A. C. A. Figueiredo

Localized bulk electron heating with ICRF mode conversion in the JET tokamak

Ion cyclotron resonance frequencies (ICRF) mode conversion has been developed for localized on-axis and off-axis bulk electron heating on the JET tokamak. The fast magnetosonic waves launched from the low-field side ICRF antennas are mode-converted to short-wavelength waves on the high-field side of the 3He ion cyclotron resonance layer in D and 4He plasmas and subsequently damped on the bulk electrons. The resulting electron power deposition, measured using ICRF power modulation, is narrow with a typical full-width at half-maximum of ?30?cm (i.e. about 30% of the minor radius) and the total deposited power to electrons comprises at least up to 80% of the applied ICRF power. The ICRF mode conversion power deposition has been kept constant using 3He bleed throughout the ICRF phase with a typical duration of 4?6?s, i.e. 15?40 energy confinement times. Using waves propagating in the counter-current direction minimizes competing ion damping in the presence of co-injected deuterium beam ions.

Impurity production from the ion cyclotron resonance heating antennas in JET

Additional heating systems can be a source of impurities in fusion plasmas. Studying the behaviour of such impurities is important to understand and minimize their effects on tokamak plasma performance. In this paper we present a detailed study of the influence of ion cyclotron resonance heating (ICRH) on the impurity content of JET plasmas. Using spectroscopic methods we monitor the Ni impurity release by the two ICRH antenna types in JET: the so-called ITER-like antenna and the A2 antennas. The release of Ni during ICRH is presented as a function of the power density, total ICRH power applied, the relative phasing of the A2 antenna straps, the D2 gas injection level and the plasma–strap distance. We also estimate the contribution of Ni impurity to Zeff and to plasma dilution. L-mode discharges are compared for different relative phasings of the four antenna straps of the A2 antennas: (0 π 0 π), (0 π π 0), (0 0 π π), (00 π/2 π/2), (0 π/2 π 3π/2), (− − π 0) and (0 π − −). We observe that for antenna phasings that allow the best coupling, the Ni impurity concentration is reduced in the central part of the plasma. The impurity content is also monitored in H-mode experiments aiming at improving the ICRH coupling at large antenna strap–separatrix distances, up to 19 cm, by injecting gas from different inlets. Interestingly, with increasing gas injection rate the coupling improves and the Ni concentration decreases, and the actual location of gas injection is found to influence the Ni concentration.

Time–frequency analysis of nonstationary fusion plasma signals: A comparison between the Choi–Williams distribution and wavelets

The continuous wavelet transform scalogram, and recently the Choi–Williams distribution, have both been used to improve upon the short-time Fourier transform spectrogram in the analysis of some nonstationary phenomena in fusion plasmas. Here, a comparison is made with real fusion plasma signals that shows the advantages of the Choi–Williams distribution over wavelets as a complementary tool to the spectrogram.

Ion heat transport studies in JET

Detailed experimental studies of ion heat transport have been carried out in JET exploiting the upgrade of active charge exchange spectroscopy and the availability of multi-frequency ion cyclotron resonance heating with (3)He minority. The determination of ion temperature gradient (ITG) threshold and ion stiffness offers unique opportunities for validation of the well-established theory of ITG driven modes. Ion stiffness is observed to decrease strongly in the presence of toroidal rotation when the magnetic shear is sufficiently low. This effect is dominant with respect to the well-known omega(ExB) threshold up-shift and plays a major role in enhancing core confinement in hybrid regimes and ion internal transport barriers. The effects of T(e)/T(i) and s/q on ion threshold are found rather weak in the domain explored. Quasi-linear fluid/gyro-fluid and linear/non-linear gyro-kinetic simulations have been carried out. Whilst threshold predictions show good match with experimental observations, some significant discrepancies are found on the stiffness behaviour.

