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Six months of stratospheric aerosol observations with the European Aerosol Research Lidar Network (EARLINET) from August 2017 to January 2018 are presented. The decay phase of an unprecedented, record-breaking stratospheric perturbation caused by wildfire smoke is reported and discussed in terms of geometrical, optical, and microphysical aerosol properties. Enormous amounts of smoke were injected into the upper troposphere and lower stratosphere over fire areas in western Canada on 12 August 2017 during strong thunderstorm–pyrocumulonimbus activity. The stratospheric fire plumes spread over the entire Northern Hemisphere in the following weeks and months. Twenty-eight European lidar stations from northern Norway to southern Portugal and the eastern Mediterranean monitored the strong stratospheric perturbation on a continental scale. The main smoke layer (over central, western, southern, and eastern Europe) was found at heights between 15 and 20 km since September 2017 (about 2 weeks after entering the stratosphere). Thin layers of smoke were detected at heights of up to 22–23 km. The stratospheric aerosol optical thickness at 532 nm decreased from values > 0.25 on 21–23 August 2017 to 0.005–0.03 until 5–10 September and was mainly 0.003–0.004 from October to December 2017 and thus was still significantly above the stratospheric background (0.001–0.002). Stratospheric particle extinction coefficients (532 nm) were as high as 50–200 Mm−1 until the beginning of September and on the order of 1 Mm−1 (0.5–5 Mm−1) from October 2017 until the end of January 2018. The corresponding layer mean particle mass concentration was on the order of 0.05–0.5 µg m−3 over these months. Soot particles (light-absorbing carbonaceous particles) are efficient ice-nucleating particles (INPs) at upper tropospheric (cirrus) temperatures and available to influence cirrus formation when entering the tropopause from above. We estimated INP concentrations of 50–500 L−1 until the first days in September and afterwards 5–50 L−1 until the end of the year 2017 in the lower stratosphere for typical cirrus formation temperatures of −55 ∘C and an ice supersaturation level of 1.15. The measured profiles of the particle linear depolarization ratio indicated a predominance of nonspherical smoke particles. The 532 nm depolarization ratio decreased slowly with time in the main smoke layer from values of 0.15–0.25 (August–September) to values of 0.05–0.10 (October–November) and < 0.05 (December–January). The decrease of the depolarization ratio is consistent with aging of the smoke particles, growing of a coating around the solid black carbon core (aggregates), and thus change of the shape towards a spherical form. We found ascending aerosol layer features over the most southern European stations, especially over the eastern Mediterranean at 32–35∘ N, that ascended from heights of about 18–19 to 22–23 km from the beginning of October to the beginning of December 2017 (about 2 km per month). We discuss several transport and lifting mechanisms that may have had an impact on the found aerosol layering structures.

