Temporal and Spatial Characterization of Ultrafast Terahertz Near-Fields for Particle Acceleration*

THz-frequency accelerating structures could provide the accelerating gradients needed for next generation particle accelerators with compact, GV/m-scale devices. One of the most promising THz generation techniques for accelerator applications is optical rectification in LiNbO₃ using the tilted pulse front method. However, accelerator applications are limited by significant losses during transport of THz radiation from the generating nonlinear crystal to the acceleration structure. In addition, the spectral properties of high-field THz sources make it difficult to couple THz radiation into accelerating structures. A better understanding of the THz near-field source properties and how they are affected by changes in the generation setup is necessary for the optimization of THz transport and coupling. We have developed a technique for detailed measurement of the THz near-fields that reveals the effects caused by varying the optical rectification setup. Using this technique we have reconstructed the full temporal 3D THz near-field close to the LN emission face. Our results will help to inform designs of novel structures for use in THz particle acceleration.