Ion viscous heating in a magnetohydrodynamically unstable Z pinch at over 2 x 10(9) Kelvin

Pulsed power driven metallic wire-array Z pinches are the most powerful and efficient laboratory x-ray sources. Furthermore, under certain conditions the soft x-ray energy radiated in a 5 ns pulse at stagnation can exceed the estimated kinetic energy of the radial implosion phase by a factor of 3 to 4. A theoretical model is developed here to explain this, allowing the rapid conversion of magnetic energy to a very high ion temperature plasma through the generation of fine scale, fast-growing m = 0 interchange MHD instabilities at stagnation. These saturate nonlinearly and provide associated ion viscous heating. Next the ion energy is transferred by equipartition to the electrons and thus to soft x-ray radiation. Recent time-resolved iron spectra at Sandia confirm an ion temperature Ti of over 200 keV (2 x 10(9) degrees), as predicted by theory. These are believed to be record temperatures for a magnetically confined plasma.     

Observations of an ion-driven instability in non-neutral plasmas confined on magnetic surfaces

The first detailed experimental study of an instability driven by the presence of a finite ion fraction in an electron-rich non-neutral plasma confined on magnetic surfaces is presented. The instability has a poloidal mode number m=1, implying that the parallel force balance of the electron fluid is broken and that the instability involves rotation of the entire plasma, equivalent to ion-resonant instabilities in Penning traps and toroidal field traps. The mode appears when the ion density exceeds approximately 10% of the electron density. The measured frequency decreases with increasing magnetic field strength, and increases with increasing radial electric field, showing that the instability is linked to the E x B flow of the electron plasma. The frequency does not, however, scale exactly with E/B, and it depends on the ion species that is introduced, implying that the instability consists of interacting perturbations of ions and electrons.     

Optical diagnostics of a low frequency instability rotating around a magnetized plasma column

An argon magnetized plasma column is created with primary energetic electrons in the Mistral device. Low frequency instabilities regularly rotating around this column are observed with an ultra-fast camera and a spectroscopic device. Experimental results coupled to a coronal code show the presence of a few percents of fast (hot) electrons inside the ejected plasma. It also shows that ultra-fast camera analysis of the ejected plasma can only give information on the primary electron population. Finally, these results suggest that the radial decrease of the light emitted by the ejected plasma is essentially due to the radial decrease of the mean energy of the hot electrons.     

Observations of Nonlinear Behavior in a Low-Pressure Discharge Column

Sudden and abrupt jumps in the plasma density and discharge current of low-pressure magnetized argon and helium plasmas are observed. These jumps are found to depend on the discharge bias voltage, the neutral gas pressure, and the magnetic field strength and occur with a substantial hysteresis in those parameters. These jumps are accompanied by the onset of intense and coherent low-frequency plasma oscillations. In addition, under certain conditions, the radial density profile of the plasma is found to be significantly different following a jump. Some possibly related plasma instabilities are discussed.     

Direct observation of confined acoustic phonons in the photoluminescence spectra of a single CdSe-CdS-ZnS core-shell-shell nanocrystal

We report on the direct observation of confined acoustic phonons in the photoluminescence spectra of single CdSe-CdS-ZnS nanocrystals, whose ligands were exchanged to poly(ethylene oxide) (PEO) before they were embedded in a PEO matrix. Modeling a nanocrystal as an elastic sphere, the confined acoustic modes can be assigned to purely radial vibrations: the breathing mode and its two first radial harmonics. In addition to acoustic modes, we also observe longitudinal optical modes of the core material and, remarkably, also of both shell materials.     

Whistler instability in an electron-magnetohydrodynamic spheromak

A three-dimensional magnetic vortex, propagating in the whistler mode, has been produced in a laboratory plasma. Its magnetic energy is converted into electron kinetic energy. Non-Maxwellian electron distributions are formed which give rise to kinetic whistler instabilities. The propagating vortex radiates whistler modes along the ambient magnetic field. A new instability mechanism is proposed.     

Linear Stability of Taylor-Couette Flows with Axial Heat Buoyancy

The linear stability is studied of flows confined between two concentric cylinders, in which the radial temperature gradient and axial gravity are considered for an incompressible Newtonian fluid. Numerical method based on the Petrov-Galerkin scheme is developed to deal with the buoyancy term in momentum equations and an additional temperature perturbation equation. Computations of the neutral stability curves are performed for different rotation cases. It is found that the flow instability is influenced by both centrifugal and axial shear instabilities, and the two instability mechanisms interact with each other. The outer cylinder rotation plays dual roles of stabilizer and destabilizer under different rotating stages with the inner cylinder at rest. For the heat buoyancyinduced axial flow, spiral structures are found in the instability modes.     

