Investigations into Dielectric Laser-Driven Accelerators using the CST and VSIM Simulation Codes

Abstract

Dielectric laser-driven accelerators (DLAs) based on gratings structures have received a lot of interests due to its high acceleration gradient up to GV/m and mature lithographic techniques for fabrication. This paper presents detailed numerical studies into the acceleration of relativistic and non-relativistic electrons in double gratings silica structures. The optimization of these structures with regards to maximum acceleration efficiency for different spatial harmonics is discussed. Simulations were carried out using the commercial CST and VSim simulation codes and results from both codes are shown in comparison. INTRODUCTION Dielectric laser-driven accelerators (DLA) have good potential to become a strong candidate for future electron accelerators. Due to a higher damage threshold than metals, these dielectric microstructures can support accelerating fields higher than what can be achieved in conventional accelerators. This can increase the acceleration gradients up to GV/m. An experiment has successfully demonstrated acceleration of relativistic electrons with an accelerating gradient of 250 MV/m in a fused silica double grating structure [1] and the acceleration of non-relativistic 28 keV electrons with a gradient of 25 MeV/m in a single grating structure was also observed [2]. This paper investigates dielectric laser-driven acceleration of electrons in a double grating structure exploiting the different spatial harmonics excited by the diffraction of the incident laser. The double grating structure was originally proposed by Plettner [3] and is shown in Figure 1. Each grating pillar adds a phase shift with respect to the adjacent vacuum space, which produces a longitudinally periodic oscillating electric field in the centre of the vacuum channel. Optimization studies into these structures by parameter variation studies have already been performed with the aim to increase the acceleration efficiency for highly relativistic electrons [4,5]. Here, we consider also the non-relativistic case where electrons are injected at an energy of 25 keV, corresponding to β=0.3, where β=v/c, v the electron velocity and c the speed of light. Different spatial harmonics were considered using the CST [6] and VSim [7] simulation codes to identify the optimum acceleration efficiency and comparing simulation results. Figure 1. Schematics of a dielectric grating structure. ACCELERATION OF HIGHLY RELATIVISTIC ELECTRONS When a double grating structure is driven by two TM polarized laser beams from opposite sides, the diffraction of the incident laser at the grating excites different spatial harmonics which can all be used in principle to accelerate the electrons, see Figure 2. Figure 2. Illustration of the first, second and third spatial harmonics for the case that one grating period is illuminated by laser from two sides. In the simulations an incoming plane wave with a wavelength of λ0 =1,550 nm was used to excite the grating structure from two sides. Silica (SiO2, refractive index n=1.528) was chosen as grating material due to its good properties in terms of transparency and field damage threshold. Figure 3 shows the acceleration efficiency for different structure parameters for a grating period of λp = 1,550 nm. With an increase of the vacuum channel width C, the acceleration efficiency η gradually decreases, as can be seen in Figure 3(a). Figure 3(b) shows that the maximum acceleration efficiency can be achieved when the pillar height H=0.87λp. For further optimization the pillar ratio A/λp was varied and Figure 3(c) shows the resulting optimum acceleration efficiency of 0.25 and 0.26 as computed by VSim and CST, respectively. ___________________________________________ *This work is supported by the EU under Grant Agreement 289191 and the STFC Cockcroft Institute core grant No.ST/G008248/1. #yelong.wei@cockcroft.ac.uk n=1