Page 246 - MEMS and Microstructures in Aerospace Applications

Osiander / MEMS and microstructures in Aerospace applications DK3181_c011 Final Proof page 237 1.9.2005 12:31pm

Micropropulsion Technologies 237

FIGURE 11.4 m-PPT (includes PPU). (Source: Busek Co.)

11.2.2.1 Principle of Operation
In a vacuum arc thruster, plasma is produced from the cathode material in vacuum.
The plasma production takes place in tiny micron-sized emission sites, so-called arc
spots. Every arc spot has a lifetime in the order of tens of nanoseconds and carries a
few amperes of arc current. A highly scalable device is produced by changing the
current, leading to a change of number of arc spots with the basic physics in the arc
spot remaining the same. The high-density plasma created in the spots produces a
very high pressure, up to 1000 atm, that accelerates the quasi-neutral plasma
outward. Due to their larger mass, the ions contribute to most of the propulsion.
For this design, any conducting material can be used as a propellant.
The performance of the vacuum arc thruster (VAT) is determined by the
propellant mass, the degree of ionization of the plasma, the angle of expansion,
the average charge state, and the ion velocity. All these parameters have been
measured repeatedly in the past and verified for numerous materials and operating
conditions. Typical values for the ion velocity vary between 10,000 and 30,000
m/sec. The average arc to ion current ratio has been shown to be approximately 8%
and a cosine distribution has been found to emulate the plasma plume expansion
very well.
With these known parameters we can predict the performance of the VAT for
various materials. The ion mass flow rate _ mm ion (kg/sec) is given in Equation (11.9),

_ m m ion ¼ rI arc m=(Ze) (11:9)

© 2006 by Taylor & Francis Group, LLC