E. Giovannozzi

Electron fishbones: theory and experimental evidence

We discuss the processes underlying the excitation of fishbone-like internal kink instabilities driven by supra-thermal electrons generated experimentally by different means: electron cyclotron resonance heating (ECRH) and by lower hybrid (LH) power injection. The peculiarity and interest of exciting these electron fishbones by ECRH only or by LH only is also analysed. Not only is the mode stability explained, but also the transition between steady state nonlinear oscillations to bursting (almost regular) pulsations, as observed in FTU, is interpreted in terms of the LH power input. These results are directly relevant to the investigation of trapped alpha particle interactions with low-frequency MHD modes in burning plasmas: in fact, alpha particles in reactor relevant conditions are characterized by small dimensionless orbits, similarly to electrons; the trapped particle bounce averaged dynamics, meanwhile, depends on energy and not mass.

Global and pedestal confinement in JET with a Be/W metallic wall

Type I ELMy H-mode operation in JET with the ITER-like Be/W wall (JET-ILW) generally occurs at lower pedestal pressures compared to those with the full carbon wall (JET-C). The pedestal density is similar but the pedestal temperature where type I ELMs occur is reduced and below to the so-called critical type I–type III transition temperature reported in JET-C experiments. Furthermore, the confinement factor H98(y,2) in type I ELMy H-mode baseline plasmas is generally lower in JET-ILW compared to JET-C at low power fractions Ploss/Pthr,08 2, the confinement in JET-ILW hybrid plasmas is similar to that in JET-C. A reduction in pedestal pressure is the main reason for the reduced confinement in JET-ILW baseline ELMy H-mode plasmas where typically H98(y,2) = 0.8 is obtained, compared to H98(y,2) = 1.0 in JET-C. In JET-ILW hybrid plasmas a similarly reduced pedestal pressure is compensated by an increased peaking of the core pressure profile resulting in H98(y,2) ≤ 1.25. The pedestal stability has significantly changed in high triangularity baseline plasmas where the confinement loss is also most apparent. Applying the same stability analysis for JET-C and JET-ILW, the measured pedestal in JET-ILW is stable with respect to the calculated peeling–ballooning stability limit and the ELM collapse time has increased to 2 ms from typically 200 µs in JET-C. This indicates that changes in the pedestal stability may have contributed to the reduced pedestal confinement in JET-ILW plasmas. A comparison of EPED1 pedestal pressure prediction with JET-ILW experimental data in over 500 JET-C and JET-ILW baseline and hybrid plasmas shows a good agreement with 0.8 < (measured pped)/(predicted pped,EPED) < 1.2, but that the role of triangularity is generally weaker in the JET-ILW experimental data than in the model predictions.

Pedestal and scrape-off layer dynamics in ELMy H-mode plasmas in JET

Pedestal and scrape-off layer (SOL) dynamics due to edge localized modes (ELMs) have been studied on JET with improved diagnostic capability. The new high resolution Thomson scattering system enables detailed measurement of the space and time evolution of the Te and ne pedestal profiles. The pedestal and SOL dynamics for type I ELMy H-mode plasmas have been studied for a wide range of plasma conditions. During a short period of <200 µs after the ELM event radial profiles of filaments in the SOL electron density and temperature have been observed. After that period the SOL density is increased and remains high for several milliseconds. During the same period the electron temperature shows no increase compared with the pre-ELM values. This SOL dynamics has been observed for a wide range of plasma parameters and is independent of plasma pedestal collisionality. For the first time on JET the convective and conductive ELM energy losses have been quantified using the new kinetic profile measurements. The findings provide detailed confirmation of earlier observations based on different measurements and analysis. The pedestal region perturbed by the ELM is the same for both density and temperature and the ELM effect extends up to about 20% of minor radius. The convective energy losses do not vary significantly and are ~5% of the pedestal stored energy (Wped) over a large range of pedestal collisionality from below to above whereas the conductive losses strongly decrease from ~20% of Wped to 5% of Wped with increasing . The experimental observations are compared with a simple model based on losses being driven by parallel transport.

Modelling of polarimetry measurements at JET

This paper presents a study aimed at validating the ability of the presently available models to predict the Cotton–Mouton (C–M) effect. The Faraday rotation and the C–M phase shift angle can be calculated by means of a rigorous numerical solution of Stokes equations. Numerical and approximated solutions are presented and compared with experimental data. A detailed comparison is done with the time traces of measurements, inside a limited dataset representative of JET regimes. A statistical analysis is then carried out on a dataset including data from 300 discharges. In general the C–M measurements are in agreement with the numerical model, and the line integral of plasma density deduced by the C–M measurements is in agreement with that measured by LIDAR Thomson scattering (well inside the experimental error, which is close to two fringes for the polarimetry measurements, 1 fringe = 1.14 × 1019 m−2).

