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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. Intense laser interaction with matter creates plasma which can act as a nonlinear optical medium. Here the authors demonstrate plasma as a refractive optics for relativistic intensity radiation, evident by the acceleration of multiple electron beams from a single laser pulse passing through the plasma.

The use of structured light to control the phase velocity of the wake in laser-wakefield accelerators has generated significant interest for its ability to mitigate electron dephasing. Combining the diffraction-free properties of Bessel beams with spatio-temporal shaping of the pulse promises to enable acceleration with an unprecedented combination of long acceleration lengths and high gradients. This would facilitate the acceleration of electrons to energies above 100 GeV in existing laser facilities. In-depth understanding of the physical mechanisms involved is critical to achieving dephasing-free electron acceleration. Here we present experimental observations of wakefields generated by beams that were spatio-temporally sculpted and then focused with a long-focal-depth mirror, known as an axiparabola, which generates a quasi-Bessel beam. The resulting wakefield was imaged using femtosecond relativistic electron microscopy. Insights into this minimally explored regime include mapping the wakefield development over the focal depth and studying the effects of spatio-temporal manipulations of the beam on the structure and phase velocity of the wakefield. Such insights pave the way towards realizing the potential of structured-light based solutions to dephasing in laser-wakefield acceleration. Spatio-temporally structured light can provide useful applications in laser-wakefield acceleration. Here, the authors demonstrate the generation of wakefields by means of quasi-Bessel beams focused by an axiparabola, imaged with femtosecond relativistic electron microscopy.

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. The first direct, non-invasive characterization of the lateral structure of a relativistic, micro-sized electron bunch generated from a laser-plasma accelerator.

Bright, energetic, and directional electron bunches are generated through efficient energy transfer of relativistic intense (~ 10 19  W/cm 2 ), 30 femtosecond, 800 nm high contrast laser pulses to grating targets (500 lines/mm and 1000 lines/mm), under surface plasmon resonance (SPR) conditions. Bi-directional relativistic electron bunches (at 40° and 150°) are observed exiting from the 500 lines/mm grating target at the SPR conditions. The surface plasmon excited grating target enhances the electron flux and temperature by factor of 6.0 and 3.6, respectively, compared to that of the plane substrate. Particle-in-Cell simulations indicate that fast electrons are emitted in different directions at different stages of the laser interaction, which are related to the resultant surface magnetic field evolution. This study suggests that the SPR mechanism can be used to generate multiple, bright, ultrafast relativistic electron bunches for a variety of applications.

Remote manipulation (triggering and guiding) of lightning in atmospheric conditions of thunderstorms has been the subject of intense scientific research for decades. High power, ultrashort-pulse lasers are considered attractive in generating plasma channels in air that could serve as conductors/diverters for lightning. However, two fundamental obstacles, namely the limited length and lifetime of such plasma channels prevented their realization to this date. In this paper, we report decisive experimental results of our multi-element broken wire concept that extends the generated plasma channels to the required tens of meters range. We obtain 13-meter-long plasma wire, limited only by our current experimental setup, with plasma conditions that could be sufficient for the leader initiation. This advance, coupled with our demonstrated method of laser heating for long time sustenance of the plasma channel, is a major, significant step towards controlling lightning.

MeV temperature electrons are typically generated at laser intensities of 10 18 W cm −2 . Their generation at non-relativistic intensities (~10 16 W cm −2 ) with high repetition rate lasers is cardinal for the realization of compact, ultra-fast electron sources. Here we report a technique of dynamic target structuring of micro-droplets using a 1 kHz, 25 fs, millijoule class laser, that uses two collinear laser pulses; the first to create a concave surface in the liquid drop and the second, to dynamically-drive electrostatic plasma waves that accelerate electrons to MeV energies. The acceleration mechanism, identified as two plasmon decay instability, is shown to generate two beams of electrons with hot electron temperature components of 200 keV and 1 MeV, respectively, at an intensity of 4 × 10 16 Wcm −2 , only. The electron beams are demonstrated to be ideal for single shot high resolution (tens of μ m) electron radiography. Typically, mJ lasers generate 50 keV temperature plasma electrons. Here, the authors use mJ laser pulses to chisel a liquid droplet surface and generate an electron temperature of 1 MeV, a feat previously possible only with 100 times more powerful lasers.

