Spectral Modification of Shock Accelerated Ions Using a Hydrodynamically Shaped Gas Target

Laser Proton Acceleration from a Near-Critical Imploding Gas Target

The interaction between relativistic intense laser pulses and near-critical-density targets has been sought after in order to increase the efficiency of laser-plasma energy coupling, particularly for laser-driven proton acceleration. To achieve the density regime for high-repetition-rate applications, one elusive approach is to use gas targets, provided that stringent target density profile requirements are met. These include reaching the critical plasma density while maintaining micron-scale density gradients. In this Letter, we present a novel scheme for achieving the necessary requirements using optical laser pulses to transversely shape the target and create a colliding shock wave in both planar and cylindrical geometries. Utilizing this approach, we experimentally demonstrated stable proton acceleration and achieved up to 5 MeV in a monoenergetic distribution and particle numbers above 10^{8}  Sr^{-1} MeV^{-1} using a 1.5 J energy on-target laser pulse. The Letter also reports for the first time an extend series of 200 consecutive shots that demonstrates the robustness of the approach and its maturity for applications. These results open the door for future work in controlling gas targets and optimizing the acceleration process for more energetic multipetawatt laser systems.

Efficient Magnetic Vortex Acceleration by femtosecond laser interaction with long living optically shaped gas targets in the near critical density plasma regime

We introduce a novel, gaseous target optical shaping laser set-up, capable to generate short scale length, near-critical target profiles via generated colliding blast waves. These profiles are capable to maintain their compressed density for several nanoseconds, being therefore ideal for laser-plasma particle acceleration experiments in the near critical density plasma regime. Our proposed method overcomes the laser-target synchronization limitations and delivers energetic protons, during the temporal evolution of the optically shaped profile, in a time window of approximately 2.5 ns. The optical shaping of the gas-jet profiles is optimised by MagnetoHydroDynamic simulations. 3D Particle-In-Cell models, adopting the spatiotemporal profile, simulate the 45 TW femtosecond laser plasma interaction to demonstrate the feasibility of the proposed proton acceleration set-up. The optical shaping of gas-jets is performed by multiple, nanosecond laser pulse generated blastwaves. This process results in steep gradient, short scale length plasma profiles, in the near critical density regime allowing operation at high repetition rates. Notably, the Magnetic Vortex Acceleration mechanism exhibits high efficiency in coupling the laser energy into the plasma in the optically shaped targets, resulting to collimated proton beams of energies up to 14 MeV.

Identification of electrostatic two-stream instabilities associated with a laser-driven collisionless shock in a multicomponent plasma

Electrostatic two-stream instabilities play essential roles in an electrostatic collisionless shock formation. They are a key dissipation mechanism and result in ion heating and acceleration. Since the number and energy of the shock-accelerated ions depend on the instabilities, precise identification of the active instabilities is important. Two-dimensional particle-in-cell simulations in a multicomponent plasma reveal ion reflection and acceleration at the shock front, excitation of a longitudinally propagating electrostatic instability due to a nonoscillating component of the electrostatic field in the upstream region of the shock, and generation of up- and down-shifted velocity components within the expanding-ion components. A linear analysis of the instabilities for a C_{2}H_{3}Cl plasma using the one-dimensional electrostatic plasma dispersion function, which includes electron and ion temperature effects, shows that the most unstable mode is the electrostatic ion-beam two-stream instability (IBTI), which is weakly dependent on the existence of electrons. The IBTI is excited by velocity differences between the expanding protons and carbon-ion populations. There is an electrostatic electron-ion two-stream instability with a much smaller growth rate associated with a population of protons reflecting at the shock. The excitation of the fast-growing IBTI associated with laser-driven collisionless shock increases the brightness of a quasimonoenergetic ion beam.

Ion acceleration at two collisionless shocks in a multicomponent plasma

Intense laser-plasma interactions are an essential tool for the laboratory study of ion acceleration at a collisionless shock. With two-dimensional particle-in-cell calculations of a multicomponent plasma we observe two electrostatic collisionless shocks at two distinct longitudinal positions when driven with a linearly polarized laser at normalized laser vector potential a_{0} that exceeds 10. Moreover, these shocks, associated with protons and carbon ions, show a power-law dependence on a_{0} and accelerate ions to different velocities in an expanding upstream with higher flux than in a single-component hydrogen or carbon plasma. This results from an electrostatic ion two-stream instability caused by differences in the charge-to-mass ratio of different ions. Particle acceleration in collisionless shocks in multicomponent plasma are ubiquitous in space and astrophysics, and these calculations identify the possibility for studying these complex processes in the laboratory.

Collisionless shock acceleration in the corona of an inertial confinement fusion pellet with possible application to ion fast ignition

Two-dimensional particle-in-cell simulations are used to explore collisionless shock acceleration in the corona plasma surrounding the compressed core of an inertial confinement fusion pellet. We show that an intense laser pulse interacting with the long scale-length plasma corona is able to launch a collisionless shock around the critical density. The nonlinear wave travels up-ramp through the plasma reflecting and accelerating the background ions. Our results suggest that protons with characteristics suitable for ion fast ignition may be achieved in this way. This article is part of a discussion meeting issue 'Prospects for high gain inertial fusion energy (part 2)'.

Development of gas jet targets for laser-plasma experiments at near-critical density

Computational fluid dynamics simulations are performed to design gas nozzles, associated with a 1000 bars backing pressure system, capable of generating supersonic gas jet targets with densities close to the critical density for 1053 nm laser radiation (10²¹ cm^-3). Such targets should be suitable for laser-driven ion acceleration at a high repetition rate. The simulation results are compared to the density profiles measured by interferometry, and characterization of the gas jet dynamics is performed using strioscopy. Proton beams with maximum energies up to 2 MeV have been produced from diatomic hydrogen gas jet targets in a first experiment.

Collisionless Shock Acceleration of High-Flux Quasimonoenergetic Proton Beams Driven by Circularly Polarized Laser Pulses

We present experimental studies on ion acceleration using an 800-nm circularly polarized laser pulse with a peak intensity of 6.9×10^{19}  W/cm^{2} interacting with an overdense plasma that is produced by a laser prepulse ionizing an initially ultrathin plastic foil. The proton spectra exhibit spectral peaks at energies up to 9 MeV with energy spreads of 30% and fluxes as high as 3×10^{12}  protons/MeV/sr. Two-dimensional particle-in-cell simulations reveal that collisionless shocks are efficiently launched by circularly polarized lasers in exploded plasmas, resulting in the acceleration of quasimonoenergetic proton beams. Furthermore, this scheme predicts the generation of quasimonoenergetic proton beams with peak energies of approximately 150 MeV using current laser technology, representing a significant step toward applications such as proton therapy.

Effects of laser polarizations on shock generation and shock ion acceleration in overdense plasmas

The effects of laser-pulse polarization on the generation of an electrostatic shock in an overdense plasma were investigated using particle-in-cell simulations. We found, from one-dimensional simulations, that total and average energies of reflected ions from a circular polarization- (CP) driven shock front are a few times higher than those from a linear polarization- (LP) driven one for a given pulse energy. Moreover, it was discovered that the pulse transmittance is the single dominant factor for determining the CP-shock formation, while the LP shock is affected by the plasma scale length as well as the transmittance. In two-dimensional simulations, it is observed that the transverse instability, such as Weibel-like instability, can be suppressed more efficiently by CP pulses.