Self-acceleration of a tokamak plasma during ohmic H mode

Core plasma rotation is observed to change from counter direction to co-current direction during the transition from low (L) to high (H) confinement mode, in Alcator C-Mod plasmas that are heated purely Ohmically and, hence, have no momentum input. The changes of the toroidal velocities, deduced independently from impurity Doppler measurements and from magnetic perturbations associated with sawteeth, agree. The magnitude of the change is consistent with the previously documented scaling for rotation in ion cyclotron rf-heated H modes. The rotation in this Ohmic experiment is obviously not an rf effect but demonstrates unequivocally a transport effect accelerating the plasma.     

Role of flow shear in the ballooning stability of tokamak transport barriers

A tokamak's confinement time is greatly increased by a transport barrier (TB), a region having a high pressure gradient and usually also a strongly sheared plasma flow. The pressure gradient in a TB can be limited by ideal magnetohydrodynamic instabilities with a high toroidal mode number n ("ballooning modes"). Previous studies in the limit n--> infinity showed that arbitrarily small (but nonzero) flow shears have a large stabilizing influence. In contrast, the more realistic finite n ballooning modes studied here are found to be insensitive to sub-Alfvénic flow shears, provided the magnetic shear s approximately 1 (typical for TBs near the plasma's edge). However, for the lower magnetic shears that are associated with internal transport barriers, significantly lower flow shears will influence ballooning mode stability, and flow shear should be retained in the analysis of their stability.     

Comparison of a low- to high-confinement transition theory with experimental data from DIII-D

From our recent theory based on the generation of shear flow and field in finite beta plasmas, the criterion for bifurcation from low to high confinement mode yields a critical parameter proportional to T(e)/square root (L(n)), where T(e) is the electron temperature and L(n) is the density scale length. The predicted threshold shows very good agreement with edge measurements on discharges undergoing low-to-high transitions in DIII-D. The observed differences in the transitions with the reversal of the toroidal magnetic field are reconciled in terms of this critical parameter. The theory also provides an explanation for pellet injection H modes in DIII-D, thereby unifying unconnected methods for accomplishing the transition.     

Active-feedback control of the magnetic boundary for magnetohydrodynamic stabilization of a fusion plasma

Stable operation with control on magnetohydrodynamic modes has been obtained in the modified reversed field experiment employing a set of 192 feedback controlled saddle coils. Improvements of plasma temperature, confinement (twofold), and pulse length (threefold) and, as a consequence of the magnetic fluctuation reduction, strong mitigation of plasma-wall interaction and mode locking are reported.     

First observation of edge localized modes mitigation with resonant and nonresonant magnetic perturbations in ASDEX Upgrade

First experiments with nonaxisymmetric magnetic perturbations, toroidal mode number n=2, produced by newly installed in-vessel saddle coils in the ASDEX Upgrade tokamak show significant reduction of plasma energy loss and peak divertor power load associated with type-I edge localized modes (ELMs) in high-confinement mode plasmas. ELM mitigation is observed above an edge density threshold and is obtained both with magnetic perturbations that are resonant and not resonant with the edge safety factor profile. Compared with unperturbed type-I ELMy reference plasmas, plasmas with mitigated ELMs show similar confinement, similar plasma density, and lower tungsten impurity concentration.     

Spatial confinement of ferromagnetic resonances in honeycomb antidot lattices

We report on a theoretical investigation of the magnetic static and dynamic properties of a thin ferromagnetic film with honeycomb lattice of circular antidots using micromagnetic simulations and analytical calculations. The theoretical model is based on the Landau–Lifshitz equations and directly accounts for the effects of the magnetic state nonuniformity. A direct calculation of local dynamic susceptibility tensor yields that the resonance spectra consist of four different quasi-uniform modes of the magnetization precession related to the confinement of magnetic domains by the hole mesh. Three of four resonant modes follow a two-fold variation with respect to the in-plane orientation of the applied magnetic field. The easy axes of these modes are mutually rotated by 60° and combine to yield the apparent six-fold configurational anisotropy. Additionally, a mode with intrinsic six-fold symmetry behavior exists, as well. Micromagnetic calculations of the local dynamic susceptibility tensor allow identifying the magnetic unit cell areas/domains responsible for each resonance mode.     

