25th IAEA Fusion Energy Conference - IAEA CN-221

Density Limit Studies in the Tokamak and the Reversed-Field Pinch

14 Oct 2014, 08:30

4h

Green 8-9 (Hotel Park Inn Pribaltiyskaya)

Poster Poster 1

Speaker

Dr Gianluca Spizzo (Consorzio RFX, Associazione EURATOM-ENEA sulla Fusione, Padova, Italy)

Description

Both in the tokamak and the reversed-field pinch (RFP), new finds show that the high density limit, which often disrupts tokamak discharges and slowly terminates RFP ones, is not governed by a unique, theoretically well-determined physical phenomenon, but by a combination of complicated mechanisms involving two-fluid effects, electrostatic plasma response to magnetic islands and plasma-wall interaction. In this paper we will show that the description in terms of the unique "Greenwald density" nG = Ip/pi a^2 should be reinterpreted in terms of edge critical density, and related to the amplitude of the equilibrium magnetic field, the resonance of islands next to the edge, and input power. Recent results in FTU point out that in discharges with a variable density peaking the line-averaged central density scales as n0 ~ B^1.5, which is a scaling with the magnetic field. The usual Greenwald-like scaling nedge = 0.35 nG holds for the edge density. The density limit depends also on the input power: recent experiments in the RFX-mod RFP with a lithized wall show that the central density increases linearly with the ohmic input power and that larger densities can be accessed, for the same input power, with better wall conditioning. An important point to raise is the role of the thermal instabilities in setting the environment for the development of the density limit: in both machines, FTU and RFX, the density limit is associated with the appearance of the multifaceted asymmetric radiation from the edge (MARFE), which is triggered by MHD activity (m/n=2/1 in FTU and 0/1 in RFX). In the RFX case, the MARFE is also linked to a well-defined flow pattern. In fact, the 0/1 island, which resonates at q = 0 in the RFP edge (r/a = 0.9), is destabilized at high density, and generates an electrostatic response in the form of a convective cell, with the same 0/1 symmetry. The toroidal flow reverses direction along the toroidal angle, with the formation of two null points of v_phi (or, equivalently, radial electric field Er): a source and a stagnation point, with the latter corresponding to the toroidally localized MARFE. The association between flow patterns and MARFE can be tested in FTU, by investigating the effect of ERCH on the MARFE and the density peaking. Initial results, on FTU as well as in ASDEX, indeed show a dependence of the disruption phenomenology on the ECRH.

Presentation materials

Poster

poster_spizzo142.pdf

summary

slide_exp1-42_v1.pdf