Investigation on suppression of vacuum breakdown in titanium materials under intense electromagnetic fields via surface self-nanocrystallization
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摘要: 为进一步提升相对论返波管高功率微波产生器的功率容量，本文提出一种采用表面自纳米化技术抑制相对论返波管强电磁场真空击穿的新思路，并通过实验研究了超声冲击和微粒子喷丸这两种技术对工业纯钛处理后的表面自纳米化效果、场致电子发射特性和强电磁场真空击穿性能。微观形貌和X射线衍射分析结果表明，超声冲击处理后在工业纯钛表层2～3 μm范围内形成梯度纳米化层，表面晶粒细化至约40 nm；微粒子喷丸处理后在工业纯钛表层30 μm范围内形成梯度纳米化层，表面晶粒细化至约48 nm。场致电子发射测试电场-电流曲线表明，表面自纳米化处理可以抑制场致电子发射，尤其是超声冲击处理可以显著抑制工业纯钛的场致电子发射。对比分析输出微波波形和结构壁损伤痕迹发现，超声冲击和微粒子喷丸处理可减弱微波尾蚀和结构壁损伤，证明其确实能够抑制慢波结构强电磁场真空击穿。Abstract:
Background Vacuum breakdown under intense electromagnetic fields in Relativistic Backward Wave Oscillator (RBWO) critically compromise the reliability of high power microwave sources. To meet increasingly demanding power capacity requirments, it is essential to further enhance the performances of resistance to vacuum breakdown. The processes of lectron emission and electron beam bombardment are closely associated with the physical interactions at the material surface. Furthermore, studies have indicated that refining the grain size of titanium suifaces may significantly increase their ability to withstand intense electromagnetic field-induced vacuum breakdown.Purpose Given that surface self-nanocrystallization is a highly effective approach for improving the mechanical properties of metallic materials, particularly through grain fefinement at the nanometer scale, this article proposes a novel strategy utilizing surface self-nanocrystallization technology to suppress vacuum breakdown in RBWO under intense electromagnetic fields. Experimental validation was conducted to evaluate the effectiveness of this method in inhibiting vacuum breakdown under such extreme conditions.Methods This paper employs commercially pure titanium (TA2) as the research object, utilizing ultrasonic peening (USP) and microshot peening (MSP) as two distinct nanoparticle surface processing techniques. The nanostructural effects were characterized through scanning electron microscopy coupled with X-ray diffraction analysis. Subsequently, field emission testing apparatus and an X-band RBWO were employed to conduct comparatice experiments on field electron emission characteristics and vacuum breakdown performance high power microwave generation, respectively.Results USP produces a gradient nanocrystalline layer extending 2-3 μm beneath the surface, with grain size refined to approximately 40 nm. MSP generates a more extensive gradient nanocrystalline layer reaching 30 μm in depth, with surface grains refined to about 48 nm. After undergoing MSP and USP treatment, TA2 exhibits a sequential reduction in the field emission current under the identical electric field, the tail erosion in the output microwave waveforms vanishes, and damage in the high-field region of the slow wave structure is mitigated.Conclusions The results demonstrate that both MSP and USP treatments successfully refine the surface grain size of TA2 to the nanoscale range of several tens of nanometers, forming a gradient nanostructured layer. Such surface self-nanocrystallization effectively suppresses field-induced electron emission and vacuum breakdown under intense electromagnetic fields. Notably, the USP treatment exhibits particularly pronounced inhibitory effects. This methodology provides technical support for further enhancing the power capacity of high power microwave generators based on RBWO. -
图 1 TA2表面自纳米化处理前后的表面微观形貌
Figure 1. The surface micromorphology of TA2 prior to and subsequent to self-nanocrystallization treatments
图 2 试样截面的金相显微组织
Figure 2. The cross-sectional microstructure of TA2 prior to and subsequent to self-nanocrystallization treatments
图 3 TA2表面自纳米化处理前后的表面XRD图谱
Figure 3. The X-ray Diffraction (XRD) patterns of TA2 prior to and subsequent to self-nanocrystallization treatments
图 4 TA2、USP-TA2、MSP-TA2的场发射电场-电流曲线
Figure 4. The field emission current-electric field characteristics of TA2, USP-TA2, and MSP-TA2
图 5 X波段RBWO强电磁场真空击穿实验装置示意图
Figure 5. Schematic diagram of the X-band RBWO intense electromagnetic field vacuum breakdown experimental device
图 6 微波功率5 GW条件下使用不同慢波结构在20 Hz刚开始连续工作600个脉冲的波形：CH1是二极管电压，CH2是二极管电流，CH3是在线微波波形，CH4是辐射场微波波形
Figure 6. Waveforms of the initial 600 pulses during the commencement of continuous operation at 5 GW microwave power with varied slow-wave structures at 20 Hz: CH1 represents the diode voltage, CH2 corresponds to the diode current, CH3 denotes the online microwave waveform, and CH4 signifies the microwave waveform of radiated field
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