2.3 PF System. WU Weiyue PF5 PF PF1

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1 2.3 PF System WU Weiyue Introduction The poloidal field (PF) system consists of fourteen superconducting coils, including 6 pieces of central selenoid coils, 4 pieces of divertor coils and 4 pieces of outer-big-rings. They are used with two types of superconducting conductors, in which the size and the configuration of cabling are different slightly. One is mm in size, 4 stages cabling and the first stage with 2 superconducting strands add 1 copper strand. It is used for selenoid coils and divertor coils. Another is mm in size, 4 stages cabling and the first stage with 1 superconducting strand add 2 copper strands. This type of CICC is used for 4 pieces of outer rings only. Figure 1 shows the cross section of the PF coils for EAST tokamak. Figure 2 shows the CICC s configuration, cabling and size. The NbTi cable-in-conduit conduct (CICC) cooled by supercritical helium at 4.5 K is chosen as superconductor for all of the PF magnets. The PF system is supported by the case of TF coils. The maximum capacity of the volt seconds for PF is about 10 Vs and the stray field in plasma Initiation region is less than 35 gauss. The peak magnetic field in the body is about 4.5 T. Four types of modes for the shape of plasma can be chosen during the operating. The plasma current is about 1MA and the duration of the plasma is about 10 seconds for ohmic heating discharge according to the design of the PF system PF9 PF PF11 PF5 PF PF Plasma Fig.1 The cross- section of the PF coils for EAST Conductors The superconducting conductor is chosen as Cable In Conduit Conductor (CICC). The strands used in the PF conductor are composed of NbTi/Cu multistage cable with about 120 strands. The maximum operating current is about 15 ka for each coils. The main parameters of the superconducting strand are: strand diameter is 0.85mm, ratio of the Cu with NbTi is 1.38, number of the filaments is 8910 and the diameter of the filament is 6 µm, twist pitch is 10 mm and the critical current at 5 T in 4.2 K is about 550 A. In order to PF

2 increase stability of the conductor, oxygen-free-high conductance copper strands are added in the conductor. A 2 µm thickness of chromium is electroplated on the surface of all strands to minimize current coupling losses during operation. The last stage (fourth-stage) cable is consisted of five third-stage sub-cables and a half overlapped wrapped with a stainless steel strip of 0.1 mm in thickness to avoid damages during cable inserting and jacket shaping. 316LN with 1.5 mm thickness is chosen as the material of the conduit. The size of the PF conductor are 20.4 mm 20.4 mm for solenoid coils and diverter coils (PF1, PF2, PF3, PF4, PF5, PF6, PF7, PF8 and PF9), and 18.6 mm 18.6 mm for two pairs of outboard ring coils (PF11, PF12, PF13 and PF14). The configuration of the cabling is shown in Fig.2. Fig.2 Two types CICC configurations Winding The PF coils are pancake wound only. We have developed two special machines for the superconducting magnet pancake winding. It can be easily to pre-winding the square conductor CICC that is quite strong and difficult to winding in normal way. The CICC was wound start from the inner circle to outer circle and continue from outer to inner. For example, each solenoid coil with seven turns in radial and 20 turns in height, so it should be wound continuously by one piece of CICC from inner to outer and outer to inner ten times. To weld the liquid helium tube also should be carefully during winding. Some inspecting and leakage detection should be done after these two processes to find any quality problem that have to be avoid fully Insulation structure The highest voltage on PF coils during the plasma initiation and disruption is about 2400 V on the diverter coils. The turn electrical insulation consists of 2 layers of kapton and multiple layers of fiberglass. During winding, the conductor was wrapped with 0.1 mm in thickness of fiberglass at first, then 1 layer of fiberglass and 1 layer of kapton were half-overlap 85

3 wrapped and finally 2 layers of fiberglass with detection wires were wrapped for the turn insulation. Between the pancakes, a 1.0 mm fiberglass carpet was inserted to improve the pancakes insulation. At the end of winding 5 mm the fiberglass of 5 mm in thickness was wrapped as ground insulation. The current connections and liquid helium tubes were set inside the coils except two pairs of the outboard ring coils Vacuum pressure impregnation (VPI) All the PF coils will be vacuum pressure impregnated (VPI) with epoxy resin to improve the capability of the insulation and structure strength Ohmic heating parameters There are three requirements for the PF system: the capability of ohmic heating voltsecond, the stray field in region of plasma initiation and the equilibrium field configuration for the plasma. The optimization of the PF system design is to reduce the total ampereturns and the magnetic energy stored in the system to be used for both buildup plasma current and equilibrium for it. The first step in defining each PF coil location is to calculate the maximum radial position and size of the solenoid. The overall layout of the PF system for EAST is shown in Table 1. In this table, I oh is the current in each coil at discharge moment of ohmic heating system, R1 and Z1 are the first turn center position in R and Z directions, Nr and Nz are the turns number for each coil in R and Z direction, Bw and Bh are the distance between two neighboring turns in R and Z direction. Table 1. Coil current and PF coils parameters No. Coil Ioh(kA) R1(cm) Z1(cm) Nr Nz Bw(cm) Bh(cm) PF PF PF PF PF PF PF In nominal cause of operating, the plasma current of EAST is 1 MA. The machine is capable of achieving this plasma current for at least 10 seconds by utilize inductive drive alone. This is accomplished with 10 volt-seconds capable of PF coil system during operating. The major radius of the plasma is 1.78 meter and the minor radius is 0.4 meter. A slight change was made to accommodate better conditions for the mechanical design of divertor and vacuum vessel. The ohmic heating system for EAST tokamak device was designed to satisfy the capability of flux swing and stray field, even the plasma initial can be helped by lower hybrid wave or electron cyclotron wave. The ohmic heating system should be provided with enough flux swing and very low stray field in the region of the plasma breakdown. An about 10 volt-seconds and quite lower stray field was designed in PF coils system. The 86

