Abstract
We have initiated an experimental programme to address some of the questions associated with the operation of a tokamak with high bootstrap current fraction under high performance conditions, without assistance from a transformer. In these discharges, stationary (or slowly improving) conditions are maintained for up to 3.7 s at βN ≈ βp approaching 3.3. The achievable current and pressure are limited by a relaxation oscillation, involving growth and collapse of an internal transport barrier at ρ ≥ 0.6. The pressure gradually increases and the current profile broadens throughout the discharge. Eventually the plasma reaches a more stable, high confinement (H89P ∼ 3) state. Characteristically these plasmas have 65–85% bootstrap current, 15–30% neutral-beam-driven current and 0–10% driven by electron cyclotron frequency electromagnetic waves.
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