為了研究史密斯-帕塞爾自由電子激光的輸出頻率和光柵槽深、光柵槽長、光柵槽寬的關(guān)系,對于基于矩形光柵的史密斯-帕塞爾自由電子激光利用粒子模擬軟件進(jìn)行模擬和理論分析。首先,利用粒子模擬軟件模擬對于基于矩形光柵的史密斯-帕塞爾自由電子激光進(jìn)行了研究,發(fā)現(xiàn)史密斯-帕塞爾自由電子激光的輸出頻率隨光柵槽深、光柵槽長、光柵槽寬的增大而減少。接著,對史密斯-帕塞爾自由電子激光的光柵槽進(jìn)行了理論分析,發(fā)現(xiàn)每個光柵槽都可以等效為一個LC諧振電路,并發(fā)現(xiàn)在史密斯-帕塞爾自由電子激光中存在兩種輻射,一種是史密斯-帕塞爾輻射,另一種是LC振蕩輻射。最后,對光柵槽的LC振蕩輻射進(jìn)行了估算,發(fā)現(xiàn)史密斯-帕塞爾自由電子激光輸出頻率的模擬值與光柵槽的LC振蕩輻射估算值的數(shù)量級均為10² GHz,且變化規(guī)律上一致。據(jù)此推測決定史密斯-帕塞爾自由電子激光輸出頻率的應(yīng)該是光柵槽,而不是諧振腔。 

【文章來源】：中國光學(xué). 2020,13(02)北大核心EICSCD

【文章頁數(shù)】：15 頁

【部分圖文】：

基于矩形光柵的SP FEL的模擬圖

圖3 在不同光柵槽深時SP FEL中電子注的動能沿z軸分布圖表2 在不同光柵槽深時SP FEL輸出頻率的模擬值Tab.2 Simulation values of output frequency of SP FEL at different depths of grating groove Parameters Values Number of periods 32 Period length of grating/mm 0.3 Slot length of grating/mm 1.5 Slot width of grating/mm 0.1 Slot depth of grating/mm 0.15 0.2 0.25 0.3 0.35 0.4 Simulation value of output frequency / GHz 723.379 529.786 483.632 436.827 400.121 370.060

where D is the grating period, n is the number of spatial harmonics, θ is the emission angle (that is, the angle between the SPR and the direction of electron motion), and β is the ratio of the velocity of electrons to the speed of light, β=v/c. According to the equation (1), the wavelength range of spontaneous SPR fundamental wave is from D( 1 β -1 )to D( 1 β +1 ) [17-22] . According to traditional SP FEL theory, the resonator surface can reflect the SPR of various wavelengths back to the electron beams, and the reflected SPR can modulate the electron beams. However, at the initial stage of electron beam injection, the SPR is not strong enough to cause the electron beams to bunching and start oscillation. When the injection time exceeds the start time to oscillate, the electron beams will begin to bunching due to the increase of SPR intensity. When the beam-wave interaction meets the oscillation conditions, the net gain of the SPR satisfying the resonance conditions in the resonator will begin to increase gradually and finally establish the steady-state oscillation in the resonator. According to the resonance conditions, the resonator size must be reduced in order to increase the output frequency of the SP FEL based on rectangular grating. However, in the simulation of SP FEL, our group found that it was difficult to increase the output frequency of the SP FEL based on rectangular grating by reducing the resonator size. We also found that the grating grooves should be changed to increase the output frequency of the SP FEL.3 Simulation and analysis of the SP FEL based on rectangular grating

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