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ITB formation and evolution with co- and counter NBI

ITB formation and evolution with co- and counter NBI. A. R. Field, R. J. Akers, M. De Bock, C. Michael, R. Scannell, M. Wisse and the MAST and NBI teams. CCFE/EURATOM Association. Motivation. High resolution kinetic and q-profile diagnostics facilitate

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ITB formation and evolution with co- and counter NBI

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  1. ITB formation and evolution with co- and counter NBI A. R. Field, R. J. Akers, M. De Bock, C. Michael, R. Scannell, M. Wisse and the MAST and NBI teams CCFE/EURATOM Association CCFE is the fusion research arm of the United Kingdom Atomic Energy Authority

  2. Motivation • High resolution kinetic and q-profile diagnostics facilitate study of ITB formation and evolution • Strong driven toroidal rotation dominates ExB flow shear • Other factors known to be involved, e.g. magnetic shear • Comparison of co- and counter-NBI cases elucidates underlying physics, e.g. changing NBI power/torque ratio • Provides discharges in which flow shear effects dominate for comparison with simulations, e.g. with GYRO or GS2

  3. Kinetic and q-profile measurements Kinetic profile diagnostics (CX & TS) with R ~ 1 cm ~ i NdYAG TS: 130 channels R ~ 1 cm, 8 x 30 Hz lasers, t ~ 4 ms CXRS: 64 tangential channels (each beam), R ~ 1 cm, t ~ 5 ms MSE: q-profile evolution: 32 ch, R ~ 2.5 cm, t ~ 0.5 ms MSE polarisation angle Ti (CXRS) and Te (TS) Vi (CXRS)

  4. Integrated analysis (MC3) Integrated analysis chain prepares TRANSP input data Re-runs EFIT, including pressure and MSE constraints Profile fitting, including rotation asymmetry Zeff analysis from visible bremsstrahlung EFIT including MSE constraint TS fitting CX fitting Zeff

  5. ITB Scenario Early NBI heating at low-density during Ip ramp favours reversed shear Higher density with counter-NBI due to increased particle confinement Absorbed power less than half with counter- compared to co-NBI but higher torque (prompt losses) Similar stored energy and toroidal rotation with co- and counter-NBI Later in discharge, confinement degraded by MHD activity Co-NBI Counter-NBI Plasma current Line-average density Central temperatures Ti Te Toroidal rotation frequency Stored energy Fast-ion energy NBI power Energy confinement time

  6. Co-NBI: Profiles and transport coefficients • Ti exceeds Te in plasma core r/a < 0.4, where i ~ iNC • Foot of ITBs in ion and momentum channels near qmin • ExB flow shear SE peaks at foot of ITB

  7. Co-NBI: ITB evolution wf • Negative magnetic shear maintained in plasma core • ITBs in ion and momentum channels form near qmin • Momentum ITB forms at smaller radius than ion ITB • ITB terminated by MHD activity at 0.27 s

  8. Ctr-NBI: Profiles and transport coefficients Ti,e much lower than with co-NBI but rotation rate similar Ti ~ Te with i ~ iNC over most of plasma radius Much broader profile of SE than with co-NBI

  9. ITB evolution with ctr-NBI wf • Similar degree of shear reversal to co-NBI case • ITBs in ion and momentum channels broader than with co-NBI • Location of ITBs further outside qminsurface than with co-NBI • Later in discharge MHD (n=2) weakens ITBs

  10. Summary and conclusions Co-NBI: ITBs in ion and momentum channels form in vicinity of qmin Momentum ITB forms at smaller radius than ion ITB ExB shear peaks at location of ITB Counter-NBI: ITBs in ion and momentum channels form outside qmin Broad ITBs with i ~ iNC over most of plasma radius Similar level of ExB flow shear in spite of lower absorbed power due to broad profile of prompt loss torque

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