Observation of weak impact of a stochastic magnetic field on fast-ion confinement

Fast ions are observed to be very well confined in the Madison Symmetric Torus reversed field pinch despite the presence of stochastic magnetic field. The fast-ion energy loss is consistent with the classical slowing down rate, and their confinement time is longer than expected by stochastic estimates. Fast-ion confinement is measured from the decay of d-d neutrons following a short pulse of a 20 keV atomic deuterium beam. Ion confinement agrees with computation of particle trajectories in the stochastic magnetic field, and is understood through consideration of ion guiding center islands.     

Bending insensitive large mode area photonic crystal fiber

A novel kind of large mode area photonic crystal fibers (PCFs) are proposed in this paper. In order to achieve large effective mode area, a novel technique is applied to seven missing air hole PCF structures. The modal characteristics of PCF structures such as effective mode area, confinement loss, chromatic dispersion properties with doped cores, are investigated by employing the full vectorial finite element method (FEM). Simulation results demonstrate that effective mode area and confinement losses of fundamental mode simultaneously improved by applying our novel technique to proposed structures. The effects of bending on confinement losses of the proposed PCFs have been thoroughly investigated. Additionally, confinement losses of first higher order modes are presented and the possibility of stripping them in a simple way is discussed.     

Improvement of confinement in tokamaks by weakening of poloidal magnetic field near boundary

Theory of turbulent equipartition and experiment indicate that density, pressure, and temperature profiles follow the poloidal magnetic field profile. Therefore, it is suggested to change the magnetic geometry between core and boundary by toroidal conductors and/or plasma current. As a result, density and temperature gradients will become steeper, and stored energy will be higher with low boundary plasma parameters. The suggested new mode of confinement may substantially simplify achieving of ignition.     

Meron pairs and fermion zero modes

Merons, conjectured as a semiclassical mechanism for color confinement in QCD, have been described analytically by either infinite action configurations or an Ansatz with discontinuous action. We construct a smooth, finite action, stationary lattice solution corresponding to a meron pair. We also derive an analytical solution for the zero mode of the meron pair Ansatz, show that it has the qualitative behavior of the exact zero mode of the lattice solution, and propose the use of zero modes to identify meron gauge field configurations in stochastic evaluations of the lattice QCD path integral.     

Novel turbulence trigger for neoclassical tearings mode in tokamaks

A stochastic trigger by microturbulence for a neoclassical tearing mode (NTM) is studied. The NTM induces a topological change of magnetic structure and has a subcritical nature. The transition rate of the probability density function for and statistically averaged amplitude of the NTM are obtained. The boundary in the phase diagram is determined as the statistical long time average of the transition conditions. The NTM can be excited by crossing this boundary even in the absence of other global instabilities.     

Enhanced confinement and quasi-single-helicity regimes induced by poloidal current drive

Enhanced confinement regimes with quasihelical symmetry are reproducibly obtained in the modified reversed field pinch eXperiment (RFX)-mod by reducing the level of internal chaos through the combination of a smooth magnetic boundary (through a virtual shell scheme) and the oscillating poloidal current drive technique. The plasma moves from a chaotic multiple helicity towards a quasi-single-helicity regime with a magnetic island showing high temperature and soft x-ray emissivity. The chaos reduction involves also the plasma outside the island allowing for a global enhanced confinement with an improvement up to 50%.     

Mean and oscillating plasma flows and turbulence interactions across the L-H confinement transition

A complex interaction between turbulence driven E × B zonal flow oscillations, i.e., geodesic acoustic modes (GAMs), the turbulence, and mean equilibrium flows is observed during the low to high (L-H) plasma confinement mode transition in the ASDEX Upgrade tokamak. Below the L-H threshold at low densities a limit-cycle oscillation forms with competition between the turbulence level and the GAM flow shearing. At higher densities the cycle is diminished, while in the H mode the cycle duration becomes too short to sustain the GAM, which is replaced by large amplitude broadband flow perturbations. Initially GAM amplitude increases as the H-mode transition is approached, but is then suppressed in the H mode by enhanced mean flow shear.     

