Pulse Shot Research Articles

Characteristics of polymer velvet as field emitters under high-current pulsed discharge

In this paper we investigated the surface morphology and emission property of polymer velvet in a cathode test system powered by a ∼400 ns, ∼400 kV pulsed generator. After a series of pulse shots, the velvet surface exhibited an obvious decrease in the amount of emitters, namely, the smoothing of microprotrusions, indicating a lower field enhancement factor or a higher turn-on electric field than that for no shots. As the velvet cathode lifetime proceeded, the beam degradation was observed in terms of the voltage pulse length, maximum emission current, and rise time of diode current. Further, the average current density significantly decreased during a 100 pulse shot test, from 280 to 160 A/cm 2. The surface discharge caused many plasma spots on the velvet surface. The cathode plasma expands towards the anode, directly leading to the diode gap closure. The degradation in the velvet performance after high-current emission may be related to this behavior of cathode plasma. Finally, the electron emission mechanisms, how to affect the surface morphology of velvet, are presented.

Design of a simple annular electron beam source and its operating characteristics in single and repetitive shot modes

In this work, design and performance of an annular carbon fiber cathode are presented. Measurements on this cathode were performed in a single high-voltage pulse generator (600 kV, 50 ns, and 50 Omega) and a repetitive one (350 kV, <10 ns, 180 Omega, and 100 Hz), respectively. In a single pulse regime, emphasis was placed on the uniformity of electron beam extracted from this cathode. It was found that this cathode could deliver uniform electron beams with current densities exceeding kA/cm(2). Cesium iodide (CsI) coating eliminated hot spots on the cathode surface, significantly improving the uniformity of electron emission. Under repetitively pulsed operation, this cathode exhibited a good shot-to-shot reproducibility at the pressure of 1.5x10(-4) Torr, suggesting an ability of surviving even in poor vacuum. However, once the base pressure rose up to 3.76x10(-4) Torr, the cathode performance gradually degraded as the pulse shot proceeded. Besides, some possible explanations for these experimental results are presented. These results show that given proper diode design, carbon fiber with CsI coating has great promise as electron emitter producing high-current electron beams.

Crystallization Processes of Amorphous Si During Excimer Laser Annealing in Complete-melting and Near-complete-Melting Conditions: A Molecular Dynamics Study

AbstractWe investigated crystallization processes of amorphous Si (a-Si) during the excimer laser annealing in the complete-melting and near-complete-melting conditions by using molecular dynamics simulations. The initial a-Si configuration was prepared by quenching liquid Si (l-Si) in a MD cell with a size of 50×50×150Å3 composed of 18666 atoms. KrF excimer laser (wavelength: 248nm) annealing processes of a-Si were calculated by taking account of the change in the optical constant upon melting during a laser pulse shot with the intensity Ioexp[−(t–t0)2/ς2] (Io: laser fluence, t: irradiation time). The refractive indices of a-Si and l-Si were set at n+ik=1.0+3.0i and n+ik=1.8+3.0i, respectively. The simulated results well reproduced the observed melting rate and the near-complete-melting and complete-melting conditions were obtained for Io = 160mJ/cm2 and 180mJ/cm2, respectively. It was found that larger grains were obtained in the near-complete-melting condition. Our MD simulations also suggest that nucleation occurs first in a-Si and subsequent crystallization proceeds toward l-Si in the near-complete-melting case.

Automatic continuous scanning and random-access switching of mid-infrared waves generated by difference-frequency mixing

We report the rapid tuning of mid-infrared waves beyond 5 microm emitted in difference-frequency mixing with an electronically tuned dual-wavelength Ti:Al2O3 laser used as a pumping source. Simultaneous rapid tuning of the dual wavelengths, which satisfy phase matching in AgGaS2, allows rapid random access switching and continuous tuning of mid-infrared wavelengths. In random-access switching, the mid-infrared wavelength is tuned every pulse shot at a repetition rate of 1 kHz. Mid-infrared wavelengths continuously tuned from 5.2 to 7.2 microm, from 7.0 to 9.1 microm, and from 8.9 to 12.0 microm are achieved at phase-matched angles of 55 degrees, 50 degrees, and 45 degrees, respectively.

Nd–Fe–B Thin Films with Perpendicular Magnetic Anisotropy and High Coercivity Prepared by Pulsed Laser Annealing

Amorphous, as-deposited Nd–Fe–B films prepared by RF-sputtering were annealed by irradiating pulsed KrF excimer laser (248 nm). A pulse of 20 ns in width and 4 kJ/m2 in power density was irradiated onto a 2 µm-thick as-deposited film kept at 400°C. It is observed that, by a single shot of the pulse, Nd2Fe14B Φ phase crystallized with prominent texturing of c-axis along the film normal. Annealed films showed excellent magnetic characteristics; Ms=0.978 T, Mr=0.906 T, iHc=1272 kA/m, and (BH)MAX=161 kJ/m3. This process is also found to be successfully applicable to a 20 µm-thick film. Using 2 µm-thick as-deposited film kept at 400°C, micro-magnet patterns of 100 µm in width and 1–2 mm in length were fabricated by irradiating a 20-ns pulse shot through a patterned mask. The patterned area revealed that magnetizations are directed along the film normal with coercivity higher than 1200 kA/m.

