[en] Highlights: • SST-1 Tokamak PFCs were fabricated using graphite tiles embedded on CuCrZr and CuZr back plates. • PFC cooling is designed considering maximum heat load up to 0.6 MW/m"2. • Cooling scheme is such that the nucleate boiling will not occur. • The required mass flow rate and velocity for cooling water in each sub-connection are found to be 0.43 kg/s and 5.5 m/s for efficient heat extraction. • The header distribution scheme is modeled using AFT fathom which is in agreement to the required parameters with maximum 5% of deviation. - Abstract: PFC of SST-1 comprising of baffles, divertors and passive stabilizers have been designed and fabricated for a maximum heat load up to 1.0 MW/m"2. In operational condition, SST-1 divertors and passive stabilizers are expected to operate with a heat load of 0.6 and 0.25 MW/m"2, respectively. During plasma operation, the heat loads on PFC are required to be removed promptly and efficiently. Thereby the design of an efficient cooling scheme becomes extremely important for an efficient operation of PFC. PFCs are also baked up to 350 °C in order to remove absorbed moistures and other gases. 3D thermal analysis of PFC using ANSYS has been carried out to ensure its thermal stability. The cooling parameters have been calculated according to average incident flux on divertors and passive stabilizers. Engineering design demonstrated the required mass flow rate and velocity for cooling water in each sub-connection are optimized to be 0.43 kg/s and 5.5 m/s for efficient heat extraction under steady state heat load. Maximum temperature which PFC could be maintained is 355 °C and is well within threshold limits of material property degradation. The header distribution, modeled using AFT fathom, resulted for required parameters within maximum 5% of deviation.

[en] SST-1 has implemented bubble type 80 K thermal shields cooled with single-phase liquid Nitrogen around the superconducting magnet system. These panels have been designed to maintain temperature uniformity within ± 5 K with pressure drops below 1.5 bar for nominal flow conditions. Another extremely critical criterion towards the functionality of these thermal shields is the leak tightness of these panels under any normal and off-normal scenarios. Integrated helium leak testing on each of these panels as well as group of assembled panels both at room temperature (RT) as well as at cold temperature of 80 K have been carried out to ensure leak-tightness. All the panels have been tested in component levels before and after thermal shocks between RT and 80 K three times. As per defined groups, these assembled panels around SST-1 vacuum vessel modules and sectors have been tested inside a dedicated high vacuum (HV) chamber having vacuum < 5.0 x 10^-5 mbar. The panel temperature was obtained to be 80 K with inlet pressure of 1.83 bar (a). Before cool down and after achieving 80 K, these panels were leak tested by pressurizing helium gas at 8.0 bar (g). RGA spectra does not show traces of helium gas indicating the leak tightness of integrated system. (author)

[en] Critical current (I_c) characteristics of 2G YBCO superconducting tape under the influence of twisting moment was experimentally investigated at varying current ramp rates in the self-field. Under a uniform twist, the degradation in the current-carrying capacity of YBCO tape up to 30% was observed at 77 K. The degradation is largely attributed to the shear stress and torsional shear strain resulting from the twisting. The superconductor to resistive transition index, n, is also found to behave in an identical manner with increase in the twisting. Finite element analysis (FEA) of the tape in the experimental configuration with twisting moment being applied on to it has been carried out in COMSOL. The torsional strain calculated analytically as per the experimental configuration matches closely with that of FEA results, which shows that the critical current degradation is a function of strain. (author)

[en] For short duration experiments, generally digitized data is transferred for processing and storage after the experiment whereas in case of steady-state experiment the data is acquired, processed, displayed and stored continuously in pipelined manner. This requires acquiring data through special techniques for storage and on the go viewing data to display the current data trends for various physical parameters. A small data acquisition set-up is developed for continuously acquiring signals from various physical parameters at different sampling rate for long duration experiment. This includes the hardware set-up for signal digitization, FPGA based timing system for clock synchronization and event/trigger distribution, time slicing of data streams for storage of data chunks to enable viewing of data during acquisition and channel profile display through down sampling etc. To store a long data stream of indefinite/long time duration, the data stream is divided into data slices/chunks of user defined time duration. Data chunks avoid the problem of non-access of data until the channel data file is closed at the end of the long duration experiment. A graphical user interface has been developed in LabVIEW application development environment for configuring the data acquisition hardware and storing data chunks on local machine as well as at remote data server for further data access. The data plotting and analysis utilities have been developed with Python software, which provides tools for further data processing. This paper describes the detailed development and implementation of data acquisition for steady-state experiment. (author)

