Contribution of ASDEX Upgrade to disruption studies for ITER

Abstract

This paper describes the most recent contributions of ASDEX Upgrade to ITER in the field of disruption studies. (1) The ITER specifications for the halo current magnitude are based on data collected from several tokamaks and summarized in the plot of the toroidal peaking factor versus the maximum halo current fraction. Even if the maximum halo current in ASDEX Upgrade reaches 50% of the plasma current, the duration of this maximum lasts a fraction of a ms. (2) Long-lasting asymmetries of the halo current are rare and do not give rise to a large asymmetric component of the mechanical forces on the machine. Differently from JET, these asymmetries are neither locked nor exhibit a stationary harmonic structure. (3) Recent work on disruption prediction has concentrated on the search for a simple function of the most relevant plasma parameters, which is able to discriminate between the safe and pre-disruption phases of a discharge. For this purpose, the disruptions of the last four years have been classified into groups and then discriminant analysis is used to select the most significant variables and to derive the discriminant function. (4) The attainment of the critical density for the collisional suppression of the runaway electrons seems to be technically and physically possible on our medium size tokamak. The CO2 interferometer and the AXUV diagnostic provide information on the highly 3D impurity transport process during the whole plasma quench.

Theory of tokamak disruptions

“…In the ASDEX Upgrade device, an axisymmetric halo current can be required to prevent Alfvénic growth during the late period of a vertical displacement. 67 The toroidal peaking factor is highly dependent on the device in which it is measured, Figure 2. The physical layout of the conducting structures that surround the plasma appears to make a difference; this difference should be calculable.…”

“…A method of mitigating the damage done by these forces is the injection of a large quantity of a radiating impurity, such as argon, into a disrupting plasma to greatly lower the electron temperature to T e $ 10 eV and shorten the time of the current quench. 66,67 On DIII-D, the early injection of pellet of a radiating impurity both reduces the strength of the halo current and makes it more axisymmetric. 66 If radiating impurities were injected at too high a level into ITER, the plasma could terminate so rapidly that melting would occur on the first wall.…”

Three-dimensional non-linear magnetohydrodynamic modeling of massive gas injection triggered disruptions in JET

“…MGI aims at spreading heat loads by radiating most of the plasma stored energy, preventing the generation of REs by increasing the electron density and controlling the CQ duration (in order to limit mechanical loads) by controlling the impurity content and thus the temperature and resistivity of the CQ plasma. A large body of experimental work on MGI exists, including experiments on JET, [6][7][8][9] ASDEX Upgrade, 10 Tore Supra, 11 DIII-D, 12 TEXTOR, 13 Alcator C-MOD, 14 and other devices. Results have shown the capability of MGI to fulfill part of the objectives of the ITER DMS, but not yet all of them.…”

Characterization of disruption halo currents in the National Spherical Torus Experiment

“…It is the purpose of this paper, however, to study a fourth major disruption consequence: the phenomenon of "halo currents" [2,3,[22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40]. The tokamak concept generally optimizes toward large elongation of the plasma cross-section, essentially due to the additional safety factor that such plasma boundary shaping provides [41][42][43][44].…”

“…Halo currents have been characterized in a large number of conventional tokamak devices, including ALCATOR C-Mod [28], ASDEX-Upgrade [30,37,38], COMPASS-D [29], DIII-D [4,22,23,26,36], JET [24,31,32,33,35], and JT-60U [27]. These devices have generally shown large halo currents, with worst-case events having up to 50% of the predisruption plasma currents flowing into the vessel and plasma facing components [2].…”