Alexander S. Pirozhkov

Review of laser-driven ion sources and their applications

For many years, laser-driven ion acceleration, mainly proton acceleration, has been proposed and a number of proof-of-principle experiments have been carried out with lasers whose pulse duration was in the nanosecond range. In the 1990s, ion acceleration in a relativistic plasma was demonstrated with ultra-short pulse lasers based on the chirped pulse amplification technique which can provide not only picosecond or femtosecond laser pulse duration, but simultaneously ultra-high peak power of terawatt to petawatt levels. Starting from the year 2000, several groups demonstrated low transverse emittance, tens of MeV proton beams with a conversion efficiency of up to several percent. The laser-accelerated particle beams have a duration of the order of a few picoseconds at the source, an ultra-high peak current and a broad energy spectrum, which make them suitable for many, including several unique, applications. This paper reviews, firstly, the historical background including the early laser-matter interaction studies on energetic ion acceleration relevant to inertial confinement fusion. Secondly, we describe several implemented and proposed mechanisms of proton and/or ion acceleration driven by ultra-short high-intensity lasers. We pay special attention to relatively simple models of several acceleration regimes. The models connect the laser, plasma and proton/ion beam parameters, predicting important features, such as energy spectral shape, optimum conditions and scalings under these conditions for maximum ion energy, conversion efficiency, etc. The models also suggest possible ways to manipulate the proton/ion beams by tailoring the target and irradiation conditions. Thirdly, we review experimental results on proton/ion acceleration, starting with the description of driving lasers. We list experimental results and show general trends of parameter dependences and compare them with the theoretical predictions and simulations. The fourth topic includes a review of scientific, industrial and medical applications of laser-driven proton or ion sources, some of which have already been established, while the others are yet to be demonstrated. In most applications, the laser-driven ion sources are complementary to the conventional accelerators, exhibiting significantly different properties. Finally, we summarize the paper.

Proton acceleration to 40 MeV using a high intensity, high contrast optical parametric chirped-pulse amplification/Ti:sapphire hybrid laser system.

Using a high-contrast (10(10):1) and high-intensity (10(21) W/cm(2)) laser pulse with the duration of 40 fs from an optical parametric chirped-pulse amplification/Ti:sapphire laser, a 40 MeV proton bunch is obtained, which is a record for laser pulse with energy less than 10 J. The efficiency for generation of protons with kinetic energy above 15 MeV is 0.1%.

Laser prepulse dependency of proton-energy distributions in ultraintense laser-foil interactions with an online time-of-flight technique

Fast protons are observed by a newly developed online time-of-flight spectrometer, which provides shot-to-shot proton-energy distributions immediately after the irradiation of a laser pulse having an intensity of ∼1018W∕cm2 onto a 5-μm-thick copper foil. The maximum proton energy is found to increase when the intensity of a fs prepulse arriving 9ns before the main pulse increases from 1014 to 1015W∕cm2. Interferometric measurement indicates that the preformed-plasma expansion at the front surface is smaller than 15μm, which corresponds to the spatial resolution of the diagnostics. This sharp gradient of the plasma has the beneficial effect of increasing the absorption efficiency of the main-pulse energy, resulting in the increase in the proton energy. This is supported by the result that the x-ray intensity from the laser plasma clearly increases with the prepulse intensity.

Soft-x-ray harmonic comb from relativistic electron spikes

We demonstrate a new high-order harmonic generation mechanism reaching the water window spectral region in experiments with multiterawatt femtosecond lasers irradiating gas jets. A few hundred harmonic orders are resolved, giving mu J/sr pulses. Harmonics are collectively emitted by an oscillating electron spike formed at the joint of the boundaries of a cavity and bow wave created by a relativistically self-focusing laser in underdense plasma. The spike sharpness and stability are explained by catastrophe theory. The mechanism is corroborated by particle-in-cell simulations.

Attosecond pulse generation in the relativistic regime of the laser-foil interaction: The sliding mirror model

Theory of the attosecond pulse generation during the interaction of a short relativistic-irradiance laser pulse with a thin overdense plasma slab is developed. The nonlinear electric current caused by the electron motion at relativistic velocity generates the high-order harmonics of the incident radiation. These harmonics are phase locked and can produce pulses with attosecond duration after spectral filtering. Conditions for the most efficient generation of single-attosecond pulses are discussed. A very efficient regime of attosecond pulse train generation without spectral filtering is proposed. The results are verified by the particle-in-cell simulations.

Temporal contrast enhancement of petawatt-class laser pulses

We demonstrate the temporal contrast enhancement in a petawatt-class Ti:sapphire chirped-pulse amplification (CPA) laser system. An extra saturable absorber, introduced downstream after a low-gain optical parametric chirped-pulse amplification (OPCPA) preamplifier, has improved the temporal contrast in the system to 1.4×10(12) on the subnanosecond time scale at 70 TW power level. We have achieved 28 J of uncompressed broadband output energy with this system, indicating the potential for reaching peak powers near 600 TW.

High-Contrast, High-Intensity Petawatt-Class Laser and Applications

A high-contrast high-intensity petawatt-class Ti:sapphire chirped-pulse amplification laser has been developed for research on high field science. A saturable absorber and a low-gain optical parametric chirped-pulse amplification preamplifier in the front-end have improved the temporal contrast in the system to ∼

Prepulse and amplified spontaneous emission effects on the interaction of a petawatt class laser with thin solid targets

2 times 10^{12}

Efficient generation of Xe K-shell x rays by high-contrast interaction with submicrometer clusters

on the subnanosecond time scale at the ∼70 TW power level. In addition to the high-contrast broadband high-energy output from the final amplifier has been achieved with a flat-top spatial profile with a filling factor of ∼70%. This is the result of pump beam spatial profile homogenization with diffractive optical elements. The system produces the uncompressed output pulse energy of 29 J, indicating the capability for reaching a peak power of ∼600 TW. We discuss in detail the design, performance, and characterization of the laser including output power, pulse duration, and spatiotemporal beam quality. We also describe the on-going upgrade of the laser system and some applications for the laser in relativistic dominated laser–matter interactions.

High order harmonics from relativistic electron spikes

Abstract When a finite contrast petawatt laser pulse irradiates a micron-thick foil, a prepulse (including amplified spontaneous emission) creates a preplasma, where an ultrashort relativistically strong portion of the laser pulse (the main pulse) acquires higher intensity due to relativistic self-focusing and undergoes fast depletion transferring energy to fast electrons. If the preplasma thickness is optimal, the main pulse can reach the target accelerating fast ions more efficiently than an ideal, infinite contrast, laser pulse. A simple analytical model of a target with preplasma formation is developed and the radiation pressure dominant acceleration of ions in this target is predicted. The preplasma formation by a nanosecond prepulse is analyzed with dissipative hydrodynamic simulations. The main pulse interaction with the preplasma is studied with multi-parametric particle-in-cell simulations. The optimal conditions for hundreds of MeV ion acceleration are found with accompanying effects important for diagnostics, including high-order harmonics generation.