Three-dimensional analysis of microwave generated plasmas with extended planar laser-induced fluorescence

We present the development and application of a diagnostic system for the analysis of microwave generated low-pressure plasmas, which might also be used for the investigation of the edge regions in magnetically confined fusion plasmas. Our method uses planar laser-induced fluorescence, which is produced by excitation of neutral metastable atoms through a short, intense, pulsed laser. The beam expansion optics consist of an uncommon setup of four lenses. By controlled shifting of an element of the optics sideways, the location of the laser sheet in the plasma is scanned perpendicular to the excitation plane. Together with a spectrometer observing different observation volumes along the beam path, we are able to map absolute three-dimensional (3D) population density distributions of the metastable ((2)P(12) (o)) 3s[12](0) (o) state of Ne I in an electron cyclotron resonance heating (ECRH) plasma. This optical tomography system was used to study the influence of the microwave power and mode on the spatial structure of the plasma. The results show that the population density of the neutral neon in this metastable state is found to be in the range of 10(16) m(-3), and that its spatial distribution is associated with the 3D structure of the magnetic field. We also report that the spatial distribution strongly varies with the mode structure, which depends on the microwave power.     

An efficient magneto-optical trap of metastable krypton atoms

We report a magneto-optical trap of metastable krypton atoms with a trap loading rate of 3×10(11) atoms/s and a trap capture efficiency of 3×10(-5). The system starts with an atomic beam of metastable krypton produced in a liquid-nitrogen cooled, radio-frequency driven discharge. The metastable beam flux emerging from the discharge is 1.5×10(14) atoms/s/sr. The flux in the forward direction is enhanced by a factor of 156 with transverse laser cooling. The atoms are then slowed inside a Zeeman slower before captured by a magneto-optic trap. The trap efficiency can be further improved, possibly to the 10(-2) level, by gas recirculation. Such an atom trap is useful in trace analysis applications where available sample size is limited.     

Studies of Hypervalent Radicals By Neutralized Ion-Beam Techniques

ABSTRACT

The advantages of obtaining both neutral fragment scattering distributions and reionized mass spectra for investigating highly reactive and/or metastable species produced by the neutralization of a high velocity precursor ion beam is outlined. Practical applications of these techniques are illustrated by results obtained recently for the neon hydride and the methanium (CH₅) radicals. In these studies a unique metastability for the ground state of NeH is identified and the previously observed metastability of CH₅ is shown to arise from the formation of high Rydberg levels of the radical rather than the ground state as first proposed. Future prospects for obtaining absorption spectra of unusual neutral species by combining optical spectroscopic techniques with those of neutralized ion beams are discussed.     

Sensitivity test of a blue-detuned dipole trap designed for parity non-conservation measurements in Fr

A dynamic blue-detuned optical dipole trap with stable (87)Rb atoms produces a differential ac Stark shift of 18 Hz in the ground state hyperfine transition, and it preserves the ground state hyperfine superpositions for a long coherence time of 180 ms. The trapped atoms undergoing microwave Rabi oscillations are sensitive to a small signal, artificially generated with a second microwave source, phase locked to the first allowing a simple and effective method for determining signal-to-noise ratio limits through interference techniques. This provides an excellent means of calibrating sensitivity in experiments such as our ongoing Fr parity non-conservation measurement.     

Metastable transfer emission spectroscopy-method and instrument for detection and measurement of trace materials in gas flows

A simple and relatively inexpensive new technique for qualitatively and quantitatively measuring various gas-phase species in a flow and the apparatus for implementation are described. Detection of atomic species has been demonstrated from concentrations greater than 10(10) atoms/cm(3) to approximately 10(4) atoms/cm(3). Several classes of molecules can also be detected quatitatively with the method, although with somewhat reduced sensitivity. The method, metastable transfer emission spectroscopy (MTES), is particularly useful in materials analysis. Possible applications including the analysis of gas, liquid, and solid samples and the determination of vapor-pressure curves are discussed.     

Measurement of xenon plasma properties in an ion thruster using laser Thomson scattering technique

This paper reports on the development of a method for measuring xenon plasma properties using the laser Thomson scattering technique, for application to ion engine system design. The thresholds of photo-ionization of xenon plasma were investigated and the number density of metastable atoms, which are photo-ionized by a probe laser, was measured using laser absorption spectroscopy, for several conditions. The measured threshold energy of the probe laser using a plano-convex lens with a focal length of 200 mm was 150 mJ for a xenon mass flow rate of 20 μg∕s and incident microwave power of 6 W; the probe laser energy was therefore set as 80 mJ. Electron number density was found to be (6.2 ± 0.4) × 10(17) m(-3) and electron temperature was found to be 2.2 ± 0.4 eV at a xenon mass flow rate of 20 μg∕s and incident microwave power of 6 W. The threshold of the probe laser intensity against photo-ionization in a miniature xenon ion thruster is almost constant for various mass flow rates, since the ratio of population of the metastable atoms to the electron number density is little changed.     

