Ziauddin Khan

Nitrogen Gas Heating and Supply System for SST-1 Tokamak

Steady State Tokamak (SST-1) vacuum vessel baking as well as baking of the first wall components of SST-1 are essential to plasma physics experiments. Under a refurbishment spectrum of SST-1, the nitrogen gas heating and supply system has been fully refurbished. The SST-1 vacuum vessel consists of ultra-high vacuum (UHV) compatible eight modules and eight sectors. Rectangular baking channels are embedded on each of them. Similarly, the SST-1 plasma facing components (PFC) are comprised of modular graphite diverters and movable graphite based limiters. The nitrogen gas heating and supply system would bake the plasma facing components at 350°C and the SST-1 vacuum vessel at 150°C over an extended duration so as to remove water vapour and other absorbed gases. An efficient PLC based baking facility has been developed and implemented for monitoring and control purposes. This paper presents functional and operational aspects of a SST-1 nitrogen gas heating and supply system. Some of the experimental results obtained during the baking of SST-1 vacuum modules and sectors are also presented here.

Performance of Joints in SST-1 Magnets

Novel sub-nano ohm leak tight joints were one of the primary objectives of the refurbishment of Steady State Superconducting Tokamak (SST-1). In total, there are 130 such shake-hand type of joints which have been fabricated on the winding packs of SST-1 Toroidal Field (TF) and Poloidal Field (PF) superconducting magnets. These joints have been validated to be performing per their design specifications in the assembled SST-1 TF and PF magnets during the first cool-down and charging of the assembled and integrated SST-1 superconducting magnet systems. This paper describes the novel aspects of design, process establishments, technology development, testing and qualifications of such joints on the single magnets in both two phase and supercritical flow conditions as well as the joints performances in the assembled SST-1 TF and PF magnets.

First Engineering Validation Results of SST-1 TF Magnets System

All SST-1 superconducting Toroidal Field (TF) and Poloidal Field (PF) magnets have been refurbished, integrated, and assembled onto the SST-1 machine shell in mid of 2012. Fabrication of sub nano-ohm, leak tight DC joints in superconducting magnets winding packs, enhancing the insulation strength in operating conditions, and test qualifying all the magnets prior to their integration in SST-1 machine shell were the primary refurbishment aspects. All assembled SST-1 TF magnets in SST-1 machine shell have since been successfully cooled down employing 1.3 kW SST-1 Helium Refrigerator/Liquefier system maintaining helium leak tightness under all temperature conditions. The TF magnets have experimentally demonstrated thermo-mechanical behavior as per the design as well as excellent flow uniformity amongst various parallel paths. Subsequently, these TF magnets have been increasingly charged so as to create a magnetic field of 1.5 T at the SST-1 major radius of 1.1 m. This is the first occasion, when cable-in-conduit wound toroidal field magnets have been successfully operating with two phase flow in a Tokamak application. The first plasma in SST-1 has been successfully obtained in June 2013 where the SST-1 TF magnets have demonstrated excellent functional characteristics. The confidence boosting engineering and functional validation test results of SST-1 TF magnets as well as its performance during the recent SST-1 plasma campaign have been elaborated in this paper.

Cryogenic Acceptance Tests of SST-1 Superconducting Coils

Toroidal field (TF) and poloidal field (PF) coils of steady-state superconducting tokamak (SST-1) have been fully refurbished to ensure cryostable and low-dc-resistance interpancake and intercoil joints, helium leak tightness of the winding pack, and ) 10 MΩ winding pack to ground insulation resistance under cold conditions. As per SST-1 mission mandate, all TF coils and PF3 Top had been cold tested at full operational parameters. A dedicated large coil test facility was integrated for these tests. Specially adapted solutions such as magnet preparation cum transport stand, demountable busbar supports, demountable sensor mountings, reusable joints for busbar connections inside cryostat, etc., were developed. These measures led to timely and successful completion of cold tests and integration of the magnet system on to the SST-1 machine shell. The functionality of various diagnostics of the magnet system was also established during these tests. Details of coil refurbishment, test facility, test campaigns, and some important test results are reported in this paper.

PXI Based Vacuum Control for Testing Various Components of SST-1

Vacuum system of steady-state Superconducting Tokamak (SST-1) has a very essential role during SST-1 plasma operation. For this purpose, its data acquisition and control system should be reliable and accurate. The PXI based faster real time data acquisition and control system were used for performing various operations like online data measurements, control, display, status indication in form of graphical visualization and storing the data for future analysis. We developed such PXI based vacuum control and implemented to our ongoing experimental set-up. This paper will describe the detailed information and guidance on PXI based platform for data acquisition and control used during the campaign.

