Enhanced laser-induced plasma channels in air

Plasma is a significant medium in high-energy density physics since it can hardly be damaged. For some applications such as plasma based backward Raman amplification(BRA), uniform high-density and large-scale plasma channels are required. In the previous experiment, the plasma transverse diameter and density are 50–200 μm and 1–2 × 10~(19)cm~(-3),here we enhance them to 0.8 mm and 8 × 10~(19)cm~(-3), respectively. Moreover, the gradient plasma is investigated in our experiment. A proper plasma gradient can be obtained with suitable pulse energy and delay. The experimental results are useful for plasma physics and nonlinear optics.     

A two-component model for the electron distribution function in ahigh-current pseudospark or back-lighted thyratron

Temperature, energy, and densities of two electron distribution function components, including an isotropic bulk part and an anisotropic beam, are analyzed for a hydrogen pseudospark and/or back-lighted thyratron switch plasma with a peak electron density of 1-3×10¹⁵ cm^-3 and peak current density of ≈10⁴ A/cm². Estimates of a very small cathode-fall width during the conduction phase and high electric field strengths lead to the injection of an electron beam with energies ⩾100 eV and density of 10¹³-10¹⁴ cm^-3 into a Maxwellian bulk plasma. Collisional and radiative processes of monoenergetic beam electrons, bulk plasma electrons and ions, and atomic hydrogen are modeled by a set of rate equations, and line intensity ratios are compared with measurements. Under these high-current conditions, for an initial density n_H2=10¹⁶ cm^-3 and electron temperature of 0.8-1 eV, the estimated beam density is ≈10¹³ -10¹⁴ cm^-3. These results suggest the possibility of producing in a simple way a very high-density electron beam     

等离子体密度对激光拉曼放大机理的影响

等离子体中的背向拉曼散射机理可以用来产生超短超强的激光脉冲. 本文采用粒子模拟方法模拟研究了等离子体密度对激光拉曼放大过程的影响. 研究发现, 过低的等离子体密度会导致等离子体波提前波破而降低能量转换效率; 而过高的等离子体密度又会导致其他不稳定性的快速增长, 限制作用距离和输出能量. 因此, 拉曼放大机理的最佳等离子体密度应处于等离子体波破的密度阈值附近, 可以获得最高的能量转换效率和能量输出. 另外, 空间频谱分析显示放大激光的强度饱和主要来自于自相位调制不稳定性的发展. 利用10¹³ W·cm^-2的抽运激光脉冲, 模拟证实拉曼放大机理可有效地将种子激光的强度从10¹³ W·cm^-2 放大到10¹⁷ W·cm^-2, 脉宽压缩到40 fs, 且能量转换效率达到58%.     

退火对Pb辐照Al2O3单晶发光特性的影响

研究了230MeV的²⁰⁸Pb²⁷⁺辐照Al₂O₃样品及随后在600,900,1100K高温条件下退火后的光致发光特性。从辐照样品的测试结果可以清楚地看到在波长为390,450nm处出现了强的发光峰。辐照量为1×10¹³ions/cm²时,样品的发光峰最强。经过600K退火2h后测试结果显示,380nm发光峰剧烈增强,而其他发光峰显示不明显。在900K退火条件下,380nm的发光峰开始减弱,而在360,510nm出现了明显的发光峰,至到1100K退火完毕后380nm的发光峰完全消失,而360,510nm的发光峰相对增强。从被辐照样品的FTIR谱中看到,波数在460~510cm^-1间的吸收是振动模式,经过离子辐照后,吸收带展宽,随着辐照量的增大,Al₂O₃振动吸收峰消失,说明Al₂O₃振动模式被完全破坏。1000~1300cm^-1之间为Al—O—Al桥氧的伸缩振动模式,辐照后吸收带向高波数方向移动,说明其振动模式受到影响。辐照剂量较小的样品,损伤程度相对较低,经退火晶化后,振动模式基本恢复到单晶状态;辐照剂量较高的样品,损伤程度大,退火处理后表面变得较粗糙,振动模式并未出现,说明结构破坏严重。     

