Analysis of metallic impurity density profiles in low collisionality Joint European Torus H-mode and L-mode plasmas

Abstract

This paper describes the behavior of nickel in low confinement (L-mode) and high confinement (H-mode) Joint European Torus (JET) discharges [P. J. Lomas, Plasma Phys. Control. Fusion 31, 1481 (1989)] characterized by the application of radio-frequency (rf) power heating and featuring ITER (International Thermonuclear Experimental Reactor) relevant collisionality. The impurity transport is analyzed on the basis of perturbative experiments (laser blow off injection) and is compared with electron heat and deuterium transport. In the JET plasmas analyzed here, ion cyclotron resonance heating (ICRH) is applied either in mode conversion (MC) to heat the electrons or in minority heating (MH) to heat the ions. The two heating schemes have systematically different effects on nickel transport, yielding flat or slightly hollow nickel density profiles in the case of ICRH in MC and peaked nickel density profiles in the case of rf applied in MH. Accordingly, both diffusion coefficients and pinch velocities of nickel are found to be systematically different. Linear gyrokinetic calculations by means of the code GS2 [M. Kotschenreuther, G. Rewoldt, and W.M. Tang, Comput. Phys. Commun. 88, 128 (1995)] provide a possible explanation of such different behavior by exploring the effects produced by the different microinstabilities present in these plasmas. In particular, trapped electron modes driven by the stronger electron temperature gradients measured in the MC cases, although subdominant, produce a contribution to the impurity pinch directed outwards that is qualitatively in agreement with the pinch reversal found in the experiment. Particle and heat diffusivities appear to be decoupled in MH shots, with χe and DD≫DNi, and are instead quite similar in the MC ones. In the latter case, nickel transport appears to be driven by the same turbulence that drives the electron heat transport and is sensitive to the value of the electron temperature gradient length. These findings give ground to the idea that in ITER it should be possible to find conditions in which the risk of accumulation of metals such as nickel can be contained.

Local effects of ECRH on argon transport in L-mode discharges at ASDEX Upgrade

“…For the most centrally heated case (10c), the central D reaches values up to one order of magnitude higher than the reference case, while the mixed heating case (10b) exhibits an increase of D up to a factor 6 at ρ ∼ 0.45, just outside the ECRH deposition radius. The maximum convection velocity is also found around the ECRH deposition radius: convection partially confirm previous experimental findings obtained using different analysis methods [3,4,5,6,7,8,9]. What is striking and novel of these results is the strong localization of these effects around the ECRH deposition position.…”

“…Perform a 1D Abel inversion and subtract the background contribution prior to the impurity injection. Finally, divide each time point of this profile by ǫ ′ to obtain the total argon density profile evolution (6). If available, re-scale this result through the impurity concentration measurement from an absolutely calibrated spectrometer (9).…”

“…Other impurity transport studies (e.g. [3,6,7,8,9]) use instead predictive approaches adjusting the D and v profiles iteratively so to reproduced the measured spectroscopic data and minimize the χ 2 of the simulated signals to the experimental ones (therefore called χ 2 -minimization methods). The new solution for the application of the G-F relation proposed here makes an integrated use of the soft X-ray (SXR) diode camera diagnostic [16] and the 1D radial impurity transport code STRAHL [17,11] for the determination of the total impurity ion density.…”

“…The use of intense central electron heating could be a possible solution since it has been observed to lead to an increase in impurity diffusivity and to a suppression of the convective pinch, or even to the rise of an outward impurity convection [2,3,4,5,6,7,8,9]. The underlying physics mechanisms of these effects are on the other hand still not clear.…”

First operation with the JET International Thermonuclear Experimental Reactor-like wall

“…In [53,54] it has been found that impurity accumulation occurs when turbulent transport in the plasma core is weak, and the neoclassical transport becomes important. The cleaning effect of large turbulent core transport is not only based on diffusive effects, but also supported by a turbulent outward pinch for the impurities requiring steep temperature gradients [55,56]. The radiation/heating balance in the core of the presented Pulse No: 81913 (Fig.5) reveals radiative cooling of similar size as the local heating power thereby decreasing the core T e and flattening the T e gradients leading to a feedback loop of decreasing transport and increasing radiation [49].…”