A major challenge facing the study of radiation in matter on ultrafast time scales is the need for an absolute timing reference. Typically, these type of experiments are performed using pulses of ions from radio frequency accelerators which have nanosecond scale pulse durations and timing jitter making high temporal resolution measurements difficult to achieve. Here, we show that a combination of a highly synchronised probe pulse with a muti-species laser-driven radiation source can allow for the absolute timing of radiation in matter. This is primarily due to the generation of a X-ray calibration fiducial enabling the study of ion-induced dynamics in matter on ultrafast timescales.
Modern laser-based accelerators for ions reach peak kinetic ion energies of > 100MeV, over 1MA of total beam currents with only a few picoseconds of bunch duration in close vicinity to the target at ≈ 1 Hz repetition rate and with a high controllability. Thus, the number of potential applications is growing rapidly. This raises a high interest in the processes of ion-matter-interactions in the energy deposition region of these ultra-intense particle bunches. In our recent experiments we investigated these interactions by single-shot time-resolved optical streaking of the energy deposition region of laser-accelerated proton bunches in liquid water. The absolute timing reference provided by the x-rays emitted from the laser-plasma-interaction and the sub-ps time resolution revealed that ionized electrons solvate > 20 ps delayed compared to experiments with lower deposited energy density. In this paper we discuss first approaches to explain these observations by micro-dosimetric considerations regarding the background molecules excitation of vibration states and polarization. This is highly relevant for applications, e.g. to understand the FLASH-effect in radio-biology. We further present the planned experiments at the Centre for Advanced Laser Applications where these phenomena will be investigated in more detail with advanced diagnostics.
The development of laser technologies to produce extreme intensities has allowed the generation of high-order harmonics from relativistic laser-plasma mirror interactions to become attainable to observe experimentally. Numerical plasma simulations are invaluable for understanding the dynamic processes underpinning this mechanism. However, accuracy in describing high-frequency electromagnetic waves is challenging. Finite Difference Time Domain methods give rise to numerical dispersion when used to solve Maxwell’s equations, inducing a dispersive change in vacuum refractive index, which causes significant errors in physical properties of the reflected field, such as an angular deviation in the harmonic spatial profiles from the predicted specular reflection. EPOCH Particle-In-Cell (PIC) code is used to perform two-dimensional (2D) simulations to extensively study and control the effects of numerical dispersion on the generated harmonics for several Maxwell solvers. Effects on angular deviation across a range of angles of incidence and strategies to mitigate dispersive effects via controlling interaction geometry are discussed.
Intense attosecond scale pulses of extreme-ultraviolet and soft X-ray light can be generated from plasma surfaces driven relativistically by intense laser pulses. The temporal profile consists of a train of pulses separated by the laser’s optical period and manifests in the spectral domain as harmonics of the laser frequency. Isolating individual attosecond pulses is a key challenge for applications of these sources to time-resolved experiments for attosecond science and plasma-based sources allow the use of ultra-high energies and intensities that can enable fully attosecond scale pump-probe measurements. Results are presented here for numerical Particle-In-Cell simulations of a scheme to angularly sweep the pulses so that one is temporally gated out from the others in the reflected direction. Using two identical laser pulses that are incident noncollinearly on the surface with a time delay causes the instantaneous wavefront to sweep between each of them with the attosecond pulses also being swept in their emission angle accordingly. This method naturally separates out the remaining reflected laser energy due to the angular gap between the incident pulses negating the need for spectral filtering after the interaction. We demonstrate clear gating of a single pulse along the reflected axis in both 2D and 3D simulations and discuss the effect of spectral isolation from the laser frequency. We extend the investigation to further examine techniques to improve the temporal gating by tailoring the laser and target properties.
The process of high-harmonic generation from solid density surfaces results in a train of ultrashort attosecond pulses. Isolating a single attosecond pulse is of interest for many reasons such as atomic and molecular time-scale pump-probe experiments.1 We highlight, via Particle-in-Cell simulations, a viable route to pulse isolation using two colour fields with off integer harmonic frequencies, which allows subtle shaping of the incident laser field which when reflected by the plasma mirror, diminishes some pulses in the train resulting in the potential to isolate a single attosecond pulse.
Ionisation dynamics on the nanoscale seed the processes that govern pathways to macroscopic equilibrium in irradiated matter. Therefore, understanding the conditions that underpin this transition is critical in a wide range of applications from healthcare to radiation science. Here we investigate these interactions in real time using a novel optical streaking technique that exploits the ultrafast nature and highly synchronous pump-probe capabilities of laser driven accelerators. This work reveals behaviour in the recovery of matter irradiated by protons and X-rays that can only be reconciled by considering the nanoscopic structure of the irradiated matter.
