Highlights and achievements of cryogenic system for SST-1

The SST-1 tokamak has sixteen TF superconducting and nine PF superconducting coils. This magnets system has been designed to cool using double phase cooling at 1.3 - 1.6 bar (a)and with forced flow supercritical helium cooling at 4 bar (a), 4.5 K using 1.3 kW / 4.5 K helium refrigerator-cum-liquefier (HRL) system. Last several campaigns, TF and PF coils could not be simultaneously cooled to desired temperatures of 4.5 K due to unexpectedly higher heat loads and higher pressure head demands from PF coils. Therefore, separate cooling of TF and PF coils were attempted. We could cool the PF coils separately at the cost of higher pressure head demands, which was balanced by sacrificing the liquefaction part of cryo plant capacity. Measurement of the helium cryo plant capacity test "As on today" basis carried out to assess the health of the SST-1 cryo system. We have also identified undue heat loads sources acting on the cold surfaces at 4.5 K. Rigorous efforts were made to mitigate those to improve the situation. Even we have upgraded integrated flow distribution and control system to have proper regulation of hydraulic resistances within the PF coils. After successful efforts, we could demonstrate the simultaneous cool down of TF and PF coils (except PF 5(L) coil). This demonstration has given us confidence and pathway to go forward for future experiments in SST-1. First time, we have achieved the major milestone for higher current (7.9 kA) and longer duration operation (6 ½ hours) of TF current leads system. Still, heat loads mitigation efforts are in progress to get more benefits wherever it is feasible within the SST-1 infrastructure. In this paper, we will focus on main highlights and achievements of SST-1 cryo system. (author)