Characterization and local modification of atomically flat gold surfaces by STM

In order to characterize the surface of epitaxial gold on mica in air we have used a scanning tunnelling microscope (STM) to image and to modify this surface. It was possible to create controlled features by applying voltage pulses on the tip while scanning. The voltage threshold for writing (about 3 V, 100 ns pulses) was dependent of the tip condition. The lowest pulses were associated with sub-50 Å feature size. We observed that at ambient temperature the written features disappeared in a time scale of half an hour for the smallest (<30 Å) to a few hours for the bigger features (～500 Å). We have used the same surface as a substrate for organic imaging. We also present images of a polymer deposited on gold.     

Chemical Degradation of a Multiply Alkylated Cyclopentane (MAC) Oil During Wear: Implications for Spacecraft Attitude Control System Bearings

The wear-induced chemical degradation process was examined using deliberately under-lubricated thrust ball bearings to create wear in a reasonable time frame. The liquid-phase products were analyzed using infrared spectroscopy and gel-permeation chromatography. Our results indicate that oil molecules react with the freshly created clean Fe surface produced during wear, and fragment into radicals by breaking C–C bonds. These free radicals produce alkenes, which can then be further attacked by radicals, leading to the formation of larger molecules (oligomers and eventually polymers). A side reaction, production of iron carboxylate salts, also occurred during wear. Additional experiments indicate that organic acids can be produced by radical reaction with the oxidized areas of the bearing surfaces and wear particles, and these acids can then attack the clean Fe surfaces created by wear to form iron carboxylate salts. We used our experimental results to estimate the amount of degradation likely to be produced in attitude control system bearing tests under conditions that produce wear and poor performance. We find that multiply alkylated cyclopentane (MAC) oil chemical degradation is an effect of wear, and is insufficient to be a cause of failure itself. Life predictions for MAC-lubricated attitude control system bearings must be based on predictions of oil transport and wear since failures are due to the failure of the lubrication system, not of the lubricant itself.     

Multichannel Spark Gaps with Control Bar Electrodes: Their Development and Application (A Review)

Multichannel low-inductance (1 nH) gas-filled spark gaps (MSGs)¹ with several tens of channels each, bar control electrodes designed for an operating voltage of 100 kV, and a switched current of up to 400 kA are reviewed. The control electrodes, made in the form of narrow thin plates, have an intermediate potential, are positioned in the gap between two common main electrodes (high-voltage and low-voltage (grounded)), and are oriented uniformly along their length. Upon a near-simultaneous change in the bars' potential in a time of <15 ns, applying a signal through trigger circuits disturbs the electric-field distribution in the gas volume. The field strength sharply increases at the electrode surfaces and especially at the edges of the bars, from which breakdowns develop synchronously from one electrode to another or simultaneously to both main electrodes. When the discharge formation is completed, the main electrodes of the MSGs are short-circuited by discharges through parallel channels (whose number is equal to the number of bars). These switches ensure the nanosecond accuracy of the operation delay relative to the trigger pulse at a breakdown-strength margin of up to 100%, determined by the pressure (>0.1 MPa) of the MSG-filling gas. Electrical circuits for initiating the discharge development in the MSGs, the transients in such circuits, and the factors affecting the parameters of processes and the gap-breakdown delay and rate are considered. Particular MSG designs, multicable systems for parallel triggering of a large number of MSGs, and the use of 48 four-channel 50-kV MSGs in the first iron-free LIA-2 linear electron accelerator (2 MeV, 25 kA, and 60 ns) created in 1967 are described. The successful operation of MSGs stimulated further studies and the development of efficient trigatrons for operating voltages of 100 and 500 kV. Up to 3000 MSGs of this type are used in new high-power linear electron accelerators. A low-impedance (0.45 ) generator of high-voltage pulses (50–200 kV) with a multicable output has been developed to synchronously trigger such large numbers of trigatrons as these.     

Morphological studies of pseudowollastonite for biomedical application

Pseudowollastonite ceramic (psW) composed of CaO·SiO₂ was found to be bioactive in a simulated body fluid environment. The chemical reaction initiated at the material surface resulted in hydroxyapatite (HA)formation. These bone-bonding properties are essential for securing the necessary physico-chemical integration of the material with living bone. Materials behaving in this way can be considered for potential biomedical application as bone tissue substitute for a natural bone repair or replacement as implant. A mechanism of hydroxyapatite formation on pseudowollastonite ceramics surface was investigated during exposure to a simulated body fluid (SBF) for a period of 3 weeks. Morphology and structure of the surface product and its original substrate was examined by thin-film X-ray diffraction, scanning electron microscopy and high-resolution transmission electron microscopy. HA crystals were found to form on an amorphous silica intermediate layer. (100) lattice planes of HA were resolved and identified. Concentration of ions in the SBF and pH of the SBF were monitored throughout the exposure. Additional pH measurements were made at the interface of psW with SBF. The HA formation occurred when there was a sudden increase of pH from 7·25 to 10·5 at the interface of psW with SBF as a result of ionic exchange between 2H⁺ and Ca²⁺ within the psW network. This ionic exchange transformed the psW crystals into an amorphous silica phase. The appropriate pH and the ion concentrations were essential for partial dissolution of the amorphous silica phase and subsequent precipitation of a Ca-P rich phase which then transformed to HA.     

