Iodine Laser Research Articles

Off-axis negative-branch unstable resonator in rectangular geometry

The application of an off-axis negative-branch unstable resonator to an active medium of rectangular geometry is examined. The presented unstable resonator consists of spherical mirrors and a scraper mirror. The adaptation to the rectangular cross section is performed by the scraper, which takes two different shapes. One shape resembles a rectangular bracket "[" and the other resembles the letter "L." The [ and L configurations correspond to a shift of the optical axis away from the center of the cross section, toward one of the edges or toward one of the corners, respectively. Both scraper setups are examined numerically and experimentally. Experiments are performed with a multikilowatt class chemical oxygen iodine laser. The active medium is characterized by a low amplification coefficient. Measured results of the intensity distribution in the far field and of the phase distribution in the near field are shown for both resonator configurations. Using the same resonator magnification, the setup with the L-shaped scraper has a lower output coupling and, therefore, a higher output power and a slightly higher beam divergence. The L-shaped scraper configuration is able to cover the gain medium completely.

Output power enhancement of all gas-phase iodine laser by addition of hydrocarbon gases

In this work, we studied the output power enhancement of an all gas-phase iodine laser (AGIL) by the addition of hydrocarbon gases. Enhancement is expected because hydrocarbon gases might scavenge Cl atoms, which are strong quenchers of the upper state of the laser medium, I(2P1/2). In AGILs, suppression of the Cl atom concentration is the key to improving the efficiency of the operation of the laser because Cl atoms are inherently generated by the self-annihilation of the energy donor, NCl(a1Δ). We found that the addition of CH4 gave the best results, because of its high scavenging rate constant and inertness to I(2P1/2). An enhancement of 10% was observed in the output power when CH4 was added at a flow rate twice that of NCl3. On the other hand, when C2H4 or C2H2 were added at the same flow rate as that of CH4, the output power reduced despite their fast removal rate of Cl atoms. The reason for the reduced output power was that the unsaturated bonds scavenged not only the Cl atoms but also the H atoms, resulting in a low density of H atoms, and this decelerated the production of NCl(a1Δ). The observed laser characteristics could reasonably be explained by numerical model calculations. To our knowledge, this is the first report of successful output power enhancement of an AGIL using a chemical agent.

Nonequilibrium population of the first vibrational level of O2(1Σ) molecules in O2 — O2(1Δ) — H2O gas flow at the output of chemical singlet-oxygen generator

The concentrations of electron-excited particles have been determined by measuring the absolute spectral irradiance in the range of 600 — 800 nm of O2 — O2(1Δ) — H2O gas mixture at the output of a chemical singlet-oxygen generator (SOG). A nonequilibrium population of the first vibrational level of O2(1Σ) molecules has been clearly observed and found to depend on the water vapour content. In correspondence with the results of these measurements and according to the analysis of kinetics processes in the O2 — O2(1Δ) — H2O mixture, the maximum number of vibrational quanta generated in the O2(1Δ) + O2(1Δ) → O2(1Σ) + O2(3Σ) reaction is 0.05 ± 0.03. It is concluded that the vibrational population of O2(1Δ) at the output of the SOG used in a chemical oxygen—iodine laser is close to thermal equilibrium value.

Kinetics of O2(1Δ) self-quenching in the O2 — O2(1Δ) — H2O gas mixture

The self-quenching kinetics of O2(1Δ) molecules in the O2 — O2(1Δ) — H2O gas mixture produced by a chemical singlet-oxygen generator for an oxygen—iodine laser is investigated. The rate of change in the O2(1Δ) concentration in continuous-flow tube is determined by measuring the absolute spectral irradiance of the O2 — O2(1Δ) — H2O gas mixture in the ranges of 600 — 800 nm and 1210 — 1330 nm. The effective rate constant of O2(1Δ) deactivation in all possible channels of the O2(1Δ) + O2(1Δ) → products reaction is found to be (8±1.2)×10-17 cm3 s-1.

The effect of flow cooling on gain and output power of an electrically excited oxygen–iodine laser

Cooling the flow in the electric discharge excited oxygen–iodine laser is used to increase gain in the supersonic cavity and laser power. The flow is cooled by injecting helium precooled in a liquid nitrogen bath downstream of the iodine injector. The highest gain achieved is 0.21% cm−1 over the gain path of 10 cm. Laser power measured in a stable resonator with 99.9% and 99.0% reflectivity mirrors is 7.8 W. Gain measured downstream of the resonator during lasing remains high, 0.15% cm−1, 25% lower than gain without lasing, demonstrating that a small fraction of power available for lasing is coupled out.

