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.     

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.     

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.     

Electron capture in an electron plasma wave

We study the influence of the radial electric field of a plasma wave on the accelerated electrons.     

双光束双曝光与四光束单曝光干涉光刻方法的比较

双光束双曝光和四光束单曝光是无掩模激光干涉光刻的两种典型方法,都容易利用现有光刻工艺,在不需掩模和高精度光刻物镜的情况下,用简单廉价光学系统在大视场和深曝光场内形成孔阵、点阵或锥阵等周期性图形。双光束双曝光法得到的阵列图形周期极限为λ/2;四光束单曝光的周期略大,为前者的2倍。模拟和实验结果表明,通过控制曝光和显影工艺,双光束双曝光较四光束单曝光能更灵活地得到孔阵或点阵,而四光束单曝光得到的图形孔与孔之间没有鞍点,较双光束双曝光形成的孔侧壁更陡。这两种方法在需要在大面积范围内形成孔或点这类周期阵列图形的微电子和光电子器件的制造领域有很好的应用前景。     

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.     

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.     

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.     

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).     

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.     

Vp×B electron linear accelerator with TE-wave assisted by plasma for beam stabilization

The V_p × B acceleration scheme with the use of a transverse electromagnetic wave is demonstrated experimentally, in which a pre-ionized plasma is supplemented for obtaining a stable electron beam. The slow wave structure employed here is a dielectric loaded waveguide, and an electron beam in the accelerator induces surfaces charges on the dielectric. The electron beam on account of acceleration also produces a dilute plasma to neutralize the surface charges. An initial energy gain of 2.5 keV for the electron beam is observed from an incident energy of 60 keV without any external vertical magnetic field. When an external vertical magnetic field of 1.5 G is applied under the same conditions for performing the V_p × B scheme, an additional 1.5 keV energy gain is observed.     

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.     

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.     

Low emittance ion beams by laser interaction

Laser ion sources offer the possibility to get ion beam utilizable to improve particle accelerators. Pulsed lasers at intensities of the order of 10⁸ W/cm² and of ns pulse duration interact with solid matter in vacuum to produce plasma of high temperature and density. The charge state distribution of the plasma generates high electric fields which accelerate ions along the normal to the target surface. The energy of emitted ions has a Maxwell Boltzmann distribution, which depends on the ion charge state. To increase the ion energy, a post-acceleration system can be employed by means of high voltage power supplies of about 100 kV. The post-acceleration system results in a good method to obtain high ion currents by an inexpensive system and the final ion beams find interesting applications in the field of the ion implantations, scientific applications and industrial use. In this work we compare the electromagnetic and geometric properties, like the emittance, of the beams delivered by a Cu and Y target. The characterization of the plasma was performed by a Faraday cup for the electromagnetic characteristics and a pepper pot system for the geometric ones. At 60 kV accelerating voltage and 5.5 mA output current the normalized beam emittance resulted in 0.22 π mm mrad for the Cu target, while under the same accelerating voltage but with 7.4 mA output current, a lower normalized beam emittance value was reached for the Y target. It resulted in 0.14 π mm mrad. The brightness of the beams was of 114 and 378 mA (π mm mrad)⁻² for the Cu and Y targets, respectively.     

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.     

Propagation factors of laser array beams in turbulent atmosphere

The propagation factors of phased locked laser array beams of radial and rectangular symmetries in a turbulent atmosphere are investigated based on the extended Huygens–Fresnel integral and the Wigner distribution function. Analytical propagation formulae for the propagation factors are derived and numerical examples are illustrated. We find that unlike their propagation invariant properties in free space, the propagation factors of laser array beams increase when propagating in turbulent atmosphere, and are closely related to the parameters of initial beams and the atmosphere.     

LD端面抽运长方形激光晶体的热效应

通过对二极管激光器端面抽运激光晶体工作特点的分析,提出了长方形激光晶体热模型。在热模型中考虑了激光晶体具有沿轴向对称加热、周边恒温等特点。利用热传导方程得出了激光晶体内部温度场和端面热形变场的一般解析表达式。研究结果表明,若使用输出功率为18W的二极管激光器端面抽运Nd:YVO4晶体(钕离子掺杂质量分数为0.5%),抽运光斑为0.4mm时,长方形激光晶体抽运端面具有267.6℃的最大温升和发生6.2μm的最大热形变量。对激光晶体产生的温度场和端面热形变场的研究为解决晶体热透镜效应、提高激光器的性能起指导作用。     

Generation of fast protons by interaction of modest laser intensities with H2O “snow” nano-wire targets

We report on the generation of protons with energies of 5.5 MeV when irradiating an H₂O nano-wire layer grown on a sapphire plate with an intensity of 5×10¹⁷ W/cm². A theoretical model is suggested in which plasma near the tip of the wire is subject to enhanced electrical fields and protons are accelerated to several MeVs.