R. Bilato

ICRF operation with improved antennas in ASDEX Upgrade with W wall

Experiments with boron-coated side limiters of two antennas operated together in 2012 showed that the side limiters are responsible for more than half of the increased W content in the plasma. Together with the contribution from the other limiter tiles, not replaced in 2012, the limiters account for at least two thirds of the W content. A modified test two-strap ion cyclotron range of frequency (ICRF) antennas in ASDEX Upgrade with broad limiters and narrow straps has shown an improved operation with full W wall in 2011/2012 campaigns with up to a 40% lower rise of W concentration allowing more stable operation at low deuterium gas injection rate. Limiter spectroscopy measurements indicate up to a 40% reduction of the rise of the W sputtering yield during ICRF power, measured under the assumption of negligible influence of geometry variations and reflections on the measurements. The boron limiters on two antennas together with the improved broad-limiter antenna allowed a successful ICRF operation in 2012. As a part of long-term strategy of antenna design development, two three-strap antennas with phase and power balance control for reduction of E|| are planned for installation in the future.

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.

Theoretical description of heavy impurity transport and its application to the modelling of tungsten in JET and ASDEX upgrade

The effects of poloidal asymmetries and heated minority species are shown to be necessary to accurately describe heavy impurity transport in present experiments in JET and ASDEX Upgrade. Plasma rotation, or any small background electrostatic field in the plasma, such as that generated by anisotropic external heating can generate strong poloidal density variation of heavy impurities. These asymmetries have recently been added to numerical tools describing both neoclassical and turbulent transport and can increase neoclassical tungsten transport by an order of magnitude. Modelling predictions of the steady-state two-dimensional tungsten impurity distribution are compared with tomography from soft x-ray diagnostics. The modelling identifies neoclassical transport enhanced by poloidal asymmetries as the dominant mechanism responsible for tungsten accumulation in the central core of the plasma. Depending on the bulk plasma profiles, turbulent diffusion and neoclassical temperature screening can prevent accumulation. Externally heated minority species can significantly enhance temperature screening in ICRH plasmas.

Gamma-ray spectroscopy measurements of confined fast ions on ASDEX upgrade

Evidence of ?-ray emission from fast ions in ASDEX Upgrade (AUG) is presented. The plasma scenarios developed for the experiments involve deuteron or proton acceleration. The observed ?-ray emission level induced by energetic protons is used to determine the effective tail temperature of the proton distribution function that can be compared with neutral particle analyser measurements. More generally the measured emission rate is used to assess the confinement of protons with energies <400?keV in discharges affected by toroidal Alfv?n eigenmode instabilities. The derived information on confined ions is combined with observations made with the AUG fast ion loss detector.

Advances in numerical simulations of ion cyclotron heating of non-Maxwellian plasmas

Coupling the full-wave solver TORIC (Brambilla 1999 Plasma Phys. Control. Fusion 41 1) and the bounce-averaged quasilinear Fokker–Planck solver SSFPQL (Brambilla 1994 Nucl. Fusion 34 1121) allows one to determine the suprathermal ion populations produced by ion cyclotron heating of tokamak plasmas, while taking into account their effects on wave propagation and absorption. By using new numerical methods for the evaluation of the coefficients of the wave equations in non-Maxwellian plasmas and the transmission of data between TORIC and SSFPQL, the interface between the two codes has been made very efficient and accurate. As an example, we have re-analysed a minority heating scenario in the ASDEX Upgrade tokamak. The results illustrate the differences between the quasilinear evolution of fundamental and first harmonic ion cyclotron heating due to the fact that the latter is a finite Larmor radius effect. They also suggest that the main missing element for fully satisfactory self-consistent simulations of ion cyclotron experiments in toroidal devices is the absence of a detailed model for the losses of suprathermal ions due, for example, to interactions with low-frequency turbulence or magnetohydrodynamic instabilities.

