Towards a 2 MW, CW, 170 GHz coaxial cavity gyrotron for ITER

The development work on coaxial cavity gyrotrons at the Forschungszentrum (FZK) Karlsruhe has been performed as an ITER task. In agreement with the final goal of the task the feasibility of manufacturing of a 2 MW, CW coaxial gyrotron operated at 170 GHz as could be used for electron cyclotron heating and current drive in the ITER tokamak has been demonstrated and all information necessary for a technical design and industrial manufacturing has been obtained. The experimental investigations have been performed on a coaxial gyrotron operated in the TE31,17 mode at 165 GHz and designed for a nominal RF output power of 1.5 MW. The gyrotron is of modular type and its technical performance limits the operation to short pulses. With this gyrotron a maximum RF output power of 2.2 MW has been reached in short pulse (typically 1 ms) operation. At the nominal RF output power of 1.5 MW an efficiency of 30 % has been obtained which has been enhanced to 48 % in operation with a single-stage depressed collector. All problems specific to the coaxial arrangement have been investigated. The stability and the losses of the coaxial insert has been measured and the influence of misalignment of the insert on the losses has been investigated. ... mehrThe amount of microwave radiation captured inside the tube has been found to be (9± 1) % of the RF output power. According to the measurements the captured stray radiation is distributed more or less uniformly inside the mirror box. In single-pulse operation the pulse length has been extended up to 17 ms. A sudden increase of the current Iins to the coaxial insert has been observed which limited the achievable pulse length. It has been found that the increase of Iins is caused by a kind of Penning discharge in the technical part of the electron gun. The occurrence of such a discharge can be avoided by a proper modification of the gun geometry. Parasitic low frequency oscillations have been successfully suppressed and stable operation has been achieved over a wide range of parameters. By applying a rapid variable bias voltage at the coaxial insert fast (~ 0.1 ms) frequency tuning has been demonstrated. In particular, step frequency tuning by ± 2.2 GHz due to switching from the nominal mode at 165 GHz to its azimuthal neighbors has been done and continuous tuning by up to 70 MHz within the bandwidth of the TE31,17 mode has been performed. Based on the experimental results a draft design of a 170 GHz coaxial gyrotron for 2 MW, CW output power has been done. The usability of the components for CW operation and their compatibility with technical restrictions has been proven. A suitable cavity mode has been selected and a collector has been designed. In addition, an integrated design of the tube has been performed.