DEVELOPMENT OF A NEUTRAL PARTICLE ANALYZER FOR THE HL-1M TOKAMAK

1　IntroductionConfinementandheatingofplasmasareele mentarytopicsinfusionresearch .Tocharacterizethefusionplasmasinauxiliaryheatingexperimentsaswellasinohmicheatingexperiments,measure mentofionenergyinplasmasbecomesnecessary .Neutralparticleanalyzersaresuchdevicesthatareaimedatdirectanalysisofneutralparticlesemittedfromfusionplasmasandareusedinmanyfusionresearchlaboratories[1～ 4] .Detailsofparticledi agnosticprincipleforfusionplasmacanbefoundinReference [5].Emissionofneutralparticlesisintrin…     

A study of density in electron-cyclotron-resonance plasma

A theory is developed for the density profile of low temperature plasmas confined by applied magnetic field and an experiment of the electron-cyclotron-resonance (ECR) plasma is conducted to compare the theoretical prediction and experimental measurements. Due to a large electron mobility along the magnetic field, electrons move quickly out of the system, leaving ions behind and building a space charge potential, which leads to the ambipolar diffusion of ions. In a steady-state condition, the plasma generation by ionization of neutral molecules is in balance with plasma loss due to the diffusion, leading to the electron temperature equation, which is expressed in terms of the plasma size, chamber pressure, and the ionization energy and cross section of neutrals. The power balance condition leads to the plasma density equation, which is also expressed in terms of the electron temperature, the input microwave power and the chamber pressure. It is shown that the plasma density increases, reaches its peak and decreases, as the chamber pressure increases from a small value (0.1 mTorr). These simple expressions of electron temperature and density provide a scaling law of ECR plasma in terms of system parameters. After carrying out an experimental observation, it is concluded that the theoretical predictions of the electron temperature and plasma density agree remarkably well with experimental data     

Determination of the Energy Flux of a Commercial Atmospheric‐Pressure Plasma Jet for Different Process Gases and Distances Between Nozzle Outlet and Substrate Surface

The energy flux of an atmospheric‐pressure plasma jet for surface treatment has been investigated by a calorimetric probe. Generally, the investigations exhibit that the main contributions of the total energy influx from the plasma to the substrate surface originate from the neutrals regarding high gas temperature coupled with a high gas flow. The use of nitrogen as process gas shows a higher energy flux compared to oxygen and air presumably caused by increased gas temperature as well as by higher molecule formation and recombination energy of N₂. Moreover, the lateral expansion of the plasma beam could be roughly determined by a spatially resolved analysis of the energy influx. A top part mounted on the nozzle, commonly used for the injection of additional precursor gases, showed a significant effect on the flow behavior and collision entailed relaxation of the excited plasma species leading to a restraining of the plasma jet. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The delay of arc starvation saturation in an intense ion source discharge

Wall neutralization of effusing energetic plasma ions results in a substantial population of very energetic neutrals which are able to readily penetrate into the intense discharge core. These fast neutrals delay arc starvation and, when reionized, accompany an increase in ion plasma temperature.     

Ablation process of silica glass induced by laser plasma soft X-ray irradiation

Silica glass can be machined by irradiation with laser plasma soft X-rays on nano- and micrometer scale. We have investigated the ablation process of silica glass induced by laser plasma soft X-ray irradiation. We observed ionic and neutral species emitted from silica surfaces after irradiation. Dominant ions and neutrals are O⁺ and Si⁺ ions and Si, O, SiO and Si₂ neutrals, respectively. The ions have kinetic energies of 13 and 25 eV, which are much higher than those of particles emitted by evaporation. The energy of laser plasma soft X-rays absorbed to silica glass at a fluence of 1.4 J/cm² is estimated to be 380 kJ/cm³, which is higher than the binding energy of SiO₂ of 76 kJ/cm³. These results suggest that the most of the bonds in silica glass are broken by absorption of laser plasma soft X-rays, that several percent of the atoms are ionized, and that neutral atoms are emitted together with repulsive ions. The process possibly enables us to fabricate nano structures.     

