Search

Laser-driven proton/ion acceleration is a rapidly developing research field attractive for both fundamental physics and applications such as hadron therapy, radiography, inertial confinement fusion, and nuclear/particle physics. Laser-driven proton/ion beams, compared to those obtained in conventional accelerators, have outstanding features such as low emittance, small source size, ultra-short duration and huge acceleration gradient of ∼1 MeV μm−1. We report proton acceleration from ultrathin polymer targets irradiated with linearly polarized, 30-fs, 1-PW Ti:sapphire laser pulses. A maximum proton energy of 45 MeV with a broad and modulated profile was obtained when a 10-nm-thick target was irradiated at a laser intensity of 3.3 × 1020 W/cm2. The transition from slow (I1/2) to fast scaling (I) of maximum proton energy with respect to laser intensity I was observed and explained by the hybrid acceleration mechanism including target normal sheath acceleration and radiation pressure acceleration in the acceleration stage and Coulomb-explosion-assisted free expansion in the post acceleration stage.

A new technique to investigate laser driven ion acceleration with fully isolated, mass-limited glass spheres with a diameter down to 8 μ m is presented. A Paul trap was used to prepare a levitating glass sphere for the interaction with a laser pulse of relativistic intensity. Narrow-bandwidth energy spectra of protons and oxygen ions have been observed and were attributed to specific acceleration field dynamics in case of the spherical target geometry. A general limiting mechanism has been found that explains the experimentally observed ion energies for the mass-limited target.

A third-order correlator suitable for detailed shape measurements of picosecond laser pulses has been developed. The working principle in both the single shot and the scanning mode is based on detection of the phase-matched difference frequency non-collinear generated signal in a non-linear crystal. This third-order OPA correlator was applied for the characterization of the specifically shaped picosecond laser pulses from the MBI CPA Nd: glass laser system.

Terawatt laser systems are affected by the problem that optical components, typically installed in vacuum, get contaminated with carbon. In this paper, we describe an “on-line” cleaning system, based on a radiofrequency discharge, to avoid carbon production or to clean optical components if carbon is already deposited. The advantage of such a cleaning system is the direct installation along the beam line which not only allows a fast and reproducible cleaning of the components without removing them from the beam line, but also allows a preventive treatment of the most sensitive optics from further contamination.

We present the design of a titanium–sapphire Ti : Sa laser system for generating 100TW laser pulses. The main aspects of the multiterawatt Ti : Sa laser architecture which can allow to reach the focusable intensity exceeding 10 20 W / cm 2 are discussed.

The satellite structure accompanying the Al Lyα emission from plasma regions close to the surface of the laser-irradiated targets is studied. In spectra produced at constant laser energy but variable pulse duration we identify the situations where the resonance line is suppressed and only the emission from long-wavelength satellites is observed. The experimental data are compared with results of calculations based on two-dimensional hydrodynamics and multilevel collisional-radiative codes. Simulations indicate that these spectra originate from cold dense plasma created inside the crater several tens of micrometers under the surface of the solid target.

While huge progress has been achieved for EUVL system design and multilayer optics during the last years the decision on the best suited source is still open. In a German basic research cooperation on short-wavelength plasma based sources some key issues also relevant for EUVL are addressed: the comparison of existing sources, investigations on the scalability of source concepts and the demonstration of key features. Preliminary results of investigations on parameters for best conversion efficiency of laser produced plasmas, concepts for high-power lasers and scalability of gas discharge based sources are presented. Comparability of results is assured by calibrated metrology tools which are cross checked with ASML’s flying circus.

For laser plasma diagnostic purposes, a specific Thomson spectrometer with a multi-pinhole array at the entrance aperture is proposed. The ion beam under investigation is thus divided into small beamlets and the obtained spectral image combines the recorded spectra of the ions, which are emitted under different angles of the source. This resembles a simultaneous measurement with many independent spectrometers. Thereby, the emission characteristic of the ion source can be reconstructed. The quantitative analysis of the ion emission spectra allows one to gain detailed information about the accelerating sheath field profile and dynamics.