Photo-ionized lithium source for plasma accelerator applications

A photo-ionized lithium source is developed for plasma acceleration applications. A homogeneous column of lithium neutral vapor with a density of 2×10^15-3 is confined by helium gas in a heat-pipe oven. A UV laser pulse ionizes the vapor. In this device, the length of the neutral vapor and plasma column is 25 cm. The plasma density was measured by laser interferometry in the visible on the lithium neutrals and by CO₂ laser interferometry on the plasma electrons. The maximum measured plasma density was 2.9×10 ¹⁴ cm^-3, limited by the available UV fluence (≈83 mJ/cm²), corresponding to a 15% ionization fraction. After ionization, the plasma density decreases by a factor of two in about 12 μs. These results show that such a plasma source is scaleable to lengths of the order of 1 m and should satisfy all the requirements for demonstrating the acceleration of electrons by 1 GeV in a 1-GeV/m amplitude plasma wake     

激光诱导等离子体对水OH伸缩振动受激拉曼散射的影响

利用532 nm脉冲激光进行水的受激拉曼散射研究,通过改变激光焦点与水-空气界面的距离,获得截然不同的OH伸缩振动受激斯托克斯和反斯托克斯谱线.焦点距水-空气界面大于20 mm时,只存在±3400cm-1的斯托克斯和反斯托克斯谱线;焦点距离水-空气界面小于20 mm时,存在±3000和±3400cm-1的斯托克斯和反斯托克斯谱线;继续缩小焦点与水-空气界面的距离,3000 cm-1谱线被增强,而3400 cm-1谱线被削弱.研究结果表明,激光诱导水产生的等离子体增强了局部水分子的氢键,导致OH伸缩振动红移,同时过剩电子增强了水的OH伸缩振动受激拉曼散射.     

水的低频受激拉曼散射

利用532 nm的脉冲激光进行了水的受激拉曼散射研究. 水的低激发光能量下主要表现为受激Stocks和反Stocks 3426 cm^-1谱线; 在能量大于140 mJ时出现低频率313 cm^-1受激Stocks谱线, 同时出现后向3389和3268 cm^-1 的受激Stocks谱线. 实验结果表明, 在较强的激光作用下, 水的结构表现为冰的Ⅷ相.     

温稠密钛电导率计算

温稠密物质是惯性约束核聚变、重离子聚变、Z箍缩动作过程中物质发展和存在的重要阶段. 其热力学性质和辐射输运参数在聚变实验和内爆驱动力学模拟过程中有至关重要的作用. 本文通过建立非理想Saha方程, 结合线性混合规则的理论方法模拟了温稠密钛从10^-5-10 g·cm^-3, 10⁴ K到3×10⁴ K区间的粒子组分分布和电导率随温度密度的变化, 其中粒子组分分布由非理想Saha方程求解得到. 线性混合规则模型计算温稠密钛的电导率时考虑了包括电子、原子和离子之间的多种相互作用. 钛的电导率的计算结果与已有的爆炸丝实验数据相符. 通过电导率随温度密度变化趋势判断, 钛在整个温度区间, 密度0.56 g·cm^-3时发生非金属相到金属相相变. 对于简并系数和耦合系数的计算分析, 钛等离子体在整个温度和密度区间逐渐从弱耦合、非简并状态过渡到强耦合部分简并态.     

Plasma guiding and wakefield generation for second-generationexperiments

A design study has been carried out for a second-generation experiment on laser guiding and wakefield excitation in a channel. From simple scaling laws for the wakefield amplitude, dephasing length, the relativistic group velocity factor γ_g, and energy gain with and without guiding, we find that the parameter regime for a compact single stage GeV accelerator favors laser systems producing short pulses (10 fs⩽τ⩽100 fs), each containing an energy on the order of 100 mJ to a few J's. Taking the dephasing length as the maximum acceleration distance, plasma channels with lengths of 1-10 cm and densities of 10¹⁷-10¹⁹ cm^-3 need to be produced; whereas the design study has been primarily concerned with diffraction and channel guiding, dephasing and depletion limits, and linear wakefield theory, aspects of the effect of the plasma wave on the evolution of the laser pulse are discussed. We find that transverse and longitudinal pulse distortions could indeed affect the generated plasma wave phase velocity and amplitude, and hence may limit the achievable energy gains over the one-dimensional (1-D) linear estimates. Some issues for experiments on prototype small accelerators (100 MeV-1 GeV, cm scale) are also discussed     

