Full-width At Half-maximum Pulse Research Articles

A Marx-based generator with adjustable FWHM using a controllable magnetic switch

The lack of turn-off ability makes the spark gap-based Marx generator uncontrollable in terminating the output pulse. In addition, there are some limitations for rise time and voltage gain improvements in the conventional critically-damped Marx design. In this paper, an efficient over-damped design of Marx circuit is proposed, which leads to a shorter rise time as well as a larger voltage gain. A chopping magnetic switch is employed to achieve the desired values of the fall time and full width at half maximum (FWHM). Moreover, adding a DC bias circuit to the magnetic switch provides the ability to adjust the pulse FWHM parameter. Therefore, an output pulse with a shorter rise time, larger voltage gain, and controllable FWHM is achieved. The finite element method is utilized to simulate the structures, and the proposed scheme is evaluated by practical experiments.

Generation of an isolated attosecond pulse via current modulation induced by a chirped laser pulse in an x-ray free-electron laser

Aiming for the future upgrade of a hard x-ray beamline at the Pohang accelerator laboratory x-ray free-electron laser (PAL-XFEL), we first analyze the scheme recently proposed for the attosecond-terawatt (TW) x-ray pulse [Phys. Rev. Lett.110, 084801 (2013)PRLTAO0031-900710.1103/PhysRevLett.110.084801]. An x-ray pulse with 32 attosecond full-width half maximum (FWHM) pulse duration and ∼4 TW in power is shown in simulations using the PAL-XFEL parameters. Furthermore, to remove neighboring radiation pulses and to generate an isolated attosecond-TW x-ray pulse, the role of optical laser frequency chirp is examined on the electron beam current modulation as well as on the x-ray pulse generation in the undulator. Our simulations show an isolated x-ray pulse with 42 attosecond FWHM pulse duration and ∼3.5 TW in power for the optimal frequency chirp of a 400 nm optical laser. Recently, in a simpler method [J. Synchrotron Radiat.23, 1273 (2016)JSYRES0909-049510.1107/S1600577516013345] and [Phys. Rev. ST Accel. Beams8, 040701 (2005)PRABFM1098-440210.1103/PhysRevSTAB.8.040701], using a frequency-chirped optical laser and the electron beam delays in between undulator sections, it has been reported that an isolated attosecond-TW x-ray pulse can be obtained at the undulator end. For optimal chirp in this simple setup, we show 50 attosecond FWHM pulse duration and ∼3 TW in power with ∼7.5×1010 photons per pulse at 0.1 nm radiation wavelength. Generation of such inherently synchronized, powerful single attosecond x-ray pulses at PAL-XFEL will be advantageous to the pump-probe experiments in the study of ultrafast dynamics.

Analyzing the effect of slotted foil on radiation pulse profile in a mode locked afterburner X-ray free electron laser

Extremely short X-ray pulses in the attosecond (as) range are important tools for ultrafast dynamics, high resolution microscopy, and nuclear dynamics study. In this paper, we numerically examine the generation of gigawatt (GW) mode-locked (ML) multichromatic X-rays using the parameters of the Pohang Accelerator Laboratory (PAL)-X-ray free electron laser (XFEL), the Korean XFEL. In this vein, we analyze the ML-FEL [Thompson and McNeil, Phys. Rev. Lett. 100, 203901 (2008)] and mode-locked afterburner (MLAB) FEL [Dunning et al., Phys. Rev. Lett. 110, 104801 (2013)] schemes on the hard X-ray beamline of the PAL-XFEL. Using the ML scheme, we numerically demonstrate a train of radiation pulses in the hard X-ray (photon energy ∼12.4 keV) with 3.5 GW power and 16 as full-width half maximum (FWHM) pulse duration. On the other hand, using the MLAB scheme, a train of radiation pulses with 3 GW power and 1 as FWHM (900 zs in RMS) pulse duration has been obtained at 12.4 keV photon energy. Both schemes generate broadband, discrete, and coherent spectrum compared to the XFEL's narrowband spectrum. Furthermore, the effect of slotted foil is also studied first time on the MLAB-FEL output. Numerical comparisons show that the temporal structure of the MLAB-FEL output can be improved significantly by the use of the slotted foil. Such short X-ray pulses at XFEL facilities will allow the studies of electron-nuclear and nuclear dynamics in atoms or molecules, and the broadband radiation will substantially improve the efficiency of the experimental techniques such as X-ray crystallography and spectroscopy, paving the way for outstanding progress in biology and material science.

