Flat-top Pulse Research Articles

Testing and Commissioning of a 135 MW Pulsed Power Supply at the Wuhan National High Magnetic Field Center

At the Wuhan National High Magnetic Field Center, two 67.5-MW pulsed power supply units have been installed to energize long-pulse magnets. Each unit based on a 12-pulse converter consists of two 6.9 kV/1.2 kV (1.0 kV) cast coil transformers and two six-pulse rectifiers connected in series. The units are connected to a 100 MVA/185 MJ inertial energy storage motor-generator set and can operate independently as well as in parallel or in series. Each unit is designed to provide no-load voltages of 3.2 kV (2.7 kV) and full-load voltages of 2.7 kV (2.25 kV) at the rated current of 25 kA for a 3-s pulse once every half hour. After completion of various performance tests, the power supply commissioning started in October 2012. At present, the power supply has energized a long-pulse magnet more than 100 times to produce flat-top pulses at various field levels, including a 50-T field pulse with 100-ms flat-top duration. In this paper, the final design of the power supply components is described in detail. Testing and commissioning results of the power supply taken with a dummy load and a long-pulse magnet are also presented.

Generation of optical frequency combs based on time‐to‐frequency conversion

Flat-top optical frequency combs (OFC) are generated using electro-optical phase modulators (PM) driven by sinusoidal signals as a time-to-frequency convertor. A linear equation is derived to predict the number of OFC lines according to the time duration of the flat-top pulse inputted to the PM and the modulation index of the PM. Both the simulated and experimental results confirm the validity of the proposed method.

Temporally flat top pulse generation from gain switched semiconductor lasers based on a polarization interferometer with variable transfer function

We propose the use of a polarization based interferometer with variable transfer function for the generation of temporally flat top pulses from gain switched single mode semiconductor lasers. The main advantage of the presented technique is its flexibility in terms of input pulse characteristics, as pulse duration, spectral bandwidth and operating wavelength. Theoretical predictions and experimental demonstrations are presented and the proposed technique is applied to two different semiconductor laser sources emitting in the 1550nm region. Flat top pulses are successfully obtained with input seed pulses with duration ranging from 40ps to 100ps.

Flat-top picosecond pulses generated by chirped spectral modulation from a Nd:YLF regenerative amplifier for pumping few-cycle optical parametric amplifiers

In this paper we present an optically synchronized Nd:YLF regenerative amplifier optimized for use as a preamplifier in a few-cycle optical parametric chirped pulse amplification pump laser. In the pump amplification process we employ a combination of spectral modulation and chirping in order to control and optimize the temporal shape of the pulses. We report on a comparative study of two methods for generating near-flat-top or custom real-time variable-shaped pump pulses using either controlled chirp and shaping of the spectrum of the pulses seeding a regenerative amplifier or intracavity spectral filtering to broaden the gain bandwidth of the system. We show that in addition to minimizing gain narrowing and B-integral, the efficiency of the cascaded nonlinear processes of the parametric amplifiers can be increased.

Temporal widening of a short polarized pulse focused with a high numerical aperture aplanatic lens

We present a theoretical analysis of the field distribution in the focal plane of a dispersionless, high numerical aperture (NA) aplanatic lens for an x-polarized short pulse. We compare the focused pulse spatial distribution with that of a focused continuous wave (CW) field and its temporal distribution with the profile of the incident pulse. Regardless of the aberration free nature of the focusing aplanatic lens, the temporal width of the focused pulse widens considerably for incident pulses with durations on the order of a few cycles due to the frequency-dependent nature of diffraction phenomena, which imposes a temporal diffraction limit for focused short pulses. The spatial distribution of the focused pulse is also affected by this dependence and is altered with respect to the diffraction limited distribution of the CW incident field. We have analyzed pulses with flat top and Gaussian spatial irradiance profiles and found that the focused pulse temporal widening is less for the Gaussian spatial irradiance pulse, whereas the spatial distribution variation is similar in both cases. We present results of the focused pulsewidth as a function of the NA for the two spatial irradiance distributions, which show that the Gaussian irradiance pulse outperforms the flat top pulse at preserving the incident pulse duration.

