Influence Of Pulse Length Research Articles

Insights into the copper HiPIMS discharge: deposition rate and ionised flux fraction

The influence of pulse length, working gas pressure, and peak discharge current density on the deposition rate and ionised flux fraction in high power impulse magnetron sputtering discharges of copper is investigated experimentally using a charge-selective (electrically biasable) magnetically shielded quartz crystal microbalance (or ionmeter). The large explored parameter space covers both common process conditions and extreme cases. The measured ionised flux fraction for copper is found to be in the range from ≈10% to 80%, and to increase with increasing peak discharge current density up to a maximum at , before abruptly falling off at even higher current density values. Low working gas pressure is shown to be beneficial in terms of both ionised flux fraction and deposition rate fraction. For example, decreasing the working gas pressure from 1.0 Pa to 0.5 Pa leads on average to an increase of the ionised flux fraction by percentage points (pp) and of the deposition rate fraction by taking into account all the investigated pulse lengths.

Influence of pulse length on heat affected zones of evaporatively-mitigated damages of fused silica optics by CO2 laser

The influence of pulse length on heat affected zones of evaporatively-mitigated damages was investigated in order to optimize the CO2 laser-based mitigation process of large aperture fused silica optics for high power laser facilities. The evolution of heat affected zones of mitigated damages with increasing pulse length from 5 μs to 90 μs indicates that the thickness of heat affected zone increases with increasing pulse length, but can be tightly confined by shortening the pulse length. The mitigated damages with confined heat affected zones show minimal redeposit, negligible residual stress and rimless. Raman spectra of heat affected zones show that local densification of fused silica occurs as a result of local fictive temperature enhancement induced by the CO2 laser pulse. The numerical simulation of heat affected zone in the form of fictive temperature modification indicates that the tightly-confined heat affected zone can be generated by shortening the pulse length.

Sonochemiluminescence observation and acoustic detection of cavitation induced by pulsed HIFU at a tissue–fluid interface

The aim of this study is to investigate the mechanism of the erosion process induced by 1.2MHz pulsed high-intensity focused ultrasound (pulsed HIFU). By using Sonochemiluminescence (SCL) photograph, the initiation and maintenance of active cavitation were observed. In order to understand the role of both inertial cavitation and stable cavitation, a passive cavitation detection (PCD) transducer was used. Since the exposure variables of HIFU are important in the controlled ultrasound tissue erosion, the influence of pulse length (PL) and duty cycle (DC, Ton:Toff) has been examined. The results of tissue hole, SCL observation and acoustic detection revealed that the erosion was highly efficient for shorter PL. For higher DCs, the area of SCL increased with increasing PL. For lower DCs, the area of SCL increased with increasing PL from 10 to 20μs and then kept constant. For all PLs, the intensity of SCL decreased with lower DC. For all DCs, the intensity of SCL per unit area (the ratio of SCL intensity to SCL area) also decreased with increasing PL from 10 to 80μs, which suggested that the higher the intensity of SCL is, the higher the efficiency of tissue erosion is. At DC of 1:10, the position of the maximum pixel in SCL pictures was distant from the tissue–fluid interface with the increasing PL because of shielding effect. By the comparison of inertial cavitation dose (ICD) and the stable cavitation dose (SCD), the mechanisms associated with inertial cavitation are very likely to be the key factor of the erosion process.

Influence of test conditions on the accuracy of laser flash measurements

The laser flash method (Parker et al, 1961 J. Appl. Phys. 32 1679-1684) is a well-known technique for the measurement of thermal diffusivity. The advantages of this non-contact measurement technique are, among others, easy sample preparation, fast measurement times, small sample dimensions, and high accuracy. The energy and shape of the laser pulse used for heating the front face of the sample can have a significant influence on the results. The influence of pulse length can be taken care of by an accurate pulse-length correction (Azumi and Takahaski, 1981 Rev. Sci. Instrum. 52 1411 -1413) based on the real shape of the laser pulse. It is shown here that, after the pulse-length correction, the thermal diffusivity is independent of the length of the employed laser pulse for samples with standard dimensions. On the other hand, the employed laser energy can have an influence on the results in the case of samples with strongly temperature-dependent thermal diffusivity. The influence of the pulse energy on the results for various kinds of carbon samples is investigated and a correction routine for its effect is presented.

RF Sparking Experimetns

Voltage breakdown limits the maximum accelerating and focusing fields and thereby the maximum beam currents in RFQ injector linacs. To determine this crucial design parameter, sparking tests have been done in a special cavity. Spark rate and voltage breakdown were investigated at different electrode geometries and materials. Mean gap fields up to 67 MV/m have been obtained. The influence of pulse length and repetition rate on voltage breakdown has been investigated.