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

Abstract

Abstract.Experimental investigations carried out in the ASDEX Upgrade tokamak under various conditions demonstrate that the ion heat flux at the plasma edge plays a key role in the L-H transition physics, while the electron heat flux does not seem to play any role. This is due to the fact that the ion heat flux governs the radial electric field well induced by the main ions which is responsible for the turbulence stabilization causing the L-H transition. The experiments have been carried out in the low density branch of the power threshold where the electron and ion heat channels can be well separated. In plasmas heated by electron heating, the edge ion heat flux has been increased to reach the L-H transition by using separately three actuators: heating power, density and plasma current. In addition, the key role of the edge ion heating has been confirmed in experiments taking advantage of the direct ion heating provided by neutral beam injection. The role of the ion heat flux explains the non-monotonic density dependence of the L-H threshold power. Based on these results, a formula for the density of the threshold minimum has been developed, which also describes well the values found in tokamaks of various size. For ITER it predicts a value which is close to the density presently foreseen to enter the H-mode and indicates that operation at half field and current would benefit from a very significantly lower density minimum and correspondingly low threshold power.

The role of turbulence–flow interactions in L- to H-mode transition dynamics: recent progress

“…Malkov and Diamond observed that the calculated power threshold attains a minimum at low density and maximum ion heating [60], but increases at low density and increased electron heating. Thus, the calculated power threshold exhibits a similar trend to the experimentally observed increase of the power threshold at low density with preferential electron heating [100,101].…”

“…L-mode outer-core turbulent transport and the relevant critical temperature/density gradients determine the ion pres sure gradient inside and across the separatrix. In AUG experiments, ion and electron thermal radial transport have been decoupled in low-density ECH plasmas, and the upturn in the L-H power threshold at low density has been attributed to the additional power loss in the electron channel at high-separatrix electron temperature [99][100][101]. After accounting for the additional electron-channel power loss, the L-H power threshold was found to increase in proportion to the ion heat flux.…”

“…In addition to an ITG linear growth rate based on a simple critical-gradient model, TEM (trapped electron mode) turbulence driven by density and electron-temperature gradients was heuristically accounted for, neglecting a possible criticalgradient effect. This work was motivated by the non-monotonic dependence of the L-H transition power threshold on plasma density, and by recent experimental results from the AUG tokamak which suggest that the increasing power threshold at low density is related to increased electron thermal loss [99][100][101]. In these experiments, employing selective electron heating, ion and electron thermal transport are collisionally decoupled at low density.…”

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

“…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]. These findings show that the occurrence of the low density dependence of the power threshold is not a universal feature in various tokamaks.…”

Recent ASDEX Upgrade research in support of ITER and DEMO

“…Previous work on the low density branch of the P LH threshold [15] had already indicated that the decoupling of ion and electron channel is responsible for the observed increase of P LH when applying ECRH at low density. Recent transport analyses in the low density branch, [16], reveal that the edge ion heat flux at the LH transition (Q i,edge ) increases linearly with density, despite the non-monotonic behavior of P LH , FIG. 2 left panel.…”