Rotational stabilization of resistive wall modes by the shear alfvén resonance

It is found that resistive wall modes with a toroidal number n = 1 in tokamaks can be stabilized by plasma rotation at a low Mach number, with the rotation frequency being lower than the ion bounce frequency but larger than the ion and electron precession drift frequencies. The stabilization is the result of the shear-Alfvén resonance, since the thermal resonance effect is negligible in this rotation frequency range. This indicates that tokamaks can operate at normalized pressure values beyond the no-wall stability limit even for low values of plasma rotation, such as those expected in fusion reactor scale devices.     

Theory and simulation of ion Coulomb crystal formation in a Penning trap

Ion Coulomb crystals (ICCs) are formed by laser-cooled ions in both radio-frequency and Penning traps. In radio-frequency traps, the crystals are generally stationary. In Penning traps, ICCs always rotate. The frequency of rotation is often set by an applied rotating wall drive that forces the crystal to rotate at the same frequency as the drive. In the absence of any applied rotating or oscillating fields, ICCs in a Penning trap can be in stable equilibrium with a range of rotation frequencies. The density and shape of the crystal adjust with the rotation frequency to ensure that equilibrium is reached. Here, we show that the parameters of the radial laser-cooling beam determine the rotation frequency of a small crystal in a Penning trap when no driving fields are present. We demonstrate, using an approximate theoretical treatment and realistic simulations, that the crystal rotation frequency is independent of the number of ions and the trap parameters, so long as the crystal radius remains smaller than the cooling laser beam waist. As the rotation frequency increases, the crystal eventually becomes a linear string, at which point it is no longer able to adjust its density. Instead, a small amplitude vibration in the zigzag mode of oscillation manifests itself as a rotation of the crystal at a fixed frequency that depends only on the applied trap potential.     

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.     

Stabilization of the resistive wall mode by a rotating solid conductor

Stabilization of the resistive wall mode (RWM) by high-speed differentially rotating conducting walls is demonstrated in the laboratory. To observe stabilization intrinsic azimuthal plasma rotation must be braked with error fields. Above a critical error field the RWM frequency discontinuously slows (locks) and fast growth subsequently occurs. Wall rotation is found to reduce the locked RWM saturated amplitude and growth rate, with both static (vacuum vessel) wall locked and slowly rotating RWMs observed depending on the alignment of wall to plasma rotation. At high wall rotation RWM onset is found to occur at larger plasma currents, thus increasing the RWM-stable operation window.     

Unstable Process Identification in a Pure Thermal Plume under Forced Rotating Conditions

A pure thermal plume development arising from a finite-size rotating heat source was analyzed experimentally. Qualitative investigation through extensive visualization has brought into focus the existence of a threshold rotation frequency (i.e., a swirl number) above which stretching effects are strengthened, thereby forcing the ascending plume motion to spiral around the geometrical axis heat source. Nevertheless, above the threshold frequency (i.e., above the swirl number), unstable processes appear through flow field pulsation in close proximity to the heat source; the pulsations literally beat and drive the flow field vicinity. From a strictly quantitative point of view, the data underscore the fact that heat source rotation presents two opposed trends. Below the threshold frequency, the higher the frequency, the more the temperature level is concentrated on the plume axis. In contrast, at the strongest rotation frequencies studied, the opposite is observed. Above the threshold rotating frequency, the characteristic rotating time scale appears to be too short to interact with the characteristic plume time scale. As a consequence, rotation of the heat source enhances transition from laminar to turbulence.     

Torque-balanced high-density steady states of single-component plasmas

Electron plasmas in a Penning-Malmberg trap are compressed radially using a rotating electric field (the "rotating-wall technique"). For large electric fields, plasmas can be compressed over a broad range of frequencies. This permits access to a novel high-density regime in which outward transport is insensitive to plasma density. The limiting density occurs when the plasma rotation frequency equals the rotating-wall frequency. Characteristics of the resulting torque-balanced steady states are described, and implications for high-density electron and positron plasma confinement are discussed.     

Toroidal rotation induced by 4.6 GHz lower hybrid current drive on EAST tokamak

Using a tangentially viewing x-ray imaging crystal spectrometer, substantial co-current rotation driven by lower hybrid current drive(LHCD) at 4.6 GHz is observed on EAST tokamak. This study presents plasma rotation behaviors with 4.6 GHz LHCD injection. Typically, the 10-20 km/s co-current rotation change and the transport of rotation velocity from edge to core are observed. The relationship between plasma parameters and rotation is also investigated, indicating that rotation decreases with increasing internal inductance(l_i) and increases with increasing safety factor(q_0). Hysteresis between rotation and T_e plasma stored energy is observed, suggesting different response times between the electron heating and rotation acceleration by LHCD. A comparison between the rotations driven by 4.6 G LHCD and 2.45 G LHCD on EAST is also presented, in which higher frequency LHCD could induce more rotation changes.     

