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An outer PF coil-only inductive plasma start-up technique on spherical tori

This study explores the use of outer PF coils to create a field null region for plasma start-up on spherical tori, enabling compact fusion reactors and eliminating the need for conventional ohmic solenoids. Numerical simulations show the feasibility and flexibility of this technique, offering sufficient access to blanket modules and potential for current ramp-up.

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An outer PF coil-only inductive plasma start-up technique on spherical tori

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  1. Supported by An outer PF coil-only inductive plasma start-up technique on spherical tori Columbia U Comp-X General Atomics INEL Johns Hopkins U LANL LLNL Lodestar MIT Nova Photonics NYU ORNL PPPL PSI SNL UC Davis UC Irvine UCLA UCSD U Maryland U New Mexico U Rochester U Washington U Wisconsin Culham Sci Ctr Hiroshima U HIST Kyushu Tokai U Niigata U Tsukuba U U Tokyo JAERI Ioffe Inst TRINITI KBSI KAIST ENEA, Frascati CEA, Cadarache IPP, Jülich IPP, Garching U Quebec Wonho Choe, Jayhyun Kim Korea Advanced Institute of Science and Technology Masayuki Ono Princeton Plasma Physics Laboratory 45th Annual Meeting of Division of Plasma Physics American Physical Society October 27 – 31, 2003 Albuquerque, New Mexico

  2. Conventional ohmic solenoid R Plasma a Out-board region In-board region Outer PF coil-only inductive plasma start-up • Ohmic solenoid has been the work horse of fusion research. • Attractive fusion CTF and power plant design requires OH elimination • Compact CTF requires elimination of OH regardless of R/a. • ARIES-AT and ARIES-ST design assumes no OH. • PF coils have been used to start-up the plasma • MAST (START): PF coils + radial compression • JT-60U: Aggressive application of rf • Plasma start-up using appropriate combination of out-board / outer PF coils.

  3. #1 Z Plasma Plasma #2 #1 R #3 a a Midplane #2 #3 Plasma R Major axis Null field region Initial plasma matched Field null region Null field generation using out-board induction coils • Small radius PF coil #1 produces a ‘peaked’ BV profile. • Large radius PF coil #2 produces a flat BV profile. • Near-midplane ‘trim’ PF coil #3 produces a follow BV profile.

  4. Case 1 Case 2 Case 5 Case 3 Case 4 #1 #1 #1 #1 #1 #2 #2 #2 #2 #2 #3_1 #3_1 #3_1 #3 #3 #3_2 #3_2 #3_2 Numerical simulation performed for NSST • Several configurations possible •  flexibility to accommodate the needs for a particular device Coil #1 down Coil #3 on mid-plane Coil #2 moved up Coil #3 off mid-plane With more contoured vacuum vessel To give sufficient access to blankets To further increase access to blankets

  5. Coil #1 (+16 MA-t) Coil #2 (-10 MA-t) Coil #3_1 (-3.7 MA-t) Coil #3_2 (+6.7 MA-t) Available flux matched Case 5 for sufficient mid-plane space for the blanket access Mid-plane vertical field profiles Net vertical field profile Field null region

  6. Radial profile of flux Mod-B contours (Gauss) Plasma axis Case 5 cont’d Flux contours • Significant amount of volt-sec available for current ramp-up: ~4.5 V-s at R0 = 1.75 m • Generation of good quality multi-pole field null • Excellent out-board access (~1.8 m vertical spacing) •  Suitable for the interchangeable blanket modules for CTF.

  7. +20 kA/turn - 20 kA/t +2.8 kA/t Simulation for NSTX Flux contours

  8. contours ~ 35 cm Time-dependent calculation with vacuum vessel eddy currents considered Evolution of B contours • Lloyd’s condition, with strong pre-ionization, ET·BT/BP ≥ 0.12  kV/m satisfied in a significant volume. (Kim, LP1.033)

  9. Null formation NSTX time-dependent calculation cont’d Coil current Loop voltage Flux vs time (at 1.4 m) Null Null • Significant V-s is available for current ramp-up. • Force balance in the initial start-up phase needs to be checked carefully (on-going). • Full ramp-up scenario will require bi-polar PF5. But initial breakdown experiment to ~100 kA should be possible with the existing power supplies.

  10. Summary • An analysis for developing an out-board PF-only inductive start-up scheme was performed. • A combination of out-board PF coils placed outside the vacuum vessel is shown to create a good quality field null region while retaining significant volt-second capability for current ramp-up. For NSST, ~5 V-s possible for ramping the current to a few MAs. For NSTX, ~0.12 V-s possible for Ip ~ a few hundred kA. • The concept provides sufficient flexibility to accommodate the needs for a particular device. • NSTX can be utilized as a test-bed for elaborating the concept. - A quality multi-pole field null can be produced at R  1.4 m with the current machine capabilities. - Dynamic calculation including structure eddy currents shows sustainment of field null for over 5 ms.

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