High speed single charge coupled device Cranz-Schardin camera

This article describes an ultrahigh speed visualization system based on a miniaturization of the Cranz-Schardin principle. It uses a set of high power light emitting diodes (LEDs) (Golden Dragon) as the light source and a highly sensitive charge coupled device (CCD) camera for reception. Each LED is fired in sequence and images the refraction index variation between two relay lenses, on a partial region of a CCD image sensor. The originality of this system consists in achieving several images on a single CCD during a frame time. The number of images is 4. The time interval between successive firings determines the speed of the imaging system. This time lies from 100 ns to 10 micros. The light pulse duration lies from 100 ns to 10 micros. The principle and the optical and electronic parts of such a system are described. As an example, some images of acoustic waves propagating in water are presented.     

Cluster analysis of soft X-ray spectromicroscopy data

Soft X-ray spectromicroscopy provides spectral data on the chemical speciation of light elements at sub-100 nm spatial resolution. When all chemical species in a specimen are known and separately characterized, existing approaches can be used to measure the concentration of each component at each pixel. In other cases (such as often occur in biology or environmental science), some spectral signatures may not be known in advance so other approaches must be used. We describe here an approach that uses principal component analysis to orthogonalize and noise-filter spectromicroscopy data. We then use cluster analysis (a form of unsupervised pattern matching) to classify pixels according to spectral similarity, to extract representative, cluster-averaged spectra with good signal-to-noise ratio, and to obtain gradations of concentration of these representative spectra at each pixel. The method is illustrated with a simulated data set of organic compounds, and a mixture of lutetium in hematite used to understand colloidal transport properties of radionuclides.     

Low-dose performance of parallel-beam nanodiffraction

We evaluate the low-dose performance of parallel nano-beam diffraction (NBD) in the transmission electron microscope as a method for characterizing radiation sensitive materials at low electron irradiation dose. A criterion, analogous to Rose's, is established for detecting a diffraction spot with desired signal-to-noise ratio. Our experimental data show that a dose substantially lower than in high-resolution bright-field imaging is sufficient to determine structure and orientation of individual nanoscale objects embedded in amorphous matrix. In an instrument equipped with a cold field-emission gun it is possible to form a probe with sub-3 nm diameter and sub-0.3 mrad convergence angle with sufficient beam current to record a diffraction pattern with less than 0.2 s acquisition time. The interpretation of NBD patterns is identical to that of selected area diffraction patterns. We illustrate the physical principles underlying good low-dose performance of NBD by means of a phase grating. The electron irradiation dose needed to detect a diffraction peak in NBD is found proportional to 1/N2, where N is the number of lattice planes contributing to the peak.     

A High-Voltage Pulse-Packet Shaper

A high-voltage nanosecond-pulse shaper based on microcircuits and high-power MOS transistors is described. The output pulse amplitude is 5 to 300 V; at a 50- load, pulse duration is 100 ns; pulse rise time is 5 ns; pulse-repetition period in a packet is 100 s; packet duration is 10 ms, and packet-repetition period is 100 ms.     

An ultra low noise telecom wavelength free running single photon detector using negative feedback avalanche diode

It is challenging to implement genuine free running single-photon detectors for the 1550 nm wavelength range with simultaneously high detection efficiency (DE), low dark noise, and good time resolution. We report a novel read out system for the signals from a negative feedback avalanche diode (NFAD) [M. A. Itzler, X. Jiang, B. Nyman, and K. Slomkowski, "Quantum sensing and nanophotonic devices VI," Proc. SPIE 7222, 72221K (2009); X. Jiang, M. A. Itzler, K. ODonnell, M. Entwistle, and K. Slomkowski, "Advanced photon counting techniques V," Proc. SPIE 8033, 80330K (2011); M. A. Itzler, X. Jiang, B. M. Onat, and K. Slomkowski, "Quantum sensing and nanophotonic devices VII," Proc. SPIE 7608, 760829 (2010)], which allows useful operation of these devices at a temperature of 193 K and results in very low darkcounts (～100 counts per second (CPS)), good time jitter (～30 ps), and good DE (～10%). We characterized two NFADs with a time-correlation method using photons generated from weak coherent pulses and photon pairs produced by spontaneous parametric down conversion. The inferred detector efficiencies for both types of photon sources agree with each other. The best noise equivalent power of the device is estimated to be 8.1 × 10(-18) W?Hz(-1∕2), more than 10 times better than typical InP/InGaAs single photon avalanche diodes (SPADs) show in free running mode. The afterpulsing probability was found to be less than 0.1% per ns at the optimized operating point. In addition, we studied the performance of an entanglement-based quantum key distribution (QKD) using these detectors and develop a model for the quantum bit error rate that incorporates the afterpulsing coefficients. We verified experimentally that using these NFADs it is feasible to implement QKD over 400 km of telecom fiber. Our NFAD photon detector system is very simple, and is well suited for single-photon applications where ultra-low noise and free-running operation is required, and some afterpulsing can be tolerated.     

