[en] A new electron injection scheme is proposed in sub-relativistic plasma wakefield accelerators. A transverse laser ionizes a dopant gas and ponderomotively accelerates the released electrons in the direction of wake propagation. This process enables electron trapping in the wakefield even for a wakefield potential below the trapping threshold. We study the scheme theoretically and by means of particle-in-cell simulations to demonstrate high-quality beam formation and acceleration with sub-micrometer normalized emittances and sub-percent uncorrelated energy spreads.

[en] Electron-bunch diagnostics are desired to utilize unambiguous, non-destructive, single-shot techniques. Various methods fulfill the latter two demands, but feature significant ambiguities and constraints in the reconstruction of a time-domain electron-bunch profile, as for example uncertainties due to the phase retrieval of coherent radiation using the Kramers-Kronig relation. We present a novel method of upconverting the THz-field spectrum of fs electron bunches at the free-electron laser FLASH into the near-infrared in an electro-optic crystal. This technique allows the single-shot detection of its longitudinal form factor in both, amplitude and phase. The spectral phase and amplitude information is measured and thus the temporal profile reconstructed using temporal analysis by dispersing a pair of light E-fields, also known as TADPOLE. This is a combination of frequency resolved optical gating (FROG) and spectral interferometry, which enables the temporal measurement of low-power laser pulses. In this experiment, a narrow-bandwidth laser pulse detecting the longitudinal electric field of an electron bunch is interfered with a broadband and FROG-characterized reference pulse. The longitudinal beam profile may therefore be unambiguously inferred from the generated interferogram and the detected spectral-phase-information of the reference pulse.

[en] The FLASHForward facility will offer unique capabilities or plasma-wakefield acceleration experiments. It uses high-quality beams from the FLASH accelerator to excite plasma wakefields for the exploration and improvement of novel and existing injection mechanisms. The unique nature of the plasma environment creates several challenges with regard to the conservation of the beam quality, partially due to the strong focusing fields present in the blowout region following a driver beam in the highly nonlinear regime. The beta function of a beam needs to be matched into the wakefield in order to avoid severe growth of the beam emittance - a crucial quality parameter for beam transport, staging and applications. Since the matched beta function is usually at least an order of magnitude lower than easily accessible for conventional accelerator optics, multiple schemes have been proposed to mitigate severe emittance growth by tailoring the plasma profile to adiabatically reduce the beta function to match the plasma wakefield. We will focus on an introduction of these techniques, before presenting initial results from numerical and theoretical analyses for the typical FLASH beam parameter space.

[en] Laser-wakefield acceleration has experienced growing scientific interest and fast development during the last decade. Short and highly intense laser pulses focused into a gas target, ionise the gas and may excite large amplitude, co-propagating plasma waves that support extreme electric fields (>10 GV/m) for acceleration of charged particles. These fields are orders of magnitude larger than in conventional radio-frequency accelerators. The REGAE facility at DESY offers the unique possibility to study the external injection of pre-accelerated electron bunches from a stable and fully controlled conventional accelerator, and their subsequent acceleration in plasma wakefields. A set of 2- and 3-dimensional numerical simulations was performed with the particle-in-cell code OSIRIS, showing a wide variety of effects which will be possible to explore in the future with REGAE, which provides 2-5 MeV electron bunches of 10 femtosecond length. Topics of particular interest are: probing of the electric wakefields, emittance evolution, bunch compression, and controlled betatron radiation emission.

[en] Highly energetic electron beams are required for various applications such as free-electron lasers and particle colliders. Nowadays these beams are almost exclusively produced in conventional radiofrequency-cavities, which are limited to typical acceleration gradients below 50 MV/m. Hence long machines on the order of 100 m are required to reach the desired energies. Laser-wakefield accelerators in plasma on the other hand are capable of providing acceleration gradients well above 10 GV/m, thereby allowing for much more compact devices on scales of centimeters. Currently, plasma-based devices are rapidly evolving and improving, but still suffer from instabilities in the generated electron-beam properties, largely due to shot-to-shot variations of laser and plasma parameters. In order to minimize possible sources for these fluctuations, a novel, more stable technique for shaping the transverse plasma-density profile in a plasma accelerator based on laser triggering and ignition of capillary discharge waveguides is presented.

