High Repetition Rate Research Articles

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.

Self-Shielding of X-ray Emission from Ultrafast Laser Processing Due to Geometrical Changes of the Interaction Zone.

Materials processing with ultrashort laser pulses is one of the most important approaches when it comes to machining with very high accuracy. High pulse repetition rates and high average laser power can be used to attain high productivity. By tightly focusing the laser beam, the irradiances on the workpiece can exceed 1013 W/cm2, and thus cause usually unwanted X-ray emission. Pulsed laser processing of micro holes exhibits two typical features: a gradual increase in the irradiated surface within the hole and, with this, a decrease in the local irradiance. This and the shielding by the surrounding material diminishes the amount of ionizing radiation emitted from the process; therefore, both effects lead to a reduction in the potential X-ray exposure of an operator or any nearby person. The present study was performed to quantify this self-shielding of the X-ray emission from laser-drilled micro holes. Percussion drilling in standard air atmosphere was investigated using a laser with a wavelength of 800 nm a pulse duration of 1 ps, a repetition rate of 1 kHz, and with irradiances of up to 1.1·1014 W/cm. The X-ray emission was measured by means of a spectrometer. In addition to the experimental results, we present a model to predict the expected X-ray emission at different angles to the surface. These calculations are based on raytracing simulations to obtain the local irradiance, from which the local X-ray emission inside the holes can be calculated. It was found that the X-ray exposure measured in the surroundings strongly depends on the geometry of the hole and the measuring direction, as predicted by the theoretical model.

Pilot Study to Demonstrate Improvement in Skin Tone and Texture by Treatment with a 1064 nm Q-Switched Neodymium-Doped Yttrium Aluminum Garnet Laser.

The 1064 nm Q-switched neodymium-doped yttrium aluminum garnet (QS Nd:YAG) laser was developed to treat unwanted pigmentation in the skin such as lentigines caused by photoaging, and tattoos from dye/ink insertion. This laser has also been used for non-ablative epidermal rejuvenation (skin toning). To evaluate changes in skin tone, skin texture and overall improvement after a series of treatments with the QS Nd:YAG laser. Participants received seven full-face treatments with M22 or Stellar M22, a 1064 nm QS Nd:YAG laser, at 2-week intervals. The investigators and participants evaluated the improvement in skin tone and texture at 1, 3 and 6 months after the last treatment. Patient satisfaction, patient discomfort, downtime and adverse events were recorded. Histological changes in the treated area were also evaluated. Thirteen women with a median age of 45 years (range, 34-61 years) were included in the study. The majority of the participants (53.9%) had skin type VI. One month after the last treatment session, 38% of participants reported good to very good improvement. This value increased to 100% participant improvement at both the 3-month and 6-month follow-up visits. The reduction in melanin index and the histological analysis demonstrated that the laser procedure contributed to a reduction in epidermal melanin content. Treatments were not associated with high levels of pain or discomfort. The most common immediate post-treatment response was erythema and edema. Most participants were satisfied with the resulting treatment outcome. Skin treatment with the 1064 nm QS Nd:YAG laser module on the M22 and Stellar M22 devices, using a large spot size, low fluence, moderately high repetition rate, improves skin tone and texture in patients with skin types II-VI.

QScatter: numerical framework for fast prediction of particle distributions in electron-laser scattering

The new generation of multi-PetaWatt laser facilities will allow tests of strong field quantum electrodynamics (QED), as well as provide an opportunity for novel photon and lepton sources. The first experiments are planned to study the (nearly) head-on scattering of intense, focused laser pulses with either relativistic electron beams or high-energy photon sources. In this work, we present a numerical framework that can provide fast predictions of the asymptotic particle and photon distributions after the scattering. The method detailed in this manuscript can include multiple features such as spatial and temporal misalignment between the laser and the scattering beam, broadband electron beams, and beam divergence. The expected mean energy, energy spread, divergence or other observables are calculated by combining an analytical description and numerical integration. This method can provide results within minutes on a personal computer, which would otherwise require full-scale 3D QED-PIC simulations using thousands of cores. The model, which has been compiled into an open-source code QScatter, may be used to support the analysis of large-size data sets from high-repetition rate experiments, leveraging its speed for optimization or reconstruction of experimental parameters.

High-power 100 W Kerr-lens mode-locked ring-cavity femtosecond Yb:YAG thin-disk oscillator.

