A nugget from FOCUS: HERCULES laser contrast reaches eleven orders of magnitude Investigators: V. Chvykov,

The HERCULES laser has recently reached record intensities, as high as 10 22 W/cm 2 [1]. Such ultrahigh intensities create exciting opportunities for experiments in the ultra-relativistic and radiation dominated [2] regime of lasermatter interactions. Laser contrast, defined as the ratio of the peak intensity of the pulse to the prepulse intensity, must be at least 11 orders of magnitude to restrict destructive preplasma dynamics. This estimate is valid for transparent targets with a high damage threshold (fused silica for example) otherwise higher contrast is required. If the contrast is smaller, a preplasma is created before the main pulse by pre-pulses, thus significantly complicating the experiments and interpretation of the results. In order to reach ultrahigh intensities, sharp focusing is required [1]. However, beam break-up into multiple filaments in an expanding pre-plasma may preclude achieving the necessary intensity on a target. In order to make experiments at 10 22 W/cm 2 possible, we have improved the HERCULES contrast dramatically from 10 8 to10 11 . This result is based on two major developments. First, the contrast without a pulse cleaner is relatively high (10 8 comparing to common values of 10 6 –10 7 ) due to the use of a long cavityring-resonator regenerative amplifier that was developed [3] for HERCULES. This contrast level is achieved with nano-joule pulses injected into the resonator. Injecting a clean pulse (contrast ~10 11 ) of micro-joule, instead of nano-joule of energy, allows for the pump fluence to be lowered to achieve the same output energy. This results in a concomitant decrease of ASE generation. We developed and implemented a simple cleaner based on direct amplification to micro-joule energy and on a modified XPWmethod [4]. The contrast was directly measured to be at least 10 11 (limited by the autocorrelator sensitivity) with the third order autocorrelator. To our knowledge HERCULES is currently the highest-contrast multiterawatt laser system.