Laser wake field acceleration: the highly non-linear broken-wave regime

We use three-dimensional particle-in-cell simulations to study laser wake field acceleration (LWFA) at highly relativistic laser intensities. We observe ultra-short electron bunches emerging from laser wake fields driven above the wave-breaking threshold by few-cycle laser pulses shorter than the plasma wavelength. We find a new regime in which the laser wake takes the shape of a solitary plasma cavity. It traps background electrons continuously and accelerates them. We show that 12-J, 33-fs laser pulses may produce bunches of 3×10¹⁰ electrons with energy sharply peaked around 300 MeV. These electrons emerge as low-emittance beams from plasma layers just 700-μm thick. We also address a regime intermediate between direct laser acceleration and LWFA, when the laser-pulse duration is comparable with the plasma period. Received: 12 December 2001 / Published online: 14 March 2002     

Ion acceleration regimes in underdense plasmas

Acceleration of large populations of ions up to high (relativistic) energies may represent one of the most important and interesting tools that can be provided by the interaction of petawatt laser pulses with matter. In this paper, the basic mechanisms of ion acceleration by short laser pulses are studied in underdense plasmas. The ion acceleration does not originate directly from the pulse fields, but it is mediated by the electrons in the form of electrostatic fields originating from channeling, double layer formation and Coulomb explosion     

Scaling electron acceleration in the bubble regime for upcoming lasers

Electron acceleration in the laser-plasma bubble appeared to be the most successful regime of laser wake field acceleration in the last decade. The laser technology became mature enough to generate short and relativistically intense pulses required to reach the bubble regime naturally delivering quasi-monoenergetic bunches of relativistic electrons. The upcoming laser technology projects are promising short pulses with many times more energy than the existing ones. The natural question is how will the bubble regime scale with the available laser energy. We present here a parametric study of laser-plasma acceleration in the bubble regime using full three dimensional particle-in-cell simulations and compare numerical results with the analytical scalings from the relativistic laser-plasma similarity theory.     

Strong-field double ionization of Ar below the recollision threshold

Nonsequential double ionization of Ar by 45 fs laser pulses (800 nm) at (4-7)x10;{13} W/cm;{2} was explored in fully differential measurements. Well below the field-modified recollision threshold we enter the multiphoton regime. Strongly correlated back-to-back emission of the electrons along the polarization direction is observed to dominate in striking contrast to all previous data. No effect of Coulomb repulsion can be found, the predicted cutoff in the sum-energy spectra of two emitted electrons is confirmed, and the potential importance of multiple recollisions is discussed.     

Additional acceleration and collimation of relativistic electron beams by magnetic field resonance at very high intensity laser interaction

For the interpretation of experiments for acceleration of electrons at interaction up to nearly GeV energy in laser produced plasmas, we present a new model using interaction magnetic fields. In addition to the ponderomotive acceleration of highly relativistic electrons at the interaction of very short and very intense laser pulses, a further acceleration is derived from the interaction of these electron beams with the spontaneous magnetic fields of about 100 MG. This additional acceleration is the result of a laser-magnetic resonance acceleration (LMRA) around the peak of the azimuthal magnetic field. This causes the electrons to gain energy within a laser period. Using a Gaussian laser pulse, the LMRA acceleration of the electrons depends on the laser polarization. Since this is in the resonance regime, the strong magnetic fields affect the electron acceleration considerably. The mechanism results in good collimated high energetic electrons propagating along the center axis of the laser beam as has been observed by experiments and is reproduced by our numerical simulations. PACS 41.75.Jv; 52.38.Kd; 52.65.Cc     

Laser-driven collisions of electron bunches

A novel scheme allowing for relativistic collisions of laser-accelerated electrons is introduced. Two spatially separated electron bunches are driven in opposite directions by two counterpropagating laser pulses until they reach the point of collision which lies within the laser fields. This method can be employed to accelerate electrons to the maximum kinetic energy which can be transferred to charged particles by plane propagating laser fields. Due to the symmetric setup, the center of momentum is at rest with respect to the laser propagation direction such that virtually the whole kinetic energy is available for particle reactions.     

Subfemtosecond K-shell excitation with a few-cycle infrared laser field

Subfemtosecond bursts of extreme ultraviolet radiation, facilitated by a process known as high-order harmonic generation, are a key ingredient for attosecond metrology, providing a tool to precisely initiate and probe ultrafast dynamics in the microcosms of atoms, molecules, and solids. These ultrashort pulses are always, and as a by-product of the way they are generated, accompanied by laser-induced recollisions of electrons with their parent ions. By using a few-cycle infrared (λ(0)=2.1 μm) driving laser, we were able to directly excite high-energy (～870 eV) inner-shell electrons through laser-induced electron recollision, opening the door to time-resolved studies of core-level and concomitant multielectron dynamics.     

