大气压管板结构纳秒脉冲放电中时域X射线研究

纳秒脉冲放电能在大气压下产生高电子能量、高功率密度的低温等离子体,由于经典放电理论无法很好地解释纳秒脉冲放电中的现象,近年来以高能逃逸电子为基础的纳秒脉冲气体放电理论受到广泛关注.纳秒脉冲放电会产生高能逃逸电子,伴随产生X射线,研究X射线的特性可以间接反映高能逃逸电子的特性.本文利用纳秒脉冲电源在大气压下激励空气放电,通过金刚石光导探测器测量放电产生的X射线,研究不同电极间隙、阳极厚度下和空间不同位置测量的X射线特性.实验结果表明,在大气压下纳秒脉冲放电能产生上升沿约1 ns,脉宽约2 ns的X射线脉冲,其产生时间与纳秒脉冲电压峰值对应,经计算探测到的X射线能量约为2.3×10-3J.当增大电极间隙时,探测到的X射线能量减弱,因为增大电极间隙会减小电场强度和逃逸电子数,从而减少阳极的轫致辐射.电极间距大于50 mm后加速减弱,同时放电模式从弥散过渡到电晕.随着阳极厚度增加,阳极后方和放电腔侧面观察窗测得的X射线能量均有所减弱,在阳极后面探测的X射线能量减弱趋势更加明显,这说明X射线主要产生在阳极内表面,因此增加阳极厚度会使穿透阳极薄膜的X射线能量减少.     

强光一号兆安电流钨丝X箍缩实验研究

为了获得更高亮度的X射线点源,在"强光一号"装置上开展钨丝X箍缩实验研究.基于能量平衡方程,估算了1 MA电流热斑等离子体平衡半径为1—10μm.实验中,测试钨丝直径为25—100μm,丝根数为2—48,负载线质量为0.18—6.9 mg/cm;负载电流峰值为1—1.4 MA,10%—90%前沿为60—70 ns."强光一号"装置上匹配的X箍缩负载为30或32根25μm钨丝X箍缩,这种负载一定概率产生单个脉冲X射线辐射,辐射时刻位于电流峰值附近,典型参数为keV X射线脉宽1 ns,辐射功率35 GW,产额40 J,热斑尺寸～30μm.然而,兆安电流X箍缩通常产生多个热斑及多个脉冲X射线辐射.keV能段首个脉冲X射线辐射时刻与负载线质量正相关,并受到负载丝直径的影响.多个X射线脉冲可能由二次箍缩和局部箍缩产生,多个热斑可能由交叉点处微Z箍缩的长波长扰动和短波长扰动引起.与百千安电流X箍缩相比,兆安电流X箍缩热斑亮度更高,但X射线辐射脉冲的单一稳定性还有待于进一步改善.     

钛空腔靶高强度keV X射线源的实验研究

利用神光-Ⅲ原型激光装置实验研究了8束ns激光脉冲从一端注入钛空腔靶产生的keV X射线源的辐射特征,发现keV X射线主要产生于腔轴附近;钛空腔靶内径过大时keV X射线能流的峰值强度较低,内径过小时keV X射线能流的持续时间较短。为了在4π空间内使钛空腔靶获得最大的X射线(4~7keV能段)转换效率,腔内径的最优值在1000~1300μm附近,此时的keV X射线转换效率为4.7%,是相同激光参数下钛平面靶的2倍左右。激光单端注入有底部钛膜和无底部钛膜的空腔靶对比实验显示,底膜能够增强keV X射线的发射。     

长脉冲keV X射线源的辐射特征

 利用神光Ⅱ第九路2 ns长脉冲激光束作用厚钛固体靶，研究了产生的keV X射线源的辐射区域和总辐射功率的时间行为。结果表明：在长脉冲激光作用厚固体靶时，硬X射线线辐射功率的时间行为以及辐射体积的时间行为与激光脉冲波形一致；长脉冲时，等离子体2维膨胀效应非常显著，keV X射线线辐射的径向辐射区域在激光焦斑尺寸附近达到饱和，导致X射线线辐射功率出现饱和，且keV X射线线辐射的辐射体积正比于焦斑尺寸的3次方。从理论和实验角度研究了在同样入射激光能量下，辐射功率随激光焦斑尺寸的变化关系，发现keV X射线线辐射的饱和辐射功率正比于焦斑尺寸的5/3次方，理论结果与实验结果一致。并讨论了相同基频输出激光能量下，keV X射线辐射总功率随激光波长的变化关系，发现即使考虑了倍频效率的影响，短波长激光仍然有利于keV X射线的发射。     

