From:
sbushman@students.uiuc.edu (stewart samuel bushman)
Newsgroups:
sci.physics.plasma
Subject: Help explain Floating Potential
Oscillations
Organization: University of Illinois at
Urbana-Champaign
Hi,
I'm doing my masters thesis on the
Pulsed Plasma Thruster, an electric
satellite thruster which uses an arc
discharge across a teflon surface to
generate a plasma, which is
electrothermally (gasdynamic) and
electromagnetically (jxB) accelerated
out of the thruster, providing
impulse.
I'm probing the plasma discharge with a triple electrostatic
probe to
determine electron density and temperature. My problem is with the
floating probe,
or rather, with the floating probe output.
I'm seeing
an oscillatory signal - the floating potential, to a
first order,
resembles a damped sinusoid.
Over a 40 microsecond time scale, the
potential starts at 0, rises
to 30V, drops to -15V, and goes through about
3 more oscillations before
settling at a constant 10V at 30 microseconds.
These oscillations
(mostly the first big one) cause the theoretical
equations to blow up,
leaving me with a discontinous plot of ne and Te vs
time. I can't really figure what the oscillatiions
mean physically in
the plasma, as I'm pretty sure the phenomenon is
real. It has been
demonstrated at
different locations in the plasma and in triple- and
single-probe setups
(to ensure the other probes were not interfering
electrically). The oscillatory signal is also highly
repeatable (although
it differs somewhat at different plasma locations,
but that is to be
expected).
Additional info:
The
current provided to the thruster pulse is unipolar (nonreversing), and
lasts
about 8 microseconds (full-width, quarter-max).
While it shouldn't
affect a floating probe, the probes are cleaned using
an incandescent
discharge via ion bombardment (they glow like the sun).
Any
thoughts?
Thanks,
-Stewart
--
|
Stewart S. Bushman |
The more I study religions the more convinced I become
sbushman@uiuc.edu | that man never worshipped anything but
himself.
| -
Sir Richard Francis Burton