High-intensity ultrashort laser pulses focused on metal targets readily generate hot dense plasmas which accelerate ions efficiently and can pave way to compact table-top accelerators. Laser-driven ion acceleration studies predominantly focus on protons, which experience the maximum acceleration owing to their highest charge-to-mass ratio. The possibility of tailoring such schemes for the preferential acceleration of a particular ion species is very much desired but has hardly been explored. Here, we present an experimental demonstration of how the nanostructuring of a copper target can be optimized for enhanced carbon ion acceleration over protons or Cu-ions. Specifically, a thin (≈0.25 μm) layer of 25–30 nm diameter Cu nanoparticles, sputter-deposited on a polished Cu-substrate, enhances the carbon ion energy by about 10-fold at a laser intensity of 1.2×10 18   W/cm 2 . However, particles smaller than 20 nm have an adverse effect on the ion acceleration. Particle-in-cell simulations provide definite pointers regarding the size of nanoparticles necessary for maximizing the ion acceleration. The inherent contrast of the laser pulse is found to play an important role in the species selective ion acceleration.

Plasma waves contribute to many fundamental phenomena, including astrophysics 1 , thermonuclear fusion 2 and particle acceleration 3 . Such waves can develop in numerous ways, from classic Langmuir oscillations carried by electron thermal motion 4 , to the waves excited by an external force and travelling with a driver 5 . In plasma-based particle accelerators 3 , 6 , a strong laser or relativistic particle beam launches plasma waves with field amplitude that follows the driver strength up to the wavebreaking limit 5 , 7 , which is the maximum wave amplitude that a plasma can sustain. In this limit, plasma electrons gain sufficient energy from the wave to outrun it and to get trapped inside the wave bucket 8 . Theory and numerical simulations predict multi-dimensional wavebreaking, which is crucial in the electron self-injection process that determines the accelerator performances 9 , 10 . Here we present a real-time experimental visualization of the laser-driven nonlinear relativistic plasma waves by probing them with a femtosecond high-energy electron bunch from another laser-plasma accelerator coupled to the same laser system. This single-shot electron deflectometry allows us to characterize nonlinear plasma wakefield with femtosecond temporal and micrometre spatial resolutions revealing features of the plasma waves at the breaking point. In a plasma-based accelerator, the amplitude of the plasma wave is constrained by the wavebreaking limit. Experiments reveal features of the plasma waves at the point at which wavebreaking occurs.

We study the stability of plasma wake wave and the properties of density-downramp injection in an electron-driven plasma accelerator. In this accelerator type, a short high-current electron bunch (generated by a conventional accelerator or a laser–wakefield acceleration stage) drives a strongly nonlinear plasma wake wave (blowout), and accelerated electrons are injected into it using a sharp density transition which leads to the elongation of the wake. The accelerating structure remains highly stable until the moment some electrons of the driver reach almost zero energy, which corresponds to the best interaction length for optimal driver-to-plasma energy transfer efficiency. For a particular driver, this efficiency can be optimised by choosing appropriate plasma density. Studying the dependence of the current of the injected bunch on driver and plasma parameters, we show that it does not depend on the density downramp length as long as the condition for trapping is satisfied. Most importantly, we find that the current of the injected bunch primarily depends on just one parameter which combines both the properties of the driver (its current and duration) and the plasma density.

High-intensity ultrashort laser pulses focused on metal targets readily generate hot dense plasmas which accelerate ions efficiently and can pave way to compact table-top accelerators. Laser-driven ion acceleration studies predominantly focus on protons, which experience the maximum acceleration owing to their highest charge-to-mass ratio. The possibility of tailoring such schemes for the preferential acceleration of a particular ion species is very much desired but has hardly been explored. Here, we present an experimental demonstration of how the nanostructuring of a copper target can be optimized for enhanced carbon ion acceleration over protons or Cu-ions. Specifically, a thin (≈0.25 μm) layer of 25–30 nm diameter Cu nanoparticles, sputter-deposited on a polished Cu-substrate, enhances the carbon ion energy by about 10-fold at a laser intensity of 1.2×10 18   W/cm 2 . However, particles smaller than 20 nm have an adverse effect on the ion acceleration. Particle-in-cell simulations provide definite pointers regarding the size of nanoparticles necessary for maximizing the ion acceleration. The inherent contrast of the laser pulse is found to play an important role in the species selective ion acceleration.