Laser plasma accelerators (LPAs) are a promising platform for compact radiation sources. For a wide range of applications, including radiotherapy, ultrafast electron diffraction and time-resolved imaging, stable operation at high repetition rates is essential in order to deliver competitive average particle flux. Here we demonstrate the first LPA driven by an industrial-grade ytterbium-doped yttrium aluminium garnet (Yb:YAG) laser, designed for high-average-power operation. The picosecond laser pulses are post-compressed in a multi-pass cell to 50 fs duration and used to drive the interaction. The electron accelerator is operated in burst mode, at repetition rates tuneable from 0.625 to 6.25 kHz, representing a substantial increase compared to the state-of-the-art. Across this range, the electron beam properties remain unchanged, with average charges of 10-12 pC per shot, divergences of 50-70 mrad, and Maxwellian-like spectra extending to a few MeV. Numerical simulations capture the key features of the experimental observations and indicate acceleration in the self-modulated regime, enabled by relativistic self-focusing in near-critical-density plasma. Combining industrial high-average-power laser technology with plasma-based acceleration, these results represent a key step toward scalable, compact high-repetition-rate electron sources for medical, imaging and industrial applications.

Energy-transfer efficiency is an important quantity in plasma-wakefield acceleration, especially for applications that demand high average power. Conventionally, the efficiency is measured using an electron spectrometer, an invasive method that provides an energy-transfer efficiency averaged over the full length of the plasma accelerator. Here, we experimentally demonstrate the use of the excess light emitted by the plasma after a beam-plasma interaction in a method that yields noninvasive, longitudinally resolved measurements of the local energy-transfer efficiency from the wake to the accelerated bunch. As we show, by diagnosing the acceleration process directly at the plasma location, this method can provide more insight into effects such as trailing bunch charge loss, which typically impose systematic errors on downstream spectrometer measurements. In the future, this method would also be suitable for the online optimization of individual stages in multistage plasma accelerators.

We report on a direct search for elastic photon-photon scattering using x-ray and γ photons from a laser-plasma based experiment. A γ photon beam produced by a laser wakefield accelerator provided a broadband γ spectrum extending to above Image 1. These were collided with a dense x-ray field produced by the emission from a laser heated germanium foil at Image 2, corresponding to an invariant mass of Image 3. In these asymmetric collisions elastic scattering removes one x-ray and one high-energy γ photon and outputs two lower energy γ photons. No changes in the γ photon spectrum were observed as a result of the collisions allowing us to place a 95% upper bound on the cross section of Image 4. Although far from the QED prediction, this represents the lowest upper limit obtained so far for Image 5.

This document expands on the Comprehensive Summary submitted to the EPPSU 2026. It contains details on aspects of the HALHF project that could not be fitted into the Summary. Some sections contain work that is still preliminary and/or status reports on current progress.

Polarized beams are indispensable for many particle, atomic, and nuclear physics experiments where spin-dependent processes are to be studied. Currently, the primary sources of polarized electron beams, such as storage rings and polarized photo-cathodes, depend on radio-frequency (RF) technology, particularly for subsequent acceleration. As a result, they are characterized by their substantial size and limited availability. Unlike RF accelerators, the accelerating fields in Laser-Plasma-Accelerators (LPA) are not limited by material breakdown. LPAs can create beams of tens to hundreds of MeV in only a millimetre, which makes them a promising alternative to conventional accelerators. Compact polarized LPAs could help make polarized sources ubiquitous to facilitate many more polarized physics experiments. The LEAP (Laser Electron Acceleration with Polarization) project at DESY aims to generate and measure spin-polarized electron beams from a compact LPA for the first time. Spin-polarized electron beams can be generated from an LPA using a pre-polarized plasma source, where a circularly polarized UV laser pulse dissociates hydrogen halide molecules. Due to the expected beam energy of tens of MeVs, photon transmission polarimetry will be used for the subsequent polarization measurement in LEAP.

>Combining high peak power and high average power has long been a key challenge of ultrafast laser technology,crucial for applications such as laser-plasma acceleration and strong-field physics. A promising solution lies in post-compressed ytterbium lasers, but scaling these to high pulse energies presents a major bottleneck. Post-compression techniques, particularly Herriott-type multi-pass cells (MPCs), have enabled large peak power boosts at high average powers but their pulse energy acceptance reaches practical limits defined by setup size and coating damage threshold. In this work, we address this challenge and demonstrate, to our knowledge, a novel type of compact, energy-scalable MPC (CMPC). By employing a novel MPC configuration and folding the beam path,the CMPC introduces a new degree of freedom for downsizing the setup length, enabling compact setups even for large pulse energies. We experimentally and numerically verify the CMPC approach, demonstrating post-compression of 8 m J pulses from 1 ps down to 51 fs in atmospheric air using a cell roughly 45 cm in length at low fluence values. Additionally, we discuss the potential for energy scaling up to 200 m J with a setup size reaching2.5 m. Our work presents a new approach to high-energy post-compression, with up-scaling potential far beyond the demonstrated parameters. This opens new routes for achieving the high peak and average powers necessary for demanding applications of ultrafast lasers.