Aerosol particles are essential constituents of the Earth’s atmosphere, impacting the earth radiation balance directly by scattering and absorbing solar radiation, and indirectly by acting as cloud condensation nuclei. In contrast to most greenhouse gases, aerosol particles have short atmospheric residence times, resulting in a highly heterogeneous distribution in space and time. There is a clear need to document this variability at regional scale through observations involving, in particular, the in situ near-surface segment of the atmospheric observation system. This paper will provide the widest effort so far to document variability of climate-relevant in situ aerosol properties (namely wavelength dependent particle light scattering and absorption coefficients, particle number concentration and particle number size distribution) from all sites connected to the Global Atmosphere Watch network. High-quality data from almost 90 stations worldwide have been collected and controlled for quality and are reported for a reference year in 2017, providing a very extended and robust view of the variability of these variables worldwide. The range of variability observed worldwide for light scattering and absorption coefficients, single-scattering albedo, and particle number concentration are presented together with preliminary information on their long-term trends and comparison with model simulation for the different stations. The scope of the present paper is also to provide the necessary suite of information, including data provision procedures, quality control and analysis, data policy, and usage of the ground-based aerosol measurement network. It delivers to users of the World Data Centre on Aerosol, the required confidence in data products in the form of a fully characterized value chain, including uncertainty estimation and requirements for contributing to the global climate monitoring system., European Commission Joint Research Centre 654109, European ERDF funds through different Spanish R&D projects of the Spanish Ministerio de Economia, Industria y Competitividad, NorthWest University, University of Helsinki, Academy of Finland 272041, Academy of Finland project Greenhouse gas 269095 296302, Korea Meteorological Administration Research and Development Program "Development of Monitoring and Analysis Techniques for Atmospheric Composition in Korea KMA2018-00522, National Research Foundation of Korea 2017R1D1A1B06032548, Korea Meteorological Administration Research and Development Program KMI2018-01111, Taiwan Environmental Protection Administration, Ministry of Research, France, French Ministry of the Environment, United States Environmental Protection Agency, MeteoSwiss (GAW-CH aerosol monitoring programme), Swiss State Secretariat for Education, Research and Innovation (SERI), Ministry of Education, Youth and Sports of CR within National Sustainability Program I (NPU I) LO1415, ERDF "ACTRISCZ RI" CZ.02.1.01/0.0/0.0/16_013/0001315 CGL2017-85344-R MINECO/AEI/FEDER, TIGAS-CM (Madrid Regional Government) Y2018/EMT-5177, AIRTECCM (Madrid Regional Government) P2018/EMT4329 REDMAAS2020 RED2018-102594-T CIENCIA, Spanish Ministry of Economy, Industry and Competitiveness, European Union (EU) CGL2016-78594-R, Generalitat de Catalunya AGAUR 2017 SGR41, National Institute for Aerospace Technology, Ministerio Espanol de Economia, Industria y Competitividad (MINECO) MIS 5021516, Competitiveness, Entrepreneurship and Innovation, NSRF, Ministry of Education, Universities and Research (MIUR), Norwegian Environment Agency, Swedish FORMAS; Swedish Research Council (VR), Magnus Bergvall foundation, Marta och Erik Holmberg foundation, Swedish EPA

Clear-sky short-wave (SW) and long-wave (LW) irradiance measurements at the surface were combined with AErosol RObotic NETwork (AERONET) sun/sky photometer aerosol products to study the desert dust impact on irradiance measurements at a Central Mediterranean site during the year 2012, by comparing measurements performed on dusty and dust-free days. Daily mean values of both the aerosol Ångström exponent (Å), calculated from the aerosol optical depths (AOD) at 440 and 870 nm retrieved from AERONET sun/sky photometer measurements, and the desert dust loading (DL) from the Barcelona Supercomputing Center Dust REgional Atmospheric Model (BSC-DREAM8b) were used to select dusty days. In particular, we have identified as dusty days the ones characterized by Å values less than 0.9 and DL values larger than 0.5 g m−2. The desert dust events occurred from March to September during the analyzed year, in which we have identified 30 and 96 clear-sky dusty and dust-free days, respectively. The daytime SW and LW downward fluxes (FDN) on average decreased by 8 % and increased by 3 %, respectively, on the dusty days with respect to the dust-free ones. These flux variations were associated with an average increase of 40 % of the AOD at 440 nm and an average decrease of 39 % of the fine mode fraction (η) at 500 nm. The daily means of SW- and LW-FDN were reasonably correlated with the corresponding AOD and η values on the dusty days, revealing that the increase of the coarse mode particle contribution was mainly responsible for the flux changes. Conversely, both the SW- and the LW-FDN values were not correlated with the corresponding AOD and η values on the dust-free days.

International audience; In the framework of the Chemistry-Aerosol Mediterranean Experiment (ChArMEx; http://charmex.lsce.ipsl.fr/) initiative, a field campaign took place in the western Mediterranean Basin between 10 June and 5 July 2013 within the ADRIMED (Aerosol Direct Radiative Impact on the regional climate in the MEDiterranean region) project. The scientific objectives of ADRIMED are the characterization of the most common ‘Mediterranean aerosols’ and their direct radiative forcing (column closure and regional scale). During 15–24 June a multi-intrusion dust event took place over the western and central Mediterranean Basin. Extra measurements were carried out by some EARLINET/ACTRIS (European Aerosol Research Lidar Network /Aerosols, Clouds, and Trace gases Research InfraStructure Network, http://www.actris.net/) lidar stations in Spain and Italy, in particular on 22 June in support to the flight over southern Italy of the Falcon 20 aircraft involved in the campaign. This article describes the physical and optical properties of dust observed at the different lidar stations in terms of dust plume centre of mass, optical depth, lidar ratio, and particle depolarization ratio. To link the differences found in the origin of dust plumes, the results are discussed on the basis of back-trajectories and air- and space-borne lidars. This work puts forward the collaboration between a European research infrastructure (ACTRIS) and an international project (ChArMEx) on topics of interest for both parties, and more generally for the atmospheric community.

