## 6 Towards all Optical \(\gamma\)-Ray Sources  
Based on the experimental results summarised above, a new experimental campaign was established aimed at demonstrating both high energy and control of electron acceleration using the FLAME laser system. In this case, the establishment of a reliable working point for acceleration with self-injection was motivated by the perspective of developing a novel, all optical \(\gamma\)-ray source based upon inverse Compton scattering of laser accelerated electrons. In fact, all-optical, Bremsstrahlung \(\gamma\)-ray sources have already been successfully tested [21] showing high efficiency and potential for laboratory applications. However, in order to access applications requiring high spectral density \(\gamma\)-ray sources like Nuclear Resonance Fluorescence (NRF), new concepts are being developed based upon the use of high energy LINACS and  
Figure 5: Electron density cut at different times, as obtained from PIC simulation, showing the growth of the wake-field structure. The observation window of the code moves at the speed of light following the laser propagation. Electron density increases from blue to red. high power lasers to generate \(\gamma\)-rays via Thomson/Inverse Compton scattering. In fact, for applications based upon NRF, the excitation of a nuclear transition requires high spectral brightness. The ideal requirements for NRF applications on the \(\gamma\)-ray beam are bandwidths as small as 0.1% to be compared with the present best values of 2%, and spectral density as high as 10\({}^{4}\) photons/sec/eV. These requirements are very challenging and new large installations based on accelerator technology are being proposed worldwide to fulfil them with estimated costs of 50-100 million Euro. However, \(\gamma\)-rays can also be generated using entirely optical techniques and based upon small scale equipment. A pioneering experiment [38] in this scenario was carried out at the Jena laser Facility in 2006. In that experiment, in spite of the low quality of laser-accelerated electron bunches, still affected by 100% energy spread, all-optical Thomson scattering in the 1 keV X-ray region was demonstrated using a compact configuration with relatively low degrees of freedom for optimisation. Since then, laser-plasma acceleration has undergone dramatic progress and laser accelerated electrons now can reach and exceed 1 GeV with energy spread well below 10%, with record values close to 1%. Moreover, new schemes are being proposed nowadays to control injection and optimise acceleration, which are now being implemented to further improve the quality of laser accelerated electrons. The demonstrated performance of laser-plasma acceleration at FLAME is already well beyond that already explored in [38] both in terms of energy and energy spread of the accelerated bunches. Even more important, considering that the energy per pulse of the FLAME laser is significantly higher, we can plan for a much higher flux of \(\gamma\)-rays in a single shot configuration. As shown in Figure 6, the proposed source is based upon a counter-propagating configuration of two ultraintense laser pulses focused in a gaseous target. In the experiment, the counter-propagating configuration is obtained by splitting the FLAME main pulse into two pulses with controlled energy and independent focusing configuration. In this configuration, one of the FLAME laser pulses generates a bunch of energetic electrons under conditions similar to those already explored in the SITE campaign. A detailed design of an experiment dedicated to all optical Thomson/Inverse Compton scattering at FLAME is available elsewhere [39]. In the following we give an overview of the FLAME laboratory, followed by a summary of the design the experiment on acceleration with self-injection (SITE). Figure 6: Schematic view of the counter-propagating configuration for all-optical \(\gamma\)-ray generation. The main FLAME laser pulse is split in two pulses that are focused in the proximity of a gas target.