表面粗糙度对热阴极本征发射度的影响

Intrinsic Emittanceof Rough Thermionic Cathode

本文推导了热阴极表面粗糙特征影响均方根本征发射度的理论形式,并结合仿真方法分析了粗糙幅度、周期以及外加电场对束流发射度的影响。首先,针对二维余弦表面形貌,首次推导了粗糙特征影响热阴极本征发射度的完整理论形式。理论解表明发射度随着粗糙幅度增加而增加,随粗糙周期增加而降低,随外加电场近似为正比关系。随后,基于有限差分粒子仿真程序YYPICMC,分析了二维余弦表面粗糙特征对本征发射度的影响,仿真结果与理论解有显著差异:当粗糙幅度增加到一定值后,发射度增长系数趋于平缓;当粗糙周期很小时,发射度增长系数也趋于一个较低的值,而非随着周期减小而迅速增大。结合发射电流随粗糙特征的变化、粒子轨迹随外电场的变化以及空间电荷效应对发射度增长的影响,对引起这种差异的机制进行了深入分析,结果表明这种差异是由理论解本身的局限性造成时。最后,针对理论模型的局限性,提出了引入“曲面比效应”和“初始粒子损失效应”改进“发射度-粗糙度”理论模型的思路。

Surface non-uniformity including temperature,work and geometric profile affect intrinsic emittance of thermionic cathode of vacuum electronics and accelerators.In order to understand how emittance is manipulated by surface profile,Firstly,a theoretical form of roughness characteristics affecting the intrinsic emissivity of thermionic cathode is derived for the first time.The theoretical solution shows that the emissivity increases with rough amplitude up,decreases with rough period down,and is approximately proportional to the applied electric field.Then,based on the finite difference particle simulation program YYPICMC,the influence of two-dimensional cosine surface roughness on intrinsic emittance was analyzed.The simulation results were significantly different from the theoretical solutions.When the roughness amplitude increased to a certain value,the emittance increase coefficient tended to grow slowly.When the rough period is very small,the emittance increase coefficient tends to a lower value rather than rapidly increasing with the period down.Combined with the variation of current,electron trajectory and the influence of space charge effect,the mechanism of this difference is analyzed,and the results show that this difference is caused by the limitation of theoretical form.Finally,according to the limitations of the theoretical model,an idea to improve the emittance-roughness theory model by introducing“surface ratio effect”and“initial particle loss effect”is proposed.