Status of radiographic X-ray source driven by 4 MV, 80 kA induction voltage adder
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摘要: 介绍了西北核技术研究院研制的4 MV脉冲X射线闪光照相装置(“剑光二号”)系统组成和实验结果。装置基于感应电压叠加器(IVA)驱动阳极杆箍缩二极管(RPD)技术,主要由前级脉冲功率源、感应电压叠加器和RPD等组成。前级脉冲功率源由两台3.2 MV低电感Marx发生器和四路同轴水介质线组成。每台Marx同时给两路脉冲形成线(特征阻抗6 Ω、电气长度30 ns)充电,充电峰值时间约370 ns。每路水介质线采用两级脉冲压缩,为感应腔馈入约1 MV/160 kA/60 ns电脉冲。电触发SF6气体开关、自击穿水开关分别用作主同步开关和脉冲陡化开关。感应电压叠加器采用四级1.5 MV感应腔串联,每级感应腔采用单点馈入结构。次级采用真空绝缘传输线实现电压叠加和功率传输,特征阻抗由30 Ω线性增大至120 Ω。采用4 MV电压下综合性能较优的RPD来产生强脉冲X射线。装置目前达到技术指标:输出电压4.3 MV、脉冲前沿(10%~90%) 21 ns、半高宽约70 ns、二极管电流85 kA,X射线半高宽约55 ns,整机延时(从Marx触发器输出到X射线产生)约749 ns,标准偏差约7 ns。当RPD阳极采用直径2 mm钨针时,正前方1 m处剂量约15.5 rad(LiF),正向焦斑约1.4 mm。Abstract: This paper presents the design details and experiment results of a 4 MV facility developed for flash X-ray radiography in Northwest Institute of Nuclear Technology (NINT). The facility is based on the technology of an induction voltage adder (IVA) driving a rod pinch diode (RPD). The facility mainly consists of the prime power source, induction voltage adder, and RPD. The prime power source consists of two 3.2 MV low-inductance Marx generators and four deionized-water coaxial lines. Each Marx generator charges two 6 Ω, 30 ns pulse forming lines (PFLs) in less than 370 ns. There exist two-stage pulse compressions for each pulseline, providing four forward waves with peak voltages of 1 MV at current of 160 kA with a duration time of 60 ns. Four electrically-triggered SF6 gas switches serve as energy transfer switches from PFLs to outlines, and then four self-breakdown water switch are used to sharpen the risetime and reduce the prepulse. The IVA consists of four-stage induction cavities stacked in series, each of which almost operates at 1.5 MV voltage. A vacuum insulated transmission line (without magnetic insulation) is used for power addition. The RPD is chosen to create X rays through bremsstrahlung. At present, the IVA could produce a 4.3 MV voltage with a risetime (10%-90%) of 21 ns and a FWHM time of 70 ns. The diode current is about 85 kA, and the FWHM time of X rays is about 55 ns. The delay time from Marx trigger to the X-ray output is about 749 ns, with a standard deviation of about 7 ns. With a 2-mm diameter tungsten rod used, the X-ray dose is about 15.5 rad (LiF) at 1 m straight ahead, and the spot size is about 1.4 mm.
