Design of a Resonant Transition Radiation Source in the Soft X-ray Range

Resonant transition radiation (RTR) can be generated from multi-layer structures when they are driven by relativistic electron beams. In consideration of using the NSRRC 90 MeV photoinjector as driver, we examined the feasibility of generating narrow-band soft x-rays from various multi-layer structures. Based on analytical theory, the expected angular-spectral distribution and photon yield of these radiators are calculated and compared. INTRODUCTION The feasibility of using multi-layer structures for generation of RTR in soft x-ray range is being investigated. We tentatively targeted the radiation energy of these structures centred at 2 keV and 620 eV. Radiation near this photon energy ranges found to be useful in research areas such as imaging of biomolecules, atom and molecular physics. The low-emittance NSRRC photoinjector system will be used to drive such RTR sources. In this system, the MeV electron beam from the laser driven photo-cathode rf gun are accelerated to its maximum energy of 90 MeV. The nominal parameters of the drive beam used in this study are listed in Table 1. Table 1: Parameters of the Electron Beam Generated From the NSRRC Photoinjector Beam energy 90 MeV Emittance 0.8 mm-mrad Bunch length 2.2 psec Bunch Charge 100 pC RESONANT TRANSITION RADIATION The existence of transition radiation (TR) was predicted by Ginzburg and Frank in 1949[2]. For a charged particle traveling across the boundary between two media of different dielectric constants, the radiation fields can be calculated with classical electromagnetic theory. In the xray range, the dielectric constant can be described by Drude model adequately. The angular-spectral distribution of transition radiation from the boundary of two media being excited by a single electron at normal incidence is [3]: 2 2 1 2 2 3 2 2 ) ( sin 4 Z Z c d d W d TR       2 e (1) where )] / ( [ 2 2 ) 2 ( 1 2 2 ) 2 ( 1      p Z     4c (2) is the plasma frequency of material 1 (or 2). For an N-layer structure which is fabricated by interleaved stacking of material 1 with thickness and material 2 with thickness , the angular-spectral distribution of RTR can be calculated from the following equation with the phase shift as well as absorption in materials are considered[3, 4]: