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 highconductivity 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.

A steady state superconducting tokamak (SST-1) has been commissioned after the successful experimental and engineering validations of its critical sub-systems. During the 'engineering validation phase' of SST-1; the cryostat was demonstrated to be leak-tight in all operational scenarios, 80 K thermal shields were demonstrated to be uniformly cooled without regions of 'thermal runaway and hot spots', the superconducting toroidal field magnets were demonstrated to be cooled to their nominal operational conditions and charged up to 1.5 T of the field at the major radius. The engineering validations further demonstrated the assembled SST-1 machine shell to be a graded, stress-strain optimized and distributed thermo-mechanical device, apart from the integrated vacuum vessel being validated to be UHV compatible etc. Subsequently, 'field error components' in SST-1 were measured to be acceptable towards plasma discharges. A successful breakdown in SST-1 was obtained in SST-1 in June 2013 assisted with electron cyclotron pre-ionization in the second harmonic mode, thus marking the 'first plasma' in SST-1 and the arrival of SST-1 into the league of contemporary steady state devices.Subsequent to the first plasma, successful repeatable plasma start-ups with E ∼ 0.4 V m −1 , and plasma current in excess of 70 kA for 400 ms assisted with electron cyclotron heating pre-ionization at a field of 1.5 T have so far been achieved in SST-1. Lengthening the plasma pulse duration with lower hybrid current drive, confinement and transport in SST-1 plasmas and magnetohydrodynamic activities typical to large aspect ratio SST-1 discharges are presently being investigated in SST-1. In parallel, SST-1 has uniquely demonstrated reliable cryo-stable high field operation of superconducting TF magnets in the two-phase cooling mode, operation of vapour-cooled current leads with cold gas instead of liquid helium and an order less dc joint resistance in superconducting magnet winding packs with high transport currents. In parallel, SST-1 is also continually getting up-graded with first wall integration, superconducting central solenoid installation and over-loaded MgB 2 -brass based current leads etc. Phase-1 of SST-1 up-gradation is scheduled by the first half of 2015, after which long pulse plasma experiments in both circular and elongated configurations have been planned in SST-1.

Abstract. The Image Space Reconstruction Algorithm (ISRA) of Daube-Witherspoon and Muehllehner is a multiplicative algorithm for solving nonnegative least squares problems. Eggermont has proved the global convergence of this algorithm. In this paper, we analyze its rate of convergence. We show that if at the minimum the strict complementarity condition is satisfied and the reduced Hessian matrix is positive definite, then the ISRA algorithm which converges to it does so at a linear rate of convergence. If, however, the ISRA algorithm converges to a minimum which does not satisfy the strict complementarity condition, then the rate of convergence of the algorithm can degenerate to being sublinear. Our results here therefor hold under more general assumptions than in the work of Archer and Titterington who assume that at a minimum point all Lagrange multipliers are zero.We provide numerical examples to illustrate our rate of convergence results and to explain why the ISRA algorithm usually appears to converge slowly. Our work here heuristically justifies why the Lee-Seung algorithm for solving nonnegative matrix factorization problems has a slow rate of convergence.Mathematics subject classification (2000): 65F20, 65Y20, 15A48, 15A23.

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 LN 2 -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. Index Terms-Baking, bubble shields, leak-tight sub-nano-ohm dc joint resistance and first plasma, Steady State Superconducting Tokamak (SST-1), supercritical helium.

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