Simulations of combined neutral beam injection and ion cyclotron heating with the TORIC-SSFPQL package

A source describing the injection of fast ions due to the ionization of high-energy neutral beams has been added to the surface-averaged quasilinear Fokker–Planck code SSFPQL (Brambilla 1994 Nucl. Fusion 34 1121). For this purpose, the multiple-beam NBI code SINBAD (Feng et al 1995 Comput. Phys. Commun. 88 161) has been included as a module in SSFPQL, with the modifications required to handle arbitrary axisymmetric equilibria. Alternatively, the neutral beam injection (NBI) source can be built using the output of a Monte Carlo NBI code. We have also added a term describing losses of fast ions during thermalization, and a subroutine evaluating the neutron production rate by nuclear reactions. With these extensions, iterations between SSFPQL and the full-wave solver TORIC (Brambilla 1999 Plasma Phys. Control. Fusion 41 1) can now be used to investigate the strong interplay between NBI and ion cyclotron (IC) heating.By comparing the predicted and measured neutron production rates from D–D reactions in a discharge with combined NBI and IC heating in ASDEX Upgrade we obtain a plausible estimate of the importance of fast-ion losses (FILs), even if their cause cannot be identified. We find, however, that the plasma composition, in particular the presence of low Z impurities, plays a more critical role than FILs in limiting the efficiency of this heating scheme.

Benchmarking ICRF full-wave solvers for ITER

Abstract Benchmarking of full-wave solvers for ICRF simulations is performed using plasma profiles and equilibria obtained from integrated self-consistent modeling predictions of four ITER plasmas. One is for a high performance baseline (5.3 T, 15 MA) DT H-mode. The others are for half-field, half-current plasmas of interest for the pre-activation phase with bulk plasma ion species being either hydrogen or He4. The predicted profiles are used by six full-wave solver groups to simulate the ICRF electromagnetic fields and heating, and by three of these groups to simulate the current-drive. Approximate agreement is achieved for the predicted heating power for the DT and He4 cases. Factor of two disagreements are found for the cases with second harmonic He3 heating in bulk H cases. Approximate agreement is achieved simulating the ICRF current drive.

Making ICRF power compatible with a high-Z wall in ASDEX Upgrade

A comparison of the ASDEX Upgrade 3-strap ICRF antenna data with the linear electro-magnetic TOPICA calculations is presented. The comparison substantiates a reduction of the local electric field at the radially protruding plasma-facing elements of the antenna as a relevant approach for minimizing tungsten (W) sputtering in conditions when the slow wave is strongly evanescent. The measured reaction of the time-averaged RF current at the antenna limiters to the antenna feeding variations is less sensitive than predicted by the calculations. This is likely to have been caused by temporal and spatial fluctuations in the 3D plasma density distribution affected by local non-linear interactions. The 3-strap antenna with the W-coated limiters produces drastically less W sputtering compared to the W-coated 2-strap antennas. This is consistent with the non-linear asymptotic SSWICH-SW calculations for RF sheaths.

Simultaneous Analysis of Ion and Electron Heat Transport by Power Modulation in JET

Heating power modulation experiments using ion cyclotron resonance heating (ICRH) in the (3)He minority scheme have been performed in the JET tokamak to investigate heat transport properties. This RF scheme provides a dominant localized ion heating, but also some electron heating, and therefore both ion and electron heat channels were modulated. This allows us to carry out a simultaneous transport analysis of ion and electron heat transport channels, including transient transport phenomena. This also provides an experimental assessment of the ICRH heat sources of the (3)He scheme. The modulation approach, so far widely used for electron transport studies, has been validated for ion heat transport in these experiments and yields results on stiffness and threshold of the ion temperature gradient (ITG)-driven ion heat transport. The results for the electron channel demonstrate the importance of the ITG-driven, off-diagonal, contribution to electron heat transport in plasmas with significant ion heating.

Simulation of ion cyclotron heating of tokamak plasmas using coupled Maxwell and quasilinear-Fokker–Planck solvers

The code TORIC solving Maxwell equations in toroidal axisymmetric plasmas in the ion cyclotron (IC) frequency range has proved to be a useful tool for the simulation of IC heating and current drive in tokamak plasmas. TORIC has now been integrated in a package which includes, among other features, an interface to the experimental data (Grad–Shafranov MHD configuration, density and temperature profiles), interfaces to quasilinear Fokker–Planck solvers for the electrons and ions and a subroutine which allows us to estimate the effects of suprathermal anisotropic minority ions populations on wave propagation and absorption. This package allows somewhat simplified but essentially self-consistent simulations of heating and current drive in this frequency domain. In this paper we summarize the physics included in TORIC, with particular emphasis on the most recent extensions, and present an example of the application of the package to the analysis of two IC heating experiments in ASDEX Upgrade and in JET.