L–H power threshold studies in JET with Be/W and C wall

Abstract

A comparison of the L–H power threshold (Pthr) in JET with all carbon, JET-C, and beryllium/tungsten wall (the ITER-like choice), JET-ILW, has been carried out in experiments with slow input power ramps and matched plasma shapes, divertor configuration and IP/BT pairs. The low density dependence of the L–H power threshold, namely an increase below a minimum density ne,min, which was first observed in JET with the MkII-GB divertor and C wall and subsequently not observed with the current MkII-HD geometry, is observed again with JET-ILW. At plasma densities above ne,min, Pthr is reduced by ∼30%, and by ∼40% when the radiation from the bulk plasma is subtracted (Psep), with JET-ILW compared to JET-C. At the L–H transition the electron temperature at the edge, where the pedestal later develops, is also lower with JET-ILW, for a given edge density. With JET-ILW the minimum density is found to increase roughly linearly with magnetic field, , while the power threshold at the minimum density scales as . The H-mode power threshold in JET-ILW is found to be sensitive both to variations in main plasma shape (Psep decreases with increasing lower triangularity and increases with upper triangularity) and in divertor configuration. When the data are recast in terms of Psep and Zeff or subdivertor neutral pressure a linear correlation is found, pointing to a possible role of Zeff and/or subdivertor neutral pressure in the L–H transition physics. Depending on the chosen divertor configuration, Pthr can be up to a factor of two lower than the ITPA scaling law for densities above ne,min. A shallow edge radial electric field well is observed at the L–H transition. The edge impurity ion poloidal velocity remains low, close to its L-mode values, ⩽5 km s−1 ± 2–3 km s−1, at the L–H transition and throughout the H-mode phase, with no measureable increase within the experimental uncertainties. The edge toroidal rotation profile does not contribute to the depth of the negative Er well and thus may not be correlated with the formation of the edge transport barrier in JET.

Experimental evidence for the key role of the ion heat channel in the physics of the L–H transition

“…We also underline that, for ASDEX Upgrade, the difference between the carbon and tungsten walls is small. It is worth noticing that the B 4/5 T of n e,min reported for JET in [22] is in good agreement with equation 3, explained by the fact that in these JET experiments I p was varied almost proportionally to B T . Our formula also explains that Ohmic H-modes could be achieved in ASDEX Upgrade with B T = 1.1 T at densities as low as n e,min ≈ 2.2 × 10 19 m −3 : the low B T value does not only yield a low P L−H through B 0.…”

“…It is worth noticing that the I p dependence is in agreement with the variation of n e,min yielded by our data. This formula can be applied to data from various devices: the recent results from Alcator C-Mod in [21] and JET with the ITER-Like Wall in [22], as well as those deduced from the ITPA threshold database and reported in [18] for DIII-D, JFT-2M and JT-60U. The comparison between the experimental and predicted values of n e,min is illustrated in figure 10.…”

“…the upwards deviation from then e 0.72 dependence of the scaling, is observed in most of the tokamaks, but with variable strength [24,25,26,19,21,20,22]. The effect is particularly strong in plasmas heated with electron cyclotron resonant heating (ECRH), [24,19], suggesting that this might be due to the decoupling between the electron and ion heat channels, but this has, so far, not been demonstrated experimentally.…”

Study on the L–H transition power threshold with RF heating and lithium-wall coating on EAST

“…The physics mechanisms underlying the low density dependence of the power threshold remain largely unknown. Recent results from JET with a Be/W and C wall show a correlation of the density roll-over to divertor geometry and divertor/wall materials [8]. On the other hand, experiments from ASDEX Upgrade demonstrate the key role played by the edge ion heat flux as an explanation for the strong increase in the L-H power threshold at low density with electron cyclotron resonant heating (ECRH) as the auxiliary heating power [33].…”

Phenomenological Classification of Turbulence States in Linear Magnetized Plasma PANTA

“…Although no ultimate way has been ever established for clarifying the nonlinear system, parameter scan experiments have succeeded to describe the nonlinear nature of the confined plasma. For example, the H-mode power threshold study is often performed by scanning the line averaged density and the input heating power [1][2][3].…”