Tomography of full sawtooth crashes on the Tokamak Fusion Test Reactor

Full sawtooth crashes in high temperature plasmas have been investigated on the Tokamak Fusion Test Reactor (TFTR) [Plasma Phys. Controlled Fusion 33, 1509 (1991)]. A strong asymmetry in the direction of major radius, a feature of the ballooning mode, and a remaining m=1 region after the crash have been observed with electron cyclotron emission image reconstructions. The TFTR data is not consistent with two-dimensional (2-D) models; it rather suggests a three-dimensional (3-D) localized reconnection arising on the bad curvature side. This process explains the phenomenon of fast heat transfer which keeps the condition q0<1

Feedback effects between resonance surface and space harmonics of external perturbations in tokamak

Resonant magnetic surfaces in a tokamak can amplify the spatial harmonics of external perturbations, which may come from other resonant surfaces, from error fields, or from a feedback system. The behavior of this active resonant media can be roughly approximated with a system of coupled Van der Pole oscillators. The effect of frequency injection locking (or spatial harmonics injection locking in the plasma frame) is typical for these nonlinear systems. It happens when the amplitude of one modes increases and this mode becomes a dominant mode. Transition into synchronized condition can occur in a time scale of ~ 50 -100 µsec. For a tokamak it means that the stability of a large scale MHD perturbation can change jumpily, because frequency (phase) lock may create a positive feedback between resonant surfaces (or between resonant surfaces and the external feedback system). This effect probably determines the explosive dynamic of the disruptive instability.Резонансна магнітна поверхня в токамаці може підсилювати просторові гармоніки збурювань інших резонансних поверхонь, збурювання просторових гармонік обмоток полоідального і тороідального полів (Error field) або обмоток зворотних зв'язків (Feedback field). Поводження цього активного резонансного середовища грубо можна апроксимувати системою зв'язаних генераторів Ван дер Поля. Ефект захоплення частоти (або захоплення просторових гармонік збурювань у системі координат, зв'язаної з плазмою), є типовим для подібних нелінійних систем. Він відбувається в тому випадку, коли амплітуда однієї з мод збільшується і ця мода стає домінантною модою. Перехід у стан захоплення (синхронізації) частоти відбувається за часи ~ 50 -100 µsec. У цей момент стійкий стан великомасштабних МГД-збурювань може стрибком стати нестійким унаслідок появи позитивного зворотного зв'язку між резонансними поверхнями (або між резонансними поверхнями і системою зворотних зв'язків). Цей ефект можливо визначає вибуховий характер розвитку нестійкості зриву.Резонансная магнитная поверхность в токамаке может усиливать пространственные гармоники возмущений других резонансных поверхностей, возмущения пространственных гармоник обмоток полоидального и тороидального полей (Error field) или обмоток обратных связей (Feedback field). Поведение этой активной резонансной среды грубо можно аппроксимировать системой связанных генераторов Ван дер Поля. Эффект захвата частоты (или захвата пространственных гармоник возмущений в системе координат, связанной с плазмой), является типичным для подобных нелинейных систем. Он происходит в том случае, когда амплитуда одной из мод увеличивается и эта мода становится доминантной модой. Переход в состояние захвата (синхронизации) частоты происходит за времена ~ 50 -100 µsec. В этот момент устойчивое состояние крупномасштабных МГД-возмущений может скачком стать неустойчивым вследствие появления положительной обратной связи между резонансными поверхностями (или между резонансными поверхностями и системой обратных связей). Этот эффект возможно определяет взрывной характер развития неустойчивости срыва

Tomography of (2, 1) and (3, 2) magnetic island structures on Tokamak Fusion Test Reactor

High-resolution electron cyclotron emission (ECE) image reconstruction has been used to observe (m,n)=(2,1) and (3, 2) island structures on Tokamak Fusion Test Reactor [Plasma Phys. Controlled. Fusion 33, 1509 (1991)], where m and n are the poloidal and the toroidal mode number, respectively. The observed island structure is compared with other diagnostics, such as soft x-ray tomography and magnetic measurements. A cold elliptic island is observed after lithium pellet injection. Evidence for the enhancement of the heat transfer due to the island is observed. A relaxation phenomenon due to the m=2 mode is newly observed in Ohmic plasmas

Neoclassical Tearing Modes in Tokamak Fusion Test Reactor Experiments Part 1. Measurements of Magnetic Islands and Delta Prime.