The Lidar/Radiometer Inversion Code (LIRIC) and the Constrained Iterative Inversion (CII) procedure combined with a graphical aerosol classification framework (GF) have been used to analyse their ability in characterizing the altitude dependence of aerosol properties and evaluate their benefits and weaknesses. LIRIC and the CII technique rely on elastic lidar signals at 355, 532, and 1064 nm and collocated Aerosol Robotic Network (AERONET) Sun/sky photometer measurements to retrieve aerosol parameter profiles at the lidar wavelengths. The aerosol GF relies on the combined analysis of the Ångström exponent at the wavelength pairs 355 and 1064 nm (A(355, 1064)) and its spectral curvature (ΔA = A(355, 532) – A(532, 1064)) to estimate the fine-modal radius and the 532 nm fine-mode fraction. The application of the LIRIC and CII-GF techniques to three selected case studies representative of Central Mediterranean aerosol scenarios has revealed that the differences between the aerosol products from LIRIC and the corresponding ones from the CII-GF procedure varied with altitude, increased with the lidar wavelength decrease, and were significantly large when aerosol from different sources and/or from different advection routes was located at the altitudes sounded by the lidar. The plot on the aerosol GF of A(355, 1064) versus the spectral curvature has indicated that the LIRIC constraint that the fine-modal radius is height independent may represent a weakness if aerosol types and hence aerosol size distributions vary with altitude. The use of lidar ratios (LRs) constant with altitude could represent one of the main weaknesses of the CII-GF technique. The combined use of both techniques should allow obtaining a better characterization of the altitude dependence of aerosol properties from three-wavelength elastic lidar signals.

In this work, the generation and the characterization of high current electron beams from rough photocathodes induced by an ultraviolet excimer laser are reported. The cathodes were rough Cu disks. In order to study the influence of the cathode surface, the electron emission from different surface roughnesses was investigated. To establish the electromagnetic and geometric parameters a Rogowski coil and a new slit–slit emittance meter were used. The laser beam utilized was obtained by a mixture of Kr and Cl which generated coherent light at 222 nm. Fixing the laser energy at 0.5 mJ and the laser spot into 4 mm2, the maximum current as well as the maximum emittance value were obtained with the cathode that had the highest surface roughness which resulted in 1 A and 46 π mm mrad, respectively. A low current and a low emittance were obtained with the cathode that had the lowest roughness, namely, 410 mA and 19 π mm mrad, respectively. A quantum efficiency value of 1.2×10−4 was reached with the cathode with t...

We report on the electron emission from Cu and diamond film cathodes irradiated with two ultraviolet excimer laser beams working at 308 nm XeCl, and 222 nm KrCl. The beam emittance measurements were performed using a new experimental setup composed of two movable slit arrays. The electron beam phase space areas were determined by splitting the beam in small beamlets and measuring the beamlet direction spread via small cups and slits. The highest emission was obtained at the lowest laser wavelength. With a 4 mm2 beam spot and 0.5 mJ laser energy the maximum currents from the Cu and diamond film cathode were 370 and 410 mA, respectively, and the corresponding emittances were 18 and 27 (π mm mrad). From these values the normalized beam brightness was estimated to be 4.6×109 A(π m rad)−2 for the Cu cathode and 2.3×109 A(π m rad)−2 for the diamond cathode. By increasing the laser energy and the spot beam the maximum extracted current was 16.4 A from the Cu cathode with the KrCl laser.

