Measurement of spatial and temporal evolution of electromagnetic fields in a 100 MHz plasma source using B dot and double dipole probes

Very high frequency plasma sources are often accompanied by plasma density nonuniformities associated with a standing-wave effect. Electron density measurements using a plasma absorption probe show density nonuniformities that can be larger than predicted by a standing wave model. These structures have been associated with harmonics of the electric fields in the plasma. The authors present the first time and phase-resolved measurements of the spatial structure of the electromagnetic waves in a 100 MHz plasma source using argon at 40 mTorr employing a B dot probe. The authors show that the harmonic structure is related to a current reversal and subsequent circulation that appears when the sheath collapses during the radio frequency cycle. The circulation is driven by inward traveling waves that are electromagnetic in nature, not plasma waves traveling at the electron thermal velocity. Double dipole probe measurements were used to validate the B dot probe electric field measurements derived from the time derivative of Β_θ which is derived from the B dot probe signal.