Prospects for laser wakefield accelerators and colliders using CO2 laser drivers

Energy directly acquired by an electron from the laser electromagnetic field is quadratically proportional to the laser wavelength. Exploiting this feature, the emerging terawatt picosecond (TWps) CO₂ lasers, having an order of magnitude longer wavelength than the well-known table-top terawatt (T³) picosecond solid state lasers, offer new opportunities for strong-field physics research. Laser accelerators serve as an example where application of the new class of lasers will result in enhancement in gas ionization, plasma wave excitation, and relativistic self-focusing. Ponderomotively strong CO₂ laser permits a 100 times reduction in the plasma density without impeding the acceleration. The improved performance of the low-pressure laser wakefield accelerators (LWFA) is potentially due to higher electric charge per accelerated bunch and better monochromaticity. The multi-kilowatt average power, high repetition rate capability of the TWps-CO₂ laser technology opens new opportunities in development of compact, 1 m long, GeV accelerators and < 1 km long high-luminosity multi-stage LWFA colliders of the TeV scale. The first TWps-CO₂ laser is under construction at the BNL Accelerator Test Facility (ATF).     

Inverse free electron lasers and laser wakefield acceleration driven by CO2 lasers

The staged electron laser acceleration (STELLA) experiment demonstrated staging between two laser-driven devices, high trapping efficiency of microbunches within the accelerating field and narrow energy spread during laser acceleration. These are important for practical laser-driven accelerators. STELLA used inverse free electron lasers, which were chosen primarily for convenience. Nevertheless, the STELLA approach can be applied to other laser acceleration methods, in particular, laser-driven plasma accelerators. STELLA is now conducting experiments on laser wakefield acceleration (LWFA). Two novel LWFA approaches are being investigated. In the first one, called pseudo-resonant LWFA, a laser pulse enters a low-density plasma where nonlinear laser/plasma interactions cause the laser pulse shape to steepen, thereby creating strong wakefields. A witness e-beam pulse probes the wakefields. The second one, called seeded self-modulated LWFA, involves sending a seed e-beam pulse into the plasma to initiate wakefield formation. These wakefields are amplified by a laser pulse following shortly after the seed pulse. A second e-beam pulse (witness) follows the seed pulse to probe the wakefields. These LWFA experiments will also be the first ones driven by a CO(2) laser beam.     

Basic concepts in plasma accelerators

In this article, we present the underlying physics and the present status of high gradient and high-energy plasma accelerators. With the development of compact short pulse high-brightness lasers and electron and positron beams, new areas of studies for laser/particle beam-matter interactions is opening up. A number of methods are being pursued vigorously to achieve ultra-high-acceleration gradients. These include the plasma beat wave accelerator (PBWA) mechanism which uses conventional long pulse ( approximately 100 ps) modest intensity lasers (I approximately 10(14)-10(16) W cm(-2)), the laser wakefield accelerator (LWFA) which uses the new breed of compact high-brightness lasers (<1 ps) and intensities >10(18) W cm(-2), self-modulated laser wakefield accelerator (SMLWFA) concept which combines elements of stimulated Raman forward scattering (SRFS) and electron acceleration by nonlinear plasma waves excited by relativistic electron and positron bunches the plasma wakefield accelerator.In the ultra-high intensity regime, laser/particle beam-plasma interactions are highly nonlinear and relativistic, leading to new phenomenon such as the plasma wakefield excitation for particle acceleration, relativistic self-focusing and guiding of laser beams, high-harmonic generation, acceleration of electrons, positrons, protons and photons. Fields greater than 1 GV cm(-1) have been generated with monoenergetic particle beams accelerated to about 100 MeV in millimetre distances recorded. Plasma wakefields driven by both electron and positron beams at the Stanford linear accelerator centre (SLAC) facility have accelerated the tail of the beams.     

The laser wakefield acceleration experiment at Ecole Polytechnique

A laser wakefield electron acceleration experiment has been set-up at Ecole Polytechnique. An electron beam with 3 MeV total energy is injected in a plasma wave generated by laser wakefield using the new LULI CPA laser (400 fs [FWHM], I < 10¹⁷ W/cm²). The first results show an effective acceleration of the order of 1 MeV, with a maximum when the electron density is close to the optimum value for which the laser pulse length is about half the plasma wavelength.     

On laser wakefield acceleration in plasma channels

The structure of the laser wakefield is analyzed for wide and narrow (in comparison with plasma wavelength) plasma channels with parabolic in radial direction plasma density distributions. The results of analytical theory are confirmed by the self-consistent nonlinear numerical modeling of laser pulse propagation and wakefield generation. In narrow plasma channels the accelerating longitudinal component of the wakefield decreases rapidly with the distance from a laser pulse. This makes possible a short single electron bunch acceleration even if the injected electron beam is much longer than a plasma wavelength.     

