NTM induced fast ion losses in ASDEX Upgrade

Abstract

The loss of fast (i.e. suprathermal) ions from a magnetically confined fusion plasma due to the interaction with magnetohydrodynamic instabilities has been experimentally characterized and interpreted by means of a numerical model. It is found that for a special class of instabilities, the so-called Neoclassical Tearing Modes, fast ions losses are increased and modulated at the same frequency of the mode. This new experimental finding is explained as a result of the drift islands formed by energetic ions in particle phase space. An eventual overlapping of these drift islands leads to an orbit stochasticity and therefore to an enhancement of the fast ion losses. This explanation is confirmed by statistical analysis of simulations of fast ions trajectories performed with the ORBIT code. The mechanism is of general importance for understanding the interaction between MHD modes and fast particles in magnetic confinement experiments. A significant fraction of plasma pressure in a magnetized fusion experiment is carried by suprathermal (fast) ions, which are produced either by fusion reactions (like α particles), injected through energetic beams, or by RF heating. In general, these fast particles play an important role in the energy balance of a fusion plasma, either for the heating and/or for current drive processes [1], [2] and [3]. The confinement of fast particles is therefore an issue of great importance, since significant losses of these ions may drastically reduce the heating as well as the current drive efficiency. In addition, an intense and localized loss of fast ions may cause damage to plasma facing components. Due to their high energy, the dynamics of fast ions in a magnetized plasma is rather different than that of thermal ions and, in many aspects, still experimentally unexplored. Several issues are still open, for example, about the interplay between a population of fast particles and a key player of magnetized fusion plasmas, like the Magnetohydrodynamic (MHD) instabilities.In this Letter we present the first measurements of fast ion losses with simultaneous high time, energy and, pitch angle resolution due to Neoclassical Tearing Modes (NTMs). We explain the measurements on the basis of drift islands and their overlap, which leads to orbit stochasticity. The main experimental phenomenology is in fact reproduced by a model simulating the guiding center orbits of fast ions. The results reported here are important for next-step fusion devices like ITER where a significant population of α-particles and of NBI and ICRH generated fast ions will be present.NTMs are metastable modes driven by the missing bootstrap current within a preexisting seed magnetic is- * Electronic address: Manuel.Garcia-Munoz@ipp.mpg.de land, provided that the plasma poloidal beta, β pol , is larger than a threshold value [4]. When a NTM grows in the plasma, global confinement is severely affected. NTMs set the limit to the maximum β pol achievable in conventional scenarios. While the NTM impact on the global confinement is rather w...

Calculation of the Non-Inductive Current Profile in High-Performance NSTX Plasmas

“…These calculations have often assumed classical physics for the beam ions, though there are cases where this assumption may not be warranted. For instance, tearing modes have been shown to modify the neutral beam current profile [37,42], consistent with observations of fast ion loss due to these instabilities [61,62]. Alfven modes have also been shown to modify the current profile [63,64].…”

Combination of fast-ion diagnostics in velocity-space tomographies

“…ASDEX Upgrade is also equipped with a suite of fast-ion diagnostics: fast-ion loss detectors (FILDs) [4][5][6], fast-ion D α (FIDA) [7], collective Thomson scattering (CTS) [8][9][10][11][12][13], neutron spectrometry [14,15], neutral particle analysers (NPA) [16,17] and γ -ray spectrometry [18]. These auxiliary heating systems and fast-ion diagnostics give unique opportunities to study fast ions in tokamak plasmas.…”

“…Velocityspace tomography is less developed [13,33,34] but could be particularly useful in studies of selective ejection or redistribution in velocity space. Several types of modes affect ions in only part of velocity space, for example sawteeth [35][36][37][38], Alfvén eigenmodes [6,[39][40][41][42][43] and neoclassical tearing modes [4,5]. Turbulent transport of fast ions also depends on the ion energy [44][45][46][47].…”

Investigation Of A Transient Energetic Charge Exchange Fux Enhancement ('spike-on-tail') Observed In Neutral-beam-heated H-mode Discharges In The National Spherical Torus Experiment

“…The TAEs can appear either as saturated modes or as bursting modes and the bursting TAEs observed in NSTX [10][11][12] can cause significant fast ion losses. In addition to Alfvénic modes, energetic ion redistribution and/or loss associated with low frequency NTM or kink-type MHD activity has been observed in many fusion experiments, for example, NSTX [13,14] and ASDEX-U [15,16].…”