Optimization of TE11/TE04 mode converters for the cold test of a 250 GHz CARM source

Abstract Electron Cyclotron ResonanceHeating (ECRH) systems in future fusion devices, like the DEMO-nstration reactor, foresee an operational frequency in the range 230–280 GHz to match the plasma characteristics. Cyclotron Auto Resonance Masers(CARMs) could represent an alternative to gyrotronsas effective source of mm and sub-mm waves and the project of a 250 GHz, 0.5 MW CARM has been undertaken at ENEA. Mode converters from the TE 10 in WR3 rectangular waveguide to the operational mode (TE 53 ) are required to perform the RF cold test of Bragg reflectors and cavities. In this paper an improved and feasible structure for the TE 11 to TE 04 conversion, in oversized circular waveguides, using only two transitions with profile and radius optimized in order to obtain high efficiencies and sufficient bandwidth, has been designed and the simulation results are presented. The first transition is a TE 11 /TE 01 serpentine mode converter in circular waveguide (efficiency of 95%) with average radius of 1.48 mm and an appropriate number of geometrical periods. The second one, a TE 01 /TE 04 rippled wall mode converter with efficiency of 97%, consists of a single conversion section with an average radius of 3 mm and sixteen sinusoidal periods instead of three conversion sections (TE 01 /TE 02 , TE 02 /TE 03 , TE 03 /TE 04 ) in cascade, thus reducing the complexity and the length of the full transition chain.