Software defined photon counting system for time resolved x-ray experiments

The time structure of synchrotron radiation allows time resolved experiments with sub-100 ps temporal resolution using a pump-probe approach. However, the relaxation time of the samples may require a lower repetition rate of the pump pulse compared to the full repetition rate of the x-ray pulses from the synchrotron. The use of only the x-ray pulse immediately following the pump pulse is not efficient and often requires special operation modes where only a few buckets of the storage ring are filled. We designed a novel software defined photon counting system that allows to implement a variety of pump-probe schemes at the full repetition rate. The high number of photon counters allows to detect the response of the sample at multiple time delays simultaneously, thus improving the efficiency of the experiment. The system has been successfully applied to time resolved scanning transmission x-ray microscopy. However, this technique is applicable more generally.     

A high voltage pulsed power supply for capillary discharge waveguide applications

We present an all solid-state, high voltage pulsed power supply for inducing stable plasma formation (density ～10(18) cm(-3)) in gas-filled capillary discharge waveguides. The pulser (pulse duration of 1 μs) is based on transistor switching and wound transmission line transformer technology. For a capillary of length 40 mm and diameter 265 μm and gas backing pressure of 100 mbar, a fast voltage pulse risetime of 95 ns initiates breakdown at 13 kV along the capillary. A peak current of ～280 A indicates near complete ionization, and the r.m.s. temporal jitter in the current pulse is only 4 ns. Temporally stable plasma formation is crucial for deploying capillary waveguides as plasma channels in laser-plasma interaction experiments, such as the laser wakefield accelerator.     

平置板式主动浮筏系统的隔振研究

建立了浮筏系统的主被动联合控制模型,推导了多机组、多扰动源的平置板式复杂浮筏隔振系统的各个子系统的导纳矩阵表达式,实现了浮筏隔振系统的主动控制和被动控制的统一数学描述。阐释了机器控制、浮筏控制和全控制等各种控制形式的概念,研究了最小功率流控制策略下不同控制形式的主动浮筏隔振系统的功率流传递特性,并给出了主动浮筏系统隔振设计的基本准则。     

二维爆震波胞格结构的数值模拟

用时间算子分裂法来分离普通流体流动和化学反应方程,采用加权基本无振荡格式构建了带有复杂状态方程的欧拉方程组;提出一种新的熵修正方法,并结合Roe平均格式来解决激波的不稳定问题。通过对带有详细化学反应的爆震波进行数值模拟,获得了独特规则的胞格结构,描述了胞格的特点及其形成的原因,胞格的长宽比和以往的实验和计算结果基本一致。研究结果表明新的熵修正方法和加权基本无振荡格式可以很好地进行爆震波结构的模拟。     

X-ray contact microscopy using an excimer laser plasma source with different target materials and laser pulse durations

Soft X-ray contact microscopy (SXCM), using a pulsed X-ray source, offers the possibility of imaging the ultrastructure of living biological systems at sub-100 nm resolution. We have developed a table-top pulsed plasma X-ray source for this application, generated by a large-volume XeCl laser, achieving a good conversion efficiency to 'water-window' X-rays (hν≈280–530 eV).
Optimum plasma conditions for SXCM are discussed, including the effect of pulse duration, target material and resist development time on image resolution. Soft X-ray contact images of Chlamydomonas dysosmos (unicellular alga) and of the cyanobacterium Leptolyngbya are shown.     

