Status of long pulse experiments in magnetic fusion devices

Achieving long-duration, high-performance discharges in magnetic fusion devices is one of the most important challenges en route to a fusion reactor. At this stage, we need to bring together many physical concepts and technological achievements that hitherto have been considered as separate issues. In the course of a long duration pulse, one encounters a sequence of progressively increasing characteristic timescales, ranging from milliseconds for MHD events, seconds for energy and particle transport times, tens of seconds for current diffusion times and up to hundreds of seconds for wall processes, such as saturation and erosion, to reach equilibrium. Although many present-day experiments have pulse lengths long enough to allow studies of the MHD and transport issues in conditions that are effectively quasi steady state, most have pulse lengths that are marginal for studying current diffusion phenomena and, generally, all are too short to study wall saturation and erosion. Very few present-day experiments bring together the necessary hardware (magnets, power supplies, heating and current drive systems, cooling loops, etc) to properly address issues on timescales greater than 10 s. This paper reviews the status of present-day long pulse experiments in tokamaks and stellarators in terms of the technology and physics. We start by defining the requirements of long pulse experiments and discussing the technology that is needed. Then, we consider the relevant physics including the important interactions between physics and technology. Finally, we consider the issues that must be addressed to go beyond long pulses in order to reach full steady-state operation.