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Particle-in-cell simulation on effect of collector outgassing on relativistic backward oscillator

Wang Hong-Guang Liu Peng-Fei Zhang Jian-Wei Li Yong-Dong Cao Yi-Bing Sun Jun

Particle-in-cell simulation on effect of collector outgassing on relativistic backward oscillator

Wang Hong-Guang, Liu Peng-Fei, Zhang Jian-Wei, Li Yong-Dong, Cao Yi-Bing, Sun Jun
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  • The relativistic backward-wave oscillator has been considered to be one of the most promising high-power microwave devices. As the output microwave power is further increased, the breakdown phenomenon inside the relativistic backward-wave oscillator, including the collector pole, becomes more and more serious, which eventually leads to the pulse shortening, becoming a biggest obstacle to the development of the device with high power and high energy. Such a problem has also been one of the important issues which constrain its development. Based on the 2.5D particle-in-cell simulation software, i.e. UNIPIC-2D developed by our research group, in this paper the dynamic gassing model is used to study the effects of the relativistic backward-wave oscillator collector breakdown process and the guiding magnetic field under different outgassing coefficients. The result of particle simulation demonstrates that as the electrons continue to bombard the collector, the surface pressure of the collector is increased, and gas ionization occurs. The generated plasma enters into the slow-wave structure along the guiding magnetic field, thus affecting the beam-wave interaction process and causing the output power to drop. With the increase of the gas release coefficient, the pulse shortening phenomenon becomes more and more obvious. In the case of low guiding magnetic field, the breakdown and pulse shortening are alleviated.
      Corresponding author: Wang Hong-Guang, wanghg@xjtu.edu.cn
    [1]

    Barker R J, Schamiloglu E 2001 High-Power Microwave Sources and Technologies (Piscataway, New Jersey: IEEE Press) pp310–380

    [2]

    宫玉彬, 张章, 魏彦玉, 孟凡宝, 范植开, 王文祥 2004 物理学报 53 3990

    Gong Y B, Zhang Z, Wei Y Y, Meng F B, Fan Z K, Wang W X 2004 Acta Phys. Sin. 53 3990

    [3]

    Li X Z, Wang J G, Song Z M, Chen C H, Sun J, Zhang X W, Zhang Y C 2012 Phys. Plasmas 19 83111

    [4]

    李小泽, 王建国, 童长江, 张海 2008 物理学报 57 4613

    Li X Z, Wang J G, Tong C J, Zhang H 2008 Acta Phys. Sin. 57 4613

    [5]

    Benford J, Benford G 1997 IEEE Trans. Plasma Sci. 25 311

    [6]

    Insepov Z, Norem J, Vetizer S, Mahalingam S 2011 AIP Conf. Proc. 1406 523

    [7]

    Cao Y B, Song Z M, Wu P, Fan Z Q, Zhang Y C, Teng Y, Sun J 2017 Phys. Plasmas 24 033109

    [8]

    Korovin S D, Mesyats G A, Pegel I V, Polevin S D, Tarakanov V P 2000 IEEE Trans. Plasma Sci. 28 485

    [9]

    Xiao R Z, Chen C H, Deng Y Q, Cao Y B, Sun J, Li J W 2016 Phys. Plasmas 23 063114

    [10]

    Zhang J, Jin Z X, Yang J H, Zhong H H, et al. 2011 IEEE Trans. Plasma Sci. 39 1438

    [11]

    Kovalev N F, Nechaev V E, Petelin M I 1998 IEEE Trans. Plasma Sci. 26 246

    [12]

    梁玉钦, 邵浩, 孙钧, 等 2014 强激光与粒子束 26 26063010

    Liang Y Q, Shao H, Sun J, et al. 2014 High Power Laser and Particle Beams 26 26063010

    [13]

    Miao T Z, Bai X C, Sun J, Zhang X W, Cao Y B, Wu P, Shi Y C, Shao H 2017 Phys. Plasmas 24 123106

    [14]

