Search

The insulator–metal transition in liquid hydrogen is an important phenomenon to understand the interiors of gas giants, such as Jupiter and Saturn, as well as the physical and chemical behavior of materials at high pressures and temperatures. Here, the path toward an experimental approach is detailed based on spectrally resolved x-ray scattering, tailored to observe and characterize hydrogen metallization in dynamically compressed hydrocarbons in the regime of carbon–hydrogen phase separation. With the help of time-dependent density functional theory calculations and scattering spectra from undriven carbon samples collected at the European x-ray Free-Electron Laser Facility (EuXFEL), we demonstrate sufficient data quality for observing C–H demixing and investigating the presence of liquid metallic hydrogen in future experiments using the reprated drive laser systems at EuXFEL.

Journal of synchrotron radiation 28(5), 28 (2021). doi:10.1107/S1600577521007335, The European XFEL delivers up to 27000 intense (>1012 photons) pulses per second, of ultrashort (≤50 fs) and transversely coherent X-ray radiation, at a maximum repetition rate of 4.5 MHz. Its unique X-ray beam parameters enable groundbreaking experiments in matter at extreme conditions at the High Energy Density (HED) scientific instrument. The performance of the HED instrument during its first two years of operation, its scientific remit, as well as ongoing installations towards full operation are presented. Scientific goals of HED include the investigation of extreme states of matter created by intense laser pulses, diamond anvil cells, or pulsed magnets, and ultrafast X-ray methods that allow their diagnosis using self-amplified spontaneous emission between 5 and 25 keV, coupled with X-ray monochromators and optional seeded beam operation. The HED instrument provides two target chambers, X-ray spectrometers for emission and scattering, X-ray detectors, and a timing tool to correct for residual timing jitter between laser and X-ray pulses., Published by Wiley-Blackwell, [S.l.]

We report on progress in the understanding of the effects of kilotesla-level applied magnetic fields on relativistic laser–plasma interactions. Ongoing advances in magnetic-field–generation techniques enable new and highly desirable phenomena, including magnetic-field–amplification platforms with reversible sign, focusing ion acceleration, and bulk-relativistic plasma heating. Building on recent advancements in laser–plasma interactions with applied magnetic fields, we introduce simple models for evaluating the effects of applied magnetic fields in magnetic-field amplification, sheath-based ion acceleration, and direct laser acceleration. These models indicate the feasibility of observing beneficial magnetic-field effects under experimentally relevant conditions and offer a starting point for future experimental design.

In this paper, we report on the acceleration of protons and oxygen ions from tens of micrometer large water droplets by a high-intensity laser in the range of 1020 W/cm2. Proton energies of up to 6 MeV were obtained from a hybrid acceleration regime between classical Coulomb explosion and shocks. Besides the known thermal energy spectrum, a collective acceleration of oxygen ions of different charge states is observed. 3D PIC simulations and analytical models are employed to support the experiential findings and reveal the potential for further applications and studies.

Creation of electrons and positrons from light alone is a basic prediction of quantum electrodynamics, but yet to be observed. Our simulations show that the required conditions are achievable using a high-intensity two-beam laser facility and an advanced target design. Dual laser irradiation of a structured target produces high-density γ rays that then create > 108 positrons at intensities of 2 × 1022 Wcm−2. The unique feature of this setup is that the pair creation is primarily driven by the linear Breit-Wheeler process (γγ → e+e−), which dominates over the nonlinear Breit-Wheeler and Bethe-Heitler processes. The favorable scaling with laser intensity of the linear process prompts reconsideration of its neglect in simulation studies and also permits positron jet formation at experimentally feasible intensities. Simulations show that the positrons, confined by a quasistatic plasma magnetic field, may be accelerated by the lasers to energies >200 MeV. Electron-positron pair generation from nonlinear quantum electrodynamics is predicted at high intensities that are, so far, beyond experimental capabilities. Here, simulations predict a high yield of positrons can be obtained from gamma-gamma photon collisions in the linear regime, using counter-propagating pulses and a microstructured target.