Two-dimensional simulation of polysilicon etching with chlorine ina high density plasma reactor

A two-dimensional fluid simulation of polysilicon etching with chlorine in an inductively-coupled high density plasma source is presented. A modular approach was used to couple in a self-consistent manner the disparate time scales of plasma and neutral species transport. This way, complex plasma chemical reactions (involving electrons, ions and neutrals) as well as surface chemistry can be included in the simulation, The power deposited into the plasma was calculated by an electromagnetics module which solves Maxwell's equations. The power deposition was used in the electron energy module to find the electron temperature and the rate coefficients of electron-impact reactions. These were in turn used as source terms in separate neutral and charged species transport modules. By iterating among the modules, a self-consistent solution was obtained. Quantities of interest, such as power deposition, species density and flux, and etch rate and uniformity were thus calculated, As power deposition was increased, the electron density increased linearly, the plasma became less electronegative, the degree of gas dissociation increased, and the plasma potential remained constant. The radial uniformity of the Cl atom flux was better than that of the ion flux. The reactivity of the wafer as compared to that of the surrounding electrode surface significantly affected the etch uniformity, despite the low pressure of 10 mtorr     

Features of the film-growth conditions by cross-beam pulsed-laser deposition

Spatial and energetic characteristics of the plasma plume by cross-beam pulsed-laser deposition (CBPLD) were investigated. Effective droplets filtering together with high efficiency of material usage are observed by this approach. Time-of-flight (TOF) technique with electrostatic ion collectors (Langmuir probes operating in the ion-collecting mode) were applied to obtain kinetic energy distribution functions of ionized particles and to compare the ionization degrees of the plasma by the CBPLD and by the conventional PLD. The average and maximum kinetic energies of the ions by the CBPLD are found to be 2–3 times lower as compared to the conventional PLD. At the same time, the fraction of ionized species and highly exited neutrals (Rydberg atoms) in the CBPLD plasma is 1.5–2 times larger in comparison to the conventional approach. Re-sputtering of the material of the growing film by fast ions is a considerable effect in both the PLD methods by the chosen experimental conditions. The angular width of the directional pattern of the plasma plume by CBPLD is comparable to that typical for the conventional PLD. Received: 21 December 1998 / Accepted: 28 March 1999 / Published online: 7 July 1999     

On the Velocity Distribution Function of Light Ions in Heavy Gases in an Electric Field

The special properties presented by the Boltzmann collision operator in the case of an induced-dipole interaction (Maxwellian interaction) between ions and neutrals are exploited to obtain in such case a proper solution of the Boltzmann equation for light ions in heavy gases in an electric field. Meanwhile it is proved that consideration of more than two terms of the spherical harmonic expansion of the ion velocity distribution requires to improve the usual accuracy of the terms deriving from the collision integral; vice versa, the improvement of the accuracy of the collision terms requires to retain more than two terms of the spherical harmonic expansion. The consistency of our procedure and results in the approximation neglecting the square of the ion-neutral mass ratio with respect to unity is discussed. Finally, the most significant velocity averages are calculated on the basis of the obtained ion distribution. In the limit of the above approximation they are shown to agree with Wannier's results.     

Experimentelle und theoretische Untersuchungen von Nichtgleichgewichtseffekten an stationären Heliumplasmen unter Normaldruck

Measurements and calculations of temperature, densities and field-strength-current-characteristics of cascaded arcs (0.15 and 0.3 cm radius) burning in Helium under normal pressure are reported. It is shown that the evaluation of measured arc data assuming Saha equilibrium is not in agreement with the detailed solution of the balance equations. The temperature of electrons and heavy particles as well as the density of electrons and neutrals must be determined as independent variables from the rate equation for ground state neutrals, from the equation of state, and from the energy balance of the electron gas and of the total plasma. The latter equation can be replaced by relations between measured intensities and the state variables. The deviations from Saha equilibrium are mainly caused by diffusion of neutral particles into the arc core and of charged particles into the opposite direction. The theoretical results derived from the balance equations are compared with spectroscopic line intensity and line width measurements. The agreement is good even if the equilibrium conditions are strongly violated.     

高能中性束注入加热

本文仔细地研究了高能中性束注入加热的有关问题,利用多束模型计算了中性束注入加热。本文的主要特点是在捕获快离子与本底离子和电子能量交换表示式中,直接用误差函数积分进行计算,而不像以前的计算中作大参量和小参量展开近似。当快离子能量比较低时,大参量展开不再适用,因此本文的计算更为精确,更为有用。给出了多束注入加热的计算结果及其结果的讨论。本计算方法可与输运方程联立求解,数值模拟等离子体行为。     

How the dynamics of an ablation plume is affected by ambient gas ionisation

The influence of inert gas ionisation on the expansion dynamics of a laser ablation plume propagating through an inert gas is studied. Charge transfer reactions between ionised ablated species and gas neutrals lead to the formation of a charged layer of ionised gas atoms in contact with the plume expansion front. The energy lost by fast ablated ions when the plume is slowed down is calculated. For the exemplary carbon ablation in helium and argon atmospheres, where background gas ionisation plays a different role, model predictions agree with the observed microstructural differences of deposited films.     