Characteristics of an all gas-phase iodine laser using molecular iodine as atomic iodine donor

The laser action of an all gas-phase iodine laser (AGIL), which uses molecular iodine as a source of iodine atoms, has been demonstrated. The laser is based on the energy transfer reaction between metastable NCl(a 1Δ) and ground state I(2P3/2) atoms, which are produced by the electric discharge of a mixture of I2 and He. At fixed flow rates of the chemical species, the laser output powers are measured at three different positions in a flow reactor. The output power is characterized by a function of the optical axis position and is in reasonable agreement with the numerical simulation. A repetitive pulse of laser output at 50 Hz with a duty factor of 40% is observed. The highest output power is 40 mW at 210 mm downstream from the mixing point of I/H/He and NCl3. This is 80% of the output power generated from the conventional system using HI as an iodine donor. The measured results of the time-resolved laser output power suggest that the output power of the I2-AGIL is more sensitive to the electric discharge plasma intensity as compared with that of the HI-AGIL. An AGIL operated using I2 could potentially have the same output power as that of an AGIL operated using HI if a continuous-wave electric discharge generator is used.

Analysis of processes participating during intense iodine-laser-beam interactions with laser-produced plasmas

The high-power iodine laser PALS was used to generate highly charged Ta ions and to study non-linear processes in laser-produced plasma. Longitudinal structures of the expanding plasma, obtained by using an X-ray streak camera on a time scale ∼ 2 ns, are presented. Various bright spots (moon-like, half-moon-like), expansion-path curvature and even their splitting were recorded. These phenomena are ascribed to the effect of the magnetic field that is self-generated at high laser intensities.

Preliminary studies on fast particle diagnostics for the future fs-laser facility at PALS

The design, construction and development of proper diagnostics systems for multi-MeV proton/ion and relativistic electron beams, typically generated by fs-laser plasmas, are under study in our laboratories. Ultra-fast plasma ion detection through classical ion collectors (ICs) in time-of-flight (TOF) configuration, shielded by a thin metallic absorber, has been theoretically studied and preliminarily tested using the 300 ps, kJ-class iodine laser operating at PALS Research Centre, in order to cut the long photo-peak contribution and analyse the ultra-fast particle signal. Processing of obtained experimental IC-TOF data, including maximum proton/ion energy estimations, is reported. Design, modelling and simulations for relativistic electron bunch detection, by means of a proper magnetic electron spectrometer, are also included. The effort is mainly devoted to match the diagnostics systems with the peculiarity of fast particle streams which could be generated by the 20 TW Ti:sapphire laser (40 fs, 1 J, 10 Hz) recently installed at PALS.

Analysis of direct-drive capsule compression experiments on the Iskra-5 laser facility

We have analyzed and numerically simulated our experiments on the compression of DT-gas-filled glass capsules under irradiation by a small number of beams on the Iskra-5 facility (12 beams) at the second harmonic of an iodine laser (λ = 0.66 μm) for a laser pulse energy of 2 kJ and duration of 0.5 ns in the case of asymmetric irradiation and compression. Our simulations include the construction of a target illumination map and a histogram of the target surface illumination distribution; 1D capsule compression simulations based on the DIANA code corresponding to various target surface regions; and 2D compression simulations based on the NUTCY code corresponding to the illumination conditions. We have succeeded in reproducing the shape of the compressed region at the time of maximum compression and the reduction in neutron yield (compared to the 1D simulations) to the experimentally observed values. For the Iskra-5 conditions, we have considered targets that can provide a more symmetric compression and a higher neutron yield.

Interaction of two plasma jets produced successively from Cu target

AbstractOur earlier papers demonstrate a very simple method of plasma jet formation, consisting in irradiating a massive planar target of a relatively high atomic number by a partly defocused laser beam. Our present interest is concentrated on interaction of the plasma jet with other media. This paper is aimed at investigations of interaction of two jets launched successively on Cu target. Our attention was paid to the role of radiative cooling in the plasma jet formation. The experiment was carried out at the PALS iodine laser facility. The laser provided a 250-ps (full width at half maximum) pulse with energy of 130 J at the third harmonic frequency (λ3 = 0.438 µm). Two successive jets were produced on a massive flat Cu target provided with a cylindrical channel 5 mm long and 400 µm in diameter. Since the focal spot diameter of the laser beam on the target surface was larger than that of the channel (800 µm), the annular irradiation of the target face resulted in creation of the first plasma jet, whereas the second jet was produced by action of the central part of laser beam on the channel wall. Three-frame interferometric system, X-ray streak camera, and a set of ion collectors were used as diagnostic tools.