The absorption and radiation of a tungsten plasma plume during nanosecond laser ablation

In this paper, the effect of absorption of the laser beam and subsequent radiation on the dynamics of a tungsten plasma plume during pulsed laser ablation is analyzed. Different laser wavelengths are taken into consideration. The absorption and emission coefficients of tungsten plasma in a pressure range of 0.1–100 MPa and temperature up to 70 000 K are presented. The shielding effects due to the absorption and radiation of plasma may have an impact on the course of ablation. The numerical model that describes the tungsten target heating and the formation of the plasma and its expansion were made for 355 nm and 1064 nm wavelengths of a Nd:YAG laser. The laser beam with a Gaussian profile was focused to a spot size of 0.055 mm2 with a power density of 1 × 109 W/cm2 (10 ns full width half maximum pulse duration). The plasma expands into air at ambient pressure of 1 mPa. The use of the shorter wavelength causes faster heating of the target, thus the higher ablation rate. The consequences of a higher ablation rate are slower expansion and smaller dimensions of the plasma plume. The higher plasma temperature in the case of 1064 nm is due to the lower density and lower plasma radiation. In the initial phase of propagation of the plasma plume, when both the temperature and pressure are very high, the dominant radiation is emission due to photo-recombination. However, for a 1064 nm laser wavelength after 100 ns of plasma expansion, the radiation of the spectral lines is up to 46.5% of the total plasma radiation and should not be neglected.

Gradual Tuning of the Current Pulse Width Within 1-0.03 ns in Gas-Filled Diodes of Nanosecond Electron Accelerators

The conditions for the generation of subnanosecond and picosecond electron beams with a tunable pulse width are studied on an SLEP-150 generator with a tubular cathode at a voltage pulse rise time of 0.3 ns. The width and shape of the beam current pulse are varied by varying the pressure and gas kind in the diode. It is shown that in the vacuum diode mode, the beam current rise time decreases from <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">${\sim}{0.5}$</tex></formula> to <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\sim}{\rm 0.2}~{\rm ns}$</tex></formula> as the interelectrode gap is decreased from 12 to <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\sim}{\rm 2}~{\rm mm}$</tex> </formula> . It is found that with a 3-mm interelectrode gap, increasing the air pressure in the diode of the SLEP-150 generator from 0.1 to 6 torr decreases the full width at half maximum (FWHM) of the current pulse from <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\sim}{1}$</tex></formula> to 0.18 ns, with the beam current amplitude greater than 400 A and the beam current rise time less delayed with respect to the voltage pulse rise time. It is confirmed that with a nanosecond voltage pulse, the residual pressure of helium, nitrogen, and air in the diode ( <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\sim}{0.1}$</tex></formula> torr or less) does not affect the beam current amplitude and pulse width. At low pressure of air in the gas diode, the attained beam current density at a beam current pulse FWHM of 0.18-1 and <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\sim}{\rm 0.1}~{\rm ns}$</tex></formula> is <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\sim}{500}$</tex></formula> and <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">${\sim}{100}~{\rm A}/{\rm cm}^{2}$</tex></formula> , respectively.

