[en] We use three-dimensional (3D) simulations with the particle-in-cell (PIC) code OSIRIS to demonstrate the theoretical production of high-quality electron bunches in beam-driven plasma wakefield accelerators (PWFA) by means of field-induced ionization injection. In these simulations, two realistic scenarios for PWFA have been considered: the FLASHForward project at DESY and the FACET experiment at SLAC. These two examples illustrate two different strategies for injection. The first one uses the transverse electric fields of the beam to induce injection, and the second constitutes a new method which utilizes only the wakefields to enable ionization and trapping of high quality electron bunches into beam driven plasma wakes. The produced bunches feature multi-kA peak currents, ∼1μm transverse normalized emittances, uncorrelated energy spreads of ≤1% on a GeV-energy scale, and few femtosecond bunch lengths

[en] Discharge capillary-based active plasma lenses are a promising new technology for strongly focusing charged particle beams, especially when combined with novel high gradient acceleration methods. Still, many questions remain concerning such lenses, including their transverse field uniformity, limitations due to plasma wakefields and whether they can be combined in multi-lens lattices in a way to cancel chromaticity. These questions will be addressed in a new plasma lens experiment at the CLEAR User Facility at CERN. All the subsystems have been constructed, tested and integrated into the CLEAR beam line, and are ready for experiments starting late 2017.

[en] The production of plasmas in saline solution at low voltage (here 225 V) is investigated. It is confirmed that this is associated with vapour layer formation on the electrode surface. The plasmas occur once the vapour layer has completely covered the electrode and reached a thickness of approximately 0.3 mm. There are several aspects of this specific environment that may influence the breakdown characteristics of the vapour layer. These include the high electric fields, approaching 10⁷ V m^-1, in the vapour layer, the presence of sodium and chlorine on the electrode surface and sodium in the vapour layer. There are generally plasmas of different character in each pulse. They can be broadly classified as of short (a few μs) or long (up to 500 μs) duration. The presence of sodium in the vapour layer is hard to explain and the possible sudden vaporization of the saline solution is considered. Vapour layer and plasma production is faster and occurs at lower voltages with negative polarity pulses. An understanding of the phenomena exhibited in the production of plasmas in saline solution is important in determining their potential new applications in areas such as plasma medicine.

[en] Experimental and finite element modelling methods are used to study the formation of vapour layers in electrical discharges through saline solutions. The experiments utilize shadowgraphic and photometric methods to observe the time dependence of thin vapour layers and plasma formation around electrodes driven by moderate voltage (<500 V) pulses, applied to an electrode immersed in a conducting saline solution. Finite element multiphysics software, coupling thermal and electrical effects, is employed to model the vapour layer formation. All relevant electrical and thermal properties of the saline are incorporated into the model, but hydrodynamic and surface tension effects are ignored. Experimental shadowgraph and modelling images are compared, as are current histories, and the agreement is very good. The comparison of experiment and modelling gives insight into both vapour layer production and subsequent plasma production. We show that, for example, superheating of the saline above its normal vaporization temperature may be playing a significant role in vapour formation. We also show that electric fields of approaching 10⁷ V m^-1 can be achieved in the vapour layer.

[en] The complex dynamics of radio-frequency driven atmospheric pressure plasma jets is investigated using various optical diagnostic techniques and numerical simulations. Absolute number densities of ground state atomic oxygen radicals in the plasma effluent are measured by two-photon absorption laser induced fluorescence spectroscopy (TALIF). Spatial profiles are compared with (vacuum) ultra-violet radiation from excited states of atomic oxygen and molecular oxygen, respectively. The excitation and ionization dynamics in the plasma core are dominated by electron impact and observed by space and phase resolved optical emission spectroscopy (PROES). The electron dynamics is governed through the motion of the plasma boundary sheaths in front of the electrodes as illustrated in numerical simulations using a hybrid code based on fluid equations and kinetic treatment of electrons.

[en] Despite enormous potential for technological applications, fundamentals of stable non-equilibrium micro-plasmas at ambient pressure are still only partly understood. Micro-plasma jets are one sub-group of these plasma sources. For an understanding it is particularly important to analyse transport phenomena of energy and particles within and between the core and effluent of the discharge. The complexity of the problem requires the combination and correlation of various highly sophisticated diagnostics yielding different information with an extremely high temporal and spatial resolution. A specially designed rf microscale atmospheric pressure plasma jet (μ-APPJ) provides excellent access for optical diagnostics to the discharge volume and the effluent region. This allows detailed investigations of the discharge dynamics and energy transport mechanisms from the discharge to the effluent. Here we present examples for diagnostics applicable to different regions and combine the results. The diagnostics applied are optical emission spectroscopy (OES) in the visible and ultraviolet and two-photon absorption laser-induced fluorescence spectroscopy. By the latter spatially resolved absolutely calibrated density maps of atomic oxygen have been determined for the effluent. OES yields an insight into energy transport mechanisms from the core into the effluent. The first results of spatially and phase-resolved OES measurements of the discharge dynamics of the core are presented.

[en] Density modulations in plasma caused by a high-intensity laser or a high charge density electron pulse can generate extreme acceleration fields. Acceleration of electrons in such fields may produce ultra-relativistic, quasi-monoenergetic, ultra-short electron bunches over distances orders of magnitudes shorter than in state-of-the-art radio-frequency accelerators. FLASHForward is a beam-driven plasma wakefield accelerator (PWFA) project at DESY with the goal of producing, characterizing, and utilizing such beams. Temporal characterization of the acceleration process is of crucial importance for improving the stability and control in PWFA beams. While measurement of the transient field of the femtosecond bunch in a single shot is challenging, in recent years novel techniques with great promise have been developed [1, 2]. This work discusses the plans and status of the transverse diagnostics at FLASHForward. (paper)

[en] The atmospheric pressure plasma jet (APPJ) is a homogeneous non-equilibrium discharge at ambient pressure. It operates with a noble base gas and a percentage-volume admixture of a molecular gas. Applications of the discharge are mainly based on reactive species in the effluent. The effluent region of a discharge operated in helium with an oxygen admixture has been investigated. The optical emission from atomic oxygen decreases with distance from the discharge but can still be observed several centimetres in the effluent. Ground state atomic oxygen, measured using absolutely calibrated two-photon laser induced fluorescence spectroscopy, shows a similar behaviour. Detailed understanding of energy transport mechanisms requires investigations of the discharge volume and the effluent region. An atmospheric pressure plasma jet has been designed providing excellent diagnostics access and a simple geometry ideally suited for modelling and simulation. Laser spectroscopy and optical emission spectroscopy can be applied in the discharge volume and the effluent region