[en] Highlights: •SST-1 Tokamak was successfully commissioned. •Vacuum vessel and cryostat were pumped down to 6.3 × 10⁻⁷ mbar and 1.3 × 10⁻⁵ mbar. •Leaks developed during baking were detected in-situ by RGA and confirmed later on. •Cryo-pumping effect was observed when LN2 thermal shields reached below 273 K. •Non-standard aluminum wire-seals have shown leak tightness < 1.0 × 10⁻⁹ mbar l/s. -- Abstract: Steady-state Superconducting Tokamak (SST-1) was installed and it is commissioning for overall vacuum integrity, magnet systems functionality in terms of successful cool down to 4.5 K and charging up to 10 kA current was started from August 2012. Plasma operation of 100 kA current for more than 100 ms was also envisaged. It is comprised of vacuum vessel (VV) and cryostat (CST). Vacuum vessel, an ultra-high (UHV) vacuum chamber with net volume of 23 m³ was maintained at the base pressure of 6.3 × 10⁻⁷ mbar for plasma confinement. Cryostat, a high-vacuum (HV) chamber with empty volume 39 m³ housing superconducting magnet system, bubble thermal shields and hydraulics for these circuits, maintained at 1.3 × 10⁻⁵ mbar in order to provide suitable environment for these components. In order to achieve these ultimate vacuums, two numbers of turbo-molecular pumps (TMP) are installed in vacuum vessel while three numbers of turbo-molecular pumps are installed in cryostat. Initial pumping of both the chambers was carried out by using suitable Roots pumps. PXI based real time controlled system is used for remote operation of the complete pumping operation. In order to achieve UHV inside the vacuum vessel, it was baked at 150 °C for longer duration. Aluminum wire-seals were used for all non-circular demountable ports and a leak tightness < 1.0 × 10⁻⁹ mbar l/s were achieved

[en] Steady State Tokamak has recently completed the 1"s"t phase of up-gradation with successful installation and integration of all its First Wall components. The First Wall of SST-1 comprises of ∼ 4500 high heat flux compatible graphite tiles being assembled and installed on 136 Cu-alloy heat sink back plates engraved with ∼ 4 km of leak tight baking and cooling channels in five major sub groups equipped with ∼ 400 sensors and weighing ∼ 6000 kg in total in thirteen isolated galvanic and six isolated hydraulic circuits. The phase-1 up-gradation spectrum also includes addition of Supersonic Molecular Beam Injection (SMBI) both on the in-board and out-board side, installation of fast reciprocating probes, adding some edge plasma probe diagnostics in the SOL region, installation and integration of segmented and up-down symmetric radial coils aiding/controlling plasma rotations, introduction of plasma position feedback and density controls etc. Post phase-I up-gradation spanning from Nov 2014 till June 2015, initial plasma experiments in up-graded SST-1 have begun since Aug 2015 after a brief engineering validation period in SST-1. The first experiments in SST-1 have revealed interesting aspects on the 'eddy currents in the First Wall support structures' influencing the 'magnetic Null evolution dynamics' and the subsequent plasma start-up characteristics after the ECH pre-ionization, the influence of the first walls on the 'field errors' and the resulting locked modes observed, the magnetic index influencing the evolution of the equilibrium of the plasma column, low density supra-thermal electron induced discharges and normal ohmic discharges etc. Presently; repeatable ohmic discharges regimes in SST-1 having plasma currents in excess of 65 KA (q_a∼3.8, B_T=1.5 T) with a current ramp rates ∼ 1.2 MA/s over a duration of ∼ 300 ms with line averaged densities ∼ 0.8 x 10"1"9 per cc and temperatures ∼ 400 eV with copious MHD signatures have been experimentally established. Further elongation of the plasma duration up to one second or more with position and density feedback as well as coupling of Lower Hybrid waves are currently being persuaded in SST-1 apart from increasing the core plasma parameters with further optimizations and with wall conditioning. This paper will elaborate the salient features of the SST-1 up-gradation spectrum, the subsequent engineering validations and important aspects of the first results in up-graded SST-1 apart from the immediate future experimental plans in SST-1. (author)