120 kV下常压空气纳秒脉冲电晕放电特性

使用上升沿15 ns、脉宽30~40 ns的重复频率纳秒脉冲电源对120 kV下大气压空气中管-板电极结构电晕放电进行了实验研究,通过电压电流测量、放电图像拍摄和X射线探测分析了纳秒脉冲电晕放电特性。结果表明:纳秒脉冲电晕放电中存在X射线辐射,但辐射强度较弱,X射线辐射计数随着气隙距离的增大而减少,随着脉冲重复频率的增大而增多;放电空间的残余电荷加强了下一个脉冲到来时的局部电场,从而导致高重复频率下易于出现分散的电晕通道。     

“阳”加速器钛丝X箍缩光源辐射特性实验研究

为了利用X箍缩产生的点光源作为背光光源对丝阵Z箍缩内爆早期的负 载内部结构进行背光照相, 在"阳"加速器(电流峰值为500-800 kA, 上升时间约80 ns)上开展了钛丝(丝交叉角度为60°) X箍缩光源辐射特性的初步实验研究. 通过X射线二极管探测器、透射光栅谱仪、晶体谱仪和狭缝相机等诊断设备获取了箍缩点光源的辐射功率达到1.5 GW左右, 辐射能量约为1 J, 光子能量为keV量级的辐射能谱范围主要集中在1-4 keV能段, 点光源尺寸小于15 μm, 其时间尺度(辐射脉冲半高宽)达到了200 ps. 对光源特征信息进行了初步分析, 同时掌握了有效获取钛丝X箍缩单脉冲点光源的方法.     

不同条件下大气压脉冲弥散放电特性

利用上升沿约0.5 s、半高宽约6 s、幅值可达40 kV的微秒脉冲电源和上升沿约150 ns、半高宽约300 ns、幅值可达50 kV的纳秒脉冲电源激励大气压弥散放电,并分别采用刀型和锯齿电极放电。通过电压电流测量和发光图像拍摄,改变施加电压种类、脉冲重复频率、高压电极结构和气隙距离等参数,研究了不同条件下弥散放电特性。实验结果表明:纳秒脉冲电源和微秒脉冲电源均能在大气压空气中激励大面积的弥散放电,弥散放电面积最大达90 cm2;放电的均匀性受脉冲参数与电极形状影响显著,其中刀型电极条件下纳秒脉冲激励的弥散放电均匀性最佳;相同条件下纳秒脉冲弥散放电的瞬时功率大于微秒脉冲弥散放电,最高可达275 kW,而纳秒脉冲弥散放电的能量小于微秒脉冲弥散放电;保持其他条件不变,弥散放电传导电流幅值随着气隙距离的增加而降低,放电强度随着脉冲重复频率的增加而增强,弥散放电的工作电压范围随着脉冲重复频率的增加显著降低。因此在低频、刀型电极结构中易于获得均匀与较大工作电压范围的大气压弥散放电。     

500 kV亚纳秒脉冲X光机研制

高能量亚纳秒脉冲X射线可用于精确定标辐射探测系统的时间响应特性和探测灵敏度,对于脉冲辐射场诊断技术研究具有重要意义。研制了一种小型化、可移动的亚纳秒脉冲X射线源,装置占地区域尺寸为1.2 m×40 cm,重142 kg。设计了双锥形绕组同轴结构变压器,采用新颖的三谐振变压技术,将纳秒脉冲形成线充电至520 kV,通过油介质自击穿开关放电,输出370 kV,3.8 ns的高压脉冲。设计了多级过匹配传输线提升输出电压,并利用油介质peaking-chopping开关进行脉冲压缩,产生了峰值520 kV、半高宽0.5 ns、功率1.8 GW的高压脉冲。基于Child-Langmuir定理,设计了刀刃阴极X光管,在520 kV电压下的运行阻抗为150 Ω,管前20 cm处的X射线峰值能量注量率约为1×1016 MeV·cm-2·s-1,累积照射量为4.1 mR。     

毛细管快放电中的管壁烧蚀

TN248.6 2004064116 毛细管快放电中的管壁烧蚀=Experimental investigation on ablation characteristics of capillary discharge[刊,中]/程元丽(哈尔滨工业大学可调谐激光技术国家级重点实验室.黑龙江,哈尔滨(150001)),赵永蓬…∥中国激光.—2004,31(5).—538-542 介绍了一台毛细管快放电软X射线激光实验装置,包括Marx发生器,Blumlein传输线和毛细管放电室等。在充入相同气压的气体(70～90Pa)和相同主脉冲(电流峰值30kA,上升沿30ns)放电条件下,通过观测其放电后的真空紫外波段辐射,对分别采用该装置固有的幅值     

