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The proposed Jupiter Icy Moons Orbiter (JIMO) mission is
recognized as NASA's first space science mission to take advantage of the
high-power and advanced propulsion capabilities under development within Project
Prometheus. Various options are being considered for the power management and
distribution (PMAD) system including a nuclear-based Brayton-alternator,
operating in the range of 1000-1500 Hz, with a 3-phase output of approximately
400 VAC; or a nuclear-based thermoelectric system with a DC based PMAD system in
the range of 200 to 600 VDC. Either spacecraft PMAD system would provide
electrical power in the range of 100-500 kW nominal, with a significant portion
of this power being required for the electric propulsion system.
All prior ion propulsion PPUs have used DC-to-DC converters to process power.
Even thought this technology is well understood it would be challenging to
develop these for JIMO because of the high voltage range being considered for a
DC PMAD. An option to overcome this problem is "stacking" or
connecting the inputs of the converters in series. This approach would allow
using lower voltage transistors for an application with higher input voltage and
obtain high efficiency.
On the other hand, an AC PMAD allows the possibility of using a
"direct-drive" approach. This consists of just using simple step-up
transformers and rectifier to generated beam power for the thruster. This
approach could significantly reduce parts count and increase efficiency. The
other thruster inputs require significantly less power so DC-to-DC converters
can still be used powered from an additional isolated transformer-rectifier
input.
This technology development program consists of fabricating a
"direct-drive" AC beam supply. Since the accelerator power supply is
critical to the transient operation of the thruster, a breadboard power supply
will also be built and integrated with the beam supply. Even though for an AC
PMAD the input voltage for the ancillary supplies can be arbitrarily selected by
transformer design, a 400 V input will be assumed for this design to also
investigate "stacked" input topologies. Integration of the high
voltage supplies with a laboratory thruster will allow characterization of
transient and fault conditions that can be detrimental to both thruster and PPU.
It will also validate the "direct-drive" approach.
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