MANUFACTURING OF MAGNETS FOR SST-1 TOKAMAK Y.C. Saxena1, S. Pradhan1, R. Bahl1,V. Bedkihale1, V. Jain1, K.J. Thomas1, S. Karthikeyan2, K.N.Das2, C..V. K. Ecka2 and BHEL Team2 1Institute for Plasma Research, Bhat, Gandhinagar, INDIA 2Bharat Heavy Electricals Limited, Piplani, Bhopal , INDIA The Magnet System of the Steady State Superconducting Tokamak, SST-1 consists of both the superconducting and resistive magnets. The manufacturing of the magnet system has been carried out at the Bharat Heavy Electricals Limited, Bhopal, India, under design and supervision of Institute for Plasma Research. Several technology and processes involving the winding of large size superconducting magnets have been developed. The cryo-compatible insulation systems. used in the superconducting magnets has been developed and tested. All superconducting PF coils, nearly half of the TF coils and all resistive coils have been manufactured. Manufacturing of precisely machined TF cases, outer inter-coil support structures and other associated support structures are in an advanced stage of fabrication.

SST1 MACHINE PARAMETERS MAJOR RADIUS : 1.1M MINOR RADIUS : 0.2 M ELONGATION : 1.7-2 TRIANGULARITY : 0.4-0.7 TOROIDAL FIELD : 3T PLASMA CURRENT : 220 kA. ASPECT RATIO : 5.2 SAFETY FACTOR : 3 AVERAGE DENSITY : 1X 1013cm-3 AVERAGE TEMP. : 1.5 keV PLASMA SPECIES : HYDROGEN PULSE LENGTH : 1000S CONFIGURATION : DOUBLE NULL : : POLOIDAL DIVERTER HEATING & CURRENT DRIVE: LOWER HYBRID : 1.0 MW NEUTRAL BEAM : 0.8 MW ICRH : 1.0 MW TOTAL INPUT POWER : 1.0 MW FUELLING : GAS PUFFING 3-D Cut View of SST-1

Supercondcting Magnets: Toroidal Field (TF) Coils : 16 Nos. SST-1 MAGNET SYSTEM Requirements: Confinement, Shaping and Equilibrium Fields Ohmic Flux Storage Feed-Back Control Supercondcting Magnets: Toroidal Field (TF) Coils : 16 Nos. Poloidal Field (PF) Coils : 9 Nos. Copper Magnets (Water Cooled) : Ohmic Transformer (TR) Coils : 7 Nos. Poloidal Field (PF ) Coils ( in-Vessel): 2 Nos. Position Control Coils ( in-Vessel) : 2 Nos.

SST-1 TOROIDAL FIELD COILS Design Drivers for TF coils: 3 Tesla Field at Plasma Center < 2% ripple at plasma edge Large Radial Ports Tangential access for NBI Human Entry in Vacuum Vessel TF COIL Winding Pack: Modified D-Shape Base Conductor: NbTI based CICC 6 # of Double pancakes (Joint less) assembly consolidated by VPI. Cross-section : 194x144 mm2 Encased in SS316L casing. PARAMETERS OF TF COILS: Total No. of Coils : 16 Turns per Coil : 108 6 Double Pancakes with 9 turns per pancake Current per turn (3T Field at Plasma Center): 10 kA Maximum Field at Conductor: 5.1 T Maximum Field Ripple : 0.35% Total Inductance (16 Coils in series) : 1.12H Total Stored Energy: 56MJ Dump Time Constant: 12 s Peak Dump Voltage: ±600V

TF MAGNETS’ CASINGS TF winding packs are encased in casings made of SS316L Each of the Casing consist of Inner ring, outer rings & two side plates Inter coil supports are provides on inner and outer legs

Assembly of TF coils TF coils nosed and bolted on in-board side to form a toroidal vault Outer Inter coils supports (2 #each ) provided between adjacent coils Coil Casings insulated from each other. Casings and inter-coils structures support PF coils