High-energy and high-intensity lasers are essential for pushing the boundaries of science. Their development has allowed leaps forward in basic research areas, including laser–plasma interaction, high-energy density science, metrology, biology and medical technology. The Helmholtz International Beamline for Extreme Fields user consortium contributes and operates two high-peak-power optical lasers using the high energy density instrument at the European X-ray free electron laser (EuXFEL) facility. These lasers will be used to generate transient extreme states of density and temperature to be probed by the X-ray beam. This paper introduces the ReLaX laser, a short-pulse high-intensity Ti:Sa laser system, and discusses its characteristics as available for user experiments. It will also present the first experimental commissioning results validating its successful integration into the EuXFEL infrastructure and viability as a relativistic-intensity laser driver.

We study the laser-driven acceleration of electrons from overdense hydrogen clusters to energies of up to 13 MeV in laser forward direction and several hundreds of keV in an outer ring-structure. The use of cryogenic hydrogen allows for high repetition-rate operation and examination of the influence of source parameters like temperature and gas flow. The outer ring-structure of accelerated electrons, originating from the interaction, that is robust against the change of laser and target parameters can be observed for low electron densities of ca 3 × 1016 cm−3. For higher electron densities, an additional central spot of electrons in the laser forward direction can be observed. Utilizing 3D particle-in-cell simulations, it is revealed that both electron populations mainly stem from ponderomotive scattering.

Using three-dimensional kinetic simulations, we examine the emission of collimated $\ensuremath{\gamma}$-ray beams from structured laser-irradiated targets with a prefilled cylindrical channel and its scaling with laser power (in the multi-PW range). The laser power is increased by increasing the laser energy and the size of the focal spot while keeping the peak intensity fixed at $5\ifmmode\times\else\texttimes\fi{}{10}^{22}\phantom{\rule{0.2em}{0ex}}\mathrm{W}/{\mathrm{cm}}^{2}$. The channel radius is increased proportionally to accommodate the change in laser spot size. The efficiency of conversion of the laser energy into a beam of MeV-level $\ensuremath{\gamma}$ rays (with a ${10}^{\ensuremath{\circ}}$ opening angle) increases rapidly with the incident laser power $P$ before it roughly saturates above $P\ensuremath{\approx}4\phantom{\rule{0.2em}{0ex}}\mathrm{PW}$. Detailed particle tracking reveals that the power scaling is a result of enhanced electron acceleration at higher laser powers. One application that directly benefits from such a strong scaling is pair production via two-photon collisions. We investigate two schemes for generating pairs through the linear Breit-Wheeler process: colliding two $\ensuremath{\gamma}$-ray beams and colliding one $\ensuremath{\gamma}$-ray beam with black-body radiation. The two scenarios project up to ${10}^{4}$ and ${10}^{5}$ pairs, respectively, for the $\ensuremath{\gamma}$-ray beams generated at $P=4\phantom{\rule{0.2em}{0ex}}\mathrm{PW}$. A comparison with a regime of laser-irradiated hollow channels corroborates the robustness of the setup with prefilled channels.

We present a study on laser-driven proton acceleration from a hydrogen cluster target. Aiming for the optimisation of the proton source, we performed a detailed parametric scan of the interaction conditions by varying different laser and the target parameters. While the underlying process of a Coulomb-explosion delivers moderate energies, in the range of 100 s of keV, the use of hydrogen as target material comes with the benefit of a debris-free, single-species proton acceleration scheme, enabling high repetition-rate experiments, which are very robust against shot-to-shot fluctuations.

