A superconducting magnet, which is used as a focusing magnet for a photocathode pattern transfer system, has been developed. The focusing magnet requires highly homogeneous and stable magnetic field to attain high resolution capability. The magnet is Helmholtz type with 880 mm average winding diameter. Each coil is placed in a cryostat which has 620 mm diameter bore. In order to obtain the field homogeneity, the relative position of two cryostats is made adjustable. Field homogeneity was 6 × 10 -5 over the 90 mm diameter after adjustment. This value was measured by nuclear magnetic resonance (NMR) method. 3k Gauss magnetic field was obtained at 188 A/mm 2 excitation current density. Superconducting persistent current was maintained for 10 -4 /hour in decreasing ratio. Consequently, the photocathode pattern transfer system with this superconducting magnet successfully transferred VLSI patterns of 0.5 μm geometry.
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.
The authors have reported the results of low n -value from a MgB 2 test coil developed a year ago. A second test coil has been developed with wire of different structure and manufacturing process. Although the n-value related voltage of the second test coil was lower than the first test coil at designed current, it still showed low n-value. A third test coil has been wound with reduced mechanical stress. It also showed very similar n-value related voltage and n-value. Investigation of voltage distribution over the coil indicated that magnetic field was the major factor causing degradation of the n-value and resulting in n -value related voltages. Since the n-value related coil voltages were on the order of 0.1 μV/cm, the usual short sample Ic test (1 μV/cm was the definition of Ic ) might not detect the n-value related voltage and might not be able to investigate the cause of low n -value. Therefore, the medium length ( ~ 10 m) samples were tested and they showed the wire's lengthwise nonuniformity both on n-value and Ic , which might be another potential cause of the low n-value of the coil. Along with the electrical investigation, the manufacturing process of the wire was carefully inspected for longitudinal uniformity. Some wire segment samples from the same batch exhibited nonuniformity in the particle size distribution resulting in nonuniform filaments. This might have occurred in the wire for the second and third test coils.
Fabrication of the central solenoid for ATLAS detector in the CERN-LHC project was completed, and the performance test has been successfully carried out in Japan. The solenoid has arrived at CERN to be assembled with the LAr calorimeter. This paper describes the fabrication and mechanical, performance of the ATLAS central solenoid.
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 authors had reported components' development of 3 T-250 mm bore MgB2 magnet system. Pre-reacted MgB2 tape wire with copper lamination had n-value related problem due to raw Boron particle size inequality, but it had been corrected. Long MgB2 wires over 3 km had been supplied. All six component coils were made with a wet winding procedure. They were tested individually with the same cooling scheme of conduction cooling as the actual magnet assembly. Though all coils could be ramped to some extent, some coils showed fairly large remnant voltage. Since the voltage distribution over the coil was not even, the uniformity along the wire length may not be good enough. The stability of the coil was verified by its no training performance even with fast ramping. The magnet assembly and its test with conduction cooling were planned. Ic of the superconducting joint with this pre-reacted wire was doubled during past one year's development.
