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    Please use this identifier to cite or link to this item: http://ir.lib.ncu.edu.tw/handle/987654321/48257


    Title: 外部反射線路對於磁旋返波振盪器影響之模擬研究;Effects of Long-line Reflection on the Dynamical Behavior of Gyrotron Backward-wave Oscillators
    Authors: 黃煒揚;Wei-yang Huang
    Contributors: 物理研究所
    Keywords: 微波物理;磁旋返波振盪器;模式選擇;磁滯現象;mode selection;microwave physics;gyro-BWO;hysteresis phenomenon
    Date: 2011-07-28
    Issue Date: 2012-01-05 14:42:44 (UTC+8)
    Abstract: 本論文主要在探討外部反射線路對磁旋返波振盪器的動態行為所產生的影響,並且嘗試藉由調整外部反射線路的反射條件,進行磁旋返波振盪器之操作-模式選擇。 我們分別使用穩態程式 (stationary code) 以及時變程式 (time-dependent, Particle-in-Cell code) 模擬外部反射線路對磁旋返波振盪器的影響。 穩態程式的計算結果顯示,在反射位置與電子注入位置的距離 (Lext) 小於14公分時,磁旋返波振盪器的振盪頻率和輸出功率會隨著Lext值的變化而發生週期性變化,其周期為1.658公分。然而隨Lext值的增長,系統開始出現重根現象 (對特定Lext值系統找到多個共振頻率),此現象會隨著Lext值的增加而更為明顯。而當我們在Lext值很大的系統改變電子束的電壓或電流,也會發現不連續調變的現象,並且也會隨Lext值的增加而更嚴重。 穩態程式得到的結果是平衡態的解,但此解不一定是穩定解。當Lext增長時,在特定Lext值求得多個振盪頻率,時變程式可以確認其中一個平衡解是穩定解,因此在真實系統中改變Lext時,會發生跳頻的現象。而藉由時變程式的計算結果,也驗證了實驗中觀察到的延遲的外部反射會造成系統振盪頻率發生不連續變化的現象。這種現象反應在模擬電壓、電流調變的過程中,就會發生類似頻率調變的磁滯現象,也就是說,在電壓或電流上升的過程中,系統振盪頻率的變化路徑,和電壓或電流下降的過程中並不相同。經過檢驗之後,我們發現磁滯現象發生的點,都是在穩態程式中發生多根的位置。因此在電壓或電流調變時,振盪頻率會由其中一個跳到另一個頻率,而且電壓或電流增加或降低走的路徑不同,看起來就像發生了磁滯現象。 另外我們也嘗試藉由外部反射線路,對磁旋返波振盪器作模式選擇。眾所周知,磁旋返波振盪器的模式競爭過程,由於基模的場型接近電子入口處,因此始終會壓制高次軸向模式,成為主宰的振盪模式。然而我們發現,在加上外部反射線路之後,若適當選擇Lext以及iris的高度 (Delta r),會出現二次模單一模式穩定振盪的現象,而其振盪功率約為基模的兩倍以上。而在另外一些這兩個值的組合下,會出現不穩定振盪的現象;進一步分析之後發現,不穩定振盪的原因是模式競爭引起的。 The gyrotron backward-wave oscillator (gyro-BWO) is a coherent microwave or millimeterwave radiation source based on the electron cyclotron maser instability. The backward wave oscillation occurs in an internal feedback loop which consists of a forward moving electron beam and a backward propagating wave. Since the gyro-BWO employs a non-resonant structure, the frequency can be tuned continuously by varying the beam voltage or magnetic field. However, discontinuous tuning induced by external reflection was observed in experiments. In this study, the effects of external reflections on the gyro-BWO were examined by using the stationary code and the particle-in-cell simulation code. The numerical results show that the dynamical behavior of the gyro-BWO is sensitive to the length of the external reflection circuit (Lext). As Lext is increased, the discontinuity in frequency tuning becomes more severe. The hysteresis phenomenon is also observed while varying the voltage or current. The mode selection in the gyro-BWO was achieved by adjusting the external reflection circuit. The gyro-BWO can be stably operated in the second axial mode by properly adjusting Lext and the height of the iris (Delta r). Moreover, the output power of the second mode is two times of the oscillating power of the fundamental mode. Nonstationary oscillations are also observed at some settings of of Lext and , and the main cause is mode competition.
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