State-of-the-Art Electron Beams for Compact Tools of Ultrafast Science

• Vitaliy Goryashko (FREIA Laboratory, Uppsala University)

In this talk, I will present a recent review of the state-of-the-art in electron beams for single-shot megaelectronvolt ultrafast electron diffraction and compact light sources [1]. The primary focus will be on sub-100 femtosecond electron bunches in the 2–30 MeV energy range. I will show that our latest simulation and experimental results enable significantly improved bunch parameters for these applications. Furthermore, we will discuss: • the regime of optimal blowout beam generation [2]; • strong bunch compression within half of an RF wavelength [3]; • phase-space degradation and aberrations arising from cathode image charge, nonlinear space-charge fields, and nonlinearities in RF and static focusing magnetic fields [4]. Finally, I will demonstrate control and reduction of slice emittance through the ponderomotive laser force, supported by direct 3D simulations [5]. References: [1] Salén, P., Opanasenko, A., Perosa, G., & Goryashko, V. (2025). State-of-the-art electron beams for compact tools of ultrafast science. Ultramicroscopy, 268, 114080. [2] Shamuilov, G., Opanasenko, A., Pepitone, K., Tibai, Z., & Goryashko, V. (2022). Emittance self-compensation in blow-out mode. New Journal of Physics, 24(12), 123008. [3] Opanasenko, A., Perosa, G., Ribbing, J., & Goryashko, V. (2023). Half-wavelength velocity bunching: non-adiabatic temporal focusing of charged particle beams. New Journal of Physics, 25(12), 123049. [4] Goryashko, V., Opanasenko, A., & Togawa, K. (2025). Self-aberration in high-brightness uniformly charged particle beams. Results in Physics, 68, 108096. [5] Ribbing, J., Perosa, G., & Goryashko, V. (2025). Relativistic ponderomotive force in the regime of extreme focusing. Optics Letters, 50(6), 2093-2096.

Ultra-high-brightness Electron Beams with the Resonant Multi-Pulse Ionization Injection (ReMPI) and First Steps towards a P-MOPA-driven ReMPI Scheme.

• Paolo Tomassini (ELI-NP/LDED, Magurele (RO))

Ultra-low emittance and GeV energy scale electron beams can be obtained in the Laser Wake Field Acceleration (LWFA) framework by employing advanced ionization injection techniques, such as the Two-Color and the Resonant Multi-Pulse Ionization injection (ReMPI) schemes. There, a tightly focused, short wavelength (ionization) pulse extracts electrons from a selected inner shells of a dopant, allowing them to be longitudinally compressed and trapped in the wake field excited by a different (driver) pulse. We demonstrate for the first time, by means of analytical results and Particle In Cell simulations, that electron beams with unprecedented brightness and tuneable rms duration as low as 340as, can be generated in a reproducible way with a simplified ReMPI scheme in which the driver is constituted by a two pulses train. By employing a 300TW Ti:Sa laser system and a delay mask to generate a two pulses driver train, 340as long electron beams with 2.3~GeV energy, 6.1pC charge, 0.15% projected energy spread, 60nm averaged normalised emittance, and projected 6D brightness in excess of 3 10^18 A/m^2/0.1%bw can be generated. Another path under active exploration is the possibility to use a P-MOPA generated pulse train as the wake field driver, thus making a P-MOPA/ReMPI scheme. We will report about the first simulations devoted at the the searching for reliable working point with state of the art technologies.

