ASIAN SUMMER SCHOOL ON LASER PLASMA ACCELERATION AND RADIATION
920(2007); http://dx.doi.org/10.1063/1.2756770View Description Hide Description
The progress in the ultra‐intense laser technologies continues to open up new fields of physics. The laser accelerator development enters a new matured stage at which it becomes possible to manipulate in a controllable way the parameters of accelerated charged particle beams. In the electron acceleration the particle injection by breaking wake waves left by the laser pulse in underdense plasmas or by interacting two laser pulses results in the quasi‐mono‐energetic beam production. When the ions are accelerated during the laser‐matter interaction the tailored multi‐layer foil targets provide conditions for the high quality proton beam generation. When the laser pulse radiation pressure is dominant, the laser energy is transformed efficiently into the energy of fast ions. Ultrahigh intense electromagnetic fields can be generated due to the laser pulse compression, carrier frequency upshifting, and focusing by a counterpropagating breaking plasma wave, relativistic flying mirrors.
920(2007); http://dx.doi.org/10.1063/1.2756771View Description Hide Description
This paper gives an overview of recent progress of laser‐driven plasma x‐ray lasers. For the recombining plasma lasers, the mechanism of generating the population inversion is explained, and the difficulties which we face are pointed out. In the collisional‐excitation lasers, substantial reduction of the pumping energy is successfully achieved for the wavelength up to 12 nm, and the x‐ray lasers are applied to wide variety of research fields such as material science, plasma diagnostics, atomic physics and x‐ray imaging. We also remark the future perspective of the x‐ray lasers, especially for the shorter wavelength x‐ray lasers.
920(2007); http://dx.doi.org/10.1063/1.2756772View Description Hide Description
In recent years high power high irradiance lasers of peta‐watt order have been or are under construction. In addition, in the next 10 years lasers of unprecedented powers, exa‐watt, could be built If lasers such as these are focused to very small spot sizes, extremely high laser irradiances will be achieved. When electrons interact with such a laser, they become highly relativistic over very short time and spatial scales. Usually the motion of an electron under the influence of electromagnetic fields is influenced to a small extent by radiation emission from acceleration. However, under such violent acceleration the amount of radiation emitted by electrons can become so large that significant damping of the electron motion by the emission of this radiation can occur. In this lecture note we will study this problem of radiation reaction by first showing how the equations of motion are obtained. Then, we will examine the problems with such equations and what approximations are made. We will specifically examine the effects of radiation reaction on the Thomson scattering of radiation from counter‐streaming laser pulses and high energy electrons through the numerical integration of the equations of motion. We will briefly address the fundamental physics, which can be addressed by using such high irradiance lasers interacting with high energy electrons.
920(2007); http://dx.doi.org/10.1063/1.2756773View Description Hide Description
Several physical processes of laser electron acceleration in plasmas are revisited. A laser beam can drive plasma waves which in turn can accelerate resonant electrons. If these plasma waves can reach amplitude limited only by wave breaking alone, then the corresponding accelerating gradient in the plasma wave is of the order of electron rest mass energy per plasma skin depth, typically about GEV per centimeter. This is several orders of magnitudes higher than the conventional RF field gradient, giving rise to the possibility of compact accelerators needed for high energy physics research as well as medical and other applications. The chirped short pulse laser, with intensity exceeding the threshold for relativistic self focusing, can generate ion bubble in its wake by expelling electrons. The electrons at the bubble boundary, surge toward the stagnation point and pile up there. As the pile acquires a critical size, these electrons are injected into the bubble and accelerated by the combined fields of ion space charge and the plasma wave to Gev in energy. Most remarkably these electrons are bunched in phase space while being accelerated to high energy, resulting in near mono‐energetic electron beam of high beam quality, with narrow energy spread. We review also other processes related to laser electron acceleration, such as acceleration in plasma wave assisted by ponderomotive force and betatron acceleration.
920(2007); http://dx.doi.org/10.1063/1.2756774View Description Hide Description
The different acceleration mechanisms of ion acceleration from a foil irradiated by a short‐pulse laser are briefly discussed, i.e., the backward and forward ion acceleration from the front side, the forward ion acceleration from the rear side, and the shock acceleration inside the target itself. A particular attention is then given to the forward ion acceleration from the rear side, as it appears presently as the most efficient mechanism. Fast electrons are first created at the front side of a thin foil by the laser‐plasma interaction, then propagate through the target and build a charge separation field at the rear side. The corresponding electric field ionizes atoms and accelerates ions. The paradigm for the plasma expansion is the self‐similar quasi‐neutral expansion of an isothermal semi‐infinite plasma into a vacuum that is first presented together with the resultant energy spectrum. The analysis of the conditions of validity of the quasi‐neutrality assumption enables to determine the structure of the ion front and the maximum ion velocity as a function of time. Various effects are then discussed which may modify the ion spectrum with respect to the simple model : (i) Electron cooling (finite plasma slab case) : the thermal electron energy is progressively converted into the kinetic energy of the ions. The ion spectrum now converges when time goes to infinity in contrast with the isothermal semi‐infinite plasma case. (ii) Two‐phase model : a refined model is presented, where the electron temperature first rises together with the laser pulse intensity, and then decreases adiabatically while the energy is transferred to the ions. (iii) Two‐temperature electron distribution function : as expected, the high energy part of the spectrum is governed by the hot electron component (iv) Existence of a finite initial ion density gradient : a wave breaking of the ion flow occurs after a finite time, with the formation of an ion front. When electron cooling is taken into account, and when the initial ion density scale length lss is larger than a few percent of the total plasma slab width, the final maximum ion velocity decreases with lss . (v) Multispecies ions: optimisation of the target structure can lead to the acceleration of quasi‐monoenergetic light ions (especially protons).
