利用质子能损诊断部分电离等离子体靶中的束缚电子密度

部分电离等离子体是惯性约束聚变燃料及天体等离子体中的重要组成部分,该等离子体的输运及流体力学等性质受到束缚电子的显著影响,然而当前基于光谱学的技术手段难以对其进行高精度诊断.本文基于中国科学院近代物理研究所低能离子束与等离子体相互作用实验平台,精确测量了100 ke V质子束穿过部分电离氢等离子体靶后的能损,该能损是质子同靶区内自由电子与束缚电子碰撞共同作用的结果.利用已有的能损理论模型,结合激光干涉诊断获得的自由电子密度信息,最终得到了部分电离氢等离子体靶中沿离子路径上的束缚电子密度,并给出了该等离子体的离化度参数.该离子束诊断技术具有在线、原位、分辨率高等优势,为解决部分电离等离子体内部束缚电子密度的诊断问题提供了新的途径.     

低能质子束在氢等离子体中的能损研究

实验测量了100 keV的质子束穿过部分电离氢等离子体靶后的能量损失. 等离子体靶由气体放电方式产生, 其自由电子密度在10¹⁶ cm^-3量级, 电子温度约1–2 eV, 维持时间在微秒量级. 研究结果表明: 质子束在等离子体靶中的能量损失与自由电子密度密切相关且明显大于在同密度条件下中性气体靶中的能量损失; 在自由电子密度达到峰值处, 通过实验结果计算得到此时的自由电子库仑对数约为10.8, 与理论计算结果符合较好, 该值比Bethe公式给出的中性气体靶中束缚电子库仑对数高4.3倍,相应的能损增强因子为2.9.     

质子轰击钨靶表面时的电子发射产额与靶温度的相关性研究

在中国科学院近代物理研究所兰州重离子加速器国家实验室测量了能量为50~250 keV 的质子入射不同温度下钨靶表面的电子发射产额。实验结果发现,不同能量的质子引起的电子发射产额均随着靶温度的升高而降低;利用功函数对温度的依赖性定性地解释了该结果。在不同靶温度下,总电子发射产额与电子能损的比值随着质子能量的增加而逐渐变小;利用靶原子不同壳层中电子之间的电离竞争机制来解释实验结果。     

极化效应对Bohr速度能区O5+离子在低密度氢等离子体中的能损影响

基于HIRFL加速器装置的低能束实验平台,实验测量了1.07 MeV(～66.9 keV/u)高电荷态O⁵⁺离子穿过中性氢气和部分电离的低密度氢等离子体靶后的能量损失,观测到等离子体中离子能损减小的新实验现象.分别考虑部分电离等离子体对炮弹离子的电荷屏蔽效应以及靶区原子的极化效应(Barkas修正),重新计算了离子能损,讨论了离子能损减小的可能物理机制.研究结果表明:在部分电离的低密度等离子体中,靶区的原子极化效应将显著影响Bohr速度能区离子的能量损失过程.     

类氦C离子诱发不同金属厚靶原子的K-X射线

利用中国原子能科学研究院HI-13MV串列加速器上提供的动能为15—55 MeV的类氦C离子分别轰击Fe, Ni, Nb和Mo金属厚靶,采用HpGe探测器测量了K-X射线,获得了相应的K-X射线的发射截面.本文中由于各个靶原子外壳层电离度的不同,类氦C离子与Fe, Ni靶原子相互作用发射的K_β与K_α X射线的分支强度比随入射离子动能增加而减小,而Nb, Mo靶原子发射的K-X射线分支强度比变化不明显.利用厚靶截面公式计算了靶原子K-X射线的发射截面,并与不同的理论模型及质子的结果进行了对比.结果表明随类氦C离子动能的增大, Fe, Ni靶原子发射的K_β与K_α X射线的总产生截面与考虑多电离的两体碰撞近似修正模型最为符合Nb, Mo靶原子发射的K_β与K_α X射线的总产生截面与平面波恩近似模型的理论值最为接近.质子与单核子C离子能量相同时,质子比类氦C离子激发不同靶的K-X射线产生截面约小3个数量级.     

