D. Farina

Effect of the magnetic field geometry on the flute-like perturbations near the divertor X point

It is shown that a very strong squeezing of the magnetic flux tube passing near the divertor X point imposes stringent limitations on the longitudinal correlation length of the flute-like perturbations near the divertor X point, and leads to a decoupling of the perturbations in the main part of the scrape-off layer from possible effects of the boundary conditions on the surface of the divertor plates

The European Integrated Tokamak Modelling (ITM) effort: achievements and first physics results

A selection of achievements and first physics results are presented of the European Integrated Tokamak Modelling Task Force (EFDA ITM-TF) simulation framework, which aims to provide a standardized platform and an integrated modelling suite of validated numerical codes for the simulation and prediction of a complete plasma discharge of an arbitrary tokamak. The framework developed by the ITM-TF, based on a generic data structure including both simulated and experimental data, allows for the development of sophisticated integrated simulations (workflows) for physics application.The equilibrium reconstruction and linear magnetohydrodynamic (MHD) stability simulation chain was applied, in particular, to the analysis of the edgeMHDstability of ASDEX Upgrade type-I ELMy H-mode discharges and ITER hybrid scenario, demonstrating the stabilizing effect of an increased Shafranov shift on edge modes. Interpretive simulations of a JET hybrid discharge were performed with two electromagnetic turbulence codes within ITM infrastructure showing the signature of trapped-electron assisted ITG turbulence. A successful benchmark among five EC beam/ray-tracing codes was performed in the ITM framework for an ITER inductive scenario for different launching conditions from the equatorial and upper launcher, showing good agreement of the computed absorbed power and driven current. Selected achievements and scientific workflow applications targeting key modelling topics and physics problems are also presented, showing the current status of the ITM-TF modelling suite.

The targeted heating and current drive applications for the ITER electron cyclotron system

A 24u2009MW Electron Cyclotron (EC) system operating at 170u2009GHz and 3600u2009s pulse length is to be installed on ITER. The EC plant shall deliver 20u2009MW of this power to the plasma for Heating and Current Drive (H&CD) applications. The EC system is designed for plasma initiation, central heating, current drive, current profile tailoring, and Magneto-hydrodynamic control (in particular, sawteeth and Neo-classical Tearing Mode) in the flat-top phase of the plasma. A preliminary design review was performed in 2012, which identified a need for extended application of the EC system to the plasma ramp-up, flattop, and ramp down phases of ITER plasma pulse. The various functionalities are prioritized based on those applications, which can be uniquely addressed with the EC system in contrast to other H&CD systems. An initial attempt has been developed at prioritizing the allocated H&CD applications for the three scenarios envisioned: ELMy H-mode (15 MA), Hybrid (∼12 MA), and Advanced (∼9 MA) scenarios. This leads to the ...

Neutron spectroscopy measurements of tritium beam transport at JET

A detailed description of the 14 MeV neutron emission in plasmas heated by neutral beam injection has been carried out by coupling Monte Carlo calculations of the neutron emission spectrum with TRANSP modelling of the beam ion energy distributions. The model is used to study tritium beam injection experiments of the JET trace tritium campaign for internal transport barrier (ITB) and H-mode discharges. For ITB discharges, the measured neutron emission spectrum is well described by modelling using as input the beam ion distribution calculated with TRANSP. For H mode discharges the neutron spectrum can be reproduced only if high energy tritons are lost from the plasma, suggesting the possible role of low frequency tearing modes on the beam ions. The presented results are of relevance for tritium beam transport studies in trace tritium experiments and, more generally, for deuterium and tritium transport studies in high power experiments using neutron emission spectroscopy.

Effect of the limiter configuration on the electron temperature gradient instability in the Tokamak scrape-off layer

A strong instability has been discovered caused by the interplay of two factors: (i) the gradient of the electron temperature across the magnetic field lines and (ii) the formation of Debye sheaths at the conducting end walls terminating the plasma along the magnetic field. The instability can be of some importance for the physics of tokamak scrape-off layers as it can determine the layer thickness. The authors generalize a previous analysis to the case of non-orthogonal intersections of the magnetic field with the wall surface, i.e. to the situation which is most typical for a tokamak with a limiter. The new effects which appear are caused by the change of the flux-tube length (between the walls) in the course of its displacement in the radial direction. Depending on the sign of the scalar product n.grad Te, where n is a unit vector normal to the wall, there can occur either suppression of the initial instability or its considerable enhancement. The possible influence of these effects on the transport coefficients in the scrape-off layer is evaluated. Under realistic assumptions on plasma parameters, this influence can be quite significant.