SST-1 Poloidal Field Coils Design Drivers: Support single & double null equilibria with wide range of Triangularity ( 0.4-0.7), Elongations ( 1.7-1.9), li (0.75 -1.4), p ( 0.01-0.85) & slot divertor configuration Limiter operation during Plasma current ramp up Parameters of PF Coils

Conductor for SST-1 Superconducting Magnets Conductor Characteristics Conductor type : CICC Dimensions : 14.8×14.8 mm2 No. of Strands : 135 Cabling Pattern : 3335 Last stage wrapped (half overlap) with 25 m thick SS304 tape. Twist Pitches: I stage : 40 mm; II stage : 75 mm III stage: 130 mm; IV stage: 290 mm Conduit Material : SS 304L Conduit thickness : 1.5 mm Void Fraction :  36 % Ic @ 5T, 4.2K : 36 kA Iop @ 5T, 4.5K : 10 kA Strand Specifications Material : NbTi in Cu Strand Dia : 0.86 mm Filament Dia. : 10 µm Filaments per strand : 1272 Cu : NbTi : : 5 : 1 Cu RRR : 100 Hysterisis Losses : <100 mJ cm-3 Cu NbTi Cu Strand Cross-section Strand Characteristics: Ic@4.5K; 5T : 232A Index ‘n’ : 45 (0.1 V / cm criteria) Hysterisis loss : 33.5 mJ cm-3 (±3T; 4.2K) CICC Cross-section

SST-1 OHMIC TRANSFORMER Design Drivers: Stress limited maximum flux storage Matching to existing power supply Plasma Break down and initial plasma current ramp up TR2 Characteristics: Water cooled copper coils Copper (OFHC) conductor 19 22.7 mm2 cross-section 8 mm dia. hole for water flow Stored Flux : 1.4 Vs @ 20 kA per turn Maximum Field in plasma Volume : 18 G TR1 TR3 TR2 Coils # Radius (m) Turns (#) X-section (mm2) Center (m) TR1 1 0.20 672 1202600 0.00 TR2 2 0.49 40 195  95  1.40 TR3 2.42 3 58  23 0.73

Constructional Features and Type of Winding of SST-1 Magnets PF1, PF2, PF3 and TR1 coils are layer wound on epoxy glass winding spools. Winding packs are vacuum pressure impregnated. TR2, TR3 and VF are single pancake wound. Consolidated and cured with internal heating

Constructional Features and Type of Winding of SST-1 Magnets TF, PF4 and PF5 are wound as double pancake (DP). Typical DP winding for TF is illustrated above.

Superconducting coils CICC Spool CICC Shaping Take-up spool for double pancakes winding TF Double-pancake winding machine

Superconducting coils Dummy TF double pancake VPI Mould for TF coils

Superconducting coils Winding machine for PF1,P2 and PF3 coils Winding of PF1 in progress Winding of PF5 in progress Winding former for PF4 and PF5 coils

Insulation System for Superconducting magnets A specially developed glass fibre based (BHELMAT-G) insulation with Bisphenol-A epoxy resin system has been used for SCMS. An extensive pre-qualification of this insulation system on CICC samples have been carried out. Test samples (length > 500 mm) : CICC bend samples of radii 200,300,400 and 500 mm CICC 4 X 4 array of straight conductors CICC S-bend samples

Insulation System for Superconducting magnets Electrical tests : Inter-turn insulation at 1 kV DC Insulation resistance between turns Insulation resistance on main insulation 2 kV AC 50 Hz proof resistance for 1 min. Thermal cycling test: 10 cycles of Heating at 1600 C for 15 minutes in an electrical oven followed by quenching in liquid Nitrogen at minus 1960 C for 5 minutes. After thermal cycling all electrical tests were done. The samples passed all the electrical tests with degradation in performance below 5% after the thermal cycling. No mechanical cracks were observed.

Water Cooled Copper Coils TR1 winding in progress Winding set up for VF coil