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Scientific reports 12(1), 16017 (2022). doi:10.1038/s41598-022-20283-8, 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., Published by Macmillan Publishers Limited, part of Springer Nature, [London]

Physical review letters 129(24), 244801 (2022). doi:10.1103/PhysRevLett.129.244801, We report on a novel, non-invasive method applying Thomson scattering to measure the evolutionof the electron beam energy inside a laser-plasma accelerator with high spatial resolution. Thedetermination of the local electron energy without altering the final beam state enabled the in-situdetection of the acting acceleration fields, in this Letter demonstrated in a plasma density rampvarying from (265 ± 119) GV/m to (9 ± 4) GV/m. The presented data shows excellent agreementwith particle-in-cell simulations. This method provides new possibilities for detecting dynamics ofplasma-based accelerators and their optimization., Published by APS, College Park, Md.

Focusing petawatt-level laser beams to a variety of spot sizes for different applications is expensive in cost, labor and space. In this paper, we propose a plasma lens to flexibly resize the laser beam by utilizing the laser self-focusing effect. Using a fixed conventional focusing system to focus the laser a short distance in front of the plasma, we can adjust the effective laser beam waist within a certain range, as if a variety of focusing systems were used with the plasma lens acting as an adjustable eyepiece in a telescope. Such a setup is a powerful tool for laser wakefield accelerator experiments in state-of-art petawatt laser projects and allows for scanning focal spot parameters.

4th European Advanced Accelerator Concepts Workshop 2019, EAAC2019, La Biodola Bay, Isola d'Elba, Italy, 15 Sep 2019 - 20 Sep 2019, FLASHForward is a beam-driven plasma wakefield accelerator located at Deutsches Elektronen Synchrotron (DESY) in Hamburg, Germany. Within the FLASHForward project, a laser-driven wakefield setup is used as testbed for the characterisation and development of diagnostics for plasma wakefield accelerators. These include different non-invasive charge diagnostics that are also tested for their usability in EMP noisy plasma environment. Another method investigated is Thomson Scattering, the interaction of relativistic electron bunches with a laser pulse resulting in x-ray beams. Results on using the dependence of the Thomson beam on the initial electron bunch parameters to study the electron bunches are presented.

Hamburg Alliance New Beams and Accelerators, Hamburg, Germany, 5 Sep 2018 - 7 Sep 2018

9th International Particle Accelerator Conference, IPAC 2018, Vancouver, Canada, 29 Apr 2018 - 4 May 2018, The FLASHForward project at DESY is an innovative beam-driven plasma-wakefield acceleration experiment in-tegrated in the FLASH facility, aiming to accelerate elec-tron beams to GeV energies over a few centimetersof ion-ised gas. These acceleratedbeams are tested for their capa-bility to demonstrate exponential free-electron laser gain; achievable only through rigorous analysis of both the driver and witness beam's phase space.The thematic priority covered in here the control system part of FLASHForward.To be able to control, read out and save data from the diagnostics into DAQ, the DOOCS control system has been integrated into FLASH Forward. Laser beam control, over 70 cameras, ADCs, timing system and motorised stages are combined into the one DOOCS control system as well as vacuum and magnet controls. Micro TCA for Physics (MTCA.4) is the solid basic computing system, supported from high power workstations for camera read-out and normal Linux computers.

60th Annual Meeting of the APS Division of Plasma Physics, APS, Portland, USA, 5 Nov 2018 - 9 Nov 2018

3rd European Advanced Accelerator Concepts Workshop, EAAC 2017, La Biodola, Isola d'Elba, Italy, 24 Sep 2017 - 30 Sep 2017, The beam-driven FLASHForward experiment 1 (X-1) aims at the generation of high-brightness electron bunches for photon science applications in several centimeters of plasma, with the plasma acting both as a cathode and accelerator.The 1 GeV electron-bunch with a peak current of 2.5 kA and a synchronized TW-laser system makes FLASHForward a unique facility[1] to study controlled electron-injection into plasma wakes.With density downramp injection, witness bunches of ~1 kA peak current at emittances well below 1 µm are achievable[2]. The sharp plasma density gradients are produced by means of controlled gas flow[3] or by localized laser ionization transverse to the electron-beam orbit[4]. Precise laser-to-electron-beam synchronization enables controlled injection as e.g. the Trojan Horse scheme[5], which is predicted to support sub-0.1-µm-emittance witness bunches.experimental installation status, planning, and prospects of the FLASHForward X-1 experiments are presented.

