Investigation Of A Folded Double Pulses-Long Pulse Accelerator And The Application Of The Laser Triggered Gas Switch

With the development of high power microwave (HPM), long pulse output of the pulse modulator and synchronous operation of different pulse power accelerators are of great interest in recent years. The folded spiral accelerator has combined the folded line methods with spiral line methods, and research of this type of accelerator is an important way to generate long pulse. Laser triggered gas switch (LTGS) has the advantages of low delay time and jitter time, stable operation, and it is capable of repetitive operation and synchronous triggering of multi switches, etc, therefore the LTGS is widely used in triggering the high power gas switch. Using one laser pulse, two main-gas switches of the folded spiral line can be synchronously triggered, thus at each load of the accelerator one pulse can be obtained. Research of this type of accelerator and its triggering method is the key technology of the synchronous operation of multi high power accelerators.Researches of this paper will focus on the following aspects:1. Theoretical analyze of the folded and spiral double pulse-long pulse modulator. The designation of the folded and spiral line is enlightened by the self-matched transmission line. Using two spiral pulse forming line (PFL) connected with a transition part, when the two switches closing at the same time, pulse with FWHM ~130 ns will be obtained at each load. Influence of the resistance of the transition part, the load and the PFL on the output pulse is analyzed in theory. The PSpice software is used to simulate the equivalent circuit of the accelerator. And the primary test of the designation is carried out with the high voltage cable at voltage 20 kV. The experimental results are in good agreement with the theoretical analyze and software simulation.2. Researches of the LTGS.The LTGS is the major component of the double pulse accelerator and the laser trigger technology is the key technology of the synchronous operation of the accelerators.In this paper, the LTGS is studied in theory. An experimental platform for test of the LTGS is set up. The delay time and jitter time of the LTGS is measured with different condition, such as laser energy, percentage of self-breakdown voltage, laser wave length, focus length, electrode material, etc. The results show that the delay and jitter time of the switch decrease as the laser energy or the percentage of self-breakdown voltage increase, and the 266nm laser has the best switching characteristic compared with 532nm and 1064nm laser.Synchronous operation of the switches is also carried out. Subnanosecond difference of the two switches delay times is obtained at 8mJ, 85% with 266nm laser. 3. Analyse of the axial length of the laser channel. Using the high speed framing camera (HSFC), the focused laser channel is captured for different laser energy without applying the voltage on the electrode. The image is analyzed by the Matlab software and the axial length of the laser channel is given. The results show that, the axial length of the laser channel is the major factor that influences the jitter time of the gas switch. When the axial length is greater than 40% of the total electrodes distance, subnanosecond jitter of the switch is obtained.4. Laser triggered transformer type accelerator.A synchronization system for laser triggered transformer-type spiral pulse modulator was designed. Using the electrical pulse to trigger the primary gas switch, the laser pulse to trigger the main gas switch of the PFL, and by controlling the working time of both the pulses, the PFL is successfully triggered by the laser pulse. The delay and jitter time of the electrical pulse generator were measured. The working time of the pulse transformer, pulse voltage at the load, laser flash-lamp and Q-switch signal were analyzed. The charging voltage of the PFL was sampled after being filtered and it is introduced to the comparison device to generate TTL pulse to trigger the Q-switch of Nd: YAG pulse laser. When the PFL is charged at voltage 795 kV, which is also the gas breakdown voltage, high power electrical pulse with voltage 402 kV, current 26 kA and pulse width 128 ns was obtained at the dummy load.The delay and jitter times of LTGS filled with N2 or SF6 are compared in a series of experiments. The results show that the N2 has better switching characteristic compared with SF6.5. Researches of the folded spiral line.Using the COMSOL Multiphysics software, the capacitance and inductance of the transition part with different medium is calculated in 3-D.The LTGS is applied on the folded spiral line. To generate the long pulse, pulse with pulse width 264 ns, 360 kV, 24 kA is obtained at the load, at the gas breakdown voltage of 720 kV. Substituting the transformer oil in the transition part with ethylene glycol, the pulse with FWHM 288ns is obtained and the flat top of the pulse is obvious better than the transformer oil's, because the impedance of the transition part match well with the spiral PFL.Using the laser to synchronously trigger two switches, at the gas breakdown voltage of 725 kV, double pulses with pulse width 132 ns, 365 kV are obtained at each load of the folded spiral line. At the 70% of the switches'self-breakdown voltage, when the laser energy is greater than 10mJ, both the switches work well, the difference of the two switches'delay time is 5ns; when the laser energy is greater than 15mJ, the difference of the two switches'delay time is smaller than 1 ns.