The understanding and predictive capability of transport physics and plasma confinement is reviewed from the perspective of achieving reactor-scale burning plasmas in the ITER tokamak, for both core and edge plasma regions. Very considerable progress has been made in understanding, controlling and predicting tokamak transport across a wide variety of plasma conditions and regimes since the publication of the ITER Physics Basis (IPB) document (1999 Nucl. Fusion 39 2137-2664). Major areas of progress considered here follow. (1) Substantial improvement in the physics content, capability and reliability of transport simulation and modelling codes, leading to much increased theory/experiment interaction as these codes are increasingly used to interpret and predict experiment. (2) Remarkable progress has been made in developing and understanding regimes of improved core confinement. Internal transport barriers and other forms of reduced core transport are now routinely obtained in all the leading tokamak devices worldwide. (3) The importance of controlling the H-mode edge pedestal is now generally recognized. Substantial progress has been made in extending high confinement H-mode operation to the Greenwald density, the demonstration of Type I ELM mitigation and control techniques and systematic explanation of Type I ELM stability. Theory-based predictive capability has also shown progress by integrating the plasma and neutral transport with MHD stability. (4) Transport projections to ITER are now made using three complementary approaches: empirical or global scaling, theory-based transport modelling and dimensionless parameter scaling (previously, empirical scaling was the dominant approach). For the ITER base case or the reference scenario of conventional ELMy H-mode operation, all three techniques predict that ITER will have sufficient confinement to meet its design target of Q = 10 operation, within similar uncertainties.

https://iopscience.iop.org/article/10.1088/0029-5515/47/6/S02/pdf

Chapter 2: Plasma confinement and transport

Tokamak discharges with improved energy confinement properties arising from internal transport barriers (ITBs) have certain attractive features, such as a large bootstrap current fraction, which suggest a potential route to the steady-state mode of operation desirable for fusion power plants. This paper first reviews the present state of theoretical and experimental knowledge regarding the formation and characteristics of ITBs in tokamaks. Specifically, the current status of theoretical modelling of ITBs is presented; then, an international ITB database based on experimental information extracted from some nine tokamaks is described and used to draw some general conclusions concerning the necessary conditions for ITBs to appear, comparing these with the theoretical models. The experimental situation regarding the steady-state, or at least quasi-steady-state, operation of tokamaks is reviewed and finally the issues and prospects for achieving such operational modes in ITER are discussed. More detailed information on the characteristics of ITBs in some 13 tokamaks (as well as helical devices) appears in the appendix.

https://iopscience.iop.org/article/10.1088/0029-5515/44/4/R01/pdf

A review of internal transport barrier physics for steady-state operation of tokamaks

The present status of zonal flow experiments is reviewed with the historical process to attain the concept of zonal flows, which provides a new framework for understanding turbulence and transport in toroidal plasmas. The existence of zonal flows is experimentally confirmed to present a new paradigm of plasma turbulence. The paper presents contemporary experiments on zonal flows as major topics with a brief presentation of the zonal flow theories, the diagnostics and data processing techniques for turbulence and zonal flows and the peripheral issues of zonal flow physics. The accumulated experimental results introduced in this review include identification of zonal flows (both stationary zonal flows and geodesic acoustic modes), nonlinear interactions between zonal flows and turbulence, quantification of turbulent Reynolds stress, flow dynamics, energy transfer dynamics between turbulent wave components and the effects of zonal flows on plasma transport. These results have given rise to a new paradigm, namely, that the plasma turbulence is a system of zonal flows and drift waves, with an emphasis on the interaction between the disparate scale structures, e.g. zonal flows (mesoscale) and turbulence (micro-scale).

https://iopscience.iop.org/article/10.1088/0029-5515/49/1/013001/pdf

A review of zonal flow experiments

This paper reviews measurements of edge plasma turbulence in toroidal magnetic fusion devices with an emphasis on recent results in tokamaks. The dominant feature of edge turbulence is a high level of broadband density fluctuations with a relative amplitude δn/n ~ 5–100%, accompanied by large potential and electron temperature fluctuations. The frequency range of this turbulence is ~10 kHz–1 MHz, and the size scale is typically ~0.1–10 cm perpendicular to the magnetic field but many metres along the magnetic field, i.e. the structure is nearly that of 2D 'filaments'. Large intermittent bursts or 'blobs' are usually observed in the scrape-off layer. Diagnostic and data analysis techniques are reviewed and the main experimental results are summarized. Recent comparisons of experimental results with edge turbulence theory are discussed, and some directions for future experiments are suggested.

http://iopscience.iop.org/article/10.1088/0741-3335/49/7/S01/pdf

Edge turbulence measurements in toroidal fusion devices

Doppler reflectometry is characterized by a finite tilt angle of the probing microwave beam with respect to the normal onto the cutoff surface. According to the Bragg condition the diagnostic selects density perturbations with wave number K⊥ in the reflecting layer. From the Doppler shift of the returning microwave the propagation velocity of these perturbations v⊥ can be obtained directly. The signal intensity contains information about the perturbation amplitude. The diagnostic potential of Doppler reflectometry is demonstrated both numerically by the use of two-dimensional full-wave codes and experimentally by an antenna system with variable tilt angle installed at the W7-AS stellarator. During stationary plasma conditions the measured profile of the propagation velocity v⊥(r) is dominated by the E×B velocity of the plasma, which is obtained from passive spectroscopy. Transient states of the plasma can be followed with a temporal resolution of less than 50 µs. Thus, Doppler reflectometry allows us to investigate the interdependence of sheared flow and turbulence on that timescale.