A new approach is introduced to characterize the dependence on altitude of the aerosol fine mode radius (Rf) and of the fine mode contribution (η) to the aerosol optical thickness (AOT) by three-wavelength lidar measurements. The introduced approach is based on the graphical method of Gobbi et al. (2007), which was applied to AERONET spectral extinction observations and relies on the combined analysis of the Ångstrom exponent (å) and its spectral curvature Δå. Lidar measurements at 355, 532 and 1064 nm were used in this study to retrieve the vertical profiles of å and Δå and to determine the dependence on altitude of Rf and η (532 nm) from the å–Δå combined analysis. Lidar measurements were performed at the Mathematics and Physics Department of Universita' del Salento, in south eastern Italy. Aerosol from continental Europe, the Atlantic, northern Africa, and the Mediterranean Sea are often advected over south eastern Italy and as a consequence, mixed advection patterns leading to aerosol properties varying with altitude are dominant. The proposed approach was applied to eleven measurement days to demonstrate its feasibility in different aerosol load conditions. The selected-days were characterized by AOTs spanning the 0.23–0.67, 0.15–0.41, and 0.04–0.25 range at 355, 532, and 1064 nm, respectively. Lidar ratios varied within the 28–80, 30–70, and 30–55 sr range at 355, 532, and 1064 nm, respectively, for the high variability of the aerosol optical and microphysical properties. å(355 nm, 1064 nm) values retrieved from lidar measurements ranged between 0.12 and 2.5 with mean value ±1 standard deviation equal to 1.4 ± 0.5. Δå varied within the −0.10–0.87 range with mean value equal to 0.1 ± 0.4. Rf and η (532 nm) values spanning the 0.02–0.30 μm and the 0.30–0.99 range, respectively were associated to the å–Δå data points. Rf and η values showed no dependence on the altitude. 72% of the data points were in the Δå–å space delimited by the η and Rf curves varying within 0.70–0.95 and 0.15–0.05 μm, respectively for the dominance of fine mode particles in driving the AOT over south eastern Italy. Volume depolarization vertical profiles retrieved from lidar measurements, aerosol products from AERONET sunphotometer measurements collocated in space and time, the BSC-DREAM model, analytical back trajectories, and satellite images were used to demonstrate the robustness of the proposed method.

Intensity-induced variations in the energy transmittivity and temporal shape of pulses emerging from a 200-μm-diameter core silica fibre have been investigated with a single-pulse XeCl laser. By coupling an XeCl laser pulse of 3mJ with a temporal width of 20 ns into a 30-m-long all-silica fibre, a total laser pulse of 1 mJ and 3 ns duration has been extracted at the fibre end. A pulse shortening larger than 80% has been observed. It is shown that the shape variations in the transmitted laser pulses are mainly due to stimulated Raman scattering processes. Substantial shape variations have been observed even at energy conversion efficiencies to the Stokes radiation of lower than 10%. The drop in energy transmittivity at higher fluences seems mainly due to two-photon absorption processes.

The performance of a long pulse XeCl laser, fitted with a negative branch unstable cavity using a phase-unifying (PU) mirror as output coupler, has been investigated. Laser beams with a divergence close to the diffraction limited value and with a relatively uniform intensity profile have been extracted by using an unstable cavity with magnification factor M = 2. It is shown that PU-unstable cavities are quite peculiar tools with respect to hard-edge unstable cavities, mainly for active media having a large cross section and requiring low magnification factors. Indeed, the beam divergence is increased by about 50% by replacing the phase-unifying mirror with a hard-edge aluminized mirror.

The performance of a high-gain, short-pulse XeCl amplifier fitted with a phase-conjugate (PC) cavity by stimulated Brillouin scattering and injected by a laser pulse provided by a XeCl oscillator without intracavity dispersive elements is investigated. A train of up to four pulses with pulse shortening was obtained from the PC cavity. The divergence of the PC cavity output beam was close to the diffraction limit and its spectral intensity distribution was equal to that provided by the free-running XeCl oscillator.

The performance of different super-Gaussian unstable resonators applied to a high-gain, short-pulse XeCl laser has been investigated. The output beam divergence measurements show that in the XeCl laser the output arises from an initial burst of spontaneous emission which is amplified as it propagates within the unstable resonator and after each round trip an output pulse with different spatial characteristics is obtained.

A XeCl laser system without dispersive elements is used to investigate near threshold reflectivity and phase-conjugation (PC) fidelity of stimulated Brillouin scattering (SBS) mirrors with SF6 as the active medium. Using different focal-length lenses to focus the broadband laser radiation into the Brillouin medium, it was found that at threshold the effective interaction length for SBS is equal to the confocal parameter and that the SBS gain is equal to its steady-state value for monochromatic pumping. High PC-fidelity values ranging from 0.8–1.0 were found under most of the experimental conditions investigated.

