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Recent ICRF Results in the Alcator C-Mod Tokamak

4 th US-PRC Magnetic Controlled Fusion Collaboration Workshop, UT Austin, May 5-6, 2008 . Recent ICRF Results in the Alcator C-Mod Tokamak. Presented by Yijun Lin With contributions from S. Wukitch, A. Binus, A. Ince-cushman, E. Marmar, M. Reinke, and J. Rice Alcator C-Mod Project

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Recent ICRF Results in the Alcator C-Mod Tokamak

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  1. 4th US-PRC Magnetic Controlled Fusion Collaboration Workshop, UT Austin, May 5-6, 2008 Recent ICRF Results in the Alcator C-Mod Tokamak Presented by Yijun Lin With contributions from S. Wukitch, A. Binus, A. Ince-cushman, E. Marmar, M. Reinke, and J. Rice Alcator C-Mod Project MIT, Plasma Science and Fusion Center Cambridge, MA 02139, USA

  2. Outline • Introduction of Alcator C-Mod • Overview of ICRF program • Recent ICRF results: • ICRF technology: fast ferrite tuning system • Fast wave direct electron heating • Mode conversion sawtooth modification • Mode conversion flow drive • Collaboration areas

  3. C-Mod Unique in World and USAmongHigh Performance Divertor Tokamaks • Unique in the World: • High field, high performance divertor tokamak • Particle and momentum source-free heating and current drive • Equilibrated electron-ion coupling • Bulk all high-Z plasma facing components • ITER level (and beyond) Scrape-Off-Layer/Divertor Power Density • Approach ITER neutral opacity, radiation trapping • Highest pressure and energy density plasmas • Exclusive in the US : • ICRF minority heating • Lower hybrid current drive • A premier major US facility for graduate student training

  4. Present ICRF System D & E antennas J antenna

  5. ITER Relevance

  6. C-Mod ICRF Research Themes • RF system R & D: • Antenna design, and real-time match for successful RF operation. • RF edge-plasma interactions: • Antenna coupling, dynamic loading, voltage and power limitations, RF sheath, and impurity production. • Wave propagation and absorption: • Fundamental minority heating, 2nd harmonic majority heating, direct fast wave, and mode conversion absorption regimes. • To validate simulation codes, scalable to ITER and reactors, to provide confidence in simulation codes used for discharge analysis. • Plasma current and flow drive: • To develop means to control plasma current profile and affect stability of MHD modes (e.g., sawtooth modification). • Investigate flow drive and application.

  7. RF Tech. R&D: Fast Ferrite Tuning System • A transmitter needs to be isolated from the antenna through impedance matching network. • Antenna loading changes with plasma conditions in real-time. • Ferrite material  varies vs. ambient B field  length variation in the line. • Digital control on the tuner current to achieve real-time matching. • Triple-stub design: one pre-match stub, and two with ferrite tuners.

  8. Fast Ferrite Tuning System • Equivalent length change up to 36 cm at 80 MHz with change of 300 A coil currents. • Power supply swing capability: 75 A/ms • Computation iteration: 250 s • Filled with SF6 for higher voltage handling • Water cooled • Optical arc detection

  9. Digital Controller and Power Supply Linux server PLC interfaces RF power/phase detector Power Supply Digital Controller

  10. FFT Performance in L- and H-modes • Under real-time digital control, the FFT system can maintain the power reflection below 2% under significant antenna load variation. • First installed as a double-stub system in 2007, re-configured to triple-stub in 2008, and has been running successfully in the entire 2008 campaign. • Max net-power handled 1.85 MW in H-mode.

  11. Latest ICRF Physics Experiments • From 04/22 to 05/02/2008 (last two weeks), we ran experiments with J-antenna at 50 MHz, and D/E antennas at 80 MHz. • In this setup we can access many scenarios other than the normal D(H) minority ICRF heating: • Bt ~ 5.3 T, D(H) plasma  J antenna fast wave direct electron heating (no IC resonance), H-minority heating with D/E antennas. • Bt ~ 5.1 T, D(H) plasma with low 3He concentration  H-minority heating with D/E antennas and 3He-minority heating from J antenna. • Bt ~ 5.1 T, D(H) plasma with moderate 3He concentration  H-minority heating with D/E antennas and D(3He) mode conversion heating/current drive/flow drive from J antenna. • Bt ~ 3.3 T,  H-minority heating with J antenna and also 2nd harmonic deuteron heating. • Really recent data: No detailed analysis, but only preliminary intepretation.

  12. Fast Wave Electron Heating • J antenna at 50 MHz does not have an ion cyclotron resonance in normal Bt = 5.3 T D(H) plasmas. • Fast wave electron heating was observed in good confinement plasmas (relatively high β) pre-heated with minority heating. Wplasma [kJ] FWEH Te [keV] J (50 MHz) Power [MW] D+E (80 MHz) Power [MW] Data from 04/23

  13. Sawteeth Period vs. MCCD 50 MHz, D(3He), Mode conversion near q = 1 surface Co-Current CD, Average sawtooth 10 ms Counter-Current CD, sawtooth 8 ms Heating phase, sawtooth 10 ms Data from 04/22 and 04/25 D+E 1.7 MW D/E 1.7 MW and J 1.7 MW

  14. Strong Rotation with Small Increase in Wp Strong co-Current rotation > 100 km/sec was observed with only 50 kJ increase in plasma energy in some mode conversion plasmas. A factor of 2 more than normal intrinsic plasma rotation scaling ΔV [km/sec] ~ 0.9* ΔW[kJ]/Ip[MA] J antenna in mode conversion regime, while D and E in minority heating regime. Toroidal rotation (km/s) Stored energy (kJ) J 50 MHz (MW) D+E 80 MHz (MW) First observed on 04/22/2008. Surprising result.

  15. Mode Conversion Flow Drive Toroidal rotation (km/s) At the power level, J antenna in mode conversion regime generated about twice core rotation than D and E antenna in minority heating. D+E J-ant Stored energy (kJ) J 50 MHz (MW) D+E 80 MHz (MW) Data from 04/25

  16. Rotation Velocity vs. RF power Blue circles: Mode conversion flow scales with the RF power. Black squares: Minority heating. Data from 04/25 and 04/29

  17. Collaboration Areas • ICRF physics and technology • LHRF physics and technology • Diagnostics • MDS-plus

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