Ablative acceleration of thin metal plates on picosecond and nanosecond time scales (comparative study by acceleration efficiency and surface profilometry) Irradiation of a metal film through a transparent substrate by a pulsed laser can produce a plasma in the substrate/metal interface and lead to launching the remainder of metal (as a flyer) at velocities up to several kilometers per second [1, 2]. Fig. 2. Experimental arrangement for flyer acceleration by two laser pulses and it ’ s velocity determination. Metal Plate acceleration is a complex process, which can be regarded as a sequence of different processes (plasma generation, absorption of laser irradiation by plasma, its expansion, flyer formation and its launching), which occur on different time scales [3]. The whole acceleration process can be characterized by comparison of acceleration efficiency (ratio of kinetic energy of flyer to total laser energy: mv 2 /2E laser ) of flyer initiated with different laser energies and pulse durations and by profile measurements of damaged metal surface as a function of above parameters.Characterization of the launch process and optimization of all possible parameters is the main objective of this research. Velocity measurement by Time Of Flight method Fig. 3. Typical signals measured for (a) different laser energies on the same distance (100μm) and for (b) different distances with the same laser energy (E laser = 49mJ). Mass can be estimated from the volume of the remained crater in the metal film multiplied by metal density Formation and acceleration of flyer are complex processes. There is an optimum pulse duration for efficient launch of the flyer. Profilometer scans show that film lifting from the surface may leave a significant portion of the irradiated film as a crater rim still on the surface. We can surmise that in those cases only jet (composed of film fragments, gaseous and liquid form of the metal) was formed. References: 1. Paisley D.L., in Shock Compression of Condensed Matter – 1989, Schmidt S.C., Johnson J.N., Davidson L.W. (ed.), Elsevier Science Publishers B.V. (1990), pp Watson S., Field J.E., J.Phys. D: Appl.Phys., (2000) 33, pp Krehl P., Schwirzke F., Cooper A.W., J. Appl.Phys., (1975) 46, pp קוגן סמיון, עבודת גמר לקבלת תואר מוסמך (2001) Fig. 5. Digital photo of the holes in metal film. Semyon Cogan, Elijah Shirman and Yehuda Haas Department of Physical Chemistry and the Farkas Center for Light Induced Processes The Hebrew University of Jerusalem, Jerusalem, Israel Experimental Details and Results Fig. 4. Determination of the initial velocity of flyer launched at different laser energies. The delay between two pulses ~2 ns. The 1 st pulse energy is ~19 mJ and the 2 nd is changing according to the column (1-6) on the figure. Fig. 6. Growth of crater diameter as a function of total laser energy. Fig. 7. Profilometric scans of the craters in Al coating. (a) Double pulse experiment 10 ns pulse width, ~ 2 ns delay between pulses, (b) single pulse, FWHM = 20 ps. Fig. 8. Relative volume of the rim to the total volume lifted for different laser energies Fig. 9. Efficiency of flyer launch (E kin /E laser ) as a function of total laser energy for different delays between two pulses 10 ns each. Fig. 10. Efficiency of flyer launch as a function of pulse width i.e. sum of FWHM and delay time for double pulse experiment and simple FWHM 30 ps single pulse experiment. Objectives Shock Propagation Flyer at different stages of flying Metal Coating Metal plasma and Vapor Expansion Fig. 1. Schematic representation of flyer formation, propagation and shocking the target Laser Beam Glass Substrate Introduction Conclusions (a)(b) Flyer formation efficiency was estimated from flight velocity and mass measurement. Acknowledgments This research was supported by the research directorate of the Ministry of Defense. The Farkas Center for Light Induced Processes is supported by the Minerva Gesellschaft mbH.