Quasi-phase matching (QPM) can be used to increase the conversion efficiency of the high harmonic generation
(HHG) process. We observed QPM with an improved dual-gas foil target with a 1 kHz, 10 mJ, 30 fs laser
system. Phase tuning and enhancement were possible within a spectral range from 17 nm to 30 nm. Furthermore
analytical calculations and numerical simulations were carried out to distinguish QPM from other effects, such
as the influence of adjacent jets on each other or the laser gas interaction. The simulations were performed with
a 3 dimensional code to investigate the phase matching of the short and long trajectories individually over a
large spectral range.
An ultra-relativistic electron beam passing through a thick, high-Z solid target triggers an electromagnetic cascade, whereby a large number of high energy photons and electron-positron pairs are produced. By exploiting this physical process, we present here the first experimental evidence of the generation of ultra-short, highly collimated and ultra-relativistic positron beams following the interaction of a laser-wakefield accelerated electron beam with high-Z solid targets. Clear evidence has also been obtained of the generation of GeV electron-positron jets with variable composition depending on the solid target material and thickness. The percentage of positrons in the overall leptonic beam has been observed to vary from a few per cent up to almost fifty per cent, implying a quasi-neutral electron-positron beam. We anticipate that these beams will be of direct relevance to the laboratory study of astrophysical leptonic jets and their interaction with the interstellar medium.
Arik Willner, Michael Schulz, Robert Riedel, Mark Yeung, Thomas Dzelzainis, Christos Kamperidis, Makis Bakarezos, Darryl Adams, Vladislav Yakovlev, Juliane Rönsch-Schulenburg, Brendan Dromey, Nektarios Papadogiannis, Michael Tatarakis, Stefan Düsterer, Siegfried Schreiber, Bart Faatz, Markus Drescher, Jörg Rossbach, Matthew Zepf, Franz Tavella
Improved performance of Free Electron Laser (FEL) light sources in terms of timing stability, pulse shape and spectral
properties of the amplified FEL pulses is of interest in many fields of science. A promising scheme is direct seeding with
High-Harmonic Generation (HHG) in a noble gas target. A Free-Electron-Laser seeded by an external XUV-source is
planned for FLASH II at DESY in Hamburg. The requirements for the XUV/soft X-ray source can be summarized as
follows: A repetition rate of at least 100 kHz in a 10 Hz burst is needed at variable wavelengths from 10 to 40 nm and
pulse energies of several nJ within single harmonics.
This application requires a laser amplifier system with exceptional parameters, mJ-level pulse energy, sub-10 fs pulse
duration at 100 kHz (1 MHz) burst repetition rate. A new OPCPA system is under development in order to meet these
requirements, and very promising results has been achieved. In parallel to this development, a new High- Harmonic
Generation concept is necessary to sustain the high average power of the driving laser system and for the need of high
conversion efficiencies. Highest conversion efficiency in High Harmonic Generation has been shown using gas-filled
capillary targets, up to now. For our application, only a free-jet target is applicable for high harmonic generation at high
repetition rate, to overcome damage threshold limitations of HHG target optics. A new multi-jet target is under
development and first tests show a good performance of this nozzle configuration.
T. Dzelzainis, D. Doria, S. White, M. Makita, G. Narsisyan, D. Marlow, R. Stefanuik, H. Ahmed, C. Seeley, D. Riley, B. Dromey, L. Romagnani, M. Zepf, M. Borghesi, C. L. S. Lewis
We report on the results of an experiment using the TARANIS laser system at Queen's University, Belfast (QUB) to
pump Ni-like X-Ray Lasers (XRLs) in the GRazing Incidence Pumped (GRIP) configuration. The system uses a long
1.2ns pulse to create a pre-plasma at the correct ionization stage, and a short, ~800fs pulse to produce a population
inversion. Strong lasing has been observed for Ni-ions of Mo and Ag. Mo exhibited gain on two laser lines, at 18.9nm
and 22.6nm, whilst only a single line, at 13.9nm, has been observed for Ag. The growth curves for both elements are
presented. The curve for Ag indicates that saturation has not been achieved. Saturation like behaviour is seen for Mo but
the small signal gain and poor fit to the Linford formula indicate that the roll-off is attributable to some effect other than
gain saturation. Axial non-uniformity in the gain and mis-match between the ASE group velocity and the traveling-wave
excitation are discussed as possible explanations for the shape of the Mo growth curve. Results of an initial application to
characterize image plate as a soft x-ray detector are presented and, finally, further possible applications, in particular the
potential for the XRL to be used as a photon source for Thomson scattering, are investigated.
The interaction of relativistically intense (Iλ2>>1.3 1018Wcm-2μm2) laser pulses with a near step-like plasma density
profile results in relativistic oscillations of the reflection point. This process results in efficient conversion of the incident
laser to a phase-locked high harmonic spectrum, which allows the generation of attosecond pulses and pulse trains.
Recent experimental results on efficiency scaling, highest harmonic generated and beam quality suggest that very high
focused intensities can be achieved opening up the possibility of ultra-intense attosecond X-ray interactions for the first
time.
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