[en] Laser accelerated proton beams have been used for field characterization in expanding plasmas. The Thomson parabola spectrometer, as a charged particles analyzer, also allows precise measurement of the charged particles' trajectories. The proton's deflections by fast changing plasma fields can be measured with the new design of the Thomson parabola spectrometer and, therefore, it can be applied for proton deflectometry. It is shown that from resulting spectrograms the plasma field dynamics can be reconstructed with high temporal resolution. In a proof-of-principle experiment, a weakly relativistic plasma expansion is studied as an example

[en] Specific ion spectra have been obtained by irradiating spherical and planar targets with 40 fs Ti:Sa laser pulses at intensities of ∼10¹⁹ W/cm². From the mass-limited spherical target, strong modulations in the proton/deuteron spectra and a high laser to ion energy conversion originate, whereas the planar target provides higher cutoff energies of protons. We compare qualitatively models in which the acceleration field is assigned to a multitemperature electron distribution and alternatively where multispecies ion acceleration is important, which both can account for the observed modulations in the spectra. The abundance of ion species and especially the observed strong suppression of the heavy ion species during the ion acceleration from planar targets are estimated on the basis of the interplay of ions with different mass during their ultrafast acceleration and the further ion-bunch propagation

[en] We report on our experiments on laser-driven ion acceleration using fully isolated mass-limited spheres with a diameter down to 8 μm for the first time. Two-dimensional (2D) particle-in-cell (PIC) and hydro-code simulations were used to show that the pre-plasma at both the front and rear sides of the target strongly affect the efficiency of the ion acceleration. The mechanism of the plasma flow around mass-limited targets has not yet been identified for laser-driven ion acceleration. Our models indicate that this effect is the cause of the observed limitation to the ion-beam energy in both previous experiments and in our own.

[en] We present a laser produced plasma (LPP) source optimized for metrology and the results of its radiometric characterization. The presented (LPP) source can be used for reflectometry and spectroscopy in the soft x-ray range. For these applications, stable operation with high spectral photon yields high reliability in continuous operation and, to reach high spectral resolution, a small source size and high source point stability is necessary. For the characterization of the source, special instruments have been designed and calibrated using the soft x-ray radiometry beamline of the Physikalisch-Technische-Bundesanstalt at BESSY. These instruments are an imaging spectrometer, a double multilayer tool for in-band power measurements, a transmission slit grating spectrograph, and a pinhole camera. From the measurements a source size of 30 μmx55 μm (2σ, horizontal by vertical) and a stability of better than 5 μm horizontally and 9 μm vertically were obtained. The source provides a flat continuous emission in the extreme ultraviolet (EUV) range around 13.4 nm and a spectral photon flux of up to 1*10¹⁴/(s sr 0.1 nm) at a pump laser pulse energy of 650 mJ. The shot-to-shot stability of the source is about 5% (1σ) for laser pulse energies above 200 mJ. It is shown that an Au-LPP source provides spectrally reproducible emission with sufficient power at low debris conditions for the operation of a laboratory based EUV reflectometer and for spectroscopy

[en] Particle-in-cell simulations of the absorption of an ultrashort and ultraintense laser pulse at structured targets are presented. The targets consist of a bunch of parallel or tapered carbon nanowires. Such targets are effective in generating large currents of relativistic electrons, propagating along the wires and following their curvature. Focusing the electron beam into one wire makes it possible to reduce the transverse size of the electron cloud in comparison to the diameter of the laser pulse. The density of the energy flux of the hot electrons in such a bunch, propagating along one nanowire, is in the excess of several times the intensity of the laser pulse. (paper)

[en] Applying a 21-channel Thomson spectrometer setup has revealed further insight to the connection between spatial and spectral beam characteristic of laser accelerated protons. Analyzing the central emission cone (plus/minus 3 degree) shows an increasing beam divergency for protons with increasing kinetic energies. This holds for protons emitted from the same source area at the target surface. The whole beam is a well ordered system with a clear functional dependence of trajectories on proton energy. This is a consequence of the source dynamics which is determined by the sheath development in time. Thus laser-driven ion beams can be advantageously manipulated for further propagation to an experiment. We demonstrate this capability with a magnetic quadrupole and obtain a nearly parallel and monochromatized beam. Furthermore we set our achievements in beam production efficiency into context with other laser systems and demonstrate the potential of very-thin target foils.

[en] The energy distribution and yield of electrons and hard x-ray photons were investigated by irradiating tungsten and tantalum targets with ∼ 30 fs pulses in the intensity range 10¹⁸ - 10¹⁹ W cm^-2 by using the Laboratoire d'Optique Appliquee (LOA) as well as the Max Born Institut (MBI) multiterawatt Ti:sapphire lasers. For the measurement of the hard x-ray emission in the energy range from 15 keV to 700 keV at the LOA a 9-channel spectrometer of calibrated thermoluminescence detectors (TLD) was used. The scaling of the hard x-rays was studied by varying the incident laser energy within one order of magnitude and the pulsewidth by a factor of 5. The hot electron output was investigated in the range 300 keV - 1 MeV with the new MBI Ti:sapphire laser by using a time-of-flight spectrometer. The results indicate a sensitive interplay between the temporal laser shape and laser intensity. (interaction of laser radiation with matter. laser plasma)

[en] An analytical distribution function is developed that describes the influence of a light ion component on the explosion of a spherically symmetric, charged cluster composed of two species, when the cluster is irradiated by an ultrashort, intense laser pulse. It is shown that the energy distribution of light ions can be used for diagnostics of the initial density profile of the plasma cluster. The evolution of the energy distribution of light ions is investigated as a function of their number in the cluster. It is possible to create a quasimonoenergetic distribution of the light ions at a specific proportion of the light ions and the degree of ionization of the heavier ion component. Analytical calculations of the explosion of 50 nm water clusters exposed by ultrashort and intense laser pulses are in good agreement with Particle-In-Cell simulations.

[en] Laser-driven ion acceleration is capable of generating ion beams of MeV energy exhibiting unique attributes such as ultralow emittance. Research is still focusing on fundamental laser-target interactions to control further beam attributes. In this Letter we present the observation of directional ion acceleration of irradiated spherical targets through proton imaging. This feature, together with an earlier observed quasimonoenergetic proton burst makes spherical targets extremely attractive candidates for high quality, high repetition rate sources of laser accelerated particles.

[en] 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.