James R. Whetstone

The Gaseous Electronics Conference radio‐frequency reference cell: A defined parallel‐plate radio‐frequency system for experimental and theoretical studies of plasma‐processing discharges

A “reference cell” for generating radio-frequency (rf) glow discharges in gases at a frequency of 13.56 MHz is described. The reference cell provides an experimental platform for comparing plasma measurements carried out in a common reactor geometry by different experimental groups, thereby enhancing the transfer of knowledge and insight gained in rf discharge studies. The results of performing ostensibly identical measurements on six of these cells in five different laboratories are analyzed and discussed. Measurements were made of plasma voltage and current characteristics for discharges in pure argon at specified values of applied voltages, gas pressures, and gas flow rates. Data are presented on relevant electrical quantities derived from Fourier analysis of the voltage and current wave forms. Amplitudes, phase shifts, self-bias voltages, and power dissipation were measured. Each of the cells was characterized in terms of its measured internal reactive components. Comparing results from different cells provides an indication of the degree of precision needed to define the electrical configuration and operating parameters in order to achieve identical performance at various laboratories. The results show, for example, that the external circuit, including the reactive components of the rf power source, can significantly influence the discharge. Results obtained in reference cells with identical rf power sources demonstrate that considerable progress has been made in developing a phenomenological understanding of the conditions needed to obtain reproducible discharge conditions in independent reference cells.

Direct measurements show decreasing methane emissions from natural gas local distribution systems in the United States.

Fugitive losses from natural gas distribution systems are a significant source of anthropogenic methane. Here, we report on a national sampling program to measure methane emissions from 13 urban distribution systems across the U.S. Emission factors were derived from direct measurements at 230 underground pipeline leaks and 229 metering and regulating facilities using stratified random sampling. When these new emission factors are combined with estimates for customer meters, maintenance, and upsets, and current pipeline miles and numbers of facilities, the total estimate is 393 Gg/yr with a 95% upper confidence limit of 854 Gg/yr (0.10% to 0.22% of the methane delivered nationwide). This fraction includes emissions from city gates to the customer meter, but does not include other urban sources or those downstream of customer meters. The upper confidence limit accounts for the skewed distribution of measurements, where a few large emitters accounted for most of the emissions. This emission estimate is 36% to 70% less than the 2011 EPA inventory, (based largely on 1990s emission data), and reflects significant upgrades at metering and regulating stations, improvements in leak detection and maintenance activities, as well as potential effects from differences in methodologies between the two studies.

Direct and Indirect Measurements and Modeling of Methane Emissions in Indianapolis, Indiana

This paper describes process-based estimation of CH4 emissions from sources in Indianapolis, IN and compares these with atmospheric inferences of whole city emissions. Emissions from the natural gas distribution system were estimated from measurements at metering and regulating stations and from pipeline leaks. Tracer methods and inverse plume modeling were used to estimate emissions from the major landfill and wastewater treatment plant. These direct source measurements informed the compilation of a methane emission inventory for the city equal to 29 Gg/yr (5% to 95% confidence limits, 15 to 54 Gg/yr). Emission estimates for the whole city based on an aircraft mass balance method and from inverse modeling of CH4 tower observations were 41 ± 12 Gg/yr and 81 ± 11 Gg/yr, respectively. Footprint modeling using 11 days of ethane/methane tower data indicated that landfills, wastewater treatment, wetlands, and other biological sources contribute 48% while natural gas usage and other fossil fuel sources contribute 52% of the city total. With the biogenic CH4 emissions omitted, the top-down estimates are 3.5-6.9 times the nonbiogenic city inventory. Mobile mapping of CH4 concentrations showed low level enhancement of CH4 throughout the city reflecting diffuse natural gas leakage and downstream usage as possible sources for the missing residual in the inventory.

Electrical measurements for monitoring and control of rf plasma processing

We have investigated the possibility of using current and voltage measurements for real-time monitoring and control of radio-frequency discharges. Specifically, we have equipped a gaseous electronics conference (GEC) rf reference cell with a computer-controlled measurement system that samples the voltage and current waveforms at the cell power input and Fourier analyzes these waveforms to obtain the amplitude and phase of their fundamental and harmonic components. The system accounts for errors introduced by the stray impedance of the cell, yielding corrected values that more accurately reflect the values of voltage and current at electrode surfaces in contact with the plasma. These corrected values are monitored to reveal changes in fundamental plasma parameters such as sheath voltages, sheath fields, and sheath (dark space) thicknesses. Furthermore, the corrected values serve as better control parameters than the raw values of voltage, current or power, measured externally. The time required for the acquisition and analysis of a pair of current and voltage waveforms is approximately one second, making these measurements suitable for real-time sensing and control applications.

