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Characterization of the laser induced damage threshold of mirrors in ...

Characterization of the laser induced damage threshold of mirrors in ...

3.8 Fit

3.8 Fit of a Gaussian distribution to a slice of the intensity distribution along one of the two main axes. . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 3.9 Flow diagram of a) the ISO procedure and b) our modification. . . . . . . 26 3.10 Example measurement for the calibration of the power determination diode. 27 3.11 Example fit of the Gaussian beam waist function on experimental data. Counts refer to the output of every pixel of the CCD or the beam profiling camera. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 3.12 Calculated relative error of the fluence on the sample as a function of the fluence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 4.1 Overview of some of the so far conducted measurements. For a better readability neither the error bars nor the actual measurement points are actually displayed. Every one of the solid lines represents a single measure- ment of the type described in this work. . . . . . . . . . . . . . . . . . . . 32 4.2 Measurements conducted on fused silica substrate. All lines represent one measurement, all five measurements have been done on the same sample. The damage sites have a distance towards each other of 1mm. . . . . . . . 33 4.3 Measurements conducted on a 100nm thick gold layer deposited on a fused silica substrate. All lines represent one measurement, all five measurements have been done on the same sample. The damage sites have a distance from each other of 1mm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 4.4 A below threshold irradiated site on a Boron Carbide single layer on fused silica substrate. The threshold fluence has been measured to be around 0.04 J cm 2 . The position has been irradiated for about 10 minutes at the displayed fluence at 500 Hz. . . . . . . . . . . . . . . . . . . . . . . . . . . 35 vi

Chapter 1 Introduction 1.1 About the necessity of high damage threshold optics Archimedes of Syracuse is said to have destroyed a complete fleet during the Roman siege of Syracuse from 212 to 214 B.C. using the sun and mirrors [13]. Today, man made light sources exceed the reachable power densities with sunlight by far. There are lasers capable of creating conditions existing naturally only in the interior of stars and planets. The National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory in California for instance was built to study nuclear fusion at extreme energy densities and pressures reaching 10 8 K and 10 11 bar. One big UV laser makes all that possible, delivering 1.8 MJ pulses with a pulse average power of 500 · 10 12 W over 3 ns [14]. In comparison, the installed electrical power in Germany 2007 was about 137 · 10 9 W [15]. In Europe the Extreme Laser Infrastructure (ELI) is planned, a laser that should provide pulses with an average pulse power of over 100 · 10 15 W and a pulse duration of some femtoseconds. This is already comparable to the average total solar power incident on the earth (174 · 10 15 W [16]). For the wavelength and pulse duration of ELI the intrinsic femto second damage intensity of the standard SiO2 based optics lies in the 12 W order of 10 · 10 cm2 [17, 5, 2]. In order to reduce the incident intensity and to avoid damage, expensive large aperture optics have to be used, see figure 1.1. In the case of NIF additional to up to meter scaled optics massive paralellization was required to reach the anticipated energy densities, resulting in 192 beam lines and leading to costs of about $4.2 billion [18]. A wide range of scientific applications has been found for high power light pulses in the past. From nuclear fusion to the creation of intense attosecond light pulses to the generation of monoenergetic electron beams, high power lasers are pushing the limits of 1

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