A stimulated Brillouin scattering mirror has been used to ‘temporally tune’ from nanoseconds to subnanoseconds the width of laser pulses generated by a free running XeCl oscillator, equipped with a super-Gaussian reflectivity unstable resonator. Pulse shortening techniques by stimulated Brillouin scattering (SBS) and by truncated SBS have been investigated. A maximum pulse shortening of about fifty has been achieved by truncated SBS.

A XeCl laser source of variable coherence length has been used to investigate stimulated Brillouin scattering in cyclohexane and methanol versus pump intensity. Peak intensity reflectivities of 48 and 55 percent have been measured in cyclohexane and methanol respectively at about 100 GW/cm2 by using a pumping beam with a bandwidth of 44 GHz. Competitive nonlinear processes reduce reflectivity at higher pump intensities. However, a poorer beam reconstruction has been observed. It has been found that the divergence of the Brillouin backscattered radiation was larger than that of the pumping beam.

Super-gaussian reflectivity unstable resonators of different magnification factor M have been applied to a high-gain, short-pulse XeCl laser, to investigate to what extent the output laser beam characteristics depend on M . It has been found that high magnification factors are required to get a fast establishment of a diffraction limited mode. However, as M is increased, the cavity feedback and then the super-gaussian mirror peak reflectivity should be also increased to get a good mode control of the laser oscillating radiation.

An oscillator-amplifier XeCl laser system has been used to experimentally investigate the effectiveness of a liquid stimulated Brillouin scattering (SBS) mirror for correcting the spatial aberrations of broad-band laser radiation in a double-pass amplifier. It has been found that the SBS amplifier performance is strongly dependent on the intensity Ip exciting the Brillouin medium. A good beam reconstruction has been attained at Ip≤1GW/cm2, whereas highly aberrated output beams have been delivered by the SBS amplifier for Ip>1 GW/cm2. By comparing the broad-band SBS amplifier performance to that of the same amplifier with a dielectrically coated flat mirror at one end, it has been found that the use of a SBS mirror is advantageous to obtain lower divergence output beams only for low energy pump beams (

The near- and far-field performance of self filtering unstable resonators, with magnification factors | M | ranging from 5 to 133 has been investigated with an uv-preionized, discharge excited XeCl laser. The mode volume increases with | M | but the extracted energy density falls out rapidly. A decrease with | M | of the output beam optical quality and brightness has also been observed.

A XeCl laser system has been used to investigate stimulated Brillouin scattering (SBS) cyclohexane and methanol with laser radiation of spectral width Delta nu /sub p/ much larger than the bandwidth of the excited acoustic wave Delta nu /sub a/. The reflectivity and the phase conjugation (PC) fidelity of the SBS reflector versus pump intensity and bandwidth have been measured. It is shown experimentally that the PC fidelity and the reflectivity are strongly dependent on the pump spectral width and intensity. However, the experimental results show that the use of a liquid SBS reflector is still effective for correcting the spatial aberrations of laser radiation with Delta nu /sub p/ approximately=30 Delta nu /sub a/ in a double-pass amplifier. >

Self-Q switching of a XeCl laser fitted with a resonator containing a stimulated-Brillouin-scattering phase-conjugate reflector is demonstrated. Q-switched laser pulses of 0.2-mJ energy and 2.5-ns duration and with a divergence close to the diffraction limit were obtained. A spectral narrowing of the two strong lines at 307.98 and at 308.19 nm exhibited by the XeCl laser spectrum was also observed.

A gaussian reflectivity resonator of magnification M=5.2 has been used in a high-gain, short-pulse excimer laser. A good optical quality, 20 mm diameter laser beam, of 60 mJ, 18.6 ns long and with a brightness of 2.4x1013 W cm-2 sr-1 has been obtained. By decreasing the active mode volume cross section, higher optical quality laser beams have been obtained.

Confocal and non-confocal negative branch unstable resonators with an intracavity spatial filter have been applied to a high-gain short-pulse UV preionized XeCl laser. The near- and far-field radiation characteristics have been investigated in both configurations. Laser beams of larger brightness have been obtained with non-confocal schemes. A maximum beam brightness of 1.3×1014 Wcm−2Sr−1 has been achieved.