Laser energy deposition in relativistic interactions with underdense plasmas

It is well established that, at sub-relativistic intensities, the absorption of laser light by underdense plasmas decreases with increasing pulse intensity as interaction enters a non-linear regime. On the other hand, as the relativistic interaction regime is reached, further absorption mechanisms can be activated which can account for a substantial energy transfer. Using the particle code WAKE, we performed numerical simulations of the relativistic interaction of intense laser pulses with underdense plasmas in conditions that can be experimentally tested. Our simulations show that, while the relativistic laser intensity generates a population of fast electrons, a considerable fraction of the pulse energy goes into a population of thermal electrons. These findings open new possibilities for a direct observation of relativistic interaction processes using high resolution soft X-ray techniques.     

Relativistic AC gyromagnetic effects in ultraintense laser-matter interaction

We demonstrate that in ultraintense ultrafast laser-matter interaction, the interplay of laser-induced oscillating space-charge fields with laser E and B fields can strongly affect whether the interaction is relativistic or not: stronger laser fields may not in fact produce more relativistic plasma interactions. We show that there exists a regime of interaction, in the relation of laser intensity and incident angle, for which the Brunel effect of electron acceleration is strongly suppressed by AC gyromagnetic fields, at a frequency different from the laser field. Analytically and with 1.5D particle-in-cell modeling, we show that from gyromagnetic effects, even in the absence of usual J x B second-harmonic contributions, there are strong effects on the harmonic emission and on the generation of attosecond pulses.     

Semiclassical Dirac theory of tunnel ionization

We present analytic tunnel ionization rates for hydrogenlike ions in ultrahigh intensity laser fields, as obtained from a semiclassical solution of the three-dimensional Dirac equation. This presents the first quantitative determination of tunneling in atomic ions in the relativistic regime. Our theory opens the possibility to study strong laser field processes with highly charged ions, where relativistic ionization plays a dominant role.     

Ion response to relativistic electron bunches in the blowout regime of laser-plasma accelerators

The ion response to relativistic electron bunches in the so called bubble or blowout regime of a laser-plasma accelerator is discussed. In response to the strong fields of the accelerated electrons the ions form a central filament along the laser axis that can be compressed to densities 2 orders of magnitude higher than the initial particle density. A theory of the filament formation and a model of ion self-compression are proposed. It is also shown that in the case of a sharp rear plasma-vacuum interface the ions can be accelerated by a combination of three basic mechanisms. The long time ion evolution that results from the strong electrostatic fields of an electron bunch provides a unique diagnostic of laser-plasma accelerators.     

Laser-driven generation of fast particles

The great progress in high-peak-power laser technology has resulted recently in the production of ps and subps laser pulses of PW powers and relativistic intensities (up to 10²¹ W/cm²) and has laid the basis for the construction of multi-PW lasers generating ultrarelativistic laser intensities (above 10²³ W/cm²). The laser pulses of such extreme parameters make it possible to produce highly collimated beams of electrons or ions of MeV to GeV energies, of short time durations (down to subps) and of enormous currents and current densities, unattainable with conventional accelerators. Such particle beams have a potential to be applied in numerous fields of scientific research as well as in medicine and technology development. This paper is focused on laser-driven generation of fast ion beams and reviews recent progress in this field. The basic concepts and achievements in the generation of intense beams of protons, light ions, and multiply charged heavy ions are presented. Prospects for applications of laser-driven ion beams are briefly discussed.     

Relativistic quantum optics

Laser pulses can reach such high intensities nowadays that atoms exposed to them ionize virtually instantaneously. The extracted electrons are then accelerated to velocities nonnegligible to the speed of light. What remains are multiply charged ions with their most loosely bound outer electrons equally undergoing relativistic dynamics. These may temporarily escape from the vicinity of the ionic core, extract relativistic energies from the laser field and later recollide with the ionic core. The extremely large energies exchanged may be emitted in the form of coherent high-frequency light which is beneficial when an upconversion of frequencies is requested. Signatures of the magnetic field component of the laser field, the relativistic mass shift and spin‐orbit coupling characterize the quantum relativistic dynamics. The existence of more than one active electron within the ion and that of more than one atom in gases or solids complicate the theoretical understanding considerably but simple pictures are often possible to a reasonable approximation.     

