繞軸旋轉電子束產生兆赫波電子迴旋脈射機制研究

Terahertz Electron Cyclotron Maser Interactions with an Axis-encircling Electron Beam

李冠德(Guan-De Li)

指導教授 ： 朱國瑞

國立臺灣大學/理學院/物理研究所/博士(2015年)

https://doi.org/10.6342/NTU.2015.01180

摘要

電子迴旋脈射,是一個作螺旋運動的電子與電磁波作用所造成的受激過程, 為了達成兩者間的同步作用,電磁波的頻率必須約略等於電子的迴旋頻率乘上諧波數s。如果要藉由基波作用產生1兆赫茲的電磁輻射,我們需要將近40特斯拉的磁場,依目前的科技這麼大的磁場還無法進行連續波操作。所以,為了達到兆赫茲電磁輻射,透過諧波作用我們可以有效地降低磁場要求。但是操作在高諧波模式時,必然會與其他模式產生諧波模式>競爭,而且諧波作用之耦合強度比基波作用還要弱,對高諧波模式而言相當不利。另一方面,為了避免結構半徑太小造成嚴重的歐姆損耗,必須要操作在高次模式以達到兆赫茲電磁輻射,但高次模式所造成的模式競爭也是另一個需要面對的挑戰。
繞軸旋轉電子束可以加強諧波作用的耦合強度,以及緩解操作在高次模式所造成模式競爭,是一個已知的好方案。在單模與不含時的假設下,線性理論的模擬可以透露出選擇操作參數的線索,結果顯示使用繞軸旋轉電子束可以比離軸旋轉電子束有更多的高諧波模式區間。考慮多模且含時的真實情況,我們從非線性理論的模擬中得到了一個模式競爭本質上的趨勢——若電子束操作在太高的電流或者電流上升太快,皆不利於高諧波模式的維持。

關鍵字

電子迴旋脈射 ； 迴旋管 ； 兆赫波 ； 繞軸旋轉電子束 ； 模式競爭 ； 諧波作用

並列摘要

Electron Cyclotron Maser (ECM) instability is a stimulated process between an electromagnetic wave and electrons in gyrational motion. For keeping sustained in- teraction, the frequency of electromagnetic wave is close to the electron cyclotron frequency multiplied by cyclotron harmonic number s. To produce ECM radiation at 1 terahertz (THz) with fundamental-harmonic interaction, a B-field of ∼ 40 T is required, which can not be for continuous wave (cw) operation nowadays. For the purpose of THz radiation, high-harmonic interaction is favorable to reduce re- quirement of the B-field by a factor s. Nevertheless, excitations of multiple modes at the same operating parameters lead to harmonic mode competitions. The high- harmonic interaction is also weaker than fundamental-harmonic interaction. On the other hand, to avoid excessive Ohmic loss from the too small rw, the high order mode operation is the only approach to THz regime. Mode competition associated with an over-moded structure is another challenge which should be overcome.
The axis-encircling electron beam is a well-known recipe to solve problems in- cluding weak high-harmonic interactions and mode competitions associated with an over-moded structure. Linear single-mode time-independent simulations are inves- tigated and give preliminary guidelines of operating parameters. There are more high-harmonic modes with an axis-encircling electron beam can be excited than with an off-axis electron beam. Nonlinear multi-mode time-dependent simulation shows an intrinsic tendency of switching over from a higher-harmonic mode to a lower-harmonic mode with a high beam current or upon a short current rise.

並列關鍵字

electron cyclotron maser ； gyrotron ； terahertz radiation ； axis-encircling electron beam ； mode competition ； harmonic interaction

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