Study On Characteristics And Applications Of Femtosecond Laser Guided High Voltage Electrode Discharges In Flow Field

With the development of laser technology,laser-based diagnostic methods are used in the field of flow field testing.Among them,femtosecond laser induced plasma spectroscopy(FLIPS)is a hot spot in international academic research.The principle is to use a femtosecond laser to self-focus the filament and excite the plasma.By measuring the filament plasma spectrum,one-dimensional composition,concentration and velocity measurement can be achieved.However,due to the low average power of the femtosecond laser,the FLIPS signal is generally weak.Therefore,how to enhance the FLIPS signal strength is a difficult problem that needs to be solved.High-voltage electrode discharge can generate a strong plasma,but because of the uncertainty of the time and space of free discharge,it is difficult to accurately measure its spectral information.Femtosecond filaments produce a weak plasma channel,the resistance of which is relatively small.When the channel is close to the high-voltage electrode pair,not only the high-voltage breakdown threshold is lowered,but also the discharge is broken down along the filament direction.Thereby,the time and space for discharging the high voltage electrode can be accurately triggered and controlled.The method of using the femtosecond laser to guide the discharge of the high voltage electrode can not only solve the problem of weak FLIPS signal,but also solve the problem of time and space uncertainty of free discharge,and make accurate measurement of plasma spectrum possible.At the same time,the high-voltage electrode discharge based on femtosecond laser can also be used in the mixture fraction and velocity measurement of the flow field.In this article,we first review the common optical speed measurement methods.This paper briefly introduces the research status of femtosecond laser guided discharge at home and abroad.The work of this paper is mainly carried out through experiments,which are divided into the following two parts:In the first part of the work,compared with the free discharge,the femtosecond laser is used to accurately control the position and discharge time of the discharge space,and at the same time,the extension of the plasma channel is realized.Compared with femtosecond filaments,in our work,the use of femtosecond laser to guide discharge can enhance the plasma spectral intensity by 840 times;guided discharge can form a one-dimensional spatially resolved emission spectrum,making one-dimensional component measurement possible;It has been found that the plasma properties can be controlled by controlling the discharge current.For example,the plasma spectrum is dominated by atomic spectra at high currents;the molecular spectrum is gradually appearing when current is reduced.This measurement of the components makes a lot of sense.The second part of the work,using femtosecond laser guided discharge can mark the characteristics of the plasma channel position,developed a speed measurement technology that can be applied in the gas flow field-Filamentary Anemometry using femtosecond Laser-extended Electric Discharge(FALED).Record the timing of the guided discharge and the continuous discharge,and explore the control of the plasma channel motion time by the FALED technology.Comparing the plasma emission spectra of filament-induced nonlinear spectroscopy(FINS),FALED and free discharge to understand the changes of particle level structure and motion state in plasma.By comparing the femtosecond laser electronic-excitation tagging(FLEET),it is verified that the technology can be applied in the air flow field,and the verification results of some speed measurement results are also verified.