Multi-section Traveling Wave Antenna for heating of large machines as DEMO

The main advantages of ICRH&CD are its ability to achieve power deposition in the center of the plasma column without any density limit along with direct heating of plasma ions. The challenge is then to couple large amount of power through the plasma boundary, where an evanescence layer has to be crossed, without exceeding the voltage standoff at the antenna. A solution presently considered is the reduction of the power density by means of antennas distributed all along the wall of the machine. In [1] we have shown that a suitable launcher can be constituted by sections of Traveling Wave Antenna (TWA) mounted in resonant ring systems. They are launching a traveling wave in one direction along the structure that leaks its energy to the plasma and is refueled periodically by generators. Each section is constituted by a series of equidistant mutually coupled grounded straps aligned in the poloidal direction which radiates its power to the plasma proportionally to the total strap number divided by their inter-strap distance. Due to the large number of radiating elements, the launched power spectrum is very selective. A detailed discussion on the multi-section antenna is made in view of its test on a mock-up. We study the influence, in ring shaped structures, of its geometrical parameters on its response along with the influence of the periodicity of sections and feedings. This extends the work done in [2]. The aim is to prepare for a proof-of-concept system to be tested in an operating tokamak machine.