920(2007); http://dx.doi.org/10.1063/1.2756775View Description Hide Description
Ion acceleration is studied both analytically and numerically. In the analytical model, a new self‐similar solution, which can be applied to any geometry (planar, cylindrical, and spherical), is employed to describe non‐relativistic expansion of a finite plasma mass into vacuum with a full account of charge separation effects. It turns out that the normalized plasma size Λ = R/λ D plays the dominant role in determining the whole ion energy spectrum and thus the maximum ion kinetic energy, where R and λ D are the plasma scale length and the Debye length, respectively. The analytical model is compared with particle simulations and experiments to show excellent agreement. It is argued that, when properly formulated, the analytical results obtained from the present model can be applied more generally than the self‐similar solution itself.
920(2007); http://dx.doi.org/10.1063/1.2756776View Description Hide Description
There is a great interest worldwide in plasma accelerators driven by ultra‐intense lasers which make it possible to generate ultra‐high gradient acceleration and high quality particle beams in a much more compact size compared with conventional accelerators. A frontier research on laser and plasma accelerators is focused on high energy electron acceleration and ultra‐short X‐ray and Tera Hertz radiations as their applications. These achievements will provide not only a wide range of sciences with benefits of a table‐top accelerator but also a basic science with a tool of ultrahigh energy accelerators probing an unknown extremely microscopic world.
Harnessing the recent advance of ultra‐intense ultra‐short pulse lasers, the worldwide research has made a tremendous breakthrough in demonstrating high‐energy high‐quality particle beams in a compact scale, so called “dream beams on a table top”, which represents monoenergetic electron beams from laser wakefield accelerators and GeV acceleration by capillary plasma‐channel laser wakefield accelerators. This lecture reviews recent progress of results on laser‐driven plasma based accelerator experiments to quest for particle acceleration physics in intense laser‐plasma interactions and to present new outlook for the GeV‐range high‐energy laser plasma accelerators.
920(2007); http://dx.doi.org/10.1063/1.2756777View Description Hide Description
Laser‐plasma wake wave can accelerate charged particles, especially electrons with an enormously large acceleration gradient. The electrons in the plasma wake wave have complicated motions in the longitudinal and transverse directions. In this paper, basic physics of the laser‐accelerated electron beam is reviewed.
920(2007); http://dx.doi.org/10.1063/1.2756778View Description Hide Description
If the index of a refraction of a dispersive medium, such as a plasma, changes in time, it can be used to change the frequency of light propagating through the medium. This effect is called photon acceleration. It has been predicted in both theory and simulations, and also been demonstrated experimentally for the case of moving ionization fronts in gases (the so‐called ionization blueshift) as well as for laser‐driven wakefields.
Here, we present studies of photon acceleration in laser‐driven plasma wakefields. The unique spectral characteristics of this process will be discussed, to distinguish it from e.g. photon acceleration by ionization fronts, frequency domain interferometry or self‐phase modulation. The dynamics of the photons in laser‐wakefield interaction are studied through both regular particle‐in‐cell and wave‐kinetic simulations. The latter approach provides a powerful, versatile, and easy‐to‐use method to track the propagation of individual spectral components, providing new insight into the physics of laser‐plasma interaction. Theory, simulations and experimental results will be brought together to provide a full understanding of the dynamics of a laser pulse in its own wakefield.
Even though the wave‐kinetic approach mentioned above has mainly been developed for the description of laser‐plasma interaction, it can be applied to a much wider range of fast wave‐slow wave interaction processes: Langmuir waves‐ion acoustic waves, drift waves‐zonal flow, Rossby waves‐zonal flow, or even photons‐gravitational waves. Several recent results in these areas will be shown, often with surprising results.
920(2007); http://dx.doi.org/10.1063/1.2756779View Description Hide Description
This chapter focuses on recent progresses in the leading femtosecond laser technology. Few‐cycle pulse generation based on Ti:sapphire laser is reviewed, and this has led to precise carrier‐envelope phase control which is crucial in optical frequency metrology and single attosecond pulse generation by high harmonic generation. The advanced concept of passive and active synchronization between two‐color femtosecond lasers has the potential to Fourier synthesizing an optical electric field and to generate an attosecond pulse train. In additional, the technology of chirped pulse amplification boost ultrashort pulses to an unprecedented peak power which is a significant issue in high field physics. With the state of the art of management of femtosecond laser pulse, we enable to open the discovery gate with the light field of covering ultrafast process, ultrabroaden spectrum, ultrastable metrology and ultraintense phenomena.
920(2007); http://dx.doi.org/10.1063/1.2756780View Description Hide Description
The physics of filaments formed by femtosecond laser pulses propagating in air is revealed both in theory and in experiment. An analytical method is used to investigate the interaction of two filaments. The interaction Hamiltonian of two filaments with different phase shifts is obtained and used to judge the properly of filaments interaction. The analytical results are in good agreement with simulation results. The influence of energy background on propagation of filaments is investigated in experiment. It is found that the characteristics of filaments can be changed by spatial and temporal control of laser pulses.