On recent results in the modelling of neoclassical-tearing-mode stabilization via electron cyclotron current drive and their impact on the design of the upper EC launcher for ITER

Electron cyclotron wave beams injected from a launcher placed in the upper part of the vessel will be used in ITER to control MHD instabilities, in particular neoclassical tearing modes (NTMs). Simplified NTM stabilization criteria have been used in the past to guide the optimization of the launcher. Their derivation is reviewed in this paper and their range of applicability clarified. Moreover, possible effects leading to a deterioration of the predicted performance are discussed. Particularly critical in this context is the broadening of the electron-cyclotron (EC) deposition profiles. It is argued that the most detrimental effect for ITER is likely to be the scattering of the EC beams from density fluctuations due to plasma turbulence, resulting in a beam broadening by about a factor of two. The combined impact of these effects with that of beam misalignment (with respect to the targeted surface) is investigated by solving the Rutherford equation in a form that retains the most relevant terms. The perspectives for NTM stabilization in the Q = 10 ITER scenario are discussed.

Potential of the ITER electron cyclotron equatorial launcher for heating and current drive at nominal and reduced fields

Electron cyclotron (EC) heating and current drive (HCD), at 170u2009GHz, 20u2009MW, is one of the heating systems foreseen to assist and sustain the development of various ITER scenarios since the early phase of ITER. It is usually assumed that EC is efficient only at magnetic fields operating around full field (5.3u2009T) and half field (2.65u2009T), and most of the analyses presented so far have focused on the ECRH&CD performances at flat-top. Here, the EC capabilities are investigated for different plasma parameters corresponding to different phases of the ITER plasma discharge, from current ramp-up up to burn, and for a wide range of magnetic fields, focusing in particular on the EC potential for heating and for L- to H-mode assist. It is found that the EC system can contribute to a wide range of heating scenarios during the ramp-up of the magnetic field, significantly increasing the applicable range as a function of magnetic field as compared with traditional views.

Crucial issues of multi-beam feed-back control with ECH/ECCD in fusion plasmas

Proof of principle of feed-back controlled Electron Cyclotron Heating and Current Drive (ECH/ECCD), aiming at automatic limitation (or suppression) of Neoclassical Tearing Modes amplitude, has been achieved in a number of present machines. In addition to Neoclassical Tearing Mode stabilization, more applications of well-localized ECH/ECCD can be envisaged (saw-tooth crash control, current profile control, thermal barrier control, disruption mitigation). However, in order to be able to take a step forward towards the application of these techniques to burning plasmas, some crucial issues should be more deeply analyzed: multi-beam simultaneous action, control of deposition radii rdep, diagnostic of plasma reaction. So far the Electron Cyclotron Emission has been the most important tool to get localized information on plasma response, essential for both rdep and risland recognition, but its use in very hot burning plasmas within automatic control loops should be carefully verified. Assuming that plasma response is appropriately diagnosed, the next matter to be discussed concerns how to control rdep, since all techniques so far used, or proposed (plasma position, toroidal field, mechanical beam steering, gyrotron frequency tuning) have limitations or drawbacks. Finally, simultaneous multiple actions on many actuators (EC beams), concurring to automatic control of one single parameter (e.g. NTM amplitude) might be a challenging task for the controller, particularly in view of the fact that any effect of each beam becomes visible only when it is positioned very close to the right radius. All these interlinked aspects are discussed in the paper.

Optimization of the ITER electron cyclotron equatorial launcher for improved heating and current drive functional capabilities

The design of the ITER Electron Cyclotron Heating and Current Drive (EC H&CD) system has evolved in the last years both in goals and functionalities by considering an expanded range of applications. A large effort has been devoted to a better integration of the equatorial and the upper launchers, both from the point of view of the performance and of the design impact on the engineering constraints. However, from the analysis of the ECCD performance in two references H-mode scenarios at burn (the inductive H-mode and the advanced non-inductive scenario), it was clear that the EC power deposition was not optimal for steady-state applications in the plasma region around mid radius. An optimization study of the equatorial launcher is presented here aiming at removing this limitation of the EC system capabilities. Changing the steering of the equatorial launcher from toroidal to poloidal ensures EC power deposition out to the normalized toroidal radius ρu2009≈u20090.6, and nearly doubles the EC driven current around m...

Inboard and outboard electron temperature profile measurements in JET using ECE diagnostics

In this paper a novel use of the heterodyne radiometer in JET is presented. The operation of the diagnostic has been extended to allow simultaneously measurement of the pedestal temperature profile at the inboard and outboard midplane with high spatial and temporal resolution. Access to the inboard midplane region is obtained by measuring ordinary mode electron cyclotron emission (ECE) (no harmonic overlap) using an antenna located on the low-field side of the torus. To assess the potential of the ECE temperature measurements in the inboard pedestal region, a detailed analysis of ECE spectrum will be discussed. The measured electron temperature profiles are presented, including a discussion of the main uncertainties in the analysis. The comparison of inboard and outboard temperature pedestal profiles in a variety of H-mode plasmas shows a reasonable agreement in both shape and magnitude.