液相射流中中Al元素单双脉冲LIBS研究

为了综合比较单双脉冲激光诱导击穿光谱技术(LIBS)在液体中重金属元素的检测效果,利用自建的液相射流单-双脉冲LIBS技术装置,对AlCl₃水溶液中的Al元素LIBS特性进行测量和分析。实验中使用两台532 nm Nd∶YAG激光器作为激发光源,等离子体辐射信号通过光谱仪和ICCD进行采集。实验研究了单脉冲下Al(396.15 nm)发射谱线的谱线强度随激光能量、ICCD门延时、门宽之间的变化关系,获得了最优化实验参数激光能量为50 mJ,ICCD门延迟为1 200 ns,门宽为150 ns。在相同的实验条件下,实验考察了Al(369.15 nm)发射谱线的谱线强度随双脉冲之间的延时,激光总能量,ICCD门延时的变化关系,获得了最优化实验参数为两双脉冲之间的延时为1 000 ns,激光总能量为50 mJ,ICCD门延时为1 100 ns。单脉冲和双脉冲条件下获得重金属Al的LIBS检测限分别为26.79和10.80 ppm,双脉冲LIBS技术使元素检测限下降2倍多。实验结果表明双脉冲可以提升LIBS技术的探测灵敏度,为LIBS技术应用于水体中重金属快速检测提供了依据。     

Experimental study of a plasma-filled backward wave oscillator

Experimental studies of a plasma-filled X-band backward-wave oscillator (BWO) are presented. Depending on the background gas pressure, microwave frequency upshifts of up to 1 GHz appeared along with an enhancement by a factor of 7 in the total microwave power emission. The bandwidth of the microwave emission increased from ⩽0.5 GHz to 2 GHz when the BWO was working at the RF power enhancement pressure region. The RF power enhancement appeared over a much wider pressure range in a high beam current case (10-100 mT for 3 kA) than in a lower beam case (80-115 mT for 1.6 kA). The plasma-filled BWO has higher power output than the vacuum BWO over a broader region of magnetic guide field strength. Trivelpiece-Gould modes (T-G modes) are observed with frequencies up to the background plasma frequency in a plasma-filled BWO. Mode competition between the T-G modes and the X-band Tm₀₁ mode prevailed when the background plasma density was below 6×10¹¹ cm^-3 . At a critical background plasma density of ≃8×10¹¹ cm^-3 power enhancement appeared in both X-band and the T-G modes. Power enhancement of the S-band in this mode collaboration region reached up to 8 dB. Electric fields measured by the Stark-effect method were as high as 34 kV/cm while the BWO power level was 80 MW. These electric fields lasted throughout the high-power microwave pulse     

INFLUENCE OF C+-IMPLANTATION DOSE ON BLUE EMISSION AND MICROSTRUCTURES OF Si-BASED POROUS β-SIC

Crystal Si were implanted with different doses of C⁺ from 10¹¹ to 10¹⁷ cm^-2 at an energy of 50 keV. β-SiC precipitates were formed by thermal annealing at 1050 ℃ for 1 h and porous structures were prepared by electrochemical anodization. Under the excitation of ultraviolet, the samples, with C⁺ dose ≥10¹⁵ cm^-2 have intense blue emission which is stronger than the photoluminescence (PL) intensity of reference porous silicon (PS), and increases as C⁺ dose increases; the samples with C⁺ dose ≤10¹⁴ cm^-2 show similar PL spectra to those of PS. The blue peak intensity in PL spectra is correlated with the TO phonon absorption strength of β-SiC in infrared absorption spectra. The transmission electron microscopy study shows that the blue peak is also correlated with the microstructures. Because porous β-SiC is nanometer in size, it is suggested that the quantum confinement effect be responsible for the blue light emission.     

A simulation study on p-doping level of polymer host material in P3HT:PCBM bulk heterojunction solar cells

