We regret overlooking two important citations relevant to the current work, and wish to add these [1, 2]. We also cite [3], which reports crucial experimental parameters pertaining to [1], e.g. chamber pressure during water target experiment. To correct the oversight of missing references, the 4th paragraph of the introduction follows with modified and additional text underlined:Liquid targets have a number of attractive features for meeting these needs. Liquid targets can be rapidly delivered into the interaction region, and mitigate debris [31,[34][35][36]. This is well illustrated by the pioneering research in [1, 3], who, for the first time combined a kHz, femtosecond laser and liquid jet targets with a long-term vision of developing integrated sources of energetic radiation and particles for future applications. They reported the production of 9.25 keV x-rays from the interaction of a kHz, 50 fs pulsed laser interacting with a liquid Ga jet target. They also reported the use of CR39 film to record the production of 500 keV protons from the interaction of the kHz laser with an intensity of 3×10 16 W cm −2 focused on a 10-30μm diameter water jet, with a background chamber pressure of 0.7-3 mbar. The proton production efficiency of 10 −5 % was reported. Prior to switching to the liquid sheet target described in our current work, we attempted to obtain protons from the interaction of 15-30μm diameter water jets with a 40 fs pulsed laser focused to an intensity of 1×10 18 W cm −2 . We recorded many tracks on the CR39 film but, when a magnetic spectrometer was used, all of the tracks were shown to be due to electrons. As noted in this paper, we later discovered that the chamber background pressure required to produce a significant flux of protons was below the freeze point pressure of water.Skip to the end of the last sentence of the paragraph and add as the last sentence of the paragraph: the ability to generate a well collimated proton beam with proton energies greater than 500 keV has recently been demonstrated [2], using a high repetition rate 0.5 kHz, 3 mJ, 55 fs laser interacting with a solid target. The focus intensity was 2×10 18 W cm −2 . A proton beam was generated at the front surface of a rotating optical quality glass disk at a chamber pressure of 3×10 −3 mbar. AbstractLaser acceleration of ions to MeV energies has been achieved on a variety of Petawatt laser systems, raising the prospect of ion beam applications using compact ultra-intense laser technology. However, translation from proof-of-concept laser experiment into real-world application requires MeV-scale ion energies and an appreciable repetition rate (>Hz). We demonstrate, for the first time, proton acceleration up to 2 MeV energies at a kHz repetition rate using a milli-joule-class short-pulse laser system. In these experiments, 5 mJ of ultrashort-pulse laser energy is delivered at an intensity neaŕ -5 10 W cm 18 2 onto a thin-sheet, liquid-density target. Key to this effort is a flowing liquid ethylene glycol target formed i...

Ultra-intense laser-matter interaction experiments (>10(18) W/cm(2)) with dense targets are highly sensitive to the effect of laser "noise" (in the form of pre-pulses) preceding the main ultra-intense pulse. These system-dependent pre-pulses in the nanosecond and/or picosecond regimes are often intense enough to modify the target significantly by ionizing and forming a plasma layer in front of the target before the arrival of the main pulse. Time resolved interferometry offers a robust way to characterize the expanding plasma during this period. We have developed a novel pump-probe interferometry system for an ultra-intense laser experiment that uses two short-pulse amplifiers synchronized by one ultra-fast seed oscillator to achieve 40-fs time resolution over hundreds of nanoseconds, using a variable delay line and other techniques. The first of these amplifiers acts as the pump and delivers maximal energy to the interaction region. The second amplifier is frequency shifted and then frequency doubled to generate the femtosecond probe pulse. After passing through the laser-target interaction region, the probe pulse is split and recombined in a laterally sheared Michelson interferometer. Importantly, the frequency shift in the probe allows strong plasma self-emission at the second harmonic of the pump to be filtered out, allowing plasma expansion near the critical surface and elsewhere to be clearly visible in the interferograms. To aid in the reconstruction of phase dependent imagery from fringe shifts, three separate 120° phase-shifted (temporally sheared) interferograms are acquired for each probe delay. Three-phase reconstructions of the electron densities are then inferred by Abel inversion. This interferometric system delivers precise measurements of pre-plasma expansion that can identify the condition of the target at the moment that the ultra-intense pulse arrives. Such measurements are indispensable for correlating laser pre-pulse measurements with instantaneous plasma profiles and for enabling realistic Particle-in-Cell simulations of the ultra-intense laser-matter interaction.

