V. Igochine

Fast-ion D-alpha measurements at ASDEX Upgrade

A fast-ion D-alpha (FIDA) diagnostic has been developed for the fully tungsten coated ASDEX Upgrade (AUG) tokamak using 25 toroidally viewing lines of sight and featuring a temporal resolution of 10 ms. The diagnostics toroidal geometry determines a well-defined region in velocity space which significantly overlaps with the typical fast-ion distribution in AUG plasmas. Background subtraction without beam modulation is possible because relevant parts of the FIDA spectra are free from impurity line contamination. Thus, the temporal evolution of the confined fast-ion distribution function can be monitored continuously. FIDA profiles during on- and off-axis neutral beam injection (NBI) heating are presented which show changes in the radial fast-ion distribution with the different NBI geometries. Good agreement has been obtained between measured and simulated FIDA radial profiles in MHD-quiescent plasmas using fast-ion distribution functions provided by TRANSP. In addition, a large fast-ion redistribution with a drop of about 50% in the central fast-ion population has been observed in the presence of a q = 2 sawtooth-like crash, demonstrating the capabilities of the diagnostic.

Interaction of energetic particles with large and small scale instabilities

Beyond a certain heating power, measured and predicted distributions of NBI driven currents deviate from each other, in a form that can be explained by the assumption of a modest diffusion of fast particles. Direct numerical simulation of fast test particles in a given field of electrostatic turbulence indicates that for reasonable parameters fast and thermal particle diffusion indeed are similar. High quality plasma edge plasma profiles on ASDEX Upgrade, used in the linear, gyrokinetic, global stability code LIGKA give excellent agreement with the eigenfunction measured by a newly extended reflectometry system for ICRH-excited TAE-modes. They support the hypothesis of TAE-frequency crossing of the continuum in the edge region as explanation of the high TAE-damping rates measured on JET.A new fast ion loss detector with 1MHz time resolution allows frequency and phase resolved correlation between low frequency magnetic perturbation, giving, together with modelling of the particle orbits, new insights into the mechanism of fast particle losses during NBI and ICRH due to helical perturbations.

Disruption studies in ASDEX Upgrade in view of ITER

Experiments on ASDEX Upgrade and other tokamaks have shown that the magnitude of mechanical forces and thermal loads during disruptions can be significantly reduced by raising the plasma density with massive injection of noble gases. This method should be applicable to ITER too. Nevertheless, the suppression of the runaway electron (RE) avalanche requires a much larger (two order of magnitude) density rise. This paper reports on recent experiments aimed at increasing the plasma density towards the critical value, needed for the collisional suppression of REs. An effective electron density equal to 24% of the critical density has been reached after injection of 3.3?bar?l of neon. However, the resultant large plasma density is very poloidally and toroidally asymmetric; this implies that several valves distributed around the plasma periphery become necessary at this level of massive gas injection to ensure a homogeneous density distribution.

Fast-ion transport induced by Alfvén eigenmodes in the ASDEX Upgrade tokamak

A comprehensive suite of diagnostics has allowed detailed measurements of the Alfven eigenmode (AE) spatial structure and subsequent fast-ion transport in the ASDEX Upgrade (AUG) tokamak [1]. Reversed shear Alfven eigenmodes (RSAEs) and toroidal induced Alfven eigenmodes (TAEs) have been driven unstable by fast ions from ICRH as well as NBI origin. In ICRF heated plasmas, diffusive and convective fast-ion losses induced by AEs have been characterized in fast-ion phase space. While single RSAEs and TAEs eject resonant fast ions in a convective process directly proportional to the fluctuation amplitude, δB/B, the overlapping of multiple RSAE and TAE spatial structures and wave–particle resonances leads to a large diffusive loss, scaling as (δB/B)2. In beam heated discharges, coherent fast-ion losses have been observed primarily due to TAEs. Core localized, low amplitude NBI driven RSAEs have not been observed to cause significant coherent fast-ion losses. The temporal evolution of the confined fast-ion profile in the presence of RSAEs and TAEs has been monitored with high spatial and temporal resolution. A large drop in the central fast-ion density due to many RSAEs has been observed as qmin passes through an integer. The AE radial and poloidal structures have been obtained with unprecedented details using a fast SXR as well as 1D and 2D ECE radiometers. GOURDON and HAGIS simulations have been performed to identify the orbit topology of the escaping ions and study the transport mechanisms. Both passing and trapped ions are strongly redistributed by AEs.

MHD induced Fast-Ion Losses on ASDEX Upgrade

A detailed knowledge of the interplay between MHD instabilities and energetic particles has been gained from direct measurements of fast-ion losses (FILs). Time-resolved energy and pitch angle measurements of FIL caused by neoclassical tearing modes (NTMs) and toroidicity-induced Alfven eigenmodes (TAEs) have been obtained using a scintillator based FIL detector. The study of FIL due to TAEs has revealed the existence of a new core-localized MHD fluctuation, the Sierpes mode. The Sierpes mode is a non-pure Alfvenic fluctuation which appears in the acoustic branch, dominating the transport of fast-ions in ICRF heated discharges. The internal structure of both TAEs and Sierpes mode has been reconstructed by means of highly resolved multichord soft x-ray measurements. A spatial overlapping of their eigenfunctions leads to a FIL coupling, showing the strong influence that a core-localized fast-ion driven MHD instability may have on the fast-ion transport. We have identified the FIL mechanisms due to NTMs as well as due to TAEs. Drift islands formed by fast-ions in particle phase space are responsible for the loss of NBI fast-ions due to NTMs. In ICRF heated plasmas, a resonance condition fulfilled by the characteristic trapped fast-ion orbit frequencies leads to a phase matching between fast-ion orbit and NTM or TAE magnetic fluctuation. The banana tips of a resonant trapped fast-ion bounce radially due to an E × B drift in the TAE case. The NTM radial bounce of the fast-ion banana tips is caused by the radial component of the perturbed magnetic field lines.

