Multi-MeV, runaway electrons pose a serious risk in tokamak plasmas, capable of inflicting catastrophic damage to the armor and vessel walls. A proper diagnosis of these runaway electrons is critical to mitigating their formation. To mitigate the shortcoming of present diagnostics, a new diagnostic with adequate temporal and spatial resolution of the runaway electrons velocity distribution is needed. Laser Inverse Compton Scattering (LICS), with time-gated detection of incident laser light up- scattered by high energy electrons into the soft x-ray regime, meets this requirement. Two key hardware systems are needed to implement LICS. The detection system, an 80-ps gated x-ray imaging camera, has been developed and demonstrated as part of the Inertial Confinement Fusion program. The second required system for LICS implementation is a high repetition rate, > 20-Hz, laser with energies-per-pulse exceeding ~1 Joule within the 80-ps detection gate time. This work will demonstrate the needed laser by building upon, and extending, the 1-kHz repetition rate laser system developed under SBIR funding. The demonstrated body of work includes an innovative 200 mJ, 80 ps laser at 1.064 microns with a repetition rate of 1 kHz, developed under an existing Phase II SBIR program. The existing system is ultra-compact, having a table footprint of less than 1 by 4 feet. The proposed work will include development of an amplifier stage which will increase the pulse energy to the multi-J-per-pulse level with an expected repetition rate of greater than 20 Hz. The limiting factor in the proposed system is the damage threshold of the Nd:YAG amplifier rod in the perspective of the maximum commercially available diameter of rods, 1 inch. In order to achieve pulse energies greater than 1 J at such short pulse durations, innovative temporal and spatial pulse shaping methods will minimize the peak intensity in the solid material. Work in Phase I will utilize the existing system to determine precise constraints on the peak intensity and to demonstrate the beam characteristics required for damage free amplification. In Phase II, a complete laser system will be constructed, incorporating design modifications to allow pulse energies greater than 1 J at the desired repetition rate. The complete laser will meet required specifications of the LICS, with the added advantage of an ultra-compact design that is suitable for use in a typical experimental tokamak facility. The laser proposed under the SBIR has many potential applications beyond the LICS diagnostic, including, but not limited to, laser machining and functioning as a pump laser for an ultra-short pulse laser system.
