圆锥磁绝缘传输线的横向空间电荷流

 根据电磁场基本理论及电子运动守恒方程，导出圆锥传输线横向空间电荷流的数值模型和磁绝缘临界条件。通过数值计算，讨论了电压及圆锥几何结构参数等对横向空间电荷流和磁绝缘性能的影响。电压较高时,无磁场的空间电荷流较大，而磁绝缘性能更好。在传输线的三个几何参数中，几何因子对传输性能影响最大。     

四种磁绝缘传输线的横向空间电荷流

 根据电磁场基本理论及电子运动守恒方程，导出平板、圆柱、圆锥和圆盘这4种常用传输线的横向空间电荷流的数值模型和磁绝缘临界条件。通过数值模拟得到:传输线的电压越高，无磁场的横向空间电荷流越大，但是磁缘性能却越显著；若分别增大圆柱、圆锥的几何结构因子，既有助于减小它们的无磁场横向空间电荷流又有助于增强它们的磁绝缘性能；对于圆锥，若电极夹角较大，内电极的极角较小，则对减小无磁场空间电荷流和增强磁绝缘性也有一定好处。     

四种磁绝缘传输线的横向空间电荷流

根据电磁场基本理论及电子运动守恒方程，导出平板、圆柱、圆锥和圆盘这4种常用传输线的横向空间电荷流的数值模型和磁绝缘临界条件。通过数值模拟得到:传输线的电压越高，无磁场的横向空间电荷流越大，但是磁缘性能却越显著；若分别增大圆柱、圆锥的几何结构因子，既有助于减小它们的无磁场横向空间电荷流又有助于增强它们的磁绝缘性能；对于圆锥，若电极夹角较大，内电极的极角较小，则对减小无磁场空间电荷流和增强磁绝缘性也有一定好处。     

求解三种导体构形中相对论电子形成的空间电荷限制流

在无横向磁场作用下，真空中损失到金属导体阳极表面的空间电荷限制（SCL）流将达到最大。在无限大平板构形导体中，这个最大的SCL流只与阴阳极电压和间距相关；而对于在磁绝缘传输线（MITL）中广泛应用的同轴圆筒和共顶点同轴圆锥构形，其最大SCL流还与导体的几何因子及极性参量有关。研究这3种典型构形的最大SCL流，是研究横向磁场对SCL流影响规律的起点，是设计同类MITL构形的基础。     

磁绝缘传输线损失电流等效电路模型

在粒子模拟的基础上,结合经典空间电荷限制流、磁绝缘临界电流的理论公式,拟合得到磁绝缘传输线中空间电荷限制流的修正计算公式,并进一步建立磁绝缘传输线损失电流的插值计算模型。该模型不需要求解广义泊松方程,计算效率高。通过对长同轴磁绝缘传输线的等效电路模拟,得到了与全尺寸粒子模拟结果基本一致的负载电压波形,插值计算模型不仅正确地反映了磁绝缘传输线中的电流损失过程,而且其等效电路模拟计算效率比粒子模拟提高3000倍以上。     

空间电荷限制流与传导电流的定量关系

 从空间电荷限制流假设、Poisson方程及电子正则动量守恒关系出发，推导了平板形、同轴圆柱和共顶点同轴圆锥三种导体构形的空间电荷流随传导电流变化的广义Poisson方程，给出了求解方法及解的基本特征，分析比较了三种导体构形空间电荷限制流的基本性质。通过推导，计算和分析可得：各种电压条件下传导电流对空间电荷限制（SCL） 流的作用效果不一样，电压越高传导电流提高磁绝缘程度的作用越显著；当几何因子(即高阻抗)较小时其它两种导体的SCL流与平板形相差较大，几何因子较大时与平板形十分接近；同样电压条件下负极性的SCL流比平板形小、正极形正好相反，而相同几何因子条件下同轴圆筒的SCL流比共顶点同轴圆锥的小；在分析研究低阻抗MITL时，采用SCL流的平板近似不会带来大的误差。在描述时变脉冲作用于MITL时，可以通过对SCL流随电压、传导电流变化的曲面函数插值的方法确定各个时刻的磁绝缘状态。     

4层圆盘锥形磁绝缘传输线的等效电路模型

 根据磁绝缘特征将PBFA Z加速器的4层圆盘锥形磁绝缘传输线结构分成几段串联的集中电感，结合磁绝缘临界电流、空间电荷限制流、Mendel磁压力平衡及丝阵内爆动力学等计算模型，并引入主要的结构参数，建立4层圆盘锥形磁绝缘传输线的等效电路模型，模型计算结果与实验结果、其它模型计算结果符合较好，模型可以用于定性或半定量设计及分析类似结构的磁绝缘传输线。     

