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

Journal of Scientific Instruments Editor: Dr H R Lang Vol 28 and Supplement No 1, 1951 Pp xvi + 388 + iii + 80 (London: Institute of Physics, 1951) Bound, £3 12s; unbound, £3

/pdf/a-review-of-scientific-instruments-50cfd4j98s.pdf

A Review of Scientific Instruments

The transport barrier which exists at the plasma edge in the high-confinement (H-mode) regime is now recognized as important to global energy confinement. Recently, detailed measurements of the width and height of barriers in electron and ion density and/or temperature have been made on several divertor tokamaks. These results are reviewed, with an emphasis on scalings of the pedestal height and width. Many experiments observe situations in which the limiting pedestal pressure gradient is well described by ideal ballooning stability. In some cases the extreme edge is in the `second stable' regime. Differences between the widths ? of pedestals in electron and ion temperature and densities are noted in several experiments. Some reported scalings are ?Ti = 3.3(?)1/2?i,pol on JT-60U, and ?Pe/R?pol,ped0.4 on DIII-D. ?Te is independent of plasma current in many experiments. Theoretical modelling of the fully developed H-mode pedestal is reviewed. Several recent predictions of the pedestal parameters and their scalings are outlined and compared with experiments.

https://iopscience.iop.org/article/10.1088/0741-3335/42/5A/302/pdf

Physics and scaling of the H-mode pedestal

Studies of the enhanced Dα H-mode (EDA) have been extended to include ohmic plasmas. No clear difference in the EDA/ELMfree boundary or in other phenomenology are seen between ohmic and ICRF-heated plasmas, suggesting that neither the effect of ion tails nor direct RF/edge plasma interaction plays a role in EDA. Edge safety factor (q95) is the principal variable which determines which regime a discharge will be in. When q95 is greater than 4.0 for standard-shaped plasmas, the discharge is almost always EDA, while when it is less than 3.5, the plasma is almost always ELMfree. New edge diagnostics have allowed measurement of pedestal profiles with resolution of the order of 1 mm. Sudden changes in profile widths are not seen when the plasma makes a transition from EDA to ELMfree; however, the widths do vary with the same parameters that determine the EDA/ELMfree boundary. Strong edge-density fluctuations are observed to accompany EDA and may be responsible for the change in particle transport which is observed. The fluctuations have a quasi-coherent component whose frequency varies inversely with the pedestal width as measured by a visible continuum diagnostic.

/pdf/studies-of-eda-h-mode-in-alcator-c-mod-3zd1ycyoyw.pdf

Studies of EDA H-Mode in Alcator C-Mod

Multi-machine comparisons of H-mode separatrix densities and edge profile behaviour in the ITPA SOL and Divertor Physics Topical Group

In this paper we summarize the results of recent measurements of the high confinement mode (H-mode) edge transport barrier in Alcator C-Mod. A number of new, high-resolution edge diagnostics provide measurements of plasma parameters (electron density and temperature, and impurity density) in the pedestal region. Pedestal parameters are measured both at the top edge of the plasma and at the midplane. Very narrow (2-5 mm) electron density and temperature and impurity density pedestals are observed. The impurity pedestal is located inside the density and temperature pedestals in the midplane measurements, but coincides with the ne pedestal at the top edge. Correlation of both width and height of the pedestals at the midplane with global plasma parameters and core confinement characteristics is observed. At the same time there are no clear indications of scaling for the electron density and impurity pedestal widths measured at the top of the plasma.

/pdf/measurements-and-scalings-of-the-h-mode-pedestal-on-alcator-x2dqisfggy.pdf

C. S. Pitcher

Papers

Measurements and scalings of the H-mode pedestal on Alcator C-Mod