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高功率微波的实用化进程要求系统能够重频、多次运行,在此基础上,做到紧凑、可靠、可适应于移动平台[1-4]。这类系统中,磁绝缘线振荡器(MILO)由于无需外加磁场,易于实现紧凑化和高功率运行[5-10]。实现吉瓦级MILO在脱离地面机组、保真空条件下的重频运行是高功率微波系统实用化的基础之一,其核心在于高功率微波器件的硬管化封装,包括强流陶瓷真空界面设计和脉冲气源下的保真空设计等。硬管化高功率微波源具有模块化、紧凑化和长寿命等优点[11-14]。与大功率微波管硬管化不同的是,高功率微波源真空封装界面通常需要耐受数百千伏级脉冲高电压,同时还需在空间、供电等有限条件下处理冷阴极、阳极等在多个脉冲过程中的高气载脉冲气源。故此,国内外关于吉瓦级高功率微波源的硬管化报道并不多见,国内曾成功研制了硬管化虚阴极以及硬管化MILO,但都为单次运行。文献[14]经特殊设计的硬管化虚阴极实现了大于100 Hz的重频运行,但输出功率仅有100 MW量级。
本文在高真空工艺的基础上,针对一种长寿命、高效MILO,研制了重频强流二极管陶瓷真空界面绝缘结构;同时,建立了微波源器件的瞬态抽气模型,应用分子流Monte-Carlo(MC)方法,模拟了脉冲放气后微波源内部真空压强在不同时刻下的三维分布和演化规律;并以此为基础优化了内置气体捕集泵的真空拓扑结构。最后,在HEART-50脉冲功率源上开展了5 Hz实验测试,陶瓷真空界面能耐受超过600 kV的脉冲电压,沿面平均绝缘场强达到40 kV/cm,重频运行可靠;微波源在脱离地面泵组后实现了重频吉瓦级输出,平衡压强小于5×10−2 Pa,微波功率大于3 GW、脉宽大于40 ns。
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吉瓦级微波源重频运行保真空的研究重点之一是获得脉冲气源演化规律[17-18]。MILO中,脉冲放气后的气团扩散主要受随机性的影响,在分子流真空环境下,气体分子到达泵口并被抽走也属于随机过程,故MC模拟方法适用于MILO瞬态抽气行为的研究[19]。模拟中,放气量为:
$${N_{\rm{p}}} = \frac{{\int {({P_{\rm{t}}} - {P_{\rm{b}}}){S_{\rm{e}}}{\rm{d}}t} }}{{kT}}$$ (1) 吸气剂吸气行为表达式为:
$${\rm{d}}{P_{\rm{t}}} = \frac{1}{V}( - {P_{\rm{t}}}(t){S_{\rm{e}}}(t){\rm{d}}t + {Q_{\rm{p}}}{\rm{d}}t)$$ (2) 式中,
${N_{\rm{p}}}$ 为放气量;${P_{\rm{t}}}$ 为瞬态压强;${P_{\rm{b}}}$ 为系统本底压强;${S_{\rm{e}}}$ 为系统有效抽速;V为系统容积;${Q_{\rm{p}}}$ 为气体吸附量;k为玻尔兹曼常数;T为开氏温度。模型模拟结果如图3所示。100 ns电脉冲后,如图3a所示,脉冲气源主要集中于阴极附近,局部最大压强接近1 Pa量级,而器件其他区域仍保持本底压强。随着气源的随机扩散,气压成梯度扩散规律,1 ms后,慢波区压强由10−6 Pa升至10−2 Pa量级,如图3b所示。当气体分子到达吸气剂表面时,它们将被吸附。100 ms后,如图3c所示,由于吸气剂的吸气效果,慢波区的压强开始下降,压强最大区域出现在阳极附近。当 200 ms后,慢波区附近的压强进一步降低,到达10−4 Pa量级,器件气压区域如真空界面以及阳极、模转区也回落到平均10−3 Pa水平,如图3d所示。由MC动态模拟可知,从保真空的角度,硬管MILO能够运行的重频水平不低于5 Hz。
Progress in a Hard-Tube, Gigawatt-Class, Repetitively Operated High-Power Microwave Source
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摘要: 针对磁绝缘线振荡器(MILO)微波源负载,在高真空工艺的基础上,研制了一种强流二极管陶瓷真空界面绝缘结构;通过对阴、阳极屏蔽、均压等电极形状和尺寸的优化,使陶瓷沿面电场和阴、阳极三结合点场强均得到了有效控制。模拟结果显示:陶瓷沿面电场分布均匀,阴、阳极三结合点场强小于30 kV/cm;同时,建立了微波源器件的瞬态抽气模型,应用分子流Monte-Carlo方法,模拟了脉冲放气后微波源内部真空压强在不同时刻下的三维分布和演化规律;并以此为基础优化了内置气体捕集泵的真空拓扑结构。最后,在HEART-50脉冲功率源上开展了5 Hz实验测试,陶瓷真空界面能耐受超过600 kV的脉冲电压,沿面平均绝缘场强达到40 kV/cm,重频运行可靠;微波源在脱离地面泵组后实现了重频吉瓦级输出,脉冲串间的真空恢复时间小于1 min,平衡压强小于5×10−2 Pa,微波平均功率大于3 GW、脉宽大于40 ns。Abstract: A compact L-band sealed-tube magnetically insulated transmission line oscillator (MILO) has been developed that does not require bulky external vacuum pump for repetitive operations. A special designed ceramic insulated vacuum interface was designed and the flashover was well controlled. A dynamic 3-D Monte-Carlo (MC) model for the molecular flow movement and collision was setup for the MILO chamber. The pulse desorption, gas evolution and pressure distribution were exactly simulated. In the 5 Hz repetition rate experiments, using a ~30 GW pulse modulator, the average radiated microwave power for 25 shots is about 3 GW in 40 ns pulse duration and the maximum equilibrium pressure is below 4.0×10−2 Pa. This sealed-tube MILO device is useful for compact and portable high-power microwave applications.
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