Abstract
The high energy densities deposited in materials by focused X-ray laser pulses generate shock waves which travel away from the irradiated region, and can generate complex wave patterns or induce phase changes. We determined the time-pressure histories of shocks induced by X-ray laser pulses in liquid water microdrops, by measuring the surface velocity of the microdrops from images recorded during the reflection of the shock at the surface. Measurements were made with ~30 µm diameter droplets using 10 keV X-rays, for X-ray pulse energies that deposited linear energy densities from 3.5 to 120 mJ/m; measurements were also made with ~60 µm diameter drops for a narrower energy range. At a distance of 15 µm from the X-ray beam, the peak shock pressures ranged from 44 to 472 MPa, and the corresponding time-pressure histories of the shocks had a fast quasi-exponential decay with positive pressure durations estimated to range from 2 to 5 ns. Knowledge of the amplitude and waveform of the shock waves enables accurate modeling of shock propagation and experiment designs that either maximize or minimize the effect of shocks.
Highlights
The deposition of energy into materials using pulsed radiation sources has many practical applications including laser surgery [1,2], laser micromachining [3,4,5], and chemical synthesis [6].Besides heating and ablation of the irradiated material, the energy deposited can lead to generation of shock waves if the time scale of energy deposition is shorter than the time in which a sound wave propagates across the irradiated region [1]
We report a more precise characterization of shock waves induced by hard X-ray free-electron laser (XFEL) pulses in water, based on measurements of the surface velocity of water microdrops during the reflection of the shock
The experimental setup and the data collection procedure are similar to the ones used to investigate cavitation and spallation induced in water by X-ray laser pulses [17], and we refer the in free space, ~1 mm away from the nozzle orifice; the thermocouples reported temperatures within
Summary
The deposition of energy into materials using pulsed radiation sources has many practical applications including laser surgery [1,2], laser micromachining [3,4,5], and chemical synthesis [6]. In samples with thicknesses much smaller than the X-ray absorption length, the shock waves will have a cylindrical symmetry Such shocks were observed using time-resolved optical imaging, initially in water microjets [24] and afterwards in liquid microdroplets [17]. The reflection of XFEL shocks at the surface of liquid jets and drops was observed to lead to spallation in droplets [17] and to the generation of shock trains in jets [24,30] In both cases these phenomena had features not observed in similar experiments with pulsed optical lasers, including the generation of very large negative pressures in drops [17], and the generation of very large sound intensities in jets [30]. Improvements in the experimental design allowed us to resolve and measure the decay of the shock pressure after the shock discontinuity, while covering a range of one and a half orders of magnitude in the XFEL pulse energy
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