Magnetic consequences of an electrostatic shield around an inductively coupled plasma discharge

Summary form only given, as follows. An electrostatic shield in the form of a hollow metal cylinder is often placed between the exciting coil and the discharge tube of an inductively coupled plasma system in order to screen the plasma from the electric field of the coil. Such fields can, under some circumstances, have a deleterious effect on the process for which the plasma is being used. It is not generally realized that the electrostatic shield can also lead to a significant reduction of the r.f. magnetic field inside the tube as well as altering its axial distribution. This effect needs to be considered in detailed modeling of inductively coupled plasma system and is of practical importance in determining the impedance the plasma presents to the r.f. generator. We have studied the effect by measuring the r.f. magnetic field inside and outside metal cylinders with different slot configurations and have developed electromagnetic models for both thin and thick shields which give good agreement with the observations. In one of the models the shield can be treated as an axisymmetric diamagnetic medium thereby allowing a 2D treatment of an intrinsically 3D problem. The thick shield is relevant to high power systems in which the discharge tube itself is a thick-walled, water cooled, slotted metal cylinder (i.e., a cold crucible). Plasma coupling in such a system has been compared with coupling in a system with dielectric tube and the predictions of the model have been verified.