Laser wakefield electron acceleration
High energy particle accelerators are essential tools to probe the nature at smallest length scales in order to find answers to fundamental scientific questions related to the physical world around us. Scientists across the world constantly have built higher and higher energy particle accelerators over the years and discovered number of fundamental particles and verified theoretically proposed atomic models. These accelerators are based on radio-frequency cavity where the acceleration field gradient is limited to < 100 MV/m, due to electric field induced breakdown on the walls of the cavity. Consequently, such accelerators are quite big in size e.g. Stanford Linear Accelerator (SLAC), USA and Large Hadron Collider (LHC), CERN, Geneva. The demand for ever increasing energy of the particles to unravel several mysteries drives the scientific community to build even bigger accelerators e.g. the proposed International Linear Collider (ILC). With the current RF based acceleration techniques, the only way to increase the energy of the particles is to increase the size (length) of the accelerators, leading to huge capital investment and therefore requires multinational collaboration, and several years to construct such accelerators. The scientific community needs less expensive and compact accelerators for variety of applications e.g. developing compact x-ray / -ray sources, medical applications e.g. in cancer therapy, sterilizing food, treating materials used in industry, and disposing of nuclear waste etc. Therefore, more advanced and efficient means of accelerating the particles are required. In this context, it was realized that ‘Plasma’ can be used to accelerate particles. Plasma as a medium for particle acceleration has an advantage that being already in the broken state, it has no electrical breakdown limit, like conventional accelerating structures, and thus supports large accelerating fields ~ 100 GV/m. Based on plasma accelerating structures one can realize compact and therefore cheaper accelerators. Although, the concept of using collective fields in a plasma to accelerate charged particles was proposed several decades back, a major development towards using ‘plasma’ as an accelerating medium for electrons happened in 1979 when Tajima and Dawson  proposed to use high power, ultra-short laser pulses to generate collective fields (electron plasma wave) in a plasma and use it for electron acceleration. This scheme is known as “Laser Wake-field Electron Acceleration (LWFA)” .
1. T. Tajima, and J. M. Dawson, “Laser electron accelerator”, Phys. Rev. Lett. 43, 267 (1979).
2. E. Esarey, C. B. Schroeder, and W. P. Leemans, “Physics of laser-driven plasma-based electron accelerators”, Rev. Mod. Phys. 81, 1229 (2009).
Basic Principle of LWFA
An ultra-high intense (>1018 W/cm2), ultra-short (few 10’s of fs) laser pulse when passes through a plasma, the plasma electrons are pushed forward at the leading edge of the laser pulse and backward at the trailing edge of the laser pulse due to "ponderomotive force", a force which is proportional to the intensity gradient (Fp ∝ - ∇ IL) of the laser pulse and arises due to non-linear part of the Lorentz force of the laser field on the electrons. The ponderomotive force pushes electrons from the region of high laser intensity to the low intensity region. As the laser pulse propagates in the plasma, the displaced electrons try to snap back to their equilibrium position to maintain charge neutrality. However, while doing so they overshoot the equilibrium position which sets up electron oscillations behind the laser pulse. This electron oscillation moves behind the laser pulse with a phase velocity equal to the group velocity of the laser in the plasma which is close to ‘c’, the velocity of light in vacuum as depicted in figure xxxx. These moving plasma oscillations are called ‘electron plasma wave’ or simply ‘plasma wave’. The plasma wave is associated with longitudinal electrostatic field which is termed as ‘Laser Wake-field’ as this field is generated in the wake of the laser pulse similar to the water waves generated in the wake of a moving speed boat in water. The electrons injected in to the plasma wave are accelerated and a high energy electron beam is produced in direction of laser propagation.
Laser wakefield generation in plasma and electron acceleration
The maximum amplitude of the wake-field is limited by “Wave-breaking” which in the limit of cold non-relativistic case is given by:
For plasma electron density of about 1X1018 cm-3, the maximum wake-field will be ~100 GV/m. This acceleration field is approximately three orders of magnitude higher than that available with conventional RF based accelerators. However, in order to excite high amplitude plasma waves, it is necessary that the laser pulse duration is short so that the corresponding ponderomotive force is stronger due to higher intensity gradient. Also, when the laser pulse length (L = cτ, where τ is laser pulse duration defined as FWHM: full width at half-maximum) matches with half the plasma wavelength (λp) i.e. L ≈ λp/2, the plasma wave is resonantly excited and produces large amplitude wake-field. For typical plasma densities suitable for LWFA in the range of 1017–1019 cm-3, this corresponds to drive laser beams with pulse lengths in the range of ~150−15 fs, i.e. in the ultra-short regime.
Experiments on LWFA at Laser Plasma Division, RRCAT, Indore
At Laser Plasma Division, we are pursuing a programme on experimental investigations on LWFA. Experiments have been carried out using a CPA based 10 TW, 45 fs Ti:sapphire laser system providing > 1018 W/cm2 of intensity on He gas jet target having electron density in the range of 1 - 10 x1019 cm-3. While working in the SM-LWFA regime, we have been able to generate quasi-monoenergetic electron beams with energy in the range of 20-50 MeV and a beam divergence of < 10 mrad, at specific values of electron density. The effect of the laser pulse chirp and laser pre-pulse on the electron beam generation was also studied. With the proper choice of focusing optics (focal length of the off axis parabola) and controlling the ASE pre-pulse intensity below the pre-plasma formation threshold, high quality, stable electron beams with almost no low energy electrons background have been generated using He gas jet target. A comparative study with different gases (He, N2, and Ar) was also performed.
Recently, we have demonstrated generation of high quality quasi-monoenergetic electron beam with peak energy ~ 12 MeV, from self-guided LWFA in a plasma plume produced by laser ablation of solid Nylon (C12H22N2O2)n target. This technique has an potential to produced accelerated electron beam at high repetition compared to gas jet targets. Further experiments are also being conducted for generation of near-GeV electron beam using a 150 TW, 25 fs Ti:sapphire laser system installed recently in our laboratory.
Schematic of the experimental setup
Schematic of the experimental setup for LWFA in plasma plume produced from solid target
Energy spectra of the quasi-monoenergetic electron beam produced from He gas-jet plasma recorded in 15 consecutive shots.