Sunil Kedia

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.

Quench Detection System for TF Coil-Test Campaigns of SST-1

Testing of all superconducting toroidal field (TF) coils of a steady-state superconducting tokamak (SST-1) in nominal currents of 10 000 A under supercritical helium-cooled conditions was an essential prerequisite of SST-1 machine reassembly and subsequent refurbishment. Testing of individual TF coils at its full operational load of >;1 MA in its winding pack necessarily demands to test these coils with all the interlocks coupled with precise protection elements. A quench detection (QD) system is an essential and integral part of this testing since any irreversible off-normal scenario leading to the magnet quench must be promptly detected, and the energy from the magnet must be extracted with equal promptness to protect the magnet within the defined dump time, avoiding thermal stresses in the winding pack. An active fail-proof electronics QD system has been developed for the detection of resistive transitions in any part of the SST-1 TF coil. The difference configuration method has been adopted and exploited in the QD system for comparing the voltage drop measured in each of the double pancakes and interpancake joints of the TF coil. This paper describes the scheme of the QD system and precautions taken ensuring enhanced reliability and redundancy, as well as the results obtained in the due course of all the 16 SST-1 TF magnet tests.

Precision Signal Conditioning Electronics for Cryogenic Temperature and Magnetic Field Measurements in SST-1

As a part of refurbishment of Steady-state Superconducting Tokamak-1 (SST-1) at the Institute for Plasma Research, India, all 16 toroidal field (TF) magnets of SST-1 have been individually tested in cold with nominal current in a dedicated experimental cryostat. The magnets were cooled down to 4.5 K using either supercritical or two-phase helium, after which they were charged up to 10 kA of transport current. Precise cryogenic temperature and magnetic field measurements in the experimental configuration were mandatory. Temperatures were required to be accurately measured at several locations in the magnet and hydraulic circuits, whereas the field measurements were carried out at few predefined locations on the magnet case. Highly accurate in-house modular signal conditioning electronics had been developed for low temperature and high magnetic field measurements for such large magnets. This paper describes the scheme of the measurement, precautions taken for enhanced reliability, precautions taken for long-term offset stability in cryogenic environment, and the results of SST-1 TF magnet tests.

Finite-Element Analysis of Dump Resistor for Prototype Superconducting Magnet Carrying 3.60 MA-t

Stability margin, transient ac responses, and protection of the magnet system in the case of abnormal quench are some of the important design drivers of superconducting magnets for fusion reactors. A prototype magnet has been designed using a cable-in-conduit conductor with a nominal operation current of 30 kA at 12 T and 5 K at the Institute for Plasma Research, Gandhinagar, India. Protection of the magnet system during off-normal scenarios leading to quench makes the operation of the magnet system more challenging. An external dump resistor is required to extract the stored magnetic energy of the magnet in case of quench. Such a dump resistor has been designed for this 3.60 MA-turns current-carrying prototype superconducting magnet. Finite-element thermal, structural, and electromagnetic analyses have been done for validating the dump resistor parameters using commercially available Ansys software. The maximum temperature rise of the dump resistor system is found to be 540 K, considering cool down by natural convection only. In addition, the maximum stress between the parallel plates of the is found to be less than 1.0 GPa, which is in the acceptable range.

Measurement of Electromagnetic and Thermal Stresses on Conduction-Cooled Joints of the SST-1 Spare TF Coil

Low-dc-resistance superconducting joints in toroidal- and poloidal-field (TF and PF, respectively) coils of the steady-state superconducting tokamak-1 (SST-1) at the Institute for Plasma Research (IPR) is under testing. The feasibility of conduction-cooled leak-tight joints made between two double pancakes in the winding pack of the TF coil is validated through experiments. The configuration of these conduction-cooled joints is comprised of a prefabricated SS304L oxygen-free high-conductivity copper leak-tight termination, into which the unconduited and soldered portion of the cable-in-conduit-conductor (CICC) is inserted. Once the cable space is inserted inside the prefabricated piece, solder filling is carried out, and the joints are realized by overlapping the mating ends and soldering them together. The supercritical helium flowing through the CICC exits prior to the termination length, and the joints are cooled by conduction. The joints are subjected to I × B-induced and bending-induced stresses during SST-1 operational scenarios. These stresses can lead to leaks in the joint region if they exceed the material strength or the brazing/welding strength. Both the thermal and electromagnetic stresses that developed at the copper-stainless steel prefabricated brazed region are measured on the SST-1 spare TF coil. These stresses are measured using the strain gauges during the cooldown and the charging of the spare TF coil up to its operational current of 10 kA at a conventional 4.5 K and 4 bar of supercritical helium forced flow. The electromagnetic-stress behavior at the time of quench that occurred accidently during the spare TF coil test at an 8 kA transport current was also studied. The signal-conditioning electronics required for this measurement are engineered and tested at the IPR before its implementation to the spare-TF-coil test campaign. The measured thermal and electromagnetic stresses are found to be in good agreement with the simulated finite-element Ansys results.

