Effect of low aspect ratio data on H-mode confinement

M Valovic, 9th ITPA CDBM Meeting, Oct 2005 Why we want to know the a/R scaling ? Preparation for ITER operation: better understanding of scaling with dimensionless parameters  *, , *. Aspect ratio is contained in their definitions and thus the a/R scaling gives additional insight into the physics. DEMO optimisation. Physics basis for Component Test Facility For ITER like shape the a/R scaling is given by MAST and NSTX data. STPP MAST NSTX

M Valovic, 9th ITPA CDBM Meeting, Oct 2005 Spherical tokamaks are naturally high  machines. START, MAST and NSTX extent the H-mode tokamak database of conventionally shaped plasmas simultaneously in  =a/R and . At the same time the ST data broadly agree with the scaling derived from conventional aspect ratio tokamaks. Thus the data make powerful constraint on  and  dependencies B  E   x   x  : Unfavourable  scaling gives positive  exponent weak  scaling gives negative  exponent M Valovič et al. Nucl. Fusion 2005 S Kaye et al. H-mode Workshop 2005  -  interplay

M Valovic, 9th ITPA CDBM Meeting, Oct 2005 Summary and plans Low aspect ratio data from NSTX, MAST are significant in determining the aspect ratio and  dependencies for ITER like shaped plasmas. Expanding the range of a/R and  raises the question of validity of log-linear scaling. It is known that * exponent is not constant across the whole * range (C C Petty). Therefore dimensionless scans at low A are important to test whether exponents depend on a/R. Beta scan on MAST is planned. Expansion of MAST H-mode dataset is planned.