    傅竹风, 胡友秋 1995 空间等离子体数值模拟 (合肥: 安徽科学技术出版社) 第433−476页

    Fu Z F, Hu Y Q 1995 Numerical Simulation of Space Plasma (Hefei: Anhui Science and Technology Publishers) pp433−476 (in Chinese)

    [15]

    Wang J G, Zhang D H, Liu C L, Li Y D, Wang Y, Wang H G, Qiao H L, Li X Z 2009 Phys. Plasmas 16 033108

    [16]

    Bird R B, Lightfoot E N, Stewart W E 1961 AIChE J. 7 5J

    [17]

    Birdsall C K 1991 IEEE Trans. Plasma Sci. 19 65

    [18]

    Wang H G, Li Y D, Liu C L, Zhou Y, Liu M Q 2010 IEEE Trans. Plasma Sci. 38 2062

    [19]

    IAEA http://www-amdis.iaea.org/ALADDIN [2019-4-15]

    [20]

    董烨, 董志伟, 周前红, 杨温渊, 周海京 2014 物理学报 63 027901

    Dong Y, Dong Z W, Zhou Q H, Yang W Y, Zhou H J 2014 Acta Phys. Sin. 63 027901

    [21]

    蔡利兵, 王建国 2011 物理学报 60 025217

    Cai L B, Wang J G 2011 Acta Phys. Sin. 60 025217

    [22]

    Vaughan R M 1988 IEEE Trans. Electron Dev. 35 1172

    [23]

    杨文晋, 李永东, 刘纯亮 2013 物理学报 62 087901

    Yang W J, Li Y D, Liu C L 2013 Acta Phys. Sin. 62 087901

    [24]

    李姝敏, 李永东, 刘震 2017 强激光与粒子束 29 29063001

    Li S M, Li Y D, Liu Z 2017 High Power Laser and Particle Beams 29 29063001

    [25]

    邵剑波, 马乔生, 谢鸿全, 李正红 2015 微波学报 31 62

    Shao J B, Ma Q S, Xie H Q, Li X H 2015 J. Microw. 31 62

  • 图 1  RBWO收集极释气的PIC/MCC模拟流程图

    Figure 1.  PIC/MCC simulation flow chart for RBWO collecting extremely outgassing.

    图 2  3.4 T引导磁场RBWO示意图

    Figure 2.  3.4 T-guide magnetic field RBWO schematic.

    图 3  无释气情况下3.4 T RBWO输出功率

    Figure 3.  3.4 T RBWO output power without outgassing.

    图 4  不同释气系数λ下输出功率对比

    Figure 4.  Output power comparison under different outgassing coefficients λ.

    图 5  无释气情况下的模拟结果 (a)电子实空间分布(b)电子z-Vz相空间分布

    Figure 5.  Simulation results without outgassing: (a) Electronic real-time spatial distribution; (b) electronic z-Vz phase spatial distribution

    图 6  释气系数λ = 2情况下的模拟结果 (a)电子实空间分布; (b)电子相空间分布; (c)−(f)分别为4, 16, 28, 40 ns时离子实空间分布; (g)收集极表面气压随时间的变化

    Figure 6.  Simulation result with outgassing coefficient λ of 2: (a) Electronic real-time spatial distribution; (b) electronic phase spatial distribution; (c)−(f) polar space distribution at 4, 16, 28, 40 ns; (g) surface pressure curve over time.

    图 7  λ = 2时慢电子占空间电子总数的比率

    Figure 7.  Ratio of slow electrons to the total number of space electrons at λ = 2.

    图 8  微波输出功率以及脉宽随释气系数λ的变化

    Figure 8.  Relationship of microwave output power and pulse width to outgassing coefficient λ.

    图 9  不同释气系数下40 ns时刻RBWO收集极表面气压(一个网格内的平均气压)的模拟结果

    Figure 9.  Simulation results of RBWO collector surface pressure (average pressure in a grid) at 40 ns with different outgassing coefficients.

    图 10  不同引导磁场下无释气情况与λ = 2时平均输出功率的对比图

    Figure 10.  Comparison of the average output power with no outgassing and λ = 2 under different guiding magnetic fields.

