Reversed shear operation and ignition in high field tokamaks

Summary form only given, as follows. In neutral beam heated tokamaks, toroidal current and q profiles that possess a minimum between the magnetic axis and the plasma edge ("reversed shear") have been seen to trigger a new regime of improved plasma confinement. The favorable confinement introduces a new scenario for deuterium-tritium (D-T) fusion ignition at low density and very low fusion power in high field, magnetically confined plasmas. In these devices, reversed shear current profiles should be attainable with modest amounts of auxiliary heating and at parameters that are well below the maximum design values that are needed for full field ohmic, or low-auxiliary-power, ignition. In the reversed shear ignition scenario, the initial phase of the discharge, the current rise or ramp, plays an important part in efficient plasma heating towards ignition, as well as in establishing the radial variation of the current profile. It is shown that reversed shear conditions can be established part way into the initial current ramp phase of a discharge, by control of the current ramp rate, plasma shape, and injected heating power alone, without the need for external current drive. Enhancement of the energy confinement during this part of the ramp provides the optimal heating path to ignition and minimizes the external heating requirement and the fusion power at ignition. Examples of reversed shear scenarios are shown for the Ignitor-Ult.