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Study of sawtooth oscillations on the HT-7 tokamak using 2D tomography of soft x-ray signal 总被引:1,自引:0,他引:1 下载免费PDF全文
It is the first time so far as we know that two arrays of multi-channel soft x-ray detectors are used to generate twodimensional (2D) images of sawtooth oscillation on the HT-7 tokamak using the Fourier-Bessel harmonic reconstruction method, and using the singular value decomposition to analyse the data from soft x-ray cameras. By these two arrays, 2D image reconstruction of soft x-ray emissivity can be obtained without assumption of plasma rigid rotation. Tomographic reconstruction of the m=1 mode structure is obtained during the precursor oscillation of the sawtooth crash. The crescent-shaped mode structure appearing on the contour map of the soft x-ray emissivity is consistent with the quasiinterchange mode. The characteristics of the m=1/n=1 mode structure observed in the soft x-ray tomography are as follows: the magnetic surface is made up of the crescent-shaped “hot core” and the circular “cold bubble”. The structure of the magnetic surface rotates in the direction of the electron diamagnetic drift and the rotation frequency is the oscillation frequency of soft x-ray signals. 相似文献
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Zernicke polynomials method was first used in HT-7 tomography. Combining the characteristics of the soft X-ray imaging system in HT-7 tokamak, some of the technical consideration in applicating the tomography method are reported in this Letter. Through analyzing experimental soft X-ray data of a typical discharge with m=1 mode superimposed on sawtooth, it is shown that in the mid-mode and precursor phases, the central magnetic surface structure is composed of a hot core and a small magnetic island. In the crash phase, the hot core is obviously pushed outside and evolves into a crescent. The magnetic island is large. In addition, we compared the reconstructions obtained with Zernicke polynomials method and the Fourier-Bessel expansion method. The comparison result shows that as the magnetic island is large, Zernicke polynomials method seems more suitable to reconstruct the central plasma emissivity distribution on HT-7 tokamak. At last, through analyzing the oscillation amplitude and island size of the m=1 mode superimposed on sawtooth, it is thought that the large oscillation amplitude is due to large pressure gradient at the vicinity of inversion surface. 相似文献
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R. M. Castro M. V. A. P. Heller I. L. Caldas R. P. da Silva Z. A. Brasilio 《Il Nuovo Cimento D》1993,15(7):983-994
Summary Electrostatic turbulence and various aspects of magnetic fluctuations have been investigated, in the edge region of the TBR-1
tokamak, by using a set of Langmuir and magnetic probes, and a triple probe. Measurements of plasma parameters such as plasma
potential, density, temperature and magnetic field were taken in order to elucidate, the effect on transport of the electrostatic
and magnetic fluctuations in the edge. The fluctuations levels are found to be higher than in most tokamaks. The particle
flux is outward and slightly higher than that calculated from Bohm diffusion, and occurs in the frequency region typical of
the macroscopic MHD oscillations. The Mirnov-oscillation frequencies in TBR-1 are higher than those observed in other tokamaks
and, consequently, there is an uncommon superposition between the Mirnov and turbulent density fluctuations spectra. This
fact and the presence of high MHD activity may contribute to elucidate the possible influence of the magnetic oscillations
on the electrostatic transport observed in the plasma edge.
