From: "David Q. King" <x.dqk@red.prmx.com>
Subject: Re: Help explain Floating Potential Oscillations
Newsgroups: sci.physics.plasma
References: <78i90l$21c$1@jupiter.cs.uml.edu>
Organization: 2alpha (2alpha.com)


Stewart,

If the 10 V floating potential is steady with respect to some ground
(what ground I don't know) at some time, then the prior oscillations sound
like noise of some sort. The noise I suspect are large voltages induced by
the 20kA or so discharge in typical PPT's. Does the discharge have current
reversal or does it use something like either a matched PFN or diode to
crowbar the ringing discharge? What are you using for a ground? Say hi to
Rod for me.

David

stewart samuel bushman <sbushman@students.uiuc.edu> wrote in article
<78i90l$21c$1@jupiter.cs.uml.edu>...
>
> 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
>
>