Neutrals' temperature in laser-generated plasma at LNS

A Nd–YAG laser operating at 532 nm with a maximum intensity of 10¹⁰ W/cm² was used to ablate aluminium and tantalum targets placed in vacuum.

A mass quadrupole spectrometer (MQS) at high sensitivity, operating in the range of 1–300 amu, with a resolution better than 1 amu, was used to analyse the atomic emission produced by the laser ablation. The neutrals' emission produced by laser-generated plasma at INFN-LNS was investigated in terms of temperature, ablation yield, angular distribution and velocity.

The neutrals' detection through the MQS permitted to measure the mass energy distribution. Results demonstrate that the maximum temperatures of the neutral species are of the order of 100 eV. The angular emission of neutrals is peaked along the normal to the target surface, as it was observed for the ions; the ablation yield increases suddenly at low laser intensity and decreases at high laser intensity, owing to the higher ionization processes; the flow velocity follows the adiabatic expansion of the plasma in vacuum and it is of the order of 10⁴ m/s.

Measurements will be presented and discussed, according to the available models.     

Neutral gas free molecular flow algorithm including ionization and walls for use in plasma simulations

Several plasma devices, including Hall and ion thrusters, operate by ionizing a low density neutral gas for which the mean free path between collisions of gas molecules is greater than typical device dimensions. In general, the discrete-particle algorithms used to calculate the neutral gas ignore velocity changes due to collisions between gas molecules. However, particle algorithms are a source of unphysical statistical noise that may detract from the study of the plasma physics, the prime purpose of most simulations. In this paper we present a new neutral gas algorithm for use in plasma simulation codes that exploits the fact that very few collisions change the velocity of neutral gas molecules. The algorithm assumes that the particle velocity distribution function for neutrals emitted from a given surface remains unchanged except for a scale factor that reflects the loss of neutrals to ionization. The sources of neutrals may be gas inlets, and isotropic, thermally accommodated, gas molecules coming off chamber surfaces including recombined ions. The algorithm is implemented in two dimensions (R–Z) with emitting surfaces represented as surfaces of revolution. The advantage of this algorithm over the conventional particle approach is the absence of statistical noise.     

Interaction of plasma facing materials for fusion devices with low energy hydrogen and helium particles

Abstract

Radiation damage formed in metal specimens exposed to long pulse tokamak plasmas of TRIAM-1M was examined by transmission electron microscopy. By comparing these results with those of low energy hydrogen ion irradiation it was concluded that the charge exchange energetic neutrals of hydrogen emitted from the core plasma caused remarkable displacement damage. The flux of the neutrals in the energy range of 0.5–3 keV which were responsible for displacement damage, was estimated to be about 1.5–3 × 10¹⁸ H/m²/s. These energetic neutrals cause not only material degradation at the sub-surface region but also change bulk properties of plasma facing components in a plasma confinement device. Effect of helium plasma was also discussed with emphasis on very strong effects on damage accumulation. Damage by He is a serious issue of plasma facing materials.     

辐射量热计在HT-6BTokamak上的测量

一、引言 在现行的托卡马克实验中,辐射损失和中性粒子损失是高温等离子体能量损失的主要通道之一。为了更好地了解托卡马克等离子体中能量平衡、辐射损失以及这些损失随等离子体参数的变化;了解等离子体中杂质含量和杂质输运以及等离子体与器壁的相互作用,辐射能量损失的时间特性、空间分布的测量显得很有必要。为     

Two-dimensional direct simulation Monte Carlo (DSMC) of reactiveneutral and ion flow in a high density plasma reactor

We present a two dimensional direct simulation Monte Carlo (DSMC) study of the rarefied reactive flow of neutrals and ions in a low pressure inductively coupled plasma reactor. The spatially-dependent rate coefficients of electron impact reactions and the electrostatic field were obtained from a fluid plasma simulation. Neutral and ion etching of polysilicon with chlorine gas was studied with emphasis on the reaction uniformity along the wafer. Substantial gradients in total gas density were observed across the reactor invalidating the commonly made assumption of constant gas density. The flow was nonequilibrium with differences in the species translational temperatures, and 100 K temperature jumps near the walls. When etching was limited by ions the etch rate was highest at the wafer center. When etching was limited by neutrals, the etch rate was highest at the wafer edge. In such case, the etch uniformity changed significantly depending on the reactivity of the ring surrounding the wafer. The ion angular distribution was several degrees off normal and it was different at the wafer edge compared to the rest of the wafer     

