A comparison of the impact of central ECRH and central ICRH on the tungsten behaviour in ASDEX Upgrade H-mode plasmas
Summary (2 min read)
- Central RF heating not only modifies the temperature, density and rotation profiles of the main plasma, but can also directly modify the impurity transport.
- In order to identify the transport mechanisms which can explain the new observations reported in this paper, power density profiles of ECRH and ICRH have been computed with the codes TORBEAM  and TORIC–SSFPQL , and complete power balance calculations have been obtained with TRANSP .
- In Section 2 the experiment is described and the experimental results are presented.
- In section 3 the observations are modelled in order to shed light on the relative role of the transport mechanisms governing the W density behaviour.
2. Experimental investigation of the W response to central ECRH and
- ICRH ICRH is applied with a H-minority heating scheme, with a H concentration around 5%, with both the 2–straps antenna with boron coated limiters and the 3–straps antenna with tungsten coated limiters .
- An interesting observation is that, during the power steps with relatively low levels of RF heating, the W behaviour can reach stationary conditions with a W density profile which is significantly more peaked than the electron density profile.
- The authors observe that plasmas with additional ICRH heating exhibit higher values of R/LT i and lower values of Te/Ti than cases with additional ECRH, consistent with an increased level of ion heating in the center produced by ICRH.
- Similar to the observations reported in , also in the heating phases examined in the present work, sawteeth are present and are characterized by the occurrence of (1, 1) modes, which appear early after the sawtooth crash and saturate over a large portion of the sawtooth period.
3. Modelling of the experimental results
- In this section the experimental results are examined with both simplified models and theoretical models in order to identify the main ingredients which are required to reproduce the experimental trends.
- In particular, the drift–kinetic code NEO [48–50] and the gyro–kinetic code GKW [51,52] are applied to compute the neoclassical and the turbulent W transport components using as inputs the measured profiles of the main plasma (deuterons and electrons).
- The authors also notice that neoclassical transport only (case with Dturb/χeff = 0) predicts too large peaking factors for most of the ECRH cases, whereas values of Dturb/χeff ≥ 1 are inconsistent with the observed accumulation at the lowest values of Qe/Qtot.
- At high additional RF heating powers, the neoclassical diffusion coefficient is smaller than the turbulent diffusion coefficient.
- The results of the combined NEO and GKW modelling are presented in Fig. 11, where predicted flux–surface–averaged density profiles are compared with those reconstructed from the experimental measurements, and in 12, where the predicted central peaking factor parameter is compared to the corresponding experimental results as a function of the fraction of electron heat flux.
- Experiments in AUG have been performed to directly compare the impact of central ECRH and central ICRH on the W behaviour.
- In these experiments both ECRH and ICRH have been applied in addition to a background of NBI heating, which is mainly delivering ion heating in the central region of the plasma.
- The absence of strong central NBI heating in an ITER plasma has some positive consequences with respect to the W behaviour.
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...In addition, the role of neoclassical W transport in the core is expected to be much smaller in a reactor plasma compared to current devices, resulting in less charge-dependent inward directed convective transport ....
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Q1. What are the contributions in "A comparison of the impact of central ecrh and central icrh on the tungsten behaviour in asdex upgrade h-mode plasmas" ?
In this paper, a comparison of the impact of additional central ECRH and ion cyclotron resonance heating on the behavior of the tungsten ( W ) density in the core of H-mode plasmas heated with neutral beam injection ( NBI ) is performed in ASDEX Upgrade.