In this study,we investigate the influence of doping on the charge transfer and device characteristics parameters in the bulk heterojunction solar cells based on poly(3-hexylthiophene)(P3HT) and a methanofuUerene derivative(PCBM).Organic semiconductors are also known to be not pure and they have defects and impurities,some of them are being charged and act as p-type or n-type dopants.Calculations of the solar cell characteristics parameters versus the p-doping level have been done at three different n-dopings(N_d) that consist of 5 × 10~(17) cm~(-3),10~(18) cm~(-3),and 5 × 10~(18) cm~(-3).We perform the analysis of the doping concentration through the drift-diffusion model,and calculate the current and voltage doping dependency.We find that at three different n-dopant levels,optimum p-type doping is about N_p = 6 × 10~(18) cm~(-3).Simulation results have shown that by increasing doping level,V_(oc) monotonically increases by doping.Cell efficiency reaches its maximum at somewhat higher doping as FF has its peak at N_p = 3 × 10~(18) cm~(-3).Moreover,this paper demonstrates that the optimum value for the p-doping is about N_p = 6 × 10~(18) cm~(-3) and optimum value for n-dopant is N_d = 10~(18) cm~(-3),respectively.The simulated results confirm that doping considerably affects the performance of organic solar cells.     

Optical emission spectroscopy investigation of an ultra-short laser induced titanium plasma reheated by a ns laser pulse

The dynamics of a titanium plasma species, induced in air by coupling a fs-ablating laser pulse with an orthogonal ns-reheating laser source placed at the fixed distance of 1.0 mm from the target surface, has been followed by temporally resolved emission spectroscopy. The temporal evolutions of plasma features such as excitation temperatures and electron densities have been evaluated by using two different laser energies of the first fs-ablating laser pulse (0.8 mJ and 3.0 mJ). Optimum inter-pulse delay times, experimentally determined, of 250 μs and 500 μs were used for the fs laser energy of 3.0 mJ and 0.8 mJ, respectively. By experimental inspections of the main plasma species electronic transitions so obtained, a strong enhancement was evaluated up to one and two orders of magnitude for Ti(I) and Ti(II), respectively. Independently from the fs laser energy employed, the plasma features showed the same temporal behaviour implying that the ns-reheating characteristics of this process belong to the reheating mechanism itself. The experimental results have been discussed and the excited species evolutions and elementary processes involved, as well as, the local thermodynamic equilibrium departures, have been outlined.     

Electrical discharge in the Teflon cavity of a coaxial pulsedplasma thruster

In this work, we analyze the physical processes of a pulsed discharge in a dielectric (Teflon) cavity. This type of discharge is generated in a coaxial pulsed plasma thruster (PPT) having a central Teflon cavity to produce a high-pressure cloud of ablation products during the discharge pulse. The primary intended role of this model is to provide upstream boundary conditions for particle simulation codes used to study the exhaust plume. The main features of the electrical discharge in the dielectric cavity include Joule heating of the plasma, heat transfer to the dielectric, decomposition of the dielectric followed by partial ionization, and acceleration of the plasma up to the sound speed at the cavity exit. We consider a diffuse type of discharge assuming that all plasma parameters are uniform in the cavity. The system of equations is based on the plasma energy balance, thermal conductivity, dielectric ablation, and mass balance. It is found that most of the energy of the plasma column is carried off by particle convection to the dielectric and by radiation. It is found that during the pulse, the electron density peaks at about 10²⁴ m^-3 and decreases to 10²¹ m^-3 toward the end of the pulse, whereas the electron temperature peaks at about 2.2 eV and decays to 1.5 eV. Teflon surface temperature peaks at about 650 K. Predicted plasma temperature and ablated mass are found to be in agreement with available experimental data     

Dense pair plasma generation by two laser pulses colliding in a cylinder channel

An all-optical scheme for high-density pair plasmas generation is proposed by two laser pulses colliding in a cylinder channel. Two dimensional particle-in-cell simulations show that, when the first laser pulse propagates in the cylinder,electrons are extracted out of the cylinder inner wall and accelerated to high energies. These energetic electrons later run into the second counter-propagating laser pulse, radiating a large amount of high-energy gamma photons via the Compton back-scattering process. The emitted gamma photons then collide with the second laser pulse to initiate the Breit–Wheeler process for pairs production. Due to the strong self-generated fields in the cylinder, positrons are confined in the channel to form dense pair plasmas. Totally, the maximum density of pair plasmas can be 4.60 × 10~(27)m~(-3), for lasers with an intensity of 4×10~(22)W·cm~(-2). Both the positron yield and density are tunable by changing the cylinder radius and the laser parameters. The generated dense pair plasmas can further facilitate investigations related to astrophysics and particle physics.     