Journal of Instrumentation 15(11), P11033 (2020). doi:10.1088/1748-0221/15/11/P11033, The von Hámos spectrometer setup at the HED instrument of the European XFEL is described in detail. The spectrometer is designed to be operated primarily between 5 and 15 keV to complement the operating photon energy range of the HED instrument. Four Highly Annealed Pyrolitic Graphite (HAPG) crystals are characterised with thicknesses of 40 μm or 100 μm and radius-of-curvature 50 mm or 80 mm, in conjunction with either an ePix100 or Jungfrau detector. The achieved resolution with the 50 mm crystals, operated between 6.5 and 9 keV, matches that reported previously: ~8 eV for a thickness of 40 μm, whereas, with an 80 mm crystal of thickness 40 μm, the resolution exceeds that expected. Namely, a resolution of 2 eV is demonstrated between 5–6 keV implying a resolving power of 2800. Therefore, we posit that flatter HAPG crystals, with their high reflectivity and improved resolving power, are a powerful tool for hard x-ray scattering and emission experiments allowing unprecedented measurements of collective scattering in a single shot., Published by Inst. of Physics, London

International audience; We present a proof-of-principle study demonstrating x-ray Raman Spectroscopy (XRS) from carbon samples at ambient conditions in conjunction with other common diagnostics to study warm dense matter, performed at the high energy density scientific instrument of the European x-ray Free Electron Laser (European XFEL). We obtain sufficient spectral resolution to identify the local structure and chemical bonding of diamond and graphite samples, using highly annealed pyrolytic graphite spectrometers. Due to the high crystal reflectivity and XFEL brightness, we obtain signal strengths that will enable accurate XRS measurements in upcoming pump-probe experiments with a high repetition-rate, where the samples will be pumped with high-power lasers. Molecular dynamics simulations based on density functional theory together with XRS simulations demonstrate the potential of this technique and show predictions for high-energy-density conditions. Our setup allows simultaneous implementation of several different diagnostic methods to reduce ambiguities in the analysis of the experimental results, which, for warm dense matter, often relies on simplifying model assumptions. The promising capabilities demonstrated here provide unprecedented insights into chemical and structural dynamics in warm dense matter states of light elements, including conditions similar to the interiors of planets, low-mass stars, and other celestial bodies.

Calculations on the dynamics of ions and electrons in near-infrared laser fields at intensities up to $3\ifmmode\times\else\texttimes\fi{}{10}^{23}\phantom{\rule{4pt}{0ex}}\mathrm{W}/{\mathrm{cm}}^{2}$ are presented. We explore the acceleration of ions in a laser focus by conservation of canonical momentum during ionization events and by the ponderomotive force in the $f$/1 focal geometry required to reach such intensity. At intensities exceeding ${10}^{23}\phantom{\rule{4pt}{0ex}}\mathrm{W}/{\mathrm{cm}}^{2}$, highly charged ions are expelled from the laser focus before they can interact with the laser pulse at peak intensity, decreasing the predicted ionization yields of deeply bound states. We consider the interaction of a tightly focused, $f$/1 laser pulse with krypton at an intensity of $3\ifmmode\times\else\texttimes\fi{}{10}^{23}\phantom{\rule{4pt}{0ex}}\mathrm{W}/{\mathrm{cm}}^{2}$ and a pulse duration of 140 fs. We find that the ions and electrons are accelerated to energies in excess of 2 MeV/nucleon and 1.4 GeV, respectively. Ponderomotive expulsion of the parent ions decreases the total number of ultrarelativistic above-threshold ionization electrons produced by tunneling ionization from the $K$-shell states of krypton but does not change their energy spectrum.

A high-power laser pulse guided through a relativistically underdense plasma channel generates a strong quasistatic magnetic field, confining the transverse motion of electrons inside the channel. This confinement allows the laser field to efficiently accelerate electrons transversely which are then deflected in forward direction and collimated by the channel's magnetic field, establishing the novel forward-sliding swing acceleration (FSSA) mechanism. Its advantage is a threshold behaviour, yielding high electron energies for sufficiently strong laser fields or initial electron momenta, regardless of a fine-tuned resonance. The achievable electron energies are demonstrated to be two orders of magnitude higher than the vacuum limit of direct laser acceleration. We study the electrons' dynamics by a simplified model analytically and confirm this model's predictions by numerical simulations. Specifically, we derive and confirm simple relations between the channel's magnetic field strength, the particles' initial transverse momenta and the laser's field strength indicating in what parameter regimes high electron energies can be achieved.

