The increasing demand for high laser powers is placing huge demands on current laser technology. This is now reaching a limit, and to realise the existing new areas of research promised at high intensities, new cost-effective and technically feasible ways of scaling up the laser power will be required. Plasma-based laser amplifiers may represent the required breakthrough to reach powers of tens of petawatt to exawatt, because of the fundamental advantage that amplification and compression can be realised simultaneously in a plasma medium, which is also robust and resistant to damage, unlike conventional amplifying media. Raman amplification is a promising method, where a long pump pulse transfers energy to a lower frequency, short duration counter-propagating seed pulse through resonant excitation of a plasma wave that creates a transient plasma echelon that backscatters the pump into the probe. Here we present the results of an experimental campaign conducted at the Central Laser Facility. Pump pulses with energies up to 100 J have been used to amplify sub-nanojoule seed pulses to near-joule level. An unprecedented gain of eight orders of magnitude, with a gain coefficient of 180 cm−1 has been measured, which exceeds high-power solid-state amplifying media by orders of magnitude. High gain leads to strong competing amplification from noise, which reaches similar levels to the amplified seed. The observation of 640 Jsr−1 directly backscattered from noise, implies potential overall efficiencies greater than 10%.
F. Hanton, D. Doria, K. Kakolee, S. Kar, S. Litt, F. Fiorini, H. Ahmed, S. Green, J. C. Jeynes, J. Kavanagh, D. Kirby, K. J. Kirkby, C. Lewis, M. Merchant, G. Nersisyan, R. Prasad, K. Prise, G. Schettino, M. Zpef, M. Borghesi
The potential that laser based particle accelerators offer to solve sizing and cost issues arising with conventional proton
therapy has generated great interest in the understanding and development of laser ion acceleration, and in investigating
the radiobiological effects induced by laser accelerated ions. Laser-driven ions are produced in bursts of ultra-short
duration resulting in ultra-high dose rates, and an investigation at Queen’s University Belfast was carried out to
investigate this virtually unexplored regime of cell rdaiobiology. This employed the TARANIS terawatt laser producing
protons in the MeV range for proton irradiation, with dose rates exceeding 109 Gys-1 on a single exposure. A clonogenic
assay was implemented to analyse the biological effect of proton irradiation on V79 cells, which, when compared to data
obtained with the same cell line irradiated with conventionally accelerated protons, was found to show no significant
difference. A Relative Biological effectiveness of 1.4±0.2 at 10 % Survival Fraction was estimated from a comparison
with a 225 kVp X-ray source.
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 evolution of the transmission of extreme ultra-violet (EUV) light from a germanium backlighter through heated thin iron targets has been measured at laser irradiances of about 8×1016 W cm-2. A rapid increase in transmission from 0 to 30% in 20 ps was observed. A two dimensional radiation hydrodynamics model was used to simulate the heating of the plasma and the transmission of EUV light as a function of time. The tamped iron targets were heated up to an average electron temperature of about 55 eV and a mass density of approximately 0.6 g cm-3. The transmission measurements are in reasonable agreement with modelling results. The experimental layout is similar to an X-ray laser experiment and therefore, for relatively low plasma temperatures, these kinds of experiments can be done in combination with X-ray laser experiments, giving transmission data for a range of wavelengths rather than a single X-ray laser wavelength.
Using the grazing incidence pumping technique with a 600 mJ, 500 ps background pulse and a 250 mJ, 200 fs main
pulse the lasing emission from a molybdenum target has been studied. A flat field spectrometer designed to observe the
X-ray laser emission in both the first and second orders was used to record the time integrated data. Time resolved data
was obtained by installing an Axis-Photonique PX1 X-ray streak camera to observe the first order output from the
spectrometer whilst retaining the time integrated second order observation. In this paper both time integrated and time
resolved data are presented for a range of grazing angles, target lengths, delays between pumping pulses and pumping
energy. Comparisons are also drawn with simulations from the Ehybrid and Medusa codes. An additional experiment is
also described in which a two colour pumping method is used to investigate lasing at short wavelength from high Z
targets (Z ≥ 62).
The Axis-Photonique PX1 fast X-ray streak camera records the temporal structure of events with picosecond accuracy.
Using a potassium iodide photocathode the streak camera has been characterised for the effects of space charge. In a
recent grazing incidence pumping X-ray laser experiment the streak camera was coupled to the output of a flat field
spectrometer to observe first order diffraction. The second order was observed using a CCD camera. In this paper data
is presented from this experiment comparing the brightness of the X-ray laser emission with the dispersion of the
streaked image both temporally and spectrally (non temporal direction). Consequently measurements of the dynamic
range of the streak camera are made. The results are compared with data from previous experiments.
