Efficient XeCl laser with a semiconductor opening switch in a pump oscillator: Theory and experiment

Semiconductor opening switches (SOS) are able to interrupt currents at density levels of up to 10 kA/cm 2 in less than 10 ns, operate at repetition rates up to 1 kHz, and possess lifetimes of more than 10 1 1 pulses. If stacked, SOS diodes can hold off voltage levels above a few 100 kV. They are therefore ideal for the design of compact high voltage pulse generators of the GW-class for industrial applications. The aim of this work was to improve our understanding of the opening process in a semiconductor diode of SOS-type with a doping profile of p + pnn + structure, obtainable through diffusion from the surfaces. To simulate the physical processes inside this diode the code POSEOSS was developed. It contains a detailed physical model of charge carrier transport under the influence of density gradients and electric fields and considers all relevant generation and recombination processes. It possess a large degree of flexibility and is easy to apply and thus allows to carry out parameter studies to determine the influence of different physical quantities, like doping and impurity levels, on the performance of the device. Applying the code some new interesting results concerning the plasma dynamics during the opening process in the switch have been found. Especially, using realistic values for the charge carrier mobility, it was found that the opening process starts first at the n-n + boundary. Also it has been possible to derive the physical conditions for the occurrence of the SOS-effect. Based on the simulation results a simplified SOS equivalent circuit model has been developed. This model can be used in the circuit simulation program PSpice. A new pulse generator scheme based on inductive storage is proposed, in which power multiplication is achieved by unloading the inductors, previously charged in series, in parallel. This scheme can be considered as the inductive equivalent of a Marx-generator. PSpice simulations of such a scheme based on semiconductor opening switches are presented. The theoretical results were compared to measurements obtained with a simple experimental set-up using two 100 kV SOS-switches. The measurements showed good agreement with the simulation results. Further improvements seem possible by adapting the SOS device structure to the specific generator circuit.

Forward pumping current is required for current interruption operation of a semiconductor opening switch (SOS). In this study, the influence of the forward pumping current on current interruption of the reverse current has been investigated experimentally. The duration and the peak value of the forward current were varied and the reverse current obtained under different conditions has been compared. As a result, it has been found that a short forward current can lead to high peak value of the reverse current. It is considered from the results that the density of the minority carriers is increased by the forward current of shorter duration because recombination consumes fewer carriers. Furthermore, when the duration of the forward current shortens with a lower peak value and depletion the peak value of the reverse current increases, the interruption of reverse current quickens. This can be explained by the fact that the time during which minority carriers are turned around by the reverse current is shorter in case of less accumulation and faster removal of minority carriers. With these investigations, it has been understood that relatively short and low forward current and high reverse current is needed for high efficiency inductive energy storage circuit using the SOS.

Explosive formed fuses (EFFs) use conducting elements that are deformed by explosive pressure (typically, against dielectric dies). This causes the fuse geometry to change, so that the conducting element cross section decreases. This enables a higher ratio of current conduction to current interrupt time than for normal fuses, and it enables more control of when current interruption occurs. In combination with a suitable output closing switch, EFF's can be used to obtain several hundred kilovolt voltage pulses from inductive stores to drive several ohm loads. With proper choices of inductive store, EFF geometry and material, and output closing switch features, such a voltage pulse can be approximately flat topped for microsecond duration and have a small fraction of microsecond risetime. We present theoretical analysis and circuit simulations which illustrate this, using scaled empirical EFF parameters for inductive stores in the 1 weber flux, several hundred nanohenry range. The circuit simulations were done using MicroCap-IV, with user defined elements. These simulations were done with static inductive stores and with explosive magnetic flux compression generators driving inductive stores.

Microwave spectra have been measured for a number of isotopically substituted forms of methyl fluoroform, methyl trifluorosilane and trifluoromethyl cyanide. By measurement of the J = 4 → 5 transitions of molecules containing partially deuterated methyl groups, it is possible to derive all three principal moments of inertia of these species with a precision sufficient for structural calculations. The structures of methyl fluoroform and trifluoromethyl cyanide obtained from these measurements are discussed. In trifluoromethyl cyanide, the CN bond is somewhat shorter than usually found for this group. Spectra of these molecules in excited vibrational states are also briefly reported.

