The Inverse Smith-Purcell Effect at Submillimeter Wavelengths

本研究计划在实验上验证逆Smith-Purcell效应的理论预测。逆Smith-Purcell效应是一种加速梯度为几GeV/m的激光驱动直线加速器的候选效应。在逆Smith-Purcell效应中，当电子束的速度与波的相速度相同时，电子束与光栅上的消失波同步相互作用。相互作用的强度和间距也随着入射激光功率、波长、逝去波的空间谐波的阶数和电子的初始速度而变化。为了从实验上研究这些问题，我们测量了电子与空间一次或二次谐波相互作用所产生的能散。这些波是由波长为496微米和185微米的两种不同驱动的亚毫米波激光器引起的。并将实验结果与实验前的计算机模拟结果进行了比较。综上所述，我们对理论进行了如下验证：(1)在实验上证实了上述速度匹配条件。(2)在我们的实验误差范围内，逝去波的幅度随离栅面的距离呈指数衰减，其衰减常数与理论预测的一致。例如，在185微米的激光波长下，这个值是1/(18；正负8微米)，而对于第一空间谐振子，这个值是1/(16.6微米)。(3)在修正系数为2的范围内，电子的能散与入射激光功率的关系与理论预测值相符。

This research project has been planned to experimentally verify the theoretical predictions on the inverse Smith-Purcell (ISP) effect which is a candidate for laser- driven linacs with an accelerating gradient of several GeV/m.In the ISP effect, an electron beam interacts synchronously with an evanescent wave on the grating when the velocity of electron is the same as the phase velocity of the wave. The interaction-strength and -space also vary with an incident laser power, the wavelength, an order of space harmonics of the evanescent wave, and an initial velocity of electron. In order to investigate these experimentally, we have measured energy spread of electron resulted from the interaction with the 1st or the 2nd space harmonics. These waves have been induced by two different driving submillimeter wave lasers with the wavelengths of 496mum and 185mum. Experimental results have been compared with computer simulated one performed before the experiment. In summary, we have obtained the following verifications for the theory.(1) The velocity matching condition described above has been confirmed experimentally for the 1st and the 2nd space harmonics of the evanescent wave.(2) The amplitude of the evanescent wave falls off exponentially with the distance from the grating surface, and its decay constant agrees with the theoretically predicted one within our experimental errors. For example, in the laser-wavelength of 185mum, this value is 1/(18<plus-minus>8mum) as compared with the theoretical one of 1/(16.6mum) for the 1st space harmoni(3) The energy spreads of electron depended on the incident laser power agree with theoretically predicted one within correction factor of 2.