Design and Fabrication of a Conduction-Cooled Superconducting Magnet for Gyrotron

A conduction-cooled superconducting magnet for gyrotron that produced the central field in 3.57 T, with less than 0.1% of field nonhomogeneity and less than 1.5% of the field error rate, was developed. The magnet consists of NbTi split-type solenoids, a cryostat with a thermal shield, a Gifford-McMahon cryocooler, and a pair of binary current leads. The magnet was installed in a vacuum cryostat with 90-mm warm bore, 645-mm depth, 370-mm width, and 220-mm height. In order to minimize the heat load to the superconducting magnet, a thermal shield was thermally attached to the first stage. In addition, thermal contact resistance was carefully considered to minimize the temperature difference between the coils and the second stage of the cryocooler. It took 20 h for the magnet to cool down from 300- to 4-K level. The temperature of the superconducting magnet was stable below 3.5 K, which corresponded to about 0.6 W of total heat loads. After charging the magnet to 88 A of current with 0.1-A/s ramp rate, it produced 3.57 T of the central field with less than 0.07% of field nonhomogeneity and 1.5% of field error rate. The detailed development of the magnet was presented in this paper.

I. Introduction

The gyrotron is a microwave source commonly used for heating plasma in nuclear fusion devices. This technology converts the rotational kinetic energy of electrons, which is accelerated by high DC voltage and magnetic field into high power energy [1]. In gyrotron, the role of the superconducting magnet is to provide the magnetic field strength enough to produce 95 GHz of electron cyclotron frequency and generate the high magnetic field uniformity to make the electron beam fired from the electron gun injected into the resonator without loss. Compared with a conventional liquid helium cooled system, the most charming character of the conduction cooled superconducting magnet lies in the operating convenience, compact size, flexibility, and mobility [2], [3]. Furthermore, due to the shortage of helium natural resources, the driver for many researchers to seek conduction-cooled solutions is that it is able to guarantee the continuity of their experiments, which is crucial and fundamental to any research program. Due to limited solid thermal conductance relative to liquid helium, the superconducting magnet system suffers from bigger temperature difference and weaker cryogenic stability. The spare cooling capacity of the cryocooler, which is equal to nominal cooling capacity minus system total heat load, can either provide low operating temperature to increase the critical current performance for the superconducting magnet.