Instabilities responsible for magnetic turbulence in laboratory rotating plasma

Instabilities responsible for magnetic turbulence in laboratory rotating plasma are investigated. It is shown that the plasma compressibility gives a new driving mechanism in addition to the known Velikhov effect due to the negative rotation frequency gradient. This new mechanism is related to the perpendicular plasma pressure gradient, while the density gradient gives an additional drive depending also on the pressure gradient. It is shown that these new effects can manifest themselves even in the absence of the equilibrium magnetic field, which corresponds to nonmagnetic instabilities.     

Off-axial plasma displacement suitable for antihydrogen production in AEgIS experiment

Antihydrogen experiments are currently based on non neutral electron, positron or antiproton plasma manipulation techniques in cylindrical Malmberg-Penning traps. An experimental study of a plasma manipulation technique based on off-axis diocotron displacement is presented. The use of the autoresonant excitation of (1, 0) diocotron mode of pure electron plasma allows a precise positioning of the plasma by moving it across the magnetic field and allows dumping such plasma in a desired angular position. The experimental procedure described here will pave the way to positron loading into an off-axial Penning trap terminated with a positronium converter target as it is proposed for the AEgIS experimental apparatus. The technique was studied over a range of confining magnetic field values and reproduces experimental conditions similar to most of the currently running antihydrogen experiments. The efficiency of the autoresonant excitation – in terms of plasma expansion rate and particle loss – is analyzed, studying the behaviour of electron plasma subjected to large off-axial displacements, showing that this method fulfills the requirements imposed by the AEgIS experiment.     

Plasma-surface reactions at a spinning wall

We report a new method for studying surface reactions and kinetics at moderately high pressures (<10 Torr) in near real time. A cylindrical substrate in a reactor wall is rotated at up to 200,000 rpm, allowing the surface to be periodically exposed to a reactive environment and then analyzed by a triple-differentially pumped mass spectrometer in as little as 150 micros thereafter. We used this method to study oxygen plasma reactions on anodized aluminum. When the substrate is spun with the plasma on, a large increase in O2 signal at m/e = 32 is observed with increasing rotation frequency, due to O atoms that impinge and stick on the surface when it is in the plasma, and then recombine over the approximately 0.7 to 40 ms period probed by changing the rotation frequency. Simulations of O2 signal versus rotation frequency indicate a wide range of recombination rate constants, ascribed to a range of O-binding energies.     

On the origin of rotation in magnetically confined plasmas

The main part of this paper surveys the mechanisms that can lead to rotation of plasma in an electrodeless discharge (commonly called a theta pinch) and in a mirror machine. Eight proposed mechanisms are examined, of which two are found to be incorrect in some way. Another mechanism, in which the reaction is purely electromagnetic, applies only to the tenuous plasma of a mirror machine. The remaining five theories apply to the highly compressed plasma of a theta pinch. These all rely fundamentally on the large difference in mass between an ion and an electron, i.e. either on Hall currents or the correction for finite ion Larmor radius in the stress tensor. The mechanisms which incorporate the Hall effect cause an equal and opposite mechanical reaction either on the wall (Roberts and Taylor), or on external conductors (Haines), or on the plasma at each end of the discharge axis (Bostick). The mechanisms which use the collisionless stress tensor predict either radial division into oppositely rotating plasma (Velikhov) or a transfer of angular momentum to the walls by collisions during the initial stage of physical contact (Haines). This last theory is new and arose as a result of a critical examination of an earlier theory (Jensen and Voorhies).

Experimental evidence for rotation has been confined mainly to high speed photographic techniques, and methods of obtaining a more definite measurement of the rotation and its origin are suggested.

The paper distinguishes carefully between the angular motions associated with the centre of mass, the guiding centres and the diamagnetic current, and also considers the stability of a rotating plasma including the Hall effect and the collisionless stress tensor.     

