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Experimental arrangement for measurement of the critical distance. A focused laser irradiates a planar target at normal incidence. A faraday cup with aperture diameter D is installed at an angle to the target normal and can be translated distances L along the path of plasma expansion. A spherical coordinate system with zenith angle and azimuthal angle with respect to the faraday cup translation axis is superimposed.
Typical signals recorded simultaneously from each of the four faraday cups are shown in (a), offset from each other but with the same vertical scale. A polar plot of mean ion energy and a least-squares curve fit is shown in (b); 0° is the target normal. The mean ion energy is calculated as , where is the ion mass (118.71 amu for Sn), is distance from each faraday cup to the target (105 cm), and , where is the measured voltage from the faraday cup as a function of time. The calculated mean ion energies in (b) were averaged from 20 shots.
as a function of distance from the faraday cup to the target. Solid circles represent data using the laser; open squares represent data using the Nd:YAG laser. Each data point is the average of least 15 shots. For the Nd:YAG data is constant within the error bars implying the critical distance is less than 55 mm. For the data is decaying over both spatial intervals investigated, implying the critical distance is greater than 622 mm.
Charge-state resolved ion energy distribution measured at a distance of 100 cm from a Sn target irradiated by a laser with parameters described in Table I.
Laser parameters used in the experiments. Our Nd:YAG laser is a Continuum Surelite II-10, and our laser is custom-built and described elsewhere.9 The focusing lens in each experiment is planoconvex with focal length f given in the table. At each laser wavelength, the focal diameter was measured directly with an appropriate imaging lens and camera.
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