Multipactor in Low Pressure Gas and in ... - of Richard Udiljak

these quantities in the range from 0 eV to about 100 eV. For the electronargon
collision cross section, the data given in Ref. [57] is used and it
covers the range up to 20 eV. An analytical formula has been devised,
which approximates the given data quite well in the measurement region
(cf. Fig. 3.5). Outside the measurement points, the cross-section
for very low energy electrons has been set to converge towards the geometrical
cross-section of the argon atom. For high energy electrons, the
cross-section is set to fall off with the same rate as for the last few eVs.
The analytical formula is given by the expression,
σc = (
1.68
+
1 + (8ɛ) 3
ɛ
1 + (0.07ɛ) 2 )1.5 × 1.15 × 10 −20 [m 2 ] (3.22)
where ɛ is the total electron energy, given by Eq. (3.15).
Total collision cross section [m 2 ]
10 −18
10 −19
10 −20
10 −3
10 −21
Buckman and Lohman
Analytical approximation
10 −2
10 −1
10 0
Electron energy [eV]
Figure 3.5: Absolute total collision cross-section for electrons scattered from
argon. The stars indicate measurement data by Buckman and
Lohmann [57] and the solid line is the analytical approximation
given by Eq. (3.22).
The ionisation cross-section increases rapidly for electron energies
slightly above the ionisation threshold and thus it is of importance to
have an accurate description in this range, especially since this is close
to the first cross-over energy of many materials. A simple function that
48
10 1
10 2