标题： 基于光纤的电子加速的基本极限$-$以及如何克服它们 显示英文标题

标题： Fundamental Limits on Fiber-Based Electron Acceleration $-$ and How to Overcome Them

摘要： 为了利用沿空心波导的电磁脉冲加速超相对论带电粒子，如电子，脉冲需要具有纵向电场分量和相速度为$c$，即真空中的光速。 我们推导出一个近似的闭式表达式，用于确定在具有介质层的金属包覆空心光纤中 TM$_{01}$模式的相速度为$c$时的波长。 该表达式随后用于推导介质材料色散所需的条件，以便同时具有$c$相速度和群速度。 结果表明，色散需要非常强烈的异常，以至于在异常色散区域的损耗会使这种粒子加速器变得无用。 然后我们提出利用介质中的两种光谱峰形式的增益来绕过原本的基本限制，并允许 TM$_{01}$脉冲具有$c$相速度和群速度，从而实现基于光纤的电子加速器的任意长度缩放。 理论上，通过进一步的增益辅助色散工程，群速度色散也可以变为零，从而允许无色散电磁脉冲与相对论粒子共传播。 显示英文摘要

摘要： To accelerate ultra-relativistic charged particles, such as electrons, using an electromagnetic pulse along a hollow-core waveguide, the pulse needs to have a longitudinal electric field component and a phase velocity of $c$, the speed of light in vacuum. We derive an approximate closed-form expression for the wavelength at which the phase velocity of the TM$_{01}$ mode in a metal-clad hollow-core fiber with a dielectric layer is $c$. The expression is then used to derive conditions for material dispersion required of the dielectric in order to simultaneously have $c$ phase and group velocity. It is shown that the dispersion would need to be so heavily anomalous that the losses in the anomalously dispersive regime would render such a particle accelerator useless. We then propose the utilization of gain in the form of two spectral peaks in the dielectric to circumvent the otherwise fundamental limits and allow for TM$_{01}$ pulses with $c$ phase and group velocity and thus arbitrary length-scaling of fiber-based electron accelerators. In theory, the group velocity dispersion could also be made zero with further gain-assisted dispersion engineering, allowing for the co-propagation of dispersionless electromagnetic pulses with relativistic particles.