Stationary high-performance discharges in the DIII-D tokamak

Discharges which can satisfy the high gain goals of burning plasma experiments have been demonstrated in the DIII-D tokamak under stationary conditions at relatively low plasma current (q95>4). A figure of merit for fusion gain (βNH89/q952) has been maintained at values corresponding to Q = 10 operation in a burning plasma for >6 s or 36τE and 2τR. The key element is the relaxation of the current profile to a stationary state with qmin>1. In the absence of sawteeth and fishbones, stable operation has been achieved up to the estimated no-wall β limit. Feedback control of the energy content and particle inventory allow reproducible, stationary operation. The particle inventory is controlled by gas fuelling and active pumping; the wall plays only a small role in the particle balance. The reduced current lessens significantly the potential for structural damage in the event of a major disruption. In addition, the pulse length capability is greatly increased, which is essential for a technology testing phase of a burning plasma experiment where fluence (duty cycle) is important.