采用靶摆式探头测量软X射线辐射材料产生的喷射冲量

 采用靶摆旋转运动式冲量探头，在“强光一号”加速器上，测量了脉冲软X射线辐射材料产生的喷射冲量。实验结果表明，在平均能量为0.21 keV、平均半高宽为36 ns和平均能注量为143 J/cm²的软X射线辐射下，测得一种复合材料的喷射冲量耦合系数为0.70 Pa·s/(J·cm^-2)。     

Investigation of the X-ray radiation produced by a diffusive discharge in the rod-plane geometry under atmospheric pressure

Spatial parameters of the X-ray radiation produced by a high-voltage nanosecond discharge evolving in air under atmospheric pressure in the rod (cathode)-plane electrode system with a 10-cm electrode spacing are studied experimentally. A ∼170-ns voltage pulse with an amplitude of ∼200 kV and 10-ns rise time is applied to the cathode. The photoelectronic method is used to study, under the same conditions, the integrated (over the gap) characteristics of the radiation, in particular, the duration of its generation. It is found that, when the size of the X-ray source is not smaller than that of the discharge region of diffusive luminescence, radiation from the cathode region of the gap is primarily observed (i.e., from the region where the electric field distribution is sharply inhomogeneous). The X-ray generation is usually observed after the bridging of the discharge gap, the X-ray pulse having a rise time of ∼3 ns, a duration of ∼10 ns, and an effective radiation energy of ∼6 keV.     

紧凑型重复频率高压纳秒脉冲电源及其仿真模型

纳秒脉冲等离子体在诸多实际的工程应用中依赖于小型化且可靠的纳秒脉冲电源实现。设计了一种紧凑型全固态高压纳秒脉冲电源,该电源主要由直流电源部分、绝缘栅双极晶体管及其驱动控制电路、可饱和脉冲变压器、磁脉冲压缩网络等组成。通过理论计算分析、PSpice电路仿真以及实验研究表明,其最终可以在800 的输出负载阻抗上获得幅值40 kV、脉冲宽度100 ns左右、脉冲上升沿约50 ns的高电压脉冲,重复频率最高可达5 kHz。     

Efficiency of a planar diode with an explosive emission cathode under the conditions of delayed plasma formation

The energy and current balances in the diode unit of a high-current pulsed electron accelerator (350–500 keV, 60 ns, 250 J per pulse) are compared for an explosive emission cathode (made of graphite, copper, or carbon felt) and a multipoint (graphite or copper) cathode. The planar diode with the continuous cathode is shown to be more efficient in terms of conversion of the applied energy to electron energy (more than 90%) despite a delay in the plasma surface formation. With the impedance of the planar diode matched to the output resistance of the nanosecond generator, the efficiency of the diode does not depend on the time of plasma formation on the cathode. In the case of the graphite cathode, the plasma formation delay reduces the fraction of slow electrons in the forming electron beam and reduces electron losses in anode foil when the beam is extracted from the vacuum space of the diode chamber into the reactor.     

Analysis of Plasmadynamic Nanosecond-Range Processes in the Formation of Shock Waves from Pulsed Discharges

We have studied the dynamics of the plasma glow of pulsed discharges (sliding surface discharge and combined volume discharge with plasma electrodes) in the nanosecond range (100–12 000 ns) in stationary air and in the flow behind the front of a plane shock wave with Mach numbers 1.7–5.0 in the shock tube channel. The temporal characteristics of the flow, the radiation spectra, and the discharge currents in air are compared in the pressure range 5–150 Torr, a pulsed voltage of 20–30 kV, and a current of about 1 kA. It is shown that the time of current under various conditions does not exceed 400 ns, and the duration of the glow can reach a few microseconds. It is shown that as a result of energy supply near the planar shock wave front, the decay of discontinuities occurs with the formation of shock waves and contact surfaces. The positions of the plasma glow regions are compared with the positions of discontinuity surfaces of numerically calculated gasdynamic parameters in the flow.     

