Theory of wakefields in a dielectric-lined waveguide
Theory of wakefields in a dielectric-lined waveguide
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DOI：
10.1103/physreve.62.1266
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发表时间：
2000-07
期刊：
影响因子：
--
通讯作者：
Park;Hirshfield
中科院分区：
文献类型：
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作者：
Park;Hirshfield
分析了沿介质衬里圆柱波导轴线方向平行运动的短电荷束对尾波场的激发。这种情况相当于在横向有界系统中产生切伦科夫辐射。尾波场被展开为一组混合电磁本征函数的正交化集合，用于该波导几何结构。首次得到了该系统的正交化关系，明显地，对于固定源和定域运动源，如电荷束，这些正交化关系是不同的。尾波场产生的力在分布的波束内部和后面都有效。束团分布偏离轴对称会导致产生幅度很大的偶极模，从而可能导致不稳定性。研究了该系统的Pointing定理，结果表明，必须从Pointing通量中减去随体库仑场能量，才能得到辐射功率。根据电场直接计算出的平衡束团阻力的这种功率，显示的方向与电荷束的方向相反。结果很容易推广到任意长度和电荷分布的束团，以及一系列这样的束团。给出了单极、偶极和四极尾场力的数值例子，并给出了样本电场模式，以帮助理解这种切伦科夫辐射的反常性质。对于长度为0的2-NC矩形传动束。在沿内外径分别为0.50和5.0 mm的氧化铝衬里波导(varepsilon=9.50)的轴向移动20 mm时，预测了155 MeV/m束流后的峰值加速梯度。这种相对较高的加速梯度表明，一旦实验室中有了低发射度、千安培、亚皮秒的电子束，一个简单的均匀介质管可能成为未来高梯度电子/正电子直线加速器的结构基础。
Excitation of wakefields from a short charge bunch moving parallel to the axis of a dielectric-lined cylindrical waveguide is analyzed. This situation amounts to generation of Cerenkov radiation in a transversely bounded system. Wakefields are expanded into an orthonormal set of hybrid electric-magnetic eigenfunctions for this waveguide geometry. The orthonormalization relations for this system are obtained, evidently for the first time, both for a stationary source and for a localized moving source such as a charge bunch; it is shown that these orthonormalization relations differ. Forces arising from wakefields are found, valid within and behind a distributed bunch. Deviation of bunch distribution from axisymmetry leads to generation of dipole modes of significant amplitude that may lead to instability. Poynting's theorem is examined for this system, and it is shown that convected Coulomb field energy must be subtracted from the Poynting flux to obtain the radiation power. This power, which balances drag on the bunch as calculated directly from the fields, is shown to flow in a direction opposite to that of the charge bunch. The results are easily generalized to bunches of arbitrary length and charge distribution, and to a train of such bunches. Numerical examples are presented for monopole, dipole, and quadrupole wakefield forces, and sample electric field patterns are shown to assist in understanding the unusual nature of this type of Cerenkov radiation. For a 2-nC rectangular drive bunch of length 0. 20 mm, moving along the axis of an alumina-lined waveguide (varepsilon=9.50) with inner and outer radii of 0.50 and 5.0 mm, a peak accelerating gradient behind the bunch of 155 MeV/m is predicted. This relatively high magnitude of accelerating gradient suggests that a simple uniform dielectric pipe could be the basis for the structure of a future high-gradient electron/positron linear accelerator, once low-emittance, kiloampere, subpicosecond electron bunches are available in the laboratory.