Controlling the intensity distribution of laser pulses in the focal region is essential for optimizing optically generated plasma waveguides and enabling advanced plasma acceleration techniques, including dephasingless Wakefield acceleration. Here, we present a method for programmatic structuring of the high-intensity focal region of a standard off-axis parabolic mirror, extending the length of this region well beyond the Rayleigh length and enabling control over the longitudinal intensity distribution. The theoretical framework is validated through numerical simulations and experimental measurements. Further, we demonstrate the use of this technique in an existing plasma accelerator system using readily available hardware components. Finally, we illustrate the potential application of this method to multi-GeV laser-plasma acceleration and the generation of flying foci, research areas that would significantly benefit from improved programmatic structuring of high-intensity laser pulses.

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.

We present the design and progress towards a 200mJ pulse energy, 5kHz rep-rate, 1kW average power, 40fs multidimensionally combined fiber laser for laser-plasma acceleration and broad applications, including overcoming challenges due to broad bandwidth.

Single-mode, record high 7.7mJ stored energy extraction from a monolithically integrated fiber amplifier is demonstrated using 85µm-core chirally-coupled-core fiber and counter pumped tapered end cap technologies, enabling integration of high-energy pulsed fiber laser arrays.

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.

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.

Broadband optical pulses with attosecond to femtosecond durations provide unique opportunities for studies of time-resolved electron dynamics. However, focusing these pulses-typically ranging from the vacuum ultraviolet to the soft-X-ray region-remains challenging. Conventional refractive lenses are not suitable owing to large dispersion and strong absorption, whereas reflective optics do not suffer from these issues but have high losses. Here we demonstrate a tunable hydrogen plasma lens to focus broadband extreme-ultraviolet attosecond pulses with energies of around 20 eV and 80 eV. Simulation results suggest that the stretching of attosecond pulses is negligible, and temporal compression is possible when atto-chirp is included. A key advantage of the plasma lens is its compatibility with nonlinear frequency conversion processes like high-harmonic generation. The different focusing properties of the fundamental and harmonic frequencies allow for an efficient separation of these components. Consequently, the transmission of high-harmonic generation beamlines can be increased to more than 80% and this approach can be suitable for applications requiring high photon flux.

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.

This document outlines a community-driven Design Study for a 10 TeV pCM Wakefield Accelerator Collider. The 2020 ESPP Report emphasized the need for Advanced Accelerator R&D, and the 2023 P5 Report calls for the “delivery of an end-to-end design concept, including cost scales, with self-consistent parameters throughout." This Design Study leverages recent experimental and theoretical progress resulting from a global R&D program in order to deliver a unified, 10 TeV Wakefield Collider concept. Wakefield Accelerators provide ultra-high accelerating gradients which enables an upgrade path that will extend the reach of Linear Colliders beyond the electroweak scale. Here, we describe the organization of the Design Study including timeline and deliverables, and we detail the requirements and challenges on the path to a 10 TeV Wakefield Collider.

Preserving the quality of positron beams in plasma-based accelerators, where wakefields are generated in electron filaments, is challenging. These wakefields are characterized by transversely non-linear focusing fields and non-uniform accelerating fields. However, a nonzero plasma temperature linearizes the transverse wakefield within the central region of the electron filament. In this study, we employ 3D particle-in-cell simulations with mesh refinement to demonstrate that beams with emittances on the order of tens of nanometers are contained within the linearized region of the transverse wakefield. This enables emittance preservation to one percent, while positron beams with the same charge and micrometer emittances, which sample the non-linear part of the transverse wakefield, experience a relative emittance growth of ten percent. Additionally, we observe a significant reduction in the growth rate of the slice energy spread for the tens of nanometers emittance beams in comparison to the micrometer emittance beams. The utilization of warm plasmas in conjunction with low-emittance beams opens up new avenues for enhancing the beam quality across various plasma-based positron acceleration approaches.

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.

Plasma-based accelerators are a promising approach for reducing the size and cost of future particle accelerators, making them a viable technology for constructing and upgrading X-ray free-electron lasers (FELs). Adding an energy booster stage to the linear accelerator of an operational X-ray FEL is recognised as a realistic near-term application of plasma accelerators, with a significant impact on the scientific reach of these facilities. Here, we discuss potential use cases of such a plasma-based energy booster and apply particle-in-cell simulations to estimate its ability to enhance the performance of existing X-ray FEL facilities.

