From: "Sir Charles W. Shults III"
Subject: Re: Home brew linear accelerator?
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
Date: Sat, 21 Sep 2002 00:16:35 GMT
NNTP-Posting-Date: Fri, 20 Sep 2002 20:16:35 EDT
Organization: RoadRunner - Central Florida
We used to keep a couple of these Lucite test blocks around. Fascinating to
see! A LINAC is not so hard to make; it is mostly a hard vacuum, a high voltage
supply, and a source of something charged. But the cyclotron is much better for
the hobbyist who wants more control and more "bang for the buck". They can be
quite compact and create powerful bursts of accelerated particles. Also, a
LINAC is a one-shot sort of machine, and requires (typically) higher potentials
than a small cyclotron. The cyclotron uses tuned RF to keep accelerating the
particles, so with a lower starting voltage you can get more output power- but
at a cost- less shots per unit time.
The first cyclotron was little more than a couple of D shaped magnets with
RF coils wrapped around it. There is plenty of room for the amateur to
experiment here, and creating the high voltage is not so difficult. Some rigs
use Van de Graaf generators, which are not hard to make or can be bought
commercially. Some use high voltage multipliers to get their charge. In any
event, you can make high voltages in many ways, many of which are more artwork
than electronics. The old spark making machinery works very nicely, like
Wimshurst machines, Van de Graafs, etc.
The vacuum is the harder part. You need a really good, hard vacuum system
to make this work properly. You can get pretty good vacuum pumps for roughing
the system down, but then you must work with a turbomolecular, cryogenic, or
diffusion pump to get where you need to be. This is where the big expense will
Your fixtures ("plumbing") should all be made of stainless steel, but good
clean copper works pretty well too. All your seals will leak a little- not
necessarily atmosphere, but molecules stuck to their surfaces. You will want to
clean, heat, exhaust, etc. until the outgassing stops. Picky, but you can do
Now, what will be the source of your charged particles? There is a lot to
choose from, and a good old fashioned alpha source will work very well.
Electrons are also good, and the careful fabricator can make a very decent
electron source from the neck of an old CRT. After all, it is already made to
inject a nice, focused stream into a picture tube! You can seal the glass with
vacuum rated epoxy right into the stainless neck that you will mount to the
Finally, the coils. You will want a tunable RF oscillator to run them. The
trick is to calculate the frequency of your oscillator to match the frequency of
your accelerator. When you find the right frequency, you inject electrons and
they fall into sync with the RF. Then you sweep the frequency up, accelerating
the electrons all the time, until you reach the energy levels you desire. This
is actually a synchro-cyclotron.
Consider a few simple principles- in a uniform magnetic field, an electron
(or positron) will orbit in perpetuity until it either collides with something
or you change the field strength. That is the basis of particle storage rings.
Also, moving charges (such as the electrons) will induce a magnetic field
back into the tubing- that is one reason you want stainless steel, since it is
not easily magnetized. It will limit your losses quite a bit. Copper tubing
will act as a brake, slowing charged particles and heating itself.
You can use a charge sensor to see where the electrons are in the tube at
any time, and this can be the basis of the feedback system to ramp up the RF
frequency or hold it stable.
My robotics, space and CGI web page - http://home.cfl.rr.com/aichip