Investigations on ionic processes and dynamics in the sheath region of helium and hydrogen discharges by laser spectroscopic electric field measurements

Temporally and spatially resolved measurements of the electric field distri bution in the sheath region of RF and de discharges provide a detailed insi ght into the sheath and ion dynamics. The electric field is directly relate d to the sheath ion and electron densities, the sheath voltage, and the dis placement current density. Under certain assumptions also the electron and ion conduction current densities at the electrode, the ion current density into the sheath from the plasma bulk, the ion energy distribution function, and the power dissipated in the discharge can be inferred. Furthermore, th e electric field distribution can give an indication of the collision-induc ed conversion between different ion species in the sheath. Laser spectrosco pic techniques allow the noninvasive in situ measurement of the electric fi eld with high spatial and temporal resolution. These techniques are based o n the spectroscopic measurement of the Stark splining of Rydberg states of helium and hydrogen atoms. Two alternative techniques we applied to RF disc harges at 13.56 MHz in helium and hydrogen and a pulsed de discharge in hyd rogen. The measured electric field profiles are analyzed, and the results d iscussed with respect to the ion densities, currents, energies, temporal dy namics and species composition.