https://iopscience.iop.org/article/10.1088/0741-3335/43/12/302/pdf

Doppler Reflectometry for the Investigation of Propagating Density Perturbations

Reflectometry applied to the measurement of density profiles on fusion plasmas has been subject to many recent developments. After a brief reminder of the principles of reflectometry, the theoretical accuracy of reflectometry measurements is discussed. The main difficulties limiting the performance, namely the plasma fluctuations and the quality of the transmission lines, are analysed. The different techniques used for reflectometry are then presented grouped into three different categories, depending on the frequency spectrum of the probing wave: single frequency, few discrete frequencies, or broad spectrum. The present status and achievements of actual implementations of these techniques are demonstrated, with an analysis of their respective limitations and merits, as well as foreseen developments. Finally, a discussion of the various reflectometry techniques is made, in particular their ability to cope with plasma fluctuations and complex transmission lines, in view of the application to next step machines and very severe environments.

https://iopscience.iop.org/article/10.1088/0741-3335/38/7/002/pdf

Reflectometry techniques for density profile measurements on fusion plasmas

A collective infra-red laser scattering diagnostic has been installed on the TORE SUPRA tokamak for the measurement of plasma density fluctuations. For the range of wavenumbers explored (3-15 cm-1), the scattering angles are very weak ( approximately 1 mrad). Consequently, the scattering signals are averaged along the whole observation chord, resulting in poor longitudinal spatial localization. By virtue of the pitch angle variation of the magnetic field lines in the tokamak, and of the perpendicularity of the turbulence wavevector to these field lines, it has been possible to obtain partial spatial resolution along the direction of the beam. Good agreement between the experimental and theoretical angular resolution of the diagnostic as well as the results of cross-correlation performed on the signals obtained by two simultaneous probing beams also justify this novel concept.

https://iopscience.iop.org/article/10.1088/0741-3335/35/1/005/pdf

Localized measurements of turbulence in the TORE SUPRA tokamak

A collective laser light scattering diagnostic ALTAIR (a french acronym for local analysis of anomalous transport using infrared light), using a CO2 laser beam (λ=10.6 μm) has been realized to measure plasma density fluctuations in the TORE SUPRA tokamak. This article describes in detail the optical setup, the signal processing, acquisition, and control systems required for this experiment. As the density fluctuations propagating in tokamaks have small wave numbers and require small scattering angles, such scattering experiments are considered as having no resolution along the beam. However, taking advantage of the pitch angle variation of the magnetic field lines around the magnetic axis along a vertical chord, it has been possible to obtain partial spatial localization of the scattering volume by rotating the direction of the analyzed wave vector in a horizontal plane. Heterodyne detection is used to determine the fluctuations propagation direction. The experiment has been tested on acoustic waves and t...

ALTAIR: An infrared laser scattering diagnostic on the TORE SUPRA tokamak

Results on confinement and turbulence from a set of Ohmic discharges in TORE SUPRA are discussed. Attention is focused on the saturation of the energy confinement time and it is emphasized that this saturation can be explained by a saturation of the electron heat diffusivity. The ion behaviour is indeed governed by dilution and equipartition effects. Although the ion heat transport is never neoclassical, there is no enhanced degradation at saturation. This behaviour is confirmed by turbulence measurements using CO2 laser coherent scattering. The level of density fluctuations follows the electron heat diffusivity variations with the average density. Waves propagating in the ion diamagnetic direction are always present in turbulence frequency spectra. Thus, the saturation cannot be explained by the onset of an ion turbulence. The existence of ion turbulence in the edge at all densities cannot be excluded. However, this ion feature in scattering spectra could be explained by a Doppler shift associated with an inversion point of the radial electric field at the edge

https://iopscience.iop.org/article/10.1088/0029-5515/32/12/I06/pdf

Turbulence and energy confinement in TORE SUPRA Ohmic discharges

A new O-mode dual frequency heterodyne reflectometer has been installed on Tore Supra [Tore Supra Team, Fusion Technol. 29, 417 (1996)] and it has greatly improved the phase determination due to its high dynamic heterodyne detection and ultrafast sweep capabilities (10 μs) provided by its solid state source HTO. This system operates with O-mode electric field polarization in the range of 26–36 GHz and has been designed for density profile measurements. The reflectometer launches two frequencies separated by 320 MHz simultaneously into the plasma. Heterodyne detection improves the dynamics up to 60 dB, and is associated with a sin/cos detection to allow separate analysis of the amplitude and phase of each reflected signal of both probing waves. Therefore, to calculate the density profile, the group delay can be defined in two different ways: (i) from the derivative of the absolute phase of one of the two probing waves or (ii) by calculating the phase difference between the two probing waves. We explain how...

C. Laviron

Papers

Reflectometry techniques for density profile measurements on fusion plasmas

Localized measurements of turbulence in the TORE SUPRA tokamak

ALTAIR: An infrared laser scattering diagnostic on the TORE SUPRA tokamak

Turbulence and energy confinement in TORE SUPRA Ohmic discharges

Ultrafast frequency sweep heterodyne reflectometer on the Tore Supra tokamak