3rd European Advanced Accelerator Concepts Workshop, EAAC 2017, La Biodola, Isola d'Elba, Italy, 24 Sep 2017 - 30 Sep 2017, The results of calibrating three independent charge measurement diagnostics with electron bunches from a laser wakefield accelerator are presented. The detectors used are phosphor screens (type DRZ), an integrated current transformer (ICT/Toroid), and a cavity-based dark current monitor (DaMon). The simultaneous use of all three measurement techniques allowed the cross-calibration of the DRZ, ICT, and DaMon for other applications, e.g. future implementation in beam driven plasma wakefield acceleration experiments at FLASHForward. Results show that the use of the DaMon permits non-destructive detection of ultrashort electron bunches with an improved dynamic range when compared to conventional diagnostics such as phosphor screens and ICTs. This originates from the insensitivity of the DaMon to the electromagnetically noisy plasma environment and its strong response to electric charge passing through its cavity. Therefore, the DaMon enables the detection of low bunch charges down to the ~10 fC regime opening up new possibilities of precise charge measurements for plasma-based acceleration

International audience; The Horizon 2020 Project EuPRAXIA ('European Plasma Research Accelerator with eXcellence In Applications') aims at producing a design report of a highly compact and cost-effective European facility with multi-GeV electron beams using plasma as the acceleration medium. The accelerator facility will be based on a laser and/or a beam driven plasma acceleration approach and will be used for photon science, high-energy physics (HEP) detector tests, and other applications such as compact X-ray sources for medical imaging or material processing. EuPRAXIA started in November 2015 and will deliver the design report in October 2019. EuPRAXIA aims to be included on the ESFRI roadmap in 2020.

3rd European Advanced Accelerator Concepts Workshop, EAAC 2017, La Biodola, Isola d'Elba, Italy, 24 Sep 2017 - 30 Sep 2017, Plasma targets required for wakefield acceleration rely on establishing specific electron density distributions in longitudinal and sometimes transverse direction to allow for control over the quality of accelerated electron bunches. The complex fragmentation dynamics of molecular gases in strong electric fields can have a crucial impact on target as well as injection properties in plasma wakefield acceleration. The importance of dissociative fragmentation during ionisation will be discussed in this contribution on the example of hydrogen. Especially in complex scenarios, in which multiple gas-species can be involved, the strength of the plasma generating source may be adjusted to accommodate for the species-specific ionisation thresholds to e.g. generate strong gradient down ramps. To predict the theoretical electron density distribution across the target, we compute the ionisation behaviour based on rate equations and ADK theory in strong electric fields. Here, the fragmentation dynamics governed by various dissociation and ionisation pathways play a crucial role in generating specific profiles. Results from these methods are benchmarked experimentally employing a multi-TW short-pulse laser. The developed understanding of the underlying processes of plasma generation allows for tailoring the electron density distribution by manipulating the focusing geometry and thus spatial laser-intensity evolution.

38th International Free-Electron Laser Conference, FEL2017, Santa Fe, USA, 20 Aug 2017 - 25 Aug 2017; 4 pp. (2017)., FLASHForward is the Future-ORiented Wakefield Accelerator Research and Development project at the DESY free-electron laser (FEL) facility FLASH. It aims to produce high-quality, GeV-energy electron beams over a plasma cell of a few centimeters. The plasma is created by means of a 25 TW Ti:Sapphire laser system. The plasma wakefield will be driven by high-current-density electron beams extracted from the FLASH accelerator. The project focuses on the advancement of plasma-based particle acceleration technology through the exploration of both external and internal witness-beam injection schemes. Multiple conventional and cutting-edge diagnostic tools, suitable for diagnosis of short electron beams, are under development. The design of the post-plasma beamline sections will be finalized based on the result of these aforementioned diagnostics. In this paper, the status of the project, as well as the progress towards achieving its overarching goal of demonstrating FEL gain via plasma wakefield acceleration, is discussed.

