Compact Plasma and Focused Ion Beams

267New Experiments in Pulsed Microwaves

Figure 16.10 shows the variation of J

e

with the radius of the multicusp at a

xed power and pressure. We see that J

e

is higher and there is a better attop

at a larger radius. This can be understood from the decay rate due to radial

diffusion (Equations 16.6 through 16.8). Since

ρ

= D/

Λ

2

, for a larger radius

(hence larger

Λ

),

ρ

will be smaller and hence the plasma will diffuse out

11

10

9

8

7

J

e

in a.u.

6

5

4

3

2

1

0

010203040

10 mTorr

100 mTorr

40 mTorr

500 mTorr

a = 2.2 cm

P

o

= 50 kW

f

= 9.45 GHz

t (

µ

s)

50 60 70 80 90

FIGURE 16.9

Temporal variation of the plasma (electron) current density for different gas pressures at a

pea ...