G igaelectron volt (GeV) electron accelerators are essential to synchrotron radiation facilities and free-electron lasers, and as modules for high-energy particle physics. Radiofrequency-based accelerators are limited to relatively low accelerating fields (10−50 MV m −1), requiring tens to hundreds of metres to reach the multi-GeV beam energies needed to drive radiation sources, and many kilometres to generate particle energies of interest to high-energy physics. Laser-wakefield accelerators 1,2 produce electric fields of the order 10-100 GV m −1 enabling compact devices. Previously, the required laser intensity was not maintained over the distance needed to reach GeV energies, and hence acceleration was limited to the 100 MeV scale 3-5. Contrary to predictions that petawatt-class lasers would be needed to reach GeV energies 6,7 , here we demonstrate production of a high-quality electron beam with 1 GeV energy by channelling a 40 TW peak-power laser pulse in a 3.3-cm-long gas-filled capillary discharge waveguide 8,9. Although it is straightforward to achieve acceleration gradients of 10−100 GV m −1 in laser-wakefield accelerators 1,2 , until recently the electron beams (e-beams) from such accelerators had energies <200 MeV with 100% energy spread 10. A breakthrough improvement in energy spread was obtained in 2004 by three groups 3-5 by interacting intense laser pulses with millimetre-scale gas jets to generate 70-200 MeV beams with percent level energy spread. For example, by using relatively large spot sizes, r s = 18 μm (1/e 2 radius of the laser intensity profile), 170 MeV e-beams were produced in 1-2-mm-long gas jets with the order of 0.5 nC bunch charge using 30 fs, 30 TW laser pulses 5. Using a 2-mm-long preformed plasma channel 2 in a gas jet to guide the driving laser beam 4,11,12 , enabled the production of 85 MeV e-beams containing 0.3 nC bunch charge, with only 9 TW of laser peak power. To scale laser-driven accelerators to GeV electron energies and beyond, two approaches had been proposed: (1) operate in initially uniform plasmas 7,13 with petawatt (PW)-scale lasers and large laser spot sizes, or (2) channel guide the laser beam over centimetre-scale distances 2,14,15. Without guiding (for example, without self-focusing or preformed channels), the laser-plasma interaction length is

Guiding of relativistically intense laser pulses with peak power of 0.85 PW over 15 diffraction lengths was demonstrated by increasing the focusing strength of a capillary discharge waveguide using laser inverse Bremsstrahlung heating. This allowed for the production of electron beams with quasi-monoenergetic peaks up to 7.8 GeV, double the energy that was previously demonstrated. Charge was 5 pC at 7.8 GeV and up to 62 pC in 6 GeV peaks, and typical beam divergence was 0.2 mrad.

Spontaneous radiation emitted from relativistic electrons undergoing betatron motion in a plasma focusing channel is analyzed and applications to plasma wakefield accelerator experiments and to the ion channel laser (ICL) are discussed. Important similarities and differences between a free electron laser (FEL) and an ICL are delineated. It is shown that the frequency of spontaneous radiation is a strong function of the betatron strength parameter a β , which plays a similar role to that of the wiggler strength parameter in a conventional FEL. For a β > ∼ 1, radiation is emitted in numerous harmonics.Furthermore, a β is proportional to the amplitude of the betatron orbit, which varies for every electron in the beam. The radiation spectrum emitted from an electron beam is calculated by averaging the single electron spectrum over the electron distribution. This leads to a frequency broadening of the radiation spectrum, which places serious limits on the possibility of realizing an ICL.03.65. 02.60.Cb, Typeset using REVT E X 1

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Physics considerations for a next-generation linear collider based on laser-plasma accelerators are discussed. The ultrahigh accelerating gradient of a laser-plasma accelerator and short laser coupling distance between accelerator stages allows for a compact linac. Two regimes of laser-plasma acceleration are discussed. The highly nonlinear regime has the advantages of higher accelerating fields and uniform focusing forces, whereas the quasilinear regime has the advantage of symmetric accelerating properties for electrons and positrons. Scaling of various accelerator and collider parameters with respect to plasma density and laser wavelength are derived. Reduction of beamstrahlung effects implies the use of ultrashort bunches of moderate charge. The total linac length scales inversely with the square root of the plasma density, whereas the total power scales proportional to the square root of the density. A 1 TeV center-ofmass collider based on stages using a plasma density of 10 17 cm À3 requires tens of J of laser energy per stage (using 1 m wavelength lasers) with tens of kHz repetition rate. Coulomb scattering and synchrotron radiation are examined and found not to significantly degrade beam quality. A photon collider based on laser-plasma accelerated beams is also considered. The requirements for the scattering laser energy are comparable to those of a single laser-plasma accelerator stage.

