Ole Gunnar Finnerud

• Finnerud, Ole Gunnar; Adli, Erik; Ariniello, R; Corde, S; Dalichaouch, T N & Emma, C [Show all 21 contributors for this article] (2026). First results from the E302 efficiency–instability experiment at the FACET-II facility. Journal of Physics: Conference Series (JPCS). ISSN 1742-6588. 3266(1). doi: 10.1088/1742-6596/3266/1/012001. Full text in Research Archive Show summary

  Abstract The beam-breakup (BBU) instability in plasma accelerators is seeded by a transverse offset between the driver and a trailing bunch. The BBU instability induces oscillations in the trailing bunch, which are detrimental to its beam quality. When the instability is large, assuming little mitigation from ion motion and energy spread, the beam suffers emittance growth, and charge can be kicked transversely out of the plasma channel. The detrimental effect on beam quality is substantially worse at high efficiencies, which places constraints on the achievable power efficiency in applications such as linear colliders, where maintaining the beam quality is required. In this paper, we present the first experimental signatures of the BBU instability in data taken in the E302 experiment at the FACET-II facility at SLAC National Accelerator Laboratory. We use a specific beam-optical setup and a novel method to probe for transverse instabilities on diagnostic screens downstream of a magnetic dipole spectrometer. We complement the analysis with full 3D particle-in-cell (PIC) simulations of the plasma interaction using similar driver and trailing bunch parameters on a simulated FACET-II spectrometer.

• Adli, Erik; Lindstrøm, Carl A.; Kalvik, Daniel; Sjobak, Kyrre Ness; Finnerud, Ole Gunnar & Chen, Jian Bin Ben [Show all 7 contributors for this article] (2025). Transverse tolerances in the plasma-wakefield acceleration blow-out regime. Proceedings of the International Particle Accelerator Conference. ISSN 2673-5490. p. 1434–1437. doi: 10.18429/JACOW-IPAC2025-TUPS011. Full text in Research Archive Show summary

  We report on recent progress in transverse instabilities and transverse tolerances for plasma-wakefield accelerators in the blow-out regime. In this regime, the transverse fields provide both strong focusing and strong deflection via transverse wakefields. The deflection effect of the wakefields on the main beam leads to limitations on the acceleration efficiency, if not mitigated. Based on comprehensive particle-in-cell simulations we summarize recent findings of the instability--efficiency relation for the blow-out regime. Ion motion and energy spread may mitigate the instability; with linac start-to-end simulations, using the recently developed ABEL framework, we demonstrate that the instability and emittance growth may be sufficiently mitigated for the colliding beams in the HALHF concept. Independent of wakefield effects, the strong focusing fields lead to very tight tolerances for the drive-beam jitter. We quantify these tolerances, using examples from HALHF start-to-end simulations. We show that the tolerances are greatly loosened by applying external magnetic fields to guide the drive-beam propagation in the plasma.

• Finnerud, Ole Gunnar; Lindstrøm, Carl Andreas; Chen, Jian Bin Ben & Adli, Erik (2025). The E302 instability-versus-efficiency experiment at FACET-II. Journal of Physics: Conference Series (JPCS). ISSN 1742-6588. 3124(1). doi: 10.1088/1742-6596/3124/1/012005. Full text in Research Archive Show summary

  We discuss plans for the E302 instability-efficiency experiment, starting in 2024 at the recently upgraded FACET-II facility at SLAC National Accelerator Laboratory. The beam-breakup instability will be the main area of study for the E302 experiment. With the imaging spectrometer at FACET-II, we introduce a novel technique for observing and quantifying the amplitude growth of the trailing bunch due to the transverse instability. Using the transverse position on the spectrometer screen and the transfer matrix of the magnetic lattice used for the spectrometer, we aim to extract a x′–E charge distribution that can be used to quantify the amplitude of the beam. By varying the trailing bunch’s charge and, hence, the beam loading of the accelerating field, we aim to adjust the wake-to-beam power transfer efficiency in the E302 experiment. We plan to quantify the amplitude for different configurations of the beam charge and, hence, investigate the relationship between the beam-breakup instability and efficiency. We use a combination of particle-in-cell (PIC) codes to simulate a beam-driven plasma wakefield accelerator from start-to-end with a FACET-II-like spectrometer and demonstrate the methodology that will be used for the instability studies at the E302 experiment.

• Chen, Jian Bin Ben; Lindstrøm, Carl Andreas; Finnerud, Ole Gunnar & Adli, Erik (2025). Modelling transverse instability in HALHF plasma stages. Journal of Physics: Conference Series (JPCS). ISSN 1742-6588. 3124(1). doi: 10.1088/1742-6596/3124/1/012015. Full text in Research Archive Show summary

  We present first results for a parameter study of the plasma linac of HALHF, a novel electron-positron collider concept combining plasma wakefield acceleration and mature RF acceleration to reach a centre of mass energy of 250 GeV. This study is a preliminary extension of the previous studies that indicated promising performance, by including transverse instability. Transverse instability was simulated using start-to-end simulations, where PIC simulations were combined with a simplified model to efficiently model transverse instability in the plasma-acceleration stages. No interstage optics were included. With a small initial transverse offset, the currently proposed parameter set will result in emittance growth and charge losses. If additional transverse misalignments are introduced between stages in the form of drive beam transverse jitter, the emittance growth is even more severe. Mitigation mechanisms to reduce the instability to an acceptable level are suggested.