We report new ab initio results for the interaction-induced dipole moments Δμ of hydrogen molecules colliding with helium atoms. These results are needed in order to calculate collision-induced absorption spectra at high temperatures; applications include modeling the radiative profiles of very cool white dwarf stars, with temperatures from 3500 K to 9000 K. We have evaluated the dipoles based on finite-field calculations, with coupled cluster methods in MOLPRO 2006 and aug-cc-pV5Z (spdfg) basis sets for both the H and He centers. We have obtained values of Δμ for eight H(2) bond lengths ranging from 0.942 a.u. to 2.801 a.u., for 15 intermolecular separations R ranging from 2.0 a.u. to 10.0 a.u., and for 19 different relative orientations. In general, our values agree well with earlier ab initio results, for the geometrical configurations that are treated in common, but we have determined more points on the collision-induced dipole surface by an order of magnitude. These results make it possible to calculate transition probabilities for molecules in excited vibrational states, overtones, and rotational transitions with ΔJ > 4. We have cast our results in the symmetry-adapted form needed for absorption line shape calculations, by expressing Δμ as a series in the spherical harmonics of the orientation angles of the intermolecular vector and of a unit vector along the H(2) bond axis. The expansion coefficients depend on the H(2) bond length and the intermolecular distance R. For large separations R, we show that the ab initio values of the leading coefficients converge to the predictions from perturbation theory, including both classical multipole polarization and dispersion effects.

The method of multifrequency laser probing is developed which can be used for diagnostics of the temperature and population of vibrational levels in gas mixtures containing CO molecules in excited vibrational states. The method is tested by studying the dynamics of the gas temperature and population of vibrational levels of the CO molecule in gas mixtures excited by a pulsed discharge. It is shown that the method provides the reduction of the gas temperature measurement error down to 3%. It is found that the population of lower vibrational levels in the CO–O2 mixture can exceed the population of levels in CO–He and CO–N2 laser mixtures by several times.

The results of an experimental study of coaxial exciplex lamps pumped by various types of discharge (the longitudinal discharge, barrier discharge, or continuous glow discharge) are presented. The mixture compositions and pressures as well as the pumping pulse parameters have been optimized for the production of radiation with λ = 193, 222, 249, 308 and 350 nm. Exciplex lamps with a radiating surface area of 570, 840, and 1300 cm2 have been created. The average radiation power achieved for λ = 222 and 308 nm is up to 100 W and for λ = 350 nm is ~20 W. The application of an inductive energy store with a semiconductor opening switch being used for pumping a longitudinal-dischargebased exciplex lamp has been demonstrated.

The results of an experimental study of coaxial exciplex lamps pumped by various types of discharge (the longitudinal discharge, barrier discharge, or continuous glow discharge) are presented. The mixture compositions and pressures as well as the pumping pulse parameters have been optimized for the production of radiation with (lambda) equals 350, 308, 249, 222, and 193 nm. Exciplex lamps with a radiating surface area of 570, 840 and 1300 cm2 have been created. The average radiation power achieved for (lambda) equals 222 nm is up to 4 W and for (lambda) approximately 308 and 350 nm is approximately 10 W. The possibility of an inductive energy store with a semiconductor opening switch being used for pumping a longitudinal-discharge-based exciplex lamp has been demonstrated.

The necessity of increasing the stability of the parameters of pulsed and cw laser radiation when operating at peak powers is substantiated. Particular attention is paid to the parameters of pulsed laser radiation in systems with semiconductor pumping. The main requirement in this case is the repeatability of the parameters of the output pulses with high accuracy. This condition is ensured by the stability of the diode supply drivers, especially at peak powers, which are required for scientific research. It was found that even a short-term excess (about 1 ms) of the upper range of the permissible current leads to damage to the laser diode, which instantly degrades the parameters of laser radiation. The developed technique can significantly reduce the likelihood of these phenomena and increase the reliability of the laser.

Today, modification of metal surfaces using laser radiation (laser ablation) is one of the most common processing methods in various scientific studies. At the same time, there is no unified approach to estimating the energy of laser radiation impact on the metal surface, which significantly complicates the comparison of the results of studies performed at different parameters of laser radiation. In the present work, a universal method of calculating the energy density of laser radiation impact on the surface is presented. The formation of relief with identical geometric characteristics at different parameters of laser radiation, but with the same energy density of radiation effect on the surface being modified, was experimentally confirmed.

