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238 MEMS and Microstructures in Aerospace Applications

where r is the ion to arc current ratio, m is the cathode material ion mass, and Z is
the average ion charge state of the discharge plasma.
Therefore, the impulse size produced is given by:

I ¼ _ mm ion vt (11:10)

with t being the duration of the impulse. Assuming t ¼ 500 msec and I arc ¼ 50 A
we can obtain a maximum impulse bit of the order 40 mN sec using Bi (Z ¼ 1.18).
This is consistent with a value of 30 mN sec measured at the JPL thrust stand for a
pulse with similar current and duration for tungsten, which is slightly lighter. Thrust
can be controlled by adjusting pulse power and repetition rates. Thrust-to-power
values are of the order of 10 mN/W.

11.2.2.2 System Requirements

The VAT was constructed using an inductive energy storage (IES) circuit PPU and
simple thruster head geometry. In the PPU, an inductor is charged through a
semiconductor switch. When the switch is opened, a voltage peak, LdI/dt,is
produced, which breaks down the thin metal film-coated anode cathode insulator
surface at relatively low voltage levels ( 200 V). The current that was flowing in
the solid-state switch (for ¼ 1 msec) is fully switched to the vacuum arc load.
Typical currents of approximately 100 A (for ~100 to 500 msec) are conducted with
voltages of approximately 25–30 V. Consequently, most of the magnetic energy
stored in the inductor is deposited into the plasma pulse. The efficiency of the PPU
may thus be greater than 90%.
Based on this inductive energy storage approach, a PPU was designed to accept
external TTL level signals to adjust the energy and the repetition rate of individual
plasma pulses. This was accomplished by adjusting the trigger signal to the semi-
conductor switch. Figure 11.5 shows an equivalent circuit diagram of the PPU. By
varying the length of the trigger signal, the level of the current in the switch and
thereby, the energy stored in the inductor can also be adjusted. This, in turn,
changes the amount of energy transferred to the arc and the impulse bit of the
individual pulse. Obviously, the repetition rate of the individual pulse can be
changed by varying the input signal as well.
The mass of the PPU is small (< 300 g) resulting in a low-mass system. The
plasma output is quasineutral; therefore, no additional neutralizer is needed. An
EMI filter might be necessary due to the noisy characteristics of the discharge, high
peak currents, and fast switching. A drawback for this technology is possible
contamination from the metal propellant. A low-mass feed mechanism is available,
therefore, even long missions can use this technology.
The design of the thruster head itself is very simple. A coaxial structure with a
center cathode, separated from a tube-like anode by an insulator, is a possibility as
well as a sandwich structure as shown in Figure 11.6.
Even smaller structures as manufactured by ChEMS are possible; however,
scaling down the PPU and the thruster will lead to inefficient operation because of

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