High-vacuum compatibility tests of SST-1 superconducting magnets

SST-1 Tokamak is under commissioning at Institute for Plasma Research in mission mode. It comprises of a toroidal doughnut shaped plasma chamber, surrounded by liquid helium cooled superconducting magnets and LN2 thermal shields, housed inside the cryostat chamber. The superconducting magnet system of SST-1 consists of toroidal field (TF) magnets and poloidal field (PF) magnets and will be operated at internal supercritical helium pressure of 4.5 bar (a) under very low temperature of 4.5 K and carrying a DC current of 10 kA. High-vacuum compatibility up to low-pressure ≤ 1 × 10−5 mbar is one of the most essential features of these superconducting magnets in order to avoid the heat losses due to conduction and convection. This paper describes the extensive tests carried out under representative conditions to ensure the high-vacuum compatibility of the SST-1 magnets before assembly to the main system.

Spinning rotor gauge based vacuum gauge calibration system at the Institute for Plasma Research (IPR)

The Steady-state Superconducting Tokamak (SST-1) is an indigenously built medium sized fusion device at IPR designed for plasma duration of 1000 seconds. It consists of two large vacuum chambers – Vacuum Vessel (16 m3) and Cryostat (39 m3) which will be pumped to UHV and HV pressures respectively using a set of turbo molecular pumps, Cryo-pumps and Roots pumps. The total as well as the partial pressure measurement in these chambers will be carried out using a set of Pirani gauges, Bayard Alpert type gauges, Capacitance manometers and Residual Gas Analyzers (RGA). A reliable and accurate pressure measurement is essential for successful operation of SST-1 machine. For this purpose a gauge calibration system is set up in SST-1 Vacuum laboratory based on Spinning Rotor Gauge which can measure absolute pressure in the range 1.0 mbar to 1.0 × 10−7 mbar. This system is designed to calibrate up to five gauges simultaneously for different gases in different operating pressure ranges of the gauges. This paper discusses the experimental set-up and the procedure adopted for the calibration of such vacuum gauges.

Baking of SST-1 vacuum vessel modules and sectors

SST-1 Tokamak is a steady state super-conducting tokamak for plasma discharge of 1000 sec duration. The plasma discharge of such long time duration can be obtained by reducing the impurities level, which will be possible only when SST-1 vacuum chamber is pumped to ultra high vacuum. In order to achieve UHV inside the chamber, the baking of complete vacuum chamber has to be carried out during pumping. For this purpose the C-channels are welded inside the vacuum vessel. During baking of vacuum vessel, these welded channels should be helium leak tight. Further, these U-channels will be in accessible under operational condition of SST-1. So, it will not possible to repair if any leak is developed during experiment. To avoid such circumstances, a dedicated high vacuum chamber is used for baking of the individual vacuum modules and sectors before assembly so that any fault during welding of the channels will be obtained and repaired. This paper represents the baking of vacuum vessel modules and sectors and their temperature distribution along the entire surface before assembly.

Overall Performance of SST-1 Tokamak Vacuum System

Vacuum system of steady-state superconducting tokamak (SST-1) is comprised of 23 m3 ultrahigh vacuum (UHV) chamber with baking channels embedded on the vacuum vessel (VV) being heated with a dedicated nitrogen gas baking facility, gas puffing system, wall conditioning system and 39 m3 high vacuum cryostat (CST) chamber housing the superconducting magnets, cold structures, 80-K thermal shields and associated auxiliary components. The pumping system for the SST-1 VV consists of two turbo-molecular pumps each of 5500 l/s (N2 gas) capacity providing a total effective pumping speed of 4675 l/s and the CST consists of three such pumps with a total effective pumping speed of 3250 l/s. To achieve UHV inside the VV, it was baked at 135 °C for a longer duration of 100 h, while four numbers of graphite based limiters were baked at 250 °C for 50 h. An ultimate pressure of 4.6 × 10-8 mbar was achieved inside VV while an ultimate vacuum achieved inside the CST was 1.1 × 10-5 mbar. The gas puffing system has performed satisfactorily during the plasma trials assisted with electron cyclotron resonance heating preionizations. This paper describes our overall experience during the entire campaign of successful operation of SST-1 during first plasma.

Characterization of a laboratory scale high-Tc ‘D-shaped’ magnet

A D-shaped laboratory scale magnet resembling the toroidal field magnet of a tokamak has been fabricated using technical grade BSCCO high-Tc superconductor tape. The I–V characteristic as a function of the current ramp rate, as well as the critical current behavior under low external transverse magnetic fields, has been experimentally investigated at liquid nitrogen temperature. Degradation up to 46% in the critical current of the magnet with respect to its virgin tape has been observed. Hysteresis behavior has also been observed during ramp-up and ramp-down. The normal zone propagation velocity for quenching the magnet has been obtained experimentally to be 2.5 cm s−1 for a transport current of 60 A at 80 K for the critical current of about 150 A.