ABSTRACT
The suppression and excitation of Alfvén eigenmodes have been experimentally obtained, for the first time, by means of externally applied 3D perturbative fields with different spatial spectra in a tokamak plasma. The applied perturbation causes an internal fast-ion redistribution that modifies the phase-space gradients responsible for driving the modes, determining, ultimately their existence. Hybrid kinetic-magnetohydrodynamic simulations reveal an edge resonant transport layer activated by the 3D perturbative field as the responsible mechanism for the fast-ion redistribution. The results presented here may help to control fast-ion driven Alfvénic instabilities in future burning plasmas with a significant fusion born alpha particle population.
ABSTRACT
The acceleration of beam ions during edge localized modes (ELMs) in a tokamak is observed for the first time through direct measurements of fast-ion losses in low collisionality plasmas. The accelerated beam-ion population exhibits well-localized velocity-space structures which are revealed by means of tomographic inversion of the measurement, showing energy gains of the order of tens of keV. This suggests that the ion acceleration results from a resonant interaction between the beam ions and parallel electric fields arising during the ELM. Orbit simulations are carried out to identify the mode-particle resonances responsible for the energy gain in the particle phase space. The observation motivates the incorporation of a kinetic description of fast particles in ELM models and may contribute to a better understanding of the mechanisms responsible for particle acceleration, ubiquitous in astrophysical and space plasmas.
ABSTRACT
Field-line localized ballooning modes have been observed at the edge of high confinement mode plasmas in ASDEX Upgrade with rotating 3D perturbations induced by an externally applied n=2 error field and during a moderate level of edge localized mode mitigation. The observed ballooning modes are localized to the field lines which experience one of the two zero crossings of the radial flux surface displacement during one rotation period. The localization of the ballooning modes agrees very well with the localization of the largest growth rates from infinite-n ideal ballooning stability calculations using a realistic 3D ideal magnetohydrodynamic equilibrium. This analysis predicts a lower stability with respect to the axisymmetric case. The primary mechanism for the local lower stability is the 3D distortion of the local magnetic shear.
ABSTRACT
In a major upgrade, the (2D) electron cyclotron emission imaging diagnostic (ECEI) at ASDEX Upgrade has been equipped with a second detector array, observing a different toroidal position in the plasma, to enable quasi-3D measurements of the electron temperature. The new system will measure a total of 288 channels, in two 2D arrays, toroidally separated by 40 cm. The two detector arrays observe the plasma through the same vacuum window, both under a slight toroidal angle. The majority of the field lines are observed by both arrays simultaneously, thereby enabling a direct measurement of the 3D properties of plasma instabilities like edge localized mode filaments.
ABSTRACT
First experiments with nonaxisymmetric magnetic perturbations, toroidal mode number n=2, produced by newly installed in-vessel saddle coils in the ASDEX Upgrade tokamak show significant reduction of plasma energy loss and peak divertor power load associated with type-I edge localized modes (ELMs) in high-confinement mode plasmas. ELM mitigation is observed above an edge density threshold and is obtained both with magnetic perturbations that are resonant and not resonant with the edge safety factor profile. Compared with unperturbed type-I ELMy reference plasmas, plasmas with mitigated ELMs show similar confinement, similar plasma density, and lower tungsten impurity concentration.
ABSTRACT
The newly installed electron cyclotron emission imaging diagnostic on ASDEX Upgrade provides measurements of the 2D electron temperature dynamics with high spatial and temporal resolution. An overview of the technical and experimental properties of the system is presented. These properties are illustrated by the measurements of the edge localized mode and the reversed shear Alfvén eigenmode, showing both the advantage of having a two-dimensional (2D) measurement, as well as some of the limitations of electron cyclotron emission measurements. Furthermore, the application of singular value decomposition as a powerful tool for analyzing and filtering 2D data is presented.
ABSTRACT
Trapped electron modes are one of the candidates to explain turbulence driven electron heat transport observed in tokamaks. This instability has two characteristics: a threshold in normalized gradient and stabilization by collisions. Experiments using modulated electron cyclotron heating in the ASDEX Upgrade tokamak demonstrate explicitly the existence of the threshold. The stabilization with increasing collisionality is evidenced by a strong decrease of the propagation of heat pulses, explained by a transition to ion temperature gradient driven transport. These results are supported by linear gyrokinetic calculations.
ABSTRACT
The electron heat transport is investigated in ASDEX Upgrade conventional L-mode plasmas with pure electron heating provided by electron-cyclotron heating (ECH) at low density. Under these conditions, steady-state and ECH modulation experiments indicate without ambiguity that electron heat transport exhibits a clear threshold in inverted Delta T(e)/T(e) and also suggest that it has a gyro-Bohm character.
ABSTRACT
Energy transport by the electrons in a tokamak is examined in steady-state and power modulation experiments using electron cyclotron heating. The results are consistent with the assumption that temperature profiles are limited by a critical gradient length, leading to "stiff" profiles. The modulation experiments show that the stiffness factor increases with temperature. They strongly suggest that turbulence driven by the electron temperature gradient may be a dominant mechanism of electron transport. Although possibly not universal, these results are valid under various plasma conditions.
ABSTRACT
Internal transport barriers have been demonstrated to exist also under conditions with T(e) approximately T(i) approximately 10 keV and predominant electron heating of the tokamak core region. Central electron cyclotron heating was added to neutral beam injection-heated ASDEX Upgrade discharges with a preexisting internal transport barrier, established through programmed current ramping leading to shear reversal. Compared to a reference internal transport barrier discharge without electron cyclotron resonance heating, the electron heat conductivity in the barrier region was found not to increase, in spite of a fivefold increase in electron heat flux, and also angular momentum and ion energy transport did not deteriorate.