We present an experimental study of the generation of ∼MeV electrons opposite to the direction of laser propagation following the relativistic interaction at normal incidence of a ∼3 mJ, 10 18 W/cm 2 short pulse laser with a flowing 30 μm diameter water column target. Faraday cup measurements record hundreds of pC charge accelerated to energies exceeding 120 keV, and energy-resolved measurements of secondary x-ray emissions reveal an x-ray spectrum peaking above 800 keV, which is significantly higher energy than previous studies with similar experimental conditions and more than five times the ∼110 keV ponderomotive energy scale for the laser. We show that the energetic x-rays generated in the experiment result from backwardgoing, high-energy electrons interacting with the focusing optic and vacuum chamber walls with only a small component of x-ray emission emerging from the target itself. We also demonstrate that the high energy radiation can be suppressed through the attenuation of the nanosecond-scale pre-pulse. These results are supported by 2D Particle-in-Cell (PIC) simulations of the laser-plasma interaction that exhibit beam-like backward-propagating MeV electrons.

High intensity laser-plasma interactions produce a wide array of energetic particles and beams with promising applications. Unfortunately, high repetition rate and high average power requirements for many applications are not satis ed by the lasers, optics, targets, and diagnostics currently employed. Here, we address the need for high repetition rate targets and optics through the use of liquids. A novel nozzle assembly is used to generate high-velocity, laminarowing liquid microjets which are compatible with a low-vacuum environment, generate li le to no debris, and exhibit precise positional and dimensional tolerances. Jets, droplets, submicron thick sheets, and other exotic con gurations are characterized with pump-probe shadowgraphy to evaluate their use as targets. To demonstrate a high repetition rate, consumable, liquid optical element, we present a plasma mirror created by a submicron thick liquid sheet. is plasma mirror provides etalon-like anti-re ection properties in the low-eld of 0.1% and high re ectivity as a plasma, 69%, at a repetition rate of 1 kHz. Practical considerations of uid compatibility, in-vacuum operation, and estimates of maximum repetition rate in excess of 10 kHz are addressed. e targets and optics presented here enable the use of relativistically intense lasers at high average power and make possible many long proposed applications. . All diode-pumped, highrepetition-rate advanced petawa laser system (hapls).

We report observation of kHz-pulsed-laser-accelerated electron energies up to 3 MeV in the −k laser (backward) direction from a 3 mJ laser interacting at normal incidence with a solid density, flowing-liquid target. The electrons/MeV/s.r. >1 MeV recorded here using a mJ-class laser exceeds or equals that of prior super-ponderomotive electron studies employing lasers at lower repetition-rates and oblique incidence. Focal intensity of the 40-fs-duration laser is 1.5 · 10 18 W cm −2 , corresponding to only ∼80 keV electron ponderomotive energy. Varying laser intensity confirms electron energies in the laser-reflection direction well above what might be expected from ponderomotive scaling in normal-incidence laser-target geometry. This direct, normal-incidence energy spectrum measurement is made possible by modifying the final focusing off-axis-paraboloid (OAP) mirror with a central hole that allows electrons to pass, and restoring laser intensity through adaptive optics. A Lanex-based, optics-free high-acquisition rate (>100 Hz) magnetic electron-spectrometer was developed for this study to enable shot-to-shot statistical analysis and real-time feedback, which was leveraged in finding optimal pre-plasma conditions. 3DParticle-in-cell simulations of the interaction show qualitative super-ponderomotive spectral agreement with experiment. The demonstration of a high-repetition-rate, high-flux source containing >MeV electrons from a few-mJ, 40 fs laser and a simple liquid target encourages development of future ≥kHz-repetition, fs-duration electron-beam applications.