DESIGN, FABRICATION AND PERFORMANCE TEST OF 10 kA CURRENT LEADS FOR SST-1 Sarkar B, Sahu A. K, Panchal P, Sonara D, Bhattacharya R, Singh M, Shukla P, Panchal R, Raju A. V, Gupta N. C, Shah N, Gupta G, Duggar R, Tank J, Saradha S, Vaghela H, R Sharma, Patel J. C and Saxena Y. C. (PRESENTATION BY J. TANK) Institute for Plasma Research, Bhat, Gandhinagar, Gujarat (India)

INTRODUCTION Current leads are used to transfer high electrical current at room temperature to superconducting magnets at low temperature. Heat leaks at low temperature becomes one of the main design criteria At cryogenic temperature material properties show highly non-linear characteristics. Closed form solution needs approximations and differs from test performance. Numerical and finite element methods (FEM) are used as tools to predict the performance. One pair leads of 10 kA capacity has been designed, fabricated and tested. Conventional type leads, helium vapor cooled, LHe bath at bottom used. Simplest fabrication techniques used.

REQUIREMENT OF LEADS FOR SST-1 Steady State superconducting tokamak (SST-1) has, * 9 poloidal field (PF) super conducting magnets * 16 toroidal field (TF) super conducting magnets 10 pairs of current leads are required to charge the TF& PF coils Operation of coils is at 4.5 K Coils are forced-flow-cooled using supercritical helium TF coils are designed for 10 kA. PF coils are designed for ~ 8 kA One pair leads for TF coils is designed for 10 kA, fabricated and tested Test results showed satisfactory performance. Other nine pairs are being fabricated on the same basis of design. Conventional type has been preferred due to not so high current rating and high non-operation to operation time ratio

ISOMETRIC VIEW OF SST-1 CURRENT FEEDER SYSTEM CLAC PF UPPER SC BUS DUCT PF LOWER SC BUS DUCT TF SC BUS DUCT SST-1 JOINT BOX

3 - D VIEW OF CURRENT LEADS WITH HEADERS CURRENT LEADS SHe HEADER FEED TUBES LHe HEADER

A CUT-VIEW OF CURRENT LEADS ASSEMBLY CHAMBER WITH SC BUS BARS, CURRENT LEADS, LHe, SHe HEADERSAND FEED TUBES CURRENT LEADS SHe HEADER SC BUS BARS LHe HEADER FEEDING TUBES

COMPONENTS OF CFS

10 KA CAPACITY CURRENT LEAD

DESIGN CRITERIA CURRENT LEADS MUST HAVE LOW ELECTRICAL RESISTANCE FOR LOW ELECTRICAL HEATING LOW THERMAL CONDUCTIVITY FOR LOW HEAT LEAK EFFICIENT HEAT EXCHANGE TO COOLING FLUID (HELIUM) EASY TO FABRICATE THE OPTIMISED CONFIGURATION IT MUST ACCOUNT THE NON-LINEAR MATERIAL PROPERTIES MUST SUSTAIN HIGH CURRENT RAMP-UP AND RAMP-DOWN REQUIRED FOR PF COILS MATERIAL MUST BE READILY AVAILABLE IN THE MARKET.

GOVERNING EQUATION ALONG THE LENGTH OF THE LEAD FOR A SMALL SEGMENT ‘dx’ ENERGY BALANCE CAN BE WRITTEN AS,……. TEMPERATURE VARIATION IN ANY CROSS SECTION IS ACCOUNTED BY THE EFFECTIVENESS OF HEAT EXCHNAGER. CLOSED FORM SOLUTION NEEDS APPROXIMATIONS OF PROPERTIES. THIS GIVES DEVIATION COMPARED TO REAL VALUES WIEDEMANN- FRANZ LAW HAS LARGE DEVIATIONS. HENCE A FINITE ELEMENT METHOD HAS BEEN ADOPTED. A COMPUTER CODE HAS BEEN DEVELOPED TO ACCOUNT EXACT VARIATION OF PROPERTIES.

