Sun Fengju

A Multi-Gap Multi-Channel Gas Switch for the Linear Transformer Driver

A multi-gap and multi-channel gas switch with convexo-convex discal planet electrodes was designed and investigated. Eight gaps are formed in series by a trigger electrode, six intermediate electrodes and two high voltage electrodes with a uniform gap length of 5 mm. The self breakdown and triggered breakdown performance of the switch are reported. Both the delay time and jitter decrease with the increase in the trigger voltage, switching coefficient and the decrease in the trigger isolating resistor. The delay time of the switch is about 40 ns, and the jitter is less than 2 ns when charged with ±85 kV and triggered by a voltage pule of −75 kV. The inductance of the switch is about 30 nH.

Experimental Study of the Multi-Gap Multi-Channel Gas Spark Closing Switch

A coaxial multi-gap multi-channel spark switch with stainless-steel-spring ring gap electrodes is designed and investigated. The switch is triggered by a pulse applied to the cylindrical electrode outside the discharging channel through a parasitic capacitance coupling. The jitter of the switch is reduced by several short-distance gas gaps in series, and its inductance is reduced by a multi-channel discharge on account of the inductance isolation between the coils of the spring ring electrode. The experimental results indicate that the switch is of low inductance (15-30 nH), low jitter (~3 ns), and stable breakdown performance.

Numerical Analysis of the Output-Pulse Shaping Capability of Linear Transformer Drivers

Output-pulse shaping capability of a linear transformer driver (LTD) module under different conditions is studied, by conducting the whole circuit model simulation by using the PSPICE code. Results indicate that a higher impedance profile of the internal transmission line would lead to a wider adjustment range for the output current rise time and a narrower adjustment range for the current peak. The number of cavities in series has a positive effect on the output-pulse shaping capability of LTD. Such an improvement in the output-pulse shaping capability can primarily be ascribed to the increment in the axial electric length of LTD. For a triggering time interval longer than the time taken by a pulse to propagate through the length of one cavity, the output parameters of LTD could be improved significantly. The present insulating capability of gas switches and other elements in the LTD cavities may only tolerate a slightly longer deviation in the triggering time interval. It is feasible for the LTD module to reduce the output current rise time, though it is not useful to improve the peak power effectively.

Numerical Simulation of Azimuthal Uniformity of Injection Currents in Single-Point-Feed Induction Voltage Adders*

In order to investigate the injection current uniformity around the induction cell bores, two fully electromagnetic (EM) models are respectively established for a single-stage induction cell and an induction voltage adder (IVA) with three cells stacked in series, without considering electron emission. By means of these two models, some factors affecting the injection current uniformity are simulated and analyzed, such as the impedances of adders and loads, cell locations, and feed timing of parallel driving pulses. Simulation results indicate that higher impedances of adder and loads are slightly beneficial to improve injection current uniformity. As the impedances of adder and loads increase from 5 Ω to 30 Ω, the asymmetric coefficient of feed currents decreases from 10.3% to 6.6%. The current non-uniformity within the first cell is a little worse than that in other downstream cells. Simulation results also show that the feed timing would greatly affect current waveforms, and consequently cause some distortion in pulse fronts of cell output voltages. For a given driving pulse with duration time of 70-80 ns, the feed timing with a time deviation of less than 20 ns is acceptable for the three-cell IVAs, just causing the rise time of output voltages to increase about 5 ns at most and making the peak voltage decrease by 3.5%.

Ultra Fast Shutter Driven by Pulsed High Current

Radiation simulation utilizing plasma radiation sources (PRS) generates a large number of undesirable debris, which may damage the expensive diagnosing detectors. An ultra fast shutter (UFS) driven by pulsed high current can erect a physical barrier to the slowly moving debris after allowing the passage of x-ray photons. The UFS consists of a pair of thin metal foils twisting the parallel axes in a Nylon cassette, compressed with an outer magnetic field, generated from a fast capacitor bank, discharging into a single turn loop. A typical capacitor bank is of 7.5 μF charging voltages varying from 30 kV to 45 kV, with corresponding currents of approximately 90 kA to 140 kA and discharging current periods of approximately 13.1 μs. A shutter closing time as fast as 38 microseconds has been obtained with an aluminium foil thickness of 100 micrometers and a cross-sectional area of 15 mm by 20 mm. The design, construction and the expressions of the valve-closing time of the UFS are presented along with the measured results of valve-closing velocities.

