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Micropropulsion Technologies 235

Certainly the achievable impulse bit, I, is a function of the specific impulse and
the energy in the pulse. Another empirical relationship, as shown in Equation
(11.7), has been formulated:

I I sp ¼ 1:7E 1:65 (11:7)

which in turn determines the achievable thrust, T, from Equation (11.8) as the
product of pulse frequency and impulse bit

T ¼ F I (11:8)

By changing the energy in the plasma pulse the exhaust velocity and the mass
ablated changes. This effect has shown influence on the scalability of the thruster.
Thrust-to-power values decrease with decreasing energies per pulse. While thrust-
to-power can reach values up to 20 mN/W for hundreds of joules, this value
decreases to 10 mN/W for pulse energies of the order 5 J. As suggested by
these parameters, achievable impulse bits can be large for large systems ( 10
mN sec) and can approach 5 mN sec for small systems.
11.2.1.2 System Requirements

The PPT can operate in a self-triggering mode or in a controlled-pulsed mode. Both
require high voltage (>1 kV) to initiate the discharge. Due to the need for high
voltage and the time necessary to charge the capacitors, repetition rates may be
limited to less than 10 Hz.
The driving circuit is usually fairly simple. A DC–DC converter connects the
PPT to the spacecraft bus. The high-voltage output of the converter is used to charge
the capacitor bank of a pulse-forming network. Once the surface breakdown voltage
along the insulator occurs, a plasma sheet is produced and starts moving. Other
ignition possibilities include the use of a switch to apply the high voltage or the use
of a preionizer like a spark plug. Once the sheet is formed, the energy left in the
capacitor drives the plasma via the Lorentz force.
A variety of different PPT configurations have been demonstrated. Many have
spring-fed propellants like the side-fed ablative PPT (APPT) or the breech-fed
APPT in which a block of Teflon is placed between two electrodes and pushed
forward with ongoing erosion. The mPPT manufactured by Busek relies on coaxial
geometries, where the propellant erosion leads to increasing recession of the plasma
source with ongoing operation as shown in Figure 11.3.
The total system mass can be quite low since the power processing electronics
are minimal. With its miniature electrode gaps, the micro-PPT benefits from the use
of lighter, lower-voltage components. Additionally, the plasma is quasineutral,
which allows for operation without an additional neutralizer. However, EMI filter-
ing may be necessary due to the pulsed high voltages. The pulsed plasma thruster
is summarized in Table 11.1 with a picture of the complete system shown in
Figure 11.4.

© 2006 by Taylor & Francis Group, LLC