Tearing-type modes are observed in most high-confinement operation regimes in TFTR. Three different methods are used to measure the magnetic island widths: external magnetic coils, internal temperature fluctuation from the electron cyclotron emission (ECE) diagnostic, and an experiment where the plasma major radius is rapidly shifted ("Jog" experiments). A good agreement between the three methods is observed. Numerical and analytic calculations of delta prime (the tearing instability index) are compared with an experimental measurement of delta prime using the tearing-mode eigenfunction mapped from the jog data. The obtained negative delta prime indicates that the observed tearing modes cannot be explained by the classical current-gradient-driven tearing theory

Phase separating binary fluids under oscillatory shear

We apply lattice Boltzmann methods to study the segregation of binary fluid mixtures under oscillatory shear flow in two dimensions. The algorithm allows to simulate systems whose dynamics is described by the Navier-Stokes and the convection-diffusion equations. The interplay between several time scales produces a rich and complex phenomenology. We investigate the effects of different oscillation frequencies and viscosities on the morphology of the phase separating domains. We find that at high frequencies the evolution is almost isotropic with growth exponents 2/3 and 1/3 in the inertial (low viscosity) and diffusive (high viscosity) regimes, respectively. When the period of the applied shear flow becomes of the same order of the relaxation time $T_R$ of the shear velocity profile, anisotropic effects are clearly observable. In correspondence with non-linear patterns for the velocity profiles, we find configurations where lamellar order close to the walls coexists with isotropic domains in the middle of the system. For particular values of frequency and viscosity it can also happen that the convective effects induced by the oscillations cause an interruption or a slowing of the segregation process, as found in some experiments. Finally, at very low frequencies, the morphology of domains is characterized by lamellar order everywhere in the system resembling what happens in the case with steady shear.Comment: 1 table and 12 figures in .gif forma

Exploration of the equilibrium operating space for NSTX-Upgrade

This paper explores a range of high-performance equilibrium scenarios available in the NSTX-Upgrade device [J.E. Menard, submitted for publication to Nuclear Fusion]. NSTX-Upgrade is a substantial upgrade to the existing NSTX device [M. Ono, et al., Nuclear Fusion 40, 557 (2000)], with significantly higher toroidal field and solenoid capabilities, and three additional neutral beam sources with significantly larger current drive efficiency. Equilibria are computed with freeboundary TRANSP, allowing a self consistent calculation of the non-inductive current drive sources, the plasma equilibrium, and poloidal field coil current, using the realistic device geometry. The thermal profiles are taken from a variety of existing NSTX discharges, and different assumptions for the thermal confinement scalings are utilized. The no-wall and idealwall n=1 stability limits are computed with the DCON code. The central and minimum safety factors are quite sensitive to many parameters: they generally increases with large outer plasmawall gaps and higher density, but can have either trend with the confinement enhancement factor. In scenarios with strong central beam current drive, the inclusion of non-classical fast ion diffusion raises qmin, decreases the pressure peaking, and generally improves the global stability, at the expense of a reduction in the non-inductive current drive fraction; cases with less beam current drive are largely insensitive to additional fast ion diffusion. The non-inductive current level is quite sensitive to the underlying confinement and profile assumptions. For instance, for BT=1.0 T and Pinj=12.6 MW, the non-inductive current level varies from 875 kA with ITER-98y,2 thermal confinement scaling and narrow thermal profiles to 1325 kA for an ST specific scaling expression and broad profiles. This sensitivity should facilitate the determination of the correct scaling of transport with current and field to use for future fully non-inductive ST devices. Scenarios are presented which can be sustained for 8-10 seconds, or (20-30)τCR, at βN=3.8-4.5, facilitating, for instance, the study of disruption avoidance for very long pulse. Scenarios have been documented which can operate with βT~25% and equilibrated qmin&gt;1. The value of qmin can be controlled at either fixed non-inductive fraction of 100% or fixed plasma current, by varying which beam sources are used, opening the possibility for feedback qmin control. In terms of quantities like collisionality, neutron emission, non-inductive fraction, or stored energy, these scenarios represent a significant performance extension compared to NSTX and other present spherical torii

Progress towards Steady State at Low Aspect Ratio on the National Spherical Torus Experiment (NSTX)

Modifications to the plasma control capabilities and poloidal field coils of the National Spherical Torus Experiment (NSTX) have enabled a significant enhancement in shaping capability which has led to the transient achievement of a record shape factor (S ≡ q95 (Iρ⁄ αΒτ)) of ∼41 (MA m−1 Τ−1) simultaneous with a record plasma elongation of κ ≡ β ⁄ α ∼ 3. This result was obtained using isoflux control and real-time equilibrium reconstruction. Achieving high shape factor together with tolerable divertor loading is an important result for future ST burning plasma experiments as exemplified by studies for future ST reactor concepts, as well as neutron producing devices, which rely on achieving high shape factors in order to achieve steady state operation while maintaining MHD stability. Statistical evidence is presented which demonstrates the expected correlation between increased shaping and improved plasma performance