[en] Remote operation of any equipment or device is implemented in distributed systems in order to control and proper monitoring of process values. For such remote operations, Experimental Physics and Industrial Control System (EPICS) is used as one of the important software tool for control and monitoring of a wide range of scientific parameters. A hardware interface is developed for implementation of EPICS software so that different equipment such as data converters, power supplies, pump controllers etc. could be remotely operated through stream protocol. EPICS base was setup on windows as well as Linux operating system for control and monitoring while EPICS modules such as asyn and stream device were used to interface the equipment with standard RS-232/RS-485 protocol. Stream Device protocol communicates with the serial line with an interface to asyn drivers. Graphical user interface and alarm handling were implemented with MEDM (Motif Editor and Display Manager) and ALH (Alarm Handler) command line channel access utility tools. This paper will describe the developed application which was tested with different equipment and devices serially interfaced to the PCs on a distributed network. (author)

[en] As a part of phase-I up-gradation of Steady-state Superconducting Tokamak (SST-1), Graphite Plasma Facing Components (PFCs) have been integrated inside SST-1 vacuum vessel as a first wall (FW) during Nov 14 and May 2015. The SST-1 FW has a total surface area of the installed PFCs exposed to plasma is ∼ 40 m"2 which is nearly 50% of the total surface area of stainless steel vacuum chamber (∼75 m"2). The volume of the vessel with the PFCs is ∼ 16 m"3. After the integration of PFCs, the entire vessel as well as the PFC cooling/baking circuits has been qualified with an integrated leak tightness of < 1.0 X 10"-"8 mbar l/s. The pumping system of the SST-1 vacuum vessel comprises of one number of Root's pump, four numbers of turbomoleculars and a cryopump. After the initial pump down, the PFCs were baked at 250 °C for nearly 200 hours employing hot nitrogen gas to remove the absorbed water vapours. Thereafter, Helium discharges cleaning were carried out towards removal of surface impurities. The pump down characteristics of SST-1 vacuum chamber and the changes in the residual gaseous impurities after the installation of the PFCs will be discussed in this paper. (author)

[en] Plasma facing components of SST-1 are designed to withstand an input heat load of 1.0 MW/m"2. They also protect vacuum vessel, auxiliary heating source i.e. RF antennas, NBI and other diagnostic in-vessel components from the plasma particles and high radiative heat loads. PFC's are positioned symmetric to mid-plane to accommodate with circular, single and double null configuration. Graphite is used as plasma facing material which is fixed on copper alloy (CuCrZr and CuZr) back plate with mechanical attachment followed with graphoil in between. SS cooling/baking tubes are brazed on copper alloy back plates for efficient heat removal of incident heat flux. Benchmarking of PFC assembly was first carried out in prototype vacuum vessel of SST-1 to develop understanding and methodology of co-ordinate measurements. Based on such hands-on-experience, the final assembly of PFC's in actually vacuum vessel of SST-1 was carried out. Initially, PFC's are to be baked at 250 °C for wall conditioning followed with cooling for heat removal of incident heat flux during long pulse plasma operation. For this purpose, the supply and return headers are designed and installed inside the vacuum vessel in such a way that it will cater water as well as hot nitrogen gas depending up on the cycle. This paper will discuss the successful installation of PFC's its operation respecting all design criteria for plasma operation. (author)

[en] Graphite plasma facing components (PFCs) were installed inside SST-1 vacuum vessel. Prior to installation, all the graphite tiles were baked at 1000 °C in a vacuum furnace operated below 1.0 X 10"-"5 mbar. However due to the porous structure of graphite, they absorb a significant amount of water vapour from air during the installation process. Rapid desorption of water vapour requires high temperature bake-out of the PFCs at ≥ 250 °C. In SST-1 the PFCs were baked at 250 °C using hot nitrogen gas facility to remove the absorbed water vapour. Also device with large graphite surface area has the disadvantage that a large quantity of hydrogen gets trapped inside it during plasma discharges which makes density control difficult. Helium (He) glow discharge cleaning (GDC) effectively removes this stored hydrogen as well as other impurities like oxygen and hydrocarbon within few nanometers from the surface by particle induced desorption. Before plasma operation in SST-1 tokamak, both baking of PFCs and He-GDC were carried out so that these impurities were removed effectively. The mean desorption yield of hydrogen was found to be 0.48. In this paper, the results of effect of baking and He-GDC experiments of SST-1 will be presented in detail. (author)