A high repetition rate target assembly for laser-plasma acceleration has been built at the Laser Laboratory for Acceleration and Applications (L2A2) of the University of Santiago de Compostela. The target consists on two linear stages combined with a rotational one to ensure the focusing and refreshment of the target material shot-by-shot. A multi-target wheel alow us to install different target materials that could accomodate few hundreds of shots at 10 Hz.
A feed-back loop procedure has been design to ensure the accurate shot-by-shot positioning of the target at focus. Focus position is determined by using the Speckle technique. Then, an accurate laser-position sensor is used to map the surface of the target wheel. The movement of the target assembly shot-by-shot is then programmed according to the target surface map. This procedure provides a target positioning at focus with an accuracy below 5 μm.
This target assembly has been used with the compact 45 TW laser of 1.2 J, 10 Hz, 25 fs and a contrast 1012 @ 20 ps at L2A2. First proton pulses were produced and their energy spectra measured with a time-of-flight detector, providing an average maximum energy above 1 MeV. Although, the maximum proton energy will be optimized in future, the most relevant feature is the stability of the proton source. Over several tens of shots the proton spectra are very stable, both in the cut-off energy and temperature. The observed differences are below 3%. The stability of the source is achieved thanks to the positioning system, which avoids the displacements introduced by the wobbling of the target.
This stable, compact, high energy source of protons could be used to produce on demand radioisotopes which could open the study of imaging techniques with short lifetime which are difficult to study with conventional cyclotrons.
|Working group||Laser-driven ion acceleration|