Analysis of Faraday rotation in JET polarimetric measurements

ThepaperdealswithJETpolarimetermeasurementsandinparticularitpresents a study of the Faraday rotation angle, which is used as a constraint in equilibrium codes. This angle can be calculated by means of the rigorous numerical solution of Stokes equations. A detailed comparison of calculations is carried outwiththetimetracesofmeasurements, insidealimiteddatasetrepresentative of JET discharges: in general, it is found that the Faraday rotation angle and Cotton‐Mouton phase shift measurements can be represented by the numerical solution to Stokes equations. To obtain this agreement in particular for Faraday rotation, a shift of the magnetic surfaces must be included. This results in an improvement of the position of the magnetic surfaces as calculated by the EFIT equilibrium code. The approximated linear models normally used can be applied only at low density and current. The Cotton‐Mouton is calculated at high plasma density including the contribution by the Faraday rotation angle. For highplasmacurrentthenon-lineartermsinthepropagationequationscanbeimportant. These conclusions have some impact on the mathematical form of the polarimetric constraints (Faraday and Cotton‐Mouton) in equilibrium codes. (Some figures in this article are in colour only in the electronic version)

A new calibration code for the JET polarimeter

An equivalent model of JET polarimeter is presented, which overcomes the drawbacks of previous versions of the fitting procedures used to provide calibrated results. First of all the signal processing electronics has been simulated, to confirm that it is still working within the original specifications. Then the effective optical path of both the vertical and lateral chords has been implemented to produce the calibration curves. The principle approach to the model has allowed obtaining a unique procedure which can be applied to any manual calibration and remains constant until the following one. The optical model of the chords is then applied to derive the plasma measurements. The results are in good agreement with the estimates of the most advanced full wave propagation code available and have been benchmarked with other diagnostics. The devised procedure has proved to work properly also for the most recent campaigns and high current experiments.

Parametric dependence of turbulent particle transport in high density electron heated FTU plasmas

In this paper we present the result of a study carried out at the Frascati Tokamak Upgrade (FTU), on a set of full non-inductive current driven, electron heated, L-mode discharges aimed at investigating the parametric dependence of the electron density profile on the electron temperature and safety factor gradients as predicted by quasi-linear drift-turbulence transport theory. Experiments in FTU allow the extension of similar studies carried out on other tokamaks to plasmas with higher density and higher magnetic field. Magnetic shear and electron temperature gradients are found to drive opposite turbulent particle flows in the gradient region (0.3 < r/a ? 0.5), while inward thermo-diffusion alone is found in the plasma core (r/a ? 0.3). Density profiles at midradius appear to be controlled by a convective term proportional to the density and independent of the gradient of temperature and magnetic shear. A linear increase in density peaking versus effective collisionality is found, differing from the scaling observed in other FTU plasma regimes.

Chapter 8: The Diagnostic Systems in the FTU

Abstract The design of diagnostics for the Frascati Tokamak Upgrade (FTU) is challenging because of the compactness of the machine (8-cm-wide ports) and the low operating temperatures requiring the presence of a cryostat. Nevertheless, a rather complete diagnostic system has been progressively installed. The basic systems include a set of magnetic probes, various visible and ultraviolet spectrometers, electron cyclotron emission (ECE) for electron temperature profiles measurements and electron tails monitoring, far-infrared and CO2 interferometry, X-ray (soft and hard) measurements, a multichord neutron diagnostics (with different type detectors), and a Thomson scattering system. Some diagnostics specific to the FTU physics program have been used such as microwave reflectometry for turbulence studies, edge-scanning Langmuir probes for radio-frequency coupling assessment, oblique ECE, and a fast electron bremsstrahlung (FEB) camera for lower hybrid current drive-induced fast electron tails. These systems are briefly reviewed in this paper. Further developments including a scanning CO2 laser two-color interferometer, two FEB cameras for tomographic analysis, a motional Stark effect system, and a collective Thomson scattering system are also described.

Mutual interaction of Faraday rotation and cotton-mouton phase shift in jet polarimetric measurements

The paper presents a study of Faraday rotation (FR) angle and Cotton–Mouton (CM) phase shift measurements to determine their mutual interaction and the validity of the linear models presently used in equilibrium codes. Comparison between time traces of measurements and model calculations leads to the result that only an exact numerical solution of Stokes equations can reproduce in all the experimental data. As a consequence, approximated linear models can be applied only in a limited range of plasma parameters. In general, the nonlinear coupling between FR and CM is important for the evaluation of polarimetry parameters.

Detection of dust on JET with the high resolution Thomson scattering system

Dust particles have been observed with Thomson scattering systems on several tokamaks. We present here the first evidence of dust particles observed by the new high resolution Thomson scattering system on JET. The system consists of filter spectrometers that analyze the Thomson scattering spectrum from 670 to 1050 nm in four spectral channels. The laser source is a 5 J Q-switched Nd:YAG laser. Without a spectral channel at the laser wavelength, only dust particles that emit broadband light could be detected; these particles have been observed on JET after disruptions. The timing of their emission is clearly different from that expected for a Thomson scattering pulse. The light pulse from dust happens after the peak of the laser light and has a long tail.