  • [1]

    Barker R J, Schamiloglu E 2001 High-Power Microwave Sources and Technologies (Piscataway, New Jersey: IEEE Press) pp310–380

    [2]

    宫玉彬, 张章, 魏彦玉, 孟凡宝, 范植开, 王文祥 2004 物理学报 53 3990

    Gong Y B, Zhang Z, Wei Y Y, Meng F B, Fan Z K, Wang W X 2004 Acta Phys. Sin. 53 3990

    [3]

    Li X Z, Wang J G, Song Z M, Chen C H, Sun J, Zhang X W, Zhang Y C 2012 Phys. Plasmas 19 83111

    [4]

    李小泽, 王建国, 童长江, 张海 2008 物理学报 57 4613

    Li X Z, Wang J G, Tong C J, Zhang H 2008 Acta Phys. Sin. 57 4613

    [5]

    Benford J, Benford G 1997 IEEE Trans. Plasma Sci. 25 311

    [6]

    Insepov Z, Norem J, Vetizer S, Mahalingam S 2011 AIP Conf. Proc. 1406 523

    [7]

    Cao Y B, Song Z M, Wu P, Fan Z Q, Zhang Y C, Teng Y, Sun J 2017 Phys. Plasmas 24 033109

    [8]

    Korovin S D, Mesyats G A, Pegel I V, Polevin S D, Tarakanov V P 2000 IEEE Trans. Plasma Sci. 28 485

    [9]

    Xiao R Z, Chen C H, Deng Y Q, Cao Y B, Sun J, Li J W 2016 Phys. Plasmas 23 063114

    [10]

    Zhang J, Jin Z X, Yang J H, Zhong H H, et al. 2011 IEEE Trans. Plasma Sci. 39 1438

    [11]

    Kovalev N F, Nechaev V E, Petelin M I 1998 IEEE Trans. Plasma Sci. 26 246

    [12]

    梁玉钦, 邵浩, 孙钧, 等 2014 强激光与粒子束 26 26063010

    Liang Y Q, Shao H, Sun J, et al. 2014 High Power Laser and Particle Beams 26 26063010

    [13]

    Miao T Z, Bai X C, Sun J, Zhang X W, Cao Y B, Wu P, Shi Y C, Shao H 2017 Phys. Plasmas 24 123106

    [14]

    傅竹风, 胡友秋 1995 空间等离子体数值模拟 (合肥: 安徽科学技术出版社) 第433−476页

    Fu Z F, Hu Y Q 1995 Numerical Simulation of Space Plasma (Hefei: Anhui Science and Technology Publishers) pp433−476 (in Chinese)

    [15]

    Wang J G, Zhang D H, Liu C L, Li Y D, Wang Y, Wang H G, Qiao H L, Li X Z 2009 Phys. Plasmas 16 033108

    [16]

    Bird R B, Lightfoot E N, Stewart W E 1961 AIChE J. 7 5J

    [17]

    Birdsall C K 1991 IEEE Trans. Plasma Sci. 19 65

    [18]

    Wang H G, Li Y D, Liu C L, Zhou Y, Liu M Q 2010 IEEE Trans. Plasma Sci. 38 2062

    [19]

    IAEA http://www-amdis.iaea.org/ALADDIN [2019-4-15]

    [20]

    董烨, 董志伟, 周前红, 杨温渊, 周海京 2014 物理学报 63 027901

    Dong Y, Dong Z W, Zhou Q H, Yang W Y, Zhou H J 2014 Acta Phys. Sin. 63 027901

    [21]

    蔡利兵, 王建国 2011 物理学报 60 025217

    Cai L B, Wang J G 2011 Acta Phys. Sin. 60 025217

    [22]

    Vaughan R M 1988 IEEE Trans. Electron Dev. 35 1172

    [23]

    杨文晋, 李永东, 刘纯亮 2013 物理学报 62 087901

    Yang W J, Li Y D, Liu C L 2013 Acta Phys. Sin. 62 087901

    [24]

    李姝敏, 李永东, 刘震 2017 强激光与粒子束 29 29063001

    Li S M, Li Y D, Liu Z 2017 High Power Laser and Particle Beams 29 29063001

    [25]

