Radial Broadening of DC potential structures in front of ICRF antennas by transverse exchange of RF currents

Measurements show that the vicinity of powered Ion Cyclotron Range of Frequency (ICRF) antennae is biased positively with respect to its environment [1] [2]. This is attributed to RF‐sheaths. The radial penetration of DC potentials into Tokamak SOL determines the power deposition on the walls and especially on the antenna structure, which is a key point for long time clean discharges. Within independent flux tube models of RF‐sheath rectification the radial penetration of DC potentials is determined by the skin depth x0 = c/ωpe for the slow wave. When self‐consistent exchanges of transverse RF currents are allowed between neighboring flux tubes, such a structure can be broadened radially up to a characteristic transverse length L. Broadening arises as soon as L>r0. A linear modeling gives a first evaluation of the theoretical length L≈(L‖ρci/)1/2. Within the ‘flute assumption’ it scales with the length L‖ of open flux tubes and the ion Larmor radius ρci. This has been confirmed by the SEM code [1] which t...