Numerical investigation of transmission probability characteristics in the first low-density region of a laser wakefield accelerator

Transmission probability is an important parameter in vacuum science and technology that needs to be accurately characterised for system design. Typically, this is computed using the test particle Monte Carlo method. However, this approach is valid only in the free molecular regime. In this work, we propose a methodology to compute transmission probability using the direct simulation Monte Carlo method, which makes possible the characterisation of vacuum devices for a wider range of operating pressure conditions from viscous to molecular flow regimes. This is applied to study the gas expansion characteristics in the first low-density drift region of a laser wakefield accelerator. Validations are first made by comparing the results against the test particle Monte Carlo method in the free molecular regime. The transmission probability is then characterised for a wide range of operating conditions, revealing interesting insights helpful towards not only a fundamental understanding but also in making design considerations.