Science and Technology on High Power Microwave Laboratory, Institute of Applied Electronics, China Academy of Engineering Physics, Mianyang 621900, China
Fund Project:Project supported by the Science Foundation of the High Power Microwave Laboratory, China (Grant Nos. 6142605180203, JCKYS2018212035, 6142605190201)
Received Date:14 August 2020
Accepted Date:09 September 2020
Available Online:24 January 2021
Published Online:05 February 2021
Abstract:The relativistic klystron amplifier (RKA) is one of the most efficient sources to amplify a high-power microwave signal due to its intrinsic merit of high-power conversion efficiency, high gain and stable operating frequency. However, the transverse dimensions of the RKA dramatically decrease when the operating frequency increases to X band, and the power capacity of the RKA is limited by the transverse dimensions. An X-band multiple-beam relativistic klystron amplifier is proposed to overcome the radiation power limitation. Each electron beam propagates in separate drift tubes and shares the same coaxial interaction cavities in the multiple-beam relativistic klystron amplifier, and the transverse dimensions of the multiple-beam relativistic klystron amplifier are free from the operating frequency restriction and a microwave power of over 1 GW is generated in the experiment. For a high-power electron device, the transmission of electron beam is critical, and the power conversion efficiency of the device is affected. In this paper, we conduct an investigation into the transmission process of the intense relativistic multiple electron beams, and the number of the multiple electron beams is set to be 16. It is found that when the multiple electron beam is transmitted in the device, the electron beam rotates around the center of the whole device, causing the electron beam to deviate from the drift tube channel. At the same time, each electron beam rotates around itself, and the cross section of the electron beam is deformed and expanded. In the improper design of electron beam and drift tube parameters, two kinds of rotating motions cause beam to lose. A multiple-electron-beam diode structure is optimized by the particle-in-cell simulation to reduce beam loss, with the effects of the related factors taken into account. Each pole of the cathodes is made up of graphite and stainless steel. The cathode head is made up of graphite, for the graphite has a lower emission threshold. The cathode base and cathode pole are made up of stainless steel, for the stainless steel has a higher emission threshold. Also the shape and structure of cathode pole, cathode head and anode are optimized to reduce the electric field intensity on the cathode pole and enhance the electric field intensity on the end face of cathode head. At the same time, the electric field distribution of the cathode head is uniform to improve the electron beam emission uniformity. The simulation results demonstrate that the transmission efficiency of multiple electron beams can reach 99%. In the experiment, the transmission efficiency of multiple electron beams is 92% with a beam voltage and beam current of 801 kV and 9.3 kA, respectively. Keywords:intense multiple electron beams/ electron beams transmission/ space charge effect/ electron beams rotation
全文HTML
--> --> -->
2.强流多注电子束传输过程空间电磁场的作用强流多注电子束在二极管与多注漂移管中的传输如图1所示, 设定电子束绕系统中心环向等间距排列, 电子束注数n = 16, 每一注电子束的半径为rb, 每一注漂移管半径为rd, 电子束中心与器件系统中心的距离为r0, 阳极半径为r1, 多注电子束在外加轴向引导磁场Bz的作用下传输. 图 1 强流多注电子束二极管结构示意图 (a) y-z截面; (b)漂移管处x-y截面 Figure1. Sketch structure of the multiple electron beams diode: (a) The y-z section; (b) the x-y section of drift tubes.
则可以得到多注电子束通过阴阳极间距的时候, 沿角向的旋转速度为${r_0}{{\dot\theta}_1}$. 设定电子束中心与器件系统中心的距离为r0 = 35 mm, 电子束电流I0 = 9 kA, 在阴阳极间距离lgap = 40 mm, 不同二极管电压U0时, 多注电子束到达阳极端面时绕系统中心沿角向的旋转距离Δl0随引导磁场的变化如图2所示. 图 2 多注电子束到达阳极端面时绕系统中心的旋转距离Δl0随引导磁场的变化 Figure2. Rotation distance Δl0 vs. Bz at different U0. Δl0 represents the rotation distance of the multi-beams around the center of the system when they reach the anode end face
设定每一注电子束半径为3 mm, 不同二极管电压下, 电子束绕自身的旋转角速度${r_{\rm{b}}}{{\dot\theta_3 }}$随引导磁场的变化如图4所示. 图 4 多注电子束绕束自身旋转角速度${r_{\rm{b}}}{{\mathop \theta \limits^. }_3}$随引导磁场的变化 Figure4. Angular velocity of the multi-beams rotation around themselves ${r_{\rm{b}}}{{\mathop \theta \limits^. }_3}$ vs. Bz at different U0. ${r_{\rm{b}}}{{\mathop \theta \limits^. }_3}$ represents the angular velocity of the multi-beams rotation around themselves.