Experimental measurements of the opacity of plasmas at densities close to solid state and temperatures ~ 60 - 300 eV using a probing X-ray laser are presented. Utilizing thin targets, opacities of iron have been measured using x-ray lasers of photon energy 89 eV created by pumping with the VULCAN RAL laser. The thin targets are separately heated by spot focus laser pulses. We have demonstrated that X-ray laser brightness is sufficient to overcome the self-emission of hot plasma so that useful opacity measurements can be made. Due to their high brightness, x-ray lasers can fulfill a useful niche in measuring opacity and other phenomena associated with laser-plasma interactions. Quantities such as opacity measured in laser-plasmas are useful elsewhere. For example, plasma opacity is important in understanding radiative transfer in the sun.
X-ray point projection absorption spectroscopy (X-PPAS) is a long established and extremely useful dynamic diagnostic of hydrodynamics, ionization balance, etc. in laser produced plasmas. It has however, to date, been exclusively the preserve of large-scale laser facilities and also used to probe plasmas formed by relatively short pulse (< 1 ns) lasers. We report here the design and selective instrumental performance measures of a new table-top scale X-ray spectrometer system which forms the core of the X-PPAS system. Our design goal is to make X-PPAS a routine diagnostic and also apply it to the measurement of deep inner-shell photoabsorption by plasma atoms and ions.
Below saturation, X-ray laser output shows a reduction in pulse duration and frequency bandwidth as the gain-length product increases. Above saturation, both quantities can be expected to rebroaden. The duration of gain can be close to an order-of-magnitude longer than the output pulse duration. With gain-length products just below saturation, X-ray lasing at 13.9 nm in nickel-like silver has been measured with a pulse duration Δt of 3 - 4 ps and an estimated frequency bandwidth Δv of 5×1011 Hz. Such values imply that the pulses are close to transform limited with Δt Δv ≈ 1.5. Measurements of x-ray laser pulse-lengths and gain duration will described in this paper.
We present the longitudinal coherence measurement of the transient inversion collisional x-ray laser for the first time. The Ni-like Pd x-ray laser at 14.68 nm is generated by the LLNL COMET laser facility and is operating in the gain-saturated regime. Interference fringes are produced using a Michelson interferometer setup in which a thin multilayer-coated membrane is used as a beam splitter. The longitudinal coherence length for the picosecond duration 4d1S0 -> 4p1P1 lasing transition is determined to be ~400 µm (1/e HW) by adjusting the length of one interferometer arm and measuring the resultant variation in fringe visibility. This is four times improved coherence than previous measurements on quasi-steady state schemes largely as a result of the narrower line profile in the lower temperature plasma. The inferred gain-narrowed linewidth of ~0.29 pm is also substantially narrower than previous measurements on quasi-steady state x-ray laser schemes. This study shows that the coherence of the x-ray laser beam can be improved by changing the laser pumping conditions. The x-ray laser is operating at 4 - 5 times the transform-limited pulse.
Metrology of XUV beams and more specifically X-ray laser (XRL) beam is of crucial importance for development of applications. We have then developed several new optical systems enabling to measure the x-ray laser optical properties. By use of a Michelson interferometer working as a Fourier-Transform spectrometer, the line shapes of different x-ray lasers have been measured with an unprecedented accuracy (δλ/λ~10-6). Achievement of the first XUV wavefront sensor has enable to measure the beam quality of laser-pumped as well as discharge pumped x-ray lasers. Capillary discharge XRL has demonstrated a very good wavefront allowing to achieve intensity as high 3*1014 Wcm-2 by focusing with a f = 5 cm mirror. The measured sensor accuracy is as good as λ/120 at 13 nm. Commercial developments are under way.
We report here some observations and preliminary findings from a study focussed on the vacuum-UV (λ, 40-60 nm) radiation emitted during the interaction of 150 ps laser pulses (100-400 mJ) with copper pre-plasmas formed by an electro-optically synchronised (0.1 - 0.8 J, 8 ns) long pulse laser. We have observed significant gains in VUV flux that scale with inter-laser delay. We also report preliminary observations on total X-ray emission from the interaction of a superintense 80 fs, 200 mJ laser pulse at the UK ASTRA laser facility with a similar pre-plasma at irradiances approaching 1019 W/cm2
This paper summarizes our recent progress achieved in the characterization and understanding of the Ni-like Ag transient x-ray laser pumped under traveling wave irradiation. At the Rutherford Laboratory CPA laser facility, we measured the temporal history of the 13.9 nm laser pulse with a high-resolution streak camera. A very short, approximately 2 ps x-ray laser pulse was directly demonstrated for the first time. More recently we carried out an experiment at the LULI CPA laser facility. Several diagnostics that recorded the plasma emission at the XRL wavelength or in the keV range indicate the presence of small-scale spatial structures in the emitting XRL source. Single-shot Fresnel interferograms at 13.9 nm were successfully obtained with a good fringe visibility. Strong lasing was also observed on the Ni-like 4f-4d line at 16 nm.