COMPUTED TEMPERATURE PROFILE FOR OPTIMISED HEAT EXCHANGER OF THE DESIGNEDLEAD

CHOOSING THE RIGHT MATERIAL AND HEAT EXCHANGER CURRENT CARRYING MATERIAL CHOOSEN IS COPPER WITH RRR=30 HEAT EXCHANGER IS A BUNDLE OF COPPER RODS EACH INSERTED IN CONCENTRIC SS TUBE HELIUM FLOWS IN THE ANNULAR SPACE BETWEEN ROD AND TUBE HEAT EXCHANGER IS JACKETED IN A JACKET OF SS304L MATERIAL SUPERCONDUCTING TRANSITION IS USED TO TRANSFER CURRENT FROM RODS TO THE BOTTOM TERMINAL THE TRANSITION IS IMMERSED IN LHe LENGTH OF THE CAN IS 465 MM AND LENGTH OF HEAT EXCHANGER IS 764 MM.

FABRICATION STAGES AND ASSEMBLY

TEST SET-UP AND INSTRUMENTS SUFFICIENT SENSORS FOR TEMPERATURE, PRESSURE, VOLTAGE DROP, PRESSURE DROP, LIQUID LEVEL AND FLOW RATE WERE USED. SENSOR POINTS WERE STRATEGICALY DECIDED TO GET MAXIMUM INFORMATION.. EACH LEAD HAS ORIFICE FLOW METER AT THE TOP TO KNOW HELIUM MASS FLOW RATE..

PID MIMIC OF THE TEST SET-UP

Cryogenic Test Facility for current Lead test

CONTROL LOGIC Programmable logic control (PLC) and supervisory control & data acquisition (SCADA) were used for the experiment Cooldown, current charging as well as warm up were done automatically and in a controlled way. Liquid level in header was controlled very precisely by inlet valve with active dependency of header level and passive dependency of level of lead. Header pressure was controlled with the outlet control valve.

COOLDOWN RESULTS Cooldown plot as a function of time

CURRENT CHARGING RESULTS

EXPERIMENTAL RESULTS SMOOTH COOLDOWN, CONTROLLED LEVEL AND CURRENT CHARGING HAS BEEN DONE UPTO 7.4 kA. FAIRLY GOOD HELIUM LEAKTIGHT AT 4.2 K, A VERY GOOD VACUUM OF 3.8  mbar WAS ACHIEVED IN THE CRYOSTAT DURING TEST RAMP-DOWN OF CURRENT WAS DONE AT A FASTER RATE OF 130 A/sec AND ~ 600 A/min. IT VERIFIED THE SUITABILITY FOR POLOIDAL FIELD CURRENT LEAD WHICH NEEDS HIGH CURRENT RAMPING. DURING CURRENT RAMP-DOWN NEITHER HELIUM FLOW FLUCTUATION NOR INCREASE IN FLOW RATE WAS OBSERVED HELIUM MASS FLOW RATE WAS 0.65 g/s PER LEAD WHEN CURRENT FLOW WAS 7.4 kA. NO TEMPERATURE FLUCTUATION WAS ALSO OBSERVED. A LOW PRESSURE DROP OF ~5 mbar HAS BEEN OBTAINED

PERFORMANCE COMPARISION : EXPERIMENTAL AND DESIGN VALUES HELIUM FLOW RATE PER LEAD (AT 7.4 kA) = 0.65 g/s THIS IS EQUIVALENT TO 5l/hr/kA/pair EXTERNAL HEAT LOAD COMING TO THE LHe CAN ARE: ELECTRICAL HEATING OF THE JOINT RADIATION HEAT LOAD ON THE JOINT AND THE SUPERCONDUCTING SHORT CALCULATED TOTAL EXTERNAL HEAT LOAD = 3 W PER LEAD EXCLUDING THIS HEAT LOAD, ACTUAL CONSUMPTION BY LEAD COMES OUT AS 3.9 l/hr/kA/pair IT IS CLOSE TO DESIGN VALUE OF 3.5 l/hr/kA/pair DROP OLTAGE, PRESSURE DROP IS ALSO CLOSE TO Design value (50 mV & 5 mbar)

CONCLUSION ONE PAIR OF 10 kA CAPACITY CURRENT LEAD HAVE BEEN DESIGNED USING FINITE ELEMENT METHOD. TESTS HAVE BEEN DONE SUCCESSFULLY. TEST RESULTS ARE CONSISTENT WITH DESIGN VALUES IT IS ROBUST TO HIGH CURRENT RAMPING THE DEVELOPED LEADS ARE SUITABLE FOR SST-1 MORE 9 PAIRS OF LEADS ARE BEING FABRICATED FOR SST-1

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