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/m2 through the plasma boundary. It uses normal-conducting coil systems, is of the external size of ASDEX and has a Bt-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 Bt 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 βpol-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.