Flattop pulse generation based on the combined action of active mode locking and nonlinear polarization rotation

A pulse-width-tunable 10GHz flattop pulse (FTP) train is generated based on the combined action of active mode locking and nonlinear polarization rotation pulse shaping. Although the setup was previously used for other applications, the mechanism of FTP generation based on it is first analyzed and confirmed in the experiment. An FTP with pulse width tunable from 12 to 20ps by changing polarization controllers is generated within the wavelength tuning range of 20nm. The generated pulse reveals good stability, with the side mode suppression ratio of 65dB, timing jitter of 92fs, and amplitude fluctuation of 0.36%.

The radiation temperature andM-band fraction inside hohlraum on the SGIII-prototype laser facility

The radiation temperature TR and M-band fraction fM inside the vacuum Au hohlraum have been experimentally determined by a shock wave technique and a broadband soft x-ray spectrometer (SXS) on the SGIII-prototype laser facility. From the results of the shock wave technique, TR is about 202 eV, and fM is about 9% for the hohlraums driven by a 1 ns flattop pulse of 6 kJ laser energy. The Continuous Phase Plate (CPP) for beam smoothing is applied in the experiment, which increases TR to 207 eV while has almost no influence on fM. Comparisons between the results from the two kinds of technologies show that TR from the shock wave technique is lower than that from SXS whether with CPP or not. However, fM from the shock wave technique is consistent with that from SXS without CPP, but obviously lower than the SXS's result with CPP. The preheat effect on exterior surface of witness plate is reduced by thicker thickness of witness plate designed for higher laser driven energy.

光纤激光器中基于偏振控制器的脉宽可调平顶光脉冲生成技术

光纤激光器中基于偏振控制器的脉宽可调平顶光脉冲生成技术

High-brightness X-ray free-electron laser with an optical undulator by pulse shaping

A normal-incident flattop laser with a tapered end is proposed as an optical undulator to achieve a high-gain and high-brightness X-ray free electron laser (FEL). The synchronic interaction of an electron bunch with the normal incident laser is realized by tilting the laser pulse front. The intensity of the flattop laser is kept constant during the interaction time of the electron bunch and the laser along the focal plane of a cylindrical lens. Optical shaping to generate the desired flattop pulse with a tapered end from an original Gaussian pulse distribution is designed and simulated. The flattop laser with a tapered end can enhance the X-ray FEL beyond the exponential growth saturation power by one order to reach 1 Gigawatt as compared to that without a tapered end. The peak brightness can reach 1030 photons/mm2/mrad2/s/0.1% bandwidth, more than 10 orders brighter than the conventional incoherent Thompson Scattering X-ray source.

Generation of bright atomic and molecular solitons in hybrid atom-molecular Bose-Einstein condensates coupled through raman photoassociation

We have explored the feasibility of designing atomic and molecular bright solitons in a hybrid atom-molecular BEC (Bose Einstein Condensate) system of 87Rb coupled through Raman Photoassociation. It is found that the atomic and molecular waves when allowed to evolve in a spherical one-dimensional trap, initially oscillate with time and assume solitonic nature as bright solitons with progress in time. After the trap is switched off, atomic and molecular waves propagate retaining its solitonic nature and the final amplitude and nature of the bright solitons depend on the time of evolution within the trap. For this study we have used system parameters which are experimentally realizable. We have imposed random perturbation (5%) on the initial atomic and molecular wavefunctions, which are the solution of time independent coupled equation (without decay) and studied the evolution of atomic and molecular solitons with trap on and off condition. A small regular or large irregular oscillation in the amplitude of atomic solitons during evolution of perturbed matter waves are imposed depending on the system parameters. However broad molecular solitons with slowly decaying amplitude are generated which oscillate symmetrically around the centre of the trap. It is found that the stability and the shape of the generated atomic and molecular bright solitons can be controlled by varying the system parameters e.g. initial number of atoms, decay rates of the excited molecule and the frequency of the trap. Since the boson-boson interaction and the atom-molecular coupling strength depend on the total number of atoms, one can control the interplay between boson-boson interaction (due to s-wave scattering) and the atom-molecular coupling (through Raman Photoassociation) by varying the total number of atoms. It is found that with the increase in trap frequency generation of atomic and molecular solitons in atom-molecular coupled system is feasible if the strength of atom-molecular coupling and two-photon detuning is increased. The shape and the stability of the bright solitonic waves depend significantly on the strength of induced decay from the excited molecular state to the two-atom continuum or to the bound vibrational levels of the ground state. Stable flat-top atomic and molecular matter wave pulse can be generated from this atom-molecular coupled system by increasing the decay rates of the excited molecules.

