Existence and Accessibility of Igniting States in a Tokamak Inferred from Its Performance in Tritiumless Discharges

The conditions for the existence and accessibility of ignited or subignited deuterium-tritium states are discussed in terms of the performance of the thermonuclear device in tritiumless discharges. The discussion includes the effects of the thermal instability of both the marginally igniting states and the nonstationary states in the start-up phase. These effects are an integral part of the problem of the accessibility to ignition under reliable conditions. Typical examples taken from the next generation of igniting tokamaks are discussed. The necessity of allowing sufficient excursion of the plasma column for a stable drive to ignition by feedback on the vertical field is underlined.