    邵剑波, 马乔生, 谢鸿全, 李正红 2015 微波学报 31 62

    Shao J B, Ma Q S, Xie H Q, Li X H 2015 J. Microw. 31 62

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  • Received Date:  16 April 2019
  • Accepted Date:  23 June 2019
  • Available Online:  01 September 2019
  • Published Online:  20 September 2019

Particle-in-cell simulation on effect of collector outgassing on relativistic backward oscillator

    Corresponding author: Wang Hong-Guang, wanghg@xjtu.edu.cn
  • 1. Key Laboratory for Physical Electronics and Devices of Ministry of Education, Xi’an Jiaotong University, Xi’an 710049, China
  • 2. School of Electronic Science and Engineering, Xi'an Jiaotong University, Xi'an 710049, China
  • 3. Northwest Institute of Nuclear Technology, Xi'an 710024, China

Abstract: The relativistic backward-wave oscillator has been considered to be one of the most promising high-power microwave devices. As the output microwave power is further increased, the breakdown phenomenon inside the relativistic backward-wave oscillator, including the collector pole, becomes more and more serious, which eventually leads to the pulse shortening, becoming a biggest obstacle to the development of the device with high power and high energy. Such a problem has also been one of the important issues which constrain its development. Based on the 2.5D particle-in-cell simulation software, i.e. UNIPIC-2D developed by our research group, in this paper the dynamic gassing model is used to study the effects of the relativistic backward-wave oscillator collector breakdown process and the guiding magnetic field under different outgassing coefficients. The result of particle simulation demonstrates that as the electrons continue to bombard the collector, the surface pressure of the collector is increased, and gas ionization occurs. The generated plasma enters into the slow-wave structure along the guiding magnetic field, thus affecting the beam-wave interaction process and causing the output power to drop. With the increase of the gas release coefficient, the pulse shortening phenomenon becomes more and more obvious. In the case of low guiding magnetic field, the breakdown and pulse shortening are alleviated.

    • 高功率微波(high power microwave, HPM)技术[1]在国防、航天、能源和通信等领域具有广阔的应用前景, 随着研究的不断深入, 正在从技术研究阶段向应用阶段加速前进. HPM源作为HPM技术的核心, 一般工作在中低真空状态(约10–2 Pa水平), 输出功率在GW量级, 电磁场功率流密度通常高达数百MW/cm2. 随着微波输出功率的不断提高, 器件内部强电磁场下的真空击穿导致微波输出功率下降以及脉冲缩短现象, 严重限制了高功率、高能量的输出, 极大地影响了器件的工作性能. 相对论返波管(relativistic backward wave oscillator, RBWO)是一种典型的HPM源, 利用电子束与慢波结构–1次空间谐波的相互作用, 在慢波结构里激励起电磁场并辐射出去. RBWO具有高功率、高效率、可重复频率运行等特点[24], 被普遍认为是最具有研究价值和应用潜力的HPM器件之一.

      在HPM的产生以及传输过程中, RBWO的高频结构、电子束收集极和介质窗三个区域是强电磁场真空击穿的薄弱环节. 强电磁场下真空击穿导致大量等离子体产生, 严重影响HPM的产生与传输条件, 造成微波功率下降以及脉冲缩短, 甚至导致结构破坏, 大幅降低HPM源的可靠性和寿命, 成为RBWO向更高功率发展的主要限制因素, 从而影响其在各个领域的推广及应用[5,6]. 电子束收集极是RBWO中的重要部分, 其主要作用是在外加引导磁场的约束下, 收集经过束-波互作用后的残余电子并且将其能量有效释放. 对于GW级输出功率的RBWO而言, 收集极不可避免地会产生大量等离子体, 其中包括解吸附气体的电子碰撞电离. 较高密度的等离子体沿着外加引导磁场向上游扩散进入高频结构, 可能导致RBWO输出功率下降甚至引发微波脉冲缩短[6,7]. 因此, 本文研究收集极等离子体的扩散及其对器件的影响机制, 对提升RBWO系统的功率、重复频率、长时间稳定工作的能力具有重要的现实意义.