At present, minipercutaneous nephrolithotripsy is a highly effective and safe method for fragmentation of stones with sizes up to 3 cm. The use for lithotripsy of a holmium: yttrium aluminum garnet laser with wavelength 2.1 μm (Ho:YAG) is a common practice today. Of late, however, more attention is paid to a new superpulse thulium fiber laser with wavelength 1.94 μm and maximum output power 40 W. The use of a thulium laser with pulse energy 0.025–6 J and a high repetition rate (to 1600 Hz) permits to obtain (compared with a holmium laser) better results of stone dusting. Objective. Selection of the optimal modes of stone fragmentation in minipercutaneous nephrolithotripsy with the use of a superpulse thulium fiber laser with wavelength 1.94 μm and maximum output power 40 W in patients with nephrolithiasis. Patients and methods. The study included patients (n = 171), who underwent minipercutaneous nephrolithotripsy using a superpulse thulium fiber laser during the period from February 2018 to July 2019. The following parameters of laser radiation were used: pulse energy 0.1-6 J, power 6-40 W, pulse rate 30-300 Hz. The impact of modes on endoscopic visualization and retropulsion was assessed intraoperatively. Results. Statistical analysis of the specificities of the most often used modes was conducted: No 1 – «0.15 J, 200 Hz, 30 W», No 2 – «0.5 J, 30 Hz, 15 W», No 3 – «0.8 J, 31.25 Hz, 25 W», No 4 – «0.8 J, 37.5 Hz, 30 W», No 5 – «1.5 J, 15 Hz, 30 W», No 6 – «1.5 J, 26.6 Hz, 40 W», No 7 – «2 J, 15 Hz, 30 W», No 8 – «2 J, 20 Hz, 40 W», No 9 – «4 J, 10 Hz, 40 W»; their effects on retropulsion and on endoscopic visualization were studies. The stone free rate was assessed on the first post-operative day according to the findings of low-dose CT. Conclusion. Study of the modes of thulium fiber lithotripsy, assessment of retropulsion and endoscopic visualization depending on the choice of emitted radiation parameters of a thulium fiber laser make it possible to improve the outcomes of operative treatment in patients with nephrolithiasis. Key words: minipercutaneous, nephrolithotripsy, nephrolithiasis, MPNL, thulium fiber lithotripsy, retropulsion, visibility, superpulse thulium fiber laser, wavelength 1.94 μm, stone free rate

An efficient electric-discharge XeCl laser is developed, which is pumped by a self-sustained discharge with a prepulse formed by a generator with an inductive energy storage device and a semiconductor current interrupter on a basis of semiconductor opening switch (SOS) diodes. An output energy up to 800 mJ, a pulse length up to 450 ns, and a total laser efficiency of 2.2% were attained by using spark UV preionisation.

One of the important tasks in optoacoustics today is the development of methods and tools for generating high-frequency ultrasound (above 1 MHz) in liquids and other media. To expand the frequency range of ultrasound, it was proposed to use coatings consisting of focusing spheres on a fiber tip. The methodology of calculating the ultrasound spectra depending on the sphere size, index of refraction, and parameters of laser radiation was developed. Two cases of small and large spheres in strongly and weakly absorbing media were simulated. The experimental results were analyzed in the approximations allowing a fairly accurate estimation of the spectrum and indicatrix of the generated ultrasound upon laser excitation through a converter based on a coating of transparent spheres. A good agreement between the model and experimental result was obtained.

Evolution of plasma plume generation on the surface of metal irradiated by laser beam with the mean radiation flux density ~ 106 W/cm2 in the external electric field with different polarity and field strength from 0 to 106 V/m was experimentally investigated. It is shown that the mean size of metal droplets carried out from the irradiated zone of target becomes materially (in several times) smaller when of the external electric field strength amplitude grow, independently to its polarity. It is essential that the mentioned differences (at the considered parameters of laser radiation) are observed only at the initial stage of the laser plume development, because after the steam-plasma cloud reaches the electrode an electric breakdown (short-circuit) occurs, and the external field in the interelectrode gap disappears. Electric breakdown leads to the spasmodic increase of electron density and temperature of plasma and to effective absorption of laser radiation by plasma torch (shielding of the target). In consequence of shielding droplets generation happens only during electric field existence. This explains decrease by several times of the characteristic size of the target substance droplets in spite of short duration of electric field existence.

The spectral, kinetic, spatial, and amplitude characteristics of emissions of pressed samples of PETN (pentaerythritole tetranitrate) by the first harmonic of a neodymium laser (1064 nm, 12 ns) are studied. It is found that at the moment of irradiation of PETN by laser pulses, hot spots are formed on the surface of the samples. The energy density of laser radiation varies in the range of 0.5–3 J/cm2. The characteristics of hot spots are determined by absorbing inhomogeneities and parameters of laser radiation. Multi-pulse excitation leads to the annealing of absorbing irregularities and increased threshold of optical breakdown. The received results are interpreted on the basis of ideas of the low-threshold optical breakdown as evolving within local (defective) regions of the PETN in the electric field of the light wave.