VOLUME AND SURFACE RF POWER ABSORPTION IN A HELICON PLASMA SOURCE

We consider the excitation and absorption of waves in a plasma cavity with parameters typical for helicon sources. rf power is shown to be t ransferred into the plasma via two channels. ?he first is realized by weakly damping helicon waves which are excited directly by the azimuth al parts of the antenna and penetrate into the bulk plasma. Electrosta tic waves arising at a plasma edge owing to the linear mode conversion form the second channel. Strongly damped electrostatic waves can reac h a plasma core al low magnetic fields only, while at high fields they deposit energy at the periphery of the plasma column. A principal fra ction of the If power is transferred into the plasma via the electrost atic channel, and so the power input turns out to be volume at low mag netic fields and surface al high fields. An enhanced volume input is p ossible at high fields in special anti-resonance regimes when the exci tation of the electrostatic wave is suppressed. The effective collisio n frequency is introduced to describe effective damping of helicon wav es arising due to their conversion into electrostatic waves. The resul ts obtained permit explanation of both measured field profiles and the high absorption efficiency in helicon sources.