Onset of breakdown and formation of cathode spots

When an increasing diode voltage is applied, enhanced field emission of electrons begins from a growing number of small spots or whiskers on the cathode surface. This stimulates desorption of weakly bound adsorbates from the surface of a whisker. As the diode voltage increases, the 100-V equipotential surface moving toward the cathode is met by the desorbed neutrals moving away from the cathode, resulting in sharp risetime for the onset of ionization of desorbed neutrals by field-emitted electrons. Positive ions produced in the ionization region a few microns from the electron emitting spot are accelerated back to it. This bombardment leads to surface heating of the spot. The onset of breakdown by this mechanism requires much less current than the Joule heating mechanism. The localized buildup of plasma above the electron emitting spot leads to pressure and electric field distributions that ignite unipolar arcs. The high current density of the unipolar arc and the associated surface heating by ions result in the explosive formation of cathode spot plasma     

Energy dependence of the ionization probability of sputtered Cu and Ni

Despite its great sensitivity, the usefulness of secondary ion mass spectrometry (SIMS) for many applications has been limited by an inadequate understanding of the probability of sputtering an atom in an ionized state. To determine this ionization probability for clean Cu and Ni surfaces, I have measured the energy distribution of sputtered neutrals and ions by quadrupole mass filtering and retarding potential analysis using potential modulation differentiation. Analysis of sputtered neutrals was accomplished by electron impact ionization. Because the neutrals outnumber the ions by at least two orders of magnitude, the ratio of sputtered ions to neutrals is an accurate measure of the ionization probability. For energies below 20 eV the dependence of the ionization probability on energy goes as P(E) α Eⁿ, where n = 0.65 for clean Cu. The absorption of oxygen on the Cu surface increases the total ion yield while causing a reduction in the value of the exponent n. Similar results are found for nickel, where n = 0.54 for the clean surface.     

Ionic recombination in a weakly ionized gas : I. A stochastic approach to the recombination rate problem

Using the framework of a stochastic approach to the reaction-kinetic problem originating from Teramoto and Shigesada equations for the phase-space evolution of the ionic part in a diluted ternary gas mixture composed of positively and negatively charged ions as well as of neutrals are derived. On the basis of the stationary solution of the reactive ion-pair equation an operator expression for the recombination constant α is defined and discussed with regard to the effect of the center-of-mass motion. By iteration a formally exact α-expansion in terms of multiple ion-pair neutral collisions is derived for the case that non-thermal center-of-mass effects can be neglected. This expansion has to be specified to meet the conditions of well-known models for ion-pair stabilization (absorbing sphere, critical energy barrier).     

激光诱导铝等离子体中原子和离子组分膨胀特性

为了了解等离子体中原子与离子组分的膨胀特性及背景气体存在状态下其运动状态的改变规律,设计了一系列实验,并进行了深入探究.采用波长为532 nm的纳秒激光剥蚀铝样品形成等离子体,并使用配有em ICCD检测器的C-T型三光栅单色仪对等离子体进行时序采集,同时使用2400 g·mm~(-1)的光栅替代窄带滤光片进行不同组分成像诊断,得到铝等离子体中Al Ⅰ (396.1 nm), Al Ⅱ (466.3 nm), Al Ⅲ (447.9 nm)的光谱分辨图像.在不同背景气压下采集了等离子体各组分光谱图像,探究背景气体对等离子体演化的影响.结果表明,在等离子体形成过程中,离子组分相对于原子组分分布在羽流前端,且角度分布较小.原子与离子组分的真空膨胀速度均处于10~4m·s~(-1)量级.等离子体中离子组分的运动速度较高,且其运动速度随着离子价态的增加而增大,但在本实验使用的能量密度范围下,随激光能量的变化波动不大.中性原子的运动速度较慢,但随能量的增加而增大.随着膨胀过程的进行,各组分羽流沿样品表面法线方向推进且发射强度逐渐降低,对应的羽流密度和温度也相应降低.环境气压逐渐增大时,各研究组分运动状态与在高真空度下时有明显区别.在气压大于1 Pa后,等离子体与环境气体发生相互渗透,膨胀前端出现的晕影,产生扰动,发生束缚缓速.且等离子羽因气压增大而收缩、与背景气体的碰撞概率增加,使得羽流发射强度加强,等离子体的寿命随之延长.提出的新颖诊断方法与实验所得结果可为等离子体组分动力学过程的研究提供参考.