HALHF is a hybrid linear collider that uses electron-driven plasma-wakefield acceleration to accelerate electrons to high energy while using radio-frequency cavity technology to accelerate positrons. The most cost-effective solution collides low-energy positrons with high-energy electrons, producing a boost to the final state in the electron direction with γ= 1.67. The current HALHF baseline design produces a luminosity comparable to that of the baseline ILC but with a greatly reduced construction and carbon footprint and hence much lower cost than the mature linear-collider designs ILC and CLIC. Costs for HALHF are evaluated, together with that for the approximate 15-year R&D programme necessary to realise HALHF. Time scales and cost for the R&D are estimated. Upgrade paths for HALHF technology from a 250 GeV Higgs factory, through 380 and 550 GeV, up to 10 TeV are sketched.

The charge contained in an electron bunch is one of the most important parameters in accelerator physics. Several techniques to measure the electron bunch charge exist. However, many conventional charge diagnostics face serious drawbacks when applied to plasma accelerators. For example, integrating current transformers (ICTs or toroids) have been shown to be sensitive to the electromagnetic pulses (EMP) originating from the plasma, whereas scintillating screens are sensitive to background radiation such as betatron radiation or bremsstrahlung and only allow for a destructive measurement of the bunch charge. We show measurements with a noninvasive, cavity-based charge diagnostic (the DaMon), which demonstrate its high sensitivity, high dynamic range and resistance towards EMP. The measurements are compared to both an ICT and an absolutely calibrated scintillating screen.

We propose a novel scheme for controlling the injection of a high-quality electron bunch into a channel-guided laser plasma accelerator. This all-optical technique, constricted waveguide injection, creates a highly tunable controlled injection structure natively within a plasma waveguide, a key requirement for efficient acceleration of high-quality multi-GeV electron beams. We describe a simple optical setup to tailor the plasma and present start-to-end simulations showing the injection structure formation and the generation of a 1.1 GeV electron beam with 10 pC of charge and 0.35 laser energy. Highly tunable tailored plasma sources, like those proposed here, enable fine control over the injection and acceleration processes and thus will be crucial for the development of application-focused laser plasma accelerators.

Experiments were performed on laser wakefield acceleration in the highly nonlinear regime. With laser powers P<250 TW and using an initial spot size larger than the matched spot size for guiding, we were able to accelerate electrons to energies Emax>2.5 GeV, in fields exceeding 500 GV m−1, with more than 80 pC of charge at energies E>1 GeV. Three-dimensional particle-in-cell simulations show that using an oversized spot delays injection, avoiding beam loss as the wakefield undergoes length oscillation. This enables injected electrons to remain in the regions of highest accelerating fields and leads to a doubling of energy gain as compared to results from using half the focal length with the same laser. Published by the American Physical Society 2024

Polarised particle beams are indispensable for the study of spin-dependent processes . The LEAP (Laser Electron Acceleration with Polarisation) project at DESY aims to demonstrate the acceleration of polarised electrons in the extremely high fields enabled by laser plasma accelerators to create high energy electron beams in ultra-compact footprint. In this proof of principle experiment, spin-polarised electron beams with energies of tens of MeV will be generated in a sub-millimetre long plasma source. For electron beams of such energies, Compton transmission polarimetry is the ideal method to measure the polarisation. Gamma rays produced by bremsstrahlung are transmitted through an iron absorber core magnetised by a surrounding solenoid, with rate and energy spectrum depending on the relative orientation of the gamma spin and the magnetisation direction of the iron. The transmission asymmetry with respect to the direction of the magnetisation is proportional to the initial electron polarisation. In this contribution, an overview of the LEAP project is presented, detailing the setup of the polarimeter as well as its implementation and commissioning status.

Multi-pass cells (MPCs) enable the accumulation of a large amount of B-integral while preserving the spatial mode of a laser beam and ensuring a uniform spectral distribution [1], [2]. In addition, MPCs used for post-compression allow achieving very large compression factors, without considerable degradation in temporal quality [3]. These qualities of MPCs, in combination with the high average power and pulse energy that Ytterbium-based lasers can deliver, open up the generation of ultrashort laser pulses with unprecedented properties [4].

Advancing ultrafast high-repetition-rate lasers to shortest pulse durations comprising only a few optical cycles while pushing their energy into the multi-millijoule regime opens a route toward terawatt-class peak powers at unprecedented average power. We explore this route via efficient post-compression of high-energy 1.2 ps pulses from an ytterbium InnoSlab laser to 9.6 fs duration using gas-filled multi-pass cells (MPCs) at a repetition rate of 1 kHz. Employing dual-stage compression with a second MPC stage supporting a close-to-octave-spanning bandwidth enabled by dispersion-matched dielectric mirrors, a record compression factor of 125 is reached at 70% overall efficiency, delivering 6.7 mJ pulses with a peak power of ∼0.3 TW. Moreover, we show that post-compression can improve the temporal contrast at multi-picosecond delay by at least one order of magnitude. Our results demonstrate efficient conversion of multi-millijoule picosecond lasers to high-peak-power few-cycle sources, prospectively opening up new parameter regimes for laser plasma physics, high energy physics, biomedicine, and attosecond science.

