Comparison of H-mode barrier width with a model of neutral penetration length

Abstract

Pedestal studies in DIII-D find that the width of the region of steep gradient in the H-mode density is comparable with the neutral penetration length, as computed from a simple analytic model. This model has analytic solutions for the edge plasma and neutral density profiles, which are obtained from the coupled particle continuity equations for electrons and deuterium atoms. In its range of validity (edge temperature between 40 and 500 eV), the analytic model quantitatively predicts the observed decrease in the width as the pedestal density increases and the observed strong increase in the gradient of the density as the pedestal density increases. The model successfully predicts that L-mode and H-mode profiles with the same pedestal density have gradients that differ by less than a factor of 2. The width of the density barrier, measured from the edge of the electron temperature barrier, is the lower limit for the observed width of the temperature barrier. These results support the hypothesis that particle fuelling is an important part of the physics that determines the structure of the H-mode transport barrier.

Advances in measurement and modeling of the high-confinement-mode pedestal on the Alcator C-Mod tokamak

“…Employing active puffing and pumping of H-modes, a wide range of pedestal densities has been reported, with trends in width and gradient fit quite well by the simple fluid model. [35] As shown in Sec. V, puffing into typical C-Mod H-modes has little effect on n e,PED or ∆ ne .…”

“…It is customary to take for v D a value consistent with the expected thermal velocity of the neutral species as they pass into the plasma. This assumption, combined with reasonable assumptions for the plasma D eff profile and LCFS boundary conditions, has been shown [35] [36] to…”

20 years of research on the Alcator C-Mod tokamak

“…169 Later similarity studies conducted in ELMy H-modes showed evidence of a mismatch in density pedestals, with the DIII-D pedestal being wider in flux space. 170 These matches required DIII-D to operate at its lowest feasible H-mode densities, in a regime known to show a dependence of the pedestal width on neutral penetration 171 that is not seen on C-Mod, suggesting that the neutral penetration range can have an effect on sufficiently transparent pedestals. 172 However, C-Mod has the highest neutral opacity of any operating tokamak and should be more prototypical of ITER/ Reactor conditions.…”

H-mode pedestal and threshold studies over an expanded operating space on Alcator C-Mod

“…To summarize these results, n e and T e pedestal widths measured using high resolution Edge Thomson Scattering (ETS) 12 are extremely narrow, typically 2-5 mm, and generally do not show systematic scalings with B T , I p , or pedestal density, n ped 13,14 , in contrast to some results elsewhere 15,16 . There are some exceptions to this rule.…”