Work partially supported by FAPESP and CNPq. 相似文献
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The oscillations of a single magnetized dust grain in electronegative plasma sheath are investigated taking into account the existence of an external magnetic field. The influence of the content of negative ions and the magnetic field intensity on the properties of the dust vibration is analysed. The result shows that the existence of the negative ions in plasma reduces the dust oscillation frequency and drops the equilibrium position of dust, whereas the magnetic field raises the equilibrium position and also reduces the dust oscillation frequency on the condition considered. 相似文献
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The plasma in tokamaks often exhibits a relaxation oscillation called the edge-localized mode (ELM), which is generally attributed to MHD instability driven by strong gradients at the plasma boundary. It is shown here that field-aligned currents flowing just outside the boundary may also play a role in the ELM process. The poloidal perturbation magnetic field during ELMs in the DIII-D tokamak calculated from measured currents can reproduce prominent observed features, including a narrow magnetic structure at the outboard midplane similar to filaments observed earlier in DIII-D and NSTX. 相似文献
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利用HT-7装置上建立的多阵列软X射线诊断系统对等离子体放电中的锯齿不稳定性进行研究,发现锯齿的出现及其行为特点与等离子体电子密度Ne、电子温度Te、边界安全因子qα、壁条件等放电参数有密切关系。在低杂波电流驱动等离子体放电条件下,观察到m/n=1/1的中间模振荡出现在锯齿的上升阶段,并迅速发展、饱和、衰减,在锯齿塌陷或先兆振荡出现以前消失。分析和讨论了欧姆加热条件下和低杂波加热条件下的锯齿行为。 相似文献
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在HT-7装置实验研究中,利用主动反馈调制低杂波(LHW)实时有效地抑制了MHD不稳定性。介绍了用DAQ2010高速采集卡为硬件的系统对HT-7装置实验中MHD的实时检测和控制。 相似文献
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Sawtooth oscillations with post-cursor were observed in LHCD plasma on HT-7 tokamak. The mode exists and decays gradually after the crash, which implies that the magnetic reconnection is incomplete and the central safety factor remains below unity after the crash. From results of the singular value decomposition (SVD) and tomographic reconstructions describing the magnetic surface structures in the crash, it was found that the m/n=1/1 mode survives in the crash. It is shown that, the appearance of the preservation of this mode is inconsistent with the secondary reconnection theory, and we conjecture that the evolving of this mode may be understood with the stochastic field model. 相似文献
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De Zeeuw D.L. Gombosi T.I. Groth C.P.T. Powell K.G. Stout Q.F. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》2000,28(6):1956-1965
A 3D parallel adaptive mesh refinement (AMR) scheme is described for solving the partial-differential equations governing ideal magnetohydrodynamic (MHD) flows. This new algorithm adopts a cell-centered upwind finite-volume discretization procedure and uses limited solution reconstruction, approximate Riemann solvers, and explicit multi-stage time stepping to solve the MHD equations in divergence form, providing a combination of high solution accuracy and computational robustness across a large range in the plasma β (β is the ratio of thermal and magnetic pressures). The data structure naturally lends itself to domain decomposition, thereby enabling efficient and scalable implementations on massively parallel supercomputers. Numerical results for MHD simulations of magnetospheric plasma flows are described to demonstrate the validity and capabilities of the approach for space weather applications 相似文献
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An expression that explicitly describes variations in the internal energy of the plasma that flows through a discontinuity is derived based on the complete system of boundary conditions for the MHD equations on the discontinuity surface. The dependence of the plasma heating on the magnetic field density and configuration in the vicinity of the discontinuity surface (i.e., on the MHD flow type) is studied. The conditions of plasma heating at discontinuities in a self-consistent analytical model of magnetic reconnection are discussed. 相似文献
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Both high and low frequency relaxation oscillations have been observed in an argon capacitive discharge connected to a peripheral grounded chamber through a slot with dielectric spacers. The oscillations, observed from time-varying optical emission of the main discharge chamber, show, for example, a high frequency (46 kHz) relaxation oscillation at 100 mTorr, with an absorbed power near the peripheral breakdown, and a low frequency (2.7-3.7 Hz) oscillation, at a higher absorbed power. The high frequency oscillation is found to ignite a plasma in the slot, but usually not in the periphery. The high frequency oscillation is interpreted by using an electromagnetic model of the slot impedance, combined with the circuit analysis of the system including a matching network. The model is further developed by using a parallel connection of variable peripheral capacitance to analyse the low frequency oscillation. The results obtained from the model are in agreement with the experimental observations and indicate that a variety of behaviours are dependent on the matching conditions. 相似文献