低能He2+-He碰撞体系转移电离机理的实验研究

利用冷靶反冲离子动量谱仪,对低能He²⁺-He碰撞反应中产生的反冲靶离子和炮弹离子进行了符合测量,根据反冲靶离子的动量,研究了转移电离过程中的电荷转移机理.实验结果表明：在20—40 keV能量范围内,靶原子上的一个电子俘获到炮弹离子的基态,另一个电子直接发射到靶的连续态的直接电离及另一个电子俘获到炮弹离子的连续态的过程(ECC)是最主要的转移电离机理,且ECC过程主要发生在大碰撞参数条件下;炮弹离子俘获两个电子处在双激发态的自电离过程的贡献很小. 关键词： 冷靶反冲离子动量谱仪 转移电离机理 离子原子碰撞     

不同离子激发Au靶的多电离效应

重离子与固体表面相互作用时,会引起靶原子内壳层的电离,相应空穴退激过程中发射的X射线对研究重离子与固体表面的相互作用有着重要意义,可为相关研究提供基础数据.目前,在K和L壳层电离方面做了一些工作,而M壳层的研究较少,本文依托兰州重离子加速器国家实验室320 kV高电荷态离子综合研究平台,测量了不同能量的H~+, Ar~(8+), Ar~(12+), Kr~(13+)和Eu~(20+)离子与Au表面作用产生的特征X射线谱及其能移,计算了X射线的产额比值.结果表明:重离子引起了靶原子内壳层的多电离,多电离效应使Au的MX射线有不同程度的能移;多电离程度取决于入射离子能量、离子的原子序数和其外壳层的空穴数量.     

高能质子在散裂靶中的能量沉积计算与实验验证

高能质子在散裂靶中的能量沉积是散裂靶中子学研究的重要内容之一,准确掌握高能质子在散裂靶中引起的能量沉积分布与瞬态变化是开展散裂靶热工流体设计的重要前提.本文采用MCNPX,PHITS与FLUKA三种蒙特卡罗模拟程序,计算并比较了高能质子入射重金属铅靶、钨靶的能量沉积分布及不同粒子对总能量沉积的占比贡献;针对高能质子入射金属钨靶的能量沉积实验数据空白,采用热释光探测器阵列测量了250 MeV质子束入射厚钨靶的能量沉积分布,实验结果表明蒙特卡罗模拟程序在散裂靶中能量沉积的计算结果具有较高的可靠性.     

拍瓦激光驱动纳米刷靶高品质质子束的产生

超强激光加速产生的高能质子束源在基础物理研究、材料科学、生物医疗等领域具有广泛应用前景。基于激光聚变研究中心的SILEX-II装置,开展了高对比度飞秒激光驱动纳米刷靶质子加速实验研究。采用等离子体镜技术进一步提升激光对比度,有效降低了预脉冲对纳米刷靶结构的影响。相比于平面靶,采用纳米刷靶质子截止能量提高到1.5倍,质子束产额增加近一个量级,成功验证了超高功率密度下纳米刷靶对激光离子加速的增强效果,并且有效提升了质子束空间分布的均匀性。研究结果为高品质质子束源的产生和应用提供了技术途径。     

HIRFL-CSR团簇靶的优化

为HIRFL-CSR团簇内靶设计加工了新的供气系统, 拆换了原有的喷嘴, 对氢气和氩气进行了新的实验, 获得了氢气和氩气的稳定团簇束, 解决了困扰团簇靶稳定运行的喷嘴堵塞问题. 获得的氢团簇束密度为1.75×10¹³ atoms/cm³, 好于德国GSI内靶对氢束所达到的1×10¹³atoms/cm³. 研究了团簇束的衰减, 测量了氢束和氩束的有效靶厚, 研究了团簇靶系统对这两种工作气体的长期运行稳定性. 对氢和氩两种工作气体, 各级气压呈现了良好的稳定性, 说明在实验的时间范围内, 团簇靶运行稳定.     