The Indianapolis Flux Experiment (INFLUX): A test-bed for developing urban greenhouse gas emission measurements

The objective of the Indianapolis Flux Experiment (INFLUX) is to develop, evaluate and improve methods for measuring greenhouse gas (GHG) emissions from cities. INFLUX’s scientific objectives are to quantify CO2 and CH4 emission rates at 1 km resolution with a 10% or better accuracy and precision, to determine whole-city emissions with similar skill, and to achieve high (weekly or finer) temporal resolution at both spatial resolutions. The experiment employs atmospheric GHG measurements from both towers and aircraft, atmospheric transport observations and models, and activity-based inventory products to quantify urban GHG emissions. Multiple, independent methods for estimating urban emissions are a central facet of our experimental design. INFLUX was initiated in 2010 and measurements and analyses are ongoing. To date we have quantified urban atmospheric GHG enhancements using aircraft and towers with measurements collected over multiple years, and have estimated whole-city CO2 and CH4 emissions using aircraft and tower GHG measurements, and inventory methods. Significant differences exist across methods; these differences have not yet been resolved; research to reduce uncertainties and reconcile these differences is underway. Sectorally- and spatially-resolved flux estimates, and detection of changes of fluxes over time, are also active research topics. Major challenges include developing methods for distinguishing anthropogenic from biogenic CO2 fluxes, improving our ability to interpret atmospheric GHG measurements close to urban GHG sources and across a broader range of atmospheric stability conditions, and quantifying uncertainties in inventory data products. INFLUX data and tools are intended to serve as an open resource and test bed for future investigations. Well-documented, public archival of data and methods is under development in support of this objective.

The second-generation NIST standard hygrometer

A second-generation standard hygrometer has been completed at the National Institute of Standards and Technology (NIST). This hygrometer measures humidity using a gravimetric method: it separates the water from the carrier gas and afterwards measures the water mass and carrier gas mass. These two measurements determine the mass ratio r (the ratio of the measured water mass to the measured dry-gas mass). The new design allows automated continuous gas collection at up to 3?L?min?1. This enables the hygrometer to collect larger amounts of gas and thereby measure humidity values lower than that measured by the previous NIST standard hygrometer. When operated in an optimal thermal environment (minimal thermal loads in the laboratory), the total expanded relative uncertainty (k = 2) of the gravimetric hygrometer is approximately 0.1% for atmospheric-pressure frost points higher than ?35??C (r = 250??g?g?1). Below this frost point the total expanded relative uncertainty gradually increases to approximately 1% at ?55??C (r = 13??g?g?1). The hygrometer has measured the humidity of gas samples produced by the NIST Hybrid Generator and the NIST Low Frost-Point Generator with dew/frost points from ?35??C to 71??C. For both generators the differences between the humidity generated and the humidity measured by the gravimetric hygrometer are less than the combined uncertainties of the generator and the hygrometer.

Tower-based greenhouse gas measurement network design—The National Institute of Standards and Technology North East Corridor Testbed

