PLASMA ADIABATICITY IN A DIVERGING MAGNETIC NOZZLE J. P. Sheehan and Benjamin W. Longmier University of Michigan Edgar A. Bering University of Houston Christopher S. Olsen, Jared P. Squire, Mark D. Carter, Franklin R. Chang Díaz, Timothy W. Glover, Andrew V. Ilin, and Leonard D. Cassady Ad Astra Rocket Company
20
Embed
PLASMA ADIABATICITY IN A DIVERGING MAGNETIC …peplweb/pdf/ICOPS2014_Sheehan2.pdf · PLASMA ADIABATICITY IN A DIVERGING MAGNETIC NOZZLE J. P. Sheehan and Benjamin W. Longmier University
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
PLASMA ADIABATICITY IN
A DIVERGING MAGNETIC NOZZLE
J. P. Sheehan and Benjamin W. Longmier University of Michigan
Edgar A. Bering University of Houston
Christopher S. Olsen, Jared P. Squire, Mark D. Carter,
Franklin R. Chang Díaz, Timothy W. Glover,
Andrew V. Ilin, and Leonard D. Cassady Ad Astra Rocket Company
ABSTRACT
We propose a fluid model for ambipolar ion acceleration in a magnetic
nozzle that preserves the adiabaticity of the plasma. This adiabatic theory
predicts that the change in average electron energy depends linearly on
the change in plasma potential, providing an important design metric for
electric propulsion devices which employ magnetic nozzles. The fluid
theory predictions were compared to measurements made in the VASIMR
VX-200 experiment which has conditions conducive to ambipolar ion
acceleration. A planar Langmuir probe was used to measure the plasma
potential, electron density, and electron temperature for a range of mass
flow rates (50 – 140 mg / s) and power levels (12 – 30 kW). The linear
relationship between electron temperature and plasma potential was
observed as predicted. The adiabatic theory relies on collisions to
rethermalize the electrons and establish a temperature gradient. Coulomb
collisions cannot account for the high collisionality but an ion acoustic
instability may enhance the collision frequency.
2 41st IEE International Conference on Plasma Science
Washington DC, May 28, 2014
HELICONS AND MAGNETIC NOZZLES FOR
APPLICATIONS IN PROCESSING AND PROPULSION
Helicons
Radio frequency
High ionizing efficiency
Electron heating
Magnetic nozzle
Functions like physical
nozzle
Accelerates ions
Converts thermal energy
into directed kinetic
energy
Applications
Materials processing
Electric propulsion
CHI KUNG
VASIMR
3 41st IEE International Conference on Plasma Science
Washington DC, May 28, 2014
CURRENT FREE DOUBLE LAYERS
IN HELICON EXPERIMENTS
Narrow layer (10s of λd) of large
potential jump (several Te/e)
Isothermal
Occurs downstream of nozzle
Current free, expanding
Accelerates ions
May be thrust mechanism in helicon
thrusters
Open question of how/why current
free double layers form
4 41st IEE International Conference on Plasma Science
Washington DC, May 28, 2014
ION ACCELERATION IN VASIMR
VASIMR: < 200 kW helicon + ICH
thruster
Went looking for double layers, but
found none!
Vp, ne, and Te derivatives coincide
Long length scales: 10,000s of λd
λd ~ 10 μm
Corroborated with RPA
B. W. Longmier et al., Plasma
Sources Sci. Technol. 20, 015007
(2011).
5 41st IEE International Conference on Plasma Science
Washington DC, May 28, 2014
PLASMA IN NOZZLE IS ADIABATIC
Assume plasma is adiabatic
Energy loss to surfaces small
Radiation loss small
Geometry dictates degrees of freedom (N)
Expanding plasma sphere: N = 3
Magnetic nozzle: N = 2
Electrons remain Maxwellian, though
temperature can change
Relies on collisions
Conservation of momentum
Relationship between average electron energy
loss and potential (→ion energy gain)
6 41st IEE International Conference on Plasma Science
Washington DC, May 28, 2014
HELICON EXPERIMENT
USING VASIMR HARDWARE
7 41st IEE International Conference on Plasma Science