Anomalous resistivity resulting from MeV-electron transport in overdense plasma

Laser produced hot electron transport in an overdense plasma is studied by three-dimensional particle-in-cell simulations. Hot electron currents into the plasma generate neutralizing return currents in the cold plasma electrons, leading to a configuration which is unstable to electromagnetic Weibel and tearing instabilities. The resulting current filaments self-organize through a coalescence process finally settling into a single global current channel. The plasma return current experiences a strong anomalous resistivity due to diffusive flow of cold electrons in the magnetic perturbations. The resulting electrostatic field leads to an anomalously rapid stopping of fast MeV electrons (almost 3 orders of magnitude stronger than that through classical collisional effects).     

Investigation of the effect of electron cyclotron heating on runaway generation in the KSTAR tokamak

Wave enhanced runaway generation is expected to play an important role in the conversion of plasma current into runaway current during major disruptions. The fast electrons created by electron cyclotron heating (ECH) were used to study this issue in KSTAR. It is found that the fast electrons driven by ECH can enhance runaway production in the flat top phase with high loop voltage. The runaway current in disruptions was not enhanced by the ECH produced fast electron population due to the strong magnetic fluctuations which inhibited the generation of runaway electrons. It is found that a complete loss of existing REs during thermal quench has occurred in KSTAR limiter configuration discharges.     

HT-7超导托卡马克调制电流改善等离子体约束

 在HT-7超导托卡马克上进行了电流调制改善等离子体约束物理实验，获得了初步的实验结果。对电流调制时的径向电场进行了数值计算和讨论。当等离子体环向电流以适当的频率和幅值周期性调制时，低模数的磁流体不稳定性得到明显的抑制，等离子体中心电子温度增加了33%，等离子体电子密度分布明显陡化，能量约束时间增加了27%~45%。杂质辐射周期性减少,粒子约束时间增加了2倍。在等离子体边界产生更强的径向负电场。     

Beam electron energy distribution at a volume nanosecond discharge in atmospheric-pressure air

The energy distributions of beam electrons and x-ray photons in a volume nanosecond discharge on atmospheric-pressure air are studied. Several groups of elevated-energy electrons are found. It is shown that electrons with an energy from several tens to several hundreds of kiloelectronvolts (which is lower than a maximal voltage across the gap) make a major contribution to the beam current measured behind thin foils. It is corroborated that fast electrons (with an energy from several kiloelectronvolts to several tens of kiloelectron-volts) arise 100–150 ps before the basic peak of the beam current, elongating the current pulse and significantly increasing its amplitude. The contribution from electrons with an anomalously high energy (exceeding a maximal voltage across the gap) to the beam current is shown to be insignificant (less than 5%). The x-ray spectra in gas-filled diodes of different design are studied. Techniques of measuring the subnanosecond electron beam current and mechanisms generating fast and runaway electrons in volume high-pressure gas discharges are analyzed.     

Ultraintense laser-produced fast-electron propagation in gas jets

We study the propagation of fast electrons in a gas at different densities. A large relativistic electron current is produced by focusing a short-pulse ultrahigh-intensity laser on a metallic target. It then propagates in a gas jet placed behind the foil. Shadowgraphy in the gas shows an electron cloud moving at sub-relativistic average velocities. The experiment shows (i) the essential role of the density of background material for allowing propagation of fast electrons, (ii) the importance of the ionization phase which produces free electrons available for the return current, and (iii) the effect of electrostatic fields on fast-electron propagation.     

无碰撞等离子体电流片中的低频波

采用两种无碰撞二维三分量不可压缩磁流体力学（MHD）模型，计入电子扰动压力张量效应，研究了电流片等离子体的色散性质和波.由于得到的一般色散关系较为复杂，只解析讨论了电流片的中心区和电子β*e=0两种特殊情况.主要结果如下：(1)在短波区（kdi＞1），存在快磁声 动理学Alfven波和斜Alfven 哨声模，电子磁流体力学模型是足够精确的MHD模型；在长波区（kdi＜1＝，存在Alfven波和离子声波，理想的MHD模型是适用的.（2）电子β*e＝0情况下的结果，显然遗漏了一些波模（如离子声波和快磁声动理 关键词： 电流片 磁流体力学 电子压力张量 色散关系     

Peak and steady-state photocurrents in a molecular diode

It has been shown that the time evolution of a photocurrent under the conditions of the irradiation of a photochromic molecule by moderate-intensity light includes the fast and slow stages of the kinetic process. The fast stage corresponds to an increase in the current and is associated with the charging of the molecule, which is in a singlet excited state after a phototransition. The slow stage includes electron transfer between electrodes involving both charged and excited (singlet and triplet) states of the molecule. When the exchange interaction of unpaired electrons on the HOMO and LUMO levels of the photoexcited molecule is weak, the steady-state photocurrent is close in magnitude to the maximum transient current, whereas the steady-state current is suppressed when the exchange interaction is strong. The reason is that a molecule in the triplet state can block electron transfer between electrodes. The conditions have been found under which such blocking is a manifestation of the peak transient photocurrent that is formed immediately after the beginning of the irradiation of the molecule.     