Compact Pulsed-Power Driver for Double Pulse Effect Studies in Nanosecond Laser Ablation

High-voltage double pulse of nanosecond duration (~ hundreds of nanoseconds) with extremely short interval (~ tens of nanoseconds) between the pulses can be used to study the double pulse effect in nanosecond laser ablation for laser-induced breakdown spectroscopy. A 0.5-GW compact pulsed-power driver is designed using the transmission line characteristics of the pulse forming line to generate two pulses with extremely short repetition interval. The impedance of the pulsed-power driver is 22 Ω, and it generates two flat top high-voltage pulses of 100 kV, 100 ns duration with a interpulse repetition interval of 30 ns across the matched load.

Experimental generation of optical flaticon pulses

We experimentally investigate the nonlinear reshaping of a continuous wave that leads to chirp-free and flat-top intense pulses or flaticons exhibiting strong temporal oscillations at their edges and a stable self-similar expansion upon propagation of their central region. This study was performed in the normal dispersion regime of a nonzero dispersion-shifted fiber and involved a sinusoidal phase modulation of the continuous wave. Our fiber optics experiment is analogous to considering the collision between oppositely directed currents near the beach, and it may open the way to new investigations in the field of hydrodynamics.

Effects of laser pulse shape and carrier envelope phase on pair production

For different fields of supercycle and subcycle laser pulses, the effects of laser pulse shape and carrier envelope phase on pair production are investigated by solving quantum Vlasov equation. By changing the pulse width and shape, the nonlinear behaviors of momentum distribution function and number density of created electron–positron pairs are obtained. It is found that there exist the multiphoton processes and stabilization phenomenon in pair production for supercycle situation. Our study may shed a light on optimizing the form of applied laser field to enhance the created pairs number, for example, when other conditions are fixed, the flat-top super-Gaussian laser pulse has advantage on pairs production.

Spatio-temporal evolution of the optical field on a hohlraum wall at the rising edge of a flat-topped pulse

AbstractConsidering the time delay in different hohlraum wall positions caused by oblique incidence, the spatio-temporal optical field distribution characteristics of a hohlraum wall, especially during the rising edge of a flat-topped pulse, is simulated by a fast Fourier transform method together with chromatography. Results demonstrate that beam propagation along the hohlraum wall is a push-broom process with complex dynamic spatial–temporal evolution. In the first few picoseconds, the optical intensity of the front position increases rapidly, while that of the rear position is relatively weak. The ratio $R$ of the optical intensity during the rising edge is smaller than that of the steady state. $R$ gradually increases and finally tends to the value of the steady state with time. Calculation also shows that, with shorter total width of the rising edge, $R$ of the optical field decreases and the difference compared to the steady state becomes larger. The evolution is more severe with smaller angle of inclination.

Performance Characteristics of an Ablative Gallium Electromagnetic Accelerator

Experimental performance measurements are presented for an ablative gallium electromagnetic accelerator driven by a flat-top current pulse, with gallium supplied to the discharge by evaporation of the central cathode. The arc impedance, exhaust velocity, and electron temperature are measured for discharge currents in the range of and an electrode radius ratio of . The arc voltage is found to vary linearly with the discharge current, giving an arc impedance of . An exhaust velocity of is found by using the mass bit and computing the electromagnetic thrust from the discharge current data. This value is invariant with the discharge current and is within the experimental error of the velocities measured using electrostatic probes. Triple probe measurements yield on-axis electron temperatures in the range of 0.8–3.8 eV and electron densities in the range of . Magnetic induction probe measurements in the interelectrode region yield a peak magnetic field of 0.8 T at 24 kA and no evidence of the spoking instability. An efficiency of 25% at 1600 s is calculated for using the mass bit and discharge current data assuming the cathode has a flat tip. The efficiency scaling predicts that changing to a conical tip cathode and increasing the radius ratio to 10 will result in an efficiency of 52% at an of 3200 s.