This thesis details experimental and theoretical work in the field of self-guided laser wakefield accelerators, characterising various aspects of the machine related to the driver laser and electron beam generation. The spectral changes to the laser pulse driving a laser wakefield accelerator were characterised. It was found that the spectral blueshift is directly correlated to the length of the plasma cavity. Spectral phase changes of the driver pulse were measured to dramatically alter the interaction. Positive second order spectral phase was measured to increase electron beam energy, its charge and the spectral blueshifting undergone by the driver pulse. The suppression of self-injection in laser wakefield accelerators operating in the highly non-linear bubble regime was observed. The use of ionisation impurity to pre-inject electrons into the plasma cavity was measured to alter the fundamental properties of the electron beam. Through particle-in-cell simulations it was shown that this effect arises from the repulsive electrostatic force from the beam load, preventing sufficient transverse momentum gain of sheath electrons. Record electron beam energies of nearly 3 GeV were measured in the self-injecting, self-guiding regime of laser wakefield accelerators. These results were obtained at higher than expected plasma densities and are thought to be a direct result of increased energy coupling due to the use of a much longer main focussing optic. Very stable injection in the self-injection regime was observed allowing for experimental measurements of peak accelerating field within the bubble. The field E = (590 ± 180) GV/m is the highest value of the electric field reported. The efficacy and long-term stability of self-guided, self-injecting laser wakefield electron acceleration was evaluated. Highest sustained laser energy to electron beam energy conversion efficiency of nearly 3% was measured. It was also shown the self-injection yields higher overall efficiencies than ionisation induced injection. Stability of injection and acceleration over more than half a thousand consecutive shots was studied and found to be directly dependent on the stability of the driving laser. Open Access

Physical review accelerators and beams 24(10), 101302 (2021). doi:10.1103/PhysRevAccelBeams.24.101302, Electron beams to be accelerated in beam-driven plasma wakes are commonly formed by a photocathode and externally injected into the wakefield of a preceding bunch. Alternatively, using the plasma itself as a cathode offers the possibility of generating ultrashort, low-emittance beams by trapping and accelerating electrons from the ambient plasma background. Here, we present a beam-driven plasma cathode realized via laser-triggered density-downramp injection, showing stable beam formation over more than a thousand consecutive events with an injection probability of 95 %. The plasma cathode is highly tunable, resulting in the injection of electron bunches of tens of pC of charge, energies of up to 79 MeV, and relative energy spreads as low as a few percent and the achieved stability allowed for the first multi-shot evaluation of the emittance of injected beams., Published by American Physical Society, College Park, MD

Physical review / X 8(3), 031004 (2018). doi:10.1103/PhysRevX.8.031004, The description of the dynamics of an electron in an external electromagnetic field of arbitrary intensityis one of the most fundamental outstanding problems in electrodynamics. Remarkably, to date, there is nounanimously accepted theoretical solution for ultrahigh intensities and little or no experimental data. Thebasic challenge is the inclusion of the self-interaction of the electron with the field emitted by the electronitself—the so-called radiation reaction force. We report here on the experimental evidence of strongradiation reaction, in an all-optical experiment, during the propagation of highly relativistic electrons(maximum energy exceeding 2 GeV) through the field of an ultraintense laser (peak intensity of $4 ×10^{20} W/cm^2$). In their own rest frame, the highest-energy electrons experience an electric field as highas one quarter of the critical field of quantum electrodynamics and are seen to lose up to 30% of their kineticenergy during the propagation through the laser field. The experimental data show signatures of quantumeffects in the electron dynamics in the external laser field, potentially showing departures from the constantcross field approximation, Published by APS, College Park, Md.