Thermal beam of metastable krypton atoms produced by optical excitation

A room-temperature beam of krypton atoms in the metastable 5s[3/2]2 level is demonstrated via an optical excitation method. A Kr-discharge lamp is used to produce vacuum ultraviolet photons at 124 nm for the first-step excitation from the ground level 4p6 1S0 to the 5s[3/2]1 level. An 819 nm Ti:sapphire laser is used for the second-step excitation from 5s[3/2]1 to 5s[3/2]2 followed by a spontaneous decay to the 5s[3/2]2 metastable level. A metastable atomic beam with an angular flux density of 3 x 10(14) s(-1) sr(-1) is achieved at the total gas flow rate of 0.01 cm3/s at STP (or 3 x 10(17) at./s). The dependences of the flux on the gas flow rate, laser power, and lamp parameters are investigated.     

湿H2S环境下16MnR钢氢鼓泡的试验研究与数值模拟

采用宏观检查、光学显微镜和扫描电镜等试验方法对16MnR钢LPG球罐在湿硫化氢环境下发生的氢鼓泡进行了取样分析，结果表明，氢鼓泡的产生与钢中沿带状珠光体分布的长条型MnS夹杂物密切相关。采用有限元软件ABAQUS，开发了氢扩散的有限元计算程序和多裂纹扩展技术，对氢向MnS夹杂扩散聚集引起氢鼓泡的行为进行有限元模拟，得到了扩散氢浓度随时间的分布以及氢鼓泡形貌。MnS／基体界面处应力集中，在湿H2s环境中，氢向界面扩散聚集，产生巨大氢压，使材料发生塑性变形，产生氢鼓泡。带状MnS夹杂物的数量越多、夹杂物越长、氢浓度越高，材料产生氢鼓泡敏感性越大。     

An apparatus for immersing trapped ions into an ultracold gas of neutral atoms

We describe a hybrid vacuum system in which a single ion or a well-defined small number of trapped ions (in our case Ba(+) or Rb(+)) can be immersed into a cloud of ultracold neutral atoms (in our case Rb). This apparatus allows for the study of collisions and interactions between atoms and ions in the ultracold regime. Our setup is a combination of a Bose-Einstein condensation apparatus and a linear Paul trap. The main design feature of the apparatus is to first separate the production locations for the ion and the ultracold atoms and then to bring the two species together. This scheme has advantages in terms of stability and available access to the region where the atom-ion collision experiments are carried out. The ion and the atoms are brought together using a moving one-dimensional optical lattice transport which vertically lifts the atomic sample over a distance of 30 cm from its production chamber into the center of the Paul trap in another chamber. We present techniques to detect and control the relative position between the ion and the atom cloud.     

用亚稳态氩原子束制作纳米结构

纳米结构制作是纳米技术的重要组成部分,原子光刻技术是纳米图形制作的一项新方法.对直流高压放电产生的亚稳态氩原子束进行准直减小其发散角,亚稳态原子在与之传播方向垂直的激光驻波场中发生淬火并沉积在基底上,破坏吸附在基底表面的SAM膜(self_assembled monolayers),结合刻蚀技术可制作出纳米量级的图形.给出该技术制作纳米图形的基本原理、方案、相关理论及模拟结果.     

冷原子光栅磁光阱的研制及 CPT 信号的探询

相干布居囚禁原子钟在小型化方面具备不可替代的优势。 由于热原子气室内部高压缓冲气体的限制,导致其频率稳定 度仍有进一步提升的空间。 利用激光冷却原子技术作为替代,可以有效提升其中长期性能。 然而,目前的冷原子物理系统仍然 相对复杂,不利于原子钟整体系统的集成化和小型化。 我们研制了高衍射效率光栅芯片、平面磁阱芯片以及微小型真空腔室, 共同构建基于平面核心器件的磁光阱,利用单光束捕获冷原子 2×10 6 个。 此外,为了简化 CPT 冷原子钟的激光系统,通过单激 光结合时分复用系统的方式,仅用单一 Rb D2 线激光实现了原子冷却与 CPT 探询。 以上的工作为将来实现微小型化高性能冷 原子 CPT 钟的最终锁定和性能评估奠定了重要理论和技术基础。     

Partial-transfer absorption imaging: A versatile technique for optimal imaging of ultracold gases

Partial-transfer absorption imaging is a tool that enables optimal imaging of atomic clouds for a wide range of optical depths. In contrast to standard absorption imaging, the technique can be minimally destructive and can be used to obtain multiple successive images of the same sample. The technique involves transferring a small fraction of the sample from an initial internal atomic state to an auxiliary state and subsequently imaging that fraction absorptively on a cycling transition. The atoms remaining in the initial state are essentially unaffected. We demonstrate the technique, discuss its applicability, and compare its performance as a minimally destructive technique to that of phase-contrast imaging.     

