Laser-Plasma Wakefield Acceleration: Concepts, Tests and Premises V
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MOYAPA01 Proceedings of EPAC 2006, Edinburgh, Scotland LASER-PLASMA WAKEFIELD ACCELERATION: CONCEPTS, TESTS AND PREMISES V. Malka, J. Faure, Y. Glinec, A. Lifschitz, LOA, ENSTA, X, CNRS, UMR 7639, 91761 Palaiseau, France Abstract known as the resonant condition. The plasma period is τ π ω The concepts of laser-plasma based accelerator and related to the electron density by p=2 / p, with 2 2 injector are discussed here. Recent tests done at LOA as ωp =nee /meε0. The resonant condition in practical units is -3 21 2 well as design studies of high-quality few GeV’s electron then given by ne(cm )=3.5 × 10 /τL (fs), for a 30 fs beam with low energy spread (1%) are presented. These laser, this gives a resonant density of 3.9 × 1018 cm-3. laser-produced particle beams have a number of On the other hand, the phase velocity of the wake interesting properties and could lend themselves to vplasma, which is equal to the group velocity of the laser applications in many fields, including medicine vglaser, increases when the electron density decreases, (radiotherapy), chemistry (radiolysis), accelerator physics, indicating that higher energy gain can be reached in lower and as a source for the production of γ-ray beams for non- density plasmas according to vplasma=vglaser= c(1- 2 2 1/2 1/2 destructive material inspection by radiography, or for ωp /ω0 ) =c(1-ne/nc) . future compact XFEL machines. Acceleration of injected electrons in laser-plasma wakefields has been successfully demonstrated using one CONCEPTS laser beam in the laser wake field regime (LWF) or two High-gradient acceleration techniques rely on electron lasers beams in the laser beat wave regime (LBW). plasma waves to efficiently accelerate particles. The Electrons injected at 3 MeV have been accelerated by concept of laser plasma acceleration was originally this scheme up to 4.7 MeV [2]. In the LBW, the plasma proposed twenty-five years ago by Tajima and Dawson wave is driven by the interference of two laser pulses (not [1]. In a plasma medium, the electric field in a plasma necessary short) at two different wavelengths. Electric wave can attain extremely high values, of the order of fields close to the GV/m level were measured for this -3 1/2 experimental configuration using mid-infrared laser E0=cmeωp/e, E0(V/m) ≈ 96 ne(cm ) (E0=190 GV/m for 18 -3 beams [3] and injected electrons have been accelerated up ne=3.9 × 10 cm ). Plasma waves can be excited by the propagation of laser pulses in a transparent plasma, i.e. in to 30 MeV at UCLA [4]. In both LWF and LBW, only - electrons injected by an external source into the wave plasmas with density below the critical density, nc (cm 3 21 2 have been accelerated. )=1.1 × 10 /λ0 (μm). In the scheme known as laser wakefield acceleration, the plasma waves are generated in In those experiments, the injected electron beam always the wake of the laser pulse, like waves in the wake of a had a duration much greater than the plasma period and as ship. Efficient energy transfer between the wave and the a consequence, the electrons of the bunch experienced the particles requires that both move at the same speed. High whole sinusoidal electric field, and had an output spectra energy gain can be reached by using low density plasmas, with a maxwellian-like distribution. The production of in which the phase velocity of the laser-excited plasma bunches with narrow energy spreads in plasma wave, which is equal to the laser pulse group velocity, is accelerating structures needs the development of bunches very close to the speed of light in vacuum. The of electrons with duration much shorter than the plasma longitudinal electric fields associated with this fast plasma period. For example, for a plasma wave structure with wave is then able to accelerate relativistic particles 100 fs the electron bunch should be 10 fs or less, in order injected into the plasma or even, if its amplitude is large to be accelerated in a constant electric field. enough, to trap particles from the plasma itself. By The interaction length is limited in homogenous “surfing” on this electrostatic wave, particles can be plasmas by the natural diffraction length, the Rayleigh boosted to high energies over very short distances. length, given by the numerical aperture of the laser beam λ π π 2 λ Laser plasma acceleration techniques have successfully and its wavelength 0, Lint= ZR, with ZR= w0 / 0 where been used either for acceleration of injected electrons or w0 is the laser waist. In the linear wakefield regime, the δ as a source