We present a review of new progress performed in several laboratories (Laboratoire pour l'Utilisation des lasers Intenses, Rutherford Appleton Laboratory, Prague Asterix Laser System, Institute of Laser Engineering, Laboratoire d'Optique Appliquee). Concerning the realization of x-ray lasers sources, using different laser pumping techniques (600 ps, 100 ps, ns/ps, OFI) and the optimization of their optical properties, using curved and plane half-cavity mirrors. In parallel of these developments, we present the main results obtained with x-ray laser in interferometry applications. These studies concern on the one hand the Michelson interferometry with an x-ray laser emitting at 13.9 nm (recently realized at LULI), and on the other hand the Fresnel bi-mirror with an x-ray laser emitting at 21.2 nm (recently realized at PALS).
Ne-like and Ni-like ions have been pumped in the transient gain regime using intense picosecond pump pulses from Vulcan in its CPA mode. High gain coefficients of at least approximately 30/cm are observed for the Ne-like ions Ti XIII, Ge XXIII and Sn XXIII at wavelengths 31.2, 19.6 and 12.0 nm respectively and approximately 20/cm for the Ni-like ion Sm XXXV at 7.3 nm. Saturated output is found in all cases for target lengths shorter than 10 mm and the effect of traveling wave pumping has been studied and unequivocably demonstrated. An experimental campaign to observe four wave mixing using a soft x-ray laser and an optical laser in a sum-difference frequency mixing scheme has been initiated. Preliminary results are described and future directions discussed.
Saturated operation of an X-ray laser is desirable as a high output irradiance is obtained with reduced shot-to-short variation. The potential of saturated X-ray laser output in probing plasma samples is first investigated. The laser pumping requirements to scale Ni-like saturated X-ray laser output to shorter wavelengths is then analyzed using published atomic physics data and a simple 4-level laser model for gain. A model of amplified spontaneous emission has been modified to accurately predict experimentally observed saturation behavior obtained in different experiments at the Rutherford Appleton Laboratory. In particular, the effects of traveling wave pumping with short duration (approximately 1 ps) laser pulses are investigated. Simulations of Ne-like Ge resonance line emission are compared to experimentally measured spectra.
Spatially and temporally varying neutral, ion and electron number densities have been mapped out within laser ablated plasma plumes expanding into vacuum. Ablation of a magnesium target was performed using a KrF laser, 30 ns pulse duration and 248 nm wavelength. During the initial stage of plasma expansion (t <EQ 100 ns) interferometry has been used to obtain line averaged electron number densities, for laser power densities on target in the range 1.3 - 3.0 X 108 W/cm2. Later in the plasma expansion (t equals 1 microsecond(s) ) simultaneous absorption and laser induced fluorescence spectroscopy has been used to determine 3D neutral and ion number densities, for a power density equal to 6.7 X 107 W/cm2. Two distinct regions within the plume were identified. One is a fast component (approximately 106 cm-1) consisting of ions and neutrals with maximum number densities observed to be approximately 30 and 4 X 1012 cm-3 respectively, and the second consists of slow moving neutral material at a number density of up to 1015 cm-3. Additionally a Langmuir probe has been used to obtain ion and electron number densities at very late times in the plasma expansion (1 microsecond(s) <EQ t <EQ 15 microsecond(s) ). A copper target was ablated using a Nd:YAG laser, 7.5 ns duration and 532 nm (2 (omega) ) wavelength, with a power density on target equal to 6 X 108 W/cm2. Two regions within the plume with different velocities were observed. Within a fast component (approximately 3 X 106 cms-1) electron and ion number densities of the order 5 X 1012 cm-3 were observed and within the second slower component (approximately 106 cms-1) electron and ion number densities of the order 1 - 2 X 1013 cm-3 were determined.
Low pump energy transient gain x-ray lasers in Ti at 32.6 nm, 30.15 nm, in V at 30.4 nm and Ge at 19.6 nm using picosecond pulse heating of a long pulse preformed plasma of neonlike ions has been realized for the first time. Gain saturation was demonstrated in Ti and Ge XRL. Results of pump consumption, x- ray divergence and output energy are given.