We demonstrate efficient post compression of 9.45 mJ, 1.2 ps pulses to 13 fs at 1 kHz repetition rate using a two stage gas filled multi pass cell system reaching near terawatt peak power.

In this work we report experimental compression of 9.4 mJ, 9.4 W, and 1.2 picosecond long pulses down to 7 mJ ,13 fs at 1 kHz in two cascaded stages with gas filled multi-pass cells (MPCs). The output from the first, argon filled MPC is compressed to <50 fs. The output pulses of the subsequent second MPC stage filled with Neon is then compressed to 13 fs using a chirped mirror compressor. The overall transmission, and compression ratio of both stages exceed 74 % and 92 times respectively.

Laser-plasma accelerators are capable of sustaining accelerating fields of 10–100 GeV/m, 100–1000 times that of conventional technology and the highest fields produced by any of the widely researched advanced accelerator concepts. Laser-plasma accelerators also intrinsically accelerate short particle bunches, several orders of magnitude shorter than that of conventional technology, which leads to reductions in beamstrahlung and, hence, savings in the overall power consumption to reach a desired luminosity. These properties make laser-plasma accelerators a promising accelerator technology for a more compact, less expensive high-energy linear collider providing multi-TeV polarized leptons. In this submission to the Snowmass 2021 Accelerator Frontier, we discuss the motivation for a laser-plasma-accelerator-based linear collider, the status of the field, and potential linear collider concepts up to 15 TeV. We outline the research and development path toward a collider based on laser-plasma accelerator technology, and highlight near-term and mid-term applications of this technology on the collider development path. The required experimental facilities to carry out this research are described. We conclude with community recommendations developed during Snowmass.

Highly polarized, multi-kiloampere-current electron bunches from compact laser-plasma accelerators are desired for numerous applications. Current proposals to produce these beams suffer from intrinsic limitations to the reproducibility, charge, beam shape and final polarization degree. In this Letter, we propose colliding pulse injection as a technique for the generation of highly polarized electron bunches from pre-polarized plasma sources. Using particle-in-cell simulations, we show that colliding pulse injection enables trapping and precise control over electron spin evolution, resulting in the generation of high-current (multi-kA) electron bunches with high degrees of polarization (up to 95 % for > 2 kA). Bayesian optimization is employed to optimize the multidimensional parameter space associated with colliding pulse injection to obtain percent-level energy spread, sub-micron normalized emittance electron bunches with 90 % polarization using 100-TW class laser systems.

Over the last decades, significant advances in high-power laser systems have enabled rapid progress in the development of laser-driven plasma accelerators. Today, the results obtained in beam stability and reproducibility present laser plasma acceleration as a viable and promising alternative to conventional accelerators. As several electron beam and secondary sources applications require high average currents, a major focus is now on increasing the beam's repetition rate. In the following, we introduce a novel plasma source for kHz electron acceleration, providing a continuous and spatially confined gas flow, while minimising the gas load in the acceleration chamber.

In recent years, Laser Plasma Acceleration (LPA) has become a promising alternative to conventional RF accelerators. However, so far, it has only been theoretically shown that generating polarized LPA beams is possible. The LEAP (Laser Electron Acceleration with Polarization) project at DESY aims to demonstrate this experimentally for the first time, using a pre-polarized plasma target. The electron polarization will be measured with photon transmission polarimetry, which makes use of the production of circularly polarized bremsstrahlung during the passage of the electron beams through a suitable converter target. The photon polarization is then measured with the aid of transmission asymmetry arising from reversing the magnetization direction of an iron absorber. In this contribution an overview of the LEAP project is presented, detailing the generation of the polarized electron beams along with the design and simulation studies of the polarimeter.

Readily available bright X-ray beams with narrow bandwidth and tunable energy promise to unlock novel developments in a wide range of applications. Among emerging alternatives to large-scale and costly present-day radiation sources which severely restrict the availability of such beams, compact laser-plasma-accelerator-driven inverse Compton scattering sources show great potential. However, these sources are currently limited to tens of percent bandwidths, unacceptably large for many applications. Here, we show conceptually that using active plasma lenses to tailor the electron bunch-photon interaction, tunable X-ray and gamma beams with percent-level bandwidths can be produced. The central X-ray energy is tunable by varying the focusing strength of the lens, without changing electron bunch properties, allowing for precision-tuning the X-ray beam energy. This method is a key development towards laser-plasma-accelerator-driven narrowband, precision tunable femtosecond photon sources, enabling a paradigm shift and proliferation of compact X-ray applications.