Studies Of Fast Electrons And Terahertz Radiation In Intense Laser-Solid Interactions

The development of diagnostics is important for intense laser-plasma interaction research apart from the laser technology.Large number of fast electrons generated when an intense laser pulse interacts with a solid target.It is very important to diagnose these fast electrons to understand the laser absorption mechanism and the fast electron beam transport in the target,as well as developing applications based on fast electron beam.There is intense terahertz radiation generated both in the target front and rear surface,which provide new route to develop intense THz sources.In the other way,diagnosing the generated THz radiation will give us more information about fast electrons.The thesis is focused on the diagnostics and study of the fast electrons and THz radiation in intense laser-plasma interactions.Several diagnostics are developed including a fast electron number and angular distribution real-time monitor,a single-shot THz spectrometer with broad band and a single-shot THz time-domain waveform measurement technology.Based on these diagnostics,the dependence of fast electrons and THz radiation on laser and target parameters is studied in detail.In addition,the laser absorption efficiency is improved with nanostructure on target surface,which enhance the generation of fast electrons and THz radiation.Electrons are accelerated both forward and backward by the intense laser field and some high-energy ones of those forward electrons can escape from the target rear sheath filed.Measuring these escaping electrons is a direct way to diagnose the fast electrons.A novel fast electron diagnostic based on Cherenkov radiation in optical fiber is developed in the thesis.The generation and propagation characteristics of Cherenkov radiation in fiber are theoretically modeled.The feasibility of the method is verified in the laser-plasma experiment.With these works,a fast electron number and angular distribution monitor based on optical fiber is built.The dependence of fast electron number and angular distribution on laser and target parameter is studied.Compared with traditional fast electron diagnostics,the Cherenkov radiation based optical fiber has advantages including high-repetition operation rate,low ion and x-ray noises and immunity against electromagnetic pluses.The shot-to-shot measurement and broad band of intense laser-plasma interactionbased terahertz sources limit the usage of traditional methods for THz spectrum diagnostic.A multichannel THz spectrometer consisting of pyroelectric detectors,highresistivity silicon THz beam splitters and band pass filters is designed which can perform single-shot and broad-band measurement of THz radiation spectrum.With the instrument,the spectrum of THz radiation from target front and rear surface are characterized in a picosecond laser-solid interaction experiment.with the dependence analysis of THz spectrum on laser and target parameters,low frequency component(< 1THz)of THz radiation from target rear and front surface is attributed to coherent transition radiations.And high frequency component(> 3THz)in the target front surface is related to linear mode conversion.And THz radiation from target rear surface between 1 and 3 THz is related to target normal sheath field radiation.The THz radiation is mainly generated by coherent transition radiation(CTR)induced by fast electrons when they cross the plasma-vacuum interface.The THz timedomain waveform is determined by fast electron pulses.As a result,it is important to measure the waveform in a single shot.A single-shot THz time-domain measurement method based a pair of reflective echelon mirrors,which has high time resolution and wide time window.In an intense femtosecond laser-solid interaction experiment,the THz time-domain pulse under different laser and target parameters is studied.Based on CTR theory,the fast electron pulse structure is obtained by analyzing the THz timedomain pulse.The dependence the optical transition radiation(OTR)from target rear surface on laser intensity and target thickness is also studied.The intensity of OTR shows exponentially decrease as the target thickness increases.It has been reported that the micro or nanostructure can improve the laser energy absorption efficiency.Using the 20 TW laser facility at Institute of physics,Chinese academy of sciences and 200 TW laser at Shanghai Jiao Tong University,the enhancement of fast electrons and THz radiation from target rear surface by copper nanorod array and copper nanorod-filled nanohole array is studied respectively.For nanorod array,as the rod length increases,THz radiation energy first increases then decreases.The fast electron number shows similar trend as THz radiation energy.PIC simulation shows laser absorption efficiency increases with nanorod or nanohole array,which enhance the fast electron and THz radiation generation.For nanohole array,there exists an optimum length corresponding to maximum enhancement under low laser intensities.The enhancement increases as the rod length increases under high laser intensities.The filled copper nanorod increase the target conductivity,which improves the fast electron transport inside the target.