Design of the Pretext Tokamak

The PRETEXT tokamak was designed to investigate the initial stages of a tokamak discharge. The machine has a major radius of 53 cm, limiter radius of 15 cm, and a toroidal field (TF) of 8 kG. Plasma currents of 40 to 60 kA with durations of 40 to 70 ms are typical. Chord average density is of the order of 10¹³ cm⁻³, the electron temperature T_e(0) ∼ 300 eV, and the effective charge Z_eff ∼ 3. The ohmic-heating (OH) transformer is unique among operating machines, having an iron core and air return. Combinations of mechanical and electronic switches in the OH and TF power supplies provide good reliability at low jitter and low cost. The design and construction of the PRETEXT tokamak are discussed in enough detail to give a feeling for the compromises that must be made and the problems that will be encountered in the building of any controlled thermonuclear reactor device.