Ultrafast And Bright Radiation Sources From Laser Plasma Acceleration

Ultrashort intense laser pulses are of great importance in fundamental research and practical application.The invention of high-intensity ultra-short lasers has opened up new frontiers of relativistic nonlinear optics,high-field physics,laser plasma physics,etc.There are various physical effects and wide applications in relativistic interaction of ultra-intense lasers with plasmas,which may offer new possibilities for applications in a variety of scientific fields.Currently,the wavelength of high-power ultra-intense ultra-short laser pulses is typically limited to the near-infrared（IR）range.However,the short-pulse intense lasers in the mid-IR regime are urgently needed for ultrafast science and attosecond physics.With the construction of multi-petawatt（PW）lasers and proposed initiatives of hundred-PW class laser facilities,light-matter interactions will enter a bran-new regime,in which the classical behaviors of charged particles are significantly modi-fied,the radiation reaction effect dominates the electron dynamics and the quantum electrodynamics（QED）effects become important.This will trig-ger many new physical effects,e.g.,pair production,strongγ-ray radiation,QED cascade and vacuum polarization,which may bring new opportunities for the studies of many frontier fields in laboratory astrophysics,high-energy physics,QED physics,etc.However,many of the physical processes and mechanisms involved in these effects are not yet well understood,thus fur-ther study and exploration are required to offer the guidance and basis for future experiments.This thesis will carry out the detailed research and dis-cussion on the following three problems and give corresponding solutions,aiming to explore the relevant physical processes and efficient generation of extremely bright radiation sources,attosecondγ-ray pulses,tunable dense GeV electron-positron beams and relativistic single-cycle mid-IR pulses.This thesis mainly involves the following three parts.In the first part（chapters 2 and 3）,we investigate the underlying physics of emission of ultra-brightγ-ray radiation from relativistic laser-plasma in-teractions.In chapter 2,we propose a new scheme to generate extremely brightγ-rays from laser-plasma accelerators,the resultingγ-ray peak bril-liance can be comparable to the level of XFEL.Although significant efforts have been made to produce high-brilliance X/γ-ray sources based-on laser-plasma accelerators,the peak brilliance and photon energy of the emitted X/γ-rays are both limited to those levels of three-generation synchrotron radiation sources.In order to overcome these restrictions,here we pro-pose a new scheme of photon emission in the quantum radiation-dominated regime from laser-driven plasma accelerators.We find that collimated ex-tremely brightγ-ray pulses are produced by focusing a multi-PW(peak intensity¹0²¹W/cm²)laser pulse into a two-stage plasma accelerator.In the fist stage,a multi-GeV dense electron beam with tens-n C charge is generated by this high-intensity laser in a relatively low density plasma.Subsequently,both the laser and electron beams enter into a higher-density plasma region in the second stage,where collimated ultrahigh brillianceγ-rays are emitted when the energetic beam electrons interact with the highly intense quasi-static electromagnetic fields self-induced in this stage.Three-dimensional particle-in-cell simulations show thatγ-rays of high energies tunable up to GeV are emitted with an unprecedented high peak brilliance>10²⁶photons/s/mm²/mrad²/0.1%BW at 1 Me V and very high energy conversion efficiency beyond 10%for photons above 1 Me V.In chapter 3,we present an all-optical new way for generating ultra-bright multi-Me V attosecondγ-ray pulses.Dense attosecond（≤170as）electron bunches with hundreds-Me V energies are produced by utilizing a circularly polarized Laguerre-Gaussian laser pulse irradiated onto a cone target.When the drive laser passes through the cone,it is significantly enhanced with an order of magnitude higher intensity.Then these hundreds-Me V electrons collide with the tightly-focused ultra-intense laser pulse which is reflected by a plasma mirror,generating ultra-bright(¹0²³photons/s/mm²/mrad²/0.1%BW at1Me V)attosecond（≤260as）γ-ray pulses with high photon energies of up to tens-of-Me V and tunable beam angular momentum.This has the potential to extend attosecond light pulses into theγ-ray radiation regime,which may offer new possibilities for attosecond nuclear physics and ultrafast science.In the second part（chapter 4）,we illustrate how to efficiently produce dense GeV electron-positron pairs and control their beam structure and/or beam duration by use of currently accessible high-intensity lasers.In recent years,various high-power laser-driven plasma-based methods have been proposed for producing GeV electron-positron pairs（e.g.,laser-irradiated solids）,however,they typically require very high-intensity lasers above10²³W/cm²that is more than an order of magnitude higher than the highest intensity level reported in experiments.Moreover,the manipulation of beam micro-structures of dense GeV pairs is challenging and their beam angular momentum control has not yet been achieved.Here we find an efficient way to generate dense GeV pairs by using ultra-intense(¹0²²W/cm²)lasers in a near-critical-density（NCD）plasma and to control their beam structure by changing laser polarization,producing collimated quasi-neutral pair sources with tunable beam angular momentum or attosecond-scale beam duration.Furthermore,it is shown that an NCD plasma channel with a transverse parabolic density profile is beneficial for enhancing the pair production.In the third part（chapters 5 and 6）,we investigate the underlying physics on the propagation and modulation of relativistic short-pulse lasers in under-dense plasmas,and show novel schemes of relativistic single-cycle mid-IR pulses generated via plasma-based optical modulation.As a relativistic short-pulse laser propagates in underdense plasmas,it will excite a nonlinear plasma wake,which,in turn,leads to strong frequency shifts and temporal shaping of laser pulses.In chapter 5,we propose a novel scheme for efficient generation of relativistic single-cycle mid-IR pulses at a high repetition rate.It utilizes two co-propagating laser pulses in an underdense plasma,where one relativistic-intensity laser as the driving pulse excites a nonlinear plasma wake as the optical modulator and the other is incident with a certain time delay as the signal pulse into the front（density up-ramp）region of the second bubble of the wake.The signal pulse can be converted into a relativistic near-single-cycle mid-IR pulse,based-on frequency downshifting modulation,with a central wavelength of⁵μm,pulse energy of multi-m J and energy conversion efficiency of as high as³0%.Since both the driving and signal laser pulses are at a few tens of millijoules at the beginning,this scheme can be thus realized with current TW-class k Hz laser systems.In chapter 6,we investigate the propagation of high-power Laguerre-Gaussian laser pulse in underdense plasmas,and show a frequency-downshifting method（plasma-based photon decelerator）to generate TW hundreds-m J near-single-cycle mid-IR twisted pulses with a long-wavelength spectral range of up to 18μm and relativistic high intensity.It is shown that the topological structure of mid-IR vortex beams is independent of the plasma,which can be controlled by the initial driving laser beam.Thus this method can be suitable for pro-ducing mid-IR vortex beams of various topological charges.In this part,we present two types of plasma-based optical modulation for generating rela-tivistic single-cycle mid-IR pulses,once they are confirmed in practical tests,which would open up relativistic nonlinear mid-IR optics and may bring new opportunities for attosecond physics and ultrafast research.