GdB3O6中Gd3+的级联发射特性

报道了Gd³⁺离子在GdB₃O₆基质中的光子级联发射特性。用Hitachi M850荧光分光光度计测定了Gd^{3+ 6}G_J能态的位置和Gd³⁺离子的光子级联发射光谱,Gd³⁺离子的第一个光子发射为⁶G_J→⁶P_J(～600nm)和⁶G_J→⁶I_J(～780nm),第二个光子的发射为⁶P_J→⁸S_7/2(～310nm)。由于⁶D_9/2与⁶I_11/2间能级差(～2900cm^-1)和⁶I_7/2与⁶P_J间能级差(～3900cm^-1)较小,多声子弛豫的几率明显超过辐射跃迁几率。因此,当用⁸S_7/2→⁶G_J(202nm)的紫外光激发时,只能观察到⁶P_J→⁸S_7/2(～310nm)、⁶G_J→⁶P_J(～600nm)和⁶G_J→⁶I_J(～780nm)的发射跃迁,未能观察到⁶I_J→⁸S_7/2(～275nm)和⁶D_J→⁸S_7/2(～250nm)的发射跃迁。     

VIOLET LIGHT-EMISSION FROM Ge+-IMPLANTED SiO2 FILMS ON Si SUBSTRATE

Germanium ions were implanted into SiO₂ films which were thermally grown on crystalline Si at an energy of 60 keV and with doses of 1×10¹⁵ and 1×10¹⁶ cm^-2.Under an ultraviolet excitation of ～5.0 eV,the implanted f ilms annealed at various temperatures exhibit intense violet luminescence with a peak at 396 nm.It is ascribed to the T₁→S₀ transition in GeO,which was formed during implantation and annealing process.     

退火对AlGaInP/GaInP多量子阱 LED外延片性能的影响

采用LP-MOCVD技术在n-GaAs衬底上生长了AlGaInP/GaInP多量子阱红光LED外延片.研究表明退火对外延片性能有重要影响.与未退火样品相比,460℃退火15min,外延片p型GaP层的空穴浓度由5.6×10¹⁸cm^-3增大到6.5×10¹⁸cm^-3,p型AlGaInP层的空穴浓度由6.0×10¹⁷cm^-3增大到1.1×10¹⁸cm^-3.但退火温度为780℃时,p型GaP层和p型AlGaInP层的空穴浓度分别下降至8×10¹⁷cm^-3和1.7×10¹⁷cm^-3,且Mg原子在AlGaInP系材料中的扩散加剧,导致未掺杂AlGaInP/GaInP多量子阱呈现p型电导.在460～700℃退火范围内,并没有使AlGaInP/GaInP多量子阱的发光性能发生明显变化.但退火温度为780℃时,AlGaInP/GaInP多量子阱的发光强度是退火前的2倍.     

Ultrabright γ-ray emission from the interaction of an intense laser pulse with a near-critical-density plasma

An efficient scheme for generating ultrabright γ-rays from the interaction of an intense laser pulse with a near-critical-density plasma is studied by using the two-dimensional particle-in-cell simulation including quantum electrodynamic effects. We investigate the effects of target shape on γ-ray generation efficiency using three configurations of the solid foils attached behind the near-critical-density plasma: a flat foil without a channel (target 1), a flat foil with a channel (target 2), and a convex foil with a channel (target 3). When an intense laser propagates in a near-critical-density plasma, a large number of electrons are trapped and accelerated to GeV energy, and emit γ-rays via nonlinear betatron oscillation in the first stage. In the second stage, the accelerated electrons collide with the laser pulse reflected from the foil and emit high-energy, high-density γ-rays via nonlinear Compton scattering. The simulation results show that compared with the other two targets, target 3 affords better focusing of the laser field and electrons, which decreases the divergence angle of γ-photons. Consequently, denser and brighter γ-rays are emitted when target 3 is used. Specifically, a dense γ-ray pulse with a peak brightness of 4.6×10²⁶ photons/s/mm²/mrad²/0.1%BW (at 100 MeV) and 1.8×10²³ photons/s/mm²/mrad²/0.1%BW (at 2 GeV) are obtained at a laser intensity of 8.5×10²² W/cm² when the plasma density is equal to the critical plasma density n_c. In addition, for target 3, the effects of plasma channel length, foil curvature radius, laser polarization, and laser intensity on the γ-ray emission are discussed, and optimal values based on a series of simulations are proposed.