High-power femtosecond pulses delivered at a high-repetition rate will aid machining throughput and improve signal-to-noise ratios for sensitive measurements. Here we demonstrate a Kerr-lens mode-locked femtosecond Yb:YAG ring-cavity thin-disk oscillator with a multi-pass scheme for the laser beam. With four passes through the thin disk, 175-fs pulses were delivered from the oscillator at an average power of 71.5 W and a repetition rate of 65.3 MHz. The corresponding intra-cavity peak power of 110 MW is ample for intra-cavity nonlinear conversion into more exotic wavelength ranges. With six passes, the average output power reached 101.3 W. To the best of our knowledge, this is the highest average output power of any mode-locked ring laser. These results confirm the viability of using multi-pass configuration on a thin-disk ring oscillator for high-throughput femtosecond applications.

Intracavity frequency doubling acousto-optic Q-switched high repetition rate high-energy Nd:YLF laser.

The ongoing advancement of Ti:sapphire femtosecond laser technology has drawn increasing attention to high repetition rate, high-energy green lasers as ideal pump sources for Ti:sapphire regenerative amplifiers. This study employed a neodymium-doped yttrium lithium fluoride (Nd:YLF) as the gain medium, supplemented with side-pumped laser diodes, acousto-optic Q-switching, and intracavity frequency doubling technologies. The results demonstrated a repetition rate ranging from 1-10kHz, a pulse width of less than 100ns, and a single pulse energy exceeding 50mJ at 527nm green light output. Furthermore, an operating stability (RMS) of ≤0.15% was maintained for 14h at a repetition rate of 1kHz and an output power of 40W.

Generation of 583 fs optical pulses at 10 GHz from a regeneratively mode-locked fiber laser combining nonlinear polarization evolution

A regeneratively mode-locked erbium fiber laser was numerically investigated and experimentally demonstrated, which was able to generate a 583 fs pulse train at 10 GHz via intracavity pulse compression with nonlinear polarization evolution (NPE). To excite the NPE at such a high repetition rate, a dispersion map was intentionally introduced to obtain short pulses accompanied by high peaks through soliton-like pulse shaping. Numerical simulations indicated that steady-state oscillation with pulses below 1 ps can be successfully established using this laser configuration. Experimentally, we obtained a pulse duration of 583 fs and a 3 dB spectral width of 4.5 nm at an average output power of 15.6 mW. Simultaneously, supermode suppression of more than 80 dB was also obtained from the appropriate biased NPE.

Cryogenic calibration for Hall probes

A superconducting undulator (SCU) prototype with a magnet length of 4m is being developed for the SHINE (Shanghai High Repetition Rate XFEL and Extreme Light Facility) project. The period length of the undulator is 16mm, the period number is 250, and the pole gap is 5mm. A point measurement system with three Hall probes will be used to measure the magnetic fields in the gap. The Hall probes fixed on a sledge will experience temperatures of about 4.2K to 50K when they move through the undulator gap. A temperature-dependent calibration system for these hall probes is necessary. The paper introduces the configuration of the cryogenic calibration system, and the result for the sensitivity and nonlinearity of one Hall probe from 2.8K to 300K over a magnetic field range of ±1.9T.

Fabrication and rejuvenation of high quantum efficiency caesium telluride photocathodes for high brightness and high average current photoinjectors

With their high quantum efficiency (QE) and excellent photoemissive properties, cesium telluride (Cs-Te) photocathodes are the current workhorse in the high average current electron accelerators around the globe. Their ability to generate high brightness and high charge electron beams has opened the doorway to numerous applications, including fundamental particle physics research, radiation therapy, high-energy physics experiments, and high repetition rate free-electron lasers (FELs). Their long-term performance is critical. In this work, we analyze the systematic production and rejuvenation of photocathodes via thermal evaporation and deposition techniques (both sequential and co-deposition), showing excellent values of QE exceeding 20% using deep ultraviolet illumination (λ=262–266 nm). To extend the photocathode lifetime, the rejuvenation process is performed via multilayer thin-film co-deposition, demonstrating the feasibility of reliably recovering the initial QE even after air exposure. We evaluated their final performance in both high gradient dc and rf gun setups and obtained consistent results of QE and lifetimes. The technique can be used at the gun level, which significantly extends the sustained accelerator operation without major interventions. These approaches significantly enhance the robustness and performance of Cs-Te photoinjectors and represent a significant advancement for reliable high average current electron accelerators and FELs. Published by the American Physical Society 2024

Tracing Photoinduced Hydrogen Migration in Alcohol Dications from Time-Resolved Molecular-Frame Photoelectron Angular Distributions.

