Physics background of the ASDEX upgrade project

Abstract

ASDEX Upgrade is intended as the next experimental step after ASDEX. It is characterized by a poloidal divertor configuration with poloidal field coils located outside the toroidal field coils, by machine parameters which allow a line density within the plasma boundary sufficient to screen fast CX particle from the plasma core, by a scrape-off layer essentially opaque to neutrals produced at the target plates, and, finally, by an auxiliary heating power high enough for producing a reactor-like power flux density of 0.3 MW/m 2 through the plasma boundary. It uses normal-conducting coil systems, is of the external size of ASDEX and has a B t-field capability of up to 3.9 T. To provide the required power flux density, ICRH heating with a deposited power of 10 MW is presently envisaged. Operation with a toroidally continuous pump limiter is also foreseen. Poloidal field design studies for both reactor-size and experimental devices have shown the single-null divertor configuration, with a concurrent elongation of the plasma cross-section b a ≌ 1.6 , to have essential advantages concerning the economy of the volume utilization and of the poloidal field system. This is therefore also the basic configuration chosen for ASDEX Upgrade, although a double-null divertor will be possible for physics studies at reduced parameters. Principal quantitative differences to ASDEX are the larger volume of the bulk plasma and the higher B t field capability, which are utilized to draw an up to 4 times larger plasma current (≤ 2 MA). This should ensure operation at high average and line-integrated densities, which are essential to simulate the boundary zone and scrape-off layer behaviour of larger devices. The plasma current in a β pol-limited situation directly determines the possible plasma parameters and their gradients. The capability for DC-operation of the tokamak system at reduced parameters will also be used for current-drive studies and particularly those scenarios requiring alternating operation at low and high densities.