This work has consisted in demonstrating that high gain can be achieved by pumping x-ray lasers (XRL) with a combination of a high intensity and short duration driving pulses (approximately 100 ps). Short pulses are very well suited for pumping collisional XRL since a high lasant ion density, electron density and temperature can be achieved simultaneously. We have successfully tested this pumping scheme on the 4d-4p (J equals 0 - 1) transition of Ni-like tin (lambda approximately 11.93 nm) and silver (lambda approximately 13.89 nm) as well as on the 3p-3s (J equals 0 - 1) Ne-like iron (lambda approximately 25.5 nm) at an intensity of approximately 2 X 1013 Wcm-2 (130 ps in duration). The driving laser (lambda equals 1.06 micrometer) was composed of three pulses (a prepulse and two main pulses). Large amplifications were demonstrated in tin and silver (respectively GL approximately 12 and GL approximately 16). Finally, the saturation of the 3p-3s (J equals 0 - 1) transition of Ne-like iron at 25.5 nm was achieved on both pumping pulses, using a prepulse of 109 Wcm-2. A gain coefficient of 15 plus or minus 3 cm-1 (GL approximately 26 plus or minus 5) on the first main pulse and 12 plus or minus cm-1 (GL approximately 23 plus or minus 2) on the second one was measured.
We report the first demonstration of saturation in nickel-like x-ray lasers, specifically nickel-like Ag, In, Sn, and Sm x- ray lasers at wavelengths of 14, 12.6, 12.0 and 7.3 nm respectively. These x-ray lasers were found to be very monochromatic x-ray sources with the laser lines completely dominating the output spectra. Using high-resolution spatial imaging and angularly resolved streaking techniques, the output source sizes as well as the time histories, divergences, energies and spatial profiles of these x-ray lasers have been fully characterized. The output intensities of these x-ray lasers were measured to be in the range of 0.7 - 2 X 1011 W (DOT) cm-2 in approximately 40 ps. The high monochromaticity, narrow divergence, short pulse duration, high efficiency and high brightness of these x-ray lasers make them ideal candidates for many applications.
The use of multi-pulse irradiation of neon-like ions has been shown to produce orders-of-magnitude enhancement of x-ray laser output. Recent results obtained at the Rutherford Appleton Laboratory are reviewed with an emphasis on understanding the reasons for the enhancement. Simulations with the fluid and atomic physics code EHYBRID are used to show that enhancement occurs because of a spatial enlargement of the gain region and consequent better propagation of the x- ray laser beam along the gain region.
Through the use of time-integrated space-resolved keV spectroscopy, we investigate line plasmas showing gain for irradiation using the prepulse technique. The experiments were conducted with the LULI laser of the Ecole Polytechnique, Palaiseau (France), at 1.06 micrometer with prepulse-to-main pulse intensity ratio ranging from 10-6 to 10-2. The particular x-ray lasers which were studied were the collisionally excited Ne-like zinc, copper and nickel systems. The effect of the prepulses on plasma conditions are inferred through spectroscopic line ratio diagnostics. It is observed that the value of the electron temperature for each system does not vary significantly with prepulse levels, nor does their spatially resolved profile along the line. The lateral width and density of the Ne-like regions in the plasmas of all three x-ray lasers are seen to increase with the prepulse level.
We describe the use of a dispersive Young's slits coherence diagnostic for determining the transverse coherence length of a soft x-ray laser source. W have studied the emission from the Ne-like germanium x-ray lasing lines at 23.2/23.6 nm for comparison with the 19.6 nm lasing line obtained with the use of a prepulse. The effective source size has been determined for both these output modes from the calculated coherence length. We conclude that the use of a prepulse leads to a significant reduction in effective source size at 19.6 nm.
Current successful approaches for achieving soft x-ray lasing typically require pumping laser pulses of duration approximately ns and energy approximately kJ (collisionally pumped schemes) or approximately ps pulses and powers of approximately several TW (recombination-pumped schemes). For applications, it is important to improve the efficiency of soft x-ray lasers and so reduce the required power of pumping lasers. The effect of pre- pulse on neon-like collisionally pumped lasers has been investigated using the LULI laser (Ecole Polytechnique, France). A small pre-pulse level approximately 10-3 of the main pulse energy was found to increase the J equals 0 minus 1 neon-like zinc laser output at 21 nm by an order-of-magnitude with a comparable increase in efficiency. A double pumping laser pulse on neon-like yttrium lasing output at 15 nm obtained with the VULCAN laser (Rutherford Appleton Laboratory, England) was also found to increase the x-ray lasing efficiency. With adiabatically cooled recombination lasing, it is shown that approximately 2 ps pulses are optimum for achieving the desired ionization balance for lasing output. The possibility of achieving recombination lasing at short wavelengths on lithium-like ions with longer pulse lasers has been investigated using the ASTERIX laser (Max-Planck Quantenoptik, Germany). These results are presented and interpreted to provide possible directions for improving the efficiency of x-ray lasers.