The recent implementation of attosecond and few-femtosecond X-ray pump/X-ray probe schemes in large-scale free-electron laser facilities has opened the way to visualize fast nuclear dynamics in molecules with unprecedented temporal and spatial resolution. Here, we present the results of theoretical calculations showing how polarization-averaged molecular-frame photoelectron angular distributions (PA-MFPADs) can be used to visualize the dynamics of hydrogen migration in methanol, ethanol, propanol, and isopropyl alcohol dications generated by X-ray irradiation of the corresponding neutral species. We show that changes in the PA-MFPADs with the pump-probe delay as a result of intramolecular photoelectron diffraction carry information on the dynamics of hydrogen migration in real space. Although visualization of this dynamics is more straightforward in the smaller systems, methanol and ethanol, one can still recognize the signature of that motion in propanol and isopropyl alcohol and assign a tentative path to it. A possible pathway for a corresponding experiment requires an angularly resolved detection of photoelectrons in coincidence with molecular fragment ions used to define a molecular frame of reference. Such studies have become, in principle, possible since the first XFELs with sufficiently high repetition rates have emerged. To further support our findings, we provide experimental evidence of H migration in ethanol-OD from ion-ion coincidence measurements performed with synchrotron radiation.

Multiline molecular tagging velocimetry of nitric oxide at 100 kHz using an injection-seeded burst-mode OPO.

An injection-seeded, burst-mode optical parametric oscillator (OPO) operating at a repetition rate of 100 kHz is used to demonstrate the multiline molecular tagging velocimetry of an underexpanded jet using nitric oxide fluorescence. The very narrow linewidth of the OPO system, along with the relatively high pulse energies of the burst-mode system, enables efficient single-photon excitation of nitric oxide along multiple laser beam lines at a high repetition rate. Simultaneous one-dimensional velocity profile measurements were obtained of an underexpanded jet system at six different locations using a reference initial image and single-shot delayed images. A methodology for calculating the uncertainty of single-shot velocity is also described. Mean and root-mean-square velocity profiles are obtained at multiple locations simultaneously over a sampling time of 1 ms. The high-repetition-rate velocity measurements also appear to capture the onset of velocity oscillations and has the potential to reveal velocity frequency content occurring in the tens of kHz. The demonstrated velocimetry technique could be paired with other emerging burst-mode laser capabilities for a quantitative multiparameter gas property or multicomponent gas velocity measurements for supersonic and hypersonic flows, especially within ground test facilities that are limited to very short run durations.

Parallel Diffractive Multi‐Beam Pulsed‐Laser Ablation in Liquids Toward Cost‐Effective Gram Per Hour Nanoparticle Productivity

Nanoparticles (NPs) generated by pulsed‐laser ablation in liquids (PLAL) have benefited many key applications due to their versatility, enlarged surface area, and high purity. However, scaling up NPs production represents one of the main requisites to commercialize this technology. The established upscaling strategy demands high power and repetition rate laser source with fast scanning systems, which are not widely available and costly. Herein, a cost‐effective alternative is proposed, the addition of static diffractive optical elements to achieve parallel processing through the multi‐beam PLAL (MB‐PLAL). In MB‐PLAL, the optimum repetition rate is reduced to compensate laser energy splitting, hence achieving a higher interpulse distance, reducing pulse shielding, and increasing NPs productivity. MB‐PLAL with 11 beams reached a factor 4 productivity increase for iron–nickel alloy (Fe50Ni50) NPs compared to the single‐beam setup (0.4–1.6 g h−1), and a factor 3 increase for gold (Au) NPs (0.32–0.94 g h−1). The scalability of the proposed MB‐PLAL technique setup is confirmed by Au and Fe50Ni50 NPs productivity experiments using 1, 6, and 11 beams, showing a linear increase in productivity.