The Hydrogen Dissociator

A performance analysis is presented for the hydrogen dissociator used in hydrogen masers to provide a beam of atomic hydrogen. An analysis of the discharge characteristics yields relations for electron temperature as a function of vessel size and gas pressure and for plasma density as a function of power input. Also a relation between ion impact energy at the wall and electron temperature is derived. For a typical dissociator (2" diameter, 0.1 Torr hydrogen pressure, and 4 watt input power) these relationships yield an electron temperature of 39,000°K, a plasma density of 1011 cm-3 and an ion impact energy of 20 volts. The dissociation rate is calculated using published cross-sections. Assuming a recombination rate of 4 × 10-3, the analysis yields an atomic hydrogen density of about 1014 cm-3, a degree of dissociation of 2%, and an atomic beam flux of 1.3 × 1018 cm-2 × sec-1 for the example quoted. This beam flux is in good agreement with estimated values for hydrogen masers. A coefficient for performance ? is derived for the hydrogen dissociator, defined as the ratio of atomic beam flux to discharge power consumption. It is shown that ? is a function of the electron temperature and has a maximum at 87,000°K. It is concluded from this analysis that the discharge in presently used hydrogen dissociators is well optimized given the pressure constraints of the system.     

Spatial and Temporal Characteristics of an Excimer Laser-Induced Lead Plasma Emission

A laser-induced plasma was generated from a lead target using an ArF excimer laser (λ = 193 nm) and characterized by time-resolved and time-integrated spatially resolved spectrometry. The ambient atmosphere (gas composition and pressure) influenced the emission intensity for both atomic and ionic lines. The emission of laser-induced lead plasma varied with time as well as the location in the plasma. Lead ion emission decayed more rapidly than lead atomic line emission. High excitation temperatures and nonlinear optical phenomena were observed in the laser-induced lead plasma. Gas breakdown and the subsequent shielding effect of the incident laser beam in different ambient gas compositions and pressures were discussed to explain the different efficiency of metal ion for matioa in the plasma. The experimental results show that spatial discrimination of the laser-induced plasma emission is desirable for direct spectrochemical analysis.     

Gas mixture effects on ion‐beam flux characteristics from dense plasma focus device: Investigation by the Vlasov–Maxwell equations and experiments

The Vlasov–Maxwell equations were numerically solved to calculate the ion‐beam flux from the plasma of argon and the plasma of mixtures of argon and neon. Some experiments were performed to measure the ion beam from the Amirkabir plasma focus (APF) device. The calculations have shown that the argon ion‐beam flux peaked up to 1.928 × 10³⁰ ions m^?2 s^?1 at the optimum pressure of 1.866 mbar while the neon‐argon mixture's ion‐beam flux reached a maximum of 4.301 × 10³⁰ ions m^?2 s^?1 for 15% neon admixture at the optimum pressure of 1.866 mbar. The calculated kinetic energy of the ion beam has shown a maximum value of 708.7 J for the mixture of 85% argon‐15% neon at the mentioned optimum pressure.     

Growth of electron energies with ion beam injection in a double plasma device

This paper reports about the observed energy growth of both high and low energetic electron species in the target plasma region with the increase in plasma potential in the source region of a double plasma device. This situation can be correlated to the injection of an ion beam from source to target plasma region. Plasma is solely produced in the source region and a low-density diffuse plasma is generated in the target region by local ionization between the neutral gas and the high energetic electrons coming from the source region. The growth of electron energy is accompanied by a decrease in diffuse plasma density. It is observed that although energy of high energetic group increases with the injected beam energy, the diffuse plasma density falls due to their decreasing population.     

静电探针测量强流电子束电离气体产生的等离子体密度

 用电离理论和核物理学中讨论电子束通过介质后的能量损耗方法分别估算了强流电子束电离中性气体产生的等离子体的密度。在实验中将静电探针应用于测量强流电子束电离氮气产生的等离子体的密度,得出等离子体密度随气压变化的曲线。实验结果表明在1～15帕气压范围内,等离子体密度在量级,与理论结果相符,证明静电探针用于诊断强流相对论电子束电离中性气体产生的等离子体的密度是可行的。     

Beam-Plasma Heating Model

A one-dimensional ionization and heating model is applied to results of several electron-beam-plasma interaction experiments. Beam energy is deposited resistively in the plasma at a rate ?j2, where j is the return current density and ? the plasma resistivity both classical and anomalous due to ion acoustic or e-e-mode turbulence. Principal energy losses include ionization, line radiation, inelastic electron impact excitation, bremsstrahlung and radiative recombination. The level of ionization and plasma heating are computed as a function of neutral gas pressure, beam rise time, pulsewidth and current density, and resistivity model. Plasma dynamics and kinetic effects such as expansion and end loss are not explicitly included in the model.     