4 ohmic heating stray field is shown in Table 2 and figure 3. The PF coils system design has been made as compact as possible. To minimize ampere-turn of the PF coils and power requirements and the overall device radius, the parameters of the ohmic heating system have been optimized to achieve maximum volt-second capability and to satisfy the constraints for structural and superconductor performance. The peak field on PF coils is about 4.5 Tesla in the final design. Table 2. Ohmic heating stray field R(cm) Z(cm) Br(T) Bz(T) Fig.3 Ohmic heating stray field and flux contour Results of plasma equilibrium EAST will be operated in a circle section of the plasma, but a non-circle cross section with a high elongation is preferred in the future of the operating. The PF system should support the inductive operation under the plasma burn pulses lasting about 10 seconds without EC wave or other conditions. Extension of the burn pulse will be towards 1000 seconds as the steady-state operation. EAST operation baseline will keep the plasma current of 1MA with the elongation k x of about 1.8 to 2.0 and the triangularility δ x of about 0.6. The EAST plasma will be shaped and controlled by PF coils system only in a pulse length of 10 seconds. There are two candidate modes of operation of the EAST plasma. In the basic mode, the plasma minor radius is 0.4 meter and the major radius is 1.78 meter. In other mode, the plasma minor radius can be 0.45 meter and the major radius can be 1.91 meter, where the required currents for plasma control are moderate and the corresponding out-of-plane loads on the TF coils are structurally tractable. The value of the poloidal beta 87

5 is about 1.6 for two modes. The calculation results are shown in Table 3 and the plasma shapes are shown in figure 4. In Table 3, R x is the distance from X-point to the center point of the tokamak device in horizontal and and Z x is the distance from X-point to the equator plane of the device. The plasma geometry and the position are given from EQT (Equilibrium of Tokamak) code and PFFIFFPS code. The PF coils system should be satisfied with the requirement of physics design and engineering constrains, in which the PF coils system should be satisfied with the following requirements: (1) the capacity of the volt-second is about 10 web, (2) the stray field of the ohmic heating system in the region of plasma initial is lower than T, (3) maximum field is limited to lower than 5 T at the superconducting magnets and (4) the current of the CICC conductor can t be more than 15 ka under the field of magnets of 4.5 T. Table 3 Parameters of three type plasma shapes R(m) a(m) K x x R x (m) Z x (m) Big-K DN Circle Big-V DN circle Single null Big-K double null Big-V double null Fig.4 Plasma shapes of EAST 2.5 Current waveforms Current waveforms Figure 5 is the inductive discharge waveform. For a fixed OH current and plasma shape, the current waveforms for PF coils are fixed. The field on coils is quite different in different time points from the current change during operating. The peak field on the PF coil occurs near the ends of the solenoid on the inboard sides and the allowable maximum field at the superconducting PF coil is 4.5T. Figure 6 shows the field on each PF coil in the inductive discharge case. The field maximum varying rate for PF coil is about 7T/s and time duration is about 60ms. A well-controlled startup was obtained with a loop voltage as low as 8 V, which among tokamaks is quite a challenging technology in experiment. 88

6 Fig.5 Typical inductive discharge scenario (R=1.79m, a=0.41m, k x =1.84, δ x =0.58) Fig.6 PF coils field in operation Support structure All the PF coils will be attached to the case of toroidal coils that will support the vertical gravity loads and magnetic forces. The solenoid coils will be bolted together with preload of about 680 T force in total and then tightly connected with the front of the toroidal coils case through the fiberglass plates. The center solenoid is self-supported against the coil radial forces and most of the vertical forces, and only its weight is supported by the toroidal field coils. The other coils were supported by the case through the supports connect parts. During assembly, these connect can be adjusted easily and inspected quickly. The current in/out and helium tube were wrapped with insulation material down to the place where the insulator was set at the bottom of the machine. The CICC joints will be used for all PF coils if need. It was about 350 mm in length and 50mm 70 mm in cross section. It was set at the bottom of the machine for each coil connects with the current leads. There are other joints for the coil, if the length of CICC is longer than 600 m. We have made several joints and will test them later. Figure 7 shows the inlets for current and liquid helium on solenoid coil. Fig.7 The inlets for current and helium tube on solenoid coil 89

7 2.3.8 Strength analysis of the magnet The main load on coils and structural are the pre-loads force and the magnetic hoop force, which creates tension in the structural material. The results from analysis showed that all PF coils were safety enough during operation at high field. It means that the stresses and displacements were acceptable. For solenoid coils, a peak stress is about 300 MPa and the displacement on the structure is about 6 mm. The detailed analysis for the solenoid has been carried out and the result is shown in Fig.8 and Fig.9. Fig.8 Analysis for solenoid on stresses and displacement MSC.Patran Feb-02 14:53:26 Fringe: Default, Static Subcase, Stress Tensor, - Max Shear, At Z Z Y X default_fringe : Max Min 1226 Fig.9 Analysis for the stresses on outboard coil and the insulation on solenoid Conclusion The engineering design of the PF coils system of EAST was completed. It is satisfied with the requirements of physics design and global constructions. It was being fabricated in industry factory now. The solenoid coils and diverter coils will be testing one by one in CASIPP. 90