The Travelling Wave Antenna: an important option for ICRH in DEMO

For the central heating of a commercial fusion reactor Ion Cyclotron Radio Frequency Heating (ICRH) is an important option as it is the only heating method capable of coupling Radiofrequency (RF) power directly to the D and T plasma ions without a cut-off density limit. However the excitation of the magnetosonic waves through the boundary layer can lead to voltage standoff problems with traditional ICRH antenna systems. In this paper we present a new antenna concept that optimizes the antenna excitation. It consists of a number of Travelling Wave Antenna (TWA) sections distributed all around the first wall, embedded in the blanket modules. The sections are fed using a resonant ring system. We will present the underlying physics of a TWA section, details of the resonant ring feeding, will show the wave patterns from antenna, through the evanescent layer, up to the bulk plasma. The proposed system is totally load resilient and is characterized by a very selective spectrum for the toroidal wavenumbers k//. Coupling of the power scales as the ratio of the total number of straps divided by the distance between the straps, for a given geometry and applied voltage.