Letteratura scientifica selezionata sul tema "Saturable pulse transformer"

Abstract Solid-state compact Marx generator using saturable pulse transformer (SPT) and fast recovery diodes has been proposed. The primary circuit is switched by three MOSFETs connected in parallel. The SPT functions as a step-up transformer to increase the voltage amplitude and as a closing switch for the secondary circuit. Meanwhile, all the SPTs share the same magnetic core to achieve a compact structure and ensure good synchronization. The energy storage capacitors on the secondary sides are charged through the unsaturated SPT. When the SPT saturates, the capacitors firstly transfer a little energy to the saturated inductors through the diodes reversely during their reverse recovery process. Currents rise quickly in these inductors until diodes totally recover to reverse blocking state. Then capacitors discharge in series to the load and high-voltage pulses are generated over the load. With the currents in the saturated inductors, the front edges of pulses are no longer affected by them but are dominated the turn-off speed of the diodes, which makes high-voltage and high-current pulses with short front edges possible. The regular and cheap fast recovery diodes in the generator act as semiconductor opening switch to sharpen the pulse front edges. Experiments were carried out with a 4-stage Marx generator prototype, 10.8-kV high-voltage pulses with a front edge of 11 ns, a pulse width of 190 ns, were obtained over a 100-Ω resistive load. The total energy efficiency is 49.8%. The proposed Marx generator using regular fast recovery diodes is compact, cheap, and efficient to generate high-voltage pulses with short front edges.

AbstractHigh-voltage pulse modulator has broad applications in industry. In order to pursue the qualities of compactness, solidification, and long life time, a high-voltage pulse modulator based on a helical Blumlein pulse forming line (HBPFL),a Marx generator and a self-coupling saturable pulse transformer (SPT) with fully cylindrical coaxial conductors is put forward and investigated in this paper. A new method that the fully cylindrical SPT simultaneously works as the charging pulse transformer and magnetic switch of the HBPFL is put forward and demonstrated. Traditional spark gap is substituted by the SPT to enable the features of solidification, compactness, and long life time of the modulator. Experimental results showed that the SPT had good response characteristics to short sinusoidal pulse and 100 ns-range square pulses. The fully cylindrical SPT driven by the 50–70 kV Marx generator can suppress the saturated inductance of the secondary windings to a level less than 500 nH, due to the strong reversed mutual induction between cylindrical windings after the core saturated. It also demonstrated that the pulse modulator was able to deliver a high-voltage pulse to a 160 Ω load, with amplitude of 148 kV, pulse duration of 130 ns, and pulse rise time ranging from 60 to 105 ns.

A fast-charging, discharge-switch-free Blumlein pulse forming line has been developed for high-voltage pulsed power generation. In the BL, a saturable charging inductor (CI) of amorphous metallic core is utilized and, as a result, fast-charging (charging time ≈220 ns) is obtained with a reduced prepulse. In addition, by using CI as a step-up transformer, the impedance of the output pulse can be converted to 4Z, Z, Z/4. By using the BL with a Marx generator of 300 kV and 1.1 kJ, an output of —580 kV at 24 kA and a pulse length of 60 ns are obtained, with a current rise time of less than 16 ns. The energy transfer efficiency of the line (output pulse energy/charging energy of a pulse forming line) is evaluated to be more than 92%.

In this paper, an all-solid-state high voltage trigger generator is developed, which is aimed at triggering a several gigawatts three-electrode spark gap of an intense electron beam accelerator (IEBA). As one of the most important parts for triggering the IEBA precisely, it is developed based on a fractional-turn ratio saturable pulse transformer and a compact six-stage Marx generator. A pulse of rising time 141 ns and amplitude 79.6 kV is obtained on the 1000 Ω dummy load. The trigger is operated at pulsed repetition frequency over 10 Hz for testing its operational stability. The jitter counted from the initial control signal to the falling edge of the pulse is 0.64 ns. In addition, experiments of three-minute continuous repetitive operations at 10 Hz and higher frequency are carried out. The results show that the trigger generator has high stability even in long-time operations. So far, it successfully applies to the main switch of IEBA with a breakdown voltage of over 500 kV, and a total system jitter of 6.7 ns is acquired.