�������ЧӦ�Ե�������Ӱ�����ֵ�о�

在考虑到捕获电子效应的情况下,对求解二维Fokker-Planck方程的编程进行了反弹平均的修改,使用了交替方向隐式法来求解方程。分析和计算了在不同扩散系数和不同共振区间的情况下,捕获电子效应对驱动电流的影响。结果显示：随着逆纵横比的增加驱动电流密度有明显的下降,在磁轴附近捕获电子效应对电流驱动影响很小;提高波功率并不能很好的改善捕获电子效应对电流驱动的影响;右移共振区间提高共振电子的速度,也不能很好的改善捕获电子效应对电流驱动的影响。所得结果与理论分析基本一致。     

Evidence for the importance of trapped particle resonances for resistive wall mode stability in high beta tokamak plasmas

Active measurements of the plasma stability in tokamak plasmas reveal the importance of kinetic resonances for resistive wall mode stability. The rotation dependence of the magnetic plasma response to externally applied quasistatic n=1 magnetic fields clearly shows the signatures of an interaction between the resistive wall mode and the precession and bounce motions of trapped thermal ions, as predicted by a perturbative model of plasma stability including kinetic effects. The identification of the stabilization mechanism is an essential step towards quantitative predictions for the prospects of "passive" resistive wall mode stabilization, i.e., without the use of an "active" feedback system, in fusion-alpha heated plasmas.     

捕获电子效应对电流驱动影响的数值研究

在考虑到捕获电子效应的情况下,对求解二维Fokker-Planck方程的编程进行了反弹平均的修改,使用了交替方向隐式法来求解方程。分析和计算了在不同扩散系数和不同共振区间的情况下,捕获电子效应对驱动电流的影响。结果显示：随着逆纵横比的增加驱动电流密度有明显的下降,在磁轴附近捕获电子效应对电流驱动影响很小;提高波功率并不能很好的改善捕获电子效应对电流驱动的影响;右移共振区间提高共振电子的速度,也不能很好的改善捕获电子效应对电流驱动的影响。所得结果与理论分析基本一致。     

Forced magnetic reconnection and field penetration of an externally applied rotating helical magnetic field in the TEXTOR tokamak

The magnetic field penetration process into a magnetized plasma is of basic interest both for plasma physics and astrophysics. In this context special measurements on the field penetration and field amplification are performed by a Hall probe on the dynamic ergodic divertor (DED) on the TEXTOR tokamak and the data are interpreted by a two-fluid plasma model. It is observed that the growth of the forced magnetic reconnection by the rotating DED field is accompanied by a change of the plasma fluid rotation. The differential rotation frequency between the DED field and the plasma plays an important role in the process of the excitation of tearing modes. The momentum input from the rotating DED field to the plasma is interpreted by both a ponderomotive force at the rational surface and a radial electric field modified by an edge ergodization.     

Toroidal plasma rotation induced by the dynamic ergodic divertor in the TEXTOR tokamak

The first results of the Dynamic Ergodic Divertor in TEXTOR, when operating in the m/n=3/1 mode configuration, are presented. The deeply penetrating external magnetic field perturbation of this configuration increases the toroidal plasma rotation. Staying below the excitation threshold for the m/n=2/1 tearing mode, this toroidal rotation is always in the direction of the plasma current, even if the toroidal projection of the rotating magnetic field perturbation is in the opposite direction. The observed toroidal rotation direction is consistent with a radial electric field, generated by an enhanced electron transport in the ergodic layers near the resonances of the perturbation. This is an effect different from theoretical predictions, which assume a direct coupling between rotating perturbation and plasma to be the dominant effect of momentum transfer.     

Compression of positron clouds in the independent particle regime

The application of an asymmetric dipolar electric field rotating at a frequency close to that of the axial bounce of a collection of trapped positrons has, in the presence of a low pressure molecular gas to provide cooling, been used to achieve compression of the cloud. A theory of this effect has been developed for a Penning trap potential, with the cooling modeled in the Stokes viscous drag approximation. Good agreement between the theory and measurements of the frequency dependence of the cloud compression rate has been found, establishing that the phenomenon is a new form of sideband cooling.     

射频击穿等离子体对高功率微波传输特性的影响

利用极化正交的高功率微波合路器,开展了等离子体对于微波传输特性的实验研究.通过改变前级源的功率和脉冲宽度,使得在合路器耦合缝处发生射频击穿,产生等离子体.等离子体扩散进入微波传输主通道,对于高功率微波的传输产生明显的影响,导致微波能量吸收和极化的偏转.初步实验结果表明,等离子体扩散到主通道中心的时间约为3μs,扩散速度约为1μs/cm,等离子体的恢复时间约为5μs.实验测得等离子体导致的微波极化方向最大偏转角度约为4.1?,此时通道内电子个数约为3.7×1015,极化偏转角度与电子数密度以及微波频率相关.     

Compression of antiproton clouds for antihydrogen trapping

Control of the radial profile of trapped antiproton clouds is critical to trapping antihydrogen. We report the first detailed measurements of the radial manipulation of antiproton clouds, including areal density compressions by factors as large as ten, by manipulating spatially overlapped electron plasmas. We show detailed measurements of the near-axis antiproton radial profile and its relation to that of the electron plasma.     

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.