Subrata Pradhan

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.

SST-1 Status and Plans

The Steady State Superconducting Tokamak (SST-1) is currently being refurbished in a mission mode at the Institute for Plasma Research with an ultimate objective of producing the first plasma in early 2012. Since January 2009, under the SST-1 Mission mandate, a broad spectrum of refurbishment activities have been initiated and pursued on several subsystems of SST-1. Developing sub-nano-ohm leak-tight joints in the magnet winding packs, developing single-phased LN2-cooled thermal shields, developing supercritical-helium-cooled 5-K thermal shields for magnet cases, ensuring thermal and electrical isolations between various subsystems of SST-1, testing of each of the SST-1 toroidal field (TF) magnets at 4.5 K with nominal currents, testing each of the modules and octants of the SST-1 machine shell in representative experimentally simulated scenarios, augmentation and reliability establishment of the SST-1 vacuum vessel baking system, time synchronizations among various heterogeneous subsystems of SST-1, large data-storage scenarios, and integrated engineering testing of the first phase of the plasma diagnostics are some of the major refurbishment activities. Presently, the SST-1 device integration is in full swing. The cold test of the assembled SST-1 TF and poloidal field magnets began in December 2011. Following the successful testing of the SST-1 superconducting magnet system and engineering validations of the machine shell, the first plasmas will be attempted in SST-1. The first plasma will be ~ 100-kA limiter assisted with the available volt-seconds and could possibly be assisted by ECCD/LHCD.

Studies on the cure parameters of cyanate ester–epoxy blend system through rheological property measurements

Thermosetting blend system of co-cured cyanate ester with epoxy resin is receiving importance for high technological applications because of its wide range of thermo-mechanical, rheological, and electrical properties. However, processing of these system warrant proper knowledge of the rheological behavior of the blend during the curing process. This article discusses the rheological behavior of the blend systems with respect to the pot life, gel time, gelation temperature and also evaluated fitting parameters for the prediction of gelation time and viscosity during the entire curing process from the isothermal rheological measurements.

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.

SST-1 Toroidal Field Magnet Tests: Some Results and Lessons Learnt

Testing each of the SST-1 Toroidal Field (TF) Magnets under representative conditions in cold with nominal currents was one of the primary and mandatory requisite in the refurbishment of the Steady-State Superconducting Tokamak (SST-1). Under this mandate prior to the SST-1 machine shell re-assembly, each of the 16 SST-1 TF magnets have been tested fully and successfully in a dedicated test stand. The campaign began on June 10, 2010 and was concluded on Jan 24, 2011 spanning over seven and half months without any interruption in-between. These campaigns ensured that all the sixteen TF magnets could be charged to their nominal currents of 10000 A in either two-phase or supercritical cooling conditions with leak-tight inter-double pancake resistances being in the range of 150 pico-ohms to 1200 pico-ohms. Sensors and diagnostics signal conditioning and data acquisitions, magnet quench detection and magnet protection aspects in these campaigns have been precise, extensive and redundant wherever feasible. Test team had many failures as well as several learning in the course of these challenging tasks. Magnet preparations for the tests, the test designs and their executions, some exciting and confidence building experimental results as well as some failure experiences have been presented in this paper.

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.

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.

Technology Developments Toward ELM Coil Manufacturing Appropriate for Tokamak

The Institute for Plasma Research (IPR) has been developing technologies appropriate toward realizing fusion-relevant edge-localized mode (ELM) magnets in Tokamaks, such as suitable for Joint European Torus (JET) and Steady State Superconducting Tokamak (SST-1). The development of characteristic winding topologies, developing an appropriate “special-purpose winding facility,” developing a high-temperature resin insulation system, developing vacuum pressure impregnation of high-temperature resin systems in a manufactured winding pack, developing profiled-case manufacturing technologies, and developing technologies at consolidating and encasing the impregnated winding packs are some of the essential aspects of realizing an ELM control coil. In these contexts, under an IN-EU joint initiative, the Magnet Technology Development Division at IPR has developed 1:1 prototypes of JET ELM control coils. These technologies would also be extended to the SST-1 ELM coils. This paper would elaborate on some of the salient technologies developed in support of JET ELM prototype coil realization.