Study of near SOL decay lengths in ASDEX Upgrade under attached and detached divertor conditions

Abstract

A database with attached, partially detached and completely detached divertors has been constructed of ASDEX Upgrade discharges in both H-mode and L-mode plasmas with Thomson Scattering data suitable for the analysis of the upstream SOL electron profiles. By comparing upstream temperature decay width, , , with the scaling of the SOL power decay width, ∥ , based on the downstream IR measurements, it is found that a simple relation based on classical electron conduction can relate , and ∥ well. The combined dataset can be described by both a single scaling and a separate scaling for H-mode and L-mode. For the single scaling, a strong inverse dependence of, , on the separatrix temperature, , , is found, suggesting the classical parallel Spitzer-Harm conductivity as dominant mechanism controlling the SOL width in both L-mode and H-mode over a large set of plasma parameters. This dependence on , explains why, for the same global plasma parameters, ∥ in L-mode is approximately twice that in H-mode and under detached conditions, the SOL upstream electron profile broadens when the density reaches a critical value. Comparing the derived scaling from experimental data with power balance, gives the cross-field thermal diffusivity as ⫠ ∝ / / , consistent with earlier studies on Compass-D, JET and Alcator C-Mod. However, the possibility of the separate scalings for different regimes cannot be excluded, which gives results similar to those previously reported for the H-mode, but here the wider SOL width for L-mode plasmas is explained simply by the larger premultiplying coefficient. The relative merits of the two scalings in representing the data and their theoretical implications are discussed.

Empirical study of gradient lengths ratio η e in the near SOL region in ASDEX Upgrade tokamak

“…These discharges with higher 𝑞 are the same ones mentioned above which will be discussed more in the next section. As observed in the previous study of SOL temperature profile [8], 𝜆 , has a strong 𝑞 dependence, 𝜆 , ∝ 𝑞 for both H-mode and L-mode plasmas. As can be seen from figure 4(a) and (b), 𝜆 , also has a strong 𝑞 dependence, 𝜆 , ∝ 𝑞 , for H-mode plasma.…”

“…Since the 2014 AUG campaign, significant improvements of the TS system have enabled good measurements of the SOL temperature and density profiles [3]. The Type І ELMy H-mode discharges under attached divertor condition (36 in total) and L-mode attached discharges (27 in total) used in the present dataset are the same discharges as those used in Ref [8]. In H-mode plasma, during ELMs the thermocurrents measured in the divertor are strongly increased, which indicates the onset of an ELM.…”

On the role of filaments in perpendicular heat transport at the scrape-off layer

“…The corresponding e-folding lengths are taken from TS data. According to recent L-mode measurements[41], this means λ Te 12 and 20 mm for the low and high collisionality cases, respectively. As can be div > 3 T e,fil /T e,back…”

Isotope dependence of the type I ELMy H-mode pedestal in JET-ILW hydrogen and deuterium plasmas

“…While so far we have investigated the pedestal stability utilising the common assumption of T e,sep = 100 eV (widely used in the literature [37,38,39,40,41,42,43]) for both H and D cases, we now assess from a qualitative point of view the effect of a potentially higher T e,sep in H on pedestal stability as suggested by the interpretative EDGE2D-EIRENE simulations presented in section 4. The simulations indicated that due to the high power and low density in the H pulse, the divertor may be in sheath limited regime when the T e,sep ∼ P 2/7 scaling breaks down and T e,sep could be much higher than 100 eV.…”