Rapid 3-D Simulation of a Bunch-Length Diagnostic for Laser Wakefield Accelerators via Coherent Transition Radiation at THz Frequencies

76105S Laser wakefield accelerator (LWFA) concepts, characterized by ultra-high gradients and ultra-short bunch lengths, show great promise for reducing the cost and size of future high-energy electron linacs. A new non-invasive, bunch-length diagnostic is critical to continuing the rapid advances in LWFA technology. Coherent transition radiation (CTR), generated as the short bunches exit the plasma, could provide this diagnostic if the effects of various secondary complications were quantified. This project will develop particle-in-cell (PIC) simulations to characterize the CTR emitted from a LWFA and self-modulated (SM) LWFA configurations. Phase I demonstrated that explicit two-dimensional (2D) PIC simulations could correctly model a full SM-LWFA interaction, including CTR emission from the micron-scale electron bunches emerging from plasmas. Also, a 2D ponderomotive-guiding-center treatment of the laser pulse was implemented within the framework of the existing PIC code. In Phase II, the ponderomotive-guiding-center model for the laser envelope will be extended to three dimensions, with improved dispersive properties. The emission of THz-scale CTR, from accelerated electron bunches (at 10 ¿m scale) emerging from the plasma, will be simulated using a numerical technique, in order to obtain far-field radiation patterns from near-field plasma currents. The simulations will be compared with on-going experimental work will clarify the effect of plasma density gradients on THz emission. Commercial Applications and Other Benefits as described by the awardee: The enhanced PIC simulation code, together with experimental measurements, should provide a uniquely powerful bunch-length diagnostic for plasma-based accelerators. Another potential benefit would be the development of a new and uniquely powerful source of THz radiation, with applications in medicine, science and national security.