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脉冲功率技术广泛应用于激光聚变、离子束、微波功率源、雷达发射机等领域,脉冲功率是通过高压脉冲电容器储存能量,闸流管或固态开关的快速导通释放能量来获取所需的高脉冲功率,而脉冲电容的高压充电电源是脉冲功率设备充电的关键[1]。高压充电电源拓扑包括电阻限流型恒压充电、LC工频谐振型充电、桥式谐振变换器等,其中,桥式谐振型中的全桥串联谐振变换器以具备恒流充电特性、负载适应性强、充电效率高、软开关、功率密度大等一系列优点得到了广泛的讨论和应用[1-8]。文献[1-3]对LC串联谐振变换器的基本原理和应用有详细介绍;文献[4]分析了采用固定占空比调制的串联谐振变换器,因工程中变压器分布电容、二极管级间电容等寄生电容的存在,实际恒流充电特性会受到影响。文献[5]基于LCC串并联谐振变换器,LCC兼有串联谐振抗负载短路能力和并联谐振抗负载开路能力的优点,更适用于脉冲电容负载;但峰值电流的减小会影响充电能力,在高重复频率应用时会受到限制;文献[9]针对双谐振拓扑,搭建样机,测试了双谐振变换器在输入电压变化时的恒流充电能力。
本文在分析串联谐振变换器的基础上,建立双谐振变换器的数学模型,根据电压电流传输特性曲线中双谐振变换器存在第二谐振点这一特征,提出一种脉冲电容的充电控制方式。该控制方式将充电过程分为两个阶段,充电阶段提高开关频率,等效为LC串联谐振,具备恒流充电、软开关等优点;充电保持阶段,降低开关频率至第二谐振点附近,通过较小的充电电流补偿脉冲电容的自放电压降,这将显著提高脉冲电容的高压稳定度。基于Matlab/simulink环境搭建仿真模型,相同参数下,通过串联谐振和双谐振变换器的仿真对比,验证了所提出的双谐振变换器的变频率调制的可行性,这对于进一步提高脉冲功率设备的脉冲高压重复稳定度提供一个可选的参考,对后期样机的研制提供理论基础。
1. LC串联谐振与双谐振电路分析
双谐振变换器的电路拓扑结构如图 1所示,其由直流侧母线电容、全桥开关管Q1~Q4、谐振电感Lr、谐振电容Cr、谐振电感Lp、变比为n的高频变压器T、续流二极管D1~D4、高频整流桥D5~D8组成,负载储能电容Cload。若不考虑谐振电感Lp,电路即为LC串联谐振变换器拓扑。
对于LC串联谐振变换器,假定负载电容较大,等效到变压器原边为n2C(若n2C≫Cr,则对谐振回路的影响可忽略),电路阻抗Z=√Lr/Cr,谐振频率为fr=1/(2π√LrCr),串联谐振充电电流平均值为I0=8UdcfsCr/n。其中,Udc为直流侧母线电压,fs为开关频率。
根据谐振频率fr与开关频率fs的关系,串联谐振有3种工作模式:(a) 当fs < fr/2,电流断续工作,软开关,实现零电流开通和关断;(b) fr/2 < fs < fr,连续工作模式,谐振回路呈容性;(c) fs>fr,连续工作模式,谐振回路呈感性。为保持高效率,一般工作在断续工作模式较多。图 2为串联谐振断续工作模式下的谐振电流波形,一个开关周期有2个谐振周期[10-12]。
相比于串联谐振,双谐振变换器在谐振电容Cr上并联谐振电感Lp。因此工作中将存在Lr、Cr、Lp的串联谐振回路和Cr、Lp组成的并联谐振回路,两个回路的谐振频率分别用fr1和fr2表示。
fr1=12π√Lp+LLpLCr (1) fr2=12π√1LpCr (2) 2. 双谐振变换器的传输特性及比较
根据等效电路,构建串联谐振电压与双谐振电路电压的传递函数,其输入输出电压传输特性分别为
Ggainl =VoVi=11+jQ(fn−1/fn) (3) Ggain2=VoVi=11+jfnQ[1+1/(k−f2n)] (4) 式中:Q为品质因数;fn=fs/fr为标幺化频率;k=Lr/Lp。串联谐振与双谐振对应的传输特性曲线如图 3所示。
由图 3可知,对于双谐振电路,当fr2 < f < fr1之间,其电压传输特性与串联谐振基本一致;因此开关频率工作在此区间时充电特性近似保持一致。当开关频率工作在谐振点fr2时,双谐振电路的电压增益为0,此时Lp、Cr并联回路的等效阻抗无穷大,回路等效为开路。
构建串联谐振和双谐振的电流传递函数,其传输特性如下
io1=ViZ=Vi[1+jQ(fn−1/fn)]R=ViZr1/Q+jZr1(fn−1/fn) (5) io2=ViZ=Vi[1+jfnQ(1+1/(k−f2n))]R=ViZr2/Q+jfnZr2[1+1/(k−f2n)] (6) 对于串联谐振和双谐振,其特征阻抗Zr1和Zr2可表示为Zr1=√Lr/Cr,Zr2=√(Lp+Lr)Lr/LpCr;其电流传输特性曲线如图 4所示。由图可知,在谐振点附近回路电流最大。当开关频率偏离谐振点时,偏离的越远,当负载变化时,电流的增益曲线基本不受影响,此时电路变现出良好的恒流特性,以及抗短路特性。而对于双谐振变换器,与电压传输特性一样,因第二个谐振点的存在,当fr2 < f < fr1之间,其电流传输特性与串联谐振基本一致;而当无线接近fr2时,电流将趋近于0,而此时变换器处于工作状态。利用这一特性,提出基于双谐振变换器的变频控制方式,即充电阶段,双谐振变换器以一定开关频率(fs < fr1/2)工作在断续模式给储能电容充电;当电压处于保持阶段时,通过频率调制,将开关频率调制到逼近fr2,用极小的电流来补偿储能电容的电压降,从而提高充电电源的精度。这对于脉冲功率设备尤其是存在大电容的固态调制器而言,对提高脉变次级的脉冲高压稳定度具有相当的优势。
3. 仿真实验验证
基于Matlab/simulink搭建仿真模型,如图 5所示,设定Udc=540 V,Lr=3.6 mH,Cr=0.6 mH,Lp=220 mH。对于串联谐振变换器,其谐振频率fr=108.346 kHz,为了实现软开关状态,实现零电流开通,电流断续工作模式,即开关频率fs < fr/2。取理想化的条件,开关频率为33 kHz,则计算得到串联谐振变换器的充电电流平均值为0.8 A;图 6为串联谐振对应的驱动、谐振电流、谐振电压的仿真结果。串联谐振采用的控制方式为充电阶段恒流给储能电容充电,因此电压曲线呈线性变化;当恒流充电到所需的高压以后,停止充电;此时,处于高压保持阶段,因电容漏电流的存在电容自放电,电容电压将以一定时间常数下降。对于双谐振,第一个谐振点fr1 =109.229 kHz,第二个谐振点fr2 =13.86 kHz,因谐振电感Lp的存在,在相同开关频率下,充电电流要略小于LC串流谐振变换器,所以相同的高压下,充电时间要略长,这与图 4所示的电流传输特性曲线相吻合。当处于高压保持阶段,因第二个谐振点的存在,则可以通过频率调制将开关频率接近该谐振点,以保持微弱的恒流充电状态从而补偿储能电容的电压降,从而保持储能电容电压的恒定,这将大幅提高后级脉冲高压的重复稳定度。图 7(b)图为该状态的放大波形,可观测到串联谐振充电的电压的阶梯增长波形,以及电压保持阶段的电压补偿效果。
图 8为双谐振变换器充电及电压保持阶段,驱动、谐振电流、谐振电容电压以及高压的波形变化曲线,可监测频率调制过程,因仿真参数较为理想,频率调制过程变化明显,变频率调制过程中储能电容保持了较好的控制精度。图 9(a)图为充电阶段,驱动、谐振电流、谐振电容电压的放大图,与图 6基本保持一致,图 9(b)为充电保持阶段,驱动、谐振电流、谐振电容电压的放大图。
4. 结论
本文针对双谐振拓扑结构变换器,根据双谐振电路存在两个谐振点的特性,提出采用频率调制的脉冲电容器充电电源充电控制方式,即充电分为两个阶段,充电阶段采用串联谐振工作模式实现快速的恒流充电,高压保持阶段降低开关频率至接近第二谐振点,实现脉冲电容自放电压降的动态补偿,从而保证充电电源的充电精度的同时,提高脉冲电容的高压稳定度,本文详细给出了双谐振充电电源的原理及工作模式。仿真结果验证了在双谐振充电电源对脉冲电容自放电压降的补偿效果,这对于更高精度脉冲电源的研制具有参考意义。
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表 1 装置连续10发次实验主要输出指标
Table 1. Key output parameters of the Jianguang-II facility over a ten shot sequence
shot output voltage/MV risetime/ns diode current/kA X-ray FWHM time/ns dose@1 m/rad(LiF) delay time/ns 2019-017 4.3 22 85.3 58 14.8 748 2019-018 4.6 24 84.1 54 15.7 749 2019-019 4.5 20 85.6 56 16.0 748 2019-020 4.5 20 80.5 54 14.5 745 2019-021 4.4 24 86.2 56 16.1 740 2019-022 3.9 20 84.9 58 14.2 741 2019-023 4.5 21 84.2 53 15.6 748 2019-024 4.2 19 87.0 52 15.4 756 2019-025 4.3 17 88.0 52 16.9 756 2019-026 4.2 21 82.5 52 15.4 762 average 4.3±0.2 21±2.1 84.8±2.2 55±2.4 15.5±0.8 749±7 表 2 剑光一号和剑光二号装置输出指标的比较
Table 2. Comparation of output parameters between the Jianguang-I and Jianguang-II facilities
output voltage/MV diode current/kA X-ray FWHM time/ns radiated dose @1 m/rad(LiF) spot diameter/mm Jianguang-I facility 2.4 51 46 3.7 1 Jianguang-II facility 4.3 85 55 15.5 1.4 -
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