A Compact, High Intensity Laser Source for Inverse Compton Scattering

A new diagnostic is currently being developed by Los Alamos National Laboratory (LANL) which will be able to provide direct measurements for the energy and density of “runaway” electrons in tokamak fusion reactors with unprecedented resolution. The multi-MeV, runaways pose a serious risk to tokamak reactors; capable of inflicting catastrophic damage to the armor and vessel walls. A proper diagnosis of the runaway electrons is critical to mitigating their formation. The diagnostic, dubbed Laser Inverse Compton Scattering (LICS), works in a way that is similar to conventional Thomson scattering. However, the scattered laser photons are upshifted in frequency to the range of soft to hard x-rays. The diagnostic leverages an 80 ps, gated x-ray framing camera developed at LANL for the Inertial Confinement Fusion (ICF) program to detect the scattered x-rays and minimize the amount of accumulated noise via the short exposure. The other requirement for the success of the LICS diagnostic is a laser source which can produce more than 1 Joule of photons inside the 80 ps gate, and in order to obtain a time history of the runaways, a modest pulse repetition rate is required.Voss Scientific has made considerable advances in laser technology, having demonstrated an innovative 200 mJ, 80 ps laser at 1.064 microns with a repetition rate of 1 kHz (developed under a prior Phase II SBIR with the Navy). In order to meet the requirements for a functional LICS diagnostic, we propose the development of a Joule-per-pulse laser system which will produce 80 ps pulses with a repetition frequency of 200 Hz. The laser will be compact and “turn-key” operable; appropriate for use in a typical tokamak laboratory. The LANL framing camera, as it exists today, only supports frame rates on the order of 10’s of Hz, but upgrades to higher frame rates are not beyond current technology. The innovative, Joule-class laser, once it is demonstrated, will have exceeded currently available commercial systems by more than a factor of 10 with respects to peak pulse power and repetition rate. During the Phase I project, testing and calculations were performed to verify that the proposed system will meet the specified requirements for LICS and operate safely at the edge of the absolute limits of the materials involved in the light amplification. The laser has many potential applications beyond the LICS diagnostic, including, but not limited to, functioning as a pump laser for an ultra-short pulse laser (USPL) system. Particularly an USPL enabled by the proposed system could produce multi-terawatt pulses with a high average power. Applications like isotope production via laser based accelerators, neutron imaging, and proton beam cancer therapy have all shown promise in research settings with low rep-rate, terawatt pulses, but they all fall short in dose rates due to the lack of a high repetition rate system like the proposed laser. In this respect, the laser could be potentially revolutionary in these commercial areas.