A planar laser pulse propagating in vacuum can exhibit an extremely large ponderomotive force. This force, however, cannot impart net energy to an electron: As the pulse overtakes the electron, the initial impulse from its rising edge is completely undone by an equal and opposite impulse from its trailing edge. Here we show that planar-like "flying focus’’ pulses can break this symmetry, imparting relativistic energies to electrons. The intensity peak of a flying focus—a moving focal point resulting from a chirped laser pulse focused by a chromatic lens—can travel at any subluminal velocity, forwards or backwards. As a result, an electron can gain enough momentum in the rising edge of the intensity peak to outrun and avoid the trailing edge. Accelerating the intensity peak can further boost the momentum gain. Theory and simulations demonstrate that these dynamic intensity peaks can backwards accelerate electrons to the MeV energies required for radiation and electron diffraction probes of high energy density materials. Vacuum laser acceleration (VLA) exploits the large electromagnetic fields of high-intensity laser pulses to accelerate electrons to relativistic energies over short distances …[1-12]. The field of an intense pulse can far surpass that in conventional radio-frequency (RF) or advanced plasma-based accelerators, and the underlying interaction—involving only an electron and the electromagnetic field—has an appealing simplicity. RF accelerators routinely improve beam quality and achieve unprecedented energies, but their low damage threshold constrains the maximum accelerating field. This necessitates physically and economically immense structures to accelerate electrons to the energies necessary for high energy density probes, radiation sources such as free electron lasers, or high-energy physics experiments …[13-16]. Wakefield accelerators, on the other hand, employ plasma to sustain accelerating fields nearly 1000x that of RF accelerators …[17-23]. The use of plasma, however, comes with its own set of challenges, such as tuning the laser or electron beam parameters to the plasma conditions, avoiding a myriad of instabilities, and creating long uniform plasma channels [24,25]. VLA avoids damage constraints and the challenges inherent to the use of plasma, but achieving competitive electron energy gains requires a bit of ingenuity. The inherent difficulty is that the accelerating waves travel at the vacuum speed of light. As a result, electrons, regardless of their speed, will encounter repeated phases of acceleration and deceleration. More specifically, the Lawson–Woodward Theorem precludes vacuum laser acceleration under the following conditions: (1) There are no boundaries or walls present. () The laser-electron interaction distance and duration are infinite. () There are no static fields. () And finally, nonlinear forces, such as the magnetic Lorentz force or ponderomotive force, are ignored …[1,26,27]. While in principle the Lawson–Woodward Theorem limits laser vacuum acceleration, in practice it classifies all laser vacuum acceleration schemes by which assumption(s) they exploit. As an example, in direct laser acceleration schemes, the linear electric field of a combination of laser pulses or an exotically polarized pulse accelerates injected relativistic …

中文翻译：

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动态激光脉冲中电子的真空加速

在真空中传播的平面激光脉冲会表现出极大的质动力。但是，此力无法将净能量赋予电子：当脉冲超过电子时，来自其上升沿的初始脉冲会被来自其后沿的相等反向的脉冲完全撤消。在这里，我们证明了类似平面的“飞行焦点”脉冲可以破坏这种对称性，为电子提供相对论能量，飞行焦点的强度峰值（由彩色透镜聚焦的chi激光脉冲产生的移动焦点）可以传播因此，电子可以在强度峰值的上升沿获得足够的动量，从而超出或避开后沿，从而加速电子的运动，从而进一步提高动量增益。理论和模拟表明，这些动态强度峰值可以使电子向后加速到高能量密度材料的辐射和电子衍射探针所需的MeV能量。真空激光加速（VLA）利用高强度激光脉冲的大电磁场在短距离内将电子加速为相对论能量…[1-12]。强脉冲的场可以远远超过传统的射频（RF）或先进的基于等离子体的加速器，并且潜在的相互作用（仅涉及电子和电磁场）具有非常吸引人的简便性。射频加速器通常可以提高光束质量并获得空前的能量，但是其低损伤阈值限制了最大加速场。这需要在物理上和经济上庞大的结构来将电子加速到高能量密度探针，诸如自由电子激光之类的辐射源或高能物理实验所需的能量……[13-16]。另一方面，Wakefield加速器采用等离子体来维持加速场，其速度是RF加速器的近1000倍……[17-23]。然而，等离子体的使用面临着一系列挑战，例如将激光或电子束参数调整为等离子体条件，避免各种不稳定性以及创建长而均匀的等离子体通道[24,25]。VLA避免了损害限制，也避免了使用等离子体所固有的挑战，但是要获得具有竞争力的电子能量，需要一些独创性。固有的困难是，加速波以光的真空速度传播。结果，电子，无论其速度如何，都将遇到加速和减速的重复阶段。更具体地说，在以下情况下，Lawson-Woodward定理排除了真空激光的加速：（1）没有边界或壁。（）激光-电子相互作用的距离和持续时间是无限的。（）没有静态字段。（）最后，非线性力，例如电磁洛伦兹力或磁动力，将被忽略……[1,26,27]。劳森-伍德沃德定理原则上限制了激光真空加速度，但实际上，它利用了所有假设来对所有激光真空加速度方案进行分类。例如，在直接激光加速方案中，激光脉冲或奇异极化脉冲的组合的线性电场会加速注入的相对论…