Investigation On The Temperature Fluctuation And The Magnetic Reconnection During Sawtooth In The Core Plasma Of HT-7 Tokamak By The Electron Cyclotron Emission Imaging (2D ECEI)

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.