由同轴线理论可知, 当二极管阳极与阴极半径的比值r1/r0 = 2.72时, 固定外导体半径的同轴线具有最大耐压. 因此在电子束中心与器件系统中心的距离r0 = 35 mm不变的情况下, 设计阳极半径由原先的r1 = 75 mm增大为r1 = 95.2 mm, 以降低阴极底座和阴极杆侧面的电场强度. 在阴极材料选择上, 多注阴极前端设计采用爆炸发射阈值较低的石墨, 多注阴极其他部分采用爆炸发射阈值较高的无磁不锈钢, 以减少阴极底座和阴极杆侧面的电子发射. 优化设计二极管阴阳极端面结构, 使阳极内导体前端突出5 mm, 即阳极内导体更靠近阴极, 阴阳极间距变为35 mm. 同时采用电磁场仿真软件通过参数扫描迭代算法, 得到最佳的阳极漂移管入口的导角尺寸. 阳极外导体漂移管入口导角为11 mm, 阳极内导体漂移管入口导角为10 mm, 石墨阴极头直径为6 mm, 阴极头前端端面导角为3 mm. 不锈钢阴极杆直径为2 mm, 不锈钢阴极底座r0 = 35 mm, 底座端面导角为2 mm. 二极管电场集中在石墨阴极头前端面且均匀分布, 使得绝大部分电子从石墨阴极头的前端面发射. 优化设计前后多注二极管阴阳极间的电场分布如图5所示. 图 5 优化设计前后的多注阴极结构与电场分布 (a)改进设计前; (b)改进设计后 Figure5. Electric field distribution of the multi-beam cathodes: (a) Before the improved design; (b) improved design.
采用三维粒子仿真软件对强流多注二极管进行多注电子束产生与传输的模拟计算. 设定二极管电压U0 = 800 kV, 轴向引导磁感应强度Bz = 1 T, 引导磁场方向为z正向(与电子束传输方向相同), 每一注阴极半径rb = 3 mm, 阳极半径r1 = 95.2 mm, 电子束注数n = 16注, 阴阳极间距离lgap = 35 mm, 电子束中心与器件系统中心的距离r0 = 35 mm, 每一注漂移管半径rd = 5 mm, 多注漂移管长度为600 mm. 粒子仿真在二极管电压U0 = 800 kV时, 爆炸发射产生总束流为9.1 kA, 传输到多注漂移管末端的总束流为8.99 kA, 传输通过率约为99%. 束流损失产生在电子束从二极管引入多注漂移管的入口处, 其原因为阴极爆炸发射产生电子过程中, 除了石墨阴极头产生电子外, 阴极底座和阴极杆侧面仍会有极少量电子发射, 而从阴极底座和阴极杆侧面发射的电子难以引入到多注漂移管中, 会轰击到阳极表面, 导致在该处存在束流损失. 之后电子束在多注漂移管中传输时没有出现束流损失. 多注电子束在二极管和漂移管中的三维传输轨迹如图6所示. 图 6 多注电子束在二极管和漂移管中传输 Figure6. Tracks of the multiple electron beams in the diode and drift tubes.
