Low-energy pulsed spark discharge characteristics of pin-plate structure in water
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摘要: 开展了J量级系统储能下电脉冲参数对水中火花放电特性影响研究。驱动源采用参数可调的固态重频纳秒脉冲电源,放电负载为水中针-板结构(间距1 mm),在低重频条件(约5 Hz)下进行实验。通过调节放电参数、拍摄高速阴影图像、光谱诊断以及声信号测量,研究水中脉冲放电的物理特性,得到不同放电参数下放电演化规律及其对声学、光谱特性影响。实验发现:在J量级储能下,放电通道连通两极后,回路电流在几百ns内快速上升至10 A左右,随后缓慢下降,持续50~60 μs。发现预设脉宽对放电影响较大,短脉宽条件下放电会被电源固态开关强制截断出现反向放电,而长脉宽条件下放电通道在后期变得不稳定甚至熄弧中断,出现气泡中二次放电现象。辐射光谱揭示了更多等离子体信息,推断通道电子密度在1018 cm−3量级,随着脉宽增加,特征谱线强度增加,表明活性粒子数密度增加,但粒子种类不变。短脉冲(<150 μs)作用下产生的脉冲声波的特征宽度在110~150 μs,而当脉宽继续增大,声波脉宽并不继续增加而是保持不变,保持在150 μs左右。研究结果对水中小能量火花放电的机理研究有一定参考价值,为水声学、液相等离子体等领域的应用提供思路。Abstract: The influence of electric pulse parameters on spark discharge characteristics in water was studied. A solid-state repetitive nanosecond pulse power supply with adjustable parameters was adopted. The discharge load was a pin-plate structure placed in water (the distance between the pin and the plate set to 1 mm). The experiment was carried out under low repetitive frequency conditions (approximately 5 Hz). The characteristics of pulse discharge in water were obtained by monitoring discharge parameters, taking high-speed shadow images, collecting optical emission spectrum, and measuring sound pressure. The evolution of pulse discharge with different parameters and its influence on acoustic and spectral characteristics were also obtained. When the energy storage on the order of a few joules after the initial discharge channel was formed between the two electrodes, the circuit current rose to approximately 10 A within a few hundred ns, followed by a rapid and then slow decline with a duration of 50−60 μs. It is found that the preset pulse width has a great influence on the spark discharge characteristics. Under short pulse width conditions, the discharge channel will be cut off by the solid-state switch of the power supply. Under long pulse width conditions, the discharge channel becomes unstable in the late stage and even interrupts the arc,and the secondary discharge appears in bubbles. The radiation spectra reveal more information. With the increase of pulse width, the intensity of the characteristic spectral lines increased, but no new spectral lines were observed. This indicates that the number of active particles increased, and their types remain the same.The channel electron density is estimated on the order of 1018 cm−3. The characteristic width of the pulse sound wave produced by a short pulse (<150 μs) is 110−150 μs. However, when the pulse width continues to increase, the sound wave pulse width does not continue to increase but remains at 150 μs. It is hoped that this research has a certain reference value for studying the mechanism of small energy spark discharge, and provides ideas for the applications of underwater acoustics, liquid phase plasma and other fields.
