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Abstract
Radio frequency (RF) breakdown can result in pulse shortening and seriously degrade the stability and reliability of relativistic backward wave oscillators (RBWOs). This paper discusses the energy range of electrons causing breakdown traces in slow-wave structures (SWSs) through particle-in-cell (PIC) simulation, numerical calculation, and experimental verification. The PIC simulation and numerical calculation results reveal that the energy of the majority of the field-induced electrons bombarding the SWS surfaces after being accelerated is less than 120 keV. Furthermore, the micro appearances of the breakdown traces in SWSs and the witness targets bombarded directly by electrons of various energy levels have been analyzed. Scanning electron microscope (SEM) shows that the breakdown traces are featured with corrugated morphologies with a wide range and a shallow depth. A mass of craters emerge in the vicinity of the corrugated morphologies. These appearances are quite similar to destructive traces impacted directly by low-energy electrons (around 160 keV). Thus, it is confirmed that the breakdown traces result from the bombardment of low-energy electrons. Therefore, the breakdown mechanism of field-emitted electrons impacting on the structure surfaces in RBWOs has been further improved.
Highlights
Relativistic backward wave oscillator (RBWO) is one of the most promising high-power microwave (HPM) devices owing to its compact structure, high-conversion efficiency, and repetitive operation [1,2,3,4,5,6,7,8]
It has been observed experimentally that the breakdown traces are more evident on the opposite sides of the locally strong field spots rather than the sides of the locally strong field, which is difficult to explain by electron emission (EEE) model [23]. erefore, a few researchers have put forward that field-emitted electrons impacting on structure surfaces may be one of the causes of radio frequency (RF) breakdown in RBWOs with a strong external guiding magnetic field [24]
During the interaction between the electrons and stainless steel (SS), part of the subsurface will reach the melting point first, resulting in the formation of molten droplets. ereafter, under the quasistatic stress generated by the temperature field gradient, the droplets break through the surface to form eruptions, and near-circular holes are formed
Summary
Relativistic backward wave oscillator (RBWO) is one of the most promising high-power microwave (HPM) devices owing to its compact structure, high-conversion efficiency, and repetitive operation [1,2,3,4,5,6,7,8]. Erefore, a few researchers have put forward that field-emitted electrons impacting on structure surfaces may be one of the causes of RF breakdown in RBWOs with a strong external guiding magnetic field [24]. Restricted by a strong guiding magnetic field, the energetic electrons bombard structure surfaces intensively and eventually cause material ablation and plasma formation. By observing the structure surfaces after different high-power microwave pulses, it has been verified experimentally that the breakdown traces on the structure surfaces originate from the bombardment of electrons [24]. The formation mechanism of breakdown traces appearing in slow-wave structures (SWSs) after HPMgeneration experiment has been analyzed through simulation and experimental verification. Is paper is organized as follows: in Section 2, field-emitted-electron movement in SWSs is studied using particle-in-cell (PIC) simulation.
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