Rescattering of electrons at laser-cluster interactions

The goal of this work is to derive the angular distributions of electrons irradiated at the outer ionization of large atomic clusters from Xe atoms by relativistic laser pulses taking into account rescattering processes. Both the magnetic field of the laser pulse and the Coulomb field of the ionized cluster significantly influence the rescattering of ejected electrons. The multiply inner ionization of atoms occurs at the leading edge of the laser pulse. The atomic ions with charge multiplicities up to Z = 26 are subsequently produced (each atomic ion with the next charge multiplicity appears in 3–5 fs) when the laser intensity increases. The measurements of the angular distributions of electrons allow us to reproduce the imaging dynamics of outer ionization of the cluster at the leading edge of the relativistic femtosecond laser pulse.     

Self-Injection and Acceleration of Monoenergetic Electron Beams from Laser Wakefield Accelerators in a Highly Relativistic Regime

Self-injection and acceleration of monoenergetic electron beams from laser wakefield accelerators are first investigated in the highly relativistic regime, using 100 TW class, 27 fs laser pulses. Quasi-monoenergetic multi- bunched beams with energies as high as multi-hundredMeV are observed with simultaneous measurements of side-scattering emissions that indicate the formation of self-channelfing and self-injection of electrons into a plasma wake, referred to as a ＇bubble＇. The three-dimensional particle-in-cell simulations confirmed multiple self-injection of electron bunches into the bubble and their beam acceleration with gradient of 1.5 GeV/cm.     

Coherent hard x rays from attosecond pulse train-assisted harmonic generation

High-order harmonic generation from atomic systems is considered in the crossed fields of a relativistically strong infrared laser and a weak attosecond pulse train of soft x rays. Due to one-photon ionization by the x-ray pulse, the ionized electron obtains a starting momentum that compensates the relativistic drift, which is induced by the laser magnetic field, and allows the electron to efficiently emit harmonic radiation upon recombination with the atomic core in the relativistic regime. This way, short pulses of coherent hard x rays of up to 40 keV energy can be generated.     

Circularly polarised attosecond pulses: generation and applications

ABSTRACT

Molecular high-order harmonic generation(MHOHG) is simulated for H⁺₂ in the nonlinear nonperturbative regime of laser-molecule interactions with ultrashort intense circularly polarised laser pulses. It is shown that combinations of co-rotating or counter-rotating pulses produce laser-induced Coriolis forces with electron-parent ion recollisions, thus enhancing circularly polarised MHOHG, the source of circularly polarised attosecond pulses. Such pulses can be used to induce electron attosecond currents for the generation of attosecond magnetic field pulses, new tools for molecular attomagnetism.     

Generation of femtosecond X-ray pulses via laser–electron beam interaction

The generation of femtosecond X-ray pulses will have important scientific applications by enabling the direct measurement of atomic motion and structural dynamics in condensed matter on the fundamental time scale of a vibrational period. Interaction of femtosecond laser pulses with relativistic electron beams is an effective approach to generating femtosecond pulses of X-rays. In this paper we present recent results from proof-of-principle experiments in which 300 fs pulses are generated from a synchrotron storage ring by using an ultrashort optical pulse to create femtosecond time structure on the stored electron bunch. A previously demonstrated approach for generating femtosecond X-rays via Thomson scattering between terawatt laser pulses and relativistic electrons is reviewed and compared with storage-ring based schemes.     

A RELATIVISTIC QUASI-STATIC MODEL FOR ELECTRONS IN INTENSE LASER FIELDS

A relativistic quasi-static model for the motion of the electrons in relativistic laser fields is proposed. Using the model, the recent experimental results about the generation of the hot electrons in relativistic laser fields can be fit quite well and the important role of the rescattering can be shown clearly.     

Angular distribution of ejected electrons at the laser-cluster interactions

The histograms of deflection angles of electrons ejected from Xe clusters irradiated by femtosecond super-intense laser pulses are presented. The dependence of the angular distribution on the peak laser intensity, the pulse duration, and the cluster position is considered. A clear relationship between the final electron energy and the deflection angle is shown. The deflection angles are calculated by solving the relativistic equation of motion taking into account the Lorentz force and the Coulomb field of the ionized cluster. The ions in the cluster undergo sequential multiple ionization up to charge multiplicity Z = 26. The measurements of the electron angular distributions allow us to reproduce the imaging dynamics of outer ionization of the cluster at the leading edge of the relativistic femtosecond laser pulse.