The North–East Corridor (NEC) Testbed project is the 3rd of three NIST (National Institute of Standards and Technology) greenhouse gas emissions testbeds designed to advance greenhouse gas measurements capabilities. A design approach for a dense observing network combined with atmospheric inversion methodologies is described. The Advanced Research Weather Research and Forecasting Model with the Stochastic Time-Inverted Lagrangian Transport model were used to derive the sensitivity of hypothetical observations to surface greenhouse gas emissions (footprints). Unlike other network design algorithms, an iterative selection algorithm, based on a k-means clustering method, was applied to minimize the similarities between the temporal response of each site and maximize sensitivity to the urban emissions contribution. Once a network was selected, a synthetic inversion Bayesian Kalman filter was used to evaluate observing system performance. We present the performances of various measurement network configurations consisting of differing numbers of towers and tower locations. Results show that an overly spatially compact network has decreased spatial coverage, as the spatial information added per site is then suboptimal as to cover the largest possible area, whilst networks dispersed too broadly lose capabilities of constraining flux uncertainties. In addition, we explore the possibility of using a very high density network of lower cost and performance sensors characterized by larger uncertainties and temporal drift. Analysis convergence is faster with a large number of observing locations, reducing the response time of the filter. Larger uncertainties in the observations implies lower values of uncertainty reduction. On the other hand, the drift is a bias in nature, which is added to the observations and, therefore, biasing the retrieved fluxes.摘要东北走廊(NEC, North-East Corridor)测试平台项目是美国国家标准与技术研究所(NIST, National Institute of Standards and Technology)的第三个温室气体排放源测试平台. 本项目旨在提高温室气体测量能力. 本文介绍了这个项目的与大气反演方法相结合的密集观测网络的设计方法. 这种方法应用 WRF(ARW 版本) 模式(Advanced ResearchWeather Research and Forecasting Model)与STILT模式(Stochastic Time-Inverted Lagrangian Transport model)相耦合来计算假定的观测对地表温室气体排放源的敏感性(足迹). 和其他观测网的设计算法的不同之处在于, 这个密集观测网络应用一个基于 k-means 聚类方法的迭代挑选算法, 以最小化每个站点的时间响应之间的相似性, 并最大化城市排放源贡献的敏感性. 一旦选择了某种配置的观测网, 将使用综合反演贝叶斯卡尔曼滤波来评估它的性能. 我们展示了由不同数量的塔和不同的位置的塔组成的不同配置的多个观测网的性能. 结果表明, 由于附加到每个站点的空间信息不能最理想的覆盖最大可能的区域, 过度密集的观测网的空间覆盖范围会减小, 而过度分散的观测网则会失去约束通量不确定性的能力. 此外, 我们还探讨了使用带有较低成本和具有较大不确定性和时间偏移的性能传感器的高密度网络的可能性. 当观测站点较多时, 分析收敛速度变快, 减少了滤波的响应时间. 观测中的较大不确定性意味着较少的不确定性的减少值. 另一方面, 自然界中存在的偏差被带入到观测中, 从而使得反演通量有偏差.

Diagnostic measurements in rf plasmas for materials processing

Radio frequency (rf) plasmas are utilized in many applications in materials processing, such as semiconductor fabrication, surface modification, and coating. Plasma processing has replaced older techniques, such as wet chemistry, because the latter could not provide the necessary characteristics as process demands increased. A good example of this is the reduction of the feature size in semiconductors. The present critical dimension for semiconductor processing is 0.8 μm and is anticipated to be ≤0.25 μm by the year 2000. At present only plasma processing exhibits the potential of producing these line widths.An important factor, as the demands on the processing of materials become more critical, is exactly how to determine that the plasma is actually performing the process as designed. One way that is being investigated is to design control diagnostics that necessarily operate in real‐time, in situ, without significantly perturbing the process. Many such diagnostic methods have been proposed and are vigor...

Construction of a high power OPO laser system for differential absorption LIDAR

Our goal is to develop and characterize optical measurement technology to enable accurate quantification of greenhouse-gas emissions from distributed sources and sinks. We are constructing a differential absorption LIDAR (DIAL) system that will be sensitive to the three primary greenhouse gases, carbon dioxide, methane, and nitrous oxide. Our system uses a high energy optical parametric oscillator (OPO) operating from 1585 nm to 1646 nm. Here we describe this OPO system and initial characterization of its output. The OPO uses a Rotated Image Singly-Resonant Twisted RectAngle (RISTRA) design. The commercially available RISTRA cavity is machined from a solid block of aluminum. The compact single piece cavity design requires no mirror adjustments and image rotation provides efficient light conversion efficiency and excellent beam quality. The injection seeded OPO has demonstrated total output energy of 50 mJ/pulse when pumped with 220 mJ/pulse of 1064 nm radiation. The pump laser has a repetition rate variable from 1 Hz to 100 Hz and a temporal pulse width of 4.2 ns. In the current configuration the seed laser is locked to a mode of the cavity.

Mass spectrometric and optical emission diagnostics for rf plasma reactors

Mass spectrometric and optical emission studies have been performed on argon discharges in a GEC rf reference reactor. Kinetic-energy distributions for ions produced in the sheath region are broad and exhibit structure, while ions produced in the bulk plasma exhibit narrow, featureless energy distributions. The addition of small amounts of O2 to an argon discharge significantly alters the observed positive-ion kinetic- energy distributions. Optical emission studies indicate increasing spatial non-uniformity in the plasma at higher pressures. Time-resolved optical emission studies indicate a varying relationship between the applied rf voltage and the time-varying optical emission with changing pressure and position between the electrodes.