摘要
我们利用在HT-7托卡马克装置上建立的一套新颖的高时空分辨的电子温度诊断设备:二维电子回旋辐射成像(2D ECEI)系统,用二维成像的方法系统观测了纯欧姆托卡马克芯部等离子体中电子温度湍流不稳定性和锯齿MHD不稳定性引起的磁重联。
我们在纯欧姆等离子体芯部的电子温度涨落的研究方面主要有如下结果。在低密度条件下的,芯部湍流的宽带电子温度涨落具有电子漂移波湍流的特征:在电子逆磁漂移方向传播,波数kθρs～0.34 ,温度梯度符合混合标长估计。同类型的TEXT-U托卡马克上的芯部湍流相对幅度有1%,而我们的湍流谱的幅度仅有0.5%甚至更弱,观测并分析到湍流的结果是很难得的。但是,当密度非常高时,这支单方向传播的电子模,在等离子体某些区域演变为双模:一支在电子逆磁方向,另一支在离子逆磁方向。这两支模应当分别是TEM模和ITG模。通过对芯部电子温度涨落的二维测量,我们发现了芯部涨落不均匀分布的普适特性。我们还发现,在宽带电子温度湍流中存在低频带状流(Zonal Flow)的迹象。
我们对锯齿MHD不稳定性做了系统的研究。目前,为了解释复杂的MHD锯齿振荡不稳定性实验现象,对于锯齿磁重联过程,理论研究中提出了多种重联模型,其中,有Kadomtsev模型,Wesson模型,气球模3-D模型,和Porcelli模型等,关于重联机理,有气球模的压强梯度和Lichtenberg的磁场随机不稳定性。ECEI诊断方法无需借助刚体旋转假设,能够比较真实反映了磁重联过程,为验证理论模型和深入揭示锯齿磁重联机制,提供了一个重要的手段。我们在HT-7托卡马克装置上,应用ECEI系统进行的实验观测,给出了清晰的二维重联过程。该结果基本符合Kadomtsev模型的图像,而和Wesson模型的图像不一致,符合气球模在低场侧的模型;该结果表明重联机理,倾向于气球模的压强梯度不稳定性,但和气球模在高场侧的结果不一致,但还不能完全排除磁场随机不稳定性的影响。其次,我们利用二维成像还观测到在q～1面附近,存在高阶模(极向模数m > 1)的多磁岛结构,以及由它们直接导致的q=1面上多点重联现象。这是一种理论模型还没有深入研究过的新现象。我们研究了不同密度和qa值条件下,高阶模,重联点,以及边界磁信号的特征和关系。对于不同的qa存在一个密度阈值,当密度超过这个阈值后,高阶模会显著的减小,m/n=1/1模代替它成为主导。这个阈值随着qa增加而减小。中心的1/1模和边界磁信号3/1模也是相关的。最后,我们还发现了前兆时间非常短暂的锯齿。这种锯齿主要在大托卡马克中观察到,破裂时间远小于Kadomtsev特征时间。
A new instrument to diagnose the electron temperature with high temporal andspatial resolution: two dimensional electron cyclotron emission imaging (2D ECEI),is developed on the HT-7 tokamak. The turbulence and the magnetic reconnectionduring the sawtooth oscillation in the core of the tokamak plasma are investigated.
The broad spectrum of the electron temperature ?uctuation has the character ofthe electron drift wave in the core at the low density ohmic plasma. The mode trans-ports in the electron diamagnetic direction, the wavenumber is kθρs～0.34, and therelation between the amplitude and the gradient of the temperature is consistent withthe mixing length scale. The relative amplitude of the ?uctuation in the core of theTEXT-U is about 1%, while the amplitude of the ?uctuation is as weak as 0.5% onthe HT-7 tokamak. However, the only one mode increases to two modes, which islocalized just outside the q=1 radius at the mid-plane at very high density in the ohmicplasma. The two modes are distinguished as they transport in completely reverse di-rections respectively: one transports in the electron diamagnetic direction, while theother in the ion diamagnetic direction. They should belong to the TEM mode andITG mode, respectively. The ?uctuation in the core is found asymmetry, which isconsistent with the universal characteristic of the ?uctuation. The evidence of the lowfrequency zonal ?ow is also observed in the broad spectral of the electron temperature?uctuation in the ohmic heated plasma.
The reconnection during the sawtooth oscillation is investigated. At present,inorder to explain the reconnection process, various reconnection models have been putforward, such as Kadomtsev model, Wesson model, ballooning 3-D model, and Por-celli model etc., while considering reconnection mechanism, there are the gradient ofthe pressure and the stochasticity of the magnetic field line proposed by Lichtenberg.Without assuming the plasma as a rigid body, the ECEI system provides true imagesof a rapid procession, and thus provides an important methods to validate the theorymodel and uncover the mechanism of the magnetic reconnection. The observations onthe HT-7 tokamak using the ECEI system, provide clear 2-D images on the reconnec- tion process. Our results are consistent with the Kadomtsev model, contradict with theWesson model, and consistent with the 3-D model on the low field side; our resultsindicate that the mechanism of the reconnection is the gradient of the pressure, notthe stochasticity of the magnetic field line. Second, the high order harmonic modes(poloidal mode m > 1) are observed on the q～1 radius, which cause multiple recon-nection points on that radius. This is a new phenomenon that is not deeply investigatedin the theoretical model. We investigate the relation between the high-order modes,the reconnection points and the perturbed magnetic signal at the edge under differentdensity and qa value. There is a density threshold depending on qa, if which are ex-ceeded, the high-order modes will decrease prominently while the m/n=1/1 mode willsubstitute them. The threshold decreases as the qa value increases. The 1/1 mode and3/1 mode measured by the magnetic probes at the edge are related. Finally, we foundthe sawtooth oscillation with the precursor of very short time. This kind of sawtooth ismainly observed on the large tokamak, which has a much shorter crash time than thecharacteristic time predicted by the Kadomtsev model.
引文
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