1.Key laboratory of Physical Electronics and Devices, Ministry of Education, School of Electronic and Informtion Engineering, Xi’an Jiaotong University, Xi’an 710049, China 2.Northwest Institute of Nuclear Technology, Xi’an 710613, China
Abstract:For a microwave device filled with dielectrics, the secondary electron (SE) emission has a very important influence on the mechanism of microwave breakdown including low pressure discharge and multipactor. In this work, the SE yields (SEYs) and the SE energy spectra of seven kinds of dielectric materials are first measured and then used to examine their effects. In the positive charging process under electron irradiation, the surface potential of the dielectric layer trends to be steady with the SEY being one. Based on the measurement data, the steady surface potential is calculated under the charging stability condition. The steady surface potential is bigger for a bigger SEY. For a given SEY, the steady surface potential is found to be proportional to the peak energy Epeak of the SE energy spectrum. Furthermore, the effect of steady surface potential on low pressure discharge and multipactor are respectively studied for a parallel plate system filled with a dielectric layer. A static electric field related to the positive charging is introduced. The electron diffusion model in low pressure discharge process is modified by considering the static electric field. The electrons drift in a fixed direction under the action of static electric field, and the electron diffusion length decreases. Consequently, the effective electrons for low discharge decreases and the threshold microwave power increases. Therefore, a dielectric material with higher SEY and bigger Epeak is helpful in suspending the inhibition of low pressure discharge. Furthermore, the effect of steady electric field on multipactor is also explored. Two effects related to dielectric material and metal are analyzed in detail. The SE emission from dielectric material is held back by the steady electric field and some low energy electrons return back to the dielectric materials. The effective SEY thus decreases. On the other hand, the electric field reduces the landing electron energy on the metal, and the corresponding SEY also decreases. The electron oscillation condition with considering both microwave field and stead electric field is derived and the threshold values for microwave power of multipactor are calculated. The susceptibility curves corresponding to different materials are plotted. Our result may be used to choose the filling dielectric materials for a microwave device. Keywords:secondary electron yield/ secondary electron energy spectrum/ surface potential/ microwave breakdown
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2.1.介质材料SEY的测量
采用单脉冲电子束照射被测样品, 并采用收集极法对SEY(用δ0表示)进行了测量. 为了消除测量过程中样品上累积的电荷对测量结果的影响, 在测量之前都对样品进行了电荷中和. 中和时, 设置电子束能量处于能使样品正带电(δ0 > 1)的状态, 同时将收集极设置为负偏压. 具体测量过程可见文献[13-16]. 图1是7种介质材料SEY与入射电子能量${E_{{\rm{pe}}}}$关系的测量结果, 其中, 点状符号代表测量结果, 光滑曲线是用“二次电子发射系数普适公式”[17,18] 图 1 7种介质材料SEY的测量结果 Figure1. The measured SEY of seven kinds of dielectric materials.
如图2所示, N(E)代表二次电子的能谱, E代表二次电子的能量, Epeak和FWHM分别为峰值处的能量和峰的半高宽. 实验中测量了多个入射能量下的能谱, 并将其进行平均, 最后测得的结果如表2所列. 图 2 能谱分布示意图 Figure2. The diagram of the secondary electron energy spectrum.
材料
PMMA
PTFE
PE
PI
Al2O3
SiO2
Mica
Epeak/eV
4.264
4.203
4.023
3.087
2.898
2.376
2.988
FWHM/eV
14.058
13.851
13.284
10.206
9.765
7.857
9.882
表27种材料的能谱特性 Table2.The characteristics of energy spectrum of seven kinds of materials.
根据(3)式可知, 稳态表面电位${V_{\rm{s}}}$只是样品二次电子发射系数${\delta _0}$和能谱的函数, 同时还可以看出稳态表面电位Vs与Epeak成正比. 图3是用(3)式计算得到的${V_{\rm{s}}}/{E_{{\rm{peak}}}}$与${\delta _0}$的关系. 图 3 稳态表面电位与SEY及能谱参数Epeak的关系 Figure3. The relationships of the steady state surface potential with the SEY and the spectrum parameter Epeak.
根据上述测量情况, 以及表1和表2的结果, 采用(1)式和(3)式, 可以计算出不同材料的稳态表面电位与入射电子能量的关系如图4所示. 图 4 稳态表面电位与入射电子能量的关系 Figure4. The relationships between the steady state surface potential and the incident electron energy.
在微放电发生的情况下, 与(16)式中最小值W1对应的是微放电的最小阈值状态, 其位置(fd)min、击穿电压${V_{{\rm{0}}\min }}$可以联合(13)式—(15)式计算出. 在不同的Vdc情况下, 本文计算了发生微放电时模式n = 1—5对应的最低击穿电压V0min与发生的位置(fd)min, 如图6中的点状符号所示. 计算时, 取W1 = 55.8 eV, 对应于表1里7种材料的平均值, 同时采用了文献[21,22]建议的k = 3. 图 6Vdc对敏感区域右边界中不同模式最低击穿点的影响 Figure6. The influence of Vdc on the minimum breakdown point at different pattern of the right boundary in suscep-tibility zone.
由图6可见, 将每个模式的最低击穿点((fd)min, V0min)相连后呈现出一个线性关系, 该线性关系可以看成是微放电敏感区域的右边界. 因此, 微放电敏感区域的右边界阈值V0与fd是线性关系, 随着fd的增加, 微放电的最低击穿阈值线性增加. 图7是计算得到的该线性关系的斜率与Vdc的关系, 随着Vdc的增加, 斜率急剧上升. 这说明, Vdc的出现, 提高了微放电击穿阈值, 使得微放电不易发生. 图 7Vdc对敏感区域右边界斜率的影响 Figure7. The influence of Vdc on the slope of the right boundary in susceptibility zone.