Edge power deposition reduction in a tokamak by replacing the Faraday screen on an RF antenna with an insulator

Abstract

Calculations show that the power coupled to the regions around the RF antenna (side, front and interior) due to the edge plasma interaction with voltages induced on the antenna structure of the Phaedrus-T tokamak was reduced from nearly 11 kW to a few hundred watts for 40 kW of RF input power by replacing the Faraday screen (FS) surrounding the antenna with insulating boron nitride (BN) side limiters. When an FS was on the antenna, calculations show that 7.7 kW of the power coupled to the edge was transferred to the electrons that passed through a potential gradient while crossing field lines and 3 kW was dissipated by ions falling through an increased sheath at the carbon limiter and FS. Experimental data of edge plasma parameters, loading resistance and intensity of VUV impurity light support the conclusion that less power was deposited in the edge plasma when the antenna had only BN side limiters than when it was covered with an FS