Active Control of Alfvén Eigenmodes by Externally Applied 3D Magnetic Perturbations

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

Parameter space of low frequency inter-ELM modes

Introduction: The ELM cycle of type-I ELMs consists of different phases characterized by the evolution of kinetic profiles on different time scales [1] and distinct MHD and turbulence activity. In the latest phase of the ELM cycle, the pressure gradients are clamped. During this phase low [2], medium [3] and high frequency [4] MHD modes develop simultaneously in the steep gradient region. The high-frequency modes are located close to the minimum of the Er well, and are measured as fluctuations in the radial magnetic field on both, low and high field side [4]. The focus of the work presented here is the low-frequency modes. The low frequency\u3cbr/\u3emodes are measured only at the low field side and are located further inwards, towards the pedestal top [5]. They appear as fluctuations in the electron density, the electron temperature and as magnetic fluctuations. These modes rotate poloidally in the electron diamagnetic direction\u3cbr/\u3ewith the velocity of the background flow at that position [5]. The nature of this mode is not yet clear. In this work, we show the dependences of the low-frequency mode characteristics on some of the plasma and ELM parameters

Radiation transport modelling for the interpretation of oblique ECE measurements

The electron cyclotron emission (ECE) diagnostic provides routinely electron temperature (Te) measurements. At ASDEX Upgrade an electron cyclotron forward model, solving the radiation transport equation for given Te and electron density profile, is used in the framework of integrated data analysis. With this method Te profiles can be obtained from ECE measurements even for plasmas with low optical depth. However, due to the assumption of straight lines of sight and an absorption coefficient in the quasi-perpendicular approximation this forward model is not suitable for the interpretation of measurements by ECE diagnostics with an oblique line of sight. Since radiation transport modelling is required for the interpretation of oblique ECE diagnostics we present in this paper an extended forward model that supports oblique lines of sight. To account for the refraction of the line of sight, ray tracing in the cold plasma approximation was added to the model. Furthermore, an absorption coefficient valid for arbitrary propagation was implemented. Using the revised model it is shown that for the oblique ECE Imaging diagnostic at ASDEX Upgrade there can be a significant difference between the cold resonance position and the point from which most of the observed radiation originates

Quantification of X3 absorption for ITER L-mode parameters in ASDEX Upgrade

For an early H-mode access in hydrogen, ITER considers operating at 1/3 of the full field using 170 GHz X-Mode for heating at the 3rd harmonic. The optical thickness for such a heating scheme depends on Te2. It is rather low in the ohmic phase (with Te about 1-2 keV), but reaches high single pass absorption for the strongly EC heated plasma with Te exceeding 10 keV. Launching ECRH into an ohmic plasma may trigger a boot-strap process on Te if the additional power absorption due to increasing Te exceeds the additional power losses due to increased transport (which often tends to increase with input power). In this contribution we present measurements of the X3 absorption for the parameter range relevant for ITER, i.e. ne 2 1019 m−3, Te 2 keV in order to back up theoretical estimates used for the modeling so far. In ASDEX Upgrade (AUG) such low densities cannot be reached in H-mode such that dominant heating with NBI is not an option. For moderate Te, it is also not an option to use X3 heating as main heating, due to the excessive stray radiation threatening in-vessel components. This dilemma is solved with the 2-frequency EC system of AUG. The main central heating is done with the lower frequency of 105 GHz at the 2nd harmonic and full single pass absorption. Up to 3.5 MW of ECRH are used at that frequency to vary Te. Two other gyrotrons are used at 140 GHz to probe the X3 interaction close to the plasma center with a sequence of short blips. The expected values of single pass absorption are calculated with TORBEAM and vary from 7% to 70%. Below 40% single pass absorption the non-absorbed power triggers an arc in the tile gaps of the inner heat shield which screens the thermo-couples from the incoming beam such that they cannot be used. Between 40% and 80% single pass absorption, the predictions and measurements agree within the uncertainty of the measurement, unless we have clear evidence for non-linear interactions, which are not described by TORBEAM and which are not expected in ITER, but are due to some specific experimental choices for an isolated subset of our results

Investigation of the coupling properties of the ion cyclotron fast wave under applied magnetic perturbations and MHD phenomena in ASDEX Upgrade

