CCFE is the fusion research arm of the United Kingdom Atomic Energy Authority

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

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

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)

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 constraintTS fitting

CX fittingZeff

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

Plasma current

NBI power

Stored energy

Fast-ion energy

Energy confinement time

Toroidal rotation frequency

Line-average density

Central temperaturesTi Te

Co-NBI Counter-NBI

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

Co-NBI: ITB evolution

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

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

ITB evolution with ctr-NBI

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 qmin surface than with co-NBI

Later in discharge MHD (n=2) weakens ITBs

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 ~ i

NC 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