Experiments of high energy gain laser wakefield acceleration

The wakefield acceleration of electrons has a great potential for the future accelerator because of its high accelerating field gradient. We have obtained over 100 MeV acceleration gain by the wakefield generated by a 2 TW Ti: sapphire laser system. In the acceleration experiment, the 17 MeV electrons from a linac were used for the injection beam. The synchronization between the RF signal and the laser pulse was achieved within the time jitter of 3.7 ps. Due to the self-focusing and ionization, a long propagation length and high field gradient were realized. The self-focusing effect of the laser was confirmed by the laser spotsize measurement along the beam axis. The plasma density oscillation was measured by using the frequency domain interferometry. The acceleration gain expected from the plasma density measurement was consistent with the result of the acceleration experiments.     

Proposal for a one GeV plasma wakefield acceleration experiment at SLAC

A plasma-based wakefield acceleration experiment E-157 has been approved at SLAC to study acceleration of parts of an SLC bunch by up to 1 GeV/m over a length of 1 m. A single SLC bunch is used to both induce wakefields in the 1 m long plasma and to witness the resulting beam acceleration. The experiment will explore and further develop the techniques that are needed to apply high-gradient plasma wakefield acceleration to large-scale accelerators. The 1 m length of the experiment is about two orders of magnitude larger than for other high gradient plasma wakefield acceleration experiments and the 1 GeV/m accelerating gradient is roughly ten times larger than that achieved with conventional metallic structures. Using existing SLAC facilities, the experiment will study high gradient acceleration at the forefront of advanced accelerator research.     

Plasma based cross-field particle acceleration with high power microwave

Electron acceleration based on the V_p × B acceleration (or the cross-field acceleration) scheme, which has a static magnetic field across the wave propagation direction, is reviewed, specifically the electron linear accelerator using a transverse mode of an EMW is introduced. Penetration and ducting of an intense microwave into overdense plasma are discussed, which show an experimental simulation and precise understanding of the concept of an optical guiding in the laser wakefield accelerator. Coherent radiation of ultrashort microwave pulse by DC-AC radiation conversion scheme has been demonstrated with use of CO₂ laser and array of capacitors. This scheme is based on the mechanism of photon acceleration.     

Near- and long-term applications of plasma-based accelerators

A variety of near-term applications has been suggested and several long-term goals, including high-energy colliders, have been discussed. Applications of laser (or beam)-driven accelerators along with applications of advanced laser itself have been considered, such as for X-ray sources. Some of the near term applications are realistic and exciting enough that they deserve serious further investigations. We point out some of these future opportunities.     

Electron bunch compression and acceleration in the laser wakefield

The compression and acceleration of an external electron bunch into the laser wakefield is studied using 3D modeling with the LAPLAC code and compared to analytical predictions. It is shown, for a laser propagating in a plasma channel, that the nonlinear laser pulse dynamics together with the finite laser spot size influence the electron bunch compression and acceleration due to the reduction of the laser pulse group velocity. The transverse bunch dynamics and loading effect determine the final bunch charge and density and restrict the compressed sizes of the trapped and accelerated electron bunch. The dynamics of the electron bunch are illustrated with a set of parameters where the accelerated bunch acquires an energy of the order of 2 GeV, and 1% energy spread with sub-micron sizes.     

Electron acceleration by single and double laser beams

The longitudinal electric field of single and double Gaussian laser beams are used to accelerate electrons. The longitudinal field of the single beam is concentrated on the axis and is favourable for acceleration. A set of two beams is considered. Beams run parallel, collinearly, overlap partially and have a phase difference iπ in between. As a result, the transverse components of fields cancel each other while the longitudinal components are double-fold. In both schemes, the electrons are accelerated in lengths of the Rayleigh range, which is common to the plasma-based accelerators.     

Laser guiding for GeV laser-plasma accelerators

Guiding of relativistically intense laser beams in preformed plasma channels is discussed for development of GeV-class laser accelerators. Experiments using a channel guided laser wakefield accelerator at Lawrence Berkeley National Laboratory (LBNL) have demonstrated that near mono-energetic 100 MeV-class electron beams can be produced with a 10 TW laser system. Analysis, aided by particle-in-cell simulations, as well as experiments with various plasma lengths and densities, indicate that tailoring the length of the accelerator, together with loading of the accelerating structure with beam, is the key to production of mono-energetic electron beams. Increasing the energy towards a GeV and beyond will require reducing the plasma density and design criteria are discussed for an optimized accelerator module. The current progress and future directions are summarized through comparison with conventional accelerators, highlighting the unique short-term prospects for intense radiation sources based on laser-driven plasma accelerators.     

Second generation beatwave experiments at UCLA

The NEPTUNE Laboratory, under construction at UCLA, will be a user facility for exploring concepts useful for advanced accelerators. The primary programmatic goal for the laboratory is to inject extremely high-quality electron bunches into a laser-driven plasma beat wave accelerator and explore ideas for extracting a high-quality ΔE/E < 0.1, < 10 π mm mrad), high-energy (100 MeV) beam from a plasma structure operating at about 1 THz and about 3 GeV/m. The lab will combine an upgraded MARS CO₂ laser and the state-of-the-art SATURNUS RF gun and linac, also undergoing an upgrade. The new MARS laser will be about 1 TW (100 J, 100 ps), up from 0.2 TW (70 J, 350 ps). This allows for doubling the spot size of the laser beam and thereby quadrupling the interaction length while still driving gradients of 3 GeV/m. The large diameter of the accelerating structure relative to the injected electron bunches (10:1 ratio) will minimize the deleterious effects of the radial dependence of the accelerating field and soften the radial focusing thus permitting, in principle, the extraction of a high-quality accelerated beam.     