Ionization of iridium ions in the Dresden EBIT studied by X-ray spectroscopy of direct excitation and radiative recombination processes

Ir^q+ ( 41≤q≤64) ions with open-shell configurations have been produced in the electron beam of the room-temperature Dresden Electron Beam Ion Trap (Dresden EBIT) at electron excitation energies from 2 keV to 13 keV. X-ray emission from direct excitation processes and radiative capture in krypton-like to aluminium-like iridium ions is measured with an energy dispersive Si(Li) detector. The detected X-ray lines are analyzed and compared with results from multiconfigurational Dirac-Fock (MCDF) atomic structure calculations. This allows to determine dominant produced ion charge states at different electron energies. The analysis shows that at the realized working gas pressure of 5×10^-9mbar for higher charged ions the maximum ion charge state is not preferently determined by the chosen electron beam energy needed for ionization of certain atomic substates, but by the balance between ionization and charge state reducing processes as charge exchange and radiative recombination. This behaviour is also discussed on the basis of model calculations for the resulting ion charge state distribution. Received 12 July 2001 and Received in final form 10 September 2001     

Material ablation and plasma state for single and collinear double pulses interacting with iron samples at ambient gas pressures below 1 bar

The influence of the ambient gas pressure on ablated material and the plasma state of laser-inducedplasmas using single and collinear double pulses is studied. A Q-switchedNd:YAG laser pulse is focused on iron samples in air at pressures ranging from 0.1 to 1013 mbar. At 1013 mbar double pulses ablate up to a factor of 4.6 more material than single pulses of the same energy. Whereas between 1013 mbar and 100 mbar a strong variation of the ablation ratio is found, the pressure dependence vanishes below 100 mbar. Electron densities in the plasma are always higher for single pulses and increase monotonously with higher ambient pressures. While the electron density for single and double pulses at 1013 mbar is close to the same value of about 10¹⁷ cm^-3, the electron temperature is more than 1500 K higher for double pulses. The plasma state achieved with double pulses at atmospheric pressure is similar to that obtained with single pulses of the same energy at a pressure of 100 mbar. Thus the effect of double pulses can be interpreted as a “laser-generated vacuum” in terms of the formation of a transient localized low particle density region in the interaction region. PACS 52.38.Mf; 78.47.+p; 42.62.Fi     

气体压强对纳秒激光诱导空气等离子体特性的影响

环境气体的压强对激光诱导等离子体特性有重要影响.基于发射光谱法开展了气体压强对纳秒激光诱导空气等离子体特性影响的研究,探讨了气体压强对空气等离子体发射光谱强度、电子温度和电子密度的影响.实验结果表明,在10-100 kPa空气压强条件下,空气等离子体发射光谱中的线状光谱和连续光谱依赖于气体压强变化,且原子谱线和离子谱线强度随气体压强的变化有明显差别.随着空气压强增大,激光击穿作用区域的空气密度增加,造成激光诱导击穿空气几率升高,从而等离子体辐射光谱强度增大.空气等离子体膨胀区域空气的约束作用,增加了等离子体内粒子间的碰撞几率以及能量交换几率,并且使离子-电子-原子的三体复合几率增加,因此造成原子谱线OⅠ777.2 nm与NⅠ821.6 nm谱线强度随着气体压强增大而增大,在80 kPa时谱线强度最高,随后谱线强度缓慢降低.而离子谱线N Ⅱ 500.5 nm谱线强度在40 kPa时达到最大值,气体压强大于40 kPa后,谱线强度随压强增加而逐渐降低.空气等离子体电子密度均随压强升高而增大,在80 kPa后增长速度变缓.等离子体电子温度在30 kPa时达到最大值,气体压强大于30 kPa后,等离子体电子温度逐渐降低.研究结果可为不同海拔高度的激光诱导空气等离子体特性的研究提供重要实验基础,为今后激光大气传输、大气组成分析提供重要的技术支持.