HIGH
ION ENERGY, HIGH POWER ION BEAM
Dr. John D. Williams
Introduction
Robert
Winglee of the University of Washington has proposed a novel plasma propulsion
concept that has received a great deal of attention at NASA’s Marshall Space
Flight Center (MSFC). The propulsion concept has been designated as Mini-Magnetospheric
Plasma Propulsion (M2P2), and numerical studies and scaling from small vacuum
chamber experiments have shown that the M2P2 system will provide high thrust
levels (I to 3 N) at very low power and propellant flow rates.
The
M2P2 concept generates thrust through the forced expansion of a dipole magnetic
field that intercepts and deflects the solar wind analogous to the way in which
a more straightforward (albeit more complicated and massive) magsail would
operate. The next stage in the development of the M2P2 concept will be
experimental testing in a very large vacuum test facility at MSFC. To simulate
the solar wind, MSFC plans on procuring several broad beam ion sources that will
be mounted at one end of the facility to be used to generate a uniform current
density argon ion beam across the entire cross‑section of the cylindrical
vacuum chamber.
The
need of MSFC for a solar wind simulator offers a unique, multi‑faceted
opportunity for the Plasma Engineering Group at Colorado State University. One
facet of this opportunity concerns the chance to apply results from the
high‑energy ion thruster research program to design a large diameter ion
optics system. Another facet concerns the opportunity to become involved in
space plasma simulation activities in a large vacuum test facility at NASA.
Another facet has to do with optimizing a large diameter discharge chamber so
that a very uniform plasma profile is produced across the end of the chamber
where the ion optics system is to be mounted. A non‑uniform or peaked ion
density profile causes the ion optics system to be overdriven near the maximum
ion density region and under‑driven near regions of lower ion density.
This problem has recently plagued both NASA and commercial entities that utilize
ion propulsion systems, and its solution (i.e., a flatter ion density profile)
will significantly increase the total impulse that is available from an ion
propulsion system.
The
objective of this project is to design an entire solar wind simulator that could
meet the needs of NASA’s Marshall Space Flight Center for testing of advanced
solar wind propulsion technologies. Specifically this effort involves (1) the
design of an advanced ion optics system that would be capable of forming a
30‑cm diameter argon ion beam with an energy range between 5 keV and 20
keV over a current density range of 1 mA/cm2 to 5 mA/cm2,
(2) the adaptation and optimization of a 30‑cm diameter discharge chamber
capable of providing a uniform plasma ion flux to the ion optics system, and (3)
the development of an advanced ion beam neutralization system capable of
servicing a cluster of up to 4 solar wind simulation sources. The Plasma
Engineering Group at CSU has developed all of the models and software codes
needed to design a solar wind simulator, which will be utilized by the graduate
and undergraduate students chosen to work on the project. The approach will
include the following steps:
1.
Optimization of the ion optics systems. This will be accomplished using
the IGX code to
simulate ion beamlet properties
under various beamlet geometries, potential biases, and ion currents.
Operational points will be identified to enable operation of the system over a
wide range of ion current densities and voltages.
2.
Generate mechanical drawings of grids to be used in the ion optics system
and design the optics mounting hardware suitable for withstanding up to 20 kV
operation.
3.
Optimization of the discharge chamber. This will be accomplished using
the HAG and PLASMA codes to design the proper magnetic field confinement scheme
and primary electron injection point that will result in the most uniform plasma
density profile over the widest possible discharge power and flow rate range.
4.
Generate mechanical drawings of the discharge chamber, cathode mount, and
expellant injection ports.
Also prepare drawings of the ground screen to be placed around the discharge
chamber to prevent plasma leakage and the mounting system to be used to attach
the ground screen structure to a vacuum test facility. Finally, prepare
electrical wiring schematics and layout drawings.
5.
Definition of the neutralization system. Model the plasma production
requirements needed to provide an electron flux sufficient to neutralize four
solar wind simulators. Provide the design of an ionization stage equipped plasma
contactor capable of providing the necessary plasma production requirements at
the lowest possible flow rates.
6. Generate mechanical drawings of the
neutralization system.
7.
Collection of the work conducted under Steps 1 through 6. Generate a
preliminary design document and prepare it for submission as part of a proposal
to MSFC.