Electric power supply systems for superconducting magnets are reviewed. Recent technical developments concerning main power supply circuits, magnet protection systems against quenches and magnet excitation controllers are described. Requirements for performances of main power supply circuits have been changed to large current output, high voltage output and magnet energy regeneration reflecting the needs for large scale superconducting magnets such as fusion application or pulsive operation. Main power supply circuits of silicon diode/transistor type have been changed to those of thyristor/transistor type due to less energy losses in circuits. However as far as transistors are used in the circuit, these power supply circuits are not appropriate for large current and pulsive operating magnets. Thyristor type power supply circuits meets all performances mentioned above. As this type of power supply circuit is able to send back magnets energy to AC line, it is suitable for pulsive operating magnets. In near future power supply circuits for the output more than 10, 000A will be changed to transformerless type, because the winding of such large current transformers is nearly impossible. The power supply circuits of silicon diode/transistor type, thyristor/transistor type, thyristor type and transformerless type would be useful for the output current of less than 1, 000A, 1, 000A to 10, 000A, approximately 10, 000A and more than 10, 000A respectively. Several methods of quench detection and magnet protection against quenches have been applied. We should select the most appropriate method corresponding to the magnet system considering inductance, current density and operating current of the load magnet. As for quench detection, the most general method bases on the principle that the bridge circuit consisting of two resistors and a magnet with a center tap loses voltage balance when a part of the magnet has resistance due to quench. In order to discriminate the quench voltage from noise voltage, the following consideration should be paid for the detection circuit. Namely the circuit, which recognizes as magnet quench when the unbalanced voltage keeps longer time than preset duration with higher voltage level than preset value, will be effective. As for magnet protection against quenches, a DC circuit breaker to cut off the load magnet from the main power supply circuit and a magnet energy absorber are necessary. The combination of a thyristor circuit breaker and a varistor absorber is one of the most efficient system for magnet protection. However the magnet protection system consisting of a no-fuse breaker and a resistor is fairly welcome from the economical point of view. A magnet excitation controller has a current sweeper which sends the output current signal to main power supply circuit. Recently electronic type sweepers instead of mechanical type are used because of easy control. Both digital type sweeper and digital control circuit of main power supply are now tried to obtain much higher control precision. By using a specially designed excitation controller, a superconducting magnet with a superconducting switch for persistent current operation can be excited speedily suppressing heat generation in the superconducting switch.
Discussions are given on various mechanism determining the critical temperature of superconductivity. The possibility is pointed out of the high critical temperature due to the excitonic mechanism along dislocations.
A new application of superconducting magnet has been opened out for a microfabrication system in future semiconductor itegrated circuits industry. Current interests are to develop electron beam pattern transfer systems which have capability of submicron patterning. A photocathode pattern transfer system has been developed in VLSI Cooperative Laboratories. In this system photoelectrons emitted from a mask with submicron VLSI patterns are accelerated by high electric field and are focused onto a silicon wafer by the highly homogeneous magnetic field. It is possible to transfer VLSI patterns onto many silicon wafers in turn with high resolution by use of this pattern transfer system. The system is superior to conventional optical projection system in regard to resolution capability. The requirements for the magnet in the system are as follows; (1) Magnetic field intensity is around 3k Gauss at the center. (2) Field homogenity is of 10-5 order over 90mm in diameter at the central part. (3) Stability of magnetic field is less than 10-4/hour. (4) The room temperature region for working space is more than 500mm in diameter. A superconducting magnet is suitable for above requirements because of its characteristics such as persistent current and high current density. Stable magnetic field will be obtained by persistent current mode operation. A Helmholtz coil with high current density will easily provide homogeneous magnetic field. The magnet developed here is of Helmholtz type. Each coil has the average diameter of 880mm and the winding crosssection of 28mm×28mm. The maximum current density over the winding crosssection is 188A/mm2. The upper and lower coils are enveloped in each cryostat and are connected by superconducting wire for persistent current mode operation. The cryostat with 620mm bore in diameter was designed in consideration of wide space for the components of pattern transfer equipments. The magnet was successfully operated in persistent current mode up to the field of 3k Gauss. Adjustment of coil separation was made to get the field homogeneity, measuring the field distribution by NMR method. Miscellaneous magnetic field due to magnetization of surrounding materials or relative displacement of two coils was fairly well cancelled out by the adjuster. As a result, 6×10-5 field homogeneity was achieved over 90mm in diameter at the center. Superconducting persistent current was kept with 10-4/hour in decreasing ratio which was enough for stability requirement. Consequently VLSI pattern resolution with less than 0.5μm geometry was successfully obtained. The superconducting magnet would give a promising result to realize VLSI like 1M bits Random Access Memory (RAM).
The structural materials of the large superconducting coils for the next engineering tokamak (NET), which is now being designed at the Japan Atomic Energy Research Institute (JAERI), have to withstand magnetic forces as large as 1400MPa which corresponds to a strain of 0.2% at liquid helium temperature. We have been developing new materials instead of 304 and 316 austenitic stainless steels because the yield strength of the austenitic stainless steels is too low for them to be used for the structure of the NET, as shown by our various mechanical test results from room temperature to liquid helium temperature. As one of the new materials, high Mn austenitic steel (32Mn-7Cr) was selected and tested in a 10-ton cryogenic tensile test machine at 4K. The test results show the material has enough strength (1100MPa) and sufficient toughness at 4K to be used as the structure of the large superconducting coils.
Recent development of superconductive magnets requires large current power supplies more than 10kA. This report describes needed properties of these power supplies suitable for the superconductive magnets, and gives brief description of the most suitable power supply. It also gives the newly developed power supply by DC-AC-DC conversion method, and its test results.