High efficiency nanosecond generator based on semiconductor opening switch

Power semiconductor devices (MOSFET, IGBT, SIThy, and SOS) have been studied for applications in repetitive pulsed power generation. These switching devices are expected to be used in pulsed high-voltage modulators for accelerators and power supplies for pulsed atmospheric discharges. MOSFET and static-induction thyristor (SIThy) are investigated for their performance in high-voltage modulation at repetition rate on the order of MHz, which is demanded by applications in high-energy particle accelerators. Although the peak voltage and current are not very high (usually a few kV and a few 10s of A respectively), the repetition rate poses fatal problems to the devices due to the thermal effect. The system cooling and waveform control had been the main efforts in this study. IGBT and semiconductor opening switch (SOS) are studied for applications to atmospheric gas discharge, which has become very attractive recently due to its applications in gas-treatment, laser pumping, and sterilization. The electrical circuit scheme is based on inductive energy storage with opening switch, which gives pulsed output high-voltage up to 100 kV for high-impedance load. The study has concentrated on repetition rate and energy efficiency. These developments are joint studies between universities, national laboratories, and industrial companies.

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.

Based on the SOS + MCL approach (SOS-generator with output Semiconductor Opening Switch; MCL-Magnetic Compression Line), a high-power solid-state picosecond system has been developed. The SOS generator forms an initial pulse with a power of 8.5 GW (0.65 MV at a load of 50Ω) and a duration of 7ns, which is fed to the input of a magnetic compressor containing 4 lines MCL1-MCL4 connected in series. Structurally, each MCL is a coaxial line and contains ferrite rings placed on its inner conductor and magnetized by an external longitudinal magnetic field. The lines are filled with transformer oil at excess pressure. Each line compresses the pulse in time and increases its peak power. At the output of the last line, MCL4, the peak power increases to 100 GW (2.2 MV at 48Ω load), and its duration decreases to 100ps. Record high values of rise rates for voltage, current, and power have been achieved. At a 48Ω load, the voltage rise rate reaches 27 MV/ns, and the power rise rate reaches 2 TW/ns. In a line with an impedance of 7Ω, a current pulse with an amplitude of 100kA and a rise rate of 1.18 MA/ns was obtained. It has been established that the duration of the pulse (full width at half maximum) generated in MCLs corresponds to the double transit time of an electromagnetic wave between the electrodes of the line in the transverse direction. Numerical models have been developed to study the operation of MCLs. Using the models, the internal structure of the process of power amplification in the line was studied, and an assessment was made on the influence of the processes of gyromagnetic precession on the operation of the lines.

Frequency nanosecond electron accelerators type URT have been developed for use in radiation technologies. The accelerators include a thyratron, a pulse transformer, and a semiconductor opening switch. The accelerating voltage is up to 0.9 MV, FWHM pulse length is up to 50 ns, and the operating frequency is up to 250 Hz. A metal ceramic cathode, which is used in the accelerator, provides an electron beam up to 160 mm in diameter at a maximum density of the pulse current equal to 2 A/cm2. A vacuum diode has been developed for two-sided irradiation of materials. The cathode holder of the vacuum diode is designed as two symmetric arms, which are connected to a common current conductor and mount two cathodes at the ends one opposite to the other. The vacuum diode with dielectric metal cathodes was tested in a URT-0.5 electron accelerator (the electron energy being up to 0.5 MeV and the average output power of the beam up to 1 kW).

The use of an electrode system with a plasma cathode and ultraviolet preionization of the discharge gap in a CO/sub 2/--air mixture at a pressure of 130 Torr produced an average output power of 500 W when the pulse repetition frequency was 250 Hz and the energy per pulse was 2 J. The addition of easily ionizable components made it possible to reduce the ratio E/p from 24 to 17 Vxcm/sup -1/xTorr/sup -1/ and to increase the efficiency from 7 to 11%. It was found that E/p and the discharge efficiency were independent of the voltage across the storage capacitor and were governed only by the composition of the active mixture. The average output power increased linearly with the pulse repetition frequency. The maximum repetition frequency was found to be 1.3 times less than the calculated value.

A ns pulse voltage was used to drive a coaxial geometry corona reactor to synthesis ozone with high energy yield. The ns pulse voltage was produced using an inductive energy storage system pulsed power generator using semiconductor opening switch (SOS) diodes. First recovery diodes were used as SOS diodes in the inductive energy storage system pulsed power generator to produce short-pulse high voltage with high-repetition rate. The pulse voltage of 12 ns width and 17 kV peak voltage was produced at charging voltage of -5 kV and was applied to a 1 mm diameter center wire electrode in the coaxial geometry reactor. The copper cylinder of 19 mm inner diameter was used as outer electrode and was connected to a ground. The ozone yield of 230 g/kWh was obtained using ns narrow pulse voltage. This value is almost 20% higher than 190 g/kWh obtained by 60 ns width pulse.