由图6可以看出, 多注电子束在二极管和多注漂移管中传输时存在绕系统中心的旋转运动和每一注电子束绕自身的旋转运动. 在二极管阴阳极区域和在多注漂移管中, 多注电子束绕系统中心的旋转运动方向相反. 图7为分别对距离石墨阴极头发射端面1, 40 mm和600 mm处多注电子束束斑的监测结果. 图 7 多注电子束在离阴极头端面不同距离处的束斑 (a)距离 1 mm; (b)距离40 mm; (c)距离640 mm Figure7. Transections of the multiple electron beams with different distance between the cathode head: (a) The distance of 1 mm; (b) the distance of 40 mm; (c) the distance of 640 mm.
在z = 1 mm处, 电子束半径为3 mm, 各注电子束中心无明显偏移. 在z = 40 mm处, 电子束截面扭变为椭圆形, 长边半径为3.23 mm, 短边半径为3 mm, 各注电子束中心逆时针偏移1.25 mm, 而按照之前理论推导公式计算得到的偏移距离为1.23 mm, 两者基本相符合. 在z = 640 mm处, 电子束形变加剧, 长边半径为3.56 mm, 短边半径为3.05 mm, 各注电子束中心变为顺时针偏移0.61 mm. 粒子仿真计算得到的电子束运动轨迹与理论分析得到的运动规律相符合.
4.强流多注电子束二极管的验证实验在长脉冲功率源平台上开展优化设计后的强流多注二极管验证实验. 脉冲功率源能够达到的最大电功率约为8 GW. 实验中每一注阴极半径rb = 3 mm, 电子束中心与器件系统中心的距离r0 = 35 mm, 阳极半径r1 = 95.2 mm, 电子束注数n = 16注, 阴阳极间距离lgap = 35 mm, 每一注漂移管半径rd = 5 mm, 多注漂移管长度为600 mm. 二极管阴极、阳极以及多注漂移管处于磁场螺旋线管线圈中, 螺旋管线圈产生的轴向引导磁感应强度为1 T, 磁场方向与电子束传输方向相同. 实验中二极管和多注漂移管内的真空度约为5 × 10–3 Pa. 采用电阻分压器测量脉冲功率源的电压, 在多注二极管前端设置一个罗戈夫斯基线圈测量脉冲功率源产生的总电流. 在多注漂移管末端采用一个法拉第筒测量达到末端的电子束总电流. 实验前采用高精度欧姆表和电流表对罗戈夫斯基线圈和法拉第筒的阻值进行校准测量, 得到相应的变比关系. 多注二极管实验开展前, 二极管阳极先接大尺寸圆柱漂移管结构(漂移管半径为45 mm), 在低功率和强磁场条件下(电子束功率约为 GW, 磁场感应强度为1 T), 对束流测量系统进行校准, 此时电子束传输通过率接近100%. 之后二极管阳极接多注漂移管开展多注二极管实验. 强流多注二极管结构与实验测量示意图如图8所示. 图 8 多注电子束束流测量实验示意图 Figure8. Sketch structure of the experimental system for multi-beams measurement.
测量得到脉冲功率源典型的电压、电流以及末端法拉第筒电流波形如图9所示. 图 9 电子束电压、电流与法拉第筒电流波形 Figure9. Voltage, current, and Faraday-cup current of the electron beam.
表1电子束电压、电流以及末端法拉第筒电流参数 Table1.Electron beam voltage, current and terminal Faraday tube current parameters.
实验连续测量3次得到电子束平均参数, 电压为801 kV, 电流为9.3 kA, 电子束功率为7.4 GW, 末端法拉第筒电流为8.6 kA, 即多注电子束通过率约为92%, 与粒子仿真结果基本相符合. 电子束电压脉宽为175 ns, 电流脉宽为171 ns, 法拉第筒电流脉宽为171 ns. 在多注漂移管末端z = 640 mm处放置无磁不锈钢靶片, 测量得到末端的电子束束斑如图10所示. 图 10 多注漂移管末端的电子束束斑 Figure10. Spots of the multiple electron beams at the end of the drift tube.
由图10可知电子束的注数为16注, 实验测量得到多注电子束的束斑形状与仿真结果相似. 各束斑长边半径与窄边半径尺寸分布如图11所示. 图 11 多注电子束束斑尺寸分布 Figure11. Size of the multiple electron beams spots