laser-driven plasma accelerator
If the intensity of a focused ultrashort-pulsed laser beam reaches 10^{18}\:{\rm W}/{\rm cm}^2, the combined electric and magnetic force of the light wave accelerates electrons along the direction of laser propagation
. The resultant separation of electrons from ions induces a giant longitudinal electric field, setting electrons into motion and resulting in a plasma wave behind the laser pulse. Tajima and Dawson proposed that the longitudinal electric field of this plasma wave can be utilized for electron acceleration.
. The resultant separation of electrons from ions induces a giant longitudinal electric field, setting electrons into motion and resulting in a plasma wave behind the laser pulse. Tajima and Dawson proposed that the longitudinal electric field of this plasma wave can be utilized for electron acceleration.Pukhov and Meyer-ter-Vehn discovered a method that allowed electron acceleration to well-defined energies in a laser-driven plasma wave. The concept requires the plasma wave to be driven so strongly that it forms a cavity void of electrons, dubbed a “bubble”, which trails the laser pulse. An artist’s illustration of bubble acceleration is sketched in Fig. 1. The laser pulse, shown in white, pushes electrons aside like a snow-plough, leaving a bubble filled only by ions behind. At sufficiently high laser intensity, electrons are trapped at the bubble's rear side and then accelerated by the longitudinal plasma field.
Bubble accelerators are ideally driven with laser pulses shorter than half the plasma wave period, which amounts to a few micrometers. LAP’s LWS-10 source has provided the first sub-10-femtosecond, multi-terawatt laser pulses required by this criterion. Fig. 2 depicts simulated characteristics of the world’s first few-cycle-laser-driven plasma electron accelerator. Fig. 3 reveals several unique characteristics: a never-before-achieved low background of low-energy electrons, an excellent beam pointing within a few milliradians, a low spread of electron energies of ~ 1 MeV. This latter value indicates the feasibility of a relative energy spread far below 1% in the energy range of several 100 MeV, where the absolute energy spread is expected to remain conserved and the divergence may even get better, as indicated by recent experiments, see Fig. 4.
has provided the first sub-10-femtosecond, multi-terawatt laser pulses required by this criterion. Fig. 2 depicts simulated characteristics of the world’s first few-cycle-laser-driven plasma electron accelerator. Fig. 3 reveals several unique characteristics: a never-before-achieved low background of low-energy electrons, an excellent beam pointing within a few milliradians, a low spread of electron energies of ~ 1 MeV. This latter value indicates the feasibility of a relative energy spread far below 1% in the energy range of several 100 MeV, where the absolute energy spread is expected to remain conserved and the divergence may even get better, as indicated by recent experiments, see Fig. 4.Further reading:
Generation of Stable, Low-Divergence Electron Beams by Laser-Wakefield Acceleration in a Steady-State-Flow Gas Cell
, Karsch et al., Phys. Rev. Lett. 101, 085002 (2008)
Few-Cycle Laser-Driven Electron Acceleration , Schmid et al., Phys. Rev. Lett. 102, 124801 (2009)
Few-Cycle Laser-Driven Electron Acceleration , Schmid et al., Phys. Rev. Lett. 102, 124801 (2009)
Fig. 1. Artist illustration of bubble acceleration.
Fig. 2. Upper illustration: Simulated characteristics of the world’s first few-cycle-laser-driven plasma electron accelerator. Grey scale: elctron density, coloured scale: instantaneous intensity, red line: longitudinal electric field.
Lower illustration: blue line instantaneous intensity on axis: black line electron density on axis with the subfemtosecond electron bunch marked in red. (© ks)
Lower illustration: blue line instantaneous intensity on axis: black line electron density on axis with the subfemtosecond electron bunch marked in red. (© ks)
Fig. 3. Unique characteristics of the first few-cycle-laser-driven plasma electron accelerator, as background-free narrow-bandwidth monoenergetic electron spectrum (black line, horizontal plot), low divergence (vertical plot). Simulated electron spectrum is the red dashed line. (© ks)
Fig. 4. False-color images of energetically dispersed electron beams generated from a capillary-discharge accelerator, which feature a tight confinement in transverse direction. Profiles along lineouts (a) and (b) display FWHM record divergences of only ∼500 μrad and∼300 μrad, respectively.