Subject: Re: Plasma Transducer
From: "Manuel Garcia" <Manuel.Garcia@quickmail.llnl.gov>
Newsgroups: sci.physics.plasma
Organization: NASA
Approved: plasma@woods.uml.edu

[Moderator's Note - This is a direct email to me so it may
not thread properly]

Plasmas can be structured so as to conduct low frequency
waves (below plasma or cyclotron frequencies). See book
by A. W. Trivelpiece, "Slow-Wave Propagation in Plasma
Waveguides," or Journal of Applied Physics, Vol. 30,
No 11, pgs 1784-1793, November, 1959, "Space-Charge Waves
in Cylindrical Plasma Columns." Here plasma is in a tube,
and within a solenoidal B-field. An e-beam, or RF can
interact with the body or surface waves propagating along
the plasma column. These electro-mechanical modes can have
frequency down to "zero." Ultrasound could be converted
into an electrical signal (microphone) then coupled to
the plasma column (for amplification?). Also see R. Gould,
"The Dynamics of Electron Beams," in book "Plasma
Dynamics," edited by F. H. Clauser, Addison-Wesley, 1960.
Gould & Trivelpiece worked on finding plasma-waveguide
amplifiers of microwaves, and modulators of e-beams.
Basically, a coax geometry with an axial B-field gives
one latitude in producing a finite plasma with the
desired dispersion characteristics. These plasmas are
low pressure gas discharges. Some "plasmas" at atmoshphere
may emit sound (and ultrasound?) directly. In natural
clouds a portion of the thunder is now believed to be
produced by the oscillating electric field from lightning
vibrating nearby charged droplets. An experiment to try
this might be to introduce radio waves into an "electro-
static precipitator" type of chamber with highly charged
mist or dust. Literature on electrostatics (e.g. "spray
electrification") may give ideas here. Why do you want
ultrasound from plasma? Have fun.

Manuel Garcia
garcia22@llnl.gov
Lawrence Livermore National Lab
POB 808, Livermore, CA 94550 USA
L-153
(510) 422-6017