H. Xie

Using fast moving electrode to achieve overvoltage breakdown of gas switch stressed with high direct voltages

A small-scale fast risetime gas switch attached to a 50 Ω pulse forming line is tested. It includes a fast moving electrode and a fixed electrode. For the applied direct voltages, such as 2.8 kV, 2.0 kV, and 1.0 kV, the risetimes of this switch are tested to be ∼3.8 ns, ∼2.3 ns, and ∼1.1 ns, respectively, while the risetimes of a switch with two fixed electrodes are about ∼10.1 ns, ∼9.0 ns, and ∼3.6 ns. The results of high-speed photography and laser interferometry reveal that the moving electrode will obviously shorten discharge spark length but almost will not change the inter-electrodes gas pressure. The reason of shortening spark length is the existence of the discharge time delay of gas switch. After moving to the static breakdown spacing, the fast moving electrode will move on for an additional distance within the discharge time delay, which makes gas switch achieve overvoltage breakdown under high direct voltages and therefore leads to shorter spark length and faster switch risetime.

Investigation of Some MHD Events in the SUNIST Spherical Tokamak

Internal reconnection event (IRE), which is characterized by a perturbation in plasma current Ip, loop voltage Vloop, Hα radiation and magnetic perturbation dBp, was observed in the SUNIST experiment. It is found that, the fluctuation before IREs is characterized by a structure of m = 2/n = 1, then changes to m = 4/n = 1 during the IREs; and, after IREs, the mode number changes to m = 3/n = 1. An analysis in the evolution of equilibrium parameters during IREs shows that a positive spike appears in the evolution of the plasmas elongation and a negative spike in the poloidal beta of plasma. A collapse in the pressure profile, corresponding to the occurrence of IREs, is also found.

Fabrication of thermal-resistant gratings for high-temperature measurements using geometric phase analysis

Grating fabrication techniques are crucial to the success of grating-based deformation measurement methods because the quality of the grating will directly affect the measurement results. Deformation measurements at high temperatures entail heating and, perhaps, oxidize the grating. The contrast of the grating lines may change during the heating process. Thus, the thermal-resistant capability of the grating becomes a point of great concern before taking measurements. This study proposes a method that combines a laser-engraving technique with the processes of particle spraying and sintering for fabricating thermal-resistant gratings. The grating fabrication technique is introduced and discussed in detail. A numerical simulation with a geometric phase analysis (GPA) is performed for a homogeneous deformation case. Then, the selection scheme of the grating pitch is suggested. The validity of the proposed technique is verified by fabricating a thermal-resistant grating on a ZrO2 specimen and measuring its thermal strain at high temperatures (up to 1300 °C). Images of the grating before and after deformation are used to obtain the thermal-strain field by GPA and to compare the results with well-established reference data. The experimental results indicate that this proposed technique is feasible and will offer good prospects for further applications.

Time-frequency analysis of non-stationary fusion plasma signals using an improved Hilbert-Huang transform

An improved Hilbert-Huang transform method is developed to the time-frequency analysis of non-stationary signals in tokamak plasmas. Maximal overlap discrete wavelet packet transform rather than wavelet packet transform is proposed as a preprocessor to decompose a signal into various narrow-band components. Then, a correlation coefficient based selection method is utilized to eliminate the irrelevant intrinsic mode functions obtained from empirical mode decomposition of those narrow-band components. Subsequently, a time varying vector autoregressive moving average model instead of Hilbert spectral analysis is performed to compute the Hilbert spectrum, i.e., a three-dimensional time-frequency distribution of the signal. The feasibility and effectiveness of the improved Hilbert-Huang transform method is demonstrated by analyzing a non-stationary simulated signal and actual experimental signals in fusion plasmas.