From: johncobb@ut-emx.cc.utexas.edu (John W. Cobb)
Newsgroups: sci.physics
Subject: Re: terminal particle energy for accelerators
Keywords: n
Date: 16 Apr 92 18:01:25 GMT
Reply-To: johncobb@ut-emx.cc.utexas.edu (John W. Cobb)
Distribution: world,local
Organization: The University of Texas at Austin

In article <1992Apr14.213121.17723@lmpsbbs.mot.com>,
capurka@ssd.comm.mot.com (Tom Capurka) writes:
|>Hello,
|>I have a question about particle accelerators:  Why does there seem to be
|>a terminal particle energy for a given particle, accelerator, related to
|>the radius of the acclerator?  Is this because the radiation caused by
|>the fact that the article is not in an inertial frame gets too large and
|>approaches equilibrium state with the energy input?

Well I'm not an accelerator physicist, but yes that's my understanding.
If you look in Jackson, you can see expressions for ultrarelativistic
synchotron radiation. The energy radiated goes down with increased
mass to charge ratio. Thus you can get higher energy protons in a ring
then you can electrons. This is why many of the electron expts. are linear
(SLAC). Note, you also get some radiation b/c of linear acceleration, but
it is usually a much smaler effect.

|>Whatever the reason, is there any possibility that there exists
|>a yet undiscovered trick around this so that one could have an SSC type
|>acclerator (as far as particle energies are concerned) in a 1 meter box?
|>Or is this impossible in principle, given the state of physics today.
Well, you coudn't put a colliding ring w/ SSC type energies into a box
with getting a great deal of radiation. So the competing effect you
describe above would be imensely increased. You might think of doing a
SLAC type accelerator in 1 meter. However, now you need to accelerate to
10 TeV in 1 meter. That means that the electric field gradient causing
the acceleration needs to be 10E13 V/m. This is a huge electric field
(it would probably be RF and not DC). BY comparison, the electric field
in an atom is on the order of 13eV/Angstrom or ~10e9 V/m. So if you try to
put a 10 TeV SLAC in a box your accelerationg field will suck electrons
off of your vacuum surfaces and you will get electrical breakdown. So
the trick you ask for seems impossible :<.

But wait, what if you use a plasma instead of a vacuum to guide your
beam. Then there is no problem with breakdown (you already are ionized).
If you send a strong Laser light pulse just ahead of your beam, it may cause
a Wake of extremely strong electric fields. Then you might be able to
get the SSC into a 1 meter box. These ideas have been around a while. It's
called the laser wakefield accelerator. If you interested, look in the
citation indices under John Dawson, UCLA; Toshi Tajima, Texas; and Chan
Joshi UCLA. My understanding is that both the theoretical and experimental
work is promising but that the problem is really difficult, so my opinion
as ot whether it will <ever> work is a definite maybe.

You might think of some other tricks also. The two big problems you need to
finesse are 1) a collider must reduce (or re-absorb?) its own radiation
so that you can tolerate a smaller radius of curvature (and thus decrease
the size);  or 2) you must increase the accelerating gradient in a linear
device to shorten the length (i.e. decrease the size).


|>Tom C.  <capurka@ssd.comm.mot.com>

John W. Cobb
jwc@fusion.ph.utexas.edu