[en] We present experimental results on a tape-drive based plasma mirror which could be used for a compact coupling of a laser beam into a staged laser driven electron accelerator. This novel kind of plasma mirror is suitable for high repetition rates and for high number of laser shots. In order to design a compact, staged laser plasma based accelerator or collider (1), the coupling of the laser beam into the different stages represents one of the key issues. To limit the spatial foot print and thus to realize a high overall acceleration gradient, a concept has to be found which realizes this in-coupling within a few centimeters (cf. Fig 1). The fluence of the laser pulse several centimeters away from the acceleration stage (focus) exceeds the damage threshold of any available mirror coating. Therefore, in reference (2) a plasma mirror was suggested for this purpose. We present experiments on a tape-drive based plasma mirror which could be used to reflect the focused laser beam into the acceleration stage. Plasma mirrors composed of antireflection coated glass substrates are usually used to improve the temporal laser contrast of laser pulses by several orders of magnitudes (3,4). This is particularly important for laser interaction with solid matter, such as ion acceleration (5,6) and high harmonic generation on surfaces (7). Therefore, the laser pulse is weekly focused onto a substrate. The main pulse generates a plasma and is reflected at the critical surface, whereas the low intensity pre-pulse (mainly the Amplified Spontaneous Emission pedestal) will be transmitted through the substrate before the mirror has been triggered. Several publications (3,4) demonstrate a conservation of the spatial beam quality and a reflectivity of about 70 %. The drawback of this technique is the limited repetition rate since for every shot a fresh surface has to be provided. In the past years several novel approaches for high repetition rate plasma mirrors have been developed (2, 8). Nevertheless, for the staged accelerator scheme a second important requirement has to be considered. Since the electron beam has to propagate through the mirror, the thickness of the substrate has to be as thin as possible to reduce the distortion of the electron beam. A tape of only several micrometer thickness can overcome these disadvantages. It can be used with a sufficient repetition rate while it allows the electron beam to propagate through with a minimum of scattering.

[en] Electron-bunch diagnostics are desired to utilize unambiguous, non-destructive, single-shot techniques. Various methods fulfill the latter two demands, but feature significant ambiguities and constraints in the reconstruction of time-domain electron-bunch profiles, e.g. uncertainties arising from the phase retrieval of coherent radiation using the Kramers–Kronig relation. We present a novel method of measuring the spectral phase. The measurement is based on upconversion in an electro-optic crystal, where the THz-field spectrum of fs-electron bunches is shifted to the near-infrared. This technique allows the single-shot detection of its longitudinal form factor in both, amplitude and phase. The spectral phase and amplitude information is measured and thus the temporal profile reconstructed using temporal analysis by dispersing a pair of light E-fields, also known as TADPOLE. This is a combination of frequency resolved optical gating (FROG) and spectral interferometry, enabling the temporal measurement of low-power laser pulses. In this procedure, a narrow-bandwidth laser pulse detecting the longitudinal variations in the transverse electric field of an electron bunch via frequency mixing is interfered with a broadband and FROG-characterized reference pulse. The longitudinal beam profile may therefore be unambiguously inferred from the generated interferogram and the detected spectral-phase-information of the reference pulse

[en] A technique has been developed to accurately align a laser beam through a plasma channel by minimizing the shift in laser centroid and angle at the channel outptut. If only the shift in centroid or angle is measured, then accurate alignment is provided by minimizing laser centroid motion at the channel exit as the channel properties are scanned. The improvement in alignment accuracy provided by this technique is important for minimizing electron beam pointing errors in laser plasma accelerators.

[en] In order to build a compact, staged laser plasma accelerator the in-coupling of the laser beam to the different stages represents one of the key issues. To limit the spatial foot print and thus to realize a high overall acceleration gradient, a concept has to be found which realizes this in-coupling within a few centimeters. We present experiments on a tape-drive based plasma mirror which could be used to reflect the focused laser beam into the acceleration stage.

[en] An injection scheme is proposed to realize electron trapping in sub-relativistic plasma wakefield accelerators. A laser under oblique angle of incidence ionizes a dopant gas in plasma and ponderomotively accelerates the released electrons into the direction of wake propagation. This process enables electron trapping in the wakefield even for a wakefield potential below the trapping threshold. We study the scheme theoretically and by means of particle-in-cell (PIC) simulations to demonstrate high-quality beam formation and acceleration with sub-micrometer normalized emittances and sub-percent uncorrelated energy spreads. (paper)