Generation of 8 nJ pulses from a dissipative-soliton fiber laser with a nonlinear optical loop mirror.

Theoretical and experimental investigations of the behavior of normal-dispersion fiber lasers with nonlinear optical loop mirrors are presented. The use of a loop mirror causes the laser to generate relatively long, flat-topped pulses. The pulse energy can be high, but the pulse duration is limited to greater than 300 fs. Experimentally, 8 nJ pulses that can be dechirped to 340 fs duration are obtained. The laser is a step toward an all-fiber, environmentally stable design.

Sensitivity Analysis and Regulation Strategy of Current Waveform for Two-Axis-Compensated Compulsators

Compulsators have been considered appropriate pulsed power supplies for electromagnetic railguns. In order to ensure a high projectile mean to peak acceleration ratio, it is desirable to provide a flat-topped pulse for the railgun. This paper analyzes a two-axis-compensated compulsator, which is a refined selective passive compulsator having an orthogonal displacement between compensating winding and field winding. During the discharge process, the quadrature-axis compensation is provided by the compensating winding, when direct-axis compensation is provided by the field winding. Compared to the traditional selective passive configuration, this arrangement is able to provide another degree of freedom to compensate the armature reaction, thus an output of a current waveform with more flexibility. In this paper, the self-excitation and discharge process of a two-axis-compensated air-core compulsator is simulated based on a finite-element model. Analysis of the current waveform sensitivity to compulsator design parameters is also carried out, as well as the regulation strategy to generate a flat-topped pulse. The research process mentioned in this paper could help to optimize the design of compulsators in future applications.

Design of Ultrafast All-Optical Signal Processing Devices Based on Fiber Bragg Gratings in Transmission

A general and practical approach for designing ultra-fast all-optical (all-fiber) signal processing devices based on chirped fiber Bragg gratings (C-FBGs) working in transmission is presented. The approach can be used to design any minimum-phase linear optical device, e.g., first and high-order all-optical time differentiators and integrators and a large variety of optical pulse shapers, significantly overcoming the bandwidth limitations of previous FBG-based designs. Processing speeds in the THz range, corresponding to sub-picosecond time features, can be achieved using readily feasible grating apodization profiles. The approach is successfully proved through the design of two relevant functionalities, namely a THz-bandwidth optical differentiator and a 1-ps flat-top optical pulse shaper.

Picosecond optical signal processing based on transmissive fiber Bragg gratings

We report the experimental realization of an ultrafast (terahertz-bandwidth) linear optical signal processor, particularly a picosecond flat-top optical pulse shaper, based on a fiber Bragg grating (FBG) working in transmission. The used FBG design technique, based on a specially apodized linearly chirped FBG, enables the synthesis of readily feasible devices with processing bandwidths well in the terahertz range. The specific device reported here is successfully demonstrated for reshaping ultrashort (400fs FWHM) optical Gaussian-like pulses into 2ps flat-top pulses.

Feasibility and Performance of a Tapered Helical Pulse-Forming Line-Based Pulse Transformer

Conventional pulsed power devices such as Marx generators, Tesla transformers, and cable-based generators are usually bulky and occupy a large volume when used for generating high-voltage pulses of duration longer than 100 ns. Cable-based generators also have the limitation of generating half the charging voltage when delivering the entire energy into a matched load. Inductive energy storage systems provide an alternate and effective scheme to overcome these issues. However, their efficiency depends upon the performance of opening switches. Also, inductive energy storage systems do not typically generate flat-top voltage pulses that are desirable for driving various pulsed power loads. In this paper, a new device, named the tapered helical pulse-forming line-based pulse transformer (THPFL-PT), is designed to overcome the above-mentioned drawbacks. The device performs two actions at the same time, i.e., it steps up the input charging voltage, and it generates a flat-top nanosecond voltage pulse. It is developed and characterized at low voltages to verify the feasibility of the concept developed and design parameters. The entire device is compact, with length of about 330 mm. It is capable of generating a voltage pulse of magnitude higher than the charging voltage with a duration of about 100 ns and almost a flat-top profile.