A fusion reactor: continuous or semi-continuous?

Based upon latest results from JET and other tokamaks and upon model projections, the operating conditions of a fusion reactor are predicted. The technical and scientific issues involved in continuous operation of such a reactor using noninductive drive are identified. Candidate techniques include injecting beams of high energy neutral particles and radio waves at various frequencies (such as fast, lower hybrid and electron cyclotron waves). The basis for a continuously operating reactor is not apparent and a convincing demonstration would require either a high current drive efficiency (above 1020 A/m2W) at a density above approximately 1020 m3; or ignition and adequate impurity control at a density of 5*1019 m-3 with moderate current drive efficiency ( approximately 0.5*1020 A/m2W); or high power operation in a regime with a dominant bootstrap current. Semi-continuous operation with inductive current drive offers the only viable alternative for long pulse reactor operation. This could use either forward current or alternating current operation, provided the central solenoid was sufficiently large. A tokamak reactor operating semi-continuously would be simpler in construction, use re-circulating power more efficiently and would likely be more reliable in operation. It is proposed that the Next Step tokamak be based on inductive semi-continuous operation.