Spatiotemporal modeling of direct acceleration with high-field terahertz pulses

We present an improved model for electron acceleration in vacuum with high-energy THz pulses that includes spatiotemporal effects. In our calculations, we examined the acceleration with 300 GHz and 3.0 THz central frequency THz pulses with properties corresponding to common sources, and compared the Gaussian and Poisson spectral amplitudes and the associated time profiles of the electric fields. Our calculation takes into account both the longitudinal field and the spatio-spectral evolution around the focus. These aspects of the model are necessary due to the tight focusing and the duration towards a single-cycle of the THz pulses, respectively. The carrier-to-envelope phase (CEP) and the tilting angle of the coincident few- or single-cycle THz pulses must be tuned in all cases in order to optimize the acceleration scheme. We reveal additionally that electron beams with different final energies and different divergences can be generated based on simulated THz pulses having different Porras factors, describing the frequency dependence of the spatiotemporal amplitude profile, which may depend strongly on the method used to generate the single-cycle THz pulses.