IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 5, NO. 2, JUNE 1995 437 ucond (tonnes) 45.4 Lcond (W 15.6 POLOIDAL FIELD SYSTEM FOR THE TOKAMAK PHYSICS EXPERIMENT 'Bmaxaf (T) 6.0 mldtinitiation (TIS) 16 V-sswing 0) 18.8 Joel H. Schultz, R.D. Pillsbury, Jr., A. Radovinsky, P.W. Wang, Massachusetts Institute of Technology J. Citrolo Princeton Plasma Physics Laboratory R.J. Bulmer, D. Lang, T. O'Connor, D. Slack, J. Zbasnik Lawrence Livermore National Laboratory L. Myatt, Stone and Webster Parameter Units Allowable fcritical 0.6 Tmareinheadroom (K) 1.0/2.0 bareinheadmom (dlcc) 300l600 hdisturbance (W/m2-K) 600 hI2R (W/m2-K) 1mNb3Sn "bTi Allowable Stress < 2/3 yield SM < 1/2 tensile Abstract--The Tokamak Physics Experiment (TPX) at Princeton will be the first tokamak with an all superconducting poloidal field (PF) magnet system. The conductors are all cable-in-conduit (CICC) superconductors with a single conduit, similar to those in the International Thermonuclear Experimental Reactor (ITER). 10 of the PF coils use Nb3Sn superconductor while 4 of them use NbTi. High noise initiation and disruptions demand the use of an advanced quench detection system. Local Membrane <1.5 SM I. INTRODUCTION < 1/3 weld tensile PLocal Membrane +Bending<l.5 SM The Tokamak Physics Experiment (TPX], a steady-state tokamak experiment to be built at the Princeton Plasma Physics Laboratory [ll will be the first tokamak in the world with a superconducting poloidal field (PF) magnet system [2]. All of the TPX magnets will use internally-cooled, cabled superconductors. TPX will thus be an important precursor of ITER, the Intemational Thermonuclear Experimental Reactor [3]. Because the TPX mission includes steady-state and extremely long pulse operation at full parameters, the use of superconducting coils was an obvious choice for the magnet system. The PF system has 14 coils, 8 in a central solenoid (CS) stack and 6 outer PF coils, providing 18.8 V-s. The TPX PF system will provide advanced shaping for double- null, high-beta and high-bootstrap-fraction plasmas, as well as single-null plasmas at a full plasma current of 2.0 MA. An isometric view of the TPX PF system is shown in Figure 1. Figure 1: Isometric View of TPX PF System A conceptual magnet design was completed and reviewed (CDR) in 1993 141. Since then, more detailed calculations of Manuscript received Oct 17, 1994. This work was supported by the US Department of Energy. machine loads and requirements have necessitated changes in both the toroidal field (TF) and PF magnet systems [51, and an Engineering Change Proposal has been submttted. This paper describes the ECP design. A contract was awarded to complete preliminary design and R&D of the PF system to an industrial team headed by Westinghouse Electric Corporation, and including NorthrUp-Grumman and Everson Electric. I. 1051-8223/95$04.00 11. REQuiREhiEms 0 1995 IEEE438 The coils are designed to remain superconducting and to recover without quench during all modes of normal operation, from all disturbances, including disruptions occurring at any time. The coils are capable of dumping all of their energy at any time without exceeding allowable voltage or temperature limits. The TF insulation systems must satisfy all protection and electrical integrity requirements, as shown in Table 111. (2) The heighvwidth ratio of the PF coils was increased in order to decrease the field at the conductor for a given PF current. (3) The central solenoid design was changed so that the eight CS modules are no longer identical. CS modules are still symmetric about the machine equator. (4) More cable space for intemal sensors, such as insulated 80 I Table Ilk Insulation and Protection System Allowables III. PF DESIGN DESCRIPTION All conductors use cable-in-conduit superconductors with cooling by forced-flow supercritical helium. The CS and PF5 coils use Nb3Sn in an outer Incoloy 908 conduit, identical to that of the US-DPC [8], while the outer PF6-7 coils use NbTi strands in a 316LN conduit. While the two conductor types use different materials, they have the same cable and conduit geometries. The Nb3Sn strands in the central solenoid and in PF5 all have a 351 ratio, while the NbTi strands are 5:l. All of the PF conductors have 360 strands, 240 superconducting composite and 120 pure copper strands, 1 in each triplet. The PF coils are symmetric about the equator to provide nominal double null operation, but they are also capable of supporting single-null operation with unbalanced currents. The topology of the TPX poloidal field system is the same as that of the ITER CDA 191. The PF support system bolts the outer six coils to the TF structure, while suspending the central solenoid from the TF coils. The Central Solenoid (CS) is supported through a single gravity support, coupled to the TF coil structure. The CS stack is connected to the TF cases through a turnbuckle support structure, connected to the TF cases, that permits differential radial motion of the two coil systems. The Central Solenoid also prevents axial tension anywhere in the interpancake insulation by the application of mechanical precompression to the stack The CS top support structure achieves the axial precompression through bolts and panels on the inside and outside of the CS stack. The CS assembly is shown in Figures 1,2, and 3. Closed loop cooling by supercritical helium is required for cooling of the coils, buswork, and structure. The coils are capable of being cooled down after a coil dump within 12 hours. The entire assembly is capable of being cooled down from 293 K to 4 K within 10 days. Since the CDR, a number of design changes have been proposed, in order to better satisfy the full range of plasma shaping flexibility and steady-state deuterium burn requirements. These include: (1) The radius of the central solenoid was increased by 10 cm when improvements in the vacuum vessel plumbing allowed an increase in PF volt-seconds and a decrease in TF peak field without increasing rbe tokamak major radius. voltage sensors, was added. (5) The conductor design was changed from 225 superconducting strands in PF1-4 and PF6-7, to 360 strands in all of the PF coils. The inner tube containing the US-DPC size cable was removed, in order to accommodate the larger number of strands. The reason for doing this was to be able to