Propagation of ion shock in solid DT target with nonlinear force-driven plasma blocks

The plasma block (piston) with pressure P ₁ is generated as a result of the nonlinear (ponderomotive) force in laser–plasma interaction. The plasma block can be used for the ignition of a fusion flame front in a solid density deuterium–tritium (DT) target by compressing the fuel that creates an ion shock propagating with velocity u _{ion? shock} in the inside of a solid DT target. The ignition is achieved by creating an ion shock during the final stages of the implosion. We estimated the effect of an ion shock in solid DT target at an early stage with no compression and at the last stage with compression, where density increases by a factor of solid-state density. According to the theoretical model, a large target with a very thin layer of fuel (high-aspect ratio target) would be ideal to obtain the very strong shocks. Results indicate that the maximum compression even by an infinitely strong single shock can never produce more than four times the initial density of DT fuel. The results reported that the threshold ignition energy in a solid DT target is reduced by a factor of 4.     

快重离子束实现点火的可行性

采用蒙特卡罗方法计算了低温下C,Si,Ar,Au和U等多种重粒子在等物质的量氘氚等离子体密度1000 g/cm3、热斑直径50 m中的电子能量损失,不同点火形式下入射能量和作用时间,以及燃料约束时间为20 ps条件下的束流强度。通过对数据的分析研究了这些重粒子辐照实现氘、氚燃料快点火的可能性。结果表明,重粒子束流加热等离子体实现快点火理论上可行,而且有一定的优势;较重的离子加热聚变等离子体的效果更好。重粒子束流加热等离子体到聚变温度需要的束流强度在MA左右;单个粒子的能量在GeV以上;相互作用时间为ps以下。     

Direct ignition and S-curve transition by in situ nano-second pulsed discharge in methane/oxygen/helium counterflow flame

A well-defined plasma assisted combustion system with novel in situ discharge in a counterflow diffusion flame was developed to study the direct coupling kinetic effect of non-equilibrium plasma on flame ignition and extinction. A uniform discharge was generated between the burner nozzles by placing porous metal electrodes at the nozzle exits. The ignition and extinction characteristics of CH₄/O₂/He diffusion flames were investigated by measuring excited OH¹ and OH PLIF, at constant strain rates and O₂ mole fraction on the oxidizer side while changing the fuel mole fraction. It was found that ignition and extinction occurred with an abrupt change of OH¹ emission intensity at lower O₂ mole fraction, indicating the existence of the conventional ignition-extinction S-curve. However, at a higher O₂ mole fraction, it was found that the in situ discharge could significantly modify the characteristics of ignition and extinction and create a new monotonic and fully stretched ignition S-curve. The transition from the conventional S-curves to a new stretched ignition curve indicated clearly that the active species generated by the plasma could change the chemical kinetic pathways of fuel oxidation at low temperature, thus resulting in the transition of flame stabilization mechanism from extinction-controlled to ignition-controlled regimes. The temperature and OH radical distributions were measured experimentally by the Rayleigh scattering technique and PLIF technique, respectively, and were compared with modeling. The results showed that the local maximum temperature in the reaction zone, where the ignition occurred, could be as low as 900 K. The chemical kinetic model for the plasma–flame interaction has been developed based on the assumption of constant electric field strength in the bulk plasma region. The reaction pathways analysis further revealed that atomic oxygen generated by the discharge was critical to controlling the radical production and promoting the chain branching effect in the reaction zone for low temperature ignition enhancement.     

Reflection of modified Korteweg-de Vries solitons in a plasmahaving negative ions

The variable coefficient modified Korteweg-de Vries (mKdV) equations for incident and reflected solitons are derived and solved to study the reflection of compressive and rarefactive ion acoustic solitons at the critical density in an inhomogeneous negative ion plasma. The polarity of the incident compressive and rarefactive solitons is not altered during the reflection process. Increasing the density gradient reinforces the reflection of both compressive and rarefactive mKdV solitons, whereas enhancement of the unperturbed plasma density weakens the reflection     

Simulation of the ion beam-plasma interaction processes for point-like ions in doped DT plasmas

The purpose of this work is to study the interaction between an ion beam and a doped deuterium-tritium (DT) plasma in a fast ignition nuclear fusion context. In order to analyze the influence of the dopants in the interaction process, we present a physical model to carry out spatial-temporal simulations of the stopping of an ion beam interacting with a doped plasma target, the plasma heating processes, and the formation of the ignition regions. We perform a set of numerical experiments where different concentrations of dopants are added to a fully ionized DT plasma. These simulations allow us to characterize the increase in the stopping power and the maximum temperatures achieved with the presence of impurities, as well as the reduction of the heated and ignition regions. This reduction in the ignition region indicates difficulties for the formation of an efficient hot spot when there are dopants in the DT plasma.     

