Impurity transport studies in ASDEX by means of neon-seeded pellets

Abstract

Neon-seeded deuterium pellets, containing 1% neon, were successfully produced and injected into ASDEX divertor discharges. The behaviour of the deposited neon ions was studied by VUV spectroscopy, using a special version of the ASDEX/JET impurity transport code for interpretation. The neutral neon source was tailored so as to resemble theoretical ablation curves. Estimates of time-scales for ionization and toroidal spreading of neon ions show that toroidal symmetry is established before ionization to Ne X takes place; however, the results for Ne VIII burnthrough must be treated with caution in a one-dimensional model. Some ionization and recombination rate coefficients were verified experimentally by investigation of the time delays between pellet entry and maximum line intensities, and of the low remaining level of Ne VIII radiation after its ionization peak. The anomalous neon diffusion coefficient was derived from the final decay of the Ne X line intensity, which was observed to be exponential, down to the 5% level. Below this level, recycling via the divertor chambers became important. A value of D ≈ 0.4 m2·s−1 was found under the assumption of an associated drift velocity vD = −2D r/a2; this is in good agreement with the results of other transport studies of ASDEX OH plasmas and similar values have been reported from other tokamaks. No apparent dependence of the diffusion coefficient on electron density was observed.