Stable, intense supercontinuum light generation at 1 kHz by electric field assisted femtosecond laser filamentation in air

Supercontinuum (SC) light source has advanced ultrafast laser spectroscopy in condensed matter science, biology, physics, and chemistry. Compared to the frequently used photonic crystal fibers and bulk materials, femtosecond laser filamentation in gases is damage-immune for supercontinuum generation. A bottleneck problem is the strong jitters from filament induced self-heating at kHz repetition rate level. We demonstrated stable kHz supercontinuum generation directly in air with multiple mJ level pulse energy. This was achieved by applying an external DC electric field to the air plasma filament. Beam pointing jitters of the 1 kHz air filament induced SC light were reduced by more than 2 fold. The stabilized high repetition rate laser filament offers the opportunity for stable intense SC generation and its applications in air.

Investigation of laser plasma instabilities driven by 527 nm laser pulses relevant for direct drive inertial confinement fusion

We report on a study of laser plasma instabilities with 527 nm laser pulses in an intensity range of 0.5×1013−1.1×1015 Wcm−2 and plasma parameters entering a regime that is relevant for direct drive inertial confinement fusion. Using the kilojoule high repetition rate L4n laser at the Extreme Light Infrastructure—Beamlines, more than 1300 shots were collected, and the onset and the growth of stimulated Brioullin scattering (SBS) and stimulated Raman scattering (SRS) were studied with a high confidence level. The measured onset intensities are 0.2×1014 Wcm−2 for SBS and 1.4×1014 Wcm−2 for SRS. At the maximum intensity, the total fraction of backscattered energy reaches 2.5% for SBS and 0.1% for SRS. These results are of high relevance for advanced concepts for inertial fusion energy, which rely on the use of 527 nm laser light to drive the implosion of the fuel target, and in particular, they can be used as a benchmark for advanced simulations.

Operation of 2 K cryogenic system with cold compressors for the SHINE test facility

The Shanghai high repetition rate X-ray free electron laser (XFEL) and extreme light facility (SHINE), a quasi-continuous wave hard XFEL facility, is currently under construction. In order to test all the superconducting components, assembled cryomodules and undulators, the SHINE test facility that consists of various test benches has been built. A superfluid cryoplant with 1 kW@2K was installed, commissioned, acceptance tested and turned into operation to support the cryogenic tests. The plant consists not only of a standard compressor system and cold box but also of a process vacuum system for 2 K operation. In the 2 K cryogenic system, three-stage cold compressors are firstly used to compress helium vapor at a mass flow of up to 50 g/s from 28 mbar to about 400 mbar. Afterwards, the sub-atmospheric helium is continually pressurize to the suction pressure of the warm compressors by five parallel process vacuum pumps. During the first eight months of 2 K operations, the tuning of the cold compressor regulation has been implemented to adapt the different static and dynamic heat loads. A helium pressure stability of superconducting cavities better than ±0.15 mbar at 31 mbar was achieved inside the vertical test bench with additional heating power of 160 W. In this paper, the pump down procedure and the operation results of cryogenic system at 2 K will be described.

Gain measurements of longitudinally excited CO2 laser using low-pressure gas without helium

The gain characteristics of a CO2 laser pumped by a longitudinal pulsed discharge were investigated to develop a compact He-free CO2 laser operating at a high repetition rate. The longitudinal discharge tube had an inner diameter of 0.8 cm and a length of 45 cm. High-voltage pulses with a discharge starting voltage of 13.4–30.2 kV at a rise time of about 321 ns were applied to the discharge tube. CO2/N2 mixture gas with mixing ratios of 1:0, 1:1, 1:2 and 1:3 was used. The small signal gain coefficient was measured by inputting a seed CO2 laser pulse into the longitudinal discharge tube. The small signal gain coefficient depended on the delay time between the start of the discharge in the longitudinal discharge tube and the input of a seed CO2 laser pulse, the gas mixing ratio, the gas pressure, the repetition rate and the input energy to the longitudinal discharge tube. In this work, in He-free gas, the maximum small signal gain coefficient at a repetition rate of 300 Hz was 2.11%/cm with a delay time of 30.4 μs, a CO2/N2 gas mixing ratio of 1:2, a gas pressure of 2.6 kPa and an input energy of 191 mJ.

Compact, ultrastable, high repetition-rate 2 μm and 3 μm fiber laser for seeding mid-IR OPCPA.