      高密度等离子体影响了微波与强相对论电子束(intense relativistic electron beams, IREBs)之间的功率转换效率, 导致微波功率下降甚至脉冲缩短[711]. 同时有文献表明引导磁场的大小对收集极上的能量沉积分布也有较大的影响[12]. 但是对于RBWO中的收集极等离子体研究较少, 近些年研究人员才开始关注收集极击穿所产生等离子体对RBWO的影响. 然而目前已有的文献均采用固定气压的静态背景气体模型, 与实际情况存在差异[13]. 为了更加接近实际, 本文基于自主研发的2.5维粒子模拟(particle-in-cell, PIC)[14]软件UNIPIC-2D[15], 采用动态释气模型研究不同引导磁场、不同释气系数下的收集极释气与电离过程, 通过粒子模拟获得RBWO的输出功率、电子实空间和相空间分布等结果.

    2.   物理模型及数值算法
    • 本文研究释气现象对RBWO收集极击穿的影响问题, 考虑到在HPM产生过程中, 与高频结构相互作用后的IREBs仍携带很高能量不断轰击到收集极上, 并且将残余电子剩余的能量以热能的形式沉积在电子束收集极的收集面内, 并轰击出二次电子, 还引起收集极表面吸附气体分子的解吸附; 当沉积在金属中的能量达到临界值时, 会引起金属材料烧蚀气化. 由于烧蚀过程过于复杂, 在本文模拟中尚未考虑, 将其近似等效为材料表面释放中性气体分子, 通过设置释气系数λ为单个电子轰击收集极表面后释放气体分子的个数来实现. 气体解吸附后将发生扩散、碰撞电离等过程, 形成的等离子体对RBWO工作产生影响, 本文通过在PIC方法中考虑气体扩散和气体碰撞过程, 对这种现象进行模拟研究, 流程图如图1所示.

      Figure 1.  PIC/MCC simulation flow chart for RBWO collecting extremely outgassing.

      对于解吸附气体的扩散过程, 采用Malthus反应[16]扩散方程描述:

      其中, ${n_{{\rm{neutral}}}}$为中性气体分子密度, ${D_{{\rm{neutral}}}}$为中性气体分子扩散系数, L为损耗项(气体电离), G增益项(电子与离子复合). 该方程可以描述低气压气体的扩散过程, 其中扩散系数与气压成反比:

      其中, ${C_1},{C_2}$为气体常数, 对于非极性气体一般都是定值; ${T_{{\rm{c,S}}}}$为中性气体分子S的临界温度(K); ${M_{\rm{S}}}$为中性气体分子S的分子质量(u); ${p_{{\rm{c,S}}}}$为中性气体分子S的临界气压(atm, 1 atm = 1.01325 × 105 Pa). 对于H2, 模拟中${p_{{\rm{c,S}}}}$取12.8, ${T_{{\rm{c,S}}}}$取33.3, ${M_{\rm{S}}}$取2.016.

      UNIPIC软件中采用蒙特卡罗碰撞(Monte Carlo collision, MCC)[17,18]模型来处理气体碰撞过程. 一般的MCC是采用碰撞时间随机的方法, 而粒子模拟方法中, 粒子推进和场推进的时间步长是固定的. 因此在PIC方法中, 采用了在一个时间步长内随机决定粒子间是否发生碰撞的方法来实现MCC. 目前, UNIPIC软件中的MCC模块能模拟带电粒子与中性原子之间的弹性碰撞、激发碰撞和电离碰撞等复杂的物理过程[4]. 在收集极等离子体中, 氢离子是主要成分之一, 并且由于质量小的离子运动快, 对器件的影响更为显著, 因此本文研究中解吸附气体采用H2, 在模拟中主要考虑了电子与H2分子的弹性碰撞、激发和电离碰撞, 所使用的碰撞截面数据来自国际原子能组织的核数据库[19]. 除此之外, 在实际过程中, 还有包括电荷交换等其他的碰撞类型, 但是因为碰撞截面较小, 故忽略不计[20,21].

