MODEL FOR CHARGE MOVEMENT AFTER THE RADIO-FREQUENCY EXCITATION IS EXTINGUISHED

A fluid equation model for charge movement in the afterglow of a parallel plate radio-frequency discharge was developed which can closely simulate the time averaged flux of negative ions to the grounded electrode of the system. While the percentage of the negative charge carried by electrons at the beginning of the afterglow influences the time dependence of the negative ion flux to the grounded electrode during the afterglow, it does not influence the time integrated negative ion flux significantly. This is because the electron attachment lifetime tends to be less than the ambipolar diffusion lifetime for electrons of several hundred microseconds. As a consequence, nearly all of the negative charge in the body of the afterglow exits as negative ions rather than as electrons independent of the initial percentage of electrons in the active glow. A change in the concentration of a negative ion during the active glow will still affect its time integrated flux to the grounded electrode during the afterglow in a nearly linear fashion provided the percentage of negative charge carried by electrons during the active glow remains nearly constant. As a consequence, measured changes in the negative ion signal from a discharge afterglow can accurately relate changes in the negative ion concentration of the active glow.