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Key words:
- discharge in water /
- plasma /
- image diagnosis /
- spectral diagnosis /
- sound and shock waves
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北京正负电子对撞机重大改造工程(BEPCⅡ)直线加速器所使用的速调管能够输出50 MW的脉冲功率[1-2],是极为昂贵的微波器件,速调管的陶瓷窗易受到表面打火和波导系统反射功率的损伤。为了更好地保护陶瓷窗,以期延长速调管的使用寿命,常用以下五种保护措施:(1)真空保护[3],速调管出口波导系统的真空度可以通过离子泵电流表征,将泵电流作为快联锁信号以便及时切断系统触发信号,但真空变化属于慢变化,且和打火位置有关;(2)ARC打火保护,通过速调管出口波导的观察窗获取打火时的光信号,再经光电转换作为快联锁信号,因在真空环境使用,需考虑带观察窗的波导制造工艺等问题;(3)在速调管出口加环形器[4],因环形器承受功率有限,仅适用于小功率速调管;(4)通过峰值功率计或专用保护装置的内部计算和报警输出进行驻波比保护[5-7]或反射保护;(5)通过低电平系统实时获取入射和反射信号,再对比驻波比或反射保护阈值给出报警信号[8-9],低电平方案可以获得μs级响应时间,但成本较高。
BEPCⅡ直线加速器使用峰值功率计测量反射功率,超阈值后输出报警信号。从直线加速器安全运行角度出发,要求当前反射信号功率超过阈值后,功率计能够在下一个脉冲到达之前发出报警信号,进而切断触发信号,实现对速调管的保护。以重复频率50 Hz为例,即要求20 ms内功率计送出报警信号。但在实际多台次测试后发现反射保护的平均响应时间为306 ms,远超20 ms的目标值。考虑到BEPCⅡ直线加速器在线运行的速调管有20台,我们提出了3种较高性价比、易实施的方案,创新性地提出“3+1”反射保护响应时间测试法,并对上述3种方案进行详细测试。最终经综合考虑,将原功率计改进升级作为实施方案。
1. 方案设计
经过前期调研,提出三种性价比高、易实施的反射保护报警解决方案:(1)PicoScope虚拟示波器的遮罩报警方式;(2)基于自带检波和报警输出功能的Linear芯片制作反射保护插件;(3)联合原国产功率计厂家对产品升级改进。本文所述反射保护报警的响应时间是指,从大于设定功率阈值的4 μs脉宽反射信号进入功率计B通道到功率计尾部BNC反射保护端输出报警信号的总时长。
1.1 PicoScope虚拟示波器的遮罩报警
PicoScope 3000D型虚拟示波器,50 MHz带宽,8 bit垂直分辨率,并具有遮罩及报警等功能[10]。利用官方软件可以设置幅度或时间遮罩,当检测到信号超过所设遮罩阈值时,自动触发虚拟示波器内部的信号发生器,进而发出报警信号。需要注意的是,报警设置时需设定报警事件的先后顺序,应将触发信号发生器作为优先报警事件,否则会影响测试结果。在使用时虚拟示波器通过USB3.0与本地工控机连接,进行数据传输与控制,无需额外供电。软件操作界面如图 1所示。
1.2 基于自带检波和报警输出功能的Linear芯片制作反射保护插件
LTC5564是Linear Technology公司的一款芯片,采用3 mm×3 mm方形扁平无引脚封装(QFN)。其内置检波器、高速比较器和放大器,具有峰值检波、超阈值快速报警、宽输入功率范围和宽输入频率范围等多重优点。通过购置官方演示板并按使用频率进行元器件替换,可以较快搭建起实验电路,如图 2所示。或以LTC5564芯片为核心,辅以外围供电电路、数控可调电源模块等,制成反射保护报警插件。比较电压即为报警阈值,需以直流电信号输入,并根据反射功率报警阈值提前确定好所需电压值。
1.3 对原国产功率计升级改进
通过对现有硬件功率计的改进与升级来降低反射保护响应时间,无需额外购置新硬件,因此是一种相对经济和省时的方法。先后对功率计进行多轮软硬件改进,每次改进后在实验室进行详细测试,并择机在BEPCⅡ直线加速器上线实验,再对测试结果进行分析并找到新改进方案,形成“实验促进改进、改进验证实验”的良性循环。
改进前响应时间过长的原因主要有两点:(1)由于之前的控制电路由两块CPU构成,一块是负责信号采集并进行相应运算和处理的控制CPU,另一块是负责面板显示及报警信号输出的面板CPU。一个报警信号的产生,需要信号采集处理后再通过内部通信传输给面板CPU判断,满足报警条件后输出反射保护报警信号。在此过程中,通信及各自程序处理增加了反射报警时间; (2)面板CPU在得到控制CPU传输来的信息后还需要进行按键轮询、串口信息处理、功率数据处理及显示输出等,从而造成不能在第一时间对反射功率进行判断并输出报警信号。针对上述情况,对功率计的软硬件进行针对性改进。硬件方面,采用一块CPU进行处理,此CPU兼顾之前面板CPU和控制CPU功能,且该CPU在处理速度上较之前两块CPU提高约1倍。此外对反射保护电路中检波电容等也进行了修改。软件方面,将报警程序与其余程序(按键轮询、显示等)分开处理。每隔一定时间(ms级)检查反射信号是否超过阈值,并进行相应处理。处理后,功率计不但能在第一时间检测到反射信号的变化,而且能排除处理其他程序对报警时间带来的影响。图 3是功率计改进后的内部结构示意图。
