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 respo nse of a radio-frequency inductively coupled plasma to a sudden change in its active power. The time-dependent equations for the conservatio n of mass, momentum, and energy, along with Maxwell's equations were s olved numerically. Results were obtained for a pressure range of 200-7 60 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 pl asma was assumed to be under steady-state condition at 20 kW. The plas ma 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 po wer was reduced, the coil current reduced significantly in 2 ms. It th en increased to a maximum before smoothly decreasing to its new steady -state value. The response of the plasma depended, to different degree s, on all the parameters considered here. Depending on the position wi thin the torch, it could vary from 2 ms to several tens of millisecond . The plasma response was fastest within the skin-depth region where p ower was dissipated. The response time was most strongly affected by t he changes in discharge pressure; and was least affected by the induct ion 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 Am erican Institute of Physics.