Plasma Physics and Controlled Fusion

Angioni, Clemente

doi: 10.1088/1361-6587/abfc9apmid: N/A

In this paper, the theory of collisional and turbulent transport of impurities in tokamak plasmas is reviewed. The results are presented with the aim of providing at the same time a historical reconstruction of the scientific progress and a complete description of the present theoretical knowledge, with a hopefully sufficiently complete reference to the works which have been published in the field in the last decades. After a general introduction on the physics challenges offered by the problem of impurity transport and their relevance for practical nuclear fusion energy, the theory of collisional transport is presented. Here a specific section is also dedicated to the transport parallel to the magnetic field lines. A complete review of the transport mechanisms produced by turbulence follows. The corresponding comparisons between theoretical predictions and experimental observations are also presented, highlighting the influence that the validation activities had in motivating further theoretical investigations. The paper is completed by a section on the direct interactions between collisional and turbulent transport and by a final specific review dedicated to the progress in the theory–based modelling activities. In the writing of this review paper, the main goal has been to combine readability with completeness and scientific rigour, providing a comprehensive list of references for deeper documentation on specific aspects.

Blackman, David R; Adak, Amitava; Singh, Prashant K; Lad, Amit D; Chatterjee, Gourab; Ridgers, Christopher P; Del Sorbo, Dario; Trines, Raoul M G M; Robinson, A P L; Nazarov, Wigen; Ravindra Kumar, G; Pasley, John

doi: 10.1088/1361-6587/abf85cpmid: N/A

The formation and evolution of post-solitons has been discussed for quite some time both analytically and through the use of particle-in-cell (PIC) codes. It is however only recently that they have been directly observed in laser-plasma experiments. Relativistic electromagnetic (EM) solitons are localised structures that can occur in collisionless plasmas. They consist of a low-frequency EM wave trapped in a low electron number-density cavity surrounded by a shell with a higher electron number-density. Here we describe the results of an experiment in which a 100 TW Ti:sapphire laser (30 fs, 800 nm) irradiates a TMPTA foam target with a focused intensity . A third harmonic ( nm) probe is employed to diagnose plasma motion for 25 ps after the main pulse interaction via Doppler-Spectroscopy. Both radiation-hydrodynamics and 2D PIC simulations are performed to aid in the interpretation of the experimental results. We show that the rapid motion of the probe critical-surface observed in the experiment might be a signature of post-soliton wall motion.

Jorge, R; Landreman, M

doi: 10.1088/1361-6587/abfdd4pmid: N/A

The stability of the ion-temperature gradient mode in quasisymmetric stellarators is assessed. This is performed using a set of analytical estimates together with linear gyrokinetic simulations. The peak growth rates, their corresponding real frequencies and wave-vectors are identified. A comparison is made between a first-order near-axis expansion model and eleven realistic designs obtained using numerical optimization methods. It is found that while the near-axis expansion is able to replicate the growth rates, real frequencies and perpendicular wave-vector at the inner core (both using simplified dispersion relations and first-principle gyrokinetic simulations), it leads to an overestimation of the growth rate at larger radii. An approximate analytic solution of the ITG dispersion relation for the non-resonant limit suggests growth rates could be systematically higher in quasi-axisymmetric (QA) configurations compared to quasi-helically (QH) symmetric ones. However except for very close to the axis, linear gyrokinetic simulations do not show systematic differences between QA and QH configurations.

Hyland, C; White, S; Kettle, B; Irwin, R; Bailie, D; Yeung, M; Williams, G; Heathcote, R; East, I; Spindloe, C; Notley, M; Riley, D

doi: 10.1088/1361-6587/ac0353pmid: N/A

Extreme ultraviolet radiation from a high harmonic source has been used to measure the free-free attenuation coefficient and real refractive index of warm dense aluminium, with sample conditions of near solid density and temperature of 0.9 ± 0.23 eV. These were compared to results from the literature, where the measured attenuation coefficients showed some consistency with the modelling and existing data from a previous experiment. The absolute values of the attenuation coefficient were found to reside between the different sets of models for the warm dense matter (WDM) attenuation coefficient, and were found to be more in line with modelling and measurements of the cold opacity from the literature. Novel measurements of the real refractive index of WDM were also achieved—while ambiguity makes these measurements consistent with all the models, they prove useful as a proof-of-concept for future WDM studies.

