Gas dynamics of impulsive jets and pressure oscillations of a laser-irradiated target

Abstract

GAS DYNAMICS OF IMPULSIVE JETS AND PRESSURE OSCILLATIONS OF A LASER-IRRADIATED TARGET N. M. Bulgakova and L. I. Kuznetsov UDC 533.6.011:535.211 I. Introduction. The interaction of an intense millisecond pulse of laser radiation (LR) with a solid target can be accompanied by pressure oscillations, recorded by a piezoelec- tric transducer on the backside of the irradiated target, with frequencies of the order of i0 kHz [i]. This phenomenon occurs on a lead target with LR intensity of about 2 MW/cm 2. In [2] it is shown that pressure oscillations are observed on different dielectric and metal- lic targets in a wide range of LR energies and is accompanied by intense vaporization of the material in the irradiation spot. Possible mechanisms for the appearance of these oscilla- tions are suggested in [3, 4]: bursts of absorption in the plasma in an unstable vaporiza- tion regime or auto-oscillatory regime of self-screening of the LR by the products of erosion. Probe measurements through a narrow channel in the target under high pressures of the sur- rounding medium have shown that oscillations of absorption of the LR in the photoerosion flame are observed in the tail part of the oscillograms in some irradiation regimes [5]. These absorption oscillations can be interpreted in a manner supporting the above-mentioned mechanism of the pressure oscillations on iradiated targets [3]. However, investigations of the attenuation of radiation in the photoerosion flame of ebonite and magnesium targets in air with LR intensities such that pressure oscillations should be observed indicate that there are no pronounced pulsations of the absorption coeffi- cient for laser radiation through the photoerosion flame [6]. Finally, simultaneous measure- ments of the pressure on the target and LR attenuation by the photoerosion flame [7] have shown that the LR transmission curves are quite smooth, and the small nonmonotonic variations on these curves are not related with the pressure oscillations on the target. These investi- gations, as well as measurements performed with different irradiation configurations [8] and change in sign of the pressure vector [2] have made it possible to propose a gas-dynamic mechanism for the appearance of the pressure oscillations on irradiated targets. In the present work, in order to check this mechanism we performed numerical modeling of the gas dynamics of an erosion flame and compared the computational and experimental re- suits in a wide range of irradiation intensities. 2. Experimental Procedure. Detailed experimental investigations of the pressure on irradiated targets were performed on a VIKA vacuum chamber [9]. The 1.06 ~m laser pulses with a width of 0.3 msec at half-height was focused on the target through a long focal length lens. The diameter of the irradiation spot ranged from 5 to 20 mm, and the target had a diam- eter of 20 man. The target was placed on a piezoelectric transducer [i0], which measured the pressure on the irradiated target. A coaxial FK-19 photocell was used to record the initial laser radiation from a beam splitter placed at the output of the laser system. The signals from the piezoelectric trans- ducer and from the FK-19 photocell were fed into an $9-8 two-channel digital oscillograph. The data were transmitted from the oscillograph to an IVK-6 measurement-computing system through a KOP interface. Processing of the signals from the piezoelectric transducer on the IVK-6 system consisted of dividing the area under the oscillogram into 10-~sec long elemen- tary sections and normalizing these elementary areas Pi twice - with respect to both the total area EP i under the oscillogram and the energy E 0, normalized in the same manner, of the initial laser radiation: ~ =