磁绝缘电压叠加器功率流的电路计算方法

以已建成的10级直线变压器驱动源系统为依托,以传输线计算方法为基础,通过引入空间电荷限制流和磁绝缘流阻抗模型,对感应电压叠加(IVA)真空功率流的电路计算方法进行了探索,开发了包含磁绝缘过程的全电路计算程序,提供了一种快速评估该IVA系统真空功率流的方法。计算结果表明:对磁绝缘状态下流阻抗的描述是该方法准确计算的关键;解决好算法的数值稳定性,避免数值振荡的发生,是将这一方法推广应用的重要前提。     

磁绝缘传输线电流损失的计算方法

 磁绝缘传输线电流损失的计算方法是丝阵Z箍缩电路模拟的关键问题之一。以传输线模拟方法TLCODE为基础,将磁绝缘传输线分成若干段有损传输线单元,每个单元由一段无损传输线及一个对地损失电阻组成,根据磁绝缘准则判断单元的磁绝缘状况,磁绝缘形成之前损失电流由空间电荷限制流与传导电流的定量关系来计算,磁绝缘形成之后则根据阻抗匹配关系及流动阻抗模型来计算;同时将丝阵负载内爆动力学方程与TLCODE表达式、流动阻抗方程进行耦合,可求解磁绝缘传输线、丝阵负载在电压脉冲作用下全时空域的动态响应特性。     

长磁绝缘传输线动态特性

基于粒子模拟研究了长磁绝缘传输线的电压电流关系、自限制流状态和损失前沿传播速度的变化规律,验证了稳态流假设下建立的不同磁绝缘传输线模型在描述长磁绝缘传输线工作状态时的精确程度。结果表明:Mendel模型在描述长磁绝缘传输线电压电流关系时较精确;Rescaled模型计算长磁绝缘传输线自限制流状态下的阳极电流较精确;损失前沿在长磁绝缘传输线中的传播速度沿着能量传输方向是动态变化的,其速度的大小与输入电压上升速率相关。     

Fragmentation of dodecanethiol molecules: application to self-assembled monolayer damage in atom lithography

Atom lithography commonly employs self-assem- bled monolayers (SAMs) of alkanethiols which act as resists to protect prepared surfaces. Metastable atomic species such as helium are used to damage the resist, enabling pattern transfer via mask lithography, followed by wet chemical etching. The damage mechanism is, however, not well understood. Here we report studies of fragmentation of dodecanethiol (DDT) molecules embedded in helium nano-droplets that have been irradiated by an electron beam. The results of the charge-transfer fragmentation process provide the first experimental data on the damage mechanisms that occur in the metastable helium/SAM interaction. Received: 20 September 1999 / Revised version: 6 December 1999 / Published online: 8 March 2000     

Magnetic hexapole lens focusing of a metastable helium atomic beam for UV-free lithography

Sources of rare gas atoms in excited metastable states have been used to expose photoresist-coated substrates to demonstrate atom lithography. These thermal atomic beams are usually created by discharge sources that also produce copious amounts of UV radiation. The UV radiation simultaneously illuminates the substrate and may play a complementary role in altering the photoresist together with the metastable atoms. In the experiments reported here, we have isolated the UV component using a magnetic hexapole lens to focus a thermal beam of metastable helium atoms around a fiducial mask that blocks the UV light. This creates an atom lithography exposure that is the result of illumination by the atoms alone. We have also modelled the performance of the magnetic hexapole lens as a potentially useful device for atom lithography. PACS 39.25.+k; 81.16.Nd     

Patterning nonanethiol protected gold films by barium atoms

Self assembled monolayers (SAM) formed from nonanethiols on thin gold films were exposed to a beam of ground state and metastable neutral barium atoms through a nickel mask. The interaction of the Ba atoms with the nonanethiol layer, followed by an etching process, creates well defined structures on the gold film, with features below 100 nm. We compared the interaction of ground state Ba atoms and SAM molecules with respect to metastable Ba atoms, finding that by using metastable atoms the Ba dose per SAM molecule is reduced. The results indicate that nanofabrication in the nanometer range with barium atoms is feasible. PACS 07.77.Gx; 42.82.Cr; 81.16.Ta     

Nanolithography with metastable helium atoms in a high-power standing-wave light field

