Micro-impulse and plasma plume produced by irradiating aluminum target with nanosecond laser pulses in double-pulse scheme

The micro-impulse generated by ablating an aluminum target in double-pulse laser bursts with different interpulse delays was investigated using a torsion pendulum.The plasma plume was simultaneously visualized using high-speed photography to analyze the coupling mechanism of the ablation impulse.The experiment was carried out using a pulsed laser with a pulse width of 8 ns and a wavelength of 1064 nm.The experimental results show that an impulse with an interpulse delay of 60 ns is roughly 60%higher than that with no delay between the two pulses,when the energy of both laser pulses is 50 m J.Therefore,double-pulse schemes could enhance the ablation impulse under certain conditions.This is because the ablation of the first laser pulse changes the optical properties of the aluminum target surface,increasing the absorptivity.However,the ablation impulse is reduced with a time delay of 20 ns when the energy of both laser pulses is100 mJ or 150 mJ.It can be concluded that the plasma produced by ablating the aluminum with the first pulse shields the second laser pulse.To summarize,the experimental results show that different delay times in a double-pulse scheme have a significant effect on the ablation impulse.The study provides a reference for the optimization of the parameters when laser ablation propulsion with a double-pulse scheme is applied in the fields of space debris removal,laser ablation thrusters,and so on.

Numerical Simulation on Laser Propulsion Capability of Polymer Target

A computational model of laser ablated polymer was established. Set the ablation criterion based on threshold energy. Put forward the polymer ablation criterion in the numerical model. It established the energy distribution equation to describe the laser ablation process. When the ablation products ejected, the target gained recoil impulse from ejection process. Get the ejection energy and the recoil momentum of target based on momentum conservation law. The numerical analysis model can reflect the propulsion capability of different polymer propellant, revealed the law of propulsion parameters in laser ablation process.

Intelligibility in Chinese English Spoken in Central China

It is widely accepted nowadays that intelligibility is the essential goal for most learners of English,and it is not necessary for them to mimic all aspects of native-speaker English in order to achieve a high level of intelligibility.However,the features that are needed in order to make oneself easily understood by listeners from elsewhere remain controversial.The current research focuses on thirteen five-minute recordings of conversations between young speakers of English in central China and an interviewer from Britain,in order to determine which features of their speech gave rise to misunderstandings.It was found that,in the 18 tokens of misunderstanding identified,4 resulted from lexical semantics(22%),3 from Chinese place names(17%),3 from grammar(17%),and 11 from pronunciation(61%)(with some tokens cross-classified).The most common phonological factors giving rise to loss of intelligibility were omission of syllables and simplification of word-initial consonant clusters.

Style-specific Rhythms of Mandarin in 4 Different Genres

This paper investigates the style-specific rhythms of Mandarin in 4 different genres using acoustic measures so as to generate more macro and micro understanding of the speech rhythms of Mandarin.It took News,Dialogue,Short story and Weather forecast as the different genres.All the data were natural and authentic materials downloaded from China CCTV websites and analyzed using 6 acknowledged rhythmic measurements:speech rate,%V,ΔC,VarcoC,rPVI-C and nPVI-V.The findings show that the rhythms of Mandarin in four genres all fall towards the"syllable-timed"ends of the timing continuum,demonstrating a broadly similar rhythmic pattern for Mandarin,but the four genres of Mandarin significantly differ from each other in speech rate,interval measures and pairwise variability indices,revealing the stylistic variations within the same rhythmic pattern,and that current rhythmic measurements vary in their powers in classifying stylistic rhythms of Mandarin with%V,ΔC and nPVI giving the best categorization in this study.