L. Schaper

Vapour layer formation by electrical discharges through electrically conducting liquids?modelling and experiment

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.

Plasma production in electrically conducting liquids

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.

High-quality electron beams from beam-driven plasma accelerators by wakefield-induced ionization injection

We propose a new and simple strategy for controlled ionization-induced trapping of electrons in a beam-driven plasma accelerator. The presented method directly exploits electric wakefields to ionize electrons from a dopant gas and capture them into a well-defined volume of the accelerating and focusing wake phase, leading to high-quality witness bunches. This injection principle is explained by example of three-dimensional particle-in-cell calculations using the code OSIRIS. In these simulations a high-current-density electron-beam driver excites plasma waves in the blowout regime inside a fully ionized hydrogen plasma of density 5×10(17)cm-3. Within an embedded 100  μm long plasma column contaminated with neutral helium gas, the wakefields trigger ionization, trapping of a defined fraction of the released electrons, and subsequent acceleration. The hereby generated electron beam features a 1.5 kA peak current, 1.5  μm transverse normalized emittance, an uncorrelated energy spread of 0.3% on a GeV-energy scale, and few femtosecond bunch length.

The FLASHForward facility at DESY

The FLASHForward project at DESY is a pioneering plasma-wakefield acceleration experiment that aims to produce, in a few centimetres of ionised hydrogen, beams with energy of order GeV that are of quality sufficient to be used in a free-electron laser. The plasma is created by ionising a gas in a gas cell with a multi-TW laser system. The plasma wave will be driven by high-current-density electron beams from the FLASH linear accelerator. The laser system can also be used to provide optical diagnostics of the plasma and electron beams due to the <30 fs synchronisation between the laser and the driving electron beam. The project will explore both external and internal witness-beam injection techniques. The operation parameters of the experiment are discussed, as well as the scientific programme.

The dynamics of radio-frequency driven atmospheric pressure plasma jets

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.

Comparing Deposition Properties in an Atmospheric Pressure Plasma System Operating in Uniform and Nonuniform Modes

A large-scale atmospheric pressure plasma has been generated in helium, and the time-resolved optical and electrical properties have been shown to produce a homogeneous dielectric barrier discharge. Introducing tetraethyl orthosilicate as a liquid aerosol into this plasma produced clear, uniform, and smooth plasma polymerized coatings. Optical imaging studies have shown that adding 1% oxygen to the gas mixture induced a switch from a homogeneous plasma to a filamentary or microdischarge mode of operation, and this has been shown to dramatically alter the morphology of the deposited coatings. Surface analysis reveals significant particulate inclusions in coatings deposited from the filamentary mode of operation.

Wakefield-Induced Ionization injection in beam-driven plasma accelerators

We present a detailed analysis of the features and capabilities of Wakefield-Induced Ionization (WII) injection in the blowout regime of beam driven plasma accelerators. This mechanism exploits the electric wakefields to ionize electrons from a dopant gas and trap them in a well-defined region of the accelerating and focusing wake phase, leading to the formation of high-quality witness-bunches [Martinez de la Ossa et al., Phys. Rev. Lett. 111, 245003 (2013)]. The electron-beam drivers must feature high-peak currents ( Ib0≳8.5 kA) and a duration comparable to the plasma wavelength to excite plasma waves in the blowout regime and enable WII injection. In this regime, the disparity of the magnitude of the electric field in the driver region and the electric field in the rear of the ion cavity allows for the selective ionization and subsequent trapping from a narrow phase interval. The witness bunches generated in this manner feature a short duration and small values of the normalized transverse emittance ( k...

Pre- to Post-discharge Behavior in Saline Solution

The development of a plasma discharge at low voltage (200-600 V) in saline solution is characterized using fast and standard CCD camera imaging. Vapor formation, plasma formation, and vapor collapse and subsequent pressure wave propagation are observed. If, with increasing voltage, the total energy deposited is kept approximately constant, the sequence and nature of events are similar but develop faster and more reproducibly at the higher voltages. This is attributed to the slower temporal evolution of the vapor layer at lower voltages which means a greater sensitivity to hydrodynamic instabilities at the vapor-liquid interface.

Status of the Transverse Diagnostics at FLASHForward

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.

FLASHForward: DOOCS Control System for a Beam-Driven Plasma-Wakefield Acceleration Experiment

The FLASHForward project at DESY is an innovative beam-driven plasma-wakefield acceleration experiment integrated in the FLASH facility, aiming to accelerate electron beams to GeV energies over a few centimeters of ionised gas. These accelerated beams are tested for their capability to demonstrate exponential free-electron laser gain; achievable only through rigorous analysis of both the driver and witness beams 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 readout and normal Linux computers. FLASH FLASH [1], a soft X-ray free-electron laser, is available to the photon science user community for experiments since 2005. Ultra-short X-ray pulses, shorter than 30 femtoseconds, are produced using the SASE process. The FLASH facility operates two SASE beamlines in parallel: FLASH1 & FLASH2 and as third beamline FLASHForward (see Fig. 1). Pulses of FLASH come in bursts of several hundred pulses with a repetition rate of 10 Hz. Figure 1: FLASH layout. DOOCS INSIDE FLASHFORWARD As FLASHForward is part of FLASH, it is obvious to us also the FLASH control system infrastructure DOOCS. The whole server architecture as existing hardware and software is a big advantage to integrate the needed detector hardware for FLASHForward. As most of the FLASHForward part is a laser system, diagnostic is slightly different as in FLASH. Laser alignment can be detected with chip cameras. Hence the amount of cameras is increasing easily to more than 70. Other components, like the timing system must be adapted in FLASHForward to get a synchronization to FLASH, which is necessary for all data taking. In the following chapters a short description of the most important components and systems are made. DOOCS CONTROL SYSTEM DOOCS, the Distributed Object Oriented Control System [2] was designed for FLASH. Currently it is extended to control the European XFEL accelerator (see Fig. 2). Recent developments for the client side applications are written in JAVA to allow them to be used on many computer platforms. This object oriented abstraction model helps for clean programming interfaces and in the overall system design including the hardware for a machine and is a significant step forward in the goal to improve software productivity and quality. Figure 2: DOOCS structure. COMPUTING HARDWARE MicroTCA.4, is a new standard form the PICMG to extend the applications of the existing μTCA crate system. This extension was developed in an international collaboration of High Energy Physics laboratories and many industrial partners within the PICMG organization. It is fully ___________________________________________ † sven.karstensen@desy.de Proceedings of IPAC2018, Vancouver, BC, Canada Pre-Release Snapshot 27-May-2018 12:00 UTC