The development at STFC’s Central Laser Facility (CLF) of multi-slab gas cooled amplifier technology based on ceramic Yb:YAG, known as DiPOLE, is well known within the field [1]. Previous successes using this laser architecture include a 10 J, 10 Hz prototype system and demonstration of the world’s first 100 J, 10 Hz, 10 ns system (DiPOLE100), supplied to and commissioned at the HiLASE facility[2]. The success of the first kW-class 100 J nanosecond pulsed laser led to development of a second system to be made available to users of the HED Instrument at the European XFEL in collaboration with HiBEF / HZDR. This system, DiPOLE-100X, will be used for fundamental research into the structure and compression of materials, such as present in extrasolar planets [3]. DIPOLE-100X will provide nanosecond, user defined, temporally shaped pulses that once frequency doubled and focussed on target, create the pressure and temperature states of matter relevant to exoplanetary interiors. These changes can then be probed using the bright x-ray pulses of the European XFEL. DIPOLE-100X is scheduled for completion at the end of summer, before shipment to the European XFEL later in the year. In this paper we will present the operational and commissioning results for DiPOLE-100X, highlighting design changes and upgrades over the first generation, along with future developments of DiPOLE technology at the CLF.

When an ultrashort laser pulse is tightly focused to a size approaching its central wavelength, the properties of the focused spot diverge from the diffraction limited case. Here we report on this change in behavior of a tightly focused Petawatt class laser beam by an F/1 off-axis paraboloid (OAP). Considering the effects of residual aberration, the spatial profile of the near field and pointing error, we estimate the deviation in peak intensities of the focused spot from the ideal case. We verify that the estimated peak intensity values are within an acceptable error range of the measured values. With the added uncertainties in target alignment, we extend the estimation to infer on-target peak intensities of $\geq$ 10$^{22}$ W/cm$^{2}$ for a target at the focal plane of this F/1 OAP., 6 pages, 4 figures

The generation of high-order harmonics in ultraviolet spectral range by targets of periodic modulation interacting with relativistic, high contrast laser pulse (Iλ2= 1020W/cm2· μm2) was investigated experimentally and numerically. The spectral intensity of the 2nd- and 3rd-order harmonic emission from grating of different periodicities (250, 410, and 480 nm) is presented. The enhancement of the 3rd harmonic order compared with 2nd was observed for a grating of 480 nm periodicity. The experimental results indicate the role of the grating periodicity on the emission efficiency of different higher order harmonics in the UV spectral range. The higher order harmonics are emitted at the grating surface separated from the specular reflection of the laser pulse, due of the interference effects. In addition, 2D numerical PIC simulations demonstrate a complex angular distribution of the higher harmonics (HH) spectral intensity and confirm the strong dependence of the HH efficiency of a specific order on the grating periodicity, as observed experimentally. These special features of the high harmonic emission by periodically modulated targets open the route toward the control of HH spectral composition and of the emission efficiency of the lower order harmonics.

With the latest configuration, the Ti:Sa laser system ARCTURUS (Düsseldorf University, Germany) operates with a double-chirped pulse amplification (CPA) architecture delivering pulses with an energy of 7 J before compression in each of the two high-power beams. By the implementation of a plasma mirror system, the intrinsic laser contrast is enhanced up to $10^{-12}$ on a time scale of hundreds of picoseconds, before the main peak. The laser system has been used in various configurations for advanced experiments and different studies have been carried out employing the high-power laser beams as a single, high-intensity interaction beam ( $I\approx 10^{20}~\text{W}/\text{cm}^{2}$ ), in dual- and multi-beam configurations or in a pump–probe arrangement.

