Studies On Relativistic Laser-plasma Produced Ultrafast Radiations

Owing to the potential ultra-high resolutions in time and space,ultra-fast high-frequency radiations ranging from extreme-ultra-violet(EUV)to x-ray spectral regions may find wide ap-plications in ultrafast science,biology,and medicine,etc.Over the past two decades,due to the rapid development of ultra-short high power laser technologies,laser-driven ultrafast radi-ations have been studied extensively.The ultrafast radiation sources are generated by different mechanisms via intense laser pulses interaction with plasmas,such as high harmonic genera-tion(HHG),betatron radiation,synchrotron radiation,Thomson scattering,and bremsstrahlung radiation and K?radiation based on electron beams interaction with solid target.Even though betatron radiation,synchrotron radiation and Thomson scattering can be used to produce well collimated table-top X-rays and?-rays,it is difficult to generate coherent x-rays with them.In this thesis,we focus on the mechanisms of ultrafast radiation and related electron ac-celeration in relativistic laser-plasma interaction under different plasma conditions(underdense and overdense)via theoretical analysis and numerical simulations by the particle-in-cell method.In Chapter 2,we describe in detail the flying mirror concept based upon laser wakefield excitation in underdense plasma.The idea of the flying mirror proposed before relies on the excitation of a laser wakefield sufficiently close to but not yet above the wave-breaking con-dition,such that the density wave crests form spikes(or attosecond electron beams)surfing in the laser direction at a speed close to the laser group velocity.Such relativistic mirrors have been envisaged to produce ultrashort high frequency radiation by backscattering and focusing a second laser counter-propagating to the mirrors.However,a main challenge for this concept to work properly is the plasma thermal effect,which largely limits the wave-breaking strength or mirror quality.Here,we propose to stabilize the mirror formation by wake excitation in a plasma tailored with an up-ramping front.We show how the thermal effect degrading the mir-ror quality is mitigated and robust flying mirror can be achieved,due to the wake behaviour in an inhomogeneous underdense plasma.In Chapter 3,we first outline the major features of laser backscattering off a dense relativis-tic electron mirror.Then,we adopt one-dimensional(1D)and 2D PIC simulations to illustrate the performance of the mirrors produced following the above scheme.The performance is inves-tigated over a large laser-plasma parameter space,indicating both the robustness and tunability of the flying mirror.Compared with using purely homogeneous and thermal plasmas,the present thermal-insensitive mirrors can provide enhanced scattering efficiency and increased spectral upshifting for a counter-propagating probe laser pulse.Another efficient method for producing coherent high-frequency radiation is based on high harmonics generation(HHG)in solid plasma surfaces irradiated by intense laser pulses.Because of the laser ponderomotive force,the surface electrons bunch as dense mirrors,oscillate at rela-tivistic speeds,and self-consistently reflect the driving laser pulses,imposing harmonics in the reflected wave form.In Chapter 4,we first outline the concept of the relativistic oscillating mir-ror(ROM)model.Then,we investigate using 2D PIC simulations how the target surface with periodic grating structures can alter the HHG properties.The HHG generation from laser inter-action with grating target generally obeys the selection rules in ROM.The period of the grating structures equals the one of laser when we set the grating constant as d=0.5?_L,and thus even harmonics are generated.The intensity of HHG reduces with larger grating constant.The HHG from graings has some fine structures due to the surface oscillations changed by the periodic structures of the gratings.On the other hand,we study the effect of transverse magnetic fields on the solid plasma surface HHG.We firstly modify ROM model,derive the revised selection rulses and compare with PIC simulations employing an external magnetic field.It is shown that the presence of transverse magnetic fields alters appreciably the surface mirror dynamics due to the induced cyclotron motion,resulting in even-order laser harmonics despite a normal laser incidence.The dependence of the coupling efficiency and hence even harmonic genera-tion with pre-plasma scale length,magnetic field strength and laser conditions are examined.The dependence differences for different magnetic field configurations(aligned either parallel or perpendicular to the drive laser polarization directions)are elucidated.For magnetic field parallel to the laser electric field,the spectral intensity of the second harmonic is proportional to the magnetic field strength in a wide range up to 160MG.The second harmonic generation due to the magnetic field also tends to increase with the plasma density scale length in a range larger than that obtained without transverse magnetic fields.These features may be used for diagnostics of either the spontaneous magnetic fields or pre-plasma expansion conditions due to pre-heating.Magnetic field generation is inherent to intense laser-overdense plasma interactions and the resulting field strength can be as high as 100 Mega-Gauss.In short-pulse interactions,target pre-heating,due to pre-pulses,low-contrast front pedestals,or a second pulse incident prior to the main pulse for controlling pre-plasma formation,may also generate large magnetic fields.Here,we outline the magnetic field generation mechanisms in laser-solid-plasma inter-actions,and then adopt multi-dimensional PIC simulations to study the spontaneous magnetic fields due to possible pre-heating effects.Besides of HHG,enormous hot electrons can be produced in intense laser interactions with solid target,and thus induce quasi-static electromagnetic fields.When the hot electrons propagate in quasi-static magnetic field,directed x-rays will be produced,similar to betatron radiation.The generation of hot electrons plays an important role in x-ray generation.In Chapter5,we study the hot electron production in intense laser-solid-plasma interactions,where a cone target is used.It is shown that,an enormous number of high-energy collimated electrons can be produced in intense laser interaction with the cone target.Smaller open angle of the cone and larger preplasma scale length are better to produce large number of collimated energetic electrons,suitable for the production of collimated X-ray radiation.