Yu.Yu. Poshekhonov

The negative triangularity tokamak: stability limits and prospects as a fusion energy system

The paper discusses edge stability, beta limits and power handling issues for negative triangularity tokamaks. The edge magnetohydrodynamic stability is the most crucial item for power handling. For the case of negative triangularity the edge stability picture is quite different from that for conventional positive triangularity tokamaks: the second stability access is closed for localized Mercier/ballooning modes due to the absence of a magnetic well, and nearly internal kink modes set the pedestal height limit to be weakly sensitive to diamagnetic stabilization just above the margin of the localized mode Mercier criterion violation. While a negative triangularity tokamak is thought to have a low beta limit with its magnetic hill property, it is found that plasmas with reactor-relevant values of normalized beta beta(N) > 3 can be stable to global kink modes without wall stabilization with appropriate core pressure profile optimization against localized mode stability, and also with increased magnetic shear in the outer half-radius. The beta limit is set by the n = 1 mode for the resulting flat pressure profile. The wall stabilization is very inefficient due to strong coupling between external and internal modes. The n > 1 modes are increasingly internal when approaching the localized mode limit, and set a lower beta in the case of the peaked pressure profile leading to a Mercier unstable core. With the theoretical predictions supported by experiments, a negative triangularity tokamak would become a prospective fusion energy system with other advantages including a larger separatrix wetted area, more flexible divertor configuration design, wider trapped particle-free scrape-off layer, lower background magnetic field for internal poloidal field coils, and larger pumping conductance from the divertor room.

Tokamak plasma equilibrium problems with anisotropic pressure and rotation and their numerical solution

In the MHD tokamak plasma theory, the plasma pressure is usually assumed to be isotropic. However, plasma heating by neutral beam injection and RF heating can lead to a strong anisotropy of plasma parameters and rotation of the plasma. The development of MHD equilibrium theory taking into account the plasma inertia and anisotropic pressure began a long time ago, but until now it has not been consistently applied in computational codes for engineering calculations of the plasma equilibrium and evolution in tokamak. This paper contains a detailed derivation of the axisymmetric plasma equilibrium equation in the most general form (with arbitrary rotation and anisotropic pressure) and description of the specialized version of the SPIDER code. The original method of calculation of the equilibrium with an anisotropic pressure and a prescribed rotational transform profile is proposed. Examples of calculations and discussion of the results are also presented.

THE SPIDER CODE. MATHEMATICAL MODELING OF TOKAMAK PLASMA EQUILIBRIUM AND EVOLUTION

80 ВАНТ. Сер. Термоядерный синтез, 2014, т. 37, вып. 1 УДК 621.039.623 ВЫЧИСЛИТЕЛЬНЫЙ КОД SPIDER. МАТЕМАТИЧЕСКОЕ МОДЕЛИРОВАНИЕ РАВНОВЕСИЯ И ЭВОЛЮЦИИ ПЛАЗМЫ ТОКАМАКА А.А. Иванов, А.А. Мартынов, С.Ю. Медведев, Ю.Ю. Пошехонов, С.В. Коновалов, Р.Р. Хайрутдинов Институт прикладной математики им. М.В. Келдыша РАН, Москва, Россия НИЦ «Курчатовский институт», Москва, Россия Эффективный расчёт условий равновесия осесимметричной плазмы произвольной формы и с произвольным распределением давления и плотности тока по радиусу является одной из первоочередных задач теоретических исследований в проблеме УТС. Данная статья посвящена описанию многомодульного вычислительного кода SPIDER, предназначенного для численного моделирования равновесия и эволюции плазмы токамака в различных вариантах постановки соответствующих задач. Разработанные методы решения прямой и обратной задачи равновесия плазмы со свободной границей позволяют точно определять положение плазмы и токи в катушках полоидального поля в токамаках. Численный расчёт квазиравновесной эволюции плазмы с учётом диффузии магнитного поля и вихревых токов в проводящих структурах может быть использован для моделирования управления положением плазмы и расчёта её динамики во время срыва тока и развития периферийных локализованных неустойчивостей (ELM). Код может быть также применён для моделирования влияния анизотропии давления и вращения на равновесие плазмы.

Influence of plasma pedestal profiles on access to ELM-free regimes in ITER

The influence of current density and pressure gradient profiles in the pedestal on the access to the regimes free from edge localized modes (ELMs) like quiescent H-mode in ITER is investigated. Using the simulator of MHD modes localized near plasma boundary based on the KINX code, calculations of the ELM stability were performed for the ITER plasma in scenarios 2 and 4 under variations of density and temperature profiles with the self-consistent bootstrap current in the pedestal. Low pressure gradient values at the separatrix, the same position of the density and temperature pedestals and high poloidal beta values facilitate reaching high current density in the pedestal and a potential transition into the regime with saturated large scale kink modes. New version of the localized MHD mode simulator allows one to compute the growth rates of ideal peeling-ballooning modes with different toroidal mode numbers and to determine the stability region taking into account diamagnetic stabilization. The edge stability diagrams computations and sensitivity studies of the stability limits to the value of diamagnetic frequency show that diamagnetic stabilization of the modes with high toroidal mode numbers can help to access the quiescent H-mode even with high plasma density but only with low pressure gradient values at the separatrix. The limiting pressure at the top of the pedestal increases for higher plasma density. With flat density profile the access to the quiescent H-mode is closed even with diamagnetic stabilization taken into account, while toroidal mode numbers of the most unstable peeling-ballooning mode decrease from n = 10−40 to n = 3−20.

Comparison of the calculations of the stability properties of a specific stellarator equilibrium with different MHD stability codes

A particular configuration of the LHD stellarator with an unusually flat pressure profile has been chosen to be a test case for comparison of the MHD stability property predictions of different three-dimensional and averaged codes for the purpose of code comparison and validation. In particular, two relatively localized instabilities, the fastest growing modes with toroidal mode number n = 2 and n = 3 were studied using several different codes, with the good agreement that has been found providing justification for the use of any of them for equilibria of the type considered.