摘要
理论预言托卡马克中的带状流在调制湍流和降低湍流引起的输运方面可以起到重要的作用,换言之,带状流和湍流的相互作用在决定湍流的饱和以及输运水平上至关重要。环形等离子体中的带状流存在两个分支:低频带状流和测地声模。本论文以低频带状流的实验研究为主。
在HL-2A托卡马克边界,使用四组朗缪尔探针阵列,我们观测到了低频带状流和测地声模共存的现象。低频带状流在悬浮电位的功率谱上表现为峰值接近零频,频谱展宽约3kHz的相干模结构。除了确定其极向和环向的对称性,我们同时也测量了径向结构和相关长度。平均径向波数的范围为kr = 0.15～0.5cm?1,波数展宽(半高全宽)为?kr = 3～4 cm?1。低频带状流的径向波数谱上存在两个几乎对称的正负峰,表明其波包沿径向的传播既向内又向外。这是实验上首次对边界等离子体中的低频带状流完整的三维谱结构的测量。
包络和双谱分析都表明低频带状流与湍流的相互作用与测地声模是类似的,暗示它们有着相同的产生机制。一点不同是包含低频带状流的三波组合的强度小于包含测地声模的三波组合。另一点不同是低频带状流与背景湍流的包络的相关系数小于测地声模。这两点都可以用边界区低频带状流的幅度较小来解释。
我们还首次运用三谱分析处理了悬浮电位的涨落信号。结果同样表明测地声模的三谱系数要高于低频带状流。另一个结果是所有四波相互作用强度的平均远小于三波作用,说明仍然是三波耦合主导着非线性相互作用。
The zonal ?ows in tokamak plasma are predicted to play an important role inregulating the turbulence and enhancing the confinement. In other words, the role ofthe mutual interaction between zonal ?ows and turbulence are emphasized as the pro-cess of determining turbulence saturation and transport level. There are two branchesfor the zonal ?ows in toroidal plasmas: the low-frequency zonal ?ows (LFZF) andGeodesic Acoustic Mode (GAM). The experimental study of LFZF is the main sub-ject of this thesis.
The LFZF, coexisting with the GAM is observed at the edge of HL-2A tokamakplasmas using four Langmuir probe arrays. It is identified as a low-frequency coher-ent mode peaking close to zero frequency and broadening about 3kHz on the powerspectral density of the ?oating potential ?uctuations. Besides the poloidal and toroidalaxisymmetry, the radial wavenumber and correlation length of the LFZF are also esti-mated. The averaged radial wavenumber is kr = 0.15～0.5cm?1 with the full widthat half maximum ?kr = 3～4 cm?1. The spectral for the LFZF radial wavenumberappear to have two peaks at approximately symmetric positive and negative values,implying the LFZF packages propagate both inwards and outwards. This is the firstcomplete measurement of three-dimension spectral structure of the LFZF in the edgeplasma.
The envelope and the bispectrum analysis both reveal that the interaction betweenthe LFZF and the ambient turbulence (AT) is similar as that between the GAM and theAT, suggesting the same generation mechanism. One difference is that the intensity ofthe wave triad interactions including the LFZF is much smaller than that including theGAM and the other difference is the coherence coefficients between the LFZF and theenvelope of the AT is also smaller that that between the GAM and the AT envelope,both of which could be explained by the smaller amplitude of the LFZF in the edgeregion.
The trispectrum analysis is firstly applied to the ?oating potential ?uctuations.The tricoherence of GAM is also larger than that of LFZF. It is also found that the averaged intensity of four-wave interactions is much smaller than that of three-waveinteractions, implying that the three-wave coupling dominates the nonlinear interac-tion process.
引文
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