We report a compact and reliable ultrafast fiber laser system optimized for seeding a high energy, 2 μm pumped, 3 μm wavelength optical parametric chirped pulse amplification to drive soft X-ray high harmonics. The system delivers 100 MHz narrowband 2 μm pulses with >1 nJ energy, synchronized with ultra-broadband optical pulses with a ∼1 μm FWHM spectrum centered at 3 μm with 39 pJ pulse energy. The 2 μm and 3 μm pulses are derived from a single 1.5 μm fiber oscillator, fully fiber integrated with free-space downconversion for the 3 μm. The system operates hands-off with power instabilities <0.2% over extended periods of time.

Generation of meter-scale hydrogen plasmas and efficient, pump-depletion-limited wakefield excitation using 10 GeV electron bunches

High repetition rates and efficient energy transfer to the accelerating beam are important for a future linear collider based on the beam-driven plasma wakefield acceleration scheme (PWFA-LC). This paper reports the first results from the Plasma Wakefield Acceleration Collaboration (E300) that are beginning to address both of these issues using the recently commissioned FACET-II facility at SLAC national accelerator laboratory. We have generated meter-scale hydrogen plasmas using time-structured 10 GeV electron bunches from FACET-II, which hold the promise of dramatically increasing the repetition rate of PWFA by rapidly replenishing the gas between each shot compared to the hitherto used lithium plasmas that operate at 1–10 Hz. Furthermore, we have excited wakes in such plasmas that are suitable for high gradient particle acceleration with high drive-bunch to wake energy transfer efficiency- a first step in achieving a high overall energy transfer efficiency. We have done this by using time-structured electron drive bunches that typically have one or more ultra-high current ( > 30 kA) femtosecond spike(s) superimposed on a longer (∼0.4 ps) lower current ( < 10 kA) bunch structure. The first spike effectively field-ionizes the gas and produces a meter-scale (30–160 cm) plasma, whereas the subsequent beam charge creates a wake. The length and amplitude of the wake depends on the longitudinal current profile of the bunch and plasma density. We find that the onset of pump depletion, when some of the drive beam electrons are nearly fully depleted of their energy, occurs for hydrogen pressure ⩾ 1.5 Torr. We also show that some electrons in the rear of the bunch can gain several GeV energies from the wake. These results are reproduced by particle-in-cell simulations using the QPAD code. At a pressure of ∼2 Torr, simulation results and experimental data show that the beam transfers about 60% of its energy to the wake.

Chemically Accurate Potential Curves for H2 Molecules Using Explicitly Correlated Qubit-ADAPT.

With the recent advances in the development of devices capable of performing quantum computations, a growing interest in finding near-term applications has emerged in many areas of science. In the era of nonfault tolerant quantum devices, algorithms that only require comparably short circuits accompanied by high repetition rates are considered to be a promising approach for assisting classical machines with finding a solution on computationally hard problems. The ADAPT approach previously introduced in Nat. Commun. 10, 3007 (2019) extends the class of variational quantum eigensolver algorithms with dynamically growing ansätze in order to find approximations to the ground and excited state energies of molecules. In this work, the ADAPT algorithm has been combined with a first-quantized formulation for the hydrogen molecule in the Born-Oppenheimer approximation, employing the explicitly correlated basis functions introduced in J. Chem. Phys. 43, 2429 (1965). By the virtue of their explicit electronic correlation properties, it is shown in classically performed simulations that chemical accuracy (<1.6 mHa) can be reached for ground and excited state potential curves using reasonably short circuits.

A Platform for Laser-Driven Ion Sources Generated with Nanosecond Laser Pulses in the Intensity Range of 1013–1015 W/cm2

An experimental platform for laser-driven ion (sub-MeV) acceleration and potential applications was commissioned at the HiLASE laser facility. The auxiliary beam of the Bivoj laser system operating at a GW level peak power (~10 J in 5–10 ns) and 1–10 Hz repetition rate enabled a stable production of high-current ion beams of multiple species (Al, Ti, Fe, Si, Cu, and Sn). The produced laser–plasma ion sources were fully characterized against the laser intensity on the target (1013–1015 W/cm2) by varying the laser energy, focal spot size, and pulse duration. The versatility and tuneability of such high-repetition-rate laser–plasma ion sources are of potential interest for user applications. Such a statistically accurate study was facilitated by the large amount of data acquired at the high repetition rate (1–10 Hz) provided by the Bivoj laser system.