When a plasma foil is irradiated by an intense laser pulse at a grazing angle, the pre-pulse of the laser evaporates the solid plasma and generates an expanding near-critical-density (NCD) layer. We consider the situation where the plasma density profile in this layer is strongly nonuniform (high-contrast laser), i.e., the density scale length is comparable to the laser spot size. In this case a relativistic electron vortex (EV) is excited in the NCD layer after the laser pulse depletion. It drifts perpendicular to the density gradient at a constant velocity, typically 0.2 - 0.3 times the speed of the light. The strong coupling between laser and NCD plasma gives rise to intense current density that generates significant magnetic field. The magnetic pressure leads to charge separation in the vortex, which can be used to accelerate protons.
The basic mechanism of acceleration is different from the magnetic vortex acceleration (MVA), where a magnetic dipole vortex is generated in a uniform NCD target and acceleration only happens when it reaches the rear side of the target. In our work, the EV, or magnetic monopolar vortex, serves as a stable slow-moving structure in which protons can be captured and gain considerable kinetic energy from the charge separation field associated with the EV. Similarly to collisionless shock acceleration, protons initially at rest can be reflected to twice the EV drift velocity.
The mechanism of EV drift in the strongly nonuniform plasma is different from the weakly nonuniform plasma that are mostly studied in the previous works. In order to obtain a deeper insight into the dynamics of EV drift, we perform 2-dimensional (2D) particle-in-cell (PIC) simulations and track the trajectories of a number of electrons that are trapped in the EV. Our results suggest the laser-driven electrons are subject to an E x B drift which drives the collective motion of the vortex structure. Based on this, an analytical model for the vortex velocity is derived in terms of laser and plasma parameters, the prediction of the model agrees well with the PIC simulations.
It demonstrates that drift velocity of EV, and therefore the maximum proton energy, can be effectively controlled by the incidence angle of the laser and the plasma density gradient.
A representative scenario is studied with 2D PIC simulations—with laser intensity at 10^21 W/cm^2 and incident at 10 degrees—is discussed, in which a quasi-monoenergetic proton beam is obtained with a mean energy 140 MeV and an energy spread of 10%. 3D effects are discussed briefly towards the end of the presentation.
The increasing demand for high laser powers is placing huge demands on current laser technology. This is now reaching a limit, and to realise the existing new areas of research promised at high intensities, new cost-effective and technically feasible ways of scaling up the laser power will be required. Plasma-based laser amplifiers may represent the required breakthrough to reach powers of tens of petawatts to exawatts, because of the fundamental advantage that amplification and compression can be realised simultaneously in a plasma medium, which is also robust and resistant to damage, unlike conventional amplifying media. Raman amplification is a promising method, where a long pump pulse transfers energy to a lower frequency, short duration counter-propagating seed pulse through resonant excitation of a plasma wave that creates a transient plasma echelon, which backscatters the pump into the probe. While very efficient, this comes at the cost of noise amplification (from plasma density fluctuations) that needs to be controlled. Here we present the results of an experimental campaign where we have demonstrated chirped pulse Raman amplification (CPRA) at high intensities. We have used a frequency chirped pump pulse to limit the growth of noise amplification, while trying to maintain the amplification of the seed. In non-optimised conditions we show that indeed noise amplification can be controlled but reducing noise scattering also limits the seed amplification factor. Finally, we show that the gross efficiency is a few percent, consistent with previous measurements of CPRA obtained in capillaries with pump pulses of duration of a few hundred picoseconds.
The increasing demand for high laser powers is placing huge demands on current laser technology. This is now reaching a limit, and to realise the existing new areas of research promised at high intensities, new cost-effective and technically feasible ways of scaling up the laser power will be required. Plasma-based laser amplifiers may represent the required breakthrough to reach powers of tens of petawatt to exawatt, because of the fundamental advantage that amplification and compression can be realised simultaneously in a plasma medium, which is also robust and resistant to damage, unlike conventional amplifying media. Raman amplification is a promising method, where a long pump pulse transfers energy to a lower frequency, short duration counter-propagating seed pulse through resonant excitation of a plasma wave that creates a transient plasma echelon that backscatters the pump into the probe. Here we present the results of an experimental campaign conducted at the Central Laser Facility. Pump pulses with energies up to 100 J have been used to amplify sub-nanojoule seed pulses to near-joule level. An unprecedented gain of eight orders of magnitude, with a gain coefficient of 180 cm−1 has been measured, which exceeds high-power solid-state amplifying media by orders of magnitude. High gain leads to strong competing amplification from noise, which reaches similar levels to the amplified seed. The observation of 640 Jsr−1 directly backscattered from noise, implies potential overall efficiencies greater than 10%.
Envelope models offer the potential to dramatically reduce the computational overhead of particle-in-cell simulations of laser-plasma interactions. However, the associated approximations inevitably limit their applicability. We here derive the governing equations for an envelope model in order to gauge those limits. The approximations for electron response are shown to be exact in one dimension for the correct initial conditions. For multidimensional geometries, limits are placed on the canonical momentum perpendicular to the laser field, and on the amplitude of the laser field relative to the laser spot size. It is shown that those conditions are readily satisfied for the case of Raman amplification.
Terahertz (THz) radiation from the interaction of ultrashort laser pulses with gases is studied both theoretically and experimentally.
