Research On The Triggered Characteristics Of Laser Triggered Vacuum Switch

Laser Triggered Vacuum Switch (LTVS), combining Electrical Triggered Vacuum Switch (ETVS) and Laser Triggered Gas Switch (LTGS) technology, is likely to be a kind of advanced pulsed power device with high voltage and current. Compared with other switches, LTVS has a lot of advantages such as large discharge current, little delay time and jitter time. In addition, it can isolate the switch with the trigger system and be trigged repeated conveniently. However, there are few reports about the trigger mechanism and characteristics of initial plasmas at home or abroad. This paper will focus on the research of trigger mechanism, characteristics and development processes of initial plasmas, arc ignition of main gap and relevant influence factors. For miniaturization requirement of the laser, the paper will optimize the performance of LTVS and reduce laser trigger energies based on target materials, structures of LTVS, the laser light path, the method of focusing and the laser waveform parameters.Firstly, in this paper five triggering mechanisms are analyzed, i.e. laser ablation, photoelectric effect, thermal ion emission, multi-photon ionization and cascade ionization. Three laser work materials, such as solid laser, gas laser and dye laser, Nd:YAG laser are considered. The target material is chosen according to the work function, activity, melting point and toxicity. It isn’t suitable to be target material which is too active, toxic, or with low melting point. If the work function of target material is less than4.66eV, it is suitable to choose the266nm laser, because of the good photoelectric effect. In order to lower the laser energy, a mixture of KCl and Ti is considered as the target material because of its good temperature-steam pressure characteristics (it will be evaporated into gas when the temperature reaches a certain value) and low ionization energy of KCl, and Ti is used to absorb the laser radiation. When the target is focused by the1064nm laser, Ti will absorb the laser energy and transfer it to KCl, which will be evaporated into gas. Initial plasmas will generate due to the thermal ion emission or multi-photon ionization. Then initial plasmas move to the electrode rapidly and hit the electrode surface. As a result, a large amount of electrons, metal ions and metal vapor will cause further collision ionization in the electric field and release more electrons, ions, atoms and particles. Finally discharge channel is formed, and the switch is conducted.Secondly, in this paper, two types of LTVS are designed, flat electrode LTVS and multi-electrode LTVS. In the previous researches, the main discharge gap is less than5mm, and it is difficult to bear high voltage. Moreover, the vacuum arc is easily to accumulate and hard to bear high current in the flat electrode LTVS. A multi-electrode structure is designed which can avoid the above problems and it increases the main discharge gap. The structure of the target material is improved to use the cone that places on the cathode, which can increase the size of light areas so that prolong the target material life and reduce the requirement for laser energy.Then the electric fields of two type switches and the development process of the initial plasmas are simulated. The measurement current circuit of initial plasmas is designed, and the development of the initial plasmas and the vacuum arc are shot.Finally, the experimental platform is set up to research the influence of different target materials, hole diameter, groove depth, switch type, focus length, defocus and polarization. Results show that these factors have effects on the triggered characteristics of switch. The speed of initial plasmas becomes faster with the increasing of the gap voltage, so that the delay time is shortened. Initial plasmas are more in larger laser energy. It is beneficial to form the cathode spots in the positive mode which can make the switch work steadily and reduce the delay time. The multi-electrode structure is useful for not only the spread of vacuum arc but also the development of initial plasmas. The interaction with the target material is stronger in elliptically polarized light compared with the linear polarized light. The light spot area is smaller by reducing the focus length and the laser power density is larger in order to produce more initial plasmas. Defocus mainly affects the spot area which is smaller when the value of defocus is smaller.The research is to reveal the characteristics of initial plasmas in LTVS and its mechanism, and it is helpful to understand the principle and application of LTVS. In addition, it can provide the guidance for the laser miniaturization and the application study of LTVS.