We have created periodic nanoscale structures in a gold substrate with a lithography process using metastable triplet helium atoms that damage a hydrophobic resist layer on top of the substrate. A beam of metastable helium atoms is transversely collimated and guided through an intense standing-wave light field. Compared to commonly used low-power optical masks, a high-power light field (saturation parameter of 10⁷) increases the confinement of the atoms in the standing wave considerably, and makes the alignment of the experimental setup less critical. Due to the high internal energy of the metastable helium atoms (20 eV), a dose of only one atom per resist molecule is required. With an exposure time of only eight minutes, parallel lines with a separation of 542 nm and a width of 100 nm (one-eleventh of the wavelength used for the optical mask) are created.PACS 32.80.Lg; 39.25.+k; 81.16.Nd     

Ultra-Slow Atomic Beam Generation by Velocity Selective Resonance

We describe a method to generate an ultra-slow atomic beam by velocity selective resonance （VSR）. A VSR experiment on a metastable helium beam in a magnetic field is presented and the results show that the transverse velocity of the deflected beam can be cooled and precisely controlled to less than the recoil velocity, depending on the magnitude of the magnetic field. We extend this idea to a cold atomic cloud to produce an ultra-slow 87Rb beam that can be used as a source of an atomic fountain clock or a space clock.     

Nanolithography with metastabile helium

We have used a self-assembling monolayer of dodecanethiole molecules as the resist for a lithography technique based on a beam of metastable helium atoms. Doses as low as 3 metastable helium atoms per 10 molecules are enough to write patterns into this resist. An edge resolution of 30 nanometers is demonstrated. The writing mechanism is based on the damage of the resist due to Penning ionization.     

Pattern generation with cesium atomic beams at nanometer scales

We have demonstrated that a cesium atomic beam can be used to pattern a gold surface using a self assembling monolayer (SAM) as a resist. A 12.5 m period mesh was used as a proximity mask for the atomic beam. The cesium atoms locally change the wetability of the SAM, which allows a wet etching reagent to remove the underlying gold in the exposed regions. An edge resolution of better than 100 nm was obtained. The experiment suggests that this method can either be used as a sensitive position detector with nanometer resolution in atom optics, or for nanostructuring in a resist technique.     

Precision microwave measurement of the 2 3P1-2 3P2 interval in atomic helium

The 2 3P1-to- 2 3P2 interval in atomic helium is measured using a thermal beam of metastable helium atoms excited to the 2 3P state using a 1.08-&mgr;m diode laser. The 2 3P1-to- 2 3P2 transition is driven by 2.29-GHz microwaves in a coaxial transmission line. Our result of 2 291 174.0+/-1.4 kHz is the most precise measurement of helium 2 3P fine structure. This measurement plays a key role in obtaining a new value for the fine-structure constant.     

An intense cold beam of metastable helium

We have produced and characterised a slow, bright and intense atomic beam of metastable helium atoms, suitable for atomic physics experiments. The maximum continuous flux attained was 2×10¹⁰ atoms/s, while a typical longitudinal peak velocity of the beam was ∼26 m/s with a divergence in the range of 15 mrad to 30 mrad. PACS 32.80.Pj; 32.80.Lg; 39.10.+j     

Excitation of helium atoms in collisions with plasma electrons in an electric field

The rate constants are evaluated for excitation of helium atoms in metastable states by electron impact if ionized helium is located in an external electric field and is supported by it, such that a typical electron energy is small compared to the atomic excitation energy. Under these conditions, atomic excitation is determined both by the electron traveling in the space of electron energies toward the excitation threshold and by the subsequent atomic excitation, which is a self-consistent process because it leads to a sharp decrease in the energy distribution function of electrons, which in turn determines the excitation rate. The excitation rate constant is calculated for the regimes of low and high electron densities, and in the last case, it is small compared to the equilibrium rate constant where the Maxwell distribution function is realized including its tail. Quenching of metastable atomic states by electron impact results in excitation of higher excited states, rather than transition to the ground electron state for the electric field strengths under consideration. Therefore, at restricted electron number densities, the rate of emission of resonant photons of the wavelength 58 nm, which results from the transition from the 2¹ P state of the helium atom to the ground state, is close to the excitation rate of metastable atomic states. The efficiency of atomic excitation in ionized helium, i.e., the part of energy of an electric field injected in ionized helium that is spent on atomic excitation, is evaluated. The results exhibit the importance of electron kinetics for an ionized gas located in an electric field.