We present our recent experimental results of monoenergetic protons accelerated from the interaction of an intense terawatt CO<sub>2</sub> laser pulse with a near-critical hydrogen gas target, with its density profile tailored by a hydrodynamic shock. A 5-ns Nd:YAG laser pulse is focused onto a piece of stainless steel foil mounted at the front edge of the gas jet nozzle orifice. The ablation launches a spherical shock into the near-critical gas column, which creates a sharp density gradient at the front edge of the target, with ~ 6X local density enhancement up to several times of critical density within ~<100 microns. With such density profile, we have obtained monoenergetic proton beams with good shot-to-shot reproducibility and energies up to 1.2 MeV.
Over the last two decades, BNL’s ATF has pioneered the use of high-peak power CO<sub>2</sub> lasers for research in advanced accelerators and radiation sources. Our recent developments in ion acceleration, Compton scattering, and IFELs have further underscored the benefits from expanding the landscape of strong-field laser interactions deeper into the midinfrared (MIR) range of wavelengths. This extension validates our ongoing efforts in advancing CO<sub>2</sub> laser technology, which we report here. Our next-generation, multi-terawatt, femtosecond CO<sub>2</sub> laser will open new opportunities for studying ultra-relativistic laser interactions with plasma in the MIR spectral domain. We will address new regimes in the particle acceleration of ions and electrons, as well as the radiations sources, ranging from THz to gamma- rays, that are enabled by the emerging ultra-fast CO<sub>2</sub> lasers.
Recent progress in using picosecond CO<sub>2</sub> lasers for Thomson scattering and ion-acceleration experiments underlines
their potentials for enabling secondary radiation- and particle- sources. These experiments capitalize on certain
advantages of long-wavelength CO<sub>2</sub> lasers, such as higher number of photons per energy unit, and favorable scaling of
the electrons' ponderomotive energy and critical plasma density. The high-flux x-ray bursts produced by Thomson
scattering of the CO<sub>2</sub> laser off a counter-propagating electron beam enabled high-contrast, time-resolved imaging of
biological objects in the picosecond time frame. In different experiments, the laser, focused on a hydrogen jet, generated
monoenergetic proton beams via the radiation-pressure mechanism. The strong power-scaling of this regime promises
realization of proton beams suitable for laser-driven proton cancer therapy after upgrading the CO<sub>2</sub> laser to sub-PW peak
power. This planned improvement includes optimizing the 10-μm ultra-short pulse generation, assuring higher
amplification in the CO<sub>2</sub> gas under combined isotopic- and power-broadening effects, and shortening the postamplification
pulse to a few laser cycles (150-200 fs) via chirping and compression. These developments will move us
closer to practical applications of ultra-fast CO<sub>2</sub> lasers in medicine and other areas.
The concept of a high-repetition-rate, high-average power γ-source is based on Compton backscattering from
the relativistic electron beam inside a picosecond CO<sub>2</sub> laser cavity. Proof-of-principle experiments combined
The picosecond CO<sub>2</sub> 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 CO<sub>2</sub> 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 CO<sub>2</sub> 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 CO<sub>2</sub> laser. High-energy, collimated particle beams originate from the rear surface of the laser-irradiated foils. The expected advantage from using a CO<sub>2</sub> 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 CO<sub>2</sub> 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 CO<sub>2</sub> laser pulses of petawatt peak power and a few-cycles duration.
We describe the physical principles and architecture of a multi-stage picosecond terawatt CO<sub>2</sub> 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 CO<sub>2</sub> laser to a multi-terawatt femtosecond regime.
A combination of a high-power CO<sub>2</sub> laser synchronized to a 70 MeV high-brightness electron linear accelerator operated at the Brookhaven Accelerator Test Facility (ATF) provides a platform for exploring novel methods of particle acceleration, x-ray generation and other advanced areas of beam physics and applications. We review the latest results from the ATF laser/e-beam interaction and plasma experiments including: staged electron laser acceleration (STELLA), Thomson scattering, laser channeling in a capillary discharge, and plasma wake study.
Practical meaningful mono-energetic laser accelerator requires the electron bunch to be within a small proportion of the period of the accelerating field. By two laser accelerator schemes, we exemplify how the emerging picosecond terawatt (ps-TW) CO<SUB>2</SUB> laser technology helps to satisfy this requirement. These include: a staged electron laser accelerator (STELLA) experiment, which is being conducted at the Brookhaven Accelerator Test Facility (ATF), and a prospective laser wakefield accelerator (LWFA), where ps-TW CO<SUB>2</SUB> laser may offer noticeable advantages over more conventional T<SUP>3</SUP> solid-state lasers.
The first terawatt picosecond CO<SUB>2</SUB> laser, PITER I, is under commissioning at the Brookhaven Accelerator Test Facility. PITER I consists of a single-mode TEA oscillator, semiconductor optical switch, and two stages of the multi- atmosphere amplifiers. We report on design, simulation, and tests of the 10 ATM final amplifier that allows multi- terawatt peak power extraction in a picosecond laser pulse.
The design, test and optimization of a picosecond CO<sub>2</sub> pulse-forming system are presented. The system switches a semiconductor's optical characteristics at 10 micrometers under the control of a synchronized 1.06-micrometers Nd:YAG picosecond laser pulse. An energy-efficient version of such a system using collimated beams is described. A simple, semi-empirical approach is used to simulate the switching process, specifically including the spatial distributions of the laser energy and phase, which are relevant for experiments in laser-driven electron acceleration.