internal transport barrier investigations in mast

M Valovic. ITPA CDBM: ITB and Transport, St Petersburg, 2003

Internal transport barrier investigations in MASTA R Field, C Challis, P G Carolan, N J Conway, G Cunningham, H Meyer, M J Walsh and the MAST team

• Introduction

• Discharge scenario

• Evidence for ITB

• Transport simulations

• Summary

Presented by Martin Valovic for:-

EURATOM/UKAEA Fusion Association, Culham Science CentreAbingdon, Oxon, OX14 3DB, UK

This work is funded jointly by the UK Department of Trade and Industry and EURATOM

ITPA CDBM: ITB and Transport, St Petersburg, 2003ITPA CDBM: ITB and Transport, St Petersburg, 2003


ITB Discharge ScenarioITB 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, 1-21019 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


Ti and V profiles, from CXR C6+, showing large thermal gradient at high velocity shear.

Electron and ion pressure profiles, showing steepgradients at position of high velocity shear.

ITB:- ITB:- TTii, , vvii (C(C6+6+) ) and and ppi,ei,e Profiles Profiles


Measured Measured TTii and Vand Vii profile evolution in ITB discharge profile evolution in ITB discharge

CXRS measurements of TTi i and Vand Vii on C on C6+6+ ions show s ions show steep Ti gradient coincides with high shear flow region.


TRANSP modelling indicates reversed magnetic shearTRANSP 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)


Criterion for presence of ITBCriterion for presence of ITB

• Velocity shear rate exceeding the turbulent growth rate:-

• or, equivalently, (CEA criterion for presence of ITB):-

014.0/ ** ITBTsT L

linE 0

ITBITB


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

2/3

22

68.0

iiinci

q

nci

nci 2/3

Preliminary Transport SimulationsPreliminary Transport Simulations


ConclusionsConclusions

• 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).


q-profile TRANSP predictions: sensitivity to Zq-profile TRANSP predictions: sensitivity to Zeffeff

r/a

q


Preliminary Transport Simulations using an Preliminary Transport Simulations using an ad hocad hoc 1D code 1D code

internal transport barrier investigations in mast

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