INSTANTANEOUS AND DELAYED-RESPONSES OF LINE-INTENSITIES TO INTERRUPTION OF THE RF POWER IN AN ARGON INDUCTIVELY COUPLED PLASMA

Instantaneous and delayed responses of line intensities to the sudden interruption of the RF power have been studied in an argon inductively coupled plasma (ICP). The instantaneous responses are caused by equilibrium shifts in the balances of elementary processes that control the populations of the excited states. It has been found that excited levels of Ar and H are predominantly populated by recombination of free electrons with ionic species, while most levels of metals such as Mg, Cd, Na, Fe, Al and Cu are populated by excitation from the ground state atom. Also charge transfer between Mg1 and Ar1 has been observed in the temporal behaviour of line intensities of two Mg+ states, quasi-resonant for charge transfer. Furthermore, we observed that in the inner part of the plasma the temperature remains constant during the recombination decay time, after an initial cooling of the electrons to the heavy particle temperature. When the power is switched on again, the electron temperature seems to increase temporarily to a value that is higher than the steady state value. The delayed responses are caused by disturbances created in the expansion zone of the plasma during and after the interruption. It was found that these disturbances travel through the plasma with a velocity of 12 m s-1.