Investigations of the ignition of arc spots on cold cathodes in anoble gas atmosphere

To investigate the ignition of arc spots on cold cathodes under defined conditions, a special experimental setup was developed. An arc ignited between horn electrodes in a pure argon gas atmosphere is blown magnetically against a third so called commutation electrode, which is negatively biased against the arc plasma. The ignition of arc spots on this cathode was investigated by electrical measurements and high-speed photography. The arc traces of short current pulses were examined by in situ optical microscopy of the cathode surface. Two different modes of arc-spot ignition were observed: an initiation by a diffuse glow discharge, which may pass into a constricted arc spot, and an immediate formation of a constricted arc spot. The two modes of arc-spot ignition at atmospheric pressure were attributed to different surface structures, which are characterized by broad or narrow distributions of local ion current density enhancement factors. Ion current density enhancement may raise the field strength and temperature on the tips of microprotrusions so far that they emit electrons. A sufficiently high density of small emission sites produces locally such a high average current density that a plasma channel and an arc spot on the cathode surface arc formed. With lower pressure, the influence of the surface structure is reduced and pushed back by Townsend-γ emission     

碰撞对两种正离子的电负性磁鞘影响的数值研究

建立包括两种正离子的电负性磁鞘的流体模型,利用四阶龙格库塔法数值求解描述一维稳态等离子体鞘层的方程组,考察离子与中性粒子碰撞对一维稳态等离子体鞘层的影响.结果表明:鞘边Ar⁺与He⁺的含量比值与碰撞参数对离子马赫数的取值范围都有影响.鞘边负离子含量越少,碰撞对鞘层中带电粒子密度的影响越明显.并且随碰撞参数的加大,鞘层中电子、负离子的密度下降越快,两种正离子的密度则呈现不同的波动变化.鞘边负离子含量越多,碰撞对鞘层中两种正离子的速度影响就越明显.此外,碰撞参数越大对鞘边δ越大的鞘层中的带电粒子密度影响越大.     

High-speed imaging of the ignition of ethanol at engine relevant conditions in a rapid compression machine

The rapid compression machine (RCM) is a great tool for investigating fuel properties under engine relevant conditions (high-pressures, low temperatures). The most common diagnostics is measuring the pressure over time and determining the ignition delay time (IDT). In this study, for the first time, the OH* luminescence of ethanol/air mixture is measured within an RCM experiment at 15 and 20?bar for Φ?=?0.5. Combining the common pressure measurements with the simultaneously recorded high-speed images (up to 74.5?kHz framerate) gives a first insight into understanding the ignition modes and the corresponding pressure traces. At 74.5?kHz, in contrast to findings in literature, the ethanol ignition did not show to be purely homogeneous. Four different propagating fronts of OH* luminescence have been recorded. Besides a flame kernel and a detonation-like ignition front two further fronts prior to main ignition have been observed. The propagating speeds of the fronts have been determined and depend on the overall IDT.     

ps-LIF measurements of minor species concentration in a counterflow diffusion flame interacting with a vortex

Interactions of vortices and flame fronts may be considered as the basic structural elements of turbulent combustion. Additionally, they play an important role in flame instabilities as well as extinction and ignition processes. An ideal geometry to study these interactions is the counterflow diffusion burner with an additional actuator-driven nozzle for the generation of a vortex ring. This burner has already been well-characterized by other methods including CARS, LDA and PLIF. We present first quantitative measurements of minor species concentration in this flame using a short-pulse laser and time- and spatially resolved fluorescence detection with a streak camera. Quench-free OH concentrations are obtained by analysis of the time-resolved profiles. The high power density of the laser pulses allowed linewise detection of hydrogen using a three-photon excitation scheme. Simultaneously, shape and position of the vortex was monitored using two-dimensional detection of flame emissions. Spatially resolved concentration profiles of H and OH are presented for different interaction heights and times in the vortex. For steady flames, comparisons with model calculations are shown. Received: 19 July 2000 / Revised version: 13 December 2000 / Published online: 21 February 2001     