This paper describes a high-current nanosecond generator providing peak power of up to 4 GW, output voltage of 0.4-1 MV, pulse length of 8-10 ns, and pulse repetition rate of 300 Hz in the continuous mode and up to 1 kHz in the burst mode of operation. The average output power is up to 30 kW at a pulse repetition rate of 1 kHz. The generator is outfitted with an all-solid-state system of energy switching. The output pulse is formed by semiconductor opening switch diodes. The electric circuit and the design of the generator have been described. Experimental results have been given. A device for the elimination of prepulses across the load has been proposed. Testing results and numerical modeling of the device have been reported

The general description, mathematical analysis and simulation research results of a satisfactory method concerning pulse repetition frequency (PRF) choice is presented in the paper. A low PRF radar with MTD signal processing is taken into consideration. The worked out method of PRF selection is satisfactory because it ensures the best quality of a target's unambiguous radial velocity measurement (URVM) under a chosen criterion. According to the chosen criterion new tools for PRF selection based on genetic algorithms are worked out. The essential comparable simulation research results are performed. They show significant improvement of the quality indexes of the radial velocity measurement that uses the PRF selection method based on genetic algorithms. The problems related to overcoming the influence of the diversity frequency mode on the URVM algorithm quality indexes are considered too. From this point of view two solutions are proposed. The first concerns modification of the genetic algorithm used for selection of the PRF; the second relates modification of the URVM algorithm. The simulation research results presented here show the limitations concerning use of both of the modifications.

In this paper, using two repetitive nanosecond generators, x-rays were detected in atmospheric air with a highly inhomogeneous electric field by a point-to- plane gap. The rise times of the generators were about 15 and 1 ns. The x-rays were directly measured by various dosimeters and a NaI scintillator with a photomultiplier tube. X-rays were detected in the continuous mode at pulse repetition frequency up to 1 kHz and a voltage pulse rise time of ∼15 ns. It is shown that the maximum x-ray intensity is attainable at different pulse repetition frequencies depending on the voltage pulse parameters and cathode design. In atmospheric pressure air the x-ray intensity is found to increase with increasing the pulse repetition frequency up to 1 kHz. It is confirmed that the maximum x-ray intensity is attained in a diffuse discharge in a point-to-plane gap.

We report the demonstration of high-average-power passively Q-switched laser oscillation from Fe2+ ions in zinc selenide. A semiconductor saturable absorber mirror was used as a passive Q-switch element. Using a 60% R outcoupler, the pump-limited output power was 515 mW. The spectral center of the laser was 4045 nm. The pulse repetition frequency (PRF) at maximum power was ~ 850 kHz with a corresponding minimum pulsewidth of 64 ns Full-Width Half-Maximum. The pulse energy and peak power were and 8.3 W, respectively. The average output power was limited only by available pump power and increased with a slope efficiency of 22%. No thermal rolloff of slope-efficiency was observed. The beam quality was measured to be M2 ≤ 2.6. The temporal stability of the pulsed output was characterized. Thermal effects were shown to play a significant role in determining the PRF of the output.

