Design and Optimization of Superconducting Magnet System for 42.0-GHz Gyrotron

In the framework of the Department of Science and Technology (DST), Government of India, a 42.2-GHz 200-kW continuous-wave/long-pulse gyrotron is envisaged to be indigenously developed. This gyrotron shall employ superconducting magnets at the interaction region and warm coils for the gun and collector region. The Institute for Plasma Research is responsible for the overall design and fabrication of the magnet system along with the required housing cryostat and auxiliary support system. The design of the appropriate magnet system is currently under progress in accordance with gyrotron physics and engineering considerations. This requires a highly homogeneous spatial field profile as well as a very steep gradient as per the compression and velocity ratios between the emission and resonator regions. These aspects demand a very precise winding of the magnets as well as the collinearity of the magnetic axis with that of the beam axis. Several technological aspects, such as accurately designing and positioning of the magnet system in space, to optimize the required field profile have been taken up in the run up to realize a highly homogeneous and stable magnet system. Different design criteria for the theoretical optimization of magnet parameters and their spatial arrangement such that the required axial magnetic field profile can be achieved have been taken up. In addition, finite-element analysis (FEA) of the optimized magnet parameter is done, and the magnetic field profile is compared with the theory. The detailed design of the guiding magnet system, the optimization of coil parameters, and the FEA simulation for the validation of the optimized parameters are presented in this paper.

I. Introduction

Gyrotrons are high-powered vacuum tubes that emit millimeter-wavelength beams by electron bunching. The output frequency of the gyrotron ranges from 20 kHz to 250 GHz. They can be designed for continuous or long-pulsed operation. A gyrotron is basically a fast-wave microwave tube based on the principle of interaction between the gyrating beam and the transverse electric (TE) mode [1], [2]. The gyrating beam is emitted from an electron gun. It has high power at millimeter-wavelength region because its dimension can be much larger than the wavelength. Gyrotrons are used in many industrial and high technology heating applications, like in nuclear fusion devices for heating plasma [3], in manufacturing industries as a fast heating tool for the processing of glass, and in annealing. The electron cyclotron wave is an effective method for plasma heating (electron cyclotron heating, ECH), on- and off-axis current drive (electron cyclotron current drive, ECCD), and plasma profile control in fusion devices. In recent years, significant progress has been made, and efforts are focused on the development of high-power, continuous-wave, or long-pulse gyrotrons as a power source for ECH/ECCD. The electron gun used for the emission of the gyrating beam is known as the magnetron injection gun (MIG). High-frequency electromagnetic waves are generated by the oscillators, such as gyrotrons outside the torus. If the waves have correct frequency and polarization, then their energy can be transferred to the charged particles in the plasma, which in turn collide with other plasma particles, thus increasing the temperature of the bulk plasma.