The all-sky direct radiative effect by anthropogenic aerosol (DREa) is calculated in the solar (0.3–4 μm) and infrared (4–200 μm) spectral ranges for six Mediterranean sites. The sites are differently affected by pollution and together reflect typical aerosol impacts that are expected over land sites of the central Mediterranean basin. Central to the simulations are aerosol optical properties from AERONET sun-/sky-photometer statistics for the year 2003. A discussion on the variability of the overall (natural+anthropogenic) aerosol properties with site location is provided. Supplementary data include MODIS satellite sensor based solar surface albedos, ISCCP products for high- mid- and low cloud cover and estimates for the anthropogenic aerosol fraction from global modelling. Since anthropogenic aerosol particles are considered to be smaller than 1 μm in size, mainly the solar radiation transfer is affected with impacts only during sun-light hours. At all sites the (daily average) solar DREa is negative all year round at the top of the atmosphere (ToA). Hence, anthropogenic particles produce over land sites of the central Mediterranean a significant cooling effect. Monthly DREa values vary from site to site and are seasonal dependent as a consequence of the seasonal dependence of available sun-light and microphysical aerosol properties. At the ToA the monthly average DREa is −(4±1) W m−2 during spring-summer (SS, April–September) and −(2±1) W m−2 during autumn-winter (AW, October–March) at the polluted sites. In contrast, it varies between −(3±1) W m−2 and −(1±1) W m−2 on SS and AW, respectively at the less polluted site. Due to atmospheric absorption the DREa at the surface is larger than at the ToA. At the surface the monthly average DREa varies between the most and the least polluted site between −(7±1) W m−2 and −(4±1) W m−2 during SS, and between −(4±3) W m−2 and −(1±1) W m−2 during AW. The DREa at infrared wavelengths is positive but negligible, especially at the ToA (

We present a very compact and very reliable laser system formed of two discharge sections both with lateral UV preionization. The discharge electrodes are contained in a single vessel and form an oscillator-amplifier system. A single spark gap switches on both circuits. By applying a generalized self-filtering unstable resonator to the oscillator and injecting the laser beam into the single pass amplifier, an output beam of 105 mJ, with a brightness of 5.5×1013 W cm−2Sr−1 has been obtained. Moreover, by applying a pulse-forming network in one discharging circuit, the duration of the discharge breakdown has been increased and output laser pulses of 40 ns FWHM, and 80 ns base width duration (at 10% points) and of energy 100 mJ are obtained.

Stimulated Brillouin scattering (SBS) of unmodified XeCl laser radiation in n‐hexane was investigated for building phase‐conjugating mirrors. It was found that the SBS mirror reflectivity could be improved by setting a holographic grating in front of the SBS mirror and by using the first order beam reflected by the grating to pump the Brillouin medium. Maximum energy and power reflectivities of 45% and 80%, respectively, were measured. The SBS mirror energy reflectivity was reduced by 45% by removing the grating. The phase conjugation fidelity of the backscattered radiation was close to one, with or without the grating.

High‐mode‐volume, good optical quality laser beams of 55 mJ, of 19 ns duration and with a brightness of 3×1013 W cm−2 Sr−1, have been extracted from a high‐gain, short‐pulse XeCl laser, with an unstable resonator using a super‐Gaussian reflectivity mirror. The misalignment angle of the output coupler to decrease the normalized output energy by 20% was 3.5 mrad.

The production of multi-wavelengths by stimulated Raman scattering (SRS) in high pressure gases is receiving a great interest in recent years, being the SRS widely used in multiparametric LIDAR systems [1] In particular, the developing of new techniques to control the energy conversion efficiency from the pump radiation to a desired Stokes order represents one of the main research objective pursued in the last years II is well known that by utilising high pressure Raman gas and a low-angle pumping geometry to reduce the four wave mixing processes, a quite high conversion to the first Stokes (Sj) can be obtained. However, as the intensity of S] becomes sufficiently high, it stimulates the second order Stokes (S2) and so on higher order Stokes beams.