      二次电子发射[18,22,23]是由于“初始”电子的碰撞从而导致材料表面发射出电子的现象. 当电子运动到收集极表面时, 有可能被反射、也可能沉积在材料中, 还有可能产生二次电子发射, 电子在表面的行为对气体电离现象有重要影响. 本文采用高能二次电子发射模型[23], 二次电子发射系数为

      其中, E0为入射能量(eV); $\varepsilon $为产生一个二次电子所需要消耗的能量, 对于同一种材料可认为$\varepsilon $是定值; $\rho $是材料密度(g/cm3); Z是原子序数; A是材料的原子量; J是材料的平均电离能(eV), $\beta $是材料对二次电子的吸收系数, b1$x = 0$时的二次电子逸出概率, $J,\;{b_1}{\rm{,}}\;\beta ,\;\varepsilon $由材料本身决定; b2E0 = 104 eV时的背散射电子发射系数, ${E_{{{\rm{1}} / {\rm{e}}}}}$为背散射电子发射系数降低到b2/e时所对应的入射电子能量值; $\theta $为入射角度.

    3.   RBWO模型及粒子模拟
    • 采用的3.4 T引导磁场RBWO模型结构[24]图2所示, 图中I为爆炸发射阴极, 在脉冲功率源驱动下发射强流相对论电子束; II, III, IV为高频结构区域, 其中II为半径2.8 cm、宽度1 cm的谐振腔反射器; III为慢波结构, 采用非均匀梯形结构以保证较高的工作效率; IV为半径3.2 cm、宽度0.7 cm的提取腔. 在模拟过程中的工作电压为700 kV, 电流为10 kA, 电子束流内外半径分别为1.9 cm和2.0 cm; 引导磁场3.4 T; 在收集极的局部区域(25 cm < z < 30 cm, r = 2.2 cm)收集电子束并产生释气和二次电子; 模拟时间为40 ns.

      Figure 2.  3.4 T-guide magnetic field RBWO schematic.

    • 在无释气情况下, RBWO输出微波功率如图3所示, 输出的平均功率为1.94 GW左右, 输出微波功率比较稳定.

      Figure 3.  3.4 T RBWO output power without outgassing.

      RBWO收集极产生释气与无释气情况下的模拟结果进行对比, 如图4所示. 相比无释气情况, 微波输出功率、脉冲宽度随着释气系数增加呈现减小趋势, 在释气系数为2时脉冲缩短现象明显, 在30 ns时器件的微波输出截止.

      Figure 4.  Output power comparison under different outgassing coefficients λ.

      图5为无释气情况下40 ns时的电子分布, 其中图5(a)为电子z-r实空间分布, 图5(b)为电子z-Vz相空间分布. 从图5可以看出, 在无释气情况下, 慢波结构区域中电子束与高频电场相互作用, 发生能量转换和速度调制; 非均匀慢波结构的前段部分对电子束进行简单的预调制, 慢波结构的后半部分与电子束相互作用, 大部分电子通过慢波结构末端实现了电子与微波场的能量交换, 从而保证了器件的高效率运行. 从相空间图上可以看到有少部分电子反向运动, 这主要是由空间电荷效应和群聚作用过程引起的.

      Figure 5.  Simulation results without outgassing: (a) Electronic real-time spatial distribution; (b) electronic z-Vz phase spatial distribution

      收集极发生释气时, 在电子束的轰击下, 表面气压升高, 形成等离子体, 离子质量较大, 运动的距离较小; 电子则沿着引导磁场进入慢波结构区域, 影响束-波相互作用, 等离子体不断增加最终导致输出功率下降. 图6为释气系数λ = 2情况下的模拟结果, 图6(a)为电子实空间分布, 图6(b)为电子相空间分布, 图6(c)(f)为离子实空间分布. 从电子相空间分布图可以看到, 相比无释气情况, 此时的低速电子(慢电子)增多, 这是因为在收集极释气时, 电离产生的电子沿着引导磁场进入慢波结构, 在高频结构区域受到驻波场的强调制[13], 扩散速度较快. 由于上游二极管电场的作用, 慢电子被限制在互作用区域, 形成慢电子累积, 在一定程度上屏蔽了电场, 影响了正常的IREBs调制, 并且从微波场中获得能量导致微波输出功率降低. 图7为释气系数λ = 2时慢电子数占空间电子总数的比率, 随着时间的推移不断增大, 在10 ns时慢电子占约40%, 接近主电子束的密度, 从图4可以看出此时微波输出功率相比无释气情况已降低了约一半; 随着慢电子比例的进一步增大, 最终导致微波截止.