Nanosecond laser ablation of silver nanoparticle film

Nanosecond laser ablation of polyvinylpyrrolidone (PVP) protected silver nanoparticle (20 nm diameter) film is studied using a frequency doubled Nd:YAG nanosecond laser (532 nm wavelength, 6 ns full width half maximum pulse width). In the sintered silver nanoparticle film, absorbed light energy conducts well through the sintered porous structure, resulting in ablation craters of a porous dome shape or crown shape depending on the irradiation fluence due to the sudden vaporization of the PVP. In the unsintered silver nanoparticle film, the ablation crater with a clean edge profile is formed and many coalesced nanoparticles of 50 to 100 nm in size are observed inside the ablation crater. These results and an order of magnitude analysis indicate that the absorbed thermal energy is confined within the nanoparticles, causing melting of nanoparticles and their coalescence to larger agglomerates, which are removed following melting and subsequent partial vaporization.

Impact of Optical Pulsewidth on the Frequency Response of P-i-N Photodiodes

An analytical model to study the influence of the finite optical pulsewidth on the frequency response of P-i-N photodiode is presented. The derivation takes into account the nonuniform carrier generation, the resistance-capacitance ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RC</i> ) parasitic effects and the Gaussian temporal profile of the generating function. For an InGaAs photodiode with 0.5-¿m intrinsic length and 10-¿ m radii excited by 5-ps full-width at half-maximum (FWHM) pulses at 1.3 ¿m , previously published theory overestimates the bandwidth by over 30% as compared to the present model. The results suggest that for the faithful measurement of the bandwidth, the FWHM of the Gaussian pulses used should be at least an order of magnitude smaller than the transit time of the slower moving carriers.

The DCU laser ion source

Laser ion sources are used to generate and deliver highly charged ions of various masses and energies. We present details on the design and basic parameters of the DCU laser ion source (LIS). The theoretical aspects of a high voltage (HV) linear LIS are presented and the main issues surrounding laser-plasma formation, ion extraction and modeling of beam transport in relation to the operation of a LIS are detailed. A range of laser power densities (I approximately 10(8)-10(11) W cm(-2)) and fluences (F=0.1-3.9 kJ cm(-2)) from a Q-switched ruby laser (full-width half-maximum pulse duration approximately 35 ns, lambda=694 nm) were used to generate a copper plasma. In "basic operating mode," laser generated plasma ions are electrostatically accelerated using a dc HV bias (5-18 kV). A traditional einzel electrostatic lens system is utilized to transport and collimate the extracted ion beam for detection via a Faraday cup. Peak currents of up to I approximately 600 microA for Cu(+) to Cu(3+) ions were recorded. The maximum collected charge reached 94 pC (Cu(2+)). Hydrodynamic simulations and ion probe diagnostics were used to study the plasma plume within the extraction gap. The system measured performance and electrodynamic simulations indicated that the use of a short field-free (L=48 mm) region results in rapid expansion of the injected ion beam in the drift tube. This severely limits the efficiency of the electrostatic lens system and consequently the sources performance. Simulations of ion beam dynamics in a "continuous einzel array" were performed and experimentally verified to counter the strong space-charge force present in the ion beam which results from plasma extraction close to the target surface. Ion beam acceleration and injection thus occur at "high pressure." In "enhanced operating mode," peak currents of 3.26 mA (Cu(2+)) were recorded. The collected currents of more highly charged ions (Cu(4+)-Cu(6+)) increased considerably in this mode of operation.

Modeling Single Event Transients in Bipolar Linear Circuits

This review paper covers modeling of single event transients (SETs) in bipolar linear circuits. The modeling effort starts with a detailed circuit model, in a program such as SPICE, constructed from a photomicrograph of the die, which is verified by simulating the electrical response of the model. A description of various approaches to generating the single event strike in a circuit element is then given. Next, validating and calibrating the output SET response for critical circuit transistors is discussed, using focused ions beams or lasers. The circuit model is validated with a heavy ion broadbeam to generate the output SET response in terms of SET peak amplitude versus full width half maximum pulse width. Finally, a system application is described and a ldquofailure raterdquo in space is calculated, based on the experimental heavy ion data, for a geostationary orbit using CREME96.