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.

Longitudinal electron-beam diagnostics play a critical role in the operation and control of x-ray free-electron lasers, which rely on parameters such as the current profile, the longitudinal phase space, or the slice emittance of the particle distribution. On the one hand, the femtosecond-scale electron bunches produced at these facilities impose stringent requirements on the resolution achievable with the diagnostics. On the other, research and development of novel accelerator technologies such as beam-driven plasma-wakefield accelerators (PWFA) demand unprecedented capabilities to resolve the centroid offsets in the full transverse plane along the longitudinal bunch coordinate. We present the beam-based commissioning of an advanced X-band transverse-deflection rf structure (TDS) system with the new feature of providing variable polarization of the deflecting force: the PolariX-TDS. By means of a comprehensive campaign of measurements conducted with the prototype, key parameters of the rf performance of the system are validated and a phase-space characterization of an electron bunch is accomplished with a time resolution of 3.3 fs. Furthermore, an analysis of second-order effects induced on the bunch from its passage through the PolariX-TDS is presented.

Radio-frequency particle accelerators are engines of discovery, powering high-energy physics and photon science, but are also large and expensive due to their limited accelerating fields. Plasma-wakefield accelerators (PWFAs) provide orders-of-magnitude stronger fields in the charge-density wave behind a particle bunch travelling in a plasma, promising particle accelerators of greatly reduced size and cost. However, PWFAs can easily degrade the beam quality of the bunches they accelerate. Emittance, which determines how tightly beams can be focused, is a critical beam quality in for instance colliders and free-electron lasers, but is particularly prone to degradation. We demonstrate, for the first time, emittance preservation in a high-gradient and high-efficiency PWFA while simultaneously preserving charge and energy spread. This establishes that PWFAs can accelerate without degradation-an essential step toward energy boosters in photon science and multistage facilities for compact high-energy particle colliders.

Linear colliders rely on high-quality flat beams to achieve the desired event rate while avoiding potentially deleterious beamstrahlung effects. Here, we show that flat beams in plasma accelerators can be subject to quality degradation due to emittance mixing. This effect occurs when the beam particles' betatron oscillations in a nonlinearly coupled wakefield become resonant in the horizontal and vertical planes. Emittance mixing can lead to a substantial decrease of the luminosity, the main quantity determining the event rate. In some cases, the use of laser drivers or flat particle beam drivers may decrease the fraction of resonant particles and, hence, mitigate emittance deterioration.

For plasma-wakefield accelerators to fulfil their potential for cost effectiveness, it is essential that their energy-transfer efficiency be maximized. A key aspect of this efficiency is the near-complete transfer of energy, or depletion, from the driver electrons to the plasma wake. Achieving full depletion is limited by the process of re-acceleration, which occurs when the driver electrons decelerate to non-relativistic energies, slipping backwards into the accelerating phase of the wakefield and being subsequently re-accelerated. Such re-acceleration is observed here for the first time. At this re-acceleration limit, we measure a beam driver depositing (56 $\pm$ 5)% of its energy into a 195-mm-long plasma. Combining this driver-to-plasma efficiency with previously measured plasma-to-beam and expected wall-plug-to-driver efficiencies, our result shows that plasma-wakefield accelerators can in principle reach or even exceed the energy-transfer efficiency of conventional accelerators.

The quest for novel technologies in the ever-evolving landscape of scientific exploration has led to the investigation of plasma lensing as a potential solution for optical matching devices for all kinds of positron sources. This research becomes increasingly significant as the need for higher event rate demands innovative concepts to increase positron yield and therefore luminosity. A prototype plasma lens has been developed and tested for the first time. Instabilities were observed during the first test trials. This paper presents the results of high-temporal resolution imaging to analyze the discharge instabilities. Furthermore, the results show splitting and bending discharge instabilities and heavy copper coating. Overcoming these challenges is pivotal for a future application of plasma lenses as an integral part of high-performance positron sources.

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.