Effect of edge neutrals on the low-to-high confinement transition threshold in the DIII-D tokamak

Abstract

To study the effect of edge neutrals on the low-to-high confinement transition threshold, a broad range of plasma discharges has been analyzed. These discharges vary by gas puffing and pumping rates, position of the X point, and line-averaged density. It is shown that the determination of the neutral density (or neutral pressure) in the scrape-off layer (SOL) can give a misleading indication of the neutral population inside the separatrix. An increase of neutral density in the SOL creates an increase of plasma density that, in turn, increases the opacity to the neutrals and results in reduced neutral penetration. At a constant magnetic field, the transition power divided by the density appears to be a function of a single parameter measuring the neutrals effect. From this analysis, this parameter cannot be uniquely identified. For instance, it may be the radial decay length of the neutral profile or the charge-exchange damping rate at about r/a≈0.95. A similar correlation exists between these neutral parameters and local plasma parameters such as electron and ion temperature. This indicates that a missing parameter linked to the neutrals exists in the power threshold scaling laws.

Studies of EDA H-mode in Alcator C-Mod

“…This non-linearity has also been recognized recently by modeling done for DIII-D discharges. 4 Thus, in assessing the influence of neutrals, the wall neutral pressure measurement can lead to wrong conclusions. Shown in Fig.…”

Effect of ion-neutral charge exchange on radial electric fields in the edge plasma

“…Neutral particles have a strong influence on the particle, momentum and energy balance in the edge plasma through processes such as charge exchange and ionization, so there are many possible physical effects. Recent studies indicate that it is the neutral population inside the separatrix, rather than the SOL neutrals, that correlates with the H-mode transition, 3,5 and that the required threshold power scales with the ion-neutral frictional damping rate. 3 As this friction is typically dominated by ion-neutral charge exchange, in this paper we will use the terms "friction"…”

“…Recently, a number of experiments have been devoted to studying the effect of neutral particle influxes on the high-confinement mode (H-mode), [1][2][3][4][5] and it has been shown that the neutrals can significantly affect the transition from the low to high confinement modes (the L-H transition). The physical mechanism by which neutrals influence this transition has not yet been identified, and it is not known whether the same mechanism is operative in all experiments.…”

Developing Models for the DIII-D Boundary Plasma

“…Since the UEDGE and OEDGE codes are the primary tools used on the DIII-D device, this report will focus on the development of these codes. Other codes have been used for analysis of DIII-D boundary plasmas [4][5][6][7][8][9][10][11][12]. Space limitations, however, prevent detailed discussion of all code results, so we have chosen to focus on the results of the codes which are most frequently 4 used.…”