Abstract:As a new kind of electromagnetic pulses with finite energy, the flying electromagnetic toroid (FET), also called as the flying electromagnetic doughnut, has significant potential applications, such as the excitation of anapole non-radiation mode and the acceleration of charged particles. To show the propagation characteristic of FET, the spatial distribution and spectrum characteristic of the transverse and longitudinal components of FET and its topology evolution in the propagation process are discussed in this paper. Without loss of generality, we theoretically research the longitudinal field and transverse field of the transverse magnetic (TM) FET based on the real part of FET’s propagation equations. The field distribution, topology, and spectrum when the FET propagates to different positions can be calculated by assigning corresponding values to the time variable in FET’s propagation equations, therefore, the propagation characteristics of FET can be studied accurately in theory. The magnetic field of TM FET is distributed into rings in the plane vertical to the propagation direction and the electric field of TM FET is rotated around the magnetic field, which means the FET has a hypertorus topology. All the field components of FET are rotationally symmetric in the plane vertical to the propagation direction. The FET’s center is the maximum position of the longitudinal electric field component and the null position of the transverse electric and magnetic field components. Maximum values of FET’s longitudinal field are always located on the central line of FET’s propagation path and decrease gradually in the propagation process. Different from the longitudinal field, the maximum value of FET’s transverse field gradually moves away from FET’s center. The theoretical research indicates that the FET spreads quite slowly in its early propagation state and spreads linearly after propagating a long distance, which has the slowly spreading propagation characteristic even in the so-called focused range with stable toroidal topological structure. The further spectrum analysis shows that the high-frequency components spread less than the low-frequency components and the high-frequency components play a vital role in the topology retention of FET in the focused range, which may provide a basis for the generation and application of FET. At present, the theoretical research on FET’s characteristics is increasingly improved. To apply the attractive characteristics of FET in actual systems, it is necessary to actually generate FET. Therefore, the generation method of FET should become the next research emphasis. Keywords:flying electromagnetic toroid/ toroidal structure/ slowly spreading/ propagation characteristic
式中, $\tau = z - ct$, $\sigma = z + ct$, ${q_1}$表示电磁飞环脉冲的有效波长, ${q_2}$表示电磁飞环的聚焦范围, ${q_1} \leqslant {q_2}$. 有效波长指的是电磁飞环在传播方向上所覆盖的空间范围, 聚焦范围指的是电磁飞环传播过程中发散速度较慢的空间范围. 当$\left| z \right| > {q_2}$时, 电磁飞环以与高斯脉冲相同的方式传播, 其有效波长为${q_1}$, 瑞利(Rayleigh)长度为${q_2}$. TM电磁飞环的电场既有横向场分量${E_\rho }$, 又有纵向场分量${E_z}$, 而其磁场只有横向场分量${H_\varphi }$. TE电磁飞环与TM电磁飞环拓扑结构相同, 电场和磁场位置互换. 从 (1)式—(3)式可知, 根据方程的实部和虚部可以分别构建两种不同的脉冲. 这两种脉冲都是麦克斯韦方程组的精确解. 实部对应的脉冲具有$1\dfrac{1}{2}$周期电场和单周期磁场, 虚部对应的脉冲具有单周期电场和$ 1\dfrac{1}{2} $周期磁场[14]. 因此, 我们将(1)式—(3)式实部和虚部对应的脉冲分别称为$1\dfrac{1}{2}$周期脉冲和单周期脉冲. 由于古伊相移作用, 单周期脉冲和$1\dfrac{1}{2}$周期脉冲会在传播过程中逐渐相互转化[12,13]. 不失一般性地, 在本文中我们以(1)式—(3)式的实部为例, 对TM电磁飞环的纵向场分量和横向场分量进行理论研究. 通过对方程中的时间变量t赋不同的值, 可以计算出电磁飞环在传播到不同位置时的场分布、拓扑结构及频谱分布, 从而在理论上准确研究电磁飞环的传播特性. 考虑参数为q2 = 100q1的TM电磁飞环, 根据(1)式—(3)式画出其在t = 0时刻的场分布, 如图2所示.电磁飞环的各场分量在垂直于传播方向(z方向)的面上(xy面)都是旋转对称的. 纵向电场Ez在z = 0附近沿+z方向, 在ρ = 0处(即x = 0, y = 0)场值最大. 随着ρ增大, 场值逐渐减小, 而后电场方向反转, 变为沿–z方向. 在z = 0平面, 横向电场Eρ与横向磁场Hθ场值均为0. 无论z取何值, Eρ与Hθ在ρ = 0处的场值均为零, 随着ρ的增大, 场值逐渐增加到最大值, 再逐渐减小为零. 图 2 电磁飞环在t = 0时刻的场分布 (a) y = 0平面Ez; (b) y = 0平面Eρ; (c) y = 0平面Hθ; (d) z = 0平面Ez; (e) z = –q1平面Eρ; (f) z = –q1平面Hθ Figure2. Field distribution of the FET when t = 0: (a) Ez on the y = 0 plane; (b) Eρ on the y = 0 plane; (c) Hθ on the y = 0 plane; (d) Ez on the z = 0 plane; (e) Eρ on the z = –q1 plane; (f) Hθ on the z = –q1 plane.
