Richard C. Westhoff

Plasma-particle interactions in plasma spraying systems

A mathematical formulation is presented to represent the interactions between the plasma jet exiting a nontransferred arc plasma torch and injected solid particles. This is a generic problem in plasma spraying operations. The calculations are based on the solution of the two-dimensional equation of motion and the thermal energy balance for the particles. Additionally, the plasma temperatures and velocities in the torch and plume are calculated using a mathematical model based on a simplified set of conservation equations. In the formulation, we allow for departure from continuum conditions, particle vaporization, and temperature gradients within the particles. The calculations are compared with previously published experimental measurements of alumina particles injected into a room-temperature, turbulent air jet and into the plume of a commercial plasma torch operating in a turbulent mode. The second set of experiments provides simultaneous measurement of particle temperature, size, and velocity and so form an excellent basis for testing our model. The comparison of the model and the measurements brings new insight into the behavior of particles in plasma jets.

A comparison of experimental measurements and theoretical predictions regarding the behavior of a turbulent argon plasma jet discharging into air

Computed results are presented describing the temperature and concentration fields obtained when an argon plasma jet is being discharged into ambient air. A previously published mathematical model for turbulent plasma plumes is used for the calculations. These predictions are compared with recent), published experimental measurements by Brossa and Pfender, performed with an enthalpy probe. The theoretical predictions appear to agree reasonably well with the measurements of both the temperature and concentration profiles, with a maximum deviation in the 10–20% range.

Radiation-hard, charge-coupled devices for the extreme ultraviolet variability experiment

The Extreme-Ultraviolet Variability Experiment (EVE) is a component of NASAs Solar Dynamics Observatory (SDO) satellite, aimed at measuring the solar extreme ultraviolet (EUV) irradiance with high spectral resolution, temporal cadence, accuracy, and precision. The required high EUV quantum efficiency (QE), coupled with the radiation dose to be experienced by the detectors during the five year mission (~1 Mrad), posed a serious challenge to the charge-coupled device (CCD) detectors. MIT Lincoln Laboratory developed the 2048 × 1024 pixel CCDs and integrated them into the detector system. The devices were back-side thinned and then back surface passivated using a thin, heavily boron-doped silicon layer grown by molecular beam epitaxy (MBE) at less than 450°C. Radiation-hardness testing was performed using the Brookhaven National Laboratorys National Synchrotron Light Source (BNL/NSLS). The MBE-passivated devices were compared against devices with back surfaces passivated with a silver charge chemisorption process and an ion-implant/furnace anneal process. The MBE devices provided both the highest QE at the required (-100°C) operating temperatures, and superior radiation hardness, exceeding the goals for the project. Several flight-ready devices were delivered with the detector system for integration with the satellite.

Low dark current, back-illuminated charge coupled devices

Dark current for back-illuminated (BI) charge-coupled-device (CCD) imagers at Lincoln Laboratory has historically been higher than for front-illuminated (FI) detectors. This is presumably due to high concentrations of unpassivated dangling bonds at or near the thinned back surface caused by wafer thinning, inadequate passivation and low quality native oxide growth. The high dark current has meant that the CCDs must be substantially cooled to be comparable to FI devices. The dark current comprises three components: frontside surface-state, bulk, and back surface. We have developed a backside passivation process that significantly reduces the dark current of BI CCDs. The BI imagers are passivated using molecular beam epitaxy (MBE) to grow a thin heavily boron-doped layer, followed by an annealing step in hydrogen. The frontside surface state component can be suppressed using surface inversion, where clock dithering reduces the frontside dark current below the bulk. This work uses surface inversion, clock dithering and comparison between FI and BI imagers as tools to determine the dark current from each of the components. MBE passivated devices, when used with clock dithering, have dark current reduced by a factor of one hundred relative to ion-implant/laser annealed devices, with measured values as low as 10-14 pA/cm2 at 20°C.

A Comparison of the Experimentally Measured and Theoretically Predicted Temperature Profiles for an Argon Plasma Jet Discharging into a Nitrogen Environment

Experimental measurements and computed results are reported, describing the behavior of a non-transferred arc plasma torch operating in a laminar mode, discharging into a nitrogen environment. The experimental measurements of the temperature fields in the vicinity of the torch exit, obtained using emission spectroscopy, were compared with theoretical predictions. The calculations were based on the solution of the axi-symmetric heat, mass, momentum, and species balance equations. The theoretical predictions were found to be in excellent agreement with measurement, with the error usually being in the 5–10% range and the maximum error being about 15%.

MBE back-illuminated silicon Geiger-mode avalanche photodiodes for enhanced ultraviolet response

We have demonstrated a wafer-scale back-illumination process for silicon Geiger-mode avalanche photodiode arrays using Molecular Beam Epitaxy (MBE) for backside passivation. Critical to this fabrication process is support of the thin (< 10 μm) detector during the MBE growth by oxide-bonding to a full-thickness silicon wafer. This back-illumination process makes it possible to build low-dark-count-rate single-photon detectors with high quantum efficiency extending to deep ultraviolet wavelengths. This paper reviews our process for fabricating MBE back-illuminated silicon Geigermode avalanche photodiode arrays and presents characterization of initial test devices.