The breakdown characteristics of a trigatron spark gap triggered by a plasma jet are studied in this paper; the development of a plasma jet and the influence of the gap distance, working coefficient, and plasma jet ejection properties on breakdown characteristics are compared. The results show that the plasma jet ejecting process can be divided into expanding, steady, and dissipating phases. The electric field distortion induced by the plasma jet is different during the expanding and dissipating phase, in which the plasma jet length increases and the radius of the curvature of the plasma jet front decreases, respectively. As the two processes have different variation rates, a faster increasing rate of the breakdown delay can be obtained during the dissipating phase. A detailed physical mode that explains the effects on the triggered breakdown process is presented. The shape of a plasma jet induced by polyethylene and ceramic surface discharge changes to a great extent during the dissipating phase; a more stable plasma jet can be obtained when induced only by polyethylene, resulting in a smaller breakdown delay under a low working coefficient. The ultraviolet radiation (UV) generated by the plasma jet is effective in reducing the critical breakdown electric field, while the electric field distortion induced by the plasma jet is the main reason behind the triggered breakdown under different working coefficients; the UV could play a limited role in the triggered breakdown process.

This paper presents experimental results on a trigatron spark gap breakdown process induced by different densities of the injected plasma. The results show that the shape of the injected plasma has a great influence on the electric field distribution. The decrease in the curvature radius of the injected plasma front can induce a breakdown during the dissipation of the injected plasma. This process is significant in explaining the sharp increase in the breakdown delay and jitter observed under very low working coefficients. The inception and propagation of the streamer are studied in detail. Due to the high intensity illumination, direct ionization between the injected plasma front and the head of the streamer can occur during the propagation of the streamer. This differs from the conventional streamer breakdown process under a uniform electric field. An explanation of the streamer inception and the direct ionization between the front of the injected plasma and the streamer head is presented. Using the Meek criterion, the effects of electric field distortion and photoionization induced by the injected plasma are analyzed. The results indicate that the effect of the photoionization changes significantly with the density and duration of the plasma injection. Combining this effect with the electric field distortion, a breakdown can occur under a very low working coefficient.

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Power sources play an important role in the characteristics and the applications of the electrothermal (ET) plasma as an edge localized mode (ELM) heat flux simulator. A repetitive high current ET plasma source with the capability of working at a 10 Hz repetition rate and peak current 7.5 kA is presented in this paper. By controlling the sequence of discharge of ten pulse power modules, a repetitive high heat flux plasma jet can be generated. A two-stage capillary structure is presented, and its repetitive trigger driving circuit based on surface flashover ignition is designed to achieve reliable and repetitive discharge. The topology of the inductive and capacitive (LC) series resonant circuit is applied to the charging system of the pulsed power source. The charging current is limited to 500 A with a charging time of 3.5 ms, and the ratio of the charging voltage to the operating voltage is 1.85. A diode and a power resistor in series are used to suppress the negative overvoltage, which is helpful to increase the thyristors’ operating reliability. Using the designed repetitive ET plasma source, the characteristics of the ET plasma jet are investigated by measuring the voltages and currents and by obtaining images of the discharges. Experimental results show that the repetitive ET plasma generator can be used as an appropriate way to simulate the ELM-like heat flux plasma.

The behavior of the transient heat flux produced by a three-electrode capillary discharge generator working at a repetitive mode is presented in this paper. The radial distribution profiles of plasma temperature, electron density, and thermal load are obtained by the optical emission spectrometry and correction algorithm. Experiments with different capillary diameters and charging voltages are carried out, and the relation between the discharge characteristics and the geometry parameters of the capillary is measured. A maximum transient thermal load of 1.42 GW·m−2 is obtained with 10 Hz, which can meet the thermal load amplitude requirement of Type-I edge localized mode heat flux in the ITER-like Tokamak.