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246 MEMS and Microstructures in Aerospace Applications

The exhaust velocity is in turn defined via the relationship seen in Equation
(11.18),
v E ¼ QC m (11:18)

where Q is the absorbed input energy of laser light per mass unit of ablated material.
It is obvious that in order to convert a certain amount of tape into a plasma, a fixed
energy is necessary. This threshold intensity can be described by the empirical
formula as a function of the laser pulse duration t:
4
F th ¼ (2:36 10 )(t) 0:45 (11:19)

Operation above this value leads to the production of higher charge states and
increasing light absorption within the plasma, which limits the amount of energy
available to ablate material.

11.2.4.2 System Requirement and Comments
While at first glance, the laser ablation thruster seems to be an extremely powerful
device, it has to be noted that it includes some overheads. Apart from using a fairly
powerful diode laser, a motor has to be used to move the fuel tape. Efficiency losses
due to these items have to be taken into account.
The total system mass may approach 1 kg, using commercially available parts,
not including the electronics that will be needed to adjust the tape speed and laser
output. However, once these control mechanisms are in place, the I sp can be
adjusted by controlling laser output parameters like intensity and pulse lengths,
which makes this thruster very versatile. The tape drive acts as a propellant feed
mechanism suitable for long missions. The plasma output is quasineutral and non-
metallic (although carbon might be produced with some propellants), which will
minimize problems associated with contamination. Due to the lack of high voltage
and the laser-produced plasma, EMI problems should be minimal.
Integration into a MEMS system might be possible with additional develop-
ment. With shrinking laser size, the output power will be reduced, which in turn will
reduce the spot size to submicron ranges and very small thrust levels. It is debatable
if at this point the system losses like the motor drive will render this technology
inefficient. The summary of the laser ablation thruster is shown in Table 11.4, with
a picture of the complete system shown in Figure 11.12.

11.2.5 MICRO-ION THRUSTER
Ion thrusters are the workhorse of electrical propulsion. 29–31 Ions created in
a plasma are extracted and accelerated electrostatically, thereby producing thrust.
In order to achieve this, an ion thruster consists of a plasma source coupled to
an extraction grid. The exhaust velocity can be adjusted by varying the
extraction voltage. Although ion engines have high thrust-to-power ratios and
are a well-developed and flight-proven technology, there are many difficulties

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