声场中空化气泡的耦合振动及形状不稳定性的研究

计算了两个具有非球形扰动的气泡所组成系统的能量,并基于Lagrange方程得到了有声相互作用的非球形气泡的动力学方程和形状稳定性方程,研究了声场中非球形气泡间相互作用力对非球形气泡的形状不稳定性和气泡形状模态振幅的影响.研究结果表明声场中具有非球形扰动的气泡之间的耦合方式有两种:形状耦合模式和径向耦合模式,气泡之间的耦合方式取决于气泡形状扰动模态.由形状耦合及径向耦合产生的气泡之间的相互作用力能够改变单个气泡的形状不稳定及形状模态振幅,具体影响因素取决于声场驱动条件、气泡形状模态、相邻气泡的初始半径.     

Thetapinch experiments with trapped antiparallel magnetic fields

Experimental evidence is presented which shows that stability for a cylindrical plasma sheet confined by antiparallel magnetic fields is established for times which are longer than theoretically predicted growth times for tearing instabilities by at least an order of magnitude. It is shown that the enhanced stability is not due to wall effects, inertia effects, too short a pinching coil, finite Larmor radius and viscosity effects. One possible explanation for the stability is shown to be related to the level of initial plasma perturbation. Further the enhanced stability can be explained by the observed plasma rotation. It is shown by simple arguments that the rotation can cancel the tearing mode's growth and that the rotation itself is a result of the diffusion of angular momentum across the neutral surface enclosing the confined plasma.     

Cross-field transport by instabilities and blobs in a magnetized toroidal plasma

The mechanisms for anomalous transport across the magnetic field are investigated in a toroidal magnetized plasma. The role of plasma instabilities and macroscopic density structures (blobs) is discussed. Examples from a scenario with open magnetic field lines are shown. A transition from a main plasma region into a loss region is reproduced. In the main plasma, which includes particle and heat source locations, the transport is dominated by the fluctuation-induced particle and heat flux associated with a plasma instability. On the low-field side, the cross-field transport is ascribed to the intermittent ejection of macroscopic blobs propagating toward the outer wall. It is shown that instabilities and blobs represent fundamentally different mechanisms for cross-field transport.     

Experimental observation of the blob-generation mechanism from interchange waves in a plasma

The mechanism for blob generation in a toroidal magnetized plasma is investigated using time-resolved measurements of two-dimensional structures of electron density, temperature, and plasma potential. The blobs are observed to form from a radially elongated structure that is sheared off by the E x B flow. The structure is generated by an interchange wave that increases in amplitude and extends radially in response to a decrease of the radial pressure scale length. The dependence of the blob amplitude upon the pressure radial scale length is discussed.     

Experimental proof of mirror instabilities in the Isar I theta pinch

In the Isar I linear theta pinchm=0 instabilities were observed and experimentally investigated. The variation of the plasma parameters over a wide range (τ _ii =10^?2... 10²μsec) showed that them=0 instabilities only occurred in sufficiently hot and thin plasmas, and so they are most probably due to the anisotropic pressure of the ions and should be mirror instabilities. Perturbations of the plasma limited in space and time were observed by measuring the continuum radiation, by 90 ° laser scattering measurements and by measuring the magnetic field and the local neutron rate. All measurements showed distinct, correlated, local perturbations of the plasma cylinder. These perturbations were accompanied by bulging of the plasma cylinder and increasing density. The extent in the axial direction was of the order of the plasma diameter. It may be assumed that the observed instabilities contribute appreciably to relaxation of the anisotropic ion pressure and increase the end losses.     

Ion velocity shear-induced drift wave and momentum transport in non-Maxwellian magnetoplasma flows

We have investigated the diamagnetic flow in a non-uniform partially ionized plasma with non-Maxwellian electron population to explain the dynamics of ion velocity shear-induced low-frequency drift mode and associated instabilities. The dispersion relations are found, and instability threshold conditions are pointed out along with ion-parallel momentum transport due to drift motion with relative phase shift of fluctuating quantities in a non-conservative system. The real frequencies and instability growth rates are studied numerically and illustrated for typical space and laboratory plasmas. This study should be useful in understanding some aspects of low-frequency time-delayed perturbations with sheared flow leading to drift instabilities and cross-field parallel ion momentum transport in nonuniform magnetoplasmas containing a non-Maxwellian electron population.     

Reduced-order modelling of thermoacoustic instabilities in a two-heater Rijke tube

The topic of thermoacoustic instabilities in combustors is well-investigated, as it is important in the field of combustion, primarily in gas-turbine engines. In recent years, much attention has been focused on monitoring, diagnosis, prognosis, and control of high-amplitude pressure oscillations in confined combustion chambers. The Rijke tube is one of the most simple, yet very commonly used, laboratory apparatuses for emulation of thermoacoustic instabilities, which is also capable of capturing the physics of the thermally driven acoustics. A Rijke tube apparatus can be constructed with an electrical heater acting as the heat source, thus making it more flexible to operate and safer to handle than a fuel-burning Rijke tube or a fuel-fired combustor. Augmentation of the heat source of the Rijke tube with a secondary heater at a downstream location facilitates better control of thermoacoustic instabilities. Along this line, much work has been reported on the investigation of thermoacoustics by using computational fluid dynamics (CFD) modelling as well as reduced-order modelling for both single-heater and two-heater Rijke tube systems. However, since reduced-order models are often designed and built upon certain empirical relations, they may not account for the dynamic behaviour of the heater itself, which is a critical factor in the analysis and synthesis of real-time robust control systems. This issue is addressed in the current paper, where modifications have been made to existing models by incorporating heater dynamics. The model results are systematically validated with experimental data, generated from an in-house (electrically heated) Rijke tube apparatus.     