Time–frequency analysis of non-stationary signals in fusion plasmas using the Choi–Williams distribution

The Choi–Williams distribution is applied to the time–frequency analysis of signals describing rapid magnetohydrodynamic modes and events in tokamak plasmas. Its effectiveness is demonstrated through a comparison with the spectrogram, which requires a compromise between time and frequency resolution, and with the Wigner distribution, which can give an unclear representation of the modes, masked by inconvenient artefacts. Examples of phenomena in the JET tokamak are shown, namely the onset of neoclassical tearing modes in discharges with ion cyclotron resonant heating, precursors of edge localized modes, and washboard modes.

Improved time-frequency visualization of chirping mode signals in Tokamak plasmas using the Choi-Williams distribution

The Choi-Williams distribution is used to create time-frequency images of chirping magnetohydrodynamic modes in the JET tokamak that have better resolution than plots based on the short-time Fourier transform spectrogram.

Gaussian short-time propagators and electron kinetics: Numerical evaluation of path-sum solutions to Fokker–Planck equations for rf heating and current drive

Knowing that short-time propagators for Fokker–Planck equations are Gaussian, and based on a path-sum formulation, an efficient and simple numerical method is presented to solve the initial-value problem for electron kinetics during rf heating and current drive. The formulation is thoroughly presented and discussed, its advantages are stressed, and general, practical criteria for its implementation are derived regarding the time step and grid spacing. The new approach is illustrated and validated by solving the one-dimensional model for lower-hybrid current drive, which has a well-known steady-state analytical solution.

Systematic linear-stability assessment of Alfvén eigenmodes in the presence of fusion α-particles for ITER-like equilibria

A systematic approach to evaluate the linear stability of Alfven eigenmodes in the presence of fusion-born -particles is reported. The techniques developed are particularly useful when dealing with scenarios where no experimental guidance is available about which eigenmodes are more easily destabilized by the supra-thermal population. The advantages and limitations of the underlying model chosen to describe the wave-particle interaction are discussed, along with the parallelization of the designed workflow in order to take advantage of massively parallel computer systems. This workflow is tested using a ITER baseline scenario, showing that it is able to routinely single out the most linearly unstable eigenmodes for a given equilibrium configuration. The eigenmodes with highest growth rate were found to be core-localised Toroidal Alfven Eigenmodes with toroidal mode number , placed near the maximum of the -particle density gradient and within a low magnetic-shear region.

On alias-free discrete time wigner distributions for time-frequency analysis of fusion plasma signals : application to jet magnetic activity data

The problem of aliasing in relation to the use of Wigner distributions to process discrete time (DT) non-stationary signals from fusion plasma diagnostics is comprehensively addressed. Three DT Wigner distributions, two of which are alias free (AF), are thoroughly studied and compared, their properties being provided as well as, making use of so-called pseudo-Wigner (p-Wigner) distributions, formulae for their efficient computation. Of the three, one is singled out as that AF form which simultaneously obeys an arguably more complete set of desirable properties for time–frequency distributions and which has a lower computational complexity. Such an AF DT Wigner distribution has been known in quantum mechanics as the rotational Wigner function, where it has been introduced to treat rotation angles and their canonically conjugate angular momenta, whose spectra have the same discrete nature as DT signals. As an illustration of the advantages and shortcomings of each of these three distributions, they are applied to the time–frequency analysis of MHD activity data. Thus, the three DT Wigner distributions, more precisely their p-Wigner counterparts, together with the spectrogram for the sake of comparison, are used to obtain representations in the time–frequency plane of non-stationary events from neoclassical tearing mode (NTM) and toroidal Alfven eigenmode (TAE) experiments in the JET tokamak. Besides aliasing, the appearance in the time–frequency plane of spurious images due to cross-term interference between different signal components is also discussed. If such undesirable artefacts are generated by negative frequencies interfering with positive frequencies, it is shown that the analytic signal can be of great help in reducing cross-term effects.

Systematic and routine analysis of radial correlation reflectometry data in JET.

This paper discusses a tool specially developed for the analysis of radial correlation reflectometry data in JET. The tool, which calculates the correlation length and coherent reflection from the raw data, has been designed for offline analysis and to assist diagnostic operators. After being verified in controlled tests using theoretical signals, the tool is validated by means of a study of ITB plasmas in JET.