Fast-ion redistribution and loss due to edge perturbations in the ASDEX Upgrade, DIII-D and KSTAR tokamaks

The impact of edge localized modes (ELMs) and externally applied resonant and non-resonant magnetic perturbations (MPs) on fast-ion confinement/transport have been investigated in the ASDEX Upgrade (AUG), DIII-D and KSTAR tokamaks. Two phases with respect to the ELM cycle can be clearly distinguished in ELM-induced fast-ion losses. Inter-ELM losses are characterized by a coherent modulation of the plasma density around the separatrix while intra-ELM losses appear as well-defined bursts. In high collisionality plasmas with mitigated ELMs, externally applied MPs have little effect on kinetic profiles, including fast-ions, while a strong impact on kinetic profiles is observed in low-collisionality, low q95 plasmas with resonant and non-resonant MPs. In low-collisionality H-mode plasmas, the large fast-ion filaments observed during ELMs are replaced by a loss of fast-ions with a broad-band frequency and an amplitude of up to an order of magnitude higher than the neutral beam injection prompt loss signal without MPs. A clear synergy in the overall fast-ion transport is observed between MPs and neoclassical tearing modes. Measured fast-ion losses are typically on banana orbits that explore the entire pedestal/scrape-off layer. The fast-ion response to externally applied MPs presented here may be of general interest for the community to better understand the MP field penetration and overall plasma response.

Kinetic Alfvén eigenmodes at ASDEX Upgrade

On the way to a comprehensive understanding of the properties of a burning plasma the physics of super-thermal particles due to external heating and fusion reactions plays a key role. In particular, Alfven and Alfven-acoustic type instabilities are predicted to strongly interact with the fast particle population and to contribute critically to the radial redistribution of the energetic ions.This paper focuses on the comparison of the kinetic dispersion relation for BAEs/GAMs (Zonca et al 1996 Plasma Phys. Control. Fusion 38 2011) with numerical results obtained by the gyrokinetic eigenvalue code LIGKA (Lauber et al 2007 J. Comput. Phys. 226/1 447–65) and experimental findings at ASDEX Upgrade. It is shown that thermal ions with a finite perpendicular energy (circulating and trapped) modify the dispersion relation significantly for low frequencies. The resulting frequency downshift together with shaping and diamagnetic effects is crucial to explain the mode frequency as measured at ASDEX Upgrade stressing the importance of a kinetic description for frequencies comparable to the thermal ion transit frequency.In the second part the BAE-frequency behaviour during a sawtooth cycle is investigated and the possibility of an accurate q-profile determination via kinetic Alfven spectroscopy is discussed.

Overview of RFX-mod results

With the exploration of the MA plasma current regime in up to 0.5 s long discharges, RFX-mod has opened new and very promising perspectives for the reversed field pinch (RFP) magnetic configuration, and has made significant progress in understanding and improving confinement and in controlling plasma stability. A big leap with respect to previous knowledge and expectations on RFP physics and performance has been made by RFX-mod since the last 2006 IAEA Fusion Energy Conference. A new self-organized helical equilibrium has been experimentally achieved (the Single Helical Axis—SHAx—state), which is the preferred state at high current. Strong core electron transport barriers characterize this regime, with electron temperature gradients comparable to those achieved in tokamaks, and by a factor of 4 improvement in confinement time with respect to the standard RFP. RFX-mod is also providing leading edge results on real-time feedback control of MHD instabilities, of general interest for the fusion community.

Local effects of ECRH on argon transport in L-mode discharges at ASDEX Upgrade

The transport of argon as trace impurity has been investigated in electron cyclotron resonance heated L-mode discharges at ASDEX Upgrade to test recent theories predicting the rise of an outward impurity convection. The profiles of the argon transport coefficients for r/a < 0.65 have been determined by analysing the linear flux–gradient dependence of the total argon ion density evolution after the puff. A new methodology to experimentally obtain the total impurity ion density from the integrated use of two spectroscopic diagnostic and the 1D impurity transport code STRAHL has been developed. Results confirm the enhancement in diffusivity and the rise of the positive convection observed in previous studies, but show for the first time how these effects are strongly localized around the electron cyclotron resonance heating deposition radius. These experimental results are found to be in promising qualitative agreement with a set of quasi-linear gyrokinetic simulations with the code GS2.

Stochastic sawtooth reconnection in ASDEX Upgrade

In this paper we investigate non-complete sawtooth reconnection in the ASDEX Upgrade tokamak. Such reconnection phenomena are associated with internal m/n = 1/1 kink mode which does not vanish after the crash phase (as would be the case for complete reconnection). It is shown that this sawtooth cannot be fully described by pure m/n = 1/1 mode and that higher harmonics play an important role during the sawtooth crash phase. We employ the Hamiltonian formalism and reconstructed perturbations to model incomplete sawtooth reconnection. It is demonstrated that stochastization appears due to the excitation of low-order resonances which are present in the corresponding q-profiles inside the q = 1 surface which reflects the key role of the q0 value. Depending on this value two completely different situations are possible for one and the same mode perturbations: (i) the resonant surfaces are present in the q-profile leading to stochasticity and sawtooth crash (q0 ≈ 0.7 ± 0.1); (ii) the resonant surfaces are not present, which means no stochasticity in the system and no crash event (q0 ≈ 0.9 ± 0.05). Accordingly the central safety factor value is always less than unity in the case of a non-complete sawtooth reconnection. Our investigations show that the stochastic model agrees well with the experimental observations and can be proposed as a promising candidate for an explanation of the sawtooth reconnection.