Experimental investigation of discharge characteristics in enhanced glow discharge plasma immersion ion implantation

In enhanced glow discharge plasma immersion ion implantation (EGD-PIII) that involves a small pointed anode and large area tabular cathode, the high negative substrate bias not only acts as the plasma producer but also supplies the implantation voltage. Consequently, an electric field is created to focus the electrons and the electron focusing field in turn enhances the glow discharge process. In this work, the discharge characteristics of EGD-PIII are investigated experimentally. The discharge initiation and extinction characteristics during pulsed biasing are discussed. The duration of the post pulse-off plasma is explained from the viewpoint of particle motion and experimentally verified by employing an auxiliary disk. Our experiments show that a dual-pulse method may be utilized to determine the remnant plasma.     

The ion streaming effect on Weibel instability in the relativistic dense plasma

The Weibel instability plays an important role in stopping hot electrons and energy deposition mechanism in fast ignition of inertial fusion process. In this paper, the ion Weibel instability in counter propagating electron‐ion plasmas is investigate. The obtained results show that the growth rate of Weibel instability will be decreased about 40% with the anisotropy velocity as v_xe = 2v_ze = 20; the ion density ratio, b = n _{0i 1}/n _{0i 2}, and density gradient, are increasing 50 and 90% respectively. The ion streaming in density gradient of dense plasma leads to increasing the Weibel instability growth rate and its amplification through ion streaming in the large wavenumber. The maximum unstable wavenumber has been decreased with decreasing the ion beam density ratio. For fixed ion density ratio, increasing 90% of the density gradient in the near of fuel plasma corona leads to reducing growth rate and unstable wavenumber about 43 and 42% respectively.     

Probe Measurements in Electronegative Plasmas: Modeling the Perturbative Effects of the Probe‐Holder

A basic property of an electronegative plasma is its separation into two distinct regions: an ion‐ion region far from boundaries, where the densities of positive and negative ions are higher then electron density, and a near‐boundary electron‐ion region, where negative ions have practically negligible density. This is due to the influence of the ambipolar electric field, which depends on electron (not negative ion) plasma parameters. This electric field “holds off” negative ions from the boundary, as the ions have lower mobility and temperature compared to the electrons. Therefore, negative ions will be repelled by any object inserted into the plasma. This can lead to errors in measurements of negative ion and electron parameters by any invasive method. Numerical modeling of electric probes in an argon‐oxygen plasma clearly demonstrates possible errors of direct measurements of negative ion probe current. This can also affect results from the photo‐detachment method and direct measurements of negative ion energy distribution (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the evaluation of ignition threshold in proton-carbon hybrid ignitor beam proposal

We may improve plasma ignition quality in a conically guided proton fast ignition scheme by the application of an extra lower intensity, secondary carbon beam. According to the experimental measurements on the number of laser-accelerated ions, the temporal evolution of the hot spot electron, ion, and radiation temperatures was examined using three-temperature plasma model, in radiation pressure acceleration (RPA) mechanisms for a proton-Carbon beam configuration, E_avg ≈ 10 MeV/nucleon. The hot spot ignition was evaluated by the well-known stopping power models proposed independently by Li–Petrasso (LP) and Brown–Preston–Singleton (BPS). Based on our analytical results, with time, plasma temperature in the LP model surpasses the BPS model. From this point of view, to compensate this deficiency in the BPS method, we will show that the density ratio of 17% is required when the energy spread is 10%. The results were also validated by the DEIRA-4 code. Moreover, the electron-ion equilibrium will decrease up to 9.3 and 4.4% for the LP and BPS methods, respectively. It is demonstrated that the key features of a higher average ion energy as well as the narrow beam profile in the RPA regime may effectively ignite hot spot much better than in the Target Normal Sheath Acceleration (TNSA) counterpart. It is estimated that the proton-Carbon beam proposal can reduce ion beam energy to 8.42 kJ, approximately saving 15% of ignitor energy.     

