High-power Nanosecond Research Articles

A compact high-power antenna for ultrawideband nanosecond bipolar pulse

In this article, the design procedures and radiation characteristics of a compact high-power antenna are discussed. The antenna is designed to efficiently radiate a high-power ultrawideband nanosecond bipolar pulse. The fabricated antenna realizes directive radiation and a wide bandwidth of 0.2–3.44 GHz (17.2:1) with dimensions of 300 × 300 × 260 mm3 (width × height × length). The measured far-voltage is 4.2 kV when the antenna is excited by a 2.5 kV amplitude bipolar pulse with a 1.3 nanosecond pulse width. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:1296–1301, 2015

Estimation of excited-state absorption and photobleaching in Fe²⁺-doped lithium sodium silicate glass under exposure to high-power nanosecond laser pulses.

Fe-doped lithium sodium silicate glasses codoped with Sn and C to promote the Fe²⁺ redox state are investigated under simultaneous excitation at the first and third harmonics of a nanosecond Nd:YAG laser. The aim is to evaluate critical parameters associated with the potential use of this material as an optical filter that transmits the third harmonic but blocks the fundamental frequency. Estimations of the excited-state absorption coefficient and photobleaching (reduction of absorption at the fundamental) are provided. The results provide insight on the design and expected operational parameters of this type of Fe-doped materials.

Amplification and formation of femtosecond laser single pulses upon the high-current electron beam pumping of Ti:Al2O3 crystals

Features of the high-power nanosecond X-ray and electronic excitation of Ti:Al2O3 crystals are investigated. It is shown that the coefficient for the single-pass amplification of femtosecond laser emission reaches 11 upon the high-current nanosecond electron beam pumping of Ti:Al2O3 crystals, along with the synchronous emission of a femtosecond laser single pulse under the action of the induced accompanying hypersonic wave.

Method of generating high-power microwave nanosecond pulses

The development of communication technology, radar, material processing, etc. stimulates the creation of powerful generators of microwave radiation with nanosecond pulse duration. A new method for generating microwave pulses of nanosecond duration is proposed. This method is based on the property of the redistribution of charges in the coaxial line with a multi-element core. The paper takes into account the time delay of fuse switching. Pulse power increases with delays of up to 5 ns. Pulse parameters depend on the profile of a multi-element core generator. On the criterion of maximum generator power the parameters of multi-element generator core construction are optimized. In view of the analysis pulse generator construction, which consists of four sections, has been created and tested. As a result, studies have shown that under the optimization of generator construction parameters the generator gain increases in power by 11 times.

High Energy and High Peak Power Nanosecond Pulses Generated by Fiber Amplifier

Generation of nanosecond pulses with 55-mJ energy and 4.8-MW peak power are demonstrated in a multistage fiber amplifier system, which is seeded with 10-nm broadband pulses from a superluminescence diode. The pulses are at low repetition of 10 Hz and with width of 10 ns. High energy and high peak power for nanosecond pulses are obtained simultaneously from fiber amplifier. In particular, the peak power is at least two times the previous peak power for nanosecond pulses.

Physics of Laser-Induced Plasma Streams Under Irradiation of Metals with Nanosecond Laser Radiation Pulses at Atmospheric Pressure

We have investigated the physics of the processes of incipient formation, development, and breakdown of vapor-plasma streams formed under the action of high-power (108–109 W/cm2) nanosecond laser pulses on massive metal targets in air. The main theoretical models of the process of laser erosion of metals by nanosecond pulses have been considered. A comparison has been made between the results of experimental and theoretical investigations of the laser erosion processes.

Modified Bridgman growth and properties of mid-infrared LiInSe2 crystal

A mid-infrared LiInSe2 (LISe) single crystal was successfully grown by the modified vertical Bridgman technique with dimensions of about ∅12mm×30mm. A large amount of high purity polycrystalline LISe used for crystal growth was synthesized in one run by a novel two-step synthesis method. The thermal properties of the crystal were carefully investigated by measuring the melting point, specific heat, and thermal diffusivity, and deriving the thermal conductivity. The transmission spectrum showed that the 50% transmittance level extends from 0.65 to 12.6μm. The excellent thermal conductivity and optical properties indicate that LISe is one of only a few promising candidates for high average power nanosecond OPOs pumped by ~1μm lasers.

