[Show abstract][Hide abstract] ABSTRACT: In many discharges at ASDEX Upgrade fast particle losses can be observed due
to Alfv\'enic gap modes, Reversed Shear Alfv\'en Eigenmodes or core-localized
Beta Alfv\'en Eigenmodes. For the first time, simulations of experimental
conditions in the ASDEX Upgrade fusion device are performed for different
plasma equilibria (particularly for different, also non-monotonic q profiles).
The numerical tool is the extended version of the HAGIS code [Pinches'98,
Br\"udgam PhD Thesis, 2010], which also computes the particle motion in the
vacuum region between vessel wall in addition to the internal plasma volume.
For this work, a consistent fast particle distribution function was implemented
to represent the strongly anisotropic fast particle population as generated by
ICRH minority heating. Furthermore, HAGIS was extended to use more realistic
eigenfunctions, calculated by the gyrokinetic eigenvalue solver LIGKA
[Lauber'07]. The main aim of these simulations is to allow fast ion loss
measurements to be interpreted with a theoretical basis. Fast particle losses
are modeled and directly compared with experimental measurements
[Garc\'ia-Mu\~noz'10]. The phase space distribution and the mode-correlation
signature of the fast particle losses allows them to be characterized as
prompt, resonant or diffusive (non-resonant). The experimental findings are
reproduced numerically. It is found that a large number of diffuse losses occur
in the lower energy range (at around 1/3 of the birth energy) particularly in
multiple mode scenarios (with different mode frequencies), due to a phase space
overlap of resonances leading to a so-called domino [Berk'95] transport
process. In inverted q profile equilibria, the combination of radially extended
global modes and large particle orbits leads to losses with energies down to
1/10th of the birth energy.
[Show abstract][Hide abstract] ABSTRACT: The medium size divertor tokamak ASDEX Upgrade (major and minor radii 1.65 m and 0.5 m, respectively, magnetic-field strength 2.5 T) possesses flexible shaping and versatile heating and current drive systems. Recently the technical capabilities were extended by increasing the electron cyclotron resonance heating (ECRH) power, by installing 2 × 8 internal magnetic perturbation coils, and by improving the ion cyclotron range of frequency compatibility with the tungsten wall. With the perturbation coils, reliable suppression of large type-I edge localized modes (ELMs) could be demonstrated in a wide operational window, which opens up above a critical plasma pedestal density. The pellet fuelling efficiency was observed to increase which gives access to H-mode discharges with peaked density profiles at line densities clearly exceeding the empirical Greenwald limit. Owing to the increased ECRH power of 4 MW, H-mode discharges could be studied in regimes with dominant electron heating and low plasma rotation velocities, i.e. under conditions particularly relevant for ITER. The ion-pressure gradient and the neoclassical radial electric field emerge as key parameters for the transition. Using the total simultaneously available heating power of 23 MW, high performance discharges have been carried out where feed-back controlled radiative cooling in the core and the divertor allowed the divertor peak power loads to be maintained below 5 MW m−2. Under attached divertor conditions, a multi-device scaling expression for the power-decay length was obtained which is independent of major radius and decreases with magnetic field resulting in a decay length of 1 mm for ITER. At higher densities and under partially detached conditions, however, a broadening of the decay length is observed. In discharges with density ramps up to the density limit, the divertor plasma shows a complex behaviour with a localized high-density region in the inner divertor before the outer divertor detaches. Turbulent transport is studied in the core and the scrape-off layer (SOL). Discharges over a wide parameter range exhibit a close link between core momentum and density transport. Consistent with gyro-kinetic calculations, the density gradient at half plasma radius determines the momentum transport through residual stress and thus the central toroidal rotation. In the SOL a close comparison of probe data with a gyro-fluid code showed excellent agreement and points to the dominance of drift waves. Intermittent structures from ELMs and from turbulence are shown to have high ion temperatures even at large distances outside the separatrix.
[Show abstract][Hide abstract] ABSTRACT: Magnetohydrodynamic instabilities like Toroidal Alfv\'en Eigenmodes or
core-localized modes such as Beta Induced Alfv\'en Eigenmodes and Reversed
Shear Alfv\'en Eigenmodes driven by fast particles can lead to significant
redistribution and losses in fusion devices. This is observed in many ASDEX
Upgrade discharges. The present work aims to understand the underlying
resonance mechanisms, especially in the presence of multiple modes with
different frequencies. Resonant mode coupling mechanisms are investigated using
the drift kinetic HAGIS code [Pinches 1998]. Simulations were performed for
different plasma equilibria, in particular for different q profiles, employing
the availability of improved experimental data. A study was carried out,
investigating double-resonant mode coupling with respect to various overlapping
scenarios. It was found that, depending on the radial mode distance,
double-resonance is able to enhance growth rates as well as mode amplitudes
significantly. Small radial mode distances, however can also lead to strong
nonlinear mode stabilization of a linear dominant mode. With the extended
version of HAGIS, losses were simulated and directly compared with experimental
loss measurements. The losses' phase space distribution as well as their
ejection signal is consistent with experimental data. Furthermore, it allowed
to characterize them as prompt, resonant or stochastic. It was found that
especially in multiple mode scenarios (with different mode frequencies),
abundant incoherent losses occur in the lower energy range, due to a broad
phase-space stochastization. The incoherent higher energetic losses are
"prompt", i.e. their initial energy is too large for confined orbits.