A high-intensity laser beam propagating through a dense plasma drives a strong current that robustly sustains a strong quasistatic azimuthal magnetic field. The laser field efficiently accelerates electrons in such a field that confines the transverse motion and deflects the electrons in the forward direction. Its advantage is a threshold rather than resonant behavior, accelerating electrons to high energies for sufficiently strong laser-driven currents. We study the electron dynamics via a test-electron model, specifically deriving the corresponding critical current density. We confirm the model's predictions by numerical simulations, indicating energy gains two orders of magnitude higher than achievable without the magnetic field.

In this paper we review the design and development of a 100 J, 10 Hz nanosecond pulsed laser, codenamed DiPOLE100X, being built at the Central Laser Facility (CLF). This 1 kW average power diode-pumped solid-state laser (DPSSL) is based on a master oscillator power amplifier (MOPA) design, which includes two cryogenic gas cooled amplifier stages based on DiPOLE multi-slab ceramic Yb:YAG amplifier technology developed at the CLF. The laser will produce pulses between 2 and 15 ns in duration with precise, arbitrarily selectable shapes, at pulse repetition rates up to 10 Hz, allowing real-time shape optimization for compression experiments. Once completed, the laser will be delivered to the European X-ray Free Electron Laser (XFEL) facility in Germany as a UK-funded contribution in kind, where it will be used to study extreme states of matter at the High Energy Density (HED) instrument.

We report on the development of a 100 J, 10 Hz cryogenic gas cooled multi-slab Yb:YAG DPSSL system capable of generating temporally shaped pulses from 2 ns to 15 ns. This system will be used for high energy density (HED) experiments at the European X-ray Free Electron Laser (XFEL) facility.

International audience; The ability of an intense laser pulse to propagate in a classically over-critical plasma through the phenomenon of relativistic transparency is shown to facilitate the generation of strong plasma magnetic fields. Particle-in-cell simulations demonstrate that these fields significantly enhance the radiation rates of the laser-irradiated electrons, and furthermore they collimate the emission so that a directed and dense beam of multi-MeV gamma-rays is achievable. This capability can be exploited for electron-positron pair production via the linear Breit-Wheeler process by colliding two such dense beams. Presented simulations show that more than $10^3$ pairs can be produced in such a setup, and the directionality of the positrons can be controlled by the angle of incidence between the beams.

The combination of powerful optical lasers and an x-ray free-electron laser (XFEL) provides unique capabilities to study the transient behavior of matter in extreme conditions. The high energy density science instrument (HED instrument) at the European XFEL will provide the experimental platform on which an unique x-ray source can be combined with various types of high-power optical lasers. In this paper, we highlight selected scientific examples together with the associated x-ray techniques, with particular emphasis on femtosecond (fs)-timescale pump–probe experiments. Subsequently, we present the current design status of the HED instrument, outlining how the experiments could be performed. First user experiments will start at the beginning of 2018, after which various optical lasers will be commissioned and made available to the international scientific community.

A high-repetition rate laser-driven proton source from a continuously operating cryogenic hydrogen cluster target is presented. We demonstrate a debris-free, Coulomb-explosion based acceleration in the 10s of kilo-electron-volt range with a stability of about 10% in a 5 Hz operation. This acceleration mechanism, delivering short pulse proton bursts, represents an ideal acceleration scheme for various applications, for example, in materials science or as an injector source in conventional accelerators. Furthermore, the proton energy can be tuned by varying the laser and/or cluster parameters. 3D numerical particle-in-cell simulations and an analytical model support the experimental results and reveal great potential for further studies, scaling up the proton energies, which can be realized with a simple modification of the target.A high-repetition rate laser-driven proton source from a continuously operating cryogenic hydrogen cluster target is presented. We demonstrate a debris-free, Coulomb-explosion based acceleration in the 10s of kilo-electron-volt range with a stability of about 10% in a 5 Hz operation. This acceleration mechanism, delivering short pulse proton bursts, represents an ideal acceleration scheme for various applications, for example, in materials science or as an injector source in conventional accelerators. Furthermore, the proton energy can be tuned by varying the laser and/or cluster parameters. 3D numerical particle-in-cell simulations and an analytical model support the experimental results and reveal great potential for further studies, scaling up the proton energies, which can be realized with a simple modification of the target.