High frequency fast magnetoacoustic modes in the plasma core

Fast magnetoacoustic modes (FMM) [known also as compressional Alfvén eigenmodes (CAE) and magnetosonic modes] with frequencies exceeding or equal the ion gyrofrequency are considered. It is shown that edge-localized FMM, which presumably are responsible for the superthermal ion cyclotron emission (ICE) observed in many experiments on tokamaks and stellarators, represent a particular case of these modes. In general, FMMs with frequencies above/about the ion gyrofrequency have different radial locations and structures. They can extend over a large part of the plasma cross section and even can have maximum amplitudes at the magnetic axis. Modes with the same frequency and the same poloidal mode number are multiple, having different radial structures. These results are obtained in the approximation of cylindrical plasma with one-ion species.     

Spiral structures in magnetized rotating plasmas

A theory of spiral structure formation has been formulated to show that spiral structures are rather basic entities in magnetized rotating plasmas subjected to various types of instabilities such as collisional drift wave instability, flute mode instability due to centrifugal force, and Kelvin-Helmhotz instability. The characteristic features of spiral structures observed experimentally in electron cyclotron resonance plasmas are reproduced by our theory.     

An experimental study into the effect of injector pressure loss on self-sustained combustion instabilities in a swirled spray burner

Combustion instabilities depend on a variety of parameters and operating conditions. It is known, especially in the field of liquid rocket propulsion, that the pressure loss of an injector has an effect on its dynamics and on the coupling between the combustion chamber and the fuel manifold. However, its influence is not well documented in the technical literature dealing with gas turbine combustion dynamics. Effects of changes in this key design parameter are investigated in the present article by testing different swirlers at constant thermal power on a broad range of injection velocities in a well controlled laboratory scale single injector swirled combustor using liquid fuel. The objective is to study the impact of injection pressure losses on the occurrence and level of combustion instabilities by making use of a set of injectors having nearly the same outlet velocity profiles, the same swirl number and that establish flames that are essentially identical in shape. It is found that combustion oscillations appear on a wider range of operating conditions for injectors with the highest pressure loss, but that the pressure fluctuations caused by thermoacoustic oscillations are greatest when the injector head loss is low. Four types of instabilities coupled by two modes may be distinguished: the first group features a lower frequency, arises when the injector pressure loss is low and corresponds to a weakly coupled chamber-plenum mode. The second group appears in the form of a constant amplitude limit cycle, or as bursts at a slightly higher frequency and is coupled by a chamber mode. Spontaneous switching between these two types of instabilities is also observed in a narrow domain.     

托卡马克高约束模边缘等离子体不稳定性研究

托卡马克高约束模运行可大幅提高磁约束核聚变等离子体约束品质,该模式下的等离子体不稳定性研究对于控制约束和保护装置有重要意义。本文主要介绍了高约束模及其边缘等离子体不稳定性的研究概况,并重点介绍了中国环流器二号A托卡马克装置上关于高约束模转换、边缘局域模特征和控制方法、台基区不稳定性和台基饱和机制等方面的研究进展。研究结果表明,实验上有望通过粒子和射频波注入等外部激励的方法,影响台基区等离子体湍流,进行控制台基动力学演化以及ELM,实现既保持高约束又降低高热负荷的等离子体稳态运行。     

Dynamics of laser-cooled Ca+ ions in a Penning trap with a rotating wall

We have performed systematic measurements of the dynamics of laser-cooled ⁴⁰Ca⁺ ions confined in a Penning trap and driven by a rotating dipole field (‘rotating wall’). The trap used is a copy of the one used in the SPECTRAP experiment located at the HITRAP facility at GSI, Germany. The size and shape of the ion cloud has been monitored using a CCD camera to image the fluorescence light resulting from excitation by the cooling laser. We have varied the experimental conditions such as amplitude and frequency of the rotating wall drive as well as the trapping parameters. The rotating wall can be used for a radial compression of the ion cloud thus increasing the ion density in the trap. We have also observed plasma mode excitations in agreement with theoretical expectations. This work will allow us to define the optimum parameters for high compression of the ions as needed for precision spectroscopy of forbidden transitions.     

Experimental evidence of intermittent convection in the edge of magnetic confinement devices

Probe measurements in the PISCES linear device indicate the presence of plasma radially far from where it is produced. We show that this is mainly caused by large-scale structures of plasma with high radial velocity. Data from the Tore Supra tokamak show striking similarities in the shape of these intermittent events as well as the fluctuation density probability distribution and frequency spectrum. The fact that intermittent, large-scale events are so similar in linear devices and tokamaks indicates the universality of convective transport in magnetically confined plasmas.