Casolari, A; Ficker, O; Grover, O; Jaulmes, F; Kripner, L; Macusova, E; Markovic, T; Peterka, M; Yanovskiy, V; ,

doi: 10.1088/1361-6587/abfbbdpmid: N/A

The non-linear dynamics of a chain of stationary, saturated magnetic islands is studied by solving a four-field system of equations that include non-ideal effects, lowest order finite Larmor radius corrections and neoclassical terms. The magnetic island rotation velocity is calculated self-consistently with the fields profiles. The solutions for the island rotation velocity and for the ion polarization current are determined as a function of the characteristic parameters of the system and the results are discussed. The results of the calculations show that island rotation velocity and the ion polarization current depend in a non-trivial way on the parameters characterizing the system, and some clear patterns emerge only in particular cases. An analysis of magnetic island rotation velocity is performed on experiments in COMPASS tokamak. Measured island rotation velocity is compared with the calculated ion and electron flow velocities, for different hypotheses on the toroidal rotation of the plasma. The comparison shows that the island rotation velocity is consistent with the ion flow velocity, under the hypothesis of slow toroidal rotation and low collisionality. Theoretical calculation of the island rotation velocity according to the model here developed suggests that the islands rotate weakly in the ion direction, in the hypothesis of slow toroidal rotation and high collisionality. The impossibility of directly measuring the plasma rotation velocity makes it difficult to distinguish between these different regimes.

Severo, J H F; Canal, G P; Ronchi, G; Andrade, N B; Fernandes, T; Ikeda, M Y; Collares, M P; Galvão, R M O; Nascimento, I C; Tendler, M

doi: 10.1088/1361-6587/abf955pmid: N/A

An overview of intrinsic plasma rotation studies in Ohmic L-mode discharges carried out in the Tokamak Chauffage Alfvén Brésilien (TCABR) tokamak is presented. Measurements of plasma poloidal and toroidal rotation, and a comparison against neoclassical theory, are presented. The results show that poloidal rotation is in good agreement with neoclassical theory while toroidal rotation is found to be anomalous. A new technique that allows for high temporal resolution measurements of plasma rotation is presented. This technique is used to test two models of intrinsic toroidal rotation: the so-called Helander model (Helander et al 2003 Physics of Plasmas 10 4396) and Rozhansky model (Rozhansky 2013 Perpendicular currents and electric fields in fully and partially ionized magnetized plasma Physics of Plasmas 24 101614). As TCABR is a relatively small device, the influence of the neutrals that form the basis of this model is expected to be enhanced. The results indicate that the mechanism proposed by Helander does not contribute significantly to the intrinsic toroidal rotation in TCABR plasmas. The measurements, however, indicate that the frictional force proposed by Rozhansky might be responsible for part of the intrinsic toroidal rotation observed in TCABR plasmas.

Wu, N; Cheng, J; Yan, L W; Huang, Z H; Xu, J Q; Wang, W C; Ke, R; Yang, Z C; Ji, X Q; Liu, L; He, X X; Jiang, M; Wen, J; Miao, X G; Zhong, W L; Yu, L M; Gao, J M; Dong, Y B; Shi, Z B;

Zito, A; Wischmeier, M; Carralero, D; Manz, P; Paradela Pérez, I; Passoni, M; ,

doi: 10.1088/1361-6587/abfcb6pmid: N/A

A desirable scenario for future fusion devices is one in which dissipative processes in the scrape-off layer (SOL) are maximized, aiming to detach the divertor plasma. The access to such a regime in current devices is thought to be correlated to the increase of the perpendicular particle transport in the SOL. In this work we investigated numerically how increasing perpendicular transport globally affects the SOL plasma through the SOLPS-ITER code package. For this we modelled one L-mode discharge, performed at the ASDEX Upgrade tokamak, trying to obtain the most accurate fit to the experimental data at the outer midplane. Studying the plasma solutions and analyzing the resulting momentum and power balances in the SOL allowed to characterize how enhancing perpendicular SOL transport leads to the experimentally observed phenomena, i.e. the formation of a density shoulder at the midplane and the partial detachment of the divertor plasma. The results suggest that strong momentum losses caused by the increase of transport are able to explain the qualitatively observed detachment in the modelled discharge. The concurrent enhanced ionization of neutrals resulting from divertor recycling, triggered by an increase of radial energy transport in the SOL, can be invoked as a cause for the shoulder formation.

Liu, Hai; Xu, Kangzhong; Xu, Yuhong; Chen, Zhipeng; Cheng, Jun; Liu, Haifeng; Wang, Xianqu; Huang, Jie; Zhang, Xin; Shao, Junren; Xiao, Chijin; Tang, Changjian; ,

doi: 10.1088/1361-6587/abf85d

Lu, Chao; Tikhonchuk, Vladimir; Weber, Stefan

doi: 10.1088/1361-6587/abf766pmid: N/A

Comprehensive understanding of nonlocal transport is mandatory for many applications of laser–plasma interaction physics, such as inertial confinement fusion and modeling of astrophysical phenomena in the laboratory. Theoretical description is important for guiding numerical simulations and experiments. In this article, an analytic approach is developed: by using a simple integral differential model, we calculate the preheating of plasma upstream of the heat front associated with nonlocal transport. A detailed comparison of various transport configurations as a function of the boundary conditions and the nonlinearity parameter is presented in the context of laser–plasma interaction. Three important results are demonstrated in this paper. First, analytical expressions for all possible self-similar solutions of local nonlinear transport are obtained. Second, a systematic comparison of various nonlocal kernels and forms of the delocalization length is performed. Third, an analytic expression for the temperature profile upstream of the heat front is obtained.