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Numerous experiments on laser-driven proton acceleration in the MeV range have been performed with a large variety of laser parameters since its discovery around the year 2000. Both experiments and simulations have revealed that protons are accelerated up to a maximum cut-off energy during this process. Several attempts have been made to find a universal model for laser proton acceleration in the target normal sheath acceleration regime. While these models can qualitatively explain most experimental findings, they can hardly be used as predictive models, for example, for the energy cut-off of accelerated protons, as many of the underlying parameters are often unknown. Here we analyze experiments on laser proton acceleration in which scans of laser and target parameters were performed. We derive empirical scaling laws from these parameter scans and combine them in a scaling law for the proton energy cut-off that incorporates the laser pulse energy, the laser pulse duration, the focal spot radius, and the target thickness. Using these scaling laws, we give examples for predicting the proton energy cut-off and conversion efficiency for state-of-the-art laser systems.

The properties of laser-accelerated ion bunches are demanding and require development of suitable beam diagnostics. In particular, the short and intense particle bunches with a broad energy spectrum emitted in conjunction with a strong electromagnetic pulse (EMP) are challenging for conventional and well established monitoring systems. An approach based on measuring the acoustic signals of particles depositing their energy in water, referred to as ionoacoustics [https://doi.org/10.1118/1.4905047] was recently developed into Ion-Bunch Energy Acoustic Tracing (I-BEAT)\cite{haffa_i-beat:_2019}. I-BEAT allows online detection of single proton bunches while being cost effective and EMP resistant. A simple water phantom equipped with only one ultrasound transducer positioned on the ion axis allows for reconstructing a rather complex energy spectrum that is typical for (manipulated) laser-accelerated ion bunches. To deduce the lateral bunch properties, additional transducers can be added, for example perpendicular to the ion beam axis. This established setup has been adapted for use closely behind the laser target and tested at the PHELIX laser at GSI. The capability of the system to retrieve information about the broad proton spectrum close to the source despite the harsh conditions has been demonstrated. Future improvements are required, most importantly the increase of dynamic range. Nevertheless, I-BEAT holds promise to evolve into an online diagnostic tool particularly suited for laser-driven source development and optimization at high repetition rates.

The evolution of dense plasmas prior to the arrival of the peak of the laser irradiation is critical to understanding relativistic laser plasma interactions. The spectral properties of a reflected laser pulse after the interaction with a plasma can be used to gain insights about the interaction itself, whereas the effect of holeboring has a predominant role. Here we developed an analytical model, describing the non-relativistic temporal evolution of the holeboring velocity in the presence of an arbitrary overdense plasma density and laser intensity profile. We verify this using two-dimensional particle-in-cell simulations, showing a major influence on the holeboring dynamic depending on the density profile. The influence on the reflected laser pulse has been verified during an experiment at the PHELIX laser. We show that this enables the possibility to determine the sub-micrometer scale length of the preplasma by measuring the maximum holeboring velocity and acceleration during the laser-plasma interaction., Experimental measurements involving multiple laser and plasma parameters are useful in understanding the relativistic laser-plasma interactions. Here the authors extend the model of holeboring for arbitrary profiles of laser pulse and plasma scale lengths.

High power laser science and engineering 8, e24 (2020). doi:10.1017/hpl.2020.23, Published by Cambridge Univ. Press, Cambridge

High power laser science and engineering 7, e62 (2019). doi:10.1017/hpl.2019.48, Published by Cambridge Univ. Press, Cambridge

AIP Advances 10(3), 035023 (2020). doi:10.1063/1.5117236, Published by American Inst. of Physics, New York, NY

We report on the development and implementation of a time resolved backscatter diagnostics for high power laser plasma experiments at the petawatt-class laser facility PHELIX. Pulses that are backscattered or reflected from overcritical plasmas are characterized spectrally and temporally resolved using a specially designed second harmonic generation frequency resolved optical gating system. The diagnostics meets the requirements made by typical experiments, i.e., a spectral bandwidth of more than 30nm with sub-nanometer resolution and a temporal window of 10ps with 50fs temporal resolution. The diagnostics is permanently installed at the PHELIX target area and can be used to study effects such as laser-hole boring or relativistic self-phase-modulation which are important features of laser-driven particle acceleration experiments.

A well-preserved isolated manus impression evidences the presence of derived aquatic adaptations in the forelimb morphology of Berriasian turtles. Size and the abundant co-occurring turtle fauna indicate that the track was left by a large-sized, bottom-walking basal pancryptodiran. The footprint shows an interesting mosaic of primitive and derived features. The basic turtle pattern is reflected by a short and broad autopodium but thin phalanges, claw reduction and extensive webbing enclosing the digits and lateral and medial margins of the distal metapodium are clearly specialized features related to the aquatic environment. Some characters including the proportional elongation of the first digit, which reached as far distally as the other digits and was probably highly mobile at the metacarpal-phalangeal joint, indicate an even higher degree of specialization. The specimen gives evidence on a hitherto unknown range of appendicular modification in Early Cretaceous turtles, and redundantly proves the aquatic habitat and locomotion of large-sized turtles from the Berriasian of northwestern Germany.