Mixing in a supersonic COIL laser: influence of trip jets

We present an experimental study of a supersonic nozzle with supersonic iodine injection. This nozzle simulates Chemical Oxygen Iodine Laser (COIL) flow conditions with non-reacting, cold flows. During the experiments, we used a laser sheet near 565 nm to excite fluorescence in iodine, which we imaged with an intensified and gated CCD camera. We captured streamwise and semi-spanwise (oblique-view) images, with fluorescence revealing the material injected into the flow. We identified the flow structures in the images, and produced quantitative characterizations of the flow morphology and of the mixing between the primary and injected flow. We considered four injection scenarios. The first scenario includes a single injector positioned downstream of the nozzle throat. To enhance the mixing between the flows, trip jets are placed in the wake of the single jet. The sonic trip jets, significantly smaller than the primary supersonic iodine jet, are intended to destabilize the counter-rotating vortex pair (CRVP) of the primary jet. We compare three different trip jet configurations for their ability to enhance mixing between the oxygen and iodine flows.

Electrode system for electric-discharge generation of atomic iodine in a repetitively pulsed oxygen — iodine laser with a large active volume

Possibilities for increasing the active medium volume of a chemical oxygen — iodine laser (CCOIL) with a pulsed electric-discharge generation of atomic iodine are studied. The reasons are analysed of the low stability of the transverse self-sustained volume discharge in electrode systems with metal cathodes under the conditions of the electric energy input into gas-discharge plasma that are typical for CCOILs: low pressure of mixtures containing a strongly electronegative component, low voltage of discharge burning, low specific energy depositions, and long duration of the current pulse. An efficient electrode system is elaborated with the cathode based on an anisotropically-resistive material, which resulted in a stable discharge in the mixtures of iodide (CH3I, n-C3H7I, C2H5I) with oxygen and nitrogen at the specific energy depositions of ∼5 J L-1, pressures of 10 — 25 Torr, and mixture volume of 2.5 L.

Numerical investigation of laser radiation conversion to X rays in experiments with spherical boxes with interior coatings of different materials

The dynamics of laser and X-ray radiation fields in experiments with spherical boxes was numerically investigated in a sector approximation using the SND-LIRA numerical code. The experiments were performed on the Iskra-5 laser facility at a wavelength λ=0.657 μm (the second harmonic of iodine laser radiation). The characteristics of X-ray generation were investigated with the inner surface of a converter box coated with different-Z materials (Au, Cu, Mg). With lowering Z, the laser energy absorption coefficient ka decreases and there occurs a lowering of the effective X-ray radiation temperature. Our calculations reveal a strong dependence of the results on the electron free-streaming flux limitation f. In particular, on lowering f from 0.1 to 0.03 for a conventional box with a gold coating, the coefficient ka decreases from 0.83 to 0.5 and the peak X-ray radiation temperature drops from 170 to 150eV. In these calculations, the rms nonuniformity of the X-ray irradiation of a capsule with thermonuclear fuel amounted to 1%—3%.

Direct spectroscopic observation of ion deceleration accompanying laser plasma–wall interaction

Interactions of plasma jets with solid surfaces are extensively studied in context with development of future fusion devices. In experiments carried out on the iodine laser system PALS, the energetic ions were produced at double-foil Al/Mg targets irradiated by one or two counter-propagating laser beams. The plasma jets from the rear surface of the laser-exploded Al foil streamed towards the Mg target representing the wall preheated by the action of the high-energy photons, particle and/or laser beams. Instead of being trapped by the cold secondary-target material, the forward-accelerated Al ions collided with the counter-propagating matter ejected from the wall. The environmental conditions in near-wall plasmas were analyzed with the high-resolution x-ray spectroscopy and temporally-resolved x-ray imaging. The deceleration of the incident Al ions in the near-wall region was directly observed and quantitatively characterized via Doppler shifts of the J-satellite from the Al Lya spectral group. The interaction scenario was modelled using the 2D arbitrary Lagrangian Eulerian hydrocode PALE and the multifluid code MULTIF.