不同成分等离子体鞘层的玻姆判据

在一维平板鞘层中采用流体模型分别研究了不同成分无碰撞等离子体鞘层的玻姆判据.通过拟牛顿法数值模拟了含有电子、离子、负离子以及二次电子的等离子体鞘层玻姆判据.结果表明二次电子发射增加了鞘层离子马赫数的临界值,且器壁发射二次电子温度越高,离子马赫数临界值越小.负离子使离子马赫数临界值减小.而在含有二次电子和负离子的等离子体鞘层中,当负离子较少时,二次电子发射对离子马赫数临界值影响较大;当负离子增加时,离子马赫数的临界值则主要受负离子的影响. 关键词： 鞘层 等离子体 玻姆判据     

Particle and power balance in fusion plasma with different fuelling scenarios and helium ash removal

The time evolution of plasma density, temperature and thermal alpha-particle density are considered through modelling of helium ash removal. It is shown that a slow variation of the helium ash density with time can be used for operation path changing in deuterium-tritium fusion plasma. Also, the effect of different fuelling rate scenarios on the plasma operation path and steady state parameters is considered. The temporal evolution of the operating point during the ignition access and ignited operation phases are analyzed analytically and numerically. The main target of the study is the optimization of the plasma operation scenario in future fusion reactors including ITER.     

DNS of spark ignition using Analytically Reduced Chemistry including plasma kinetics

In order to guarantee good re-ignition capacities in case of engine failure during flight, it is of prime interest for engine manufacturers to understand the physics of ignition from the spark discharge to the full burner lightning. During the ignition process, a spark plug delivers a very short and powerful electrical discharge to the mixture. A plasma is first created before a flame kernel propagates. The present work focuses on this still misunderstood first instants of ignition, i.e., from the sparking to the flame kernel formation. 3D Direct Numerical Simulations of propane-air ignition sequences induced by an electric discharge are performed on a simple anode-cathode set-up. An Analytically Reduced Chemistry (ARC) including 34 transported species and 586 irreversible reactions is used to describe the coupled combustion and plasma kinetics. The effect of plasma chemistry on the temperature field is found to be non-negligible up to a few microseconds after the spark due to endothermic dissociation and ionization reactions. However, its impact on the subsequent flame kernel development appears to be weak in the studied configuration. This tends to indicate that plasma chemistry does not play a key role in ignition and may be omitted in numerical simulations.     

Effect of the reversibility of the chemical reaction on triple flames

We examine a new aspect of triple flames, namely the effect of the reversibility of the chemical reaction on flame propagation. The study is carried out in the configuration of the two-dimensional strained mixing layer formed between two opposing streams of fuel and oxidiser. The chemical reaction is modelled as a single reversible reaction following an Arrhenius law in the forward and backward directions. The problem is formulated within the constant-density (thermo-diffusive) approximation, the main non-dimensional parameters relevant to this study being a reversibility parameter R (equal to zero in the irreversible case), a non-dimensional measure of the strain rate ? and a stoichiometric parameter S. We provide analytical (asymptotic) expressions for the local burning speed of the triple flame in terms of ?, S, and R. In particular we describe how the propagation speed of the front is decreased by an increase in R and how the location of its leading edge is affected by reversibility. For example, it is found that the leading edge shifts towards the fuel stream for S > 1 and towards the oxidiser if S < 1, as R is increased. A detailed numerical study is conducted covering all propagation regimes ranging from weakly stretched positively propagating (ignition) fronts to thick negatively propagating (extinction) fronts. In the weakly stretched cases we show that the numerics are in good agreement with the asymptotic findings. Furthermore, the results allow the determination of the domains of the distinct propagation regimes, mainly in the terms of R and ?. In line with our physical intuition, it is found that reversibility reduces the domain of ignition fronts and promotes extinction. The results provide a systematic investigation which can be considered as a first step when considering a more realistic chemistry, or poorly explored aspects (such as the existence of a temperature gradient in the unburnt mixture), when analyzing triple flames.     

