Yu Chang-Xuan

Chaotic behavior and self-oscillation in DC discharge plasma

Three routes to chaos in a DC discharge plasma under different initial conditions are reported. It is shown that these chaotic phenomena come from the self-oscillation of plasma and the plasma sheaths have contribution to this instability especially in negative space potential.

Deposition of Diamond-Like Carbon Films with Femtosecond KrF Laser Pulses

Hydrogen-free diamond-like carbon (DLC) films with the optical band gaps of 2.0-2.7 eV and the microhardness of 40-55 GPa are deposited by femtosecond KrF laser pulses. The film microstructure of nanometer spheroid is observed. The film microhardness is improved with the increase of ion kinetic energy of plasma plume from 300 to 600 eV. The characteristics of DLC film deposition with fs pulses is discussed.

One-Dimensional Vertical ECEI Diagnostic for HT-7 Tokamak

At the first stage of the electron cyclotron emission imaging (ECEI) diagnostic project on HT-7, a 16-channel vertical-resolved ECEI diagnostic has been developed and installed on HT-7 tokamak to measure electron cyclotron emission with a temporal resolution of 0.5 μsec. The system works at a fixed frequency of 97.5 GHz. The sample volumes of the system are aligned vertically with a vertical channel spacing of 11 mm, and can be shifted across the plasma cross-section by varying the toroidal magnetic field. The high spatial resolution of the system is achieved by utilizing a low-cost linear mixer/receiver array and an optical imaging system. The focus location may be shifted horizontally when translating one of the optical imaging elements. The details of the system design and laboratory testing of the ECEI optics are presented together with the preliminary experimental results.

Electron temperature fluctuation in the HT-7 tokamak plasma observed by electron cyclotron emission imaging

The fluctuation of the electron temperature has been measured by using the electron cyclotron emission imaging in the Hefei Tokamak-7 (HT-7) plasma. The electron temperature fluctuation with a broadband spectrum shows that it propagates in the electron diamagnetic drift direction, and the mean poloidal wave-number θ is calculated to be about 1.58 cm−1, or θρs ≈ 0.34. It indicates that the fluctuation should come from the electron drift wave turbulence. The linear global scaling of the electron temperature fluctuation with the gradient of electron temperature is consistent with the mixing length scale qualitatively. Evolution of spectrum of the fluctuation during the sawtooth oscillation phases is investigated, and the fluctuation is found to increase with the gradient of electron temperature increasing during most phases of the sawtooth oscillation. The results indicate that the electron temperature gradient is probably the driver of the fluctuation enhancement. The steady heat flux driven by electron temperature fluctuation is estimated and compared with the results from power balance estimation.

Modification of plasma edge fluctuations using electrostatic probes in Keda Tokamak-5C

The experiments to characterize the modification of edge fluctuations using electrostatic probes have been performed on Keda Tokamak-5C (KT-5C) [World Survey of Activities in Controlled Fusion Research, Nuclear Fusion Special Supplement (International Atomic Energy Agency, Vienna, 1991), p. 190]. The results show that the externally excited perturbation propagates in the direction of electron directional movement. The observed phenomena are mainly related to the electrons, and can be well explained by the proposed dynamic theory of the ballistic mode. The experimental correlation coefficient and the longitudinal dispersion relation are in good agreement with the proposed ballistic model.

Multistability phenomena in discharge plasma

Multistability phenomena in V-I characteristic of discharge plasma have been observed in a Double Plasma device. These effects are explained using catastrophe theory together with the consideration of the negative resistance of plasma.

Self-Organized Criticality Properties of the Turbulence-Induced Particle Flux at the Plasma Edge of the HT-6M Tokamak

The power spectrum and the probability distribution function (PDF) of the turbulence-induced particle flux Γ in the velocity shear layer of the HT-6M edge region have been measured and analysed. Three regions of frequency dependence (f 0, f-1, f-4) have been observed in the spectrum of the flux. The PDF of the flux displays a Γ-1 scaling over one decade in Γ. Using the rescaled-range statistical technique, we find that the degree of the self-similarity (Hurst exponent) of the particle flux in the measured region ranges from 0.64 to 0.83. All of these results may mean that the plasma transport is in a state characterized by self-organized criticality.

Internal Magnetic Configuration Measured by ECE Imaging on EAST Tokamak

ECE imaging (electron cyclotron emission imaging) is an important diagnostic which can give 2D imaging of temperature fluctuation in the core of tokamak. A method based on ECE imaging is introduced which can give the information of the position of magnetic axis and the structure of internal magnetic surface for EAST tokamak. The EFIT equilibrium reconstruction is not reliable due to the absence of important core diagnostic at the initial phase for EAST, so the information given by ECE imaging could help to improve the accuracy of EFIT equilibrium reconstruction.

Identification of Low-Frequency Zonal Flow in a Linear Magnetic Plasma Device

A low-frequency (f < 2 kHz) potential structure (LFPS) is observed in a linear magnetic plasma device using Langmuir probe arrays. The center frequency of this structure is near zero. This structure has azimuthal and axial symmetries while with a finite radial wavenumber. The complete 3D spectra features of this structure have been identified to have the characteristics expected for zonal flows.

Properties of Density Fluctuations Induced by Geodesic Acoustic Mode in the Edge of HT-7 Tokamak

Properties of the geodesic acoustic mode (GAM) density fluctuations are studied using two toroidally separated Langmuir triple-probe arrays on the top of HT-7 tokamak. The GAM scenario is identified in the potential fluctuations with the toroidally symmetric structure (n = 0) and satisfying the temperature scaling of GAM mode frequency. Some theoretical predictions about the mode features of GAM density fluctuations are verified in our experiments: the toroidal mode number of GAM density fluctuations is n = 0; its amplitude is consistent with the theoretical prediction in a factor of 2; the density and potential fluctuations of GAM is in anti-phase at the top of plasma cross-section. Strong nonlinear interactions are found between GAM density fluctuations and ambient turbulence (AT). The results support the conclusions that the envelope modulation of potential fluctuations is dominantly caused by the direct regulation of GAM in the generation processing, and the envelope modulation of density fluctuation is due to the GAM shear effect.