Sylvain Fourmaux , Stéphane Payeur , Philippe Lassonde , Jean - Claude Kieffer and François Martin

Extreme laser peak intensities can be produced with current laser technology, using both the chirped pulse amplification (CPA) technique (Strickland & Mourou, 1985) and Ti:Sapphire amplification crystals (Le Blanc et al., 1993). Laser systems using this technology are commercially providing instantaneous power in excess of 100 TW with a laser pulse duration ∼30 fs and energy per pulse of several Joules. By focusing these pulses to a few μm spot size, high intensity laser matter interaction studies are now routinely performed at peak intensities of 1018 − 1020 W/cm2. For fundamental physics research, increased laser intensities enhances current interaction processes and can lead to new and more efficient interaction regimes. A peak intensity above 1022 W/cm2 has already been reported by Yanovski et al. (Yanovsky et al., 2008) and a facility such as the Extreme Light Infrastructure (ELI) (Gerstner, 2007) envisions peak intensities in the range of 1023 W/cm2 which are needed for experiments on radiation reaction effects (Zhidkov et al., 2002). For applications development, recent progress of laser systems combining high intensity and high repetition rate have attracted considerable interest for the production of solid target based secondary sources where high mean brightness is required. In high field science, this includes bright x-ray sources (Chen et al., 2004; Schnürer et al., 2000; Teubner et al., 2003; Thaury et al., 2007), high energy particle acceleration (Fritzler et al., 2003; Steinke et al., 2010; Zeil et al., 2010) and nuclear activation (Grillon et al., 2002; Magill et al., 2003). To illustrate this interest for high peak intensities, recently published scaling laws for laser based proton acceleration on thin film solid targets (Fuchs et al., 2006) have shown that an important increase of the on target laser intensity is necessary to reach the expected energy required for biomedical application in the proton therapy field (60 250 MeV). Moreover, intensities greater than 1020 W/cm2 will allow access to the non collisional shock acceleration regime where >100 MeV maximum energy protons could be produced (Silva et al., 2004). For currently available peak intensities (1018 − 1020 W/cm2 range), where the field strength is sufficient to accelerate particle to relativistic energies, the laser pulse contrast ratio (LPCR) is a crucial parameter to take into consideration. Considering the laser pulse intensity temporal profile, the LPCR is the ratio between its maximum (peak intensity) and any fixed delay before it. A low contrast ratio can greatly modify the dynamics of energy coupling between the Laser Pulse Contrast Ratio Cleaning in 100 TW Scale Ti: Sapphire Laser Systems 7