Imaging with neutral atoms—a new matter-wave microscope

Matter‐wave microscopy can be dated back to 1932 when Max Knoll and Ernst Ruska published the first image obtained with a beam of focussed electrons. In this paper a new step in the development of matter‐wave microscopy is presented. We have created an instrument where a focussed beam of neutral, ground‐state atoms (helium) is used to image a sample. We present the first 2D images obtained using this new technique. The imaged sample is a free‐standing hexagonal copper grating (with a period of about 36 μm and rod thickness of about 8 μm). The images were obtained in transmission mode by scanning the focussed beam, which had a minimum spot size of about 2.0 μm in diameter (full width at half maximum) across the sample. The smallest focus achieved was 1.9 ± 0.1 μm. The resolution for this experiment was limited by the speed ratio of the atomic beam through the chromatic aberrations of the zone plate that was used to focus. Ultimately the theoretical resolution limit is set by the wavelength of the probing particle. In praxis, the resolution is limited by the source and the focussing optics.     

Improving metastable impact electron spectroscopy and ultraviolet photoelectron spectroscopy signals by means of a modified time-of-flight separation

The separation of ultraviolet photoelectron spectroscopy (UPS) and metastable impact electron spectroscopy (MIES) is usually performed by a time-of-flight (ToF) separation using pre-set ToF for both types of signal. In this work, we present a new, improved ex situ signal separation method for the separation of MIES and UPS for every single measurement. Signal separation issues due to changes of system parameters can be overcome by changing the ToF separation and therefore allowing for the application of a wider range of measuring conditions. The method also enables to identify and achieve separation of the two signals without any time consuming calibration and the use of any special material for the calibration. Furthermore, changes made to the discharge source are described that enable to operate an existing MIES/UPS source over a broader range of conditions. This allows for tuning of the yield of UV photons and metastable rare gas atoms leading to an improved signal to noise ratio. First results of this improved setup are well in agreement with spectra reported in literature and show increased resolution and higher signal intensities for both MIE and UP spectra compared to the previous, non-optimized setup.     

Measurement of probe displacement to the thermal resolution limit in photonic force microscopy using a miniature quadrant photodetector

A photonic force microscope comprises of an optically trapped micro-probe and a position detection system to track the motion of the probe. Signal collection for motion detection is often carried out using the backscattered light off the probe-however, this mode has problems of low S/N due to the small backscattering cross sections of the micro-probes typically used. The position sensors often used in these cases are quadrant photodetectors. To ensure maximum sensitivity of such detectors, it would help if the detector size matched with the detection beam radius after the condenser lens (which for backscattered detection would be the trapping objective itself). To suit this condition, we have used a miniature displacement sensor whose dimensions makes it ideal to work with 1:1 images of micrometer-sized trapped probes in the backscattering detection mode. The detector is based on the quadrant photo-integrated chip in the optical pick-up head of a compact disc player. Using this detector, we measured absolute displacements of an optically trapped 1.1 μm probe with a resolution of ～10 nm for a bandwidth of 10 Hz at 95% significance without any sample or laser stabilization. We characterized our optical trap for different sized probes by measuring the power spectrum for each probe to 1% accuracy, and found that for 1.1 μm diameter probes, the noise in our position measurement matched the thermal resolution limit for averaging times up to 10 ms. We also achieved a linear response range of around 385 nm with cross talk between axes ?4% for 1.1 μm diameter probes. The detector has extremely high bandwidth (few MHz) and low optical power threshold-other factors that can lead to its widespread use in photonic force microscopy.     

Versatile cold atom target apparatus

We report on a compact and transportable apparatus that consists of a cold atomic target at the center of a high resolution recoil ion momentum spectrometer. Cold rubidium atoms serve as a target which can be operated in three different modes: in continuous mode, consisting of a cold atom beam generated by a two-dimensional magneto-optical trap, in normal mode in which the atoms from the beam are trapped in a three-dimensional magneto-optical trap (3D MOT), and in high density mode in which the 3D MOT is operated in dark spontaneous optical trap configuration. The targets are characterized using photoionization.     