Picosecond pulse generation using a saturable absorber section of grating-coupled surface-emitting laser

We report on a passively and hybridly mode-locked grating-coupled surface-emitting laser (GCSEL) using the unpumped section of the GCSEL as a saturable absorber. We obtain 8.8-ps full-width at half-maximum (FWHM) autocorrelation pulses in passive mode-locked operation and an FWHM pulse duration of 3.4 ps in hybrid mode-locked operation, which are the shortest pulses from a GCSEL external cavity. With hybrid mode-locking, a peak power of 0.26 W and a spectral bandwidth of 0.6 nm are obtained. These results demonstrate the potential of multisegment GCSELs in ultrashort pulse generation.

High-power source of 200–350 nm spontaneous emission excited by unipolar current pulses

Spectral, energy, and temporal characteristics of pulse discharge in xenon have been experimentally studied. Upon passing from an oscillatory regime to unipolar pulses of discharge current, the power of emission in the wavelength interval 200–350 nm increases, while the emission pulse full width at half maximum (FWHM) decreases. A quartz pulsed discharge lamp excited by a generator based on high-current high-voltage diodes radiates in the 200–350 nm interval at a peak radiant intensity above 65 kW/sr and at a pulse FWHM ∼ 2 μs.

Stabilised 10 GHz rationally mode-locked erbium fibre laser and its use in a 40 Gbit/s RZ data transmission system over 240 km of standard fibre

Mode-locked fibre lasers are a promising pulse source for use in high-speed data communication systems. Optical pulses at high repetition rates can be achieved by mode-locking the laser cavity at a rational harmonic of the fundamental mode-locking frequency. Using this technique, optical pulse streams at very high rates can be obtained by using longer laser cavities and slower optoelectronic components. A 10 GHz rationally mode-locked laser is presented, which is the fastest stabilised rationally mode-locked laser reported. The pulse source produces stable soliton-like pulses, with a full-width half-maximum pulse width of 5 ps and a back-to-back receiver sensitivity of -19.7 dBm for a bit error rate (BER) of 10/sup -9/. Using optical time division multiplexing, a 40 Gbit/s data stream is generated from the 10 GHz optical pulse source with a back-to-back receiver sensitivity of -18.9 dBm. This 40 Gbit/s transmitter is used for transmitting data over 240 km of dispersion-managed fibre. This incurs a sensitivity penalty of only 0.4 dB for a BER of 10/sup -9/.

A distinct class of isolated intracloud lightning discharges and their associated radio emissions

Observations of radio emissions from thunderstorms were made during the summer of 1996 using two arrays of sensors located in northern New Mexico. The first array consisted of three fast electric field change meters separated by distances of 30 to 230 km. The second array consisted of three broadband (3 to 30 MHz) HF data acquisition systems separated by distances of 6 to 13 km. Differences in signal times of arrival at multiple stations were used to locate the sources of received signals. Relative times of arrival of signal reflections from the ionosphere and Earth were used to determine source heights. A distinct class of short‐duration electric field change emissions was identified and characterized. The emissions have previously been termed narrow positive bipolar pulses (NPBPs). NPBPs were emitted from singular intracloud discharges that occurred in the most active regions of three thunderstorms located in New Mexico and west Texas. The discharges occurred at altitudes between 8 and 11 km above mean sea level. NEXRAD radar images show that the NPBP sources were located in close proximity to high reflectivity storm cores where reflectivity values were in excess of 40 dBZ. NPBP electric field change waveforms were isolated, bipolar, initially positive pulses with peak amplitudes comparable to those of return stroke field change waveforms. The mean FWHM (full width at half maximum) of initial NPBP field change pulses was 4.7 μs. The HF emissions associated with NPBPs were broadband noise‐like radiation bursts with a mean duration of 2.8 μs and amplitudes 10 times larger than emissions from typical intracloud and cloud‐to‐ground lightning processes. Calculations indicate that the events represent a distinct class of singular, isolated lightning discharges that have limited spatial extents of 300 to 1000 m and occur in high electric field regions. The unique radio emissions produced by these discharges, in combination with their unprecedented physical characteristics, clearly distinguish the events from other types of previously observed thunderstorm electrical processes.