Ultrashort electron beam and volume high-current discharge in air under the atmospheric pressure

Conditions are studied under which an electron beam and a volume discharge with a subnanosecond rise time of a voltage pulse are produced in air under atmospheric pressure. It is shown that the electron beam appears in a gas-filled diode at the front of the voltage pulse in ∼0.5 ns, has a half-intensity duration of ≤0.4 ns and an average electron energy of ∼0.6 of the voltage across the gas-filled diode, and terminates when the voltage across the gap reaches its maximum value. The electron beam with an average electron energy of 60 to 80 keV and a current amplitude of ≥70 A is obtained. It is assumed that the electron beam is formed from electrons produced in the gap due to gas ionization by fast electrons when the intensity of the field between the front of the expanding plasma cloud and the anode reaches its critical value. A nanosecond volume discharge with a specific power input of ≥400 MW/cm³, a density of the discharge current at the anode of up to 3 kA/cm², and specific energy deposition of ∼1 J/cm³ over 3 to 5 ns is created.     

Mechanically driven millimeter source of nanosecond X-ray pulses

The emission of nanosecond pulses of ≈20 keV photons having a total energy of GeVs which are generated by peeling millimeter wide strips of pressure sensitive adhesive (PSA) tape in a partial pressure of air (≈10⁻³ Torr) is demonstrated. The X-ray spectrum is similar to that obtained by peeling much wider bands of PSA, implying that the characteristic length for the sequence of processes that govern this phenomenon is less than 1 mm. These experiments demonstrate that MEMS-type X-ray generators are technologically feasible.     

基于连续谱的氩气纳秒脉冲放电中电子温度的研究

采用了一种针对针的放电结构,将其放置在一个高纯氩气的密闭腔室中,通过施加正极性的过电压产生可重复的大气压纳秒脉冲放电,并提出建立大气压放电的连续辐射模型来诊断氩气纳秒脉冲放电中的电子温度。实验利用电压和电流探头分别获取放电过程中的电压和电流波形图,其放电脉宽约为20 ns。通过消色差透镜、单色仪和ICCD等光学系统的组合来测量放电正柱区在不同时刻（0<t<20 ns）的时间分辨发射光谱。结果表明,放电中连续谱的强度随时间先增加（0<t<10 ns）后减小（10 ns<t<20 ns）,但是氩原子的谱线强度则随时间的增加而一直增大。研究表明连续谱强度与电子密度成正相关,因而电子密度随着时间也是先增加而后减小,这与放电电流的变化规律是完全一致的。根据连续谱模型拟合得到放电过程中（0<t<10 ns）的电子温度为(1.4±0.2) eV。随着驱动电压的下降（10 ns<t<20 ns）,电子温度逐步减小至0.9 eV。在0<t<10 ns中,激发态氩原子主要是由电子碰撞激发产生的,因而谱线强度随着电子密度的增加而增大。然后,随着电子温度的减小,Ar+₂复合反应速率激增,导致电子与离子的复合过程主导产生激发态氩原子,即谱线强度继续增大。通过加入0.5%的水蒸气以获取OH的振转光谱。实验发现,OH(A)的产生机制使其偏离玻尔兹曼平衡分布,本文采用了双温的OH(A-X)光谱模型来考察气体温度。在放电过程中,气体温度保持不变,大约为400 K。此外,水蒸气的加入使得短波长的连续谱发生显著增强。光谱分析认为H₂O在放电中能够解离产生H₂,继而与氩原子的亚稳态发生能量转移生成激发态H₂(a3Σ+_g)。H₂(a3Σ+_g)将会自发辐射跃迁到排除态H₂(b3Σ+_u),同时发射短波长的连续谱。由于短波长的连续谱对电子温度（T_e>1 eV）的响应较为灵敏,所以载气中少量的水蒸气将会对连续谱诊断电子温度带来较大的影响。     

Possible application of a volume avalanche discharge initiated by an electron beam for designing a krypton dimer laser

The emissive and amplifying properties of volume nanosecond discharge plasma formed at high pressures in krypton have been investigated theoretically and experimentally. It is theoretically shown that lasing can be obtained in this type of discharge at krypton pressures exceeding 6–7 atm. In the discharge gap with a cathode of small curvature radius, volume discharge without preliminary ionization is obtained at a high pressure and intense krypton dimer emission that peaks at a wavelength of 146 nm is detected. It is shown that, upon excitation by a volume avalanche discharge initiated by an electron beam, no less than 90% of the energy in the range 120–540 nm is emitted by krypton dimers. At a krypton pressure of 1.5 atm, an energy of ～30 mJ of spontaneous emission into the total solid angle and a radiation pulse width at half maximum of ～240 ns are obtained.