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当前国内光学条纹相机所用的扫描变像管通常为轴对称型六电极扫描变像管[13-14 ],其结构如图1所示,主要由阴极(C)、栅极(M)、第一聚焦极(F1)、第一阳极(A1)、第二聚焦极(F2)、第二阳极(A2)、偏转电极(D)和荧光屏(P)组成,这种变像管阴极高压最高可达到−16.5 kV,此时栅极高压可设置为−3.5 kV。
为降低环境杂散光引入的噪声,可使变像管阴极发射电子在无待测信号时处于反向截止状态,而在有用信号到达变像管时,阴极发射电子可被正常聚焦成像和扫描,并到达荧光屏。如此,应使变像管在无待测信号到来时,阴极处电压高于变像管后端某处位置电压,此时变像管的电压加载情况会使各种杂散光引起的阴极发射电子不能到达变像管的荧光屏;而在有用信号到达变像管时,通过在阴极上作用矩形脉冲高压,使变像管的各电极电压达到正常工作时的电压值,可对待测信号辐照光阴极而发射的光电子进行正常聚焦扫描。通常情况下,为使杂散光导致的阴极发射电子处于反向截止状态,一般使阴极电压高于栅极电压,如可将阴极电极电压设置为−3 kV,而为使待测信号使阴极发射的光电子能够正常到达荧光屏并聚焦成像,需要在阴极上施加的矩形门脉冲幅度需高达−13.5 kV,如此高幅度的矩形门脉冲制作难度较大,因此当前条纹相机扫描变像管只加载直流电压,无法有效抑制环境杂散光产生的噪声。
通过对变像管各电极电压加载情况进行详细分析,发现只需使阴极轴心位置处电压高于后端电极F2轴心处电压即可实现对阴极发射电子的反向截止。图2为计算得到的变像管轴心上电压分布情况,可以看出当阴极电压为−16.5 kV时,阴极处轴心位置电压为−16.5 kV,而后端F2电极位置处轴心电压约为−11.79 kV,因此,为使阴极电子能够反向截止,只需使阴极位置处电压高于−11.79 kV即可,如可将阴极电压设置为−11 kV,此时阴极出射的电子就将在F2电极处截止。而为使变像管正常工作,只需施加−5.5 kV的门控高压脉冲即可,显然这种幅度的门控高压脉冲制作相对容易。
为验证变像管的电子截止情况,分别对阴极加载−16.5 kV和−11 kV电压时变像管的电子运动轨迹进行了计算,如图3所示,图3(a)为变像管阴极加载−16.5 kV时的电子运动轨迹,可以看到此时变像管为正常工作状态,电子能够正常聚焦成像到荧光屏上;图3(b)为阴极电压−11 kV时的电子运动轨迹,可以看出,电子运动到F2电极位置时即被截止,无法到达荧光屏。 因此,根据变像管的这一特性制定了如图4所示的门控技术方案,首先在阴极上施加−11 kV直流电压,其他电极施加正常工作电压,而在待测信号到达阴极时间段内,在阴极上叠加−5.5 kV的矩形门控脉冲,即可使环境光辐照光阴极发射的电子无法到达荧光屏,而降低变像管噪声水平。
当扫描变像管阴极叠加矩形门控脉冲时,变像管将类似于一种高速快门,此时需要考虑变像管的开启时间τ。如果变像管不能快速开启,会难以同步待测信号到达阴极的时间与矩形门脉冲加载时间,导致待测信号到达相机阴极时,相机依然处于截止状态而无法对有用信号进行测量。理论上扫描管的开启时间τ与阴极面电阻R以及阴栅极间电容C有如下关系:
$$\tau = {{RC}}/\pi $$ 通过增加多碱导电层薄膜厚度的方法可降低面电阻,但会较大程度上降低入射信号的透过率而降低阴极光谱响应灵敏度。因此,为进一步降低开启时间τ,需要降低阴栅极间电容C。由于阴栅间类似于平板电容器,电容可按下述公式计算:
$$ C = Q/U = {{\varepsilon }} \cdot {{S}}/4\pi {{kd}} $$ 式中:C为板间电容值;ε为两板间相对介电常数;k为静电力常数;S为阴栅正对面积;d为阴栅间距。介电常数主要为真空介电常数,难以改变,而阴栅间距d的增大会降低变像管的时间分辨率,因此减少阴栅正对面积是主要方法。如图5所示,通过对变像管阴栅电极结构的优化,将阴栅面积减少了5倍以上。最后,为获得超快的阴极高压门控,需尽可能减少传输线长和寄生电感,采用从阴极位置处就近引出高压线的方式使得阴极脉冲高压引出线缩短到了100 mm以下。
Optical streak camera with gated photocathode
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摘要: 为满足惯性约束聚变(ICF)物理实验诊断中对高信噪比低噪声光学条纹相机的需求,基于六电极同轴型扫描变像管发展了一种阴极门控光学条纹相机。该相机通过阴极预置直流高压叠加门控脉冲电压的方法,使相机变像管电压只有在门控脉冲加载时间内处于正常工作状态,才能对阴极发射的电子进行正常聚焦扫描,而在非加载门控脉冲时间内,变像管的第二聚焦电极处电压低于阴极电压,从而使阴极发射的光电子被反向截止,因此相机只对在门控脉冲加载时间内到达的有用信号进行测量,从而有效抑制了光学条纹相机受环境光辐照而引入的背景噪声,提升了相机的信噪比。验证实验表明,通过在阴极上加载幅度−5.5 kV、脉宽203 ns的门控脉冲电压,即可较大程度降低相机噪声,同时可维持相机的空间分辨率。Abstract: In order to meet the demand for high signal-to-noise ratio and low-noise optical streak camera in inertial confinement fusion (ICF) physical experiment diagnosis, a cathode-gated optical streak camera with six electrodes streak image tube was developed. To suppress the camera noise, a method that loading DC high voltage while superimposing gated high voltage pulse to the photocathode was used. In this way, the streak image tube of the camera was in the normal working state only during the gated pulse loading time, and the electrons emitted by the cathode could be normally focused and scanned. When the control pulse was not loaded, the voltage at the second focusing electrode could be lower than the cathode voltage, so that the photoelectrons emitted by the cathode would be reversely cut off. Therefore, the camera only measured the useful signals that arriving within the gated pulse loading time, thereby effectively suppressing the background noise introduced by the optical streak camera irradiated by ambient light, and improving the camera's signal-to-noise ratio. The verification experiment shows that by loading the gated pulse voltage with amplitude of − 5.5 kV and pulse width of 203 ns on the cathode, the camera noise can be greatly reduced, and the spatial resolution of the camera can be maintained.
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Key words:
- optical streak camera /
- streak image tube /
- gated cathode /
- noise
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