Characteristics of amine-based all-gas-phase iodine laser

The characteristics of an amine-based all-gas-phase iodine laser (AGIL) are studied. At constant flow rates of the chemical species, the small-signal gain and laser output power are measured at three different positions in the flow reactor. It is clarified that the positive gain exceeding the threshold (2.3×10−3%/cm) is maintained over a region more than 80 mm long along the flow. The highest small-signal gain of 7.8×10−3%/cm and laser output power of 50 mW are observed at 170 mm downstream from the mixing point of HI and H. Numerical simulations suggest that the long positive-gain region is attributable to the following characteristics of the present apparatus: inefficient mixing and inefficient H2 dissociation. It is confirmed experimentally and numerically that these limitations are beneficial for extending the positive-gain region to the downstream; however, the optimum HI flow rate is limited a small value and results in a much lower gain than the theoretical limit. In order to achieve a higher gain, fast mixing and efficient H2 dissociation must be simultaneously achieved so as to increase the optimum HI flow rate. Numerical simulation results suggest that a positive gain of 5.4×10−2%/cm, a 7-times higher gain than that indicated by the present experimental results, should be obtained with the same NCl3 flow rate.

Recent advances in the development of discharge‐pumped oxygen‐iodine lasers

AbstractChemical oxygen‐iodine lasers are unique in their ability to generate high‐power beams with near diffraction limited beam quality. The operating wavelength, 1.315 µm is readily transmitted by the atmosphere and compatible with fiber optics beam delivery systems. However, applications of the laser are severely limited by logistical problems associated with the complex chemistry used to power the device. Electrical or microwave discharge excitation of oxygen‐iodine lasers offers an attractive alternative that eliminates the chemical power generation problems and has the possibility of closed‐cycle operation. A discharge oxygen‐iodine laser was first demonstrated in 2005. Since that time the power of the device has been improved by a factor of 400 and much has been learned concerning the physics and chemistry of the discharge driven system. Although our current understanding of the chemical kinetics is incomplete, parametric studies of laser performance show considerable promise for further scaling. This article reviews the basic principles of the discharge oxygen iodine laser, summarizes the most recent advances, and outlines some of the unresolved questions regarding the production and removal of excited species in the gas flow.

Measure of precursor electron density profiles of laser launched radiative shocks

We have studied the dynamics of strong radiative shocks generated with the high-energy subnanosecond iodine laser at Prague Asterix Laser System facility (Prague) over long time scales, up to 100 ns. These shock waves are characterized by a developed radiative precursor, a radiation driven ionization wave in front of the density jump of the shock. Electronic density profiles are measured at different times after the laser pulse and at different distances from the axis of the shock tube. A new feature, described as a split precursor, has been observed. Comparisons with two-dimensional computations are shown.

Pressure recovery studies on a supersonic COIL with central ejector configuration

A chemical oxygen–iodine laser (COIL) is an electronic transition, low pressure, high throughput system. The field use of this laser demands the development of suitable pressure recovery systems. Ejector based pressure recovery systems form a potent alternative for open cycle COIL operation. The two possible configurations of motive gas injection in ejectors are peripheral and central. The present paper focuses on the investigation of a central injection low pressure ejector operated with a small scale supersonic COIL (SCOIL). The ejector handles a motive flow of nearly 120 g s −1 and an entrained laser flow of nearly 3 g s −1. The predicted geometry using integral methods has been validated numerically by employing Fluent 6.1 software in a 2-d axisymmetric viscous turbulent flow formulation. The numerical predictions have been experimentally validated, which indicate a pressure recovery of 63 Torr at design conditions. The results also show that the recovered pressure improves to 75 Torr for an off-design condition of higher motive flow rate.

Experimental study of radiative shocks at PALS facility

AbstractWe report on the investigation of strong radiative shocks generated with the high energy, sub-nanosecond iodine laser at PALS. These shock waves are characterized by a developed radiative precursor and their dynamics is analyzed over long time scales (~50 ns), approaching a quasi-stationary limit. We present the first preliminary results on the rear side XUV spectroscopy. These studies are relevant to the understanding of the spectroscopic signatures of accretion shocks in Classical T Tauri Stars.

Laser transition in atomic iodine for passive and active SETI

It is proposed to use the wavelength 1.315 μm of the laser transition 2P 1/2→ 2P 3/2 in atomic iodine both for passive and active SETI. The search for an extraterrestrial signal at this wavelength is promising because an active quantum filter (AQF) with a quantum sensitivity limit and with strong fixing in the spectrum luminescence line of the width less than 0.01 cm −1, has been developed for the noted wavelength. Such AQF is capable of receiving the laser signals, consisting of only a few photons, against the background of emission from a star under study. In addition, the high-energy iodine lasers emitting diffraction-limited nanosecond pulses of energy of 2 kJ in a single beam at the wavelength 1.315 μm have been created. The spectral tuning of radiation of these lasers allows one to compensate the frequency Doppler shift. A weak absorption of the 1.315 μm radiation in the Earth atmosphere allows the search for extraterrestrial signals by using the ground optical telescopes equipped with adaptive optical systems.