The requirements of a next step large steady state tokamak

After a decision by the ITER parties to investigate the possibility of desi gning a reduced cost version of ITER several possible machine layouts with different aspect ratios were studied. Relatively early in this process it b ecame clear that there is no significant cost difference between different aspect ratios and that there is a maximum realistically possible aspect rat io for a machine with 6 m major radius and rather high plasma shaping. Foll owing this study a machine with an intermediate aspect ratio (3.1) called t he ITER Fusion Energy Advanced Tokamak (ITER FEAT) was chosen as the basis for the outline design of a reduced cost ITER. Several potential steady sta te scenarios can be investigated in ITER FEAT, i.e. monotonic or reversed s hear at full or reduced minor radius. In addition, so-called hybrid dischar ges, are feasible where a mixture of inductive and non-inductive current dr ive as well as bootstrap current allows long pulse discharges of the order of 2500 s. The beta(N) values and H factors required for these discharges a re in the same range as those observed on present machines, which provides confidence that such discharges can be studied in ITER FEAT. However, due t o uncertainties in physics knowledge, for example the current drive efficie ncy off-axis, it is impossible at present to generate a completely self-con sistent scenario taking all boundary conditions, for example engineering or heating system constraints, into account. In addition, all of these regime s have a potential problem with divertor operation compatibility (low edge density) and with helium exhaust which has to be addressed in existing expe riments. For the engineering design of the in-vessel components and for the balance of the plant there is practically no difference between inductive (500 s) and steady state operation. However, the choice of heating systems and the distribution of power between them will be strongly influenced by t he envisaged steady state scenarios.