Test of a single stage 1-MV prototype induction voltage cavity

A single stage 1-MV prototype induction voltage cavity has been tested detailedly. Low-voltage principle experiments with injection currents of several kiloampere, and high-voltage experiments with feed pulses up to 800 kV/150 kA have been conducted. The major objective of the principle experiments is to measure the feed current symmetry. With the cavity fed by a single current, azimuthal distributions of feed currents under several typical azimuthal line configurations are measured. Meanwhile, the influence of the current asymmetry on cavity equivalent inductance is analyzed. With the cavity fed by double currents, the effects of asynchronous feed currents on current symmetry and current addition efficiency are investigated experimentally. When the cavity is single-point driven by high-power pulses, the insulator stack made of three different dielectrics is tested. The results show that the final flashover voltage of the two newly developed materials is slightly higher than that of the common PMMA.

Trigger method based on internal bricks within cavities for Linear Transformer Drivers

A novel trigger method based on an internal brick and azimuthal line in cavities is presented for Linear Transformer Drivers (LTD). The triggered brick is similar as the other normal bricks in cavities, which has the same charge voltage and the switch gas pressure. When the switch in triggered brick is closed by an external pulse, a high voltage pulse is produced and transmitted to other bricks along an azimuthal line installed in the middle insulator of cavities. Therefore, one external trigger pulse is enough for a 1 MA LTD cavity. This novel trigger manner has been applied successfully in 800 kA cavity with 36 bricks. Based on this novel trigger manner, a trigger method is proposed for a LTD module with multi-stage cavities stacked in series. Only several upstream cavities are triggered by external trigger pulses, and the residual cavities are triggered by the high pulses produced from the particular-location of upstream cavities. This trigger method will greatly simplify the requirements of trigger system, which has a promising application for future 60-MA LTD accelerator.

Modeling Methods for the Main Switch of High Pulsed-Power Facilities Based on Transmission Line Code

Based on the transmission line code (TLCODE), a circuit model is developed here for analyses of main switches in the high pulsed-power facilities. With the structure of the ZR main switch as an example, a circuit model topology of the switch is proposed, and in particular, calculation methods of the dynamic inductance and resistance of the switching arc are described. Moreover, a set of closed equations used for calculations of various node voltages are theoretically derived and numerically discretized. Based on these discrete equations and the Matlab program, a simulation procedure is established for analyses of the ZR main switch. Voltages and currents at different key points are obtained, and comparisons are made with those of a PSpice L-C model. The comparison results show that these two models are perfectly in accord with each other with discrepancy less than 0.1%, which verifies the effectiveness of the TLCODE model to a certain extent.

Optimization of Cavity Combination for 20-MA LTD-Based Accelerators

Based on a transmission line code, a circuit model is proposed that could serve as the basic method for the analysis of linear transformer driver (LTD)-based accelerators. By using 1 MA, 100 kV LTD cavities, the peak load current is optimized for a total of N cavities between 500 and 1200. The simulation results suggest that, with the same number of cavities, the peak current changes obviously with the types of combinations, and the maximum change can be as large as 1.2 MA. The results also show that, for the cases considered, the optimized peak current as a function of the total number of cavities agrees with the exponential associate, and the peak current for one level LTD cannot be enhanced infinitely. Furthermore, it is found that, to obtain a 20 MA peak load current, at least 1029 LTD cavities (49 in series and 21 in parallel connection) are needed. Finally, the typical parameters of the optimized design are compared to those of the existing Z accelerator.

Influences of Switching Jitter on the Operational Performances of Linear Transformer Drivers-Based Drivers

A whole circuit model of a linear transformer drivers (LTD) module composed of 60 cavities in series was developed in the software PSPICE to study the influence of switching jitter on the operational performances of LTDs. In the model, each brick in each cavity is capable of operating with jitter in its switch. Additionally, the manner of triggering cables entering into cavities was considered. The performances of the LTD module operating with three typical cavity-triggering sequences were simulated and the simulation results indicate that switching jitter affects slightly the peak and starting time of the output current pulse. However, the enhancement in switching jitter would significantly lengthen the rise time of the output current pulse. Without considering other factors, a jitter lower than 10 ns may be necessary for the switches in the LTD module to provide output current parameters with an acceptable deviation.