Observations on the W-transport in the core plasma of JET and ASDEX Upgrade

Abstract

The W-transport in the core plasma of JET is investigated experimentally by deriving the W-concentration profiles from the modelling of the signals of the soft x-ray cameras. For the case of pure neutral beam heating W accumulates in the core (r/a < 0.3) approaching W-concentrations of 10 −3 in between the sawtooth crashes, which flatten the W-profile to a concentration of about 3 × 10 −5 . When central Ion cyclotron resonant heating is additionally applied the core W-concentration decays in phases that exhibit a changed mode activity, while also the electron temperature increases and the density profile becomes less peaked. The immediate correlation between the change of magnetohydrodymanic (MHD) and the removal of W from the plasma core supports the hypothesis that the change of the MHD activity is the underlying cause for the change of transport. Furthermore, a discharge from the ASDEX Upgrade is investigated. In this case the plasma profiles exhibit small changes only, while the most prominent change occurs in the W-content of the confined plasma caused by the reduction of the fuelling deuterium gas puff. Concomintantly, the W-concentration profiles in the core plasma r/a < 0.2 steepen up reminescent to the well-known connection between central radiation and transport during cases with strong, established W-accumulation, while in the present analysis such a causality between the two during the onset of W-accumulation could not be pinned down. Both case studies exemplify that small changes of the core parameters of a plasma my influence the W-transport in the plasma core drastically.

Experimental data on low energy electron impact ionisation of W

“…We fit expression 2 to the experimental data using g l and v w á ñ as free parameters ( figure 4, left figure 5 together with other data sets from literature. The present results compare well to the only other existing experimental data by Steinbrink [6], which were independently obtained using a different approach 2 . Both experimental data sets are, however, significantly lower than their widely used theoretical pendants based on the Born approximation (Vainstein [4]) or the the distorted wave approximation (ADAS [5], Pindzola and Griffin [13]).…”

“…However, due to its high nuclear charge number W can significantly degrade the plasma performance and energy confinement when abundant as intrinsic impurity. Therefore, a thorough survey and control of the W concentration in the plasma core is of crucial importance [2]. A key step for understanding the W behaviour in the plasma is to quantify the W source at the plasma edge, which can readily be accessed spectroscopically on the basis of SX/B values [3].…”

Modification of impurity transport in the presence of saturated (m,n) = (1,1) MHD activity at ASDEX Upgrade

“…Despite having been qualitatively described in more than one publication in the past decades (see e.g. [18,19,20,21]) only one attempt to quantify these changes in transport have been made for JET plasmas [21].…”

A new ion cyclotron range of frequency scenario for bulk ion heating in deuterium-tritium plasmas: How to utilize intrinsic impurities in our favour

“…3(a) and (c), is that for the given ICRF frequency f = 37 MHz Be absorption is the dominant The presence of high-Z impurities like tungsten has a small impact on the (Be)-D-T scenario. A typical charge state for W in the plasma core is Z ≃ 40 − 50 [37], and such impurities have (Z/A) W ≈ 0.2 − 0.3, which is very different to (Z/A) Be . On the contrary, impurities with a charge-to-mass ratio close to that of Be ions, e.g.…”