Enhanced x-ray emissions from Au-Gd mixture targets ablated by a high-power nanosecond laser

As an important x-ray source, enhancement of x-ray emissions from laser-produced plasmas is imperative for various applications. High-Z Au-Gd mixture targets are proposed to enhance the laser to x-ray conversion efficiency compared to pure Au target. In the experiments, a 1 ns frequency-tripled (351 nm wavelength) laser light was used to obtain an intensity of 3×1014 W/cm2 on the targets. The x-ray spectra, total absolute x-ray emissions of all space, M-band fraction and backscattering from pure Au and Au-Gd mixture have been measured, respectively. It is shown that the absolute laser to x-ray conversion efficiency for the Au-Gd mixture containing 60% gold by atom is 47.7%, which has a 15% enhancement compared with that of the pure Au target. The experimental results are consistent with the radiation hydrodynamic simulations.

High-power Yb-doped continuous-wave and pulsed fibre lasers

High-power laser generation using Yb-doped double-clad fibres with conversion efficiencies in excess of 80% have attracted much attention during the last decade due to their inherent advantages in terms of very high efficiency, no misalignment due to in-built intracore fibre Bragg gratings, low thermal problems due to large surface to volume ratio, diffraction-limited beam quality, compactness, reliability and fibre-optic beam delivery. Yb-doped fibres can also provide a wide emission band from ∼1010 nm to ∼1170 nm, which makes it a versatile laser medium to realize continuous-wave (CW), Q-switched short pulse, and mode-locked ultrashort pulse generation for various applications. In this article, a review of Yb-doped CW and pulsed fibre lasers along with our study on self-pulsing dynamics in CW fibre lasers to find its role in high-power fibre laser development and the physical mechanisms involved in its generation has been described. A study on the generation of high-power CW fibre laser of 165 W output power and generation of high peak power nanosecond pulses from acousto-optic Q-switched fibre laser has also been presented.

Experimental simulation of materials degradation of plasma-facing components using lasers

The damage and erosion of plasma-facing components (PFCs) due to extremely high heat loads and particle bombardment is a key issue for the nuclear fusion community. Currently high current ion and electron beams are used in laboratories for simulating the behaviour of PFC materials under ITER-like conditions. Our results indicate that high-power nanosecond lasers can be used for laboratory simulation of high heat flux PFC material degradation. We exposed tungsten (W) surfaces with repetitive laser pulses from a nanosecond laser with a power density ∼ a few GW cm−2. Emission spectroscopic analysis showed that plasma features at early times followed by intense particle emission at later times. Analysis of laser-exposed W surface demonstrated cracks and grain structures. Our results indicate that the typical particle emission features from laser-irradiated tungsten are consistent with high-power particle beam simulation results.

A compact radiator of high-power electromagnetic pulses

The design and parameters of a compact radiator of high-power electromagnetic nanosecond pulses designed for simulation of ultra-wideband exposures in electromagnetic compatibility problems are described. It is shown that the effective pulse power of the radiator is 3 GW in the forward traveling wave and 15 GW in the wave reflected from the reflector. The effective density of the continuous energy spectrum is 17 mJ/MHz at 0.3 GHz.

Picosecond and nanosecond pulse delivery through a hollow-core Negative Curvature Fiber for micro-machining applications

We present high average power picosecond and nanosecond pulse delivery at 1030 nm and 1064 nm wavelengths respectively through a novel hollow-core Negative Curvature Fiber (NCF) for high-precision micro-machining applications. Picosecond pulses with an average power above 36 W and energies of 92 µJ, corresponding to a peak power density of 1.5 TWcm⁻² have been transmitted through the fiber without introducing any damage to the input and output fiber end-faces. High-energy nanosecond pulses (>1 mJ), which are ideal for micro-machining have been successfully delivered through the NCF with a coupling efficiency of 92%. Picosecond and nanosecond pulse delivery have been demonstrated in fiber-based laser micro-machining of fused silica, aluminum and titanium.

Dynamic cratering of graphite: Experimental results and simulations

The cratering process in brittle materials under hypervelocity impact (HVI) is of major relevance for debris shielding in spacecraft or high-power laser applications. Amongst other materials, carbon is of particular interest since it is widely used as elementary component in composite materials. In this paper we study a porous polycrystalline graphite under HVI and laser impact, both leading to strong debris ejection and cratering. First, we report new experimental data for normal impacts at 4100 and 4200 m s−1 of a 500-μm-diameter steel sphere on a thick sample of graphite. In a second step, dynamic loadings have been performed with a high-power nanosecond laser facility. High-resolution X-ray tomographies and observations with a scanning electron microscope have been performed in order to visualize the crater shape and the subsurface cracks. These two post-mortem diagnostics also provide evidence that, in the case of HVI tests, the fragmented steel sphere was buried into the graphite target below the crater surface. The current study aims to propose an interpretation of the results, including projectile trapping. In spite of their efficiency to capture overall trends in crater size and shape, semi-empirical scaling laws do not usually predict these phenomena. Hence, to offer better insight into the processes leading to this observation, the need for a computational damage model is argued. After discussing energy partitioning in order to identify the dominant physical mechanisms occurring in our experiments, we propose a simple damage model for porous and brittle materials. Compaction and fracture phenomena are included in the model. A failure criterion relying on Weibull theory is used to relate material tensile strength to deformation rate and damage. These constitutive relations have been implemented in an Eulerian hydrocode in order to compute numerical simulations and confront them with experiments. In this paper, we propose a simple fitting procedure of the unknown Weibull parameters based on HVI results. Good agreement is found with experimental observations of crater shapes and dimensions, as well as debris velocity. The projectile inclusion below the crater is also reproduced by the model and a mechanism is proposed for the trapping process. At least two sets of Weibull parameters can be used to match the results. Finally, we show that laser experiment simulations may discriminate in favor of one set of parameters.