      Figure 6.  Simulation result with outgassing coefficient λ of 2: (a) Electronic real-time spatial distribution; (b) electronic phase spatial distribution; (c)−(f) polar space distribution at 4, 16, 28, 40 ns; (g) surface pressure curve over time.

      Figure 7.  Ratio of slow electrons to the total number of space electrons at λ = 2.

      为了分析收集极产生等离子体对RBWO输出微波性能的影响规律, 在其他条件均保持不变的前提下改变释气系数, 得到最大输出功率、脉宽与释气系数的关系, 如图8所示. 结果表明, 当收集极产生释气时, RBWO的输出微波功率开始出现下降, 随着释气系数的增大, 单位时间内产生的气体分子增加, 等离子体密度上升, 输出功率迅速减小, 微波脉冲宽度缩短. 在3.4 T引导磁场下, 当释气系数为5时输出微波几乎完全截止, 可以推测此时器件已经发生严重的击穿.

      Figure 8.  Relationship of microwave output power and pulse width to outgassing coefficient λ.

      图9为不同释气系数下40 ns时刻RBWO收集极表面气压的模拟结果, 显示出随着λ的增大表面气压也随之增大, 结合图8可知当收集极表面气压大于0.0105 Torr (1 Torr = 1.33322 × 102 Pa)时, 对器件影响较大; 收集极表面气压气压小于0.0105 Torr时, 在40 ns内器件输出功率的影响较小.

      Figure 9.  Simulation results of RBWO collector surface pressure (average pressure in a grid) at 40 ns with different outgassing coefficients.

      为了进一步分析释气对RBWO收集极击穿的影响, 在该器件的适用引导磁场范围内, 计算了不同引导磁场下该器件收集极产生释气后击穿的模拟情况. 图10所示为该器件在不同引导磁场下无释气情况与释气系数λ = 2时平均输出功率的对比. 可以看出在无释气情况下, 随着引导磁场的减小, 器件输出功率递减; 而在释气系数λ = 2时, 随着引导磁场的减小, 器件微波输出功率基本呈递增状态, 这可能是因为随着引导磁场的降低, 对电子束的约束变小, 轰击收集极的面积增大, 从而释气气压降低; 同时引导磁场对等离子体的约束变小, 等离子体分布范围变大, 而密度降低, 对主电子束的影响减小, 所以输出微波功率逐渐增加. 随着HPM技术的不断发展, 采用较低引导磁场是实现HPM产生系统小型化和低功耗的重要手段, 基于低磁场的永磁包装器件是当前发展方向之一[25]. 由模拟结果可知, 减小引导磁场也能够降低收集极释气对输出功率的影响.

      Figure 10.  Comparison of the average output power with no outgassing and λ = 2 under different guiding magnetic fields.

    4.   结 论
    • 本文采用2.5维UNIPIC软件, 基于动态释气与气体扩散模型研究了RBWO在不同引导磁场、不同释气系数下的收集极击穿过程, 模拟得到了RBWO的输出功率、电子实空间分布、相空间分布、等离子体分布和气压变化等结果. 模拟结果表明, 随着电子不断轰击收集极, 收集极表面气压升高, 轰击产生的二次电子以及收集极表面气体解吸附, 产生等离子体扩散到互作用区域, 使得RBWO发生输出功率下降和脉冲缩短现象; 随着收集极释气系数的不断增大, RBWO的脉冲缩短现象越来越严重; 低引导磁场在一定程度上可以缓解脉冲缩短现象.

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