João P. S. Bizarro

A lower hybrid current drive system for ITER

A 20 MW/5 GHz lower hybrid current drive (LHCD) system was initially due to be commissioned and used for the second mission of ITER, i.e. the Q = 5 steady state target. Though not part of the currently planned procurement phase, it is now under consideration for an earlier delivery. In this paper, both physics and technology conceptual designs are reviewed. Furthermore, an appropriate work plan is also developed. This work plan for design, R&D, procurement and installation of a 20 MW LHCD system on ITER follows the ITER Scientific and Technical Advisory Committee (STAC) T13-05 task instructions. It gives more details on the various scientific and technical implications of the system, without presuming on any work or procurement sharing amongst the possible ITER partners(b). This document does not commit the Institutions or Domestic Agencies of the various authors in that respect.

Improved confinement in high li lower hybrid driven steady state plasmas in TORE SUPRA

The global energy confinement of combined ohmic and lower hybrid driven TORE SUPRA plasmas has been analysed at various densities. In contradiction to the L mode ITER scaling law, this analysis indicates that the global energy confinement time depends strongly on the plasma density. Furthermore, the thermal electron energy content of steady state discharges is found to be in good agreement with the global Rebut-Lallia-Watkins (RLW) scaling law. Current ramp experiments show an enhancement of the global energy confinement with the internal inductance, li. These results have been extended to steady state regimes with lower hybrid current drive. Improved confinement has been obtained in a high li steady state plasma (li=1.7), where the modification of the current profile by lower hybrid waves leads to an increase in the central value of the safety factor (qψ(0) ≈ 2). In this case, the global confinement time is shown to exceed the value predicted by the RLW scaling law by 40%

On ray stochasticity during lower‐hybrid current drive in tokamaks

Using a combined ray‐tracing and Fokker–Planck code, a comprehensive and detailed analysis is presented on the importance of toroidally induced ray stochasticity for the modeling of lower‐hybrid (LH) current drive in tokamaks and for the dynamics of the launched power spectrum. The injected LH power distribution in poloidal angle and in parallel wave index is accurately represented by taking into account the poloidal extent of the antenna and by efficiently covering the full range of its radiated spectrum. The influence of the balance between the wave damping and the exponential divergence of nearby ray trajectories in determining the shape and robustness of the predicted LH power deposition profiles is emphasized. When stochastic effects are important, code predictions are shown to be stable with respect to small changes in plasma parameters and initial conditions, and to be consistent with experimental data, provided a sufficiently large number of rays is used. Sensitivity studies indicate that the comp...

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.

On self‐consistent ray‐tracing and Fokker–Planck modeling of the hard x‐ray emission during lower‐hybrid current drive in tokamaks

A detailed investigation is presented on the ability of combined ray‐tracing and Fokker–Planck calculations to predict the hard x‐ray (HXR) emission during lower‐hybrid (LH) current drive in tokamaks when toroidally induced ray stochasticity is important. A large number of rays is used and the electron distribution function is obtained by self‐consistently iterating the appropriate power deposition and Fokker–Planck calculations. It is shown that effects due to radial diffusion of suprathermal electrons and to radiation scattering by the inner wall can be significant. The experimentally observed features of the HXR emission are fairly well predicted, thus confirming that combined ray‐tracing and Fokker–Planck codes are capable of correctly modeling the physics of LH current drive in tokamaks.

Magnetic ripple and the modeling of lower-hybrid current drive in tokamaks

Using ray tracing, a detailed investigation of the lower‐hybrid (LH) wave propagation in presence of toroidal magnetic field ripple is presented. The local ray behavior is first depicted for a cylindrical equilibrium periodically modulated along the axial direction. Variations along ray trajectories in the component of the wave vector parallel to the equilibrium magnetic field are observed, with a maximum relative amplitude that is locally of the order of the ripple level. For the full rippled toroidal equilibrium, a similar local behavior is found when the ray trajectory crosses a high ripple region. Despite the modest amplitude of the local ray perturbation, its global influence on ray trajectories may be strong, as a consequence of the combined effects of toroidal and poloidal inhomogeneities. By coupling ray tracing with a one‐dimensional relativistic Fokker–Planck code, simulations of LH experiments have been performed for the TORE SUPRA tokamak [Equipe TORE SUPRA, in Proceedings of the 15th Conferen...

On the dynamics of the launched power spectrum during lower hybrid current drive in tokamaks

Using a combined ray tracing and Fokker-Planck code, an investigation is made on the propagation and absorption of the launched power spectrum during lower hybrid (LH) current drive in tokamaks, in a situation where toroidally induced ray stochasticity is important. Stochastic effects are correctly taken into account by tracing a large number of rays. The influence of the balance between the wave damping and the stochastic divergence of nearby ray trajectories in governing the power spectrum dynamics and in establishing the characteristics of the LH power deposition patterns is illustrated. It is shown that, when strong wave damping prevails, the launched power spectrum behaves regularly: the LH power deposition is localized and the absorbed power spectrum is very similar in shape to the launched one, although some broadening and shifting in the parallel wave index generally occur. If the wave damping is weak and stochastic effects are important, the rays end by covering the entire plasma cross-section and the power spectrum dynamics follows a diffusive pattern: the LH power deposition turns out to be extended and the absorbed power spectrum is much broader than the launched one, as a result of strong spreading towards the high absolute values of the parallel wave index

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.

On the Behavior of the Continuous-Time Spectrogram for Arbitrarily Narrow Windows

The behavior of the continuous-time (CT) spectrogram, in particular of its first frequency moment (FFM) and instantaneous bandwidth (IB), in the limit of infinitely thin windows is revisited: new insight is gained and novel quantitative results are presented. Power series in the width of the short-time Fourier transform (STFT) window are derived for the CT spectrogram and its moments, which are valid for any reasonably regular narrow window, and which universally yield the instantaneous frequency (IF) as the first term in the FFM expansion. The well-known result, for CT signals, according to which the spectrogram FFM tends to the IF when the square of the STFT window approaches a Dirac delta (delta) function is thus extended. Indeed, upon introduction of functional sequences depending on a small width parameter, it is shown that the spectrogram FFM always tends to the IF when the STFT window becomes arbitrarily narrow (with no actual need for the latter, or any of its powers, to converge to a delta function). It is also checked, within the framework here developed, that the spectrogram IB behaves as expected, going to infinity when the FFM approaches the IF. It is argued that, when estimating the IF of CT signals with the spectrogram, it may be eventually more convenient to work with functions that become arbitrarily close to a delta function than to its square root (whatever mathematical object this might be!). In addition, the results of the analysis on the IF and the IB are illustrated with a few examples of standard signal and window forms

Thermodynamics with friction. I. The Clausius inequality revisited

The different forms of the Clausius inequality are generalized to explicitly include frictional work, which can be partly dissipated as heat in the surroundings and thus contribute to increase its entropy so, in a cyclic process, the net entropy leaving the surroundings with the heat directly exchanged with the system is no longer constrained by the second law to be nonpositive. Likewise, the lower bound for the system’s entropy change in an arbitrary transformation is not now determined solely by the heat transferred from the surroundings but can be smaller by a term given by frictional dissipation in the latter. A wide range of applications is discussed, from thermal engines to mesoscopic devices, including the efficiency of dithermal engines and the problem of information erasure. Moreover, it is verified that the macroscopic approach utilized to extend standard macroscopic equilibrium theory in such a manner as to incorporate friction can be cast as well in the continuous, fieldlike formulation of non...