Dans les systèmes de hautes puissances pulsées, le stockage inductif présente un avantage indéniable vis-à-vis du stockage capacitif du fait de sa plus forte densité d’énergie. L’exploitation de cet avantage nécessite toutefois l'utilisation d'un interrupteur à ouverture pour générer l'impulsion de tension. En outre, compte tenu de la demande croissante de générateurs impulsionnels fiables, en particulier pour les applications industrielles, il devient indispensable de recourir aux composants semi-conducteurs. La diode SOS (Semiconductor Opening Switch), développée dans les années 1990 à l'Institute of Electrophysics en Russie, est un candidat idéal pour la commutation état solide à ouverture, de par sa capacité à générer des impulsions de haute puissance de manière fiable et répétitive, tout en offrant une longue durée de vie et un fonctionnement exempt de maintenance. Cependant, le manque de fabricants de diodes SOS limite leur utilisation à grande échelle. Par conséquent, cette thèse se concentre sur l’étude de diodes disponibles dans le commerce (OTS : Off-The-Shelf) capables de commuter rapidement des courants élevés et de générer des tensions nanosecondes pouvant atteindre 500 kV. Plusieurs types de diodes, incluant les diodes de redressement, à avalanche, à temps de récupération rapide et de suppression de tension transitoire (TVS) ont été étudiés en tant qu’interrupteurs à ouverture, en comparaison avec les diodes SOS de référence. Pour mener à bien cette étude, des bancs d’essai à basse, moyenne et haute énergie (respectivement 25 mJ, 10 J et 40 J) ont été mis au point. Afin d’augmenter leur efficacité énergétique, ces bancs utilisent un circuit basé sur un élément magnétique unique : un transformateur impulsionnel saturable. Plusieurs noyaux magnétiques nanocristallins ont été examinés sur le banc de 10 J dans le but d’optimiser les performances du transformateur. Parmi les diodes étudiées sur les bancs de 25 mJ et 10 J, les diodes TVS et les diodes de redressement ont émergé du lot, démontrant des performances de temps de commutation de l'ordre de la nanoseconde et de tensions générées de plusieurs kilovolts. Enfin, un prototype de générateur de hautes puissances pulsées de 40 J (GO-SSOS) basé sur un interrupteur OTS composé de diodes de redressement a été développé. Le rendement énergétique du système varie de 35% à 70% selon la valeur de la charge, et la puissance crête obtenue est supérieure à 300 MW. Sur une charge de 1 kΩ, l'impulsion de tension générée atteint une amplitude de 500 kV avec un temps de montée de 36 ns et une largeur à mi-hauteur de 80 ns. La reproductibilité des impulsions à une fréquence de répétition de 60 Hz est démontrée, ainsi qu’une application de génération de décharges couronnes. Les travaux prouvent la fiabilité des diodes OTS en mode SOS, ne révélant aucune dégradation après quelques milliers d'impulsions générées. Ils ouvrent également la voie à l’utilisation de cette technologie pour des applications industrielles telles que la stérilisation par faisceau d’électrons
In pulsed power systems, inductive energy storage has an advantage over capacitive storage because of its higher energy density. Exploiting this advantage requires the use of an opening switch to generate the voltage pulse. Moreover, the growing need for reliable pulsed power generators, particularly for industrial applications, strongly supports the adoption of solid-state solutions. The Semiconductor Opening Switch (SOS) diode developed in the 1990s at the Institute of Electrophysics in Russia is an ideal candidate for solid-state opening switching because of its ability to reliably generate high-power pulses at high repetition rates while offering long lifetime and maintenance-free operation. However, the lack of SOS diode manufacturers prevents their widespread use. This thesis is therefore devoted to the study of off-the-shelf (OTS) diodes capable of rapidly switching high currents and generating nanosecond voltages of up to 500 kV. The research includes the investigation of various diode types including rectifier, avalanche, fast recovery, and transient voltage suppression (TVS) diodes as opening switches in comparison with state-of-the-art SOS diodes. Low, medium, and high-energy (25 mJ, 10 J, and 40 J respectively) test benches are developed for the experiments. Their circuits use a single magnetic element – a saturable pulse transformer – resulting in high energy efficiency. Several nanocrystalline cores are examined for optimum transformer performance at an energy of 10 J. Among the diodes investigated at 25 mJ and 10 J energy, the TVS and rectifying diodes stand out particularly promising with nanosecond switching time and generated voltages in the kilovolt range. Finally, a 40 J pulsed power generator prototype (GO-SSOS) based on an OTS opening switch consisting of rectifier diodes is developed. The GO-SSOS achieves a peak power of more than 300 MW with an energy efficiency ranging from 35% to 70% depending on the load value. Across a 1 kΩ load, the voltage pulse generated reaches 500 kV amplitude with a rise time of 36 ns and a pulse width of 80 ns. The system shows high reproducibility at a repetition rate of 60 Hz and is used to demonstrate a corona discharge application. The work proves the reliability of the OTS diodes in SOS mode, revealing no degradation after thousands of pulses. It also offers the prospect of using this technology in industrial applications such as electron-beam sterilization