SP0021 How to Optimize Doppler Settings for Fast Flow

A continuous-wave (CW) and Q-switched operation of a Ho:YLF laser resonantly, single-pass pumped by a Tm: fiber laser operating at a wavelength of 1940nm is presented. At room temperature, for an output coupler of 40% transmission, a maximum CW output power of 11.5W for 32.5W of incident pump power was achieved, corresponding to a slope efficiency of 40%. Applying Q-switching operation, and under CW pumping regime, the pulse repetition frequency (PRF) was changed from 1 to 10kHz. The maximum average output power of 11.23W at the PRF of 10kHz was obtained. Lowering the PRF to 1kHz allows achieving pulse energy of 4.97mJ in a 19ns pulse corresponding to the maximum peak power of 260kW. The Ho:YLF laser operated at the wavelength of 2050.5nm with a FWHM line width of 1.09nm. The output parameters of the laser radiation are sufficient enough to be used for range finding or for pumping mid-infrared optical parametric oscillators. Full Text: PDF References K. Scholle, S. Lamrini, P. Koopmann, P. Fuhrberg, chapter 21 in Frontiers in Guided Wave Optics and Optoelectronics, INTECH, ISBN 978-953-7619-82-4, 2010. J. Kwiatkowski, J.K. Jabczynski, W. Zendzian, L. Gorajek, M. Kaskow, "High repetition rate, Q-switched Ho:YAG laser resonantly pumped by a 20 W linearly polarized Tm: fiber laser", Appl. Phys. B 114, 395 (2014). CrossRef W. Koen, C. Bollig, H. Strauss, M. Schellhorn, C. Jacobs, M.J.D. Esser, "Compact fibre-laser-pumped Ho:YLF oscillator?amplifier system", Appl. Phys. B 99, 101 (2010). CrossRef Y.J. Zhang, G. Li, Y.Y. Zhao, Y.N. Chen, "In-band pumped Ho:YAP laser by Tm-Doped fiber laser at 1936 and 1948 nm, respectively", Laser Phys. 22, 415 (2012). CrossRef H. Chen, D.Y. Shen, X.D. Xu, T. Zhao, X.F. Yang, D.H. Zhou, J. Xu, "High-power 2.1 ?m Ho:Lu 1.5 Y 1.5 Al 5 O 12 laser in-band pumped by a Tm fiber laser ", Laser Phys. Lett. 9, 26 (2012). CrossRef J.W. Kim, J.I. Mackenzie, D. Parisi, S. Veronesi, M. Tonelli, W.A. Clarkson, "Efficient in-band pumped Ho:LuLiF 4 2?m laser", Opt. Lett. 35, 420 (2010). CrossRef B.Q. Yao, X.M. Duan, Y.L. Ju, Y.Z. Wang, "Continuous wave operation of singly-doped Ho:LSO laser at room temperature", Laser Phys. 21, 1888 (2011). CrossRef X.J. Cheng, J.Q. Xu, M.J. Wang, B.X. Jiang, W.X. Zhang, Y.B. Pan, "Ho:YAG ceramic laser pumped by Tm:YLF lasers at room temperature", Laser Phys. Lett. 7, 351 (2010). CrossRef A.S. Kurkov, V.A. Kamynin, E.M. Sholokhov, A.V. Marakulin, "Mid-IR supercontinuum generation in Ho-doped fiber amplifier", Laser Phys. Lett. 8, 754 (2011). CrossRef B.M. Walsh, G.W. Grew, N.P. Barnes, "Energy levels and intensity parameters of Ho 3+ ions in GdLiF 4 , YLiF 4 and LuLiF 4 ", J. Phys.: Condens. Matter 17, 7643 (2005). CrossRef

Continuous-wave and high repetition rate Q-switched operation of Ho:YLF laser in-band pumped by a linearly polarized Tm:fiber laser

3D LIDARs (Light Detection and Ranging) with 1.5μm nanosecond pulse lasers have been increasingly used in different applications. The main reason for their popularity is that these LIDARs have high performance while at the same time can be made eye-safe. Because the laser hazard effect on eyes or skin at this wavelength region (<1.4μm) is mainly from the thermal effect accumulated from many individual pulses over a period of seconds, scanning can effectively reduce the laser beam hazard effect from the LIDARs. Neptec LIDARs have been used in docking to the International Space Station, military helicopter landing and industrial mining applications. We have incorporated the laser safety requirements in the LIDAR design and conducted laser safety analysis for different operational scenarios. While 1.5μm is normally said to be the eye-safe wavelength, in reality a high performance 3D LIDAR needs high pulse energy, small beam size and high pulse repetition frequency (PRF) to achieve long range, high resolution and high density images. The resulting radiant exposure of its stationary beam could be many times higher than the limit for a Class 1 laser device. Without carefully choosing laser and scanning parameters, including field-of-view, scan speed and pattern, a scanning LIDAR can’t be eye- or skin-safe based only on its wavelength. This paper discusses the laser safety considerations in the design of eye-safe scanning LIDARs, including laser pulse energy, PRF, beam size and scanning parameters in two basic designs of scanning mechanisms, i.e. galvanometer based scanner and Risley prism based scanner. The laser safety is discussed in terms of device classification, nominal ocular hazard distance (NOHD) and safety glasses optical density (OD).

Mobile weather radars at high frequencies (C, X, K, and W bands) often collect data using staggered pulse repetition time (PRT) or dual pulse repetition frequency (PRF) modes to extend the effective Nyquist velocity and mitigate velocity aliasing while maintaining a useful maximum unambiguous range. These processing modes produce widely dispersed “processor” dealiasing errors in radial velocity estimates. The errors can also occur in clusters in high shear areas. Removing these errors prior to quantitative analysis requires tedious manual editing and often produces “holes” or regions of missing data in high signal-to-noise areas. Here, data from three mobile weather radars were used to show that the staggered PRT errors are related to a summation of the two Nyquist velocities associated with each of the PRTs. Using observations taken during a mature mesoscale convective system, a landfalling tropical cyclone, and a tornadic supercell storm, an algorithm to automatically identify and correct staggered PRT processor errors has been developed and tested. The algorithm creates a smooth profile of Doppler velocities using a Savitzky–Golay filter independently in radius and azimuth and then combined. Errors are easily identified by comparing the velocity at each range gate to its smoothed counterpart and corrected based on specific error characteristics. The method improves past dual PRF correction methods that were less successful at correcting “grouped” errors. Given the success of the technique across low, moderate, and high radial shear regimes, the new method should improve research radar analyses by affording the ability to retain as much data as possible rather than manually or objectively removing erroneous velocities.