Newsgroups: sci.physics.plasma
From: mdp1@crux2.cit.cornell.edu (Mark)
Organization: Cornell University
Subject: Re: Short pulse group velocity Q

William S. Lawson (lawson@pax.llnl.gov) wrote:
[my original post deleted]
: Short-pulse propagation in a vacuum is like any other kind of propagation in a
: vacuum -- there is no distortion of any kind, and the group velocity is c. A
: plasma is more complicated, but at optical frequencies, it shouldn't be too
: bad. Any book on plasma theory should give you the dielectric tensor you need
: -- Bill Lawson

Are you sure about this? I thought that the vacuum diffraction of the pulse
causes the group velocity to be less than c along the axis of propagation.
If you look at in terms of rays and phase fronts, the fronts are curved both
before and after focus, so that the direction of propagation of parts of the
pulse is not parallel to the axis of propagation of the whole pulse.

Mark
mdp1@crux2.cit.cornell.edu