Laser plasma accelerators 1 have produced high-quality electron beams with GeV energies from cm-scale devices 2 and are being investigated as hyperspectral fs light sources producing THz to γ-ray radiation 3-5 , and as drivers for future highenergy colliders 6,7 . These applications require a high degree of stability, beam quality and tunability. Here we report on a technique to inject electrons into the accelerating field of a laser-driven plasma wave and coupling of this injector to a lower-density, separately tunable plasma for further acceleration. The technique relies on a single laser pulse powering a plasma structure with a tailored longitudinal density profile, to produce beams that can be tuned in the range of 100-400 MeV with per-cent-level stability, using laser pulses of less than 40 TW. The resulting device is a simple stand-alone accelerator or the front end for a multistage higher-energy accelerator.Producing high-quality electron beams from an accelerator requires electron injection into the accelerating field to be localized in time and space. For laser plasma accelerators (LPAs) that rely on homogeneous plasmas driven with single laser pulses, continuous injection can occur when driving large-amplitude plasma waves (wakefields), resulting in large energy spread. Lower energy spread can be achieved through termination of injection by operating near the injection threshold or by injecting enough charge to suppress the wake amplitude (that is, beam loading). Subsequent termination of the accelerating process at dephasing (that is, when electrons are starting to outrun the accelerating wave) minimizes energy spread. These mechanisms have produced per-cent-level energy-spread beams 2,8-10 , but small changes in parameters can result in large changes in beam quality. As a result, tunability has been limited, necessitating the development of a simple, robust and controlled injection technique combined with an independently controllable accelerating stage.In general, injection of electrons into a plasma wave occurs when the velocity of background electrons approaches the wake phase velocity. Laser-based methods for boosting the electron velocity have been proposed 11,12 and implemented 13,14 to achieve tunable electron beams, but require sophisticated alignment and synchronization of the multiple laser pulses. Injection can also be triggered by introducing electrons into the correct phase of the wake through ionization 15 , but so far the technique has resulted in broad energy-spread beams with high divergence 16,17 . A different approach, that relies on a single laser pulse for powering the LPA, is to momentarily slow down the wake phase velocity to facilitate trapping 18 . The control of the wake phase velocity can be achieved by tailoring the nonlinear plasma wavelength λ p (z) along the longitudinal coordinate z, through control of the electron density n e and the laser parameters. Specifically, λ p (z) = λ p0 (z)F , where the linear plasma wavelength λ p0 (µm) ≈ 3.3 × 10 10 / √ n e (cm −3 ) and F ...

In conventional accelerators, energy from RF electromagnetic waves in vacuum is transformed into kinetic energy of particles driven by the electric field. In high-energyphysics colliders, some of that kinetic energy is in turn transformed into short-lived exotic particles. The new crown jewel of colliders is the recently completed Large Hadron Collider at CERN (see the Quick Study by Fabiola Gianotti and Chris Quigg in PHYSICS TODAY, September 2007, page 90). The LHC, a 27-km-circumference ring for accelerating and storing countercirculating beams of 7-TeV protons, has a stored beam energy exceeding 300 MJ. The collider's design luminosity (collision rate per unit scattering cross section) of 10 34 s −1 cm-2 is two orders of magnitude greater than that of its immediate predecessor, the Tevatron collider at Fermilab. Accelerator-based light sources rely on the fact that when beams of GeV electrons interact with magnetic fields or materials, they emit radiation at frequencies ranging from microwaves to gamma rays. A new generation of freeelectron lasers (FELs) powered by ebeams from linear accelerators will soon be delivering x-ray beams of unprecedented peak brightness, orders of magnitude greater than one gets from conventional synchrotron light sources. The Linac Coherent Light Source (LCLS) at SLAC, for example, is powered by a high-quality beam of 14-GeV electrons from a kilometer-long segment of the laboratory's venerable twomile linear accelerator. Passing finally through a 100-m-long undulator-a periodic sequence of bending magnets-the ebeam generates subpicosecond bursts of coherent 8-keV x rays (see PHYSICS TODAY, May 2005, page 26). The technology and physics of the accelerators that power today's light sources and colliders have been developed and improved over many decades. As the LHC and LCLS become operational, they will equip scientists with powerful new capabilities for answering key questions. Those machines will also point to what is needed next. One thing is certain: Scientists will want higher energy, luminosity, and brightness. Which brings us to the central issue of size and cost: At some point, society will decide that bigger accelerators built with today's technology are simply unaffordable. To survive, we must develop new technologies to make accelerators more compact and more economical.

Coherent radiation in the 0.3-3 THz range has been generated from femtosecond electron bunches at a plasma-vacuum boundary via transition radiation. The bunches produced by a laser-plasma accelerator contained 1.5 nC of charge. The THz energy per pulse within a limited 30 mrad collection angle was 3-5 nJ and scaled quadratically with bunch charge, consistent with coherent emission. Modeling indicates that this broadband source produces about 0.3 microJ per pulse within a 100 mrad angle, and that increasing the transverse plasma size and electron beam energy could provide more than 100 microJ/pulse.