在t = 0时刻, 上述电磁飞环的纵向电场Ez的最大值位于直线ρ = 0上, 横向电场Eρ的最大值位于柱面ρ = 4.5q1上, 这两个位置上的场分量如图3所示. 由图可知, 在直线ρ = 0上, Ez的最大值位于z = 0处, 此处Eρ恒为零. 在柱面ρ = 4.5q1上, 横向电场Eρ的最大值和最小值分居z = 0两侧, 即横向电场Eρ在z = 0处改变方向; 纵向电场Ez在柱面ρ = 4.5q1上的最大值仍位于z = 0处, 即Eρ的0值位置. 图 3 在t = 0时刻的z方向上的场分布 (a) 直线ρ = 0处; (b) 柱面ρ = 4.5q1上直线(x = 4.5q1, y = 0)处 Figure3. Field distribution along z direction when t = 0: (a) on the line ρ = 0; (b) on the line (x = 4.5q1, y = 0) of cylindrical surface ρ = 4.5q1.
表1电磁飞环传输距离z与环半径ρ的扩散关系 Table1.Relation between propagation distance and toroid radius of FET.
图 4 电磁飞环在传播过程中场分布的演化 (a) 纵向电场Ez; (b)横向电场Eρ, 黑线表示电磁飞环传播到不同位置时横向电场最大值所在位置 Figure4. Evolution of the field distribution of FET: (a) Longitudinal electric field Ez; (b) transverse electric field Eρ, the black line indicates the position of maximum transverse electric field when the FET propagates to different positions.
为了更清晰地展示电磁飞环的缓扩散传输特性, 我们分别使用线性方程和对数方程对其传播到不同位置时横向电场最大值所在的位置轨迹进行了拟合, z < q2区间的线性拟合曲线和对数拟合曲线分别为(4)式和(5)式, z > q2区间的线性拟合曲线和对数拟合曲线分别为(6)式和(7)式, 拟合的决定系数记为R2, 拟合曲线与理论计算曲线的对比如图5所示. 由拟合结果可知, 当z < q2时, 电磁飞环传播到不同位置时横向电场最大值所在的位置轨迹与对数曲线更吻合, 其中, 当z < 20q1, 即z < 0.2q2时, 电磁飞环的实际传播轨迹比对数曲线发散还要慢; 当z > q2时, 电磁飞环传播到不同位置时横向电场最大值所在的位置轨迹与线性曲线更吻合. 也就是说, 电磁飞环在初始传播阶段发散非常缓慢, 在传播较远距离后接近线性发散. 图 5 电磁飞环传播到不同位置时横向电场最大值所在位置 (a) z < q2; (b) z > q2 Figure5. Position of maximum transverse electric field when the FET propagates to different positions: (a) z < q2; (b) z > q2.
前文已经从时域讨论了电磁飞环的拓扑结构和传播特性, 接下来展示电磁飞环的频域特性. 根据(1)式—(3)式, 在空间中各点提取电磁飞环传播过程中经过该点的场值, 即获得该点对应的电磁飞环的时域信号, 然后对该信号做傅里叶变换, 即可得到电磁飞环在该位置处的频谱. 在z = 0平面上记录电磁飞环各分量对应的时域场并做傅里叶变换, 得到的频谱分布如图6所示. 由图可知, 纵向电场Ez的频谱在所有频点的最大值均位于ρ = 0处. 随着ρ的增大, 纵向场分量Ez各频点的谱值先减小后增加, 在频谱图上形成一条谱值为零的带状区域. 横向电场Eρ的频谱在ρ = 0处为零, 随着ρ的增大, 各频点的值先增大后减小. 比较电磁飞环的横向电场和纵向电场的空间分布可以知道, 各频点的电场均在ρ = 0附近沿z方向穿过电磁飞环的中心, 然后沿ρ方向逐渐变为横向场, 再经由电磁飞环外边缘(即ρ较大的位置)演变为沿z方向的纵向场, 由此形成电场闭环, 即形成图1所示的超环结构. 图 6 电磁飞环经过z = 0平面时的归一化频谱分布(a)纵向电场Ez; (b)横向电场Eρ Figure6. Normalized spectrum distribution when the FET propagates through the z = 0 plane: (a) Longitudinal electric field Ez; (b) transverse electric field Eρ.
从图6(a)可以, 纵向电场Ez的频谱在各个频点的最大值的位置都为ρ = 0, 那么横向电场Eρ频谱在各个频点的最大值的位置是否保持不变呢? 答案是否定的. 在上述q2 = 100q1的TM电磁飞环传播过程中, 计算垂直于传播方向的每个面的频谱, 并记录各个频点最大值的坐标, 如图7所示. 图7中最外侧曲线对应频率为c/15q1的最大值的坐标, 越往内侧的曲线频率越高, 最内侧曲线对应频率为c/q1的最大值的坐标. 每一个频点的最大值坐标都随着z增大(即随着电磁飞环的传播)逐渐向外移动, 并且频率越低的曲线向外偏转的幅度越大. 这表明即使在z < q2时, 电磁飞环也是缓慢向外扩散的, 这与前文通过时域分析的结论是一致的. 此外, 图7还表明, 在电磁飞环中, 频率越高的频谱分量扩散的速度越慢, 此部分频谱分量对保持电磁飞环在聚焦段内的拓扑结构起到关键作用. 图 7 横向电场Eρ各个频点最大值的位置曲线 Figure7. Position curves of maximum value of each frequency of the transverse electric field Eρ.