There has been considerable interest in the use of resonators with a phase-conjugate mirror to enhance the performance of pulsed lasers. Stimulated Brillouin scattering (SBS) is often chosen as the phase conjugation mechanism because of its relative simplicity and high efficiency. A further interesting feature of SBS mirrors used in phase-conjugate resonators (PCR) is represented by their capability of providing passive self-Q-switching because of their intensity-dependent reflectivity.

The temporal evolution of the beam-quality factor M2 is investigated in plane-parallel cavities of different Fresnel numbers with a Gaussian reflectivity mirror and applied to a Xe–Cl excimer laser. The Fresnel number is changed by varying resonator length. Laser radiation of higher spatial coherence is provided by the cavity of greater length (2.5 m) and then of lower Fresnel number after the first transit. Indeed, the beam spatial coherence decreases with cavity length. It is also shown that the Gaussian mirror affects the properties of the oscillating laser radiation only at the very first cavity round trips. The near- and far-field spatial characteristics of the laser radiation do not vary after a few transits inside the cavity. Experimental results are supported by a theory formerly developed for the evolution of the parameters of Gaussian Schell-model beams propagating in Gaussian cavities.

A one-dimensional code for the numerical simulation of negative branch unstable resonators with an intracavity aperture that are applied to high-gain, short-pulse XeCl lasers is described. The model predicts near- and far-field performance of the output laser beams. The intracavity aperture size is shown as an important parameter for control of the output beam energy and divergence. A comparison with experimental measurements is presented.

The evolution of the main parameters (spot size, radius of curvature, and far-field divergence) of the beam provided by a high-gain, short-pulse Xe–Cl laser equipped with a plane-parallel Gaussian resonator has been investigated both theoretically and experimentally as a function of the number of round trips. It is shown that a theoretical analysis, formerly developed to study the growth of coherence of a Schell–Gauss model beam propagating through a periodic sequence of Gaussian apertures, provides a satisfactory description of the temporal evolution of the Xe–Cl laser beam. A good agreement between theoretical and experimental data has been found.

Near-diffraction-limited laser beams of high energy have been achieved with a long-pulse (110-ns) XeCl laser fitted with a super-Gaussian unstable cavity of magnification M = 2.7. It has been found that when the super-Gaussian output coupler is replaced with a hard-edge aluminized mirror of radius equal to the super-Gaussian mirror spot size, the output laser energy is reduced by 10%, whereas the beam divergence increases more than 50%.

The authors present the output characteristics of an excimer laser (XeCl and KrCl) with delayed double UV preionisation. He- and Ne-based mixtures were used for the XeCl laser and Ne-based mixtures for the KrCl laser, at pressures up to 7*105 Pa. For both lasers, maximum output energies (1.1 J/pulse at 308 nm and 0.3 J/pulse at 222 nm) were obtained for a delay time of about 290 ns between the onset of the preionisation and the breakdown of the laser gas. At a charging voltage of 50 kV, by varying the delay time from 140 ns to 290 ns, an improvement in the output energy of 16% for the XeCl laser and 22% for the KrCl laser was obtained. The improvement in respect of a single preionisation is about 18% for the XeCl laser with the He-based mixture, and about 14% for the XeCl laser with the Ne-based mixture.

Report of a study of the shape and length of the output pulse of a molecular nitrogen laser, excited by an intense relativistic electron beam, is described. The rate equations are computer solved, at first ignoring the spontaneous emission during the excitation process. Afterwards the rate equations are solved taking into account excitation functions of various shapes and lengths, related to electron‐beam pulses of a few kA and a few nsec. Laser power output, energy, and peak‐time, i.e., the time at which the gain reaches its saturated value, are given and discussed as functions of the intensity and rise time of the excitation functions.

The unsaturated losses α, and the saturation intensityIs, were measured in an uv-preionized KrCl laser for optimized He and Ne based laser gas mixtures. The measurements were made as a function of the specific power loading and of the total pressure of the laser mixtures. Higher values for α andI s were found for the Ne-based laser mixture than for the He-based mixture. At 45 kV charging voltage and at 355 kPa of total pressure we measured α=0.053 cm−1 andI s =9.4MW/cm2 for the Ne based mixture, and α=0.035 cm−1 andI s =5.1MW/cm2 for the He based mixture.