Experimental study of noise properties of a Ti:sapphire femtosecond laser.

The fidelity of a coherent link between optical and microwave frequencies is largely determined by noise processes in a mode-locked femtosecond laser. This work presents an experimental study of the noise properties of a Ti:sapphire femtosecond laser. It includes measurements of pulse repetition rate fluctuations and shot noise exhibited by the Ti:sapphire femtosecond laser. Based on the results of noise measurements, the fractional frequency stability of a microwave signal produced by the femtosecond laser has been evaluated.

パルスＹＡＧレーザによるマイクロ補修溶接の研究 １ 溶接部の硬さと強さに及ぼすパルスショット数の影響

In this study, the effect of laser pulse shot number on the hardness of weld metal and strength of welds was investigated. In the repair welding for such micro part as the cutter, laser welding was carried out to investigate the effects of pulse shot number for the hardness and strength of micro welded zone by the methods of changing pulse shot number under the condition that pulse output energy is constant. The following results were obtained in this study.In the case of laser welding under the condition that pulse output energy is 30 J/P and pulse shot number change from 10 to 60 pulse, the hardness of weld metal was increased with increasing pulse shot number. However, the hardness of weld metal is low against the hardness of base metal.The structure of martensite in the weld metal was decreased with increasing the pulse shot number, and pulse shot number is more than 30 pulse, the martensite in the weld metal disappeared from the weld metal.The strength of welds was increased with increasing the pulse shot number, but the rate of increasing at the strength of welded joint were moderated under the condition that pulse shot number is more than 30 pulse.

Laser ablation at solid–liquid interfaces: An approach from optical emission spectra

The emission spectra from the solid–liquid interface irradiated by a pulsed laser were studied. The solid target used in this study was graphite and boron nitride, and the liquid in which the target was immersed was water, benzene, n-hexane, and carbon tetrachloride. The results showed strong continuous spectrum immediately after a pulse shot, whereas after ≈100 ns later from the irradiation it was greatly reduced, and instead, the emission from small molecules dominated the spectra. The line spectra of small molecules observed in the later time range indicate the chemical reaction between the ablated species and the species originated from the liquid molecules. The intensity of the continuous spectrum was very prominent compared to what has been observed for solid–gas interfaces. This is due to rapid electron ion recombination or bremsstrahlung due to highly confined interface plasma.

Photoinduced phase transition in a mixed-valence gold complex

We have found a photoinduced phase transition (PIPT) in the mixed-valence (MV) gold complex ${\mathrm{Cs}}_{2}{\mathrm{Au}}_{2}{\mathrm{Br}}_{6}$ from the MV state (ambient pressure phase) to the single-valence (SV) phase (high-pressure phase) in the pressure region from 6.4 to 6.8 GPa. We have observed an accumulation of the photoirradiation effect of each pulse shot, and have ascribed it to stabilization of the photogenerated SV clusters due to the cooperative Jahn-Teller instability of ${\mathrm{Au}}^{2+}$ ions. The PIPT shows a distinct threshold behavior against the photon density as well as the shots number, which has been interpreted in terms of the critical nucleation of the photoinjected SV clusters. We further found that the phototransformed SV phase is not stable below 5.5 GPa, and changes into the former MV phase after a long dead time of $\ensuremath{\sim}{10}^{4}$ s.

Instantaneous measurement system of the effective photon energy for a single shot x-ray pulse radiation

For a short pulse x-ray radiation with extremely broad spectrum such as Bremsstrahlung, the effective photon energy cannot be determined by conventional x-ray detectors with only a single x-ray pulse shot. An instantaneous measurement system capable of determining the energy with a single shot has been developed. The system is made of a metal–oxide–semiconductor linear array image sensor, a scintillation phospher, and an absorber material, and a control system of the sensor. The energy of a hard x-ray pulse with a short pulse duration of 500 ns is determined with this system. It is shown that the system can determine the energy more quickly and accurately than with the conventional method.

Initiation of flux jump in SC composite by heat pulse

Nonisothermal diffusion of magnetic flux after a heat pulse shot on the surface of SC (superconductor) composite has been studied numerically taking into account the smoothed transition characteristic of the SC. It is shown that for an SC composite with poor stabilization, the current and heat redistribution change significantly the estimations of stability based on steady-state functions of heat generation and heat transfer. The critical pulsed energy strongly depends on the initial current distribution over the conductor cross section, and the energy may be much less for a conductor with growing current than for a conductor with the same current in steady state. It has been found that undercritical heat pulses only slightly affect current density profile, but stability increases as time delay increases between the current input halt and the pulse shot. It has also been found that for a SC composite with poor stabilization, the transverse thermal conductivity is more important than the electrical resistivity of the matrix from the stability standpoint. >