Characterization of plasma accelerators with RF linac terminology

The physics of plasma acceleration is described by using RF linac terminology such as shunt impedance, filling time, transit time factor, etc. It is shown that some differences between conventional RF accelerators and plasma accelerators make it difficult to import the RF linac terminology directly into the new field. For example, the shunt impedance is of limited use and the filling time is no use in wake-field accelerators with single-drive beams or single-pump pulses. The beatwave accelerator, a driven oscillator system, has in a sense more similarity to RF linacs than wake-field accelerators. It was shown that plasma wave decay due to collisions and modulational instability seriously deteriorate the quality factor.     

Particle acceleration and coherent radiation by subcycle laser pulses

Electron acceleration by subcycle laser pulses is studied. It is shown in the particle simulations that the irradiation of an intense subcycle pulse on a thin plasma layer gives rise to a pickup of all plasma electrons on the spot and accelerate them to lead to a formation of the high-energy bunched electrons. The condition of generating coherent synchrotron radiation from the bunched electrons is estimated to apply to a bright X-ray source.     

Transverse beam dynamics in plasma-based linacs

The transverse beam dynamics in plasma channels of possible future plasma-based linacs is discussed. We represent the transverse focusing of both a beam-driven and a laser-driven plasma wakefield accelerator by a uniform focusing channel. The transverse beam sizes and a basic offset tolerance are calculated, finding that sub-micron beams must be transported with even smaller offset tolerances. The results emphasize the need to pursue further ideas for plasma structures with high-acceleration gradients but reduced transverse wakefields.     

Radiation sources based on laser-plasma interactions

Plasma waves excited by intense laser beams can be harnessed to produce femtosecond duration bunches of electrons with relativistic energies. The very large electrostatic forces of plasma density wakes trailing behind an intense laser pulse provide field potentials capable of accelerating charged particles to high energies over very short distances, as high as 1GeV in a few millimetres. The short length scale of plasma waves provides a means of developing very compact high-energy accelerators, which could form the basis of compact next-generation light sources with unique properties. Tuneable X-ray radiation and particle pulses with durations of the order of or less than 5fs should be possible and would be useful for probing matter on unprecedented time and spatial scales. If developed to fruition this revolutionary technology could reduce the size and cost of light sources by three orders of magnitude and, therefore, provide powerful new tools to a large scientific community. We will discuss how a laser-driven plasma wakefield accelerator can be used to produce radiation with unique characteristics over a very large spectral range.     

Plasma wake-field acceleration of high energies: Physics and perspectives

Requirements of high-energy physics impose some restrictions on parameters of future plasma-based accelerators; these restrictions are analyzed for the accelerator driven by particle beams. With two-dimensional simulations of driver dynamics it is shown that properly prepared 10 GeV electron driver can accelerate up to 5 × 10⁹ particles with energy gain of about 10 GeV in 10 m long plasma section and with energy spread less than 5%. On the basis of these simulations, the draft concept of high-gradient linear collider of TeV-range energy is briefly considered.

To prove the possibility of precise driver handling and to test simulation results, the experiment is under preparation at Budker Institute, Novosibirsk. The modulated electron beam (0.8 GeV) is expected to excite the electric field up to 0.5 GeV/m at the distance of about 1 m.     

Injection and dynamics of accelerated beams

This paper is a summary of the discussions undertaken by the working group on injection and accelerated beam dynamics at the 1st ICFA Novel and Advanced Accelerator Workshop on Second Generation Plasma Accelerators. The second generation of work on plasma accelerators is aimed to bring the accelerated beams up to the quality needed for applications such as high-energy physics linear colliders. To begin, first generation, or proof-of-principle, experiments and concepts were reviewed. To map the work needed in the second generation of development, the demands of the applications were examined, and an improved framework for discussing the viability of plasma accelerators was constructed. In particular, the issues scaling applications to the short wavelengths characteristic of plasma accelerators was discussed, as was the appropriate characterization of the beam quality in these devices, and the connection between plasma accelerator and conventional accelerator design. Within this framework, the working group discussed electron sources and injectors, the effects of drive beam evolution on accelerated beam dynamics, this effects of nonlinear plasma wave fields on beam phase space, stochastic processes, spatial and temporal beam-plasma wave matching, and future second-generation experimental goals and techniques.     

One GeV acceleration with laser wakefield in the linear regime

We derive an expression for the maximum energy gain of an accelerated electron, in the limit that the plasma wave created by a laser wake is linear both along the longitudinal direction and in the transverse plane, and with a maximum laser power lower than the critical power for relativistic self-focusing. With an available power of 300 TW, the energy gain is of 1 GeV.