The admittance expressions of small signal electron beam in the 27t-mode double-gap output cavity are derived by analytic method in this paper, and a formula of starting oscillation current is obtained based on the condition of forming monotron oscillation in a double-gap output cavity. The circuit admittance of a 27t-mode double-gap outqut cavity loaded with rectangular waveguide is calculated quantitatively by 3D simulation method, then the starting oscillation current and the frequency of monotron oscillation in the 2;t-mode double-gap output cavity are determined.These calculations results show that under the condition of invariant accelerative voltage of electron beam, when beam current is increased to a certain value, a harmful monotron oscillation will be brought on in the double-gap output cavity.Moreover,the monotron oscillation frequency has complicated relationships with the accelerative voltage and the current of electron beam.Meanwhile,the possibility of forming monotron oscillation in the double-gap output cavity has been demonstrated further using the MAGIC PIC simulation program, and the starting oscillation current is obtained. The results of particle simulation are consistent with the results of analytic method. These research conclusions about forming process and regularities of harmful monotron oscillations in a double-gap output cavity will have a vital significance for improving the stability of double-gap cavity output circuits of klystrons.%本文采用解析方法推导了2π模双间隙输出腔中电子注的小信号导纳公式,并根据双间隙输出腔形成单腔管振荡的条件,得到了单腔管振荡的起振电流计算公式.利用二维仿真方法定量计算了加载矩形波导的2π模双间隙输出腔的线路导纳,并以此确定2π模双间隙输出腔形成“单腔管振荡”的起振电流和振荡频率.计算结果表明:在电子注加速电压一定的情况下,电子注电流增大到一定程度,双间隙输出腔将会形成危害性较大的单腔管振荡；而且双间隙输出腔的单腔管振荡频率与电子注加速电压和电子注电流存在复杂的关系.与此同时,我们采用MAGIC粒子模拟程序进一步验证了双间隙输出腔中产生单腔管振荡的可能性问题,并得出其起振电流值,这一粒子模拟结果与解析计算结论基本相吻合.这些关于双间隙输出腔中有害的单腔管振荡形成过程和规律的研究结论,对于改善速调管双间隙腔输出电路的稳定性将具有重要意义.
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