The main focus of this work is to show influences of the cooling rate on the microstructure and wear performance of Fe–Cr–C hardfacings. The coatings were produced by welding. The weld overlays were cooled from the molten stage in selected variations: (i) standard cooling – cooling on air at the steel welding table; (ii) passive cooling – cooling of the base material during the hardfacing process with a water cooled cooper plate with 12 °C; (iii) active cooling – cooling of the welding seam behind the main arc with CO 2 gas at temperature of −80 °C. For each cooling situation the temperature of the weld seam was measured in the melt pool with help of thermocouple. It was shown, that different cooling conditions can lead to the significant changes in the cooling-off time t 8/5 , dilution of the hardfacing with substrate and microstructural features of the hardfacings under investigation. Three different wear tests (three-body abrasion according to ASTM G65 standard, impact/abrasion and solid particle erosion) were performed to study the wear performances of Fe–Cr–C hardfacing. Testing results have shown that if the cooling-off time t 8/5 is too short it influences the brittleness and increases the residual stresses of the Fe–Cr–C hardfacing, which lowers its impact resistance. The effect of the cooling rate is in good correlation with wear test results and gives promise for further microstructure and process optimisation for the improved tribological performance.

We apply Fourier-transform spectral interferometry (FTSI) to study the interaction of intense laser pulses with ultrathin targets. Ultrathin submicrometer-thick solid CH targets were shot at the PHELIX laser facility with an intensity in the mid to upper 10^{19} W/cm^{2} range using an innovative double-pulse structure. The transmitted pulse structure was analyzed by FTSI and shows a transition from a relativistic transparency-dominated regime for targets thinner than 500 nm to a hole-boring-dominated laser-plasma interaction for thicker targets. The results also confirm that the inevitable preplasma expansion happening during the rising slope of the pulse, a few picoseconds before the maximum of the pulse is reached, cannot be neglected and plays a dominant role in laser-plasma interaction with ultrathin solid targets.

Laser-driven neutrons are generated by the conversion of laser-accelerated ions via nuclear reactions inside a converter material. We present results from an experimental campaign at the PHELIX laser at GSI in Darmstadt where protons and deuterons were accelerated from thin deuterated plastic foils with thicknesses in the μm and sub-μm range. The neutrons were generated inside a sandwich-type beryllium converter, leading to reproducible neutron numbers around 1011 neutrons per shot. The angular distribution was measured with a high level of detail using up to 30 bubble detectors simultaneously. It shows a laser forward directed component of up to 1.42 × 1010 neutrons per steradian, corresponding to a dose of 43 mrem scaled to a distance of 1 m from the converter.

A monolithic integrated modulator driver with a data decision function for high-speed optical fiber links is presented. The integrated circuit (IC) was manufactured in a 0.2-/spl mu/m gate length AlGaAs/InGaAs high electron mobility transistor technology with an f/sub T/ of 68 GHz. The modulator driver IC features differential configuration and operates up to 40 Gb/s with a clock phase margin of 210/spl deg/ and an output voltage swing of 2.9 V/sub p-p/ at each output. The maximum slew rate of the output signal is 200 mV/ps. The power dissipation of the circuit is 1.6 W using a single supply voltage of -5 V.

Two integrated circuits, a modulator driver and a photoreceiver integrating a metal-semiconductor-metal (MSM) photodetector, a differential transimpedance amplifier and two limiting amplifier stages for high-speed optical-fiber links are presented. The IC's were manufactured in a 0.2 /spl mu/m gate-length AlGaAs-GaAs high-electron mobility transistor (HEMT) technology with a f/sub T/ of 60 GHz. The modulator driver IC operates up to 25 Gb/s with an output voltage swing of 3.3 V/sub p-p/ at each output. The 1.3-1.55 /spl mu/m wavelength monolithically integrated photoreceiver optoelectronic integrated circuit (OEIC) has a bandwidth of 17 GHz with a high transimpedance gain of 12 k/spl Omega/. Eye diagrams are demonstrated at 20 Gb/s with an output voltage of 1 V/sub p.p/.

Vertically compact Al x Ga 1-x As/GaAs laser structures with Al 0.3 Ga 0.7 As/AlAs SPSL-cladding layers were grown on top of HEMT structures by MBE and investigated with regard to series resistance. We found an exponential dependence of series resistance R s with AlAs and Al 0.3 Ga 0.7 As SPSL-layer thicknesses at ambient temperature. Laser-HEMT structures for monolithic integration were grown and lasers were processed. For 3 x 200 μm 2 3-QW-lasers, suitable for high-frequency performance, threshold currents of 20 mA and series resistances below 12 Ω were obtained. The slope of the linear regression of the I th values as a function of mesa width yielded a low threshold current density j th of 480 A/cm 2 . Reduction of the p-cladding thickness from 600 nm down to 450 nm shows no increase in threshold current density if the p-dopants are kept from diffusing into the active region. This clearly demonstrates that laser structures for monolithic integration can be designed very compact without loosing performance.