Journal of Physics Conference Series 11/2012; 401(1).
[Show abstract][Hide abstract] ABSTRACT: In future fusion devices fast particles must be well confined in order to
transfer their energy to the background plasma. Magnetohydrodynamic
instabilities like Toroidal Alfv\'en Eigenmodes or core-localized modes such as
Beta Induced Alfv\'en Eigenmodes and Reversed Shear Alfv\'en Eigenmodes, both
driven by fast particles, can lead to significant losses. This is observed in
many ASDEX Upgrade discharges. The present study applies the drift-kinetic
HAGIS code with the aim of understanding the underlying resonance mechanisms,
especially in the presence of multiple modes with different frequencies. Of
particular interest is the resonant interaction of particles simultaneously
with two different modes, referred to as 'double-resonance'. Various mode
overlapping scenarios with different q profiles are considered. It is found
that, depending on the radial mode distance, double-resonance is able to
enhance growth rates as well as mode amplitudes significantly. Surprisingly, no
radial mode overlap is necessary for this effect. Quite the contrary is found:
small radial mode distances can lead to strong nonlinear mode stabilization of
a linearly dominant mode.
[Show abstract][Hide abstract] ABSTRACT: The phase-space of convective and diffusive fast-ion losses induced by shear Alfvén eigenmodes has been characterized in the ASDEX Upgrade tokamak. Time-resolved energy and pitch-angle measurements of fast-ion losses correlated in frequency and phase with toroidal Alfvén eigenmodes (TAEs) and Alfvén cascades (ACs) have allowed to identify both loss mechanisms. While single ACs and TAEs eject resonant fast-ions in a convective process, the overlapping of AC and TAE spatial structures leads to a large fast-ion diffusion and loss. The threshold for diffusive fast-ion losses depends on the ion energy (gyroradius). Diffusive fast-ion losses with gyroradius ≈70 mm have been observed with a single TAE for local radial displacements of the magnetic field lines larger than ≈2 mm. Multiple frequency chirping ACs cause an enhancement of the diffusive losses. The ACs and TAEs radial structures have been reconstructed by means of cross-correlation techniques between the fast-ion loss detector and the electron cyclotron emission radiometer.
[Show abstract][Hide abstract] ABSTRACT: We present here the first phase-space characterization of convective and diffusive energetic particle losses induced by shear Alfvén waves in a magnetically confined fusion plasma. While single toroidal Alfvén eigenmodes (TAE) and Alfvén cascades (AC) eject resonant fast ions in a convective process, an overlapping of AC and TAE spatial structures leads to a large fast-ion diffusion and loss. Diffusive fast-ion losses have been observed with a single TAE above a certain threshold in the fluctuation amplitude.
[Show abstract][Hide abstract] ABSTRACT: 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, Alfvén and Alfvén-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 Alfvén spectroscopy is discussed.
Plasma Physics and Controlled Fusion 11/2009; · 2.37 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The closed ASDEX Upgrade Divertor II, `LYRA', is capable of handling heating powers of up to 20 MW or P/R of 12 MW/m, owing to a reduction of the maximum heat flux to the target plates by more than a factor of 2 compared with the open Divertor I. This reduction is caused by high radiative losses from carbon and hydrogen inside the divertor region and is in agreement with B2-EIRENE modelling predictions. At medium densities in the H mode, the type I ELM behaviour shows no dependence on the heating method (NBI, ICRH). ASDEX Upgrade-JET dimensionless identity experiments showed compatibility of the L-H transition with core physics constraints, while in the H mode confinement, inconsistencies with the invariance principle were established. At high densities close to the Greenwald density, the MHD limited edge pressures, the influence of divertor detachment on separatrix parameters and increasing edge transport lead to limited edge densities and finally to temperatures below the critical edge temperatures for H mode. This results in a drastic increase of the H mode threshold power and an upper H mode density limit with gas puff refuelling. The H mode confinement degradation approaching this density limit is caused by the ballooning mode limited edge pressures and `stiff' temperature profiles relating core and edge temperatures. Repetitive high field side pellet injection allows for H mode operation well above the Greenwald density; moreover, higher confinement than with gas fuelling is found up to the highest densities. Neoclassical tearing modes limit the achievable beta depending on the collisionality at the resonant surface. In agreement with the polarization current model, the onset beta is found to be proportional to the ion gyroradius in the collisionless regime, while higher collisionalities are stabilizing. The fractional energy loss connected with saturated modes at high pressures is about 25%. A reduction of neoclassical mode amplitude and an increase of beta have been demonstrated by using phased ECRH and ECCD in the O point of islands. Advanced tokamak operation with internal transport barriers for both ions and electrons has been achieved with flat shear profiles and q0 > 1 or with reversed shear and qmin > 2. With flat shear a stationary H mode scenario was maintained for 40 confinement times and several internal skin times with betaN = 2 and HITERL-89P = 2.4, where fishbones keep q0 at 1. betaN is limited by either neoclassical tearing modes in the case of flat shear or kink modes with reversed shear.