The Horizon 2020 Project EuPRAXIA ('European Plasma Research Accelerator with eXcellence In Applications') aims at producing a design report of a highly compact and cost-effective European facility with multi-GeV electron beams using plasma as the acceleration medium. The accelerator facility will be based on a laser and/or a beam driven plasma acceleration approach and will be used for photon science, high-energy physics (HEP) detector tests, and other applications such as compact X-ray sources for medical imaging or material processing. EuPRAXIA started in November 2015 and will deliver the design report in October 2019. EuPRAXIA aims to be included on the ESFRI roadmap in 2020., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

[Proceedings of the 8th International Particle Accelerator Conference : IPAC2017 14 - 19 May 2017, Bella Center, Copenhagen, Denmark]Editors: Volker RW Schaa (GSI, Darmstadt, Germany); Gianluigi Arduini (CERN, Geneva, Switzerland); Juliana Pranke (ESS, Lund, Sweden); Mike Seidel (PSI, Villigen, Switzerland); Mats Lindroos (ESS, Lund, Sweden) 8th International Particle Accelerator Conference, IPAC2017, Copenhagen, Denmark, 14 May 2017 - 19 May 2017 ; Geneva, Switzerland : JACoW, 1265-1268(2017). doi:10.18429/JACoW-IPAC2017-TUOBB3, Published by JACoW, Geneva, Switzerland

Proceedings of the National Academy of Sciences of the United States of America 115(25), 6335 - 6340 (2018). doi:10.1073/pnas.1802314115, In the fieldof X-raymicrocomputedtomography(μCT) thereis a growingneedto reduceacquisitiontimesat highspatialresolution(approximatemicrometers)to facilitatein vivoandhigh-throughputoperations.The stateof the art representedbysynchrotron lightsources is not practicalfor certainapplications,andtherefore the developmentof high-brightnesslaboratory-scalesources is crucial.We presenthere imagingof a fixedembryonicmousesampleusinga compactlaser–plasma-basedX-raylightsource andcompare the resultsto imagesobtainedusinga commercial X-rayμCT scanner. The radiationis generatedby the betatron motionof electrons insidea diluteandtran-sientplasma,whichcircumventsthe flux limitationsimposedbythe solidor liquidanodesusedin conventionalelectron-impactX-raytubes.This X-raysource is pulsed(duration1010photonsper pulse),small(diameter15 keV. StableX-rayperformanceenabledtomographicimagingof equivalentqualityto thatof theμCTscanner, an importantconfirmationof the suitabilityof the laser-drivensource for applications.TheX-rayfluxachievablewiththis approachscaleswiththe laserrepetitionratewithoutcom-promisingthe source size,whichwillallowthe recordingofhigh-resolutionμCT scansin minutes., Published by National Acad. of Sciences, Washington, DC

Masterarbeit, University of Hamburg, 2020; 62 pp. (2020). = Masterarbeit, University of Hamburg, 2020, The goal of this thesis was to enable high repetition rate Laser Wakefield Acceleration(LWFA) by solving heat induced grating deformation issues in a Chirped PulseAmplification (CPA) based 25 TW Ti:sapphire laser system centred at 800 nm. Thesepreviously limited the intensity and wavefront quality of the pulse. An increase of maximumrepetition rate, limited by heat induced grating deformation, from 1 to 10 Hz wasachieved. Through simulations it was shown that the grating deforms due to heatingup from absorbed laser energy which induces spatial-distortions in the wavefront. Byreplacing the NZK-7 based grating with one made from ZERODURR , a glass ceramicproduced by the Schott AG with a vanishing thermal expansion coefficient a, it wasexpected, and later verified through measurements, that the introduction of such distortionsinto the beam wavefront could be inhibited. During operating at 10 Hz, thefocal spot size of the far field was measured to increase from 51 mm to 204 mm inthe vertical-, and 61 mm to 202 mm in the horizontal axis within 5 minutes usingthe NZK-7 grating and kept growing to 244 mm in the vertical-, and 434 mm inthe horizontal axis over 8 hours. With the Zerodur grating installed it stayed constantin both axis at 51 mm during the whole measurement. It was also possible to reducethe change in position of the focus by one order of magnitude. Lastly, a RadioFrequency (RF) plasma cleaning setup was successfully tested and implemented intothe compressor chamber to remove the Carbon layer that was coated onto the gratingsurface by the high intensity of the laser. Heat induced grating deformation is one ofthe things that inhibit high repetition rate LWFA experiments. In summary, this thesispresents a possibility to resolve these challenges.