The modulation of the ion cyclotron fast wave coupling to the plasma due to non-axisymmetric changes of the distance antenna-R-cutoff is studied. These changes can arise when magnetic perturbation (MP) fields are used, or when MHD activity is present. The application of MP fields can excite a low field side midplane plasma kink response that amplifies the vacuum perturbation field, leading to appreciable 3D plasma displacements. This effect is studied via NEMEC simulations. Rigid rotation of the MP field is found to produce a coherent antenna loading resistance modulation, suggesting an interplay between the non-axisymmetric magnetic field structure and the wave coupling properties. MHD modes are shown to introduce similar loading resistance oscillations, coherent with the mode rotation frequency. The case of a (2,1) mode is presented

Investigation of the coupling properties of the ion cyclotron fast wave under applied magnetic perturbations and MHD phenomena in ASDEX Upgrade

The modulation of the ion cyclotron fast wave coupling to the plasma due to non-axisymmetric changes of the distance antenna-R-cutoff is studied. These changes can arise when magnetic perturbation (MP) fields are used, or when MHD activity is present. The application of MP fields can excite a low field side midplane plasma kink response that amplifies the vacuum perturbation field, leading to appreciable 3D plasma displacements. This effect is studied via NEMEC simulations. Rigid rotation of the MP field is found to produce a coherent antenna loading resistance modulation, suggesting an interplay between the non-axisymmetric magnetic field structure and the wave coupling properties. MHD modes are shown to introduce similar loading resistance oscillations, coherent with the mode rotation frequency. The case of a (2,1) mode is presented

Non-linear modeling of the threshold between ELM mitigation and ELM suppression by resonant magnetic perturbations in ASDEX upgrade

\u3cp\u3eThe interaction between Edge-Localized Modes (ELMs) and Resonant Magnetic Perturbations (RMPs) is modeled with the magnetohydrodynamic code JOREK using experimental parameters from ASDEX Upgrade discharges. According to the modeling, the ELM mitigation or suppression is optimal when the amplification of both tearing and peeling-kink responses results in a better RMP penetration. The ELM mitigation or suppression is not only due to the reduction of the pressure gradient but predominantly arises from the toroidal coupling between the ELMs and the RMP-induced mode at the plasma edge, forcing the edge modes to saturate at a low level. The bifurcation from ELM mitigation to ELM suppression is observed when the RMP amplitude is increased. ELM mitigation is characterized by rotating modes at the edge, while the mode locking to RMPs is induced by the resonant braking of the electron perpendicular flow in the ELM suppression regime.\u3c/p\u3

Electron temperature fluctuation levels of the quasi-coherent mode across the plasma radius

EDA H-mode is an ELM-free regime in which the edge quasi-coherent mode (QCM) replaces the ELMs. The estimated location of the quasi-coherent mode is in a partly optically thin region of steep gradients localized between ρpol = 0.96 -1. Relative fluctuations of radiation temperature between 15 and 80 kHz are about 7% with significant density contribution. In the electron cyclotron emission (ECE) channels with resonances in the plasma core, a mode with the same frequency as the quasi-coherent mode is measured. The peak amplitude of both core and edge modes matches the strongest electron temperature gradient in the core and the edge, respectively. The ECE core and edge signals are out of phase. The radiation transport forward model (ECRad) shows that the refraction explains the phase relation between the edge and the core ECE channels. The phase correlates with the sign of the core Te. The amplitude of the fluctuations in the core decreases with decreasing gradients, which is the trend seen in the experiment. The amplitude ratio of the core and edge fluctuation is a factor of five in the experiment; this ratio remains a factor of a hundred in the modeling

Investigation of the coupling properties of the ion cyclotron fast wave under applied magnetic perturbations and MHD phenomena in ASDEX Upgrade

The modulation of the ion cyclotron fast wave coupling to the plasma due to non-axisymmetric changes of the distance antenna-R-cutoff is studied. These changes can arise when magnetic perturbation (MP) fields are used, or when MHD activity is present. The application of MP fields can excite a low field side midplane plasma kink response that amplifies the vacuum perturbation field, leading to appreciable 3D plasma displacements. This effect is studied via NEMEC simulations. Rigid rotation of the MP field is found to produce a coherent antenna loading resistance modulation, suggesting an interplay between the non-axisymmetric magnetic field structure and the wave coupling properties. MHD modes are shown to introduce similar loading resistance oscillations, coherent with the mode rotation frequency. The case of a (2,1) mode is presented