Yuanzineng kexue jishu (Oct 2024)
Studies on Novel Cs2Te Photocathode Based on Surface Plasmon Polaritons
Abstract
Cathodes serve as sources of electron beams, and their performance plays a great part of beam quality in electron accelerators. The introduction of surface plasmon polaritons (SPPs) into different photocathodes has been investigated to increase their quantum efficiency. Currently, the SPPs excitation is predominantly achieved in the infrared and visible bands due to limitations in the permittivity of common noble metals. Exciting SPPs in the ultraviolet band typically involve attaching aluminium nano-particles to the material surface, which can impact the optical properties of the material. Therefore, it is important to develop a method for exciting SPPs in the ultraviolet band without affecting optical properties of the material. The cesium telluride (Cs2Te) photocathode represents a lifetime of several months, a quantum efficiency of up to 10%, a response time of less than 1 ps, a thermal emittance of less than 1 mm·mrad/mm, and moderate operating vacuum requirement of 10−7 Pa. These characteristics make Cs2Te an exceptional photocathode material for generating high-current electron beams. However, a drawback of Cs2Te photocathodes is that they need to be excited by high-power ultraviolet lasers, which are more challenging to obtain compared to green lasers. Consequently, there is a continued demand for increasing the quantum efficiency of Cs2Te to reduce the laser power requirements. In this paper, accuracy of the Monte Carlo model to simulate the Cs2Te photoemission was verified. SPPs were introduced into Cs2Te photocathode by means of micro-nano grating structure, which increases substantially the absorption of incident photons and hence form a local field enhancement to regulate initial photoelectrons distribution. The Monte Carlo model was modified according to the theory of SPPs to simulate the photoelectric emission process of the Cs2Te photocathode after the introduction of SPPs. The simulation results show that the quantum efficiency of Cs2Te photocathode increases by 60% while the thermal emittance is basically unchanged, with the introduction of SPPs. This is due to the photoelectrons population, resulting from the additional laser absorption induced by the SPPs, decreases exponentially from the Cs2Te/Al interface to the vacuum/Cs2Te interface. The majority of these increased photoelectrons are distributed near the deeper interface, resulting in higher scattering probabilities during their transport from inside the photocathode to the surface, leading to a reduction of thermal emittance of the emitted electrons. Combined with conventional methods for controlling the thermal emittance of photocathodes, such as reducing the energy of incident laser photons and operating at low temperatures, it is anticipated that a photocathode with higher performance, characterized by low thermal emittance and high quantum efficiency, can be achieved. This advancement holds significant implications for the future development of high-brightness electron sources.
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