a good counter to measure
>radiation in small amounts and can be used to detect X rays. These
>are on Ebay all the time.

I've been given to understand that CD type geiger counters have a 
severe failing in that they don't detect low energy soft x-rays very 
well (ones that can't penetrate the GM tube envelope).  If you're 
working at lowish voltages (tens of kV), one could have a fairly high 
X-ray flux that wouldn't register.

I'm not a X-ray dosimetry expert  by any means, but it's probably 
worth consulting someone who is, and, in particular, someone who's 
familiar with low energies.


>Frank
>
> >Quoting Tesla list <tesla@xxxxxxxxxx>:
> >
> > >
> > > ---------- Forwarded message ----------
> > > Date: Mon, 13 Aug 2007 07:52:30 -0400