Effect of gain switching frequency on ultrashort pulse generation from laser diodes

In this work, the effects of gain switching frequency on ultrashort pulse generation are investigated using a model based on the multi-mode rate equations. In addition to the commonly used laser diode parameters, the key parameters corresponding to the different laser diode materials and structures are included in the model. Effects of some laser diode parameters, and d.c. and RF drive conditions on the full width at half-maximum (FWHM) of gain switched pulses are investigated for the 1–15 GHz gain switching frequency range. Multi-pulse generation and the cut-off frequency of gain switching are also determined for this frequency range. It is found that the FWHM of pulses decrease as the RF current is increased, at a constant frequency. However, the FWHM of pulses either decrease or increase as the d.c. bias is increased, depending especially on the gain compression and gain peak. As the frequency is increased, pulse widths start to decrease or increase depending on the laser diode parameters. The FWHM of pulses for shorter cavity lasers are found to be almost insensitive to the frequency. Among the parameters affecting the multi-pulse generation and cut-off, gain compression, gain constant, and d.c. and RF current amplitudes are found to be the most effective. All calculations are also carried out using single-mode rate equations and results of multi-mode and single-mode solutions are compared.

Twin traveling-wave tube amplifiers driven by a relativistic backward-wave oscillator

Experiments demonstrate stable frequency and relative-phase angle output from twin traveling-wave tube (TWT) amplifiers driven with the redirected signal from a high-power backward-wave oscillator (BWO). The experimental X-band apparatus employs a single generator to produce three independent electron beams which simultaneously drive the BWO and TWT sources. The BWO spontaneously generates up to 14.1 MW peak, 25 to 15-ns long pulses over a current-tunable bandwidth of 9.6%. The BWO power extracted upstream is split and redirected into twin TWT's for amplification. The TWT's produce up to 9.0 MW pulses over an 800 MHz instantaneous bandwidth. Across the amplifier's full-width half-maximum pulse duration of 10 to 20 ns, a relative-phase angle of better than 15/spl deg/ is maintained between TWT's for an 11.0 to 11.7 GHz range. Experiments characterize the gain, relative-phase angle, and efficiency of twin-TWT output as a function of RF-drive frequency and beam current. These experiments are the first to demonstrate the feasibility of relativistic TWT's for phased-array applications, and increase the limited data base documenting relativistic-TWT operation.

Improvements to a home-built nitrogen laser

Several major design improvements to a previously described nitrogen laser are presented. The active region has a larger cross section but shorter length resulting in an output beam of better optical quality and a laser channel which is cheaper and easier to build. The gas flow is transverse to the channel, and the pressure is more uniform. These and other improvements result in a 9-nsec pulse FWHM (full width at half-maximum) whose peak power exceeds 0.7 MW when the laser is supplied with 24 kV and pulsed at 10 Hz.

Cerenkov pulse dispersion in two low-loss fibers

We have measured the material and mode dispersion of a 50-psec Cerenkov light pulse generated in 500-m lengths of low-loss graded and step-index Corning fibers. The initial 15.7-nsec full width at half-maximum (FWHM) transmitted pulse in the graded fiber was reduced to less than 2 nsec with narrow-band optical filtering. Parallel measurements in the step-index fiber gave a value of less than 4 nsec for an initial 16.3-nsec FWHM pulse. Fluorescence induced in the fiber by the electron radiation was less than 1% of the amplitude of the transmitted Cerenkov light in the graded fiber and approximately 2% in the step fiber.