We describe an experiment demonstrating XUV amplification following collisional excitation in a capillary discharge plasma irradiated by a picosecond IR laser pulse. Guiding and temporally resolved transmission of the pump laser beam are also demonstrated and analysed. The short pump laser pulse heated rapidly the electrons producing amplification in the 3 p 1 S 0 –3 s 1 P 1 transition of Ne-like sulphur at 60.84 nm. The estimated gain–length product was equal to 6.8, while the beam divergence reached 2.5 mrad for 30 mm capillary. This new, hybridly pumped collisional soft X-ray laser with the transient gain offers a new way towards efficient table-top XUV sources.

Simulations are presented for the collisionally pumped germanium X-ray laser working in the transient gain scheme. The influence of the shape, length, intensity and energy of the preforming pulse on the gain and X-ray output are analysed. The hydrodynamics simulations post-processed with 3D raytracing give additional information about the effective gain and the output signals determined by the refraction due to the plasma density gradients. The modelling presented shows to what extent the plasma preforming plays a deciding role for the improvement of the lasing conditions. Dependence on the main pulse delay is tested as well. There are indications that significant gain and output signal could be created with a subpicosecond main driving pulse of reduced total energy provided the preforming pulse is optimised.

We have measured the absolute photon number of single-shot high-order harmonic spectra generated by focusing intense short pulsed laser radiation into a gas target with a calibrated extreme ultraviolet (EUV) spectrometer. For the calibration, we used a compact hollow-cathode source as a radiometric transfer standard providing radiation from rare gas plasma emission lines of known radiant intensity in the spectral range from 13.1 nm to 58.4 nm. The photon number measurement is based on the comparison of the detected signals of the high-order harmonics and the hollow-cathode emission lines with nearly identical spectrometer set-ups. This technique provides a measurement of the harmonics photon number with a relative uncertainty of less than 50% (1σ).

Lasing in a titanium XII plasma at 32.6 nm and ∼ 30 nm as well as in vanadium XIII at 30.4 nm using a short pulse driven transient inversion population has been realized. Gain performances and temporal characteristics of the X-ray signal output at only few Joules of pump energy are described.

A hybrid Ti:Sa Nd:glass laser is described delivering two synchronized laser pulses with duration of 0.7 ps and 1.2 ns and output energies of 4 J in the short pulse and 8 J in the long pulse. The system was successfully used to drive efficiently a transient inversion XUV laser on [Ne]-like ions [see Nickles et al., SPIE Vol. 2520].

We present a compact FROG system for determination of both pulse shape and phase of sub-picosecond pulses in a single-shot measurement. The system utilizes third-order auto-correlation due to the induced optical Kerr effect in heavy flint glass (SF6). The optical layout allows to use single Calcite wedges as high-contrast polarizers. Instead of using a separate spectrgraph, we incorporated a grating and appropriate imaging optics directly into the optical scheme. The FROG system has proven to be a convenient measurement tool to optimally align the grating compressor of CPA laser systems.

A 1.5 picosecond laser pulse with an intensity of more than 1018 W/cm2 was used to heat solid state magnesium targets. The unique characteristics of this plasma source together with a high resolution recording system ( λ Δλ = 10 4 ) allowed us to study multicharged ions with L- and M-shell dielectronic satellite structures in the wavelength range of 9.2–9.5 A. Ten spectral lines determined by 1s2p4l–1s24l and 1s2p3l–1s23l transitions in Mg X and of 43 spectral lines of Mg IX were identified and measured. Parameters of the plasma in the process of its evolution were deduced.

It is shown that a compact, all glass laser using fiberless CPA technique is a potential system for generation of extreme high contrast picosecond pulses in the terawatt power range. At present the system delivers 1.5 ps pulses of 1.5 J energy with an intensity contrast of 4×109.

We report on a specially designed actively-passively modelocked Nd-glass laser. Negative feedback control and frequency tuning by a Lyot filter allows the generation of subpisecond pulses if the generated wavelength is shifted to the red side of the fluorescence spectrum of the active material. Experimental results and possible explanations for the observed spectral broadening and pulse shortening are given and discussed in the approximation of a linear amplification model. The oscillator should be useful for the chirped pulse amplification experiments.