Development of a stable source of atomic oxygen with a pulsed high-voltage discharge and its application to crossed-beam reactions

To investigate the reactions of oxygen atoms with ethene and silane in a crossed-beam condition, we developed a stable, highly intense, and short-pulsed source of atomic oxygen with a transient high-voltage discharge. Mixtures of O(2) and He served as discharge media. Utilizing a crossed molecular-beam apparatus and direct vacuum-ultraviolet ionization, we measured the temporal profiles of oxygen atoms and the time-of-flight spectra of reaction products. With O(2) 3% seeded in He as a discharge medium, oxygen atoms might have a full width as small as 13.5 micros at half maximum at a location 193 mm downstream from the discharge region. Most population of oxygen atoms is in the ground state (3)P but some in the first excited state (1)D, depending on the concentration of precursor O(2). This discharge device analogously generates carbon, nitrogen, and fluorine atoms from precursors CO, N(2), and F(2), respectively.     

基于辐射压力控制的微型元件的加工与检测

We have been developing new fabrication tools based on optical radiation pressur e and related phenomena to develop aflexible and accurate microfabrication tec hnology. In this paper, the laser trapping probe for the nano-CMM for assessment, in addition to micromachining technique using a small particle controlled by optical radiation pressure and laser aggregation technique are discussed. As the positional detection probe for the nano-CMM, an optically trapped silica particle with 8 mm diameter in forced oscillation state is used. A probe sphere retains a stable position when applied with trapping force by Nd:YAG laser light formed an nu lar and is forced to oscillate by the driving force changed by modulating the in tensity of LD emission. Experintal results show that this vibrational microprobe h as the possibility to achieve positional sensing accuracy of less than 25 nm. As a new micromachining technique, nano-removal process using an optically trapped micro-grain is proposed. The laser trapping force enables not only to stably trap the diamond grain with asymmetrical shape but also to freely control the positi on with spinning. Using this micro machining tool, the machining experiments of h ydrocarbon film are performed. AFM observation confirmed that the fine groove wi th depths of about 3～4 nm can be fabricated. As an additive process based on ra diation pressure, a laser microstructure fabrication using laser agglomeration p h enomena of colloidal particles aided by radiation pressure is investigated. By c ontrolling laser beam scanning in slurry containing KOH solution and SiO₂ par ticles with a diameter of 140 nm, colloidal particles are aggregated and adhered firmly to a silicon wafer substrate. Using this laser agglomerating process, two-dimensional grid microstructures at the pitch of 5 mm can be fabricated.     

Continuous wave cavity ring down spectroscopy measurements of velocity distribution functions of argon ions in a helicon plasma

We report continuous wave cavity ring down spectroscopy (CW-CRDS) measurements of ion velocity distribution functions (VDFs) in low pressure argon helicon plasma (magnetic field strength of 600 G, T(e) ≈ 4 eV and n ≈ 5 × 10(11) cm(-3)). Laser induced fluorescence (LIF) is routinely used to measure VDFs of argon ions, argon neutrals, helium neutrals, and xenon ions in helicon sources. Here, we describe a CW-CRDS diagnostic based on a narrow line width, tunable diode laser as an alternative technique to measure VDFs in similar regimes but where LIF is inapplicable. Being an ultra-sensitive, cavity enhanced absorption spectroscopic technique; CW-CRDS can also provide a direct quantitative measurement of the absolute metastable state density. The proof of principle CW-CRDS measurements presented here are of the Doppler broadened absorption spectrum of Ar II at 668.6138 nm. Extrapolating from these initial measurements, it is expected that this diagnostic is suitable for neutrals and ions in plasmas ranging in density from 1 × 10(9) cm(-3) to 1 × 10(13) cm(-3) and target species temperatures less than 20 eV.     

A dark-line two-dimensional magneto-optical trap of (85)Rb atoms with high optical depth

We describe the apparatus of a dark-line two-dimensional (2D) magneto-optical trap (MOT) of (85)Rb cold atoms with high optical depth (OD). Different from the conventional configuration, two (of three) pairs of trapping laser beams in our 2D MOT setup do not follow the symmetry axes of the quadrupole magnetic field: they are aligned with 45° angles to the longitudinal axis. Two orthogonal repumping laser beams have a dark-line volume in the longitudinal axis at their cross over. With a total trapping laser power of 40 mW and repumping laser power of 18 mW, we obtain an atomic OD up to 160 in an electromagnetically induced transparency (EIT) scheme, which corresponds to an atomic-density-length product NL = 2.05 × 10(15) m(-2). In a closed two-state system, the OD can become as large as more than 600. Our 2D MOT configuration allows full optical access of the atoms in its longitudinal direction without interfering with the trapping and repumping laser beams spatially. Moreover, the zero magnetic field along the longitudinal axis allows the cold atoms maintain a long ground-state coherence time without switching off the MOT magnetic field, which makes it possible to operate the MOT at a high repetition rate and a high duty cycle. Our 2D MOT is ideal for atomic-ensemble-based quantum optics applications, such as EIT, entangled photon pair generation, optical quantum memory, and quantum information processing.