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The use of a hollow plasma channel in plasma-based acceleration has ben-eficial properties for the acceleration of electron and positron bunches. Inthe scope of the FLASHForward facility at DESY, the generation of sucha plasma structure is examined. Therefore, the generation of a ring-shapedlaser intensity profile with different techniques is analyzed.From the obtained intensity profiles the electron density of a hollow plasmachannel is simulated in the focal region. Different parameters are scanned tounderstand their influence on the electron density distribution - an importantparameter being, for example, the radius of the central region of the channel.In addition to the simulations, experiments are presented, during which alaser pulse is transformed into a hollow beam with a spiral phase plate. Sub-sequently, it forms a plasma during the interaction with hydrogen, where theplasma is imaged with interferometry. For energies above0.9mJ a hollowplasma structure can be observed at the location of first plasma formation.Die Verwendung eines hohlen Plasmakanals bei der plasmabasiertenBeschleunigung hat vorteilhafte Eigenschaften für die Beschleunigung vonElektronen- und Positronenbündeln. Im Rahmen der FLASHForward-Anlagebei DESY wird die Entstehung einer solchen Plasmastruktur untersucht. Eswird die Erzeugung eines ringförmigen Laserintensitätsprofils mit verschiede-nen Techniken analysiert.Aus den erhaltenen Intensitätsprofilen wird die Elektronendichte eines hohlenPlasmakanals im Fokusbereich simuliert. Verschiedene Parameter werdengescannt, um ihren Einfluss auf die Elektronendichteverteilung zu verstehen- ein wichtiger Parameter ist zum Beispiel der Radius des zentralen Bereichsdes Kanals.Neben den Simulationen werden Experimente vorgestellt, bei denen einLaserpuls mit einer Spiralphasenplatte in einen Hohlstrahl umgewandeltwird. Anschließend bildet dieser bei der Wechselwirkung mit Wasserstoff einPlasma, wobei das Plasma interferometrisch abgebildet wird. Für Energienüber0.9mJ kann eine hohle Plasmastruktur am Ort der ersten Plasmabil-dung beobachtet werden.

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

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.

8th International Particle Accelerator Conference, IPAC17, Copenhagen, Denmark, 15 May 2017 - 19 May 2017; Geneva : JACoW, 1692-1695 (2017). doi:10.18429/JACoW-IPAC2017-TUPIK006, FLASHForward is a beam-driven plasma wakefield acceleration facility, currently under construction at DESY (Hamburg, Germany), aiming at the stable generation of electron beams of several GeV with small energy spread and emittance. High-quality 1 GeV-class electron beams from the free-electron laser FLASH will act as the wake driver. The setup will allow studies of external injection as well as density-downramp injection. With a triangular-shaped driver beam electron energies of up to 5 GeV from a few centimeters of plasma can be anticipated. Particle-In-Cell simulations are used to assess the feasibility of each technique and to predict properties of the accelerated electron bunches. In this contribution the current status of FLASHForward, along with recent experimental developments and upcoming scientific plans, will be reviewed., Published by JACoW, Geneva

3rd European Advanced Accelerator Concepts Workshop, EAAC 2017, La Biodola, Isola d'Elba, Italy, 24 Sep 2017 - 30 Sep 2017, One of the key issues for generation of stable and reproducible beams in plasma particle accelerators is control of the initial plasma density distribution. Not only the longitudinal but also the transverse density profile, the species distribution as well as fragmentation and ionisation dynamics can have significant impact on the generated beams. Especially increasingly complex scenarios, e.g. specific electron injection and laser beam transport mechanisms, precise knowledge of these parameters is vital. Currently established methods, like interferometry or gas density diagnostics using scattering processes, only allow for gas targets operated at densities well above 10^17 cm-3 and lack versatility.Here, electron density measurements based on stark broadening are presented. For a wide parameter range laser and discharge generated plasmas have been investigated regarding longitudinal and transverse electron density distributions, plasma lifetime, and electron temperature. In addition an absolute density calibration has been performed for the parameter range investigated, since literature values proved to be of insufficient accuracy in the transient situations in targets required for plasma acceleration

58th Annual Meeting of the APS Division of Plasma Physics, San Jose, USA, 31 Oct 2016 - 4 Nov 2016 ; (2016)., Laser driven plasma wakefield accelerators have been explored as a potential compact, reproducible source of relativistic electron bunches, utilising an electric field of many GV/m. Control over injection of electrons into the wakefield is of crucial importance in producing stable, mono-energetic electron bunches. Density tailoring of the target, to control the acceleration process, can also be used to improve the quality of the bunch. By using gas jets to provide tailored targets it is possible to provide good access for plasma diagnostics while also producing sharp density gradients for density down-ramp injection. OpenFOAM hydrodynamic simulations were used to investigate the possibility of producing tailored density targets in a supersonic gas jet. Particle-in-cell simulations of the resulting density profiles modelled the effect of the tailored density on the properties of the accelerated electron bunch. Here, we present the simulation results together with preliminary experimental measurements of electron and x-ray properties from LPWA experiments using gas jet targets and a 25 TW, 25 fs Ti:Sa laser system at DESY.