The Ghost Collider

• Andrew Hutton (Thomas Jefferson National Accelerator Laboratory (TJNAF))

(Presented by Andrew Hutton for Peter Williams, Robert Apsimon, William Clampitt, Todd Satogata and Balša Terzić) Present and future high-energy electron-positron colliders are limited by two effects: the energy required to accelerate the beams and the electromagnetic beam-beam disruption. The Ghost Collider is an innovative concept that circumvents both problems, based on Energy Recovery Linac (ERL) technology. The first key innovation is the placement of electron and positron bunches within the same RF bucket – one being accelerated, the other decelerated. The buckets are therefore electrically neutral, beam position monitors will register no current (hence “ghost”) and crucially when the beams are decelerated, the energy is recovered, a vital part of making the collider more sustainable. The second key innovation is at the interaction point (IP); an electron bunch and a positron bunch collide with an electron bunch and a positron bunch from the opposite direction. These collisions are electrically neutral so there is no beam-beam disruption at the IP to first order, eliminating the primary limit on luminosity in all other collider concepts. time as accelerated positrons from one bucket behind. Recent advances will be presented.

Muon Colliders - Challenges from Collective Effects

• David Amorim (EPFL)

In the framework of the International Muon Collider Collaboration, a 10TeV centre-of-mass muon collider ring is being studied, with a possible first collider stage at 3TeV. Generating two counter-rotating high-intensity low-emittance muon bunches of opposite charge for the experiments requires a specific complex of accelerators. First, a proton driver produces a high-power proton beam which hits a target to generate pions. The pions then decay into muons and anti-muons, and these bunches undergo 6D ionization cooling to reduce their transverse emittance by a factor ∼1000. When the target emittance is reached, the two counter-rotating bunches must be quickly accelerated in a series of Recirculating Linacs (RLAs) and Rapid Cycling Synchrotrons (RCS) to minimize the intensity loss from muon decay. The two bunches then reach the collider ring and collide at two interaction points. The whole complex operates at a 5 Hz repetition rate to increase the integrated luminosity. The bunch intensity must reach 43 × $10^{12}$ muons or anti-muons at the start of the ionization cooling to meet the intensity target of 1.8 × $10^{12}$ in the 10TeV collider ring. Coherent effects are therefore a concern in all the accelerators of the chain as they could lead to instabilities, therefore limiting the intensity reach and increasing the transverse emittance. Because of the variety of machines present in the complex, a diverse range of coherent effects will be present, such as coherent direct space-charge, beam break-up, head-tail instability or beam-beam effects. Impedance models were developed for the RCS and collider rings, estimating the impact of the RF cavities and beam chamber on transverse beam stability. Simulation tools were developed to perform tracking simulations from the start of the acceleration chain to its end, through four different accelerators. In the RCS, studies on the impedance and eddy current effects in the vacuum chambers of the pulsed magnets have been undertaken and are driving the chamber design. Several other effects are still being investigated, such as the direct space charge in the proton driver, the impact of beam loading in the muon cooling and the two-beam wakefields in the RCS.

Beamline background simulations and Beam Delivery Simulation (BDSIM)

• Stewart Boogert (Cockcroft Institute of Accelerator Science and Technology)

Typically, in accelerator simulations, the interaction of beam particles with material is either neglected or implemented only for specific physics processes. However, beam-material interactions can sometimes be beneficial—for example, in the cooling of muons, the creation of secondary or tertiary beams, or the energy degradation of therapeutic proton beams. In other cases, these interactions represent beam losses and associated backgrounds that need to be minimised. Another important application is beam instrumentation which relies on the beam interacting with material, for example, screens or wires. This seminar surveys the field of simulating beam interactions with accelerator materials. It describes the radiation transport simulation tools typically used (e.g., Geant4 and FLUKA), coupled with beam tracking codes such as XSuite. Stewart Boogert has co-developed multiple open-source codes to achieve efficient and easy-to-use simulations of beam interactions with matter. One example is Beam Delivery Simulation (BDSIM), a Geant4-based code that will be described in the seminar. BDSIM is unique as it is routinely used for a broad range of accelerators from a 250 MeV proton therapy machine all the way up in energy to HL-LHC and FCC-ee. Another recent example is PYthon FLUka Beam Line (PYFLUBL), a Python package for generating beamline models for FLUKA. The seminar will conclude with an outlook on the future of such coupled simulations and potential research and development directions.