X-Ray photoelectron spectroscopy-based analysis of change in the composition and chemical state of atoms on chalcopyrite and sphalerite surface before and after the nanosecond electromagnetic pulse treatment

Using X-ray photoelectron spectroscopy, the authors analyze the change in the surface layer composition and chemical state of atoms of chalcopyrite and sphalerite as a result of treatment by high-power nanosecond electromagnetic pulses. General regular patterns and distinctions of modification of the mineral surface and floatability under high-power pulse treatment are found. The research findings are supported by the results of experimental improvement gained in selective separation of sulfides after the high-power nanosecond electromagnetic treatment.

A high-power nanosecond pulse generator based on solid-state switches

A high-power nanosecond pulse generator based on a Lewis transformer and ultrafast IGBT-transistors is described. The generator ensures the formation of square pulses at a 50-Ω matched load with a repetition rate of up to 2 kHz. The pulse duration may be freely varied from 20 to 200 ns, and the pulse power may vary from 200 W to 2 MW.

Hybrid master oscillator power amplifier high-power narrow-linewidth nanosecond laser source at 257 nm

We report on a high-power narrow-linewidth pulsed laser source emitting at a wavelength of 257 nm. The system is based on a master oscillator power amplifier architecture, with Yb-doped fiber preamplifiers, a Yb:YAG single crystal fiber power amplifier used to overcome the Brillouin limitation in glass fiber and nonlinear frequency conversion stages. This particularly versatile architecture allows the generation of Fourier transform-limited 15 ns pulses at 1030 nm with 22 W of average power and a diffraction-limited beam (M(2)<1.1). At a repetition rate of 30 kHz, 106 μJ UV pulses are generated corresponding to an average power of 3.2 W.

Low repetition rate broadband high energy and peak power nanosecond pulsed Yb-doped fiber amplifier

We demonstrate a broadband fiber amplifier system emitting 10nm pulses with 33mJ pulse energy at repetition rates between 1 and 20Hz. In combination with a pulse width of 10ns, a corresponding peak output power over than 3MW was achieved. This record performance was enabled by fiber with core diameter of 200μm. Furthermore, in order to suppress amplified spontaneous emission (ASE), multi-stage pulse pump technology was applied and different pumping characteristics had been measured. Finally, 15% extraction efficiency was obtained when output energy reached 33mJ. Output capability of the amplifier was not limited by SBS and ASE but the pump peak power.

High Energy Terahertz Parametric Oscillator Based on Surface-Emitted Configuration

We experimentally demonstrate a high power nanosecond pulsed terahertz (THz)-wave parametric oscillator (TPO) by using a wide pump beam. A surface emitted cavity configuration is employed to reduce the THz absorption in MgO:LiNbO3 crystal. The THz wave can be tuned from 1 THz to 3 THz. A maximum THz output energy of 438 nJ/pulse is achieved at 1.56 Hz using a 4.5-mm-diameter pump beam with a pulse energy of 226 mJ pump energy with the repetition of 10 Hz, corresponding to the energy conversion efficiency of 1.94 × 10−6.

Subnanosecond Semiconductor Opening Switch Based on 4H-SiC Junction Diode

Mesa-epitaxial 4H-SiC p+-p-no-n+-diodes were fabricated and their reverse recovery characteristics were measured in pulse regimes to be relevant to DSRD- and SOS-modes of operation [I.V. Grekhov, G.A. Mesyats, Physical basis for high-power semiconductor nanosecond opening switches, IEEE Transactions on Plasma Science 28 (2000) 1540-1544]. It has been found that after short pumping the diodes by forward current pulse (5-ns duration, 200-A/cm2 peak current density) followed by applying the reverse voltage pulse (rise time 2 ns) the diodes are able to interrupt the reverse current density of 3.5 - 25 kA/cm2 in a time less than 0.3 ns.

高功率纳秒光纤激光器抽运锥形光子晶体光纤产生超连续谱

高功率纳秒光纤激光器抽运锥形光子晶体光纤产生超连续谱