强电流铁电阴极电性能参数与发射结果的关系研究

 在实验结果的基础上,从理论上阐述了材料的电滞回线、介电常数、压电常数等性能对电流发射性能的明显影响,并通过材料的掺杂改性提高材料的性能参数,从而达到了提高发射电流密度的目的。     

HL－1M感应与低杂波组合电流驱动研究

本文结合ＨＬ－１Ｍ的基本参数，利用准线性的低杂波电流驱动理论和等离子体的电回路方程．研究了在控制等离子体总电流不变情形下欧姆感应和低杂波注入组合驱动电流的问题。结果表明，这一组合驱动方案对ＨＬ－１Ｍ装置的运行是可行的，其驱动电流分布可以通过改变低杂波注入功率、波谱形状、等离子体电子温度、密度以及总等离子体电流等加以控制。组合驱动的电流分布将优于欧姆驱动的电流分布，并可能抑制诸如锯齿振荡等一些ＭＨＤ不稳定性。     

Fast electron energy deposition in aluminium foils: Resistive vs. drag heating

The high current electron beam losses have been studied experimentally with 0.7 J, 40 fs, 6 10¹⁹ Wcm^-2 laser pulses interacting with Al foils of thicknesses 10-200 μm. The fast electron beam characteristics and the foil temperature were measured by recording the intensity of the electromagnetic emission from the foils rear side at two different wavelengths in the optical domain, ≈407 nm (the second harmonic of the laser light) and ≈500 nm. The experimentally observed fast electron distribution contains two components: one relativistic tail made of very energetic (T _h ^tail ≈ 10 MeV) and highly collimated (7° ± 3°) electrons, carrying a small amount of energy (less than 1% of the laser energy), and another, the bulk of the accelerated electrons, containing lower-energy (T _h ^bulk=500 ± 100 keV) more divergent electrons (35 ± 5°), which transports about 35% of the laser energy. The relativistic component manifests itself by the coherent 2ω₀ emission due to the modulation of the electron density in the interaction zone. The bulk component induces a strong target heating producing measurable yields of thermal emission from the foils rear side. Our data and modeling demonstrate two mechanisms of fast electron energy deposition: resistive heating due to the neutralizing return current and collisions of fast electrons with plasma electrons. The resistive mechanism is more important at shallow target depths, representing an heating rate of 100 eV per Joule of laser energy at 15 μm. Beyond that depth, because of the beam divergence, the incident current goes under 10¹² Acm^-2 and the collisional heating becomes more important than the resistive heating. The heating rate is of only 1.5 eV per Joule at 50 μm depth.     

γ射线在大气中产生康普顿电流的快速计算方法

康普顿电流是激励核电磁脉冲的主要机制。为了提高核电磁脉冲数值模拟中γ射线产生康普顿电流过程的计算效率,发展了一种γ射线在均匀大气中产生康普顿电流的快速计算方法。首先通过计算康普顿电子在出射方向上的衰减特征,得到在电子出射方向净电子流与其射程和出射距离的拟合关系,然后结合康普顿散射微分截面,给出单能γ射线在大气中产生的康普顿电流密度的解析数值计算方法。和蒙特卡罗程序的计算结果比较,用该方法计算得到的电流密度值差异在10%以内,计算速度提高两个量级以上。该计算方法可用于大气中康普顿电流的快速估算。     

Potential of an insulated electrode in a high-energy electron flow under a gas pressure of 0.1–1.0 Pa

We analyze the variation of the floating potential of an insulated metallic electrode in a flow of electrons with an energy of up to 300 eV under a gas pressure of 0.1–1.0 Pa at a current density lower than 0.1 A/cm². It is shown that the dependence of the floating potential on the initial electron energy is non-monotonic; this fact is explained by the variation of the ratio of the ion current density to the density of fast electron current in the plasma. The balance of the electron and ion currents on the surface of an insulated electrode is ensured by the cutoff of the low-energy part of the electron flow at the level determined by the magnitude of the floating potential. The maximal value of the floating potential increases upon a decrease in the gas pressure; this is due to a decrease in the ion current density. The interval of energy variation in which the floating potential decreases from the maximal value (50–250 eV) to 5–6 eV increases with the electron current density and the gas pressure. The electrode material and the type of the gas do not noticeably affect the variation of the floating potential.     

Electron impact ionization of large krypton clusters

We show that the detection of ionization of very large van der Waals clusters in a pulsed jet or a beam can be realized by using a fast ion gauge. Rapid positive feedback electron impact ionization and fragmentation processes, which are initially ignited by electron impact ionization of the krypton clusters with the electron current of the ion gauge, result in the appearance of a progressional oscillation-like ion spectrum, or just of a single fast event under critical conditions. Each line in the spectrum represents a correlated explosion or avalanche ionization of the clusters. The phenomena have been analysed qualitatively along with a Rayleigh scattering experiment of the corresponding cluster jet.     

