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arXiv physics/0612077physics.acc-ph
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超高场梯度加速电子束的纵向尾流场

Longitudinal Wake Field for an Electron Beam Accelerated through a Ultra-High Field Gradient

Gianluca Geloni, Evgeni Saldin, Evgeni Schneidmiller, Mikhail Yurkov

摘要:

具有越来越高的纵向场梯度的电子加速器是期望的,因为它们允许通过紧凑且便宜的设置来产生高能束。新的激光等离子体加速技术似乎构成了这个方向上更有前途的突破,提供前所未有的场梯度高达TV/m。本文定量地描述了纵向尾流场对电子注的影响。我们的论文是基于麦克斯韦方程组的纵向场的解决方案。当加速距离远小于超越距离时,我们的结论是有效的,超越距离是电子作为光信号从束团尾部超越束团头部的距离。该条件在激光等离子体器件中得到了很好的验证。我们计算了一个封闭的阻抗和尾流函数,可以进行数值计算。结果表明,由于辐射相互作用,聚束中的能量损失率等于通过远区相干辐射发射的能量。此外,发现一个表达式的渐近极限的电子束从加速器的一个大的距离相比,超越长度。这样的表达式使我们能够计算高斯横向和纵向束团形状的解析解。最后,我们研究了基于100 MeV和1 GeV激光等离子体加速器驱动的真空紫外和X射线台式自由电子激光的可行性。数值计算表明,纵向尾迹引起的能量随时间变化的影响对这些装置的运行构成了严重的威胁。

英文摘要:

Electron accelerators with higher and higher longitudinal field gradients are desirable, as they allow for the production of high energy beams by means of compact and cheap setups. The new laser-plasma acceleration technique appears to constitute the more promising breakthrough in this direction, delivering unprecedent field gradients up to TV/m. In this article we give a quantitative description of the impact of longitudinal wake fields on the electron beam. Our paper is based on the solution of Maxwell's equations for the longitudinal field. Our conclusions are valid when the acceleration distance is much smaller than the the overtaking length, that is the length that electrons travel as a light signal from the tail of the bunch overtakes the head of the bunch. This condition is well verified for laser-plasma devices. We calculate a closed expression for the impedance and the wake function that may be evaluated numerically. It is shown that the rate of energy loss in the bunch due to radiative interaction is equal to the energy emitted through coherent radiation in the far-zone. Furthermore, an expression is found for the asymptotic limit of a large distance of the electron beam from the accelerator compared with the overtaking length. Such expression allows us to calculate analytical solutions for a Gaussian transverse and longitudinal bunch shape. Finally, we study the feasibility of Table-Top Free-Electron Lasers in the Vacuum Ultra-Violet (TT-VUV FEL) and X-ray range (TT-XFEL), respectively based on 100 MeV and 1 GeV laser-plasma accelerator drivers. Numerical estimations presented in this paper indicate that the effects of the time-dependent energy change induced by the longitudinal wake pose a serious threat to the operation of these devices.

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