During the past five years numerous mixed signal integrated circuits (ICs) have been designed, processed, and characterized based on our 0.2 /spl mu/m gate length AlGaAs/GaAs quantum well HEMT technology. Utilizing the inherent advantages of the AlGaAs/GaAs material system, optical, analog, microwave, and digital functions have been integrated monolithically. Examples are a chip set for 40 Gb/s optoelectronic data transmission systems, 15 and 34 GHz PLLs, a 35 GHz phase shifter for phased array antenna applications, a 2-kb ROM with subnanosecond access time for direct digital signal synthesis, and a 6-k gate array.

Monolithic integration of lasers and photodetectors with electronic circuits promises improved manufacturability, smaller size, lower power, and hence lower costs. This paper reviews the activities of the Fraunhofer IAF on optoelectronic integrated circuits (OEICs) for serial and parallel optical links.

The successful application of direct laser modulation for very-high-speed digital transmission or microwave/millimeter-wave analog optical links requires: reductions in the drive currents required to achieve high modulation bandwidths, increases in the maximum intrinsic modulation bandwidths, reductions in laser chirp under high-speed direct modulation, and high-speed laser structures which can be monolithically integrated with high-speed transistors. Due to the maturity of GaAs transistor and circuit fabrication technologies, we have pursued the development of GaAs-based MQW lasers which address a11 of the above requirements. In addition, we have demonstrated the feasibility of utilizing the impurity-free interdiffusion (IFID) process to fabricate multi-wavelength, high-speed, pseudomorphic laser arrays; such components promise substantial increases in both single-fiber transmission capacity and network flexibility via wavelength division multiplexing (WDM).

A molecular beam epitaxy (MBE) grown (GaAs/AlGaAs multi-quantum well laser, monolithically integrated with a laser driver, was realised on 2 inch GaAs substrate wafers. In an optical data communication setup, performance up to data rates of 7.4 Gbit/s was demonstrated.

Monolithic integration of lasers and photodetectors with electronic circuits promises higher bandwidth, improved manufacturability, smaller size, lower power and hence lower costs. This paper reviews the activities of the Fraunhofer IAF on optoelectronic integrated circuits (OEICs) for serial and parallel optical links.

The on-going developments in laser acceleration of protons and light ions, as well as the production of strong bursts of neutrons and multi-$$\hbox {MeV}$$ MeV photons by secondary processes now provide a basis for novel high-flux nuclear physics experiments. While the maximum energy of protons resulting from Target Normal Sheath Acceleration is presently still limited to around $$100 \, \hbox {MeV}$$ 100 MeV , the generated proton peak flux within the short laser-accelerated bunches can already today exceed the values achievable at the most advanced conventional accelerators by orders of magnitude. This paper consists of two parts covering the scientific motivation and relevance of such experiments and a first proof-of-principle demonstration. In the presented experiment pulses of $$200 \, \hbox {J}$$ 200 J at $$\approx \, 500 \, \hbox {fs}$$ ≈ 500 fs duration from the PHELIX laser produced more than $$10^{12}$$ 10 12 protons with energies above $$15 \, \hbox {MeV}$$ 15 MeV in a bunch of sub-nanosecond duration. They were used to induce fission in foil targets made of natural uranium. To make use of the nonpareil flux, these targets have to be very close to the laser acceleration source, since the particle density within the bunch is strongly affected by Coulomb explosion and the velocity differences between ions of different energy. The main challenge for nuclear detection with high-purity germanium detectors is given by the strong electromagnetic pulse caused by the laser-matter interaction close to the laser acceleration source. This was mitigated by utilizing fast transport of the fission products by a gas flow to a carbon filter, where the $$\upgamma$$ γ -rays were registered. The identified nuclides include those that have half-lives down to $$39 \, \hbox {s}$$ 39 s . These results demonstrate the capability to produce, extract, and detect short-lived reaction products under the demanding experimental condition imposed by the high-power laser interaction. The approach promotes research towards relevant nuclear astrophysical studies at conditions currently only accessible at nuclear high energy density laser facilities.

In this presentation the various technology steps for the monolithic integration of GaAs quantum well lasers with Double Pulse Doped AlGaAs/GaAs/AlGaAs Quantum Well (DPDQW) E/D HEMT electronics on a single substrate in one process run are described. All layers are grown by molecular beam epitaxy. The laser structure, consisting of three 74 Å GaAs quantum wells between two AlGaAs cladding layers, are grown on top of the electronic structure. The laser mesas and contact areas are defined by a combined wet and dry etch process. Apart from the transistor gates which are exposed by electron beam lithography, all lithography steps are performed using contact printing. A two layer metallization is used to interconnect the devices whereby air-bridges are used to connect the laser mesas to the electronics. First results showed laser action of laser diodes of area 3 x 300 μm2 at a threshold current of less than 60 mA, as well as the operation of different electronic devices on wafers which have been processed in this way. These include a laser diode driver, and an optoelectronic receiver with a MSM photo diode, both devices operating at a data rate of 5 Gbit/sec. These results indicate that the process sequence described is suitable for the integration of laser diodes and HEMT electronics.