of electron beam, the later will be called plasma waves amplitude, Emax/E0 (or n/n), is related to “plasma injector”. a0, the laser strength parameter according to ≈ 2 Excitation of relativistic plasma waves by intense laser Emax/E0 0.8×a0 , which is related to the laser peak -10 2 1/2 pulses is made via the ponderomotive force (related to the intensity : a0≈8.6×10 (ILλ ) (IL and λ0 are respectively “light pressure”), which expels electrons from regions of expressed in W/cm2 and in μm). In order to get intense high laser intensity. Since this force is proportional to the but linear plasma waves, a0 must be of the order of unity, laser intensity gradient, large amplitude plasma waves can in other words, the laser peak intensity (for λ0=800 nm) be excited using relatively modest laser energies by using must be of the order of 1018 W/cm2. With current laser very short pulses. In the wake field regime, the optimum technology (in the level of tens of TW), laser beam laser pulse duration τL is half of the period τp, relationship focused with an f/20 aperture can generate a plasma wave 10 03 Linear Colliders, Lepton Accelerators and New Acceleration Techniques A03 Linear Colliders Proceedings of EPAC 2006, Edinburgh, Scotland MOYAPA01 in the linear regime (a0 ~ 1) with interaction lengths of a modern conventional accelerators [10]. Whereas few millimetres. This value has to be compared to the accelerated electron distributions previously suffered dephasing length, which is the maximum length over from a very large energy spread, a new and extremely 2 which electrons are accelerated: Ld≈γp λp where the exciting milestone has been reach recently, when three 2 2 -1/2 relativistic factor of the plasma wave γp= (1-vp /c ) is groups, from Imperial College London[11], Lawrence close to λp/λ0 for tenuous plasmas. The plasma Berkeley National Lab[12] (LBNL), and LOA[13], independently produced quasi mono-energetic, high wavelength is related to the electron density by λp 10 -3 -1/2 energy electron beams directly by focusing a laser pulse (μm)≈3.3×10 ne(cm ) . For a 30 fs laser pulse, the dephasing length, L ≈λ 3/λ 2, at the resonance is of the onto an homogenous plasma [11,12] or into a plasma d p 0 channel [13], as predicted by numerical simulations order of 7 mm. The maximum energy gain Wmax ≈ 2 2 [14,15]. 4γp (EMax/E0)mc is then given by or Wmax(GeV) ≈ 1.6 nc/ne 2 Experimental tests on laser-plasma acceleration were a which is close to 700 MeV. In the above mentioned 0 made recently at LOA by focusing a 20 TW laser with a experimental studies of acceleration of externally injected f/18 off axis parabola onto a gas jet target. The focal spot electrons, the energy gain was limited by the length of had a radius at full width half maximum of 18 μm and the interaction, which was much smaller than the dephasing 18 2 length. corresponding laser intensity was I=3.2×10 W/cm . Since the physics of laser interaction with underdense This choice of small aperture gave a long Rayleigh length plasma in the weakly relativistic regime is mainly of about 1.3 mm, which is the range of interest for high governed by linear processes, the corresponding energy gain as mentioned above. Measurements of the accelerating structures have very good stability, only electron beam distribution, in energy and in space are a limited by the laser or target shot to shot fluctuations. The powerful way to gain a better understanding of processes problem of stability in these schemes does not seem to be involved in laser-plasma interaction. We developed a new a real one for future accelerators due to the rapid progress method of beam characterisation based on the use of a in laser and target technologies. On the other hand, when compact spectrometer consisting of a 0.45 Tesla, 5 cm the laser power exceeds the self-focusing power, plasma long permanent magnet and a LANEX phosphor screen. waves can reach very high levels and trap plasma Doing so, we were able to measure, in a single shot, the electrons directly without external injection. Several whole spectrum and to record its image on a 16 bit experiments have been performed in these conditions, Charged Coupled Device (CCD) camera. Since we do not with laser powers in the range of tens of TW and laser use a collimator, the vertical direction on the LANEX energies (and durations) in the range of 0.1 J (~30 fs) to screen corresponds to the angular aperture of the electron 100 J (ps). Those “plasmas injectors” were the subject of beam. The LANEX screen was protected by a 100 µm intense research efforts during these last years. thick Aluminium foil in order to avoid direct exposure to the laser light.