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Internal transport barrier investigations in MAST

ITPA CDBM: ITB and Transport, St Petersburg, 2003. Internal transport barrier investigations in MAST. Presented by Martin Valovic for:-. A R Field, C Challis, P G Carolan, N J Conway, G Cunningham, H Meyer, M J Walsh and the MAST team. EURATOM/UKAEA Fusion Association, Culham Science Centre

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Internal transport barrier investigations in MAST

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  1. ITPA CDBM: ITB and Transport, St Petersburg, 2003 Internal transport barrier investigations in MAST Presented by Martin Valovic for:- A R Field, C Challis, P G Carolan, N J Conway, G Cunningham, H Meyer, M J Walsh and the MAST team EURATOM/UKAEA Fusion Association, Culham Science Centre Abingdon, Oxon, OX14 3DB, UK • Introduction • Discharge scenario • Evidence for ITB • Transport simulations • Summary This work is funded jointly by the UK Department of Trade and Industry and EURATOM

  2. ITB Discharge Scenario • Aim for weak/reversed magnetic shear and high toroidal flow shear: • 4MA/s Ip ramp from initial 100 kA MC plasma • Early NBI heating, 2 MW • Low density, 1-21019 m-3 • Broad initial j(r) indicated by low Li ~ 0.6 • Significant NBI heating after 0.1 s due to • poor initial fast ion confinement

  3. ITB:- Ti, vi(C6+) and pi,e Profiles Ti and Vf profiles, from CXR C6+, showing large thermal gradient at high velocity shear. Electron and ion pressure profiles, showing steep gradients at position of high velocity shear.

  4. Measured Tiand Vi profile evolution in ITB discharge CXRS measurements of Ti and Vi on C6+ ions show steep Ti gradient coincides with high shear flow region.

  5. TRANSP modelling indicates reversed magnetic shear • Inputs to TRANSP • Te(r,t), ne(r,t) from Thomson scattering • TC6+(r,t), VC6+(r,t) from CXRS of C 6+ • EFIT determined boundary shape • Assumed Zeff profiles • Outputs • Ti (r,t), Vi (r,t) of fuel ions (deuterium) • neutron flux (compared with measurements) • Vloop , li , etc. • q(r,t) and j(r,t) • Some indications that ITB formation is associated with weak or reversed magnetic shear (e.g. in #7051, TRANSP suggests q  0 for extreme limits of Zeff ~ 1.5 to 3.0) but wider “causality” investigations and more detailed measurements are required; e.g. 200Hz TS and 200 chord Zeff now available)

  6. Criterion for presence of ITB • Velocity shear rate exceeding the turbulent growth rate:- • or, equivalently, (CEA criterion for presence of ITB):- ITB

  7. Preliminary Transport Simulations • A simple 1D ad hoc time dependent code which uses:- • Neo-classical thermal diffusivity: • Momentum diffusivity smaller by 3/2: • gives:- • Reasonable agreement with measured central C6+ temperature • and velocity (from CXRS) • Close coupling of D+ and C6+ toroidal flow

  8. Conclusions • Initial attempts at ITB formation on MAST have been successful. • Strongly sheared toroidal flow are accompanied by increases in pressure gradients. • More experiments are underway (e.g. density scans, co- and counter-NBI) with improved diagnostics (e.g. 200 Hz TS and 200 chord Zeff). • More in-depth tranport analyses are progressing (e.g. TRANSP).

  9. q r/a q-profile TRANSP predictions: sensitivity to Zeff

  10. Preliminary Transport Simulations using an ad hoc 1D code

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