We theoretically study the THz generation based on transient ionization current model and give the relation
between the final THz field and the initial transient ionization current. Recent experimental results on optimization of THz
radiation in laser air interaction are also shown. We find by use of a simple aperture to change the laser field distribution,
the terahertz wave amplitudes can be enhanced by more than eight times than those of aperture-free cases. We use two
dimensional particle-in-cell codes to simulate the experiments and give possible explanations.
The relativistic Doppler effect offers a fundamental means of frequency upconverting electromagnetic radiation.
In 1993, Esarey et al.1 mentioned the possibility of scattering light at fast moving electrons to upconvert its
frequency. For the process to be efficient, one needs to have a highly compressed bunch of electrons, since only
then the scattering process can become coherent. The condition for coherency is, that the scale length of the
electron bunch or its density gradient needs to be on the order of the wavelength to be generated or smaller. This
is much tinier than what can be reached by commonly known techniques, including conventional accelerators as
well as laser-plasma accelerators.
Therefore, electrons are extracted from a small droplet or a thin foil by a highly relativistic driver laser
(a0 = eA0/mc2 ⪆⪆ 1). The electron bunch becomes accelerated and at the same time compressed by the forces of
the laser field. The acceleration can be achieved either by the relativistic ponderomotive force of a conventional
laser pulse, as suggested in,6 or by the longitudinal field on the optical axis of a radially polarized pulse, as
suggested in.8 In both cases, the bunch is compressed because of the fundamental snowplough effect of the
co-moving force, i.e. the laser pulse. Spacecharge forces are counteracting the compression, thus limiting the
amount of charge to be compressed. Nevertheless, in a wide range of parameters the edges of the electron bunches
density profile remain sharp, enabling coherent Thomson scattering.
We use analytic models and PIC simulations to describe and analyze thoroughly the effects occurring and
finally estimate the conversion efficiency that can be reached by this scheme. Techniques to increase the efficiency
and gain further control over the generated radiation are suggested and discussed. Reaching best possible control
over temporal envelope of the driver pulse appears to be the most important issue here.
The picosecond CO2 gas laser has proven a valuable tool in strong-field physics applications. We review the merits of this approach, taking as an example, the Brookhaven Accelerator Test Facility (ATF) that affords a platform for exploring novel methods of particle acceleration and radiation sources. To carry out this mission, the ATF is equipped with a picosecond terawatt CO2 laser system, PITER-I. We describe the physical principles and architecture of this multi-stage laser system and its application in two high-energy physics projects. The first is the intense Thomson scattering of the CO2 beam from 60 MeV electrons with production of one x-ray photon per electron that opened the possibility for a Compton gamma-source generating a polarized positron beam for the next generation of electron-positron colliders, such as the International Linear Collider (ILC) and the Compact Linear Collider (CLIC). The second is our new study of a high-brightness multi-MeV ion- and proton-beam source energized by this picosecond CO2 laser. High-energy, collimated particle beams originate from the rear surface of the laser-irradiated foils. The expected advantage from using a CO2 laser for this application, rather than an ultra-fast solid state laser, is the 100-fold increase in the electron ponderomotive potential due to the tenfold longer wavelength of the CO2 laser. This innovation promises to substantially enhance energy efficiency and particle yield, and will facilitate the advancement of laser-driven ion accelerators towards practical applications. Finally, we address possibilities for generating CO2 laser pulses of petawatt peak power and a few-cycles duration.
We describe the physical principles and architecture of a multi-stage picosecond terawatt CO2 laser system, PITER-I,
operational at Brookhaven National Laboratory (BNL). The laser is a part of the DOE user's facility open for
international scientific community. One of the prospective strong-field physics applications of PITER-I is the
production of proton- and heavy-ion beams upon irradiating thin-film targets and gas jets. We discuss the possibilities
for upgrading a CO2 laser to a multi-terawatt femtosecond regime.
Using our 3D PIC code VLPL (Virtual Laser-Plasma Laboratory) we study Laser-Wake Field Acceleration (LWFA) of electrons by laser pulses shorter than or comparable with the plasma wavelength. When driven into the highly non-linear wave breaking regime the plasma wave mutates to a solitary bubble that generates ultra-short dense bunches of electrons with quasi-monoenergetic energy spectra. The electron bunches may have density high enough to forward-scatter the tail of the laser pulse. The forward scattering results in blue shift of the laser pulse after interaction. The energetic electrons make betatron oscillations in transverse fields of the plasma wave and emit hard X- and γ-rays. We show that an extremely bright source of GeV γ-quanta can be built due to the combination of an external electron beam and the laser wake field. The GeV γ-source can be particularly used as an efficient plant for positron production.
We have studied the influence of the target properties on laser-accelerated proton and ion beams generated by the LULI multi-terawatt laser. A strong dependence of the ion emission on the surface conditions, conductivity, shape and material of the thin foil targets were observed. We have performed a full characterization of the ion beam using magnetic spectrometers, Thompson parabolas, radiochromic film and nuclear activation techniques. The strong dependence of the ion beam acceleration on the conditions on the target back surface was found in agreement with theoretical predictions based on the target normal sheath acceleration (TNSA) mechanism. Proton kinetic energies up to 25 MeV have been observed.