2. “3+1”反射保护响应时间测试法
“3+1”反射保护响应时间测试法中,“3”代表反射保护的3种状态,“1”代表建立一种基于功分器、微波开关的反射保护响应时间测试方法。
反射保护响应时间测试分为三种情况: 常态运行保护、连续保护和极端情况下的反射保护报警,可简称为常态、连续和极端保护。所谓常态运行时的反射保护是指,调制器稳定运行在指定高压时,功率计探测到的反射功率也维持在一个稳定数值。当一个超过2 MW设置阈值的反射信号突然传到功率计时,功率计输出保护信号,即由高电平变为低电平。这也是最常见的反射保护形式,故称之为常态运行时的反射保护,简称常态保护。连续保护,即前一个突然变大的信号超过阈值造成反射保护,紧跟着又一个超阈值的信号进入到功率计,造成二次保护。极端情况下的反射保护,即调制器高压到位后,开启触发信号开关的瞬间出现反射保护。比如直线加速器开机或某位置调试后的开机瞬间。连续保护和极端保护都属于小概率事件,但也是功率计反射保护性能必须考察的两个方面。反射保护三种状态示意图见图 4。
图 5是一种基于功分器、微波开关的反射保护响应时间测试方法,通过合理方法控制信号源和微波开关,可以实现实验室内常态、连续和极端三种反射保护测试。图中的功率计可以替换为PicoScope虚拟示波器、反射保护插件。信号源也可以替换为直线加速器的反射信号,即可进行在线反射保护响应时间测试,且微波开关的引入,可以除去调制器的高压调节对测试带来的影响,结果更加合理可信。Mini-Circuits公司的微波开关可以通过电脑程序控制通道切换,以模拟触发开关的过程。
3. 实验结果
按照图 5的方法在实验室对PicoScope虚拟示波器、基于Linear芯片的反射报警插件和改进型国产功率计进行3种状态下的反射保护响应时间测试,测试环境为重复频率50 Hz、脉冲宽度4 μs,采取多次测量求平均值方式,测试结果见表 1。
表 1 实验室50 Hz, 4 μs脉宽下三种方案的反射保护响应时间测试结果Table 1. Reflection protection response time results of the three schemes tested at 50 Hz with 4 μs RF pulse width in the laboratorynormal protection/ms continuous protection/ms extreme protection/ms picoscope virtual oscilloscope 72 — — reflection protection plug-in 8×10-5 7×10-5 8×10-5 upgraded RF power meter 5.2 5.5 5.9 虚拟示波器自身功能较多,遮罩报警功能仅是众多功能之一,并不是首要或主流功能,因而其常态保护的响应时间较长,仅为72 ms。这标志着它失去了进行连续和极端保护的测试机会。经过六轮改进的国产功率计在常态、连续和极端三种情况下,反射保护响应时间的测试结果基本相同,都在5~6 ms之间,体现了较强的稳定性,已满足指标要求。图 6是改进型功率计在极端保护情况下反射保护响应时间的测试截图。基于Linear LTC5564芯片的反射保护插件则表现更为突出,可实现三种保护状态下的几十ns级报警响应时间。
就测试数据而言,无疑基于Linear芯片的反射保护插件胜出,且为数量级的优势。但该插件也有不足之处:造价大概是功率计升级费用的2~3倍;对内置的开关稳压电源要求较高;插件输出的报警信号是高电平脉冲信号,与国产功率计相反,即与BEPCⅡ现有的报警联锁盒子不兼容;未经历BEPCⅡ长期在线测试,无法保证复杂电磁环境下的稳定运行。综上,反射保护插件暂不适合BEPCⅡ直线加速器在线运行使用。
4. 结论
针对降低反射保护响应时间以保护速调管陶瓷窗这一命题,进行了细致的选型调研和对比试验,创新性提出“3+1”反射保护响应时间测试法。研制了基于Linear LTC5564芯片的反射保护插件,测试结果优异。对国产功率计进行多轮软硬件改进,最终实现了实验室模拟常态、连续和极端三种情况下反射保护响应时间均为5~6 ms之间的结果。综合考虑成本、兼容性和稳定性等因素,国产功率计升级较反射保护插件更具优势,故在2017年8月~9月的暑期检修期间对直线加速器所有峰值功率计进行专项升级改进。经过2017~2018年一整轮BEPCⅡ运行结果显示,上述改进型功率计运行稳定可靠,每月测试反射保护响应时间均达标,因而改进工作符合预期。
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