3 research outputs found
Amending the reflected shock phase of the Lee code
No full textThe radial reflected shock phase begins when the inward shock front (IS) which is collisional hits the central axis and a reflected shock (RS) is produced that moves radially outward. This phase ends and the pinch phase begins when the outward moving RS front meets the inward moving magnetic piston. The Lee code approximates the RS phase by taking an estimated constant RS speed in the formulation of the code. In this paper we analyse the RS situation in more detail. A physical mechanism is used to obtain the RS speed as functions of position and time during its propagation towards the incoming piston. This is an important amendment to this phase of the code. In computing the RS speed, the temperature variation during the RS phase is also obtained. The speed and temperature versus time profiles are presented demonstrating a more realistic picture than the constant speed and temperature assumption. The temperature-radius profile across the pinch at its start is also derived
Plasma synthesis of nanodiamonds in ethanol
No full textIn this work, we examined the characteristics of the plasma formed by fs laser filamentation in ethanol that affected the nanodiamonds formation by optical emission spectroscopy. Molecular and atomic C species were detected in the plasma as the precursors to the nanodiamonds formed; above the threshold laser energy of 360 μJ. Thus, the generation of homogeneous nanodiamonds was identified to be occurred within laser energy of 360–550 μJ where atomic C, ionized C and C2 clusters coexisted. The process of fs laser filamentation is monitored with in-situ absorbance measurement of ethanol. The intensity of the absorbance peak of the sample at ∼228 nm, corresponds to intrinsic absorbance of diamond (σ→ σ* transition) was observed to increase with irradiation time. The prepared samples are characterized by using Raman spectroscopy, XPS and TEM. Nanodiamonds of <5 nm in size were produced. Photoluminescence measurements show that the sample fluoresce at ∼490 nm, when excited by 266 nm, 355 nm, 368 nm and 406 nm laser while additional photoluminescence peak is detected at 329 nm when excited by 266 nm laser
Thermal-induced effects on ultrafast laser filamentation in ethanol
No full textFs laser filamentation in ethanol is studied for bottom-up nanomaterials synthesis at different laser repetition rate. Measurements of energy loss, visible conical images, transmitted beam profile and the optical emission spectra show that laser repetition rate affects ultrafast laser filamentation in ethanol significantly because of the induced thermal processes. Firstly, near the threshold laser energy for self-focusing, high laser repetition rate could interfere with the process, via thermal defocusing that inhibits the onset of filamentation. An enlarged, near flat-top laser beam is produced for the conditions that favor thermal lensing. Secondly, at high laser energy, thermal defocusing effect is saturated as convective heat dissipation occurs. Thus, self-focusing prevailed and led to filamentation at all repetition rates. The strong convection at high laser repetition rate increases the rate of reactions for nanodiamonds synthesis, as the flow of fresh ethanol precursor to the localized filamentation zone is enhanced
