The ASDEX divertor tokamak

ASDEX is a large tokamak (R = 1.65 m, a = 0.4 m, Ip ≤ 500 kA) that started operation in 1980. Its distinctive features are a double- (or single-) null poloidal divertor, the capability for long-pulse operation (up to 10 s) and high-power neutral-beam (4.3 MW), ICRH (3 MW) and LH (2.4 MW) heating systems. Several highly significant experimental findings have been obtained, facilitated by the large flexibility of the machine. The high-recycling divertor regime is considered the most viable solution for handling the large power fluxes envisaged for the next generation of tokamak devices. Studies of impurity transport in the bulk plasma led to the postulation of an additional inward particle drift, while gas puffing experiments revealed the importance of thermoelectric forces along field lines for impurity retainment by the divertor. The high-confinement regime (H-mode) of neutral-beam-heated plasmas, discovered on ASDEX, is also intimately connected with the divertor configuration. The H-mode.confinement, in turn, enabled the investigation of β-limits resulting in the experimental scaling ≅ 2.8 Ip/(a BT) [%; MA, m, T], in excellent agreement with theoretical predictions. The most important results from recent experiments with RF heating and current drive include the recharging of the OH transformer at constant plasma current, and the observed high heating efficiency obtained with combined second-harmonic ICRH and neutral-injection heating.