Microchip lasers are the most compact (~1mm3) and the simplest diode pumped solid state lasers The mirrors of the optical cavity are directly deposited on the polished faces of a thin (-1mm) laser material. They are fabricated using collective fabrication processes allowing a low cost mass production. A microchip laser has a typical size of 1×1×0.5 mm3. It is very reliable and simple to use. The microchip laser is a kind of optical transformer which transforms a poor quality laser diode beam to a diffraction limited TEM00 and single frequency laser beam Moreover, by active or passive Q-switching , very short pulses (-0.4 to 2 ns) with very high peak power (0.5 to 50 kW) can be obtained. Standard wavelengths are: 1μm or 1.3μm with Nd doped materials such as YAG, YVO4, LMA, etc.; 1.5μm with Er and Yb co-doped phosphate glass; 2μm with Tm doped materials. The laser threshold is generally low, i.e as low as 9mW for 1.5μm microchip laser with a slope efficiency of 38%. Passively Q-switched Nd:YAG microchip laser can be fabricated using a novel process. A thin film (~30-50μm) of Cr4+:YAG, which is a saturable absorber at 1064nm, is epitaxially grown on 25mm diameter and 0.5 to 1mm thick Nd:YAG subtrates. The absorption coefficient of the epitaxial thin film saturable absorber is ~20-25 cm-1, about five times higher than the bulk Cr4+:YAG. The saturable absorption is adjusted by polishing the thin film. This process has the advantage to keep the monolithic structure of the microchip laser without any optical interface between the active and passive materials, and allows a great flexibility for the design of Q-switched lasers. Typical pulse energies are between 0.5 to 5μJ using low power diode pumps, and the pulse width is 0.4 to 2ns. Repetition rates as high 100kHz have been obtained. The pulse energy, width and peak power are independant of the pump power, which acts only on the pulse repetition rate which increases linearly with the diode pump power. The plane-parallel Fabry-Perot cavity of the microchip laser is optically stabilized by thermal effects due to the heating by the pump power. The laser mode is also defined by thermal effects. In order to avoid this problem, stable plano-concave optical cavities are fabricated using photolithography and ion beam etching technologies such as used currently in microelectronics. Small microlenses with 100-200μm in diameter and 1μm thick are directly fabricated with a collective process on laser materials such as Nd:YAG or Er,Yb:glass. The microlense acts as a concave micromirror for the microchip laser cavity, with a radius of curvature of few mm, so that the cavity is optically stable and the laser mode is well defined. The laser threshold is drastically reduced, i.e. less than 2mW incident pump power in CW mode Moreover, this technology allows also the fabrication of micro-optical components, such as microlenses on silica substrates, which can be used for the fabrication of optical microsystems including microchip lasers. For exemple such microlenses are used for the fiber coupling of a stable cavity microchip laser. The low cost, the re lability, the excellent beam quality and the high peak power of the microchip lasers are very useful for several industrial applications such as: time of flight range finding; micro-marking of materials; compact green laser for alignment; injection locking; etc.

Mode locking with an antiresonant ring is a powerful technique for producing stable trains of ultrashort pulses. This structure1 was first applied to visible dye lasers by placing a saturable absorber within an antiresonant ring that forms the high-reflectivity end mirror of a linear laser cavity. Nearly transform-limited 65 fs pulses have been generated with enhanced pulse stability.2 By using the additive pulse mode-locking (APM) representation, we describe the antiresonant ring (ARR) as a pulse-shaping reflector, which returns a temporally narrowed pulse to the main laser cavity. When a nonlinear element is incorporated within the ARR, an intensity-dependent reflectivity is predicted. A nonlinear element introduces temporal or spectral pulse shaping of the two counterpropagating fields within the ring. If the pulse shaping is unbalanced for the two fields, then preferential reflection of the high-intensity portions of the incident pulse can occur, resulting in a temporally narrowed pulse being returned to the laser gain medium. Results will be presented for a saturable absorber, a saturable amplifier, and a Kerr-type nonlinearity, such as an optical fiber.

