Compact Plasma and Focused Ion Beams

186 Compact Plasma and Focused Ion Beams

In the pressure dependence study (Figures 11.1 and 11.2), it is noted that

J

me

and I

op

reach a maximum as the pressure is increased from 10

−3

to 10 Torr.

This may be understood qualitatively from the pressure dependence of the

energy gain by the electrons from the oscillating electric eld. At lower pres-

sures, the energy gain efciency is poor because electrons undergo many

oscillations before undergoing a collision. There is an optimum pressure

at which the energy gain is maximum. As the plasma becomes more and

more collisional, the energy gain efciency once again decreases. It is also

remarked that ...