The main feature of x-ray laser research at LULI is the development of a saturated laser at 212 angstrom with a relatively small pump laser of 0.4 kJ in 600 ps. The laser works with the 3p- 3s J equals O yields 1 transition of neon-like zinc, by using the double-pass of amplified radiation in the active medium. Plasma parameters (temperature, density, homogeneity), and x-ray laser emission properties (intensity, pointing angle, divergence, and coherence) have been studied. Lasing action needs the main laser pulse to be preceded by a ten-prepulse train (contrast ratio less than 103) due to the remnant oscillator. The effect of a single prepulse was investigated as a function of contrast ratio and delay between the prepulse and the main pulse.
It is shown that curved slab targets are effective in compensating x-ray refraction due to electron density gradient in the expanding plasma. Significant improvement in the beam divergence and laser intensity has been observed. Soft x-ray laser of 1 mrad divergence has been generated in double- pass amplification of a collisionally-excited Ne-like Ge laser with the curved target. Generation of a polarized beam with a polarizing half cavity is described. Initial results of in-line holography as well as Fourier transform holography using the Ge laser as the light source are also presented.
Michael Key, W. Blyth, Gerald Cairns, A. Damerell, A. Dangor, Colin Danson, J. Evans, Graeme Hirst, M. Holden, Chris Hooker, J. Houliston, J. Krishnan, Ciaran Lewis, J. Lister, Andrew MacPhee, Z. Najmudin, David Neely, Peter Norreys, Allen Offenberger, Karoly Osvay, Geoffrey Pert, S. Preston, Stuart Ramsden, Ian Ross, Wilson Sibbett, Gregory Tallents, C. Smith, Justin Wark, Jie Zhang
An injector-amplifier architecture for XUV lasers has been developed and demonstrated using the Ge XXIII collisional laser. Results are described for injection into single and double plasma amplifiers. Prismatic lens-like and higher order aberrations in the amplifier are considered. Limitations on ultimate brightness are discussed and also scaling to operation at shorter wavelengths. A preliminary study has been made of UV multiphoton ionization using 300 fs pulses at high intensity.
We have successfully demonstrated double pass enhancement of amplified spontaneous emission of soft x rays, 23.2 and 23.6 nm of 3p - 3s transitions in Ne-like Ge, using an x-ray multilayer mirror. In this paper, we report on the fabrication of the mirror and analysis of its damage suffered during the experiments. The mirror used was a Mo-Si multilayer mirror with the reflectivity of 35% at the wavelength of 23.6 nm, deposited by an rf-sputtering system. In the damaged area of the mirror, only the multilayer was locally evaporated and the bare substrate underneath appeared. The size of the damaged area corresponded to the aperture size. We carried out the simulation on the spatial and temporal distribution of the mirror temperature during the experiment. Assuming that thermal x rays enter the mirror with the largest amount of energy among all the fluxes at the early stage of the enhancement, the result of the simulation can explain the damage feature and the temporal profile of the intensity of the amplified spontaneous emissions.
Basic characteristics of soft x-ray lasers generated as amplified spontaneous emission are described. Experimental results on soft x-ray amplification in neon-like germanium ions, obtained recently at the Institute of Laser Engineering, are reported. By comparing the experimental results with a simplified model on amplified spontaneous emission, basic parameters such as coherence and brightness of the Ge soft x-ray laser are evaluated.
Systematic study of the collisionally pumped Ne-like germanium 3p-3s laser system has led to the observation of saturated output on the J = 2-1 lines at 23.2 and 23.6 nm when a double slab target is double passed using a normal incidence concave mirror. The about-1-MW output power on the same lines from a simple double plasma has been used to record preliminary images with a Schwarzchild condenser and zone plate microscope arrangement. Initial analysis of measurements on the 4d-4p J = 1-0 line at 7.3 nm from nickel-like samarium indicate a gain coefficient of between 0.4/cm and 0.8/cm for an incident irradiance of about 2 x 10 exp 13 W/sq cm using a l.06-micron heating laser on slab targets of SmF3.
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