Scale size of magnetic turbulence in tokamaks probed with 30-MeV electrons

Measurements of synchrotron radiation emitted by 30-MeV runaway electrons in the TEXTOR-94 tokamak show that the runaway population decays after switching on neutral beam injection (NBI). The decay starts only with a significant delay, which decreases with increasing NBI heating power. This delay provides direct evidence of the energy dependence of runaway confinement, which is expected if magnetic modes govern the loss of runaways. Application of the theory by Mynick and Strachan [Phys. Fluids 24, 695 (1981)] yields estimates for the "mode width" (delta) of magnetic perturbations: delta<0.5 cm in Ohmic discharges, increasing to delta = 4.4 cm for 0. 6 MW NBI.     

HIAF中环形质量谱仪SRing的线性设计

为了精确测量短寿命原子核质量,提出了在强流重离子加速器装置(HIAF)上建造高精度环形质量谱仪SRing。SRing长188.7 m,最大设计磁刚度为1 3 T·m,主要由磁聚焦结构、注入系统、引出系统、随机冷却以及探测系统等组成。SRing将运行在等时性模式和收集模式下用于短寿命原子核质量的精确测量和放射性次级束流收集并纯化。详细介绍了SRing的线性光学设计,并给出两种模式下的光学设计、注入及引出系统的设计等,设计参数优化完毕后,机器测量精度有望提高到10~6。     

Stability criteria of Rayleigh ‐ Taylor modes in magnetized plasmas

The Rayleigh ‐ Taylor (RT) instability is investigated analytically in an inhomogeneous plasma in an external magnetic field. For the case of two distinct fluid layers separated by a sharp boundary and for a fluid of a continuously varying density, RT dispersion relations have been obtained and analyzed. Stability criteria of the excited modes are disscussed with respect to the mode propagation relative to the applied magnetic field. The magnetic field is found to act as a stabilizer up to a threshold value that can be determined from the dispersion relation.     

gamma-Ray wavelength standard for atomic scales

H-mode operation has been achieved in high current (I(p)>200 kA) plasmas in the START spherical tokamak for both neutral-beam-injection-heated and Ohmic discharges. The transition to H mode features the development of well-defined edge pedestals in density and temperature, which signifies the formation of an edge-transport barrier, and associated edge-localized modes. Recent operation at plasma currents exceeding 250 kA shows that these features are accompanied by increases in energy confinement time. This is the first clear demonstration of the H-mode regime in a spherical tokamak.     

InGaAs/InAlAs多量子阱脊形波导及定向耦合器光波特性准矢量分析

提出了一种基于级数展开的三维准矢量束传播法(SE-QV-BPM)用以分析由InGaAs/InAlAs多量子阱构成的脊形光波导及定向耦合器.结果表明，刻蚀深度相同时，TM模比TE模在水平方向限制强，且TM模的模场在波导角上出现畸变；波导间距相同时，定向耦合器TM波的耦合长度大于TE波的耦合长度，对偏振态敏感.分析获得了浅/深刻脊形光波导承载的准矢量TE/TM基模、定向耦合器承载的TE/TM偶/奇模的模场分布及其有效折射率，模拟了光场在定向耦合器中的传输演变情况.另外，SE-QV-BPM导出矩阵小，计算效率高 关键词： 级数展开 准矢量束传播法 多量子阱 InGaAs/InAlAs     

Direct observation of current in type-I edge-localized-mode filaments on the ASDEX Upgrade tokamak

Magnetically confined plasmas in the high confinement regime are regularly subjected to relaxation oscillations, termed edge localized modes (ELMs), leading to large transport events. Present ELM theories rely on a combined effect of edge current and the edge pressure gradients which result in intermediate mode number (n?10-15) structures (filaments) localized in the perpendicular plane and extended along the field lines. By detailed localized measurements of the magnetic field perturbation associated to type-I ELM filaments, it is shown that these filaments carry a substantial current.     

Dynamics of Ring-to-Volume Discharge Transition in H Mode in Inductively Coupled Plasma Torches at Atmospheric Pressure

The transition process in ring-to-volume discharge in H mode in inductively coupled plasma torches at atmospheric pressure is investigated by analyzing the time resolved image taken by a high speed camera. The effects of input power, plasma working gas flow rate, and its composition on the transition dynamics are also discussed.The results show that the discharge plasma has experienced ring discharge, and the development stage diffused from the boundary to the center in the confinement tube, and steady volume discharge after entering the H mode. Increasing input power, sheath gas flow rate and hydrogen contents in plasma working gas are all able to lessen the time consumed in the transition process in ring-to-volume discharge.