A Multichannel Automatized System for Measuring Current Pulses in the LIA-30 Electron Accelerator

In the LIA-30 high-power linear pulsed induction electron accelerator (40 MeV, 100 kA, 25 ns), the energy is stored, and pulses of the accelerating voltage are shaped by 288 water-insulated radial lines arranged in succession along a common air-free acceleration channel. The lines are simultaneously charged up to 500 kV from 72 shielded Marx generators. To measure the parameters (amplitude, pulse shape, pulse rise time, pulse fall time, and pulse duration) of the synchronized pulses of the charging current with amplitudes as high as 60 kA and duration of 0.85 s in each of the 72 charging circuits, an automatized measuring system is used. The current pulse is sensed at the output of each generator by a self-integrating Rogovsky coil galvanically isolated from the generator. The signal from the coil is transmitted over a cable to an analog-to-digital converter, sampled with a period of 50 ns, and recorded in memory. Upon operating the accelerator, the signals are reproduced in succession or selectively on the display screen, and their shapes are compared to the shape of a standard pulse.     

Application of microwave spectroscopy to chemical analysis.

Traditional analytical techniques have been sucessfully employed on a great variety of systems for identifying stable reaction products as well as for purity analysis and reaction efficiency. However, most analytical techniques generally can only be applied to "well-behaved" chemical systems. Since gas phase reaction products do not always follow the "well-behaved" guideline, an alternate analytical tool, such as microwave spectroscopy, could prove beneficial for analyzing such systems and may well provide some new routes to synthetic chemistry. A general review of the present state-of-the-art for applications of microwave techniques to analytical studies will be presented. An attempt will be made to describe the limitations and advantages of microwave spectroscopy for probing chemical systems for product identification and for optimization of the efficiency of gas phase chemical reactions. As an illustration, some recent results obtained in our laboratory on the complex pyrolytic decomposition reactions of ethylamine will be described. Further examples of possible applications will be taken from typical industrial processes that employ vapor phase chemical synthesis techniques.     

A Pseudospark Gap in the Inductive-Energy-Storage Circuit

A pseudospark gap is described that is capable of interrupting a current of up to 1 kA and shaping voltage pulses with an amplitude of up to 110 kV and rise time of 100 ns. Analytical expressions for calculating the energy released in the device at the switching-off stage and the efficiency of the generator with an inductive energy storage system are obtained. The characteristics of the pseudospark gap and of a similar-design thyratron connected in the same circuit are compared. It is found that the turn-off time of the pseudospark gap is shorter than that of the thyratron by a factor of 2.5.     

Time- and angle-resolved x-ray diffraction to probe structural and chemical evolution during Al-Ni intermetallic reactions

We present novel time- and angle-resolved x-ray diffraction (TARXD) capable of probing structural and chemical evolutions during rapidly propagating exothermic intermetallic reactions between Ni-Al multilayers. The system utilizes monochromatic synchrotron x-rays and a two-dimensional (2D) pixel array x-ray detector in combination of a fast-rotating diffraction beam chopper, providing a time (in azimuth) and angle (in distance) resolved x-ray diffraction image continuously recorded at a time resolution of ~30 μs over a time period of 3 ms. Multiple frames of the TARXD images can also be obtained with time resolutions between 30 and 300 μs over three to several hundreds of milliseconds. The present method is coupled with a high-speed camera and a six-channel optical pyrometer to determine the reaction characteristics including the propagation speed of 7.6 m/s, adiabatic heating rate of 4.0 × 10(6) K/s, and conductive cooling rate of 4.5 × 10(4) K/s. These time-dependent structural and temperature data provide evidences for the rapid formation of intermetallic NiAl alloy within 45 μs, thermal expansion coefficient of 1.1 × 10(-6) K for NiAl, and crystallization of V and Ag(3)In in later time.     

Time-resolved imaging of laser-induced refractive index changes in transparent media

We describe a method to visualize ultrafast laser-induced refractive index changes in transparent materials with a 310 fs impulse response and a submicrometer spatial resolution. The temporal profile of the laser excitation sequence can be arbitrarily set on the subpicosecond and picosecond time scales with a pulse shaping unit, allowing for complex laser excitation. Time-resolved phase contrast microscopy reveals the real part of the refractive index change and complementary time-resolved optical transmission microscopy measurements give access to the imaginary part of the refractive index in the irradiated region. A femtosecond laser source probes the complex refractive index changes from the excitation time up to 1 ns, and a frequency-doubled Nd:YAG laser emitting 1 ns duration pulses is employed for collecting data at longer time delays, when the evolution is slow. We demonstrate the performance of our setup by studying the energy relaxation in a fused silica sample after irradiation with a double pulse sequence. The excitation pulses are separated by 3 ps. Our results show two dimensional refractive index maps at different times from 200 fs to 100 μs after the laser excitation. On the subpicosecond time scale we have access to the spatial characteristics of the energy deposition into the sample. At longer times (800 ps), time-resolved phase contrast microscopy shows the appearance of a strong compression wave emitted from the excited region. On the microsecond time scale, we observe energy transfer outside the irradiated region.