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van Alfven current

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THORSTEN WESP

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Jun 17, 1998, 3:00:00 AM6/17/98
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Can anyone tell me, what the van Alfven current

I = 4 pi eps0 m0 c^3 / e = 17 kA

is? It occurs for example in the small signal gain
formula of a free elctron laser. Here it is often
said to be a current limit. But for what?


Steve Lidia

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Jun 21, 1998, 3:00:00 AM6/21/98
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THORSTEN WESP (th....@gmx.de) wrote:

: Can anyone tell me, what the van Alfven current

With a few other details, you will find that the
Alfven current as defined above is a soft limit on
the amount of charge that can be transported. Without
some form of charge neutralization, the current will
produced a retarding voltage that inhibits flow. Electrons
in high intensity beam like this will stop.

This number is non-relativistic, so there is a correction
for higher energy beams - more current is transportable. Also,
propagating a high intensity beam through a plasma (or a neutral
gas which ionizes), with neutralize some of the beam's charge,
allowing the self-magnetic field of the beam to compensate for
the electric field, thus enhancing self-focusing effects.

This method was necessary in the FEL experiments conducted at
Lawrence Livermore in the 1980's when they were transporting
50 MeV, 10 kA beams through the 50m long PALADIN wiggler.

For most FEL applications, however, the Alfven current is
merely a convenient numerical parameter to non-dimensionalize
your equations - it appears in the 'rho' parameter (see the
work of Bonifacio/Cassagrande/Pelegrini) - as I assume you
know.

- Steve

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