In depth fusion flame spreading with a deuterium-tritium plane fuel density profile for plasma block ignition

<正>Solid-state fuel ignition was given by Chu and Bobin according to the hydrodynamic theory at x = 0 qualitatively. A high threshold energy flux density,i.e.,E^* = 4.3×10¹² J/m²,has been reached.Recently,fast ignition by employing clean petawatt-picosecond laser pulses was performed.The anomalous phenomena were observed to be based on suppression of prepulses.The accelerated plasma block was used to ignite deuterium-tritium fuel at solid-state density. The detailed analysis of the thermonuclear wave propagation was investigated.Also the fusion conditions at x≠0 layers were clarified by exactly solving hydrodynamic equations for plasma block ignition.In this paper,the applied physical mechanisms are determined for nonlinear force laser driven plasma blocks,thermonuclear reaction,heat transfer, electron-ion equilibration,stopping power of alpha particles,bremsstrahlung,expansion,density dependence,and fluid dynamics.New ignition conditions may be obtained by using temperature equations,including the density profile that is obtained by the continuity equation and expansion velocity.The density is only a function of x and independent of time.The ignition energy flux density,E_t^*,for the x≠0 layers is 1.95×10¹² J/m².Thus threshold ignition energy in comparison with that at x = 0 layers would be reduced to less than 50 percent.     

Hydrodynamic and chemical scaling for blow-off dynamics of lean premixed flames stabilized on a meso-scale bluff-body

Direct numerical simulations were conducted to investigate the effect of two parameters, density ratio and laminar flame speed, on the conditions of the onset of local extinction and blow-off of lean premixed flames, stabilized on a meso-scale bluff-body in hydrogen-air and syngas–air mixtures. A total of six simulation cases were considered as isolated comparison of the two parametric effects of the fluid dynamic instability and flame time scale. For all cases under study, the general flame development towards the blow-off limit showed a sequence of five distinct modes, with possible cyclic patterns among the different modes for a range of velocity conditions. The onset of local extinction was observed during the asymmetric vortex shedding and vortex street mode. As the density ratio is decreased, the flow inunder reviewstability is promoted through the increased sinuous mode, and such behavior was properly scaled by the Strouhal number. Although the blow-off velocity is altered by the fluid dynamic effects, the condition for the onset of local extinction and blow-off was mainly dictated by the competition between flow residence time associated with the lateral flame motion and ignition delay of the local mixtures. Time scale analysis supported the validity of the findings across all the cases investigated.     

基于光谱诊断的氩-氮P-TIG焊引弧的动态电弧物理特性研究

焊接电弧等离子体的物理特性直接决定了焊接接头的成形形貌,分析双组分保护气体的脉冲钨极惰性气体保护焊（P-TIG）动态电弧物理特性,为深入开展混合气体保护焊的焊缝成形物理过程研究提供理论基础。氩-氮混合气体保护焊电弧具有高热特性可以增加熔深,但在焊接前混合均匀的保护气体,引弧后气体浓度会重新分布,使电弧等离子体物理特性的实时动态变化特点变得复杂。光谱诊断是电弧等离子体物理特性测量的最重要手段,但对双组分气体保护的P-TIG焊电弧特性的研究仍需深入进行,特别是对于易引起缺陷的起弧过程,其动态物理特性亟需深入分析。针对氩-氮混合气体P-TIG焊的引弧过程,以P-TIG焊产生的氩-氮双组分电弧等离子体为研究对象,提出利用窄带滤光片与CCD相结合的高速摄影实验系统采集双组分电弧等离子的动态光谱信息,获取特征谱Ar Ⅰ 794.8 nm和N Ⅰ 904.6 nm的P-TIG焊电弧光谱强度动态分布;提出利用双元素双组分标准温度法计算P-TIG焊引弧过程中距离钨极下方1,2,3和4 mm位置处电弧等离子体的动态温度及浓度,定量分析80%Ar+20%N₂保护的P-TIG焊从引弧至电弧稳定过程的电弧等离子体物理特性实时分布。实验结果表明,80%Ar+20%N₂保护的P-TIG焊电弧强度、电弧温度及浓度的变化均与脉冲电流的变化同步,焊接电流在3 ms内达到稳定状态,而电弧等离子体的强度、温度及浓度需要更长时间达到平衡状态。从起弧到电弧等离子稳定燃烧的过程中,基值期间和峰值期间的电弧等离子体强度均呈现先升高再降低的趋势;由于阴极的热传导及电流密度的变化,使得电弧等离子体轴向位置的峰值温度及基值温度均出现迅速升高再缓慢降低的现象;由于粒子间碰撞及摩擦力的影响,使得电弧等离子体的峰值及基值期间氩的浓度均呈迅速减小再缓慢增加的趋势,且氩的浓度均低于焊前浓度。