Towards stabilizing laser-plasma accelerators through non-perturbative focus diagnostics of 100-TW-class laser pulses

High power laser systems are attracting great interest due to their compact footprint compared to alternative technologies. However, the non-linear physics at play in the high-power laser plasma interaction makes applications highly sensitive to laser alignment. We demonstrate an accurate non-perturbative high-power laser diagnostic, allowing shot-tagged laser delivery information on position and angle by the installation of a specialty final steering mirror (the last optic delivering the converging laser beam from the parabola towards focus), where its rear surface reflection of the main beam is used to provide online non perturbative monitoring of the high-power laser focus position and angle at a two-camera setup. We refer to the beam reflected from the rear surface as the "witness beam". It is a fully correlated copy of the high-power beam since it shares the same mirror reflections from the oscillator all the way to the final steering mirror and final focus, limiting the source of the uncorrelated factors. The demonstrated correlation of the high-power laser pulse train to the non-amplified background pulse train highlights the option for feedback integration. If high-power laser systems were to become available at kHz repetition rates and the focus position and pointing angle fluctuation dominated by frequency are well below 500 Hz, fast feedback mechanisms using the witness beam technique demonstrated here could be able to correct. This would allow significant improvements in stability, quality, and applicability of high-power laser systems.