Recent progress in laser diodes with an ion-implanted fast saturable absorber resulted in generation of ultrashort optical pulses both in the gain-switching and in the passive mode-locking regimes.1-3 Normally, an implanted laser generates low intensity non-transform-limited pulses or pulse trains. Application of a fiber amplifier to increase the signal intensity and to improve the pulse quality is a promising way of obtaining soliton-like pulses.

We examine, experimentally and theoretically, the nonlinear dynamics of a laser which generates pulses as short as 27 fs directly from the laser (1), Fig. 1 . The laser shapes the intracavity pulses by a balanced combination of self phase modulation, group velocity dispersion, saturable absorption, and saturable gain which appears different from the shaping mechanisms in earlier lasers where the pulse shaping was due principally to saturable absorption and gain (2), or to soliton-like mechanisms in an optical fiber (3). The shaping which we discuss also offers advantages over prior modelocking mechanisms in that it yields enhanced stability, a close approach to transform limit, and pulses which are, to our knowledge, the shortest yet generated from a laser.

The combination of a GaAs semiconductor saturable absorbing mirror (SESAM)1 and a low-threshold laser geometry2 has allowed the efficient production of stable low-noise femtosecond pulses. The highly asymmetric 4-mirror Cr3+LiSAF laser was pumped by 2 polarisation-combined narrow-stripe AlGaInP laser diodes each providing a maximum available power of 40mW incident on the crystal facet. The cw oscillation threshold of the laser was 16mW and the pulse repetition frequency was 153MHz. When output coupling through a high reflecting (HR) end mirror, the laser produced stable, sub 70 fs transform limited pulses at an average output power of 1mW. Modelocking could be sustained with pump powers as low as 21 mW.

We have doubled the tuning range of a passively mode-locked NaCl color-center laser by introducing a low-loss intracavity frequency filter and properly preparing the NaCl gain crystal. By using a collection of bulk and multiple-quantum-well III–V saturable absorbers and II–VI quantum-well saturable absorbers, we obtained passive mode-locked operation over the wavelength range of 1.5–1.7 µm. To provide sufficient wavelength restriction without introducing excess bandwidth limiting or loss, we replaced the biréfringent plate of previous systems1 with a tunable frequency filter,2 formed by a pair of equilateral flint glass prisms (spaced 22 cm apart) and followed by a knife edge. We tune the laser by horizontal translation of the knife edge. The laser produces pedestal-free, near-transform-limited 280 fs pulses, which is the same pulse width obtained with birefringent plates. To obtain lasing at wavelengths shorter than 1.51 µm, the NaCl YAG crystal must be annealed to reduce scattering from presence of colloidal particles formed during coloration. With annealed crystals we obtain lasing at wavelengths as short as 1.38 µm, and laser power at 1.5 µm is only 30% below the maximum. In summary, we demonstrate a stable, single-cavity solidstate, subpicosecond pulse source of 1.5–1.7 µm.

In recent years, many solid-state lasers have been mode-locked successfully, using the additive pulse mode-locking (APM) principle. It has been shown that, under appropriate approximations, the theory of APM is congruent with that of saturable absorber mode-locking, supplemented by self-phase modulation and group velocity dispersion. When the shortest pulses are achieved, the pulses behave like transform-limited solitons, perturbed by the gain-bandwidth limiting and the APM action. In this limit, a perturbation theory is feasible that is closely related to that of soliton perturbation theory. Noise is handled by perturbation theory; an adiabatic perturbation of the pulse changes its amplitude, phase, carrier frequency and timing. Contrary to soliton perturbations, amplitude and frequency perturbations of mode-locked solitary pulses do not persist but shed some of their energy into the continuum. This is a consequence of the gain saturation and gain-bandwidth limitation not encountered with solitons (nonlinear Schrodinger equation). The timing experiences a random walk, and so does the phase. The latter leads to a Lorentzian line-shape of the resonant cavity modes, similar to that of a free-running oscillator. The spectrum of the amplitude and frequency are Lorentzian because both have nonzero relaxation times. The frequency relaxes to the center frequency of the gain, the amplitude to the value required to equate gain to loss.