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.

Real-time visualization of the laser-plasma wakefield dynamics.

The exploration of new acceleration mechanisms for compactly delivering high-energy particle beams has gained great attention in recent years. One alternative that has attracted particular interest is the plasma-based wakefield accelerator, which is capable of sustaining accelerating fields that are more than three orders of magnitude larger than those of conventional radio-frequency accelerators. In this device, acceleration is generated by plasma waves that propagate at nearly light speed, driven by intense lasers or charged particle beams. Here, we report on the direct visualization of the entire plasma wake dynamics by probing it with a femtosecond relativistic electron bunch. This includes the excitation of the laser wakefield, the increase of its amplitude, the electron injection, and the transition to the beam-driven plasma wakefield. These experimental observations provide first-hand valuable insights into the complex physics of laser beam-plasma interaction and demonstrate a powerful tool that can largely advance the development of plasma accelerators for real-time operation.

High repetition rate relativistic laser-solid-plasma interaction platform featuring simultaneous particle and radiation detection.

We report on a uniquely designed high repetition rate relativistic laser-solid-plasma interaction platform, featuring the first simultaneous measurement of emitted high-order harmonics, relativistic electrons, and low divergence proton beams. This versatile setup enables detailed parametric studies of the particle and radiation spatio-spectral beam properties under a wide range of controlled interaction conditions, such as pulse duration and plasma density gradient. Its array of complementary diagnostics unlocks the potential to unravel interdependencies among the observables and should aid in further understanding the complex collective dynamics at play during laser-plasma interactions and in optimizing the secondary beam properties for applications.

Refractive plasma optics for relativistic laser beams.

The high intensities reached today by powerful lasers enable us to explore the interaction with matter in the relativistic regime, unveiling a fertile domain of modern science that is pushing far away the frontiers of plasma physics. In this context, refractive-plasma optics are being utilized in well established wave guiding schemes in laser plasma accelerators. However, their use for spatial phase control of the laser beam has never been successfully implemented, partly due to the complication in manufacturing such optics. We here demonstrate this concept which enables phase manipulation near the focus position, where the intensity is already relativistic. Offering such flexible control, high-intensity high-density interaction is becoming accessible, allowing for example, to produce multiple energetic electron beams with high pointing stability and reproducibility. Cancelling the refractive effect with adaptive mirrors at the far field confirms this concept and furthermore improves the coupling of the laser to the plasma in comparison to the null test case, with potential benefits in dense-target applications.

Femtosecond electron microscopy of relativistic electron bunches.

The development of plasma-based accelerators has enabled the generation of very high brightness electron bunches of femtosecond duration, micrometer size and ultralow emittance, crucial for emerging applications including ultrafast detection in material science, laboratory-scale free-electron lasers and compact colliders for high-energy physics. The precise characterization of the initial bunch parameters is critical to the ability to manipulate the beam properties for downstream applications. Proper diagnostic of such ultra-short and high charge density laser-plasma accelerated bunches, however, remains very challenging. Here we address this challenge with a novel technique we name as femtosecond ultrarelativistic electron microscopy, which utilizes an electron bunch from another laser-plasma accelerator as a probe. In contrast to conventional microscopy of using very low-energy electrons, the femtosecond duration and high electron energy of such a probe beam enable it to capture the ultra-intense space-charge fields of the investigated bunch and to reconstruct the charge distribution with very high spatiotemporal resolution, all in a single shot. In the experiment presented here we have used this technique to study the shape of a laser-plasma accelerated electron beam, its asymmetry due to the drive laser polarization, and its beam evolution as it exits the plasma. We anticipate that this method will significantly advance the understanding of complex beam-plasma dynamics and will also provide a powerful new tool for real-time optimization of plasma accelerators.

Determination of the Ion Temperature in a High-Energy-Density Plasma Using the Stark Effect.

We present the experimental determination of the ion temperature in a neon-puff Z pinch. The diagnostic method is based on the effect of ion coupling on the Stark line shapes. It was found, in a profoundly explicit way, that at stagnation the ion thermal energy is small compared to the imploding-plasma kinetic energy, where most of the latter is converted to hydromotion. The method here described can be applied to other highly nonuniform and transient high-energy-density plasmas.