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

Predictive models are developed for the temperature at the top at the edge of type 1 ELMy (edge localized mode) H-mode (high-confinement mode) plasmas. Theory-motivated models are used for the pedestal width and pressure gradient, while the pedestal density is obtained from experimental data in this study. The pedestal pressure gradient is assumed to be limited by the ballooning mode instability and is expressed in terms of the magnetic shear and geometrical factors. The effect of the bootstrap current, which reduces the magnetic shear in the steep pressure gradient region at the edge of the H-mode plasma, is included in the determination of the magnetic shear. Approaches for calculating the magnetic shear, combined with proposed models for the pedestal width, are used to determine the pedestal temperature. The computed pedestal temperatures are compared with more than 500 measured pedestal temperatures for type 1 ELMy H-mode discharges in four tokamaks. Some of the uncertainties in these results are disc...

Models for the pedestal temperature at the edge of H-mode tokamak plasmas

The International Thermonuclear Experimental Reactor FEAT design for a long-pulse tokamak burning plasma experiment (R=6.2 m, a=2 m, B=5.3 T, I=15 MA) is intended to achieve extended burn in inductively driven deuterium–tritium plasmas with the ratio of fusion power to auxiliary heating power, Q, of at least 10 and a nominal fusion power output of ∼500 MW. It also aims to demonstrate steady-state plasma operation using noninductive current drive with a Q of at least 5. Particular features of the design are: a significant operating window for Q=10 inductive operation; long inductive pulses (several hundred seconds burn); a capability for studying steady-state scenarios, specifically in cases where α-particles make a significant contribution to the plasma pressure; disruption physics processes which are comparable to those expected at the reactor scale; and an α-particle density and heating power which permit the key issues of α-particle confinement and α-particle driven magnetohydrodynamic instabilities to be investigated under conditions appropriate to a reactor.

The physics of the International Thermonuclear Experimental Reactor FEAT

The article treats the recent development of quasi-steady ELMy high ?p H?mode discharges with enhanced confinement and high ? stability, where long sustainment time, an increase in absolute fusion performance and extension of the discharge regime towards low q95 (~3) are emphasized. After modification to the new W shaped pumped divertor, a long heating time (9?s) with a high total heating energy input of 203?MJ became possible without a harmful increase in impurity and particle recycling. In addition, optimization of the pressure profile characterized by the double transport barriers, optimum electron density and/or high triangularity ? made it possible to extend the performance in long pulses. The DT equivalent fusion gain QeqDT ? 0.1 (? = 0.16) was sustained for ~9?s (~50?E, ~10?*p) and QDTeq ? 0.16 (? = 0.3) for 4.5?s at Ip = 1.5?MA. In the latter case with higher ?, an H?factor ( = ?E/?ITER89PLE) of ~2.2, ?N ? 1.9 and ?p ? 1.6 were sustained with 60-70% of the non-inductively driven current. In the low q95 (~3) region, the ? limit was improved by the high ? (~0.46) shape, where ?N ? 2.5-2.7 was sustained for ~3.5?s with the collisionality close to that of ITER-FDR plasmas. The limit of the edge ? parameter in the ELMy phase increases with ?, which is the main reason behind the improved ? limit in a long pulse at high ?. The sustainable value of ?NH also increases with ?. Sustainable ?N is limited by the onset of low n resistive modes. Direct measurement of island width shows agreement with the neoclassical tearing mode theory.

Long sustainment of JT-60U plasmas with high integrated performance

Characteristics of the internal transport barrier (ITB) were studied. The region of steep and , i.e. the ITB front, propagated from the core outwards. The thickness of the ITB front was about 3 cm. The ITB worked as a particle transport barrier as well as a thermal transport barrier for ions. The threshold heating power for ITB formation strongly increased with electron density and was independent of the toroidal magnetic field. ITB with was sustained for twice the global energy confinement time . A repetitive relaxation phenomenon at ITB was observed, which induced spikes like ELMs but had a different poloidal distribution.

Internal transport barrier with improved confinement in the JT-60U tokamak

Improvement of an enhanced confinement state in a high poloidal beta (βp) regime without sawtooth activity has been achieved in JT-60U. A confinement mode has been demonstrated where both the edge and the core confinement are improved. The attainable βp was also extended to higher values in this improved mode, because of its broader pressure profile. As a result of the improvement in confinement and in attainable βp, the highest value of the fusion triple product has been extended by a factor of 2.5 over that achieved in the 1992 experiments; it has reached (1.1 ± 0.3) × 1021 m-3.s.keV with a central ion temperature of about 37 keV. The D-D neutron emission rate has also been doubled in these experiments and has reached (5.6 ± 0.6) × 1016 s-1

https://iopscience.iop.org/article/10.1088/0029-5515/34/7/I09/pdf

M. Mori

Papers

Achievement of high fusion triple product in the JT-60U high βp H mode