Transient response of the radio frequency inductively coupled plasma to a sudden change in power

A two-dimensional, axisymmetric model was developed to study the response of a radio-frequency inductively coupled plasma to a sudden change in its active power. The time-dependent equations for the conservation of mass, momentum, and energy, along with Maxwell's equations were solved numerically. Results were obtained for a pressure range of 200-760 Torr, a frequency range of 1-3 MHz; torch diameters between 40 and 75 mm; and, argon/hydrogen flow rates of 40-75 slpm. Initially, the plasma was assumed to be under steady-state condition at 20 kW. The plasma power was then reduced to 10 kW for 35 ms and, the response of the plasma fields and the coil current were predicted numerically. When power was reduced, the coil current reduced significantly in 2 ms. It then increased to a maximum before smoothly decreasing to its new steady-state value. The response of the plasma depended, to different degrees, on all the parameters considered here. Depending on the position within the torch, it could vary from 2 ms to several tens of millisecond. The plasma response was fastest within the skin-depth region where power was dissipated. The response time was most strongly affected by the changes in discharge pressure; and was least affected by the induction frequency. The response time increased with pressure and/or torch diameter, and decreased with frequency and/or flow rate. We also found that, depending on the magnitude of induction frequency, an increase in plasma power may cause an oscillatory plasma behavior. (C) I 998 American Institute of Physics.