The a.c. impedance of plasma discharges in mercury vapour

Whenever an alternating electric field of small amplitude is applied to a d.c.-sustained plasma discharge, a corresponding alternating component will appear in the discharge current. The discharge path behaves as a conductor of inductive impedance; the inductance and resistance depend on the frequency of the superimposed quantities. The paper deals with the effect of the behaviour of electron and thermal ionization on the impedance of the low- and high-pressure positive columns of a plasma discharge in mercury vapour. The particular characteristics of the positive column in each case are outlined and an expression for the effective impedance per unit length of the column is derived. Typical theoretical and experimental curves of the variation of both components with frequency are given for the low-pressure case. For the high-pressure column the theoretical curves are of a shape similar to those obtained experimentally by Magnold for an arc discharge in mercury vapour. Although Magnold did not give sufficient particulars of the arc discharge investigated for an exact comparison to be made, the theory seems to explain the physical phenomena that take place in the high-pressure positive column.