Investigations of energy transfer of high intensity (I = 5 × 1019 W/cm2), ultrashort (

The interaction of high-intensity laser pulses with matter releases instantaneously ultra-large currents of highly energetic electrons, leading to the generation of highly-transient, large-amplitude electric and magnetic fields. We report results of recent experiments in which such charge dynamics have been studied by using proton probing techniques able to provide maps of the electrostatic fields with high spatial and temporal resolution. The dynamics of ponderomotive channeling in underdense plasmas have been studied in this way, as also the processes of Debye sheath formation and MeV ion front expansion at the rear of laser-irradiated thin metallic foils. Laser-driven impulsive fields at the surface of solid targets can be applied for energy-selective ion beam focusing.

We have completed a pulse contrast upgrade on the Texas Petawatt Laser. This improvement enables the use of thin and reduced mass targets for ion acceleration, and reduces pre-plasma effects on all experiments. The new design starts with two BBO-based OPCPA stages pumped by an optically synchronized 8-ps laser. These stages amplify slightly chirped few ps pulses by six orders of magnitude. Next there are two LBO-based OPCPA stages that are pumped by 4 ns pulses. With much less gain than before, parametric fluorescence has been reduced by about three orders of magnitude. Prior to the upgrade, lenses caused pencil beam prepulses. Since tilting or wedging lenses was not a viable option, we replaced all lenses in the glass amplifiers with off-axis parabolic mirrors. There are still weak prepulses that we attribute to surface scattering. We eliminated thin transmissive optics to avoid post pulses that would result in prepulses by nonlinear (B-integral) conversion. This required us to reduce from eight to four passes in the 64-mm glass amplifier and to add a two-pass 25-mm "booster amplifier." As a final upgrade we added an Acousto-Optic Programmable Dispersive-Filter (AOPDF) to improve higher order dispersion and steepen the rising edge of the compressed pulse.

Dilute plasmas surrounding mass‐limited targets provide sufficient current for influencing strong fields, which are built up due to the interaction of an intense and ultrafast laser pulse. Such situation occurs, where evaporation of the target surface is present. The high‐intensity laser pulse interacts with the quasi‐isolated mass‐limited target and the spatial wings of the intensity distribution account for ionization of the ambient plasma. A fast change of strong electrical fields following intense laser irradiation of water droplets (16 micron diameter) has been measured with proton imaging. An analytical model explains charge transport accounting for the observation.

The proton probing technique has been used to investigate the incidence of a mid-l019 W cm-2 pulse with metalhc wire and laminar foam targets. Electric fields ∼ l010 V-m-1 are measured on the surface of the 125 μm-diameter wire in the wake of the laser interaction as it charges and discharges within a 20 ps temporal window, whilst the employment of a novel experimental technique permits the observation of the propagation of a charging front at ∼ c away from the point of interaction. In the foam shots, meanwhile, the behaviour of the hot electrons generated by the interaction pulse is probed inside the target. Evidence of electric inhibition effects and filamentation is found.