电子回旋波与快波协同效应的数值研究

在考虑了电子的反弹平均效应及高能电子的相对论效应下,通过数值求解电子的Fokker-Planck方程,研究了托卡马克等离子体中快波与电子回旋波电流驱动问题。采用九点差分法对方程进行离散,应用近似LU因子分解法对方程进行了数值求解。计算结果表明快波与电子回旋波联合电流驱动具有很强的协同效应,在两波联合电流驱动下,驱动电流大于单波电流驱动之和。     

A Linear Model for the Interpretation of Optical Emission Spectroscopy Measurements in Low Pressure DC Magnetron Sputtering

For a balanced dc magnetron discharge a linear model for the formation of excited and ionized species is presented. The consideration is based on collisional kinetic theory and a correlation of discharge current with ionization rates, and leads to normalized rate coefficients. The electron energy distribution changes according to the determining external parameters, but needs neither to be known in detail nor to be of Maxwellian type. The model calculations are consistent with measurements done by optical emission spectroscopy (OES) on the magnetron sputtering of aluminium in argon discharges. The dependences of optical emission intensities on discharge current and on argon pressure show that both, excited argon atoms as well as excited argon ions, are generated from neutral argon by direct, fast electron impact. It is additionally shown that at least for low argon pressure the excitation of aluminium atoms is also caused by direct electron impact.     

电子辐照聚合物带电特性多参数共同作用的数值模拟

电子辐照聚合物样品的带电特性是扫描电子显微镜成像、电子束探针微分析以及空间器件辐照效应等领域的一个重要研究课题. 通过建立基于蒙特卡罗方法的电子散射和时域有限差分法的电子输运的数值模型, 并采用高效的多线程并行计算, 模拟了电子非透射辐照聚合物样品的带电特性, 得到了带电稳态下的样品底部泄漏电流密度、表面负电位以及样品总电荷密度等带电特征量受入射电子能量、入射电流密度、样品材料的电子迁移率、样品厚度等相关参数共同作用的影响. 结果表明, 一个参数的变化使表面负电位增强时, 其他参数对负电位的影响将增强. 样品的带电稳态特征量在同一个电流平衡的模式下受参数影响的变化是单调的. 当电流平衡模式发生变化时, 如在入射电子能量较低的条件下, 样品内部的总电荷量会随着样品厚度的增大而先增加后减小, 出现局部极大值. 样品底部的泄漏电流密度随着入射电流密度的增大而近线性成比例地增大. 研究结果对于揭示电子辐照聚合物的带电规律及微观机理、预测不同条件下的样品带电状态具有重要科学意义.     

Improved lower hybrid current drive efficiency with IBW heating in the HT-7 tokamak

Lower hybrid current drive (LHCD) efficiency in the HT-7 tokamak is observed to increase about 30% with ion Bernstein wave (IBW) heating. The current drive efficiency as high as has been reached in HT-7 thanks to an LHCD/IBW synergy. The IBWs are significantly coupled to the fast electrons produced by the lower hybrid waves, thus results in higher fast electron photon temperature in the combined LHCD + IBW scenario. The interaction of LHCD + IBW improved current drive efficiency.     

Influence of the state of the electron gas on dispersion curves of ionization waves in neon

On the example of neon the onset of a transition is described from a non-hydrodynamic state of the electron gas, which exists at low pressures and currents, to a state where a local hydrodynamics is applicable. In the non-hydrodynamic state the ionization waves are influenced by resonant properties of the electron gas which means that several varieties can be selected on each of the two dispersion curves. With growing current the two parts of dispersion curve of the fast varieties (r and s) merge and this is interpreted as disappearance of the spatial resonance of the electron gas due to an enhancement of the electron Coulomb collisions.The author expresses his gratitude to K.Rohlena and L.Pekárek for helpful discussions.     

Can climate sensitivity be estimated from short-term relationships of top-of-atmosphere net radiation and surface temperature?

Increasing the knowledge in climate radiative feedbacks is critical for current climate studies. This work focuses on short-term relationships between global mean surface temperature and top-of-atmosphere (TOA) net radiation. The relationships may be used to characterize the climate feedback as suggested by some recent studies. As those recent studies, an energy balance model with ocean mixed layer and both radiative and non-radiative heat sources is used here. The significant improvement of current model is that climate system memories are considered.Based on model simulations, short-term relationship between global mean surface temperature and TOA net radiation (or the linear striation feature as suggested by previous studies) might represent climate feedbacks when the system had no memories. However, climate systems with the same short-term feedbacks but different memories would have a similar linear striation feature. This linear striation feature reflects only fast components of climate feedbacks and may not represent the total climate feedback even when the memory length of climate systems is minimal. The potential errors in the use of short-term relationships in estimations of climate sensitivity could be big. In short time scales, fast climate processes may overwhelm long-term climate feedbacks. Thus, the climate radiative feedback parameter obtained from short-term data may not provide a reliable estimate of climate sensitivity. This result also suggests that long-term observations of global surface temperature and TOA radiation are critical in the understanding of climate feedbacks and sensitivities.