Role of low-energy electrons in Ar emission from low-pressure radio frequency discharge plasma

Optical emission spectra from a low-pressure Ar plasma were studied with high spatial resolution. It has been shown that the intensity ratios of Ar lines excited through metastable levels to those excited directly from the ground state are sensitive to the shape of electron energy distribution function. From these measurements, important information on the spatial variation of plasma parameters can be obtained. (C) 1999 American Institute of Physics. [S0003-6951(99)01629-0].

Comparison of plasma transport theory with measurements in a H- discharge

This paper and its companion paper describe the comparison between a one-dimensional theoretical model of a hydrogen discharge in a magnetic multipole plasma source and experimental measurements of the plasma parameters. The discharge chamber, described here, has been designed to produce significant densities of H- ions by incorporating a weak transverse field through the discharge to obtain electron cooling so as to maximize H- production. Langmuir probes are used to monitor the plasma, determining the ion density, the electron density and temperature and the plasma potential. The negative density is measured by photo-detachment of the extra electron using an intense laser beam. The model, described in the companion paper, uses the presented source geometry to calculate these plasma quantities as a function of the major are parameters; namely the are current and voltage and gas pressure. Good agreement is obtained between theory and experiment as a function of position and arc parameters.

TEMPORALLY AND SPATIALLY-RESOLVED PLASMA PARAMETERS AND EEDF MEASUREMENTS IN A LOW-FREQUENCY DISCHARGE

A time-resolved Langmuir probe technique is used to measure the dependence of the electron density, electron temperature, plasma potential and electron energy distribution function (EEDF) on the phase of the driving voltage in a RF driven parallel plate discharge. The measurements were made in a low-frequency (100-500 kHz), symmetrically driven, radio frequency discharge operating in H-2, D-2 and Ar at gas pressures of a few hundred millitorr. The EEDFs could not be represented by a single Maxwellian distribution and resembled the time averaged EEDFs reported in 13.56 MHz discharges. The measured parameters showed structure in their spatial and temporal dependence, generally consistent with a simple oscillating sheath model. Electron temperatures of less than 0.1 eV were measured during the phase of the RF cycle when both electrodes are negative with respect to the plasma.

STUDY OF THE DISCHARGE OF A MULTICUSP ION-SOURCE OPERATING WITH H2 AND D2

The plasma parameters and relative positive and negative ion concentrations in a small, filtered, multicusp ion source, operating at low plasma density (

TIME-RESOLVED ELECTRON-ENERGY DISTRIBUTION FUNCTION MEASUREMENTS IN A PULSED MAGNETIC MULTIPOLE HYDROGEN DISCHARGE

The time evolution of measured plasma parameters, including the electron energy distribution function (EEDF), in the discharge and post-discharge regime of a pulsed hydrogen magnetic multipole plasma is presented. The time necessary for the plasma to reach equilibrium has been established as 160-mu-s. The present results clarify the mechanisms which initiate the discharge. The decay rates of the charged-particle density and energy in the post-discharge have been measured. These measurements indicate that particle transport to the wall is the dominant loss mechanism for both charged-particle density and energy. The time-resolved EEDF is found to be non-Maxwellian in the discharge and Maxwellian in the late post-discharge.

ENHANCED VOLUME PRODUCTION OF NEGATIVE-IONS IN THE POST DISCHARGE OF A MULTICUSP HYDROGEN DISCHARGE

In this paper we demonstrate a new concept in the production of negative hydrogen ions in a low-pressure multicusp discharge. The discharge voltage is modulated to produce a non-Maxwellian, hot-electron plasma during the current pulse, followed by a cool Maxwellian electron plasma in the post discharge. This procedure, of separating in time the required hot and cold electron plasmas required for volume H- production, is called a temporal filter. The time evolution of the electron energy distribution function is measured using the time-resolved second derivative of a Langmuir probe characteristic. Time-resolved measurements of the negative ion density are made using laser photodetachment. The measurements show that the negative ion density in the center of the source, at a gas pressure of 0.07 Pa, increases by a factor of 2 when the discharge is switched off. At this low pressure the average H- beam current extracted from the source, when operated with a discharge current of 1 A in the pulse modulated mode exceeds the H- beam current from a 5 A continuously operated source. The increase in efficiency of the pulsed source is explained in terms of a two-step H- production mechanism.

A constraints-based resource discovery model for multi-provider cloud environments

Increasingly infrastructure providers are supplying the cloud marketplace with storage and on-demand compute resources to host cloud applications. From an application user's point of view, it is desirable to identify the most appropriate set of available resources on which to execute an application. Resource choice can be complex and may involve comparing available hardware specifications, operating systems, value-added services, such as network configuration or data replication, and operating costs, such as hosting cost and data throughput. Providers' cost models often change and new commodity cost models, such as spot pricing, have been introduced to offer significant savings. In this paper, a software abstraction layer is used to discover infrastructure resources for a particular application, across multiple providers, by using a two-phase constraints-based approach. In the first phase, a set of possible infrastructure resources are identified for a given application. In the second phase, a heuristic is used to select the most appropriate resources from the initial set. For some applications a cost-based heuristic is most appropriate; for others a performance-based heuristic may be used. A financial services application and a high performance computing application are used to illustrate the execution of the proposed resource discovery mechanism. The experimental result shows the proposed model could dynamically select an appropriate set of resouces that match the application's requirements.