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Instability Parameters of Optical Oscillation Frequency in Plasma Central Discharge and Periphery Region 下载免费PDF全文
We have observed relaxation oscillations in a capacitive discharge in Ar gas, connected to a peripheral ground chamber. The plasma oscillations observed from time-varying optical emission from the main discharge chamber show, for example, a high frequency (75.37kHz) relaxation oscillation, at lOOmTorr and 8 W absorbed power, and a low frequency (2.72Hz) relaxation oscillation, lOO mTorr and 325 W absorbed power. Time-varying optical emission intensity and plasma density are also detected with a Langmuir probe. The theoretical result agrees well with experiments. 相似文献
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在HT-7装置上建立了一套高速CCD可见光成像诊断,测量了边界等离子体的可见光辐射成像。在HT-7装置放电中,首次观察到在等离子体边界区域存在一条极向旋转的可见光辐射带,由CCD诊断系统得到其极向旋转的频率为858Hz。根据多道Hα阵列测量得到极向旋转频率为952Hz。多道磁探针信号测量发现,等离子体内部存在m/n=3/1的电磁模,该模的旋转频率为972Hz。从电子回旋辐射诊断系统得到的电子温度剖面发现该模的磁岛宽度约为2.5cm。 相似文献
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The ordinary differential magnetic field line equations are solved numerically; the tokamak magnetic structure is studied on Hefei Tokamak-7 Upgrade (HT-TU) when the equilibrium field with a monotonic q-profile is perturbed by a helical magnetic field. We find that a single mode (m, n) helical perturbation can cause the formation of islands on rational surfaces with q=m/n and q=(m±1,±2,±3,...)/n due to the toroidicity and plasma shape (i.e. elongation and triangularity), while there are many undestroyed magnetic surfaces called Kolmogorov-Arnold-Moser (KAM) barriers on irrational surfaces. The islands on the same rational surface do not have the same size. When the ratio between the perturbing magnetic field Br(r) and the toroidal magnetic field amplitude Bφ0 is large enough, the magnetic island chains on different rational surfaces will overlap and chaotic orbits appear in the overlapping area, and the magnetic field becomes stochastic. It is remarkable that the stochastic layer appears first in the plasma edge region. 相似文献
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V. S. Belyaev V. I. Vinogradov A. S. Kurilov A. P. Matafonov V. S. Lisitsa V. P. Gavrilenko A. Ya. Faenov T. A. Pikuz I. Yu. Skobelev A. I. Magunov S. A. Pikuz Jr. 《Journal of Experimental and Theoretical Physics》2004,99(4):708-718
We present the results of our measurements of the spectra for multicharged ions in a plasma produced by moderately intense (about 1017Wcm?2) picosecond laser pulses. They suggest the existence of intense plasma oscillations with a frequency appreciably lower than the frequency of the laser radiation. The observed spectrum for the plasma satellites of the Lyman Lyα doublet of the hydrogenic F IX ion in a dense plasma was modeled theoretically. The resulting doublet profile was shown to have a complex structure that depends non-trivially both on the plasma density and on the frequency and amplitude of the plasma oscillations. The positions of the satellites and their separations allowed them to be associated with intense electrostatic oscillations with an amplitude of (4–6)×108Vcm?1 and a frequency near (0.7–1)×1015s?1. Assuming the oscillation frequency to be determined by the strength of the magnetic field B generated in the plasma, we obtained an estimate of B that is in reasonable agreement with other measurements and estimates of this quantity. Our theoretical analysis allowed explanation of the emission spectra observed when flat fluoroplastic targets were heated by intense picosecond laser pulses. 相似文献
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Experimental evidence of parametric excitation, by an intense external H.F. field, of an electron surface mode and an ion wave is presented. The pumping electromagnetic energy density is equal to or slightly larger than the thermal energy density of the electrons. The value of fpc/f0 (electron plasma frequency/external field frequency) is that for an electron surface wave. Depending on the pressure and field intensity, this decay instability can lead to three types of low frequency oscillations, with frequencies close to the ion plasma frequency. Two of these are described by Aliev and Silin's intense field theory: one is the volume ion plasma oscillation and the other a surface ion plasma oscillation. The third corresponds to no known ion eigenmode. Several other features of the theory by Aliev and co-workers are also confirmed experimentally, such as the harmonic excitation of the instability (nf0 ≈ fpe/√2, where n is an integer), the instability amplitude as a function of fpe/f0 (above threshold conditions), the value of the mismatch parameter as a function of field strength and ion mass, and the existence of a fine structure corresponding to the symmetric and antisymmetric electron surface oscillations. Even at high pump field strengths, the decay products are nearly monochromatic i.e. the plasma does not become turbulent. 相似文献
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在HT-7装置上建立了一套高速CCD可见光成像诊断,测量了边界等离子体的可见光辐射成像.在HT-7装置放电中,首次观察到在等离子体边界区域存在一条极向旋转的可见光辐射带,由CCD诊断系统得到其极向旋转的频率为858Hz.根据多道Hα阵列测量得到极向旋转频率为952Hz.多道磁探针信号测量发现,等离子体内部存在m/n=3/1的电磁模,该模的旋转频率为972Hz.从电子回旋辐射诊断系统得到的电子温度剖面发现该模的磁岛宽度约为2.5cm. 相似文献