Compact Plasma and Focused Ion Beams

43Plasma and Wave Diagnostics

a nearly uniform sheath forms on the mesh near the orice. This electrode

is oated so that it acquires the oating potential V

f

(usually negative) of the

plasma. The second electrode is held at a positive potential of

VV

As

~(),40 V >

which attracts the electrons by a potential difference of (V

A

− V

f

) and repels

the ions. The third electrode acts as a discriminator, and is swept between

0 and −100 V to retard the electrons so that only electrons having energy

greater than the discriminator voltage (V

D

) can pass through it. The energy-

discriminated electrons are collected by the last electrode held at V

c

~ 40 V,