The interaction of an extremely intense laser beam with the dense plasma generated on a solid target can lead tocoherent electron dynamics at the vacuum plasma interface which causes the emission of ultra short light burstswithin each optical cycle of the driving laser pulse. Therefore, the resulting pulse train consists of high harmonicsof the laser fundamental frequency. We report on the, to our knowledge, “rst observation of surface-harmonicsup to the 42nd order without any contrast improvement by a plasma mirror. In the spectral region from the25th to the 40th order approximately 10 7 photons were detected in a solid angle of 4 ·10 Š 4 sr.Keywords: surface harmonics, attosecond pulses, laser-plasma interaction 1. INTRODUCTION Phase locked dynamics of electrons in laser “elds can lead to the emission of attosecond light bursts and opennovel opportunities to probe matter at on previously unaccessible short timescales. 1,2 High-harmonic generationin gases is a well-known way to generate these short pulses.

Due to the envisioned advantages of mass‐limited targets for laser driven ion beams, which are high efficiency and high cut‐off energies, their field dynamics are of special interest. Micro‐water droplets can be used as mass‐limited targets with a high repetition rate. Our investigations show that the surrounding dilute plasma of such liquid spheres influences the interaction. We review our experimental findings together with computer simulations and conclude on the different processes in electron transport and related acceleration fields for mass‐limited targets and foils, respectively.

The experimental arrangement for the investigation of high-field laser-induced processes using a broadband proton probe beam has been modified to enable the detection of the ultrafast motion of field fronts. It is typical in such experiments for the target to be oriented perpendicularly with respect to the principal axis of the probe beam. It is demonstrated here, however, that the temporal imaging properties of the diagnostic arrangement are altered drastically by placing the axis (or plane) of the target at an oblique angle to the transverse plane of the probe beam. In particular, the detection of the motion of a laser-driven field front along a wire at a velocity of (0.95+/-0.05)c is described.

International audience; The past few years have seen remarkable progress in the development of laser-based particle accelerators. The ability to produce ultrabright beams of multi-megaelectronvolt protons routinely has many potential uses from engineering to medicine, but for this potential to be realized substantial improvements in the performances of these devices must be made. Here we show that in the laser-driven accelerator that has been demonstrated experimentally to produce the highest energy protons, scaling laws derived from fluid models and supported by numerical simulations can be used to accurately describe the acceleration of proton beams for a large range of laser and target parameters. This enables us to evaluate the laser parameters needed to produce high-energy and high-quality proton beams of interest for radiography of dense objects or proton therapy of deep-seated tumours.

The proton acceleration from a thin foil irradiated by a laser pulse at relativistic intensities is a process highly dependent on the electron dynamic at the rear side of the foil. By reducing the lateral size of the laser irradiated foil the hot electrons are confined in a small volume leading to an enhancement of both the maximum proton energy and the conversion efficiency in the target normal sheath acceleration regime. In this paper we demonstrate that an optimal lateral size of the target can be found. While a smaller target surface leads to a better hot electron confinement and enhances the Debye sheath accelerating the protons, it also leads to an increase of preplasma formation due to limited laser contrast available experimentally and hence to a decrease of the proton acceleration. The experimentally found optimum is in good agreement with analytic theory and 2D particle in cell simulations. In addition, the maximum proton energy as a function of pulse duration has been investigated. The experimental results fit to an analytical model.

Spatially and temporally resolved measurements of runaway electrons have been performed in the TEXTOR tokamak. The main diagnostic for the investigations is a newly developed runaway probe which is mounted at a reciprocating mechanism. In addition, the runaway electrons are studied in the plasma by synchrotron emission. The scanning probes consists of fluorescing crystals covered by a graphite housing and is located at different radial positions. In this study, two generations of the probes are presented, a simple one for proving the principle and a more advanced one which allows the measurement of the energy distribution of the runaways. A strong decay of the runaway electrons at the plasma edge was observed. Runaway electrons have been measured at different plasma densities by synchrotron radiation and the probe. The reduction of the number of runaways have been measured with increasing plasma densities during low density plasma discharges by both diagnostics. The spectrum of runaway electrons produced during low density plasma discharges was measured. Finally, measurements of runaways with high temporal resolution of 0.05 ms have been carried out during a disruption discharge.