Development of 440-V 800-A Resistive-Type Modular Superconducting Fault Current Limiter With YBCO Tapes

CG Global R&D is developing a resistive-type superconducting fault current limiter (SFCL) under the Ministry of Power, National Perspective Plan for R&D in Indian Power Sector. A resistive-type 440-V 800-A single-phase SFCL is developed using YBCO tape in stage I of the project, and an 11-kV 1250-A three-phase SFCL will be developed in stage II. The 440-V 800-A SFCL is assembled with seven parallel modules. Each module has series-connected YBCO tapes to accommodate the design length. The SFCL is tested at 77 K for continuous current of few hours, fault currents at low voltage, and fault currents at 440 V up to 100 ms (5 cycles). The evaluated data will be used to develop an 11-kV 1250-A three-phase SFCL in stage II.

Time-Varying Magnetic Field Coupled Noise Reduction in Low-Voltage Measurements in Superconductors

Noise generated by time-varying magnetic fields interferes in the measurements of low voltages particularly with superconductor materials. The measured values can alter significantly if it is not compensated appropriately or if suitable protocols are not followed. Measurements on inductive loads, like superconducting wires and coils, considerably generate high magnetic field in the surrounding, which couples electromagnetic noise through measurement leads and circuits. Twisted pair cables are adopted and used as a classical solution to avoid magnetically induced voltages. However, the magnetic field produced by large inductive load couples imposed electromagnetic interference (EMI) with the cable loop created by voltage taps, which are soldered over part of the load. The effects are severe when there is a big loop area due to large sample size or when there is a high rate of change in the magnetic field due to high-current ramp rates. A finite uncompensated flux is invariably linked in such cases. Three simple, yet effective, techniques were implemented for the cancellation of EMI noise which couples in a cable loop formed by voltage-measuring leads of superconductor sample in low-temperature experiments. In all these methods, the basic principles are to detect and measure signal portion and coupled-noise portion with dedicated voltage taps, use subtraction mathematics to cancel the noise portion, and derive actual voltage-drop signal across a superconducting load. These techniques were tested successfully on low-resistance joint of high-temperature superconducting Bi2Sr2Ca2 Cu3O10 tape in a bath-cooled condition (77 K). EMI noise coupling was simulated with background field coil and a permanent magnet. This paper describes the detailed experimental procedure, results, and observations. The concept can be extended to DC joint-resistance measurement of Toroidal and Poloidal field coils of Steady State Superconducting Tokamak-1.

Precision signal conditioning and front-end electronics for temperature and field measurements in SST-1 TF magnets

As a part of refurbishment of Steady-state Super-conducting Tokamak-1 (SST-1) at Institute for Plasma Research, India, all sixteen Toroidal Field (TF) magnets of SST-1 have been individually tested in cold with nominal current in a dedicated experimental cryostat. The magnets were cooled down to 4.5 K using either super critical or two phase helium, after which they were charged upto 10 kA of transport current. Precise temperature and field measurements in the experimental configuration were mandatory. Temperatures were required to be measured accurately at several locations in the magnet and hydraulic circuits, where as the field measurements were carried out at few predefined locations on the magnet case. Highly accurate in-house modular signal conditioning electronics had been developed for accurate temperature and field measurements. This paper describes the scheme of the measurement diagnostics, precautions taken for enhanced reliability and long term offset stability in cryogenic environment, and the results of TF magnet tests.

A Resistive-Type Modular Superconducting Fault Current Limiter Fabricated With Stainless-Steel-Stabilized YBCO Tapes

A resistive-type superconductor fault current limiter (SFCL) is experimentally studied with stainless-steel-stabilized 12-mm-wide YBCO tapes. The SFCL is assembled with six noninductive modules, each rated for 120 V, 100 A. The module has several YBCO tapes connected in series that are arranged such that the flow of current in one tape is in the opposite direction from its neighboring tapes. The modules can be connected for different SFCL configurations with series and parallel combinations. In this paper, the SFCL is assembled in two configurations and is tested for continuous, overload, and short-circuit currents. The first configuration is assembled to evaluate current carrying performance in parallel connected modules at 600 A. The second configuration is assembled for short-circuit performance of two series-three parallel connected modules at 240 V, 300 A. The evaluated data will be useful to develop higher rating SFCL with modular construction.

DC and AC Characteristics of Copper Clamp-Type Mechanical Joints of YBCO-Coated Conductors

CG Global R&D is currently working on the design and validation of resistive-type superconducting fault current limiters using second-generation-YBa2Cu3O7 coated conductors (YBCO CC). Noninductive modules are made using several series-connected YBCO tapes to accommodate the required design length. Copper clamp-type mechanical bridge joints are employed between these YBCO tapes. These joints are made by sandwiching two parallel tapes between copper plates. This type of configuration gives additional ruggedness to the joint compared to soldered superconducting bridge joints with the superconducting bridging element. The dc and ac resistive characteristics of these joints are studied as a function of the operating current. The ac losses in the YBCO tape are also measured as a function of applied current with line frequency of ~50 Hz.