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: Introduction and experimental features In many ICRF heated discharges (hydrogen minority heating) at ASDEX-Upgrade a vari- ety of fast particle driven modes can be observed. In order to determine their nature and their possible impact on fast particle transport, a broad set of diagnostics is employed: the Mirnov coils show clearly the footprints of all electromagnetic modes with finite perturbation at the plasma edge and allow for a reliable mode number identification. The soft x-ray cameras de- liver valuable information about the radial mode position. Finally, the energy and the pitch angle of expelled particles can be directly measured by the fast-ion-loss detector (FILD) diagnostic allowing to reconstruct possible resonance conditions and the mode amplitude evolution. During an IRCF power ramp, the toroidal Alfvén Eigenmodes (TAE) with typical mode num- bers n = 4...7 at typically 200 280 kHz appear first, indicating that they are the least damped modes under these experimental conditions. At higher heating power another electromagnetic mode at about 70 110kHz is observed, a so-called 'BAE' (1, 2) or 'sierpes' (3) mode. At ASDEX-Upgrade this mode seems to be closely connected with the appearance of a q = 1 sur- face and therefore with sawtooth oscillations (see Fig. 2). Its mode numbers are n = 4 with a dominant m = 4 component and it is located radially at ρ pol � 0.2 0.4 (SXR reconstruction with the MHD-IC code (4)). It has been demonstrated that this mode is non-Alfvenic since neither B-field ramps nor density changes influence the mode frequency. Furthermore, the ap- pearance of the 'BAE' mode enhances the FILD-losses induced by the TAE mode (3) suggesting a possible 'channelling' effect between the 'BAE' and the TAE. As in earlier publications (2), no clear scaling with only the ion sound speed or only the dia-
[Show abstract][Hide abstract] ABSTRACT: ASDEX Upgrade was operated with fully W-covered wall in 2007 and 2008. Stationary H-modes at the ITER target values and improved H-modes with H up to 1.2 were run without any boronisation. The boundary conditions set by the full W-wall (high enough ELM frequency, high enough central heating and low enough power density arriving at the target plates) require significant scenario development, but will apply to ITER as well. D retention has been reduced and stationary operation with saturated wall conditions has been found. Concerning confinement, impurity ion transport across the pedestal is neoclassical, explaining the strong inward pinch of high-Z impurities in between ELMs. In improved H-mode, the width of the temperature pedestal increases with heating power, consistent with a β pol,ped 1/2 scaling. In the area of MHD instabilities, disruption mitigation experiments using massive Ne injection reach volume averaged values of the total electron density close to those required for Runaway suppression in ITER. ECRH at the q=2 surface was successfully applied to delay density limit disruptions. The characterisation of fast particle losses due to MHD has shown the importance of different loss mechanisms for NTMs, TAEs and BAEs. Specific studies addressing the first ITER operational phase show that O1 ECRH at the HFS assists reliable low-voltage breakdown. During ramp-up, additional heating can be used to vary l i to fit within the ITER range. Confinement and power threshold in He 2 OV/2-3 are more favourable than in H, suggesting that He operation could allow to assess H-mode operation in the non-nuclear phase of ITER operation.
22nd IAEA Fusion Energy Conference, Geneva (Switzerland); 10/2008
[Show abstract][Hide abstract] ABSTRACT: The ability to predict the stability of fast-particle-driven Alfvén eigenmodes in burning fusion plasmas requires a detailed understanding of the dissipative mechanisms that damp these modes. In order to address this question, the linear gyro-kinetic, electromagnetic code LIGKA is employed to investigate their behaviour in realistic tokamak geometry. The eigenvalue formulation of LIGKA allows to calculate self-consistently the coupling of large-scaled MHD modes to the gyroradius scale-length kinetic Alfvén waves. Therefore, the properties of the kineticly modified TAE mode in or near the gap (KTAE, radiative damping or `tunnelling') and its coupling to the continuum close to the edge can be analysed numerically. In addition, an antenna-like version of LIGKA allows for a frequency scan, analogous to an external antenna. The model and the implementation of LIGKA were recently extended in order to capture the coupling of the shear Alfvén waves to the sound waves. This coupling becomes important for the investigation of kinetic effects on the low-frequency phase of cascade modes, where e.g. geodesic acoustic effects play a significant role.