• Zhang, Chaojie; Storey, Douglas; Knetsch, Alexander; O’Shea, Brendan D.; Ariniello, Robert & Cao, Jiawei [Show all 22 contributors for this article] (2025). Plasma-wakefield accelerator simultaneously boosts electron beam energy and brightness. Nature Communications. 16(1). doi: 10.1038/s41467-025-65742-8. Full text in Research Archive Show summary

  High-energy particle colliders and X-ray free-electron lasers demand electron beams with qualities currently achieved only in kilometer-scale radio-frequency accelerators. Plasma accelerators promise a compact alternative but have faced challenges in delivering the needed beam quality at relevant energies. Here, we demonstrate that a plasma-wakefield accelerator operating in the nonlinear regime acts as a transformer to simultaneously boost the energy and brightness of an electron bunch injected from the plasma. Using a 10-GeV drive bunch and a three-stage meter-scale plasma source, we generated electron bunches exceeding 20 GeV with sub-percent energy spread, 2 mm·mrad normalized emittance, and multi-kA peak current. A significant number of drive-bunch electrons lost over 90% of their energy, a prerequisite for high energy-conversion efficiency. This demonstration of an energy transformer ratio exceeding two and a brightness enhancement over an order of magnitude opens a path towards cost-effective accelerators for future colliders and light sources.

• Finnerud, O. G.; Lindstrøm, Carl A. & Adli, Erik (2025). Parametric mapping of the efficiency–instability relation in plasma-wakefield accelerators. Physical Review Accelerators and Beams. ISSN 2469-9888. 28(7). doi: 10.1103/72qb-517d. Full text in Research Archive Show summary

  High efficiency is essential for plasma-wakefield accelerators to be a cost-effective alternative in high-power applications, such as a linear collider. However, in a plasma-wakefield accelerator, the beam-breakup instability can be seeded by a transverse offset between the driver and trailing bunch. This instability, which rapidly increases the oscillation amplitude of the trailing bunch, grows with higher power-transfer efficiency from the driver to the trailing bunch [V. Lebedev et al., Phys. Rev. Accel. Beams 20, 121301 (2017)]. In this paper, we use particle-in-cell simulations to investigate the efficiency–instability relation that constrains the driver-to-trailing-bunch power-transfer efficiency in beam-driven plasma accelerators. We test the relation using a grid of simulations across all parameters that affect the beam-breakup instability, assuming a uniform accelerating field (optimal beam loading) and no ion motion. We find that the previously proposed efficiency–instability relation represents a lower limit on the strength of the instability for a given efficiency. For each normalized wake radius, only a certain accelerating field reaches this lowest value of the transverse instability; deviating from this point can increase the growth rate by several orders of magnitude. Finally, we highlight how the oscillation-amplitude growth of the trailing bunch can be reduced or damped with an initial uncorrelated energy spread and the presence of ion motion.

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• Chen, Jian Bin Ben; Adli, Erik; Drobniak, Pierre; Finnerud, Ole Gunnar; Hørlyk, Eir Eline & Kalvik, Daniel [Show all 9 contributors for this article] (2025). ABEL: The adaptable beginning-to-end linac simulation framework. doi: 10.18429/JACOW-IPAC2025-TUPS012. Full text in Research Archive Show summary

  We introduce ABEL, the Adaptable Beginning-to-End Linac simulation framework developed for agile design studies of plasma-based accelerators and colliders. ABEL’s modular architecture allows users to simulate particle acceleration across various beamline components. The framework supports specialised codes such as HiPACE++, Wake-T, ELEGANT, GUINEA-PIG and CLICopti, which facilitate precise modelling of complex machine components. Key features include simplified models for addressing transverse instabilities, radiation reactions, and ion motion, alongside comprehensive diagnostics and optimisation capabilities. Our simulation studies focus on the HALHF plasma linac, examining tolerances for drive beam jitter, including effects of self-correction mechanisms. Simulation results demonstrate ABEL's ability to model emittance growth under transverse instability and ion motion, highlighting the framework’s adaptability in balancing simulation fidelity with computational efficiency. The findings point towards ABEL’s potential for advancing compact accelerator designs and contribute to the broader goals of enhancing control and precision in plasma-based acceleration.

• Finnerud, Ole Gunnar; Adli, Erik & Lindstrøm, Carl A. (2026). Beam-breakup instability and efficiency in plasma accelerators. Universitetet i Oslo. Full text in Research Archive Show summary

  Particle colliders have been important tools in particle physics since the early 20th century. However, the growing demand for larger and higher-energy particle accelerators is driving up their cost. Plasma-based colliders have emerged as a candidate for reducing their size and cost thanks to their higher accelerating gradients. A major obstacle for this emerging technology is the efficiency—instability relation, which states that at higher efficiencies, the beam becomes more unstable, eventually breaking apart. This thesis tackles this issue through numerical simulations and experimental studies. The efficiency—instability relation is shown to be a lower bound on the strength of the instability, which varies by a large amount depending on the accelerator parameters, indicating the importance of choosing an optimal operating point. The effect of mitigation strategies is shown to be significant while also dependent on the parameters chosen. An experiment is carried out to probe the instability using novel measurement techniques. The results show indications of the instability, its correlation with efficiency, and mitigating effects. The efficiency—instability relation is complicated to measure and not fully understood in theory, but studies like the ones outlined in this thesis are important milestones towards a better understanding of it.

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