Segmented Anode Research Articles

Performance and Active Thermal Control of a 2-kW Hall Thruster with Segmented Electrodes

The performance and plume characteristics of a 2-kW Hall thruster with segmented anodes for thermal control of the main anode are characterized. Diagnostics performed included thrust, specific impulse, and efficiency, as well as the ion current density in the plume. The apparatus included an inverted-pendulum thrust stand for performance measurements and a shielded Faraday probe for ion current density characterization. Data were taken at several operating points and at several ratios of current to the main anode vs the shim electrodes. The data show a general increase in centerline current density, thrust, specific impulse, and efficiency as discharge current is moved to the shims. Thermal measurements show up to 50° C control authority in the main anode temperature.

Reaction Dynamics in a Parallel Flow Channel PEM Fuel Cell

The spatiotemporal dynamic response of a segmented anode parallel channel polymer electrolyte membrane (PEM) fuel cell was monitored following changes in flow rate, temperature and load resistance. Autohumidified operation with dry feeds at 1 bar pressure was achieved at temperatures below 70°C, where the convective transport of water vapor was less than the water production by the fuel cell current. The current could be “ignited” by a single injection of water into the anode feed, or by reducing the temperature and external load resistance. Co-current flow of the hydrogen and oxygen resulted in current ignition at the outlets of the flow channels, followed by a wave of high current density propagating toward the inlets. Counter-current flow of the hydrogen and the oxygen resulted in ignition near the center of the flow channels; over time the ignition front fanned out. The spatio-temporal dynamics of the current ignition along the flow channels can be effectively predicted from a model of a set of coupled differential fuel cells in series. Liquid water condensing in the flow channels gives rise to complex spatio-temporal variations in the current density; these variations are strongly dependent on orientation of the fuel cell with respect to gravity.

The pixel readout of Micro Patterned Gaseous Detectors

The use of pixel readout chips as highly segmented anodes of gaseous detectors offers high granularity and low noise at the input of each channel. This readout can be applied to TPCs for high energy particle tracking as well as for low energy recoil detection where a small charge is created in the gas volume. Detectors combining GEM or Micromegas amplification stages with pixel readout chips will be presented and their tracking capabilities described.

Spatially-resolved current and impedance analysis of a stirred tank reactor and serpentine fuel cell flow-field at low relative humidity

A 20 cm(2) segmented anode fuel cell is used to investigate the performance of a hydrogen-air fuel cell at 1 atm. with two different flow-fields using spatially-resolved current and impedance measurements. A self-draining stirred tank reactor (STR) fuel cell and a single-channel serpentine fuel cell are compared with humidified and dry feed conditions. The current density distribution, impedance distribution, heat distribution and water evolution are compared for the two different flow-fields. With inlet feed dew points of 30 degrees C, the STR fuel cell and serpentine system performed comparably with moderate current gradients. With drier feeds, however, the STR fuel cell exhibited superior overall performance in terms of a higher total current and lower current, impedance and temperature distribution gradients. The STIR fuel cell design is superior to a single-channel serpentine design under dry conditions because its open channel design allows the feed gases to mix with the product water and auto-humidify the cell. (c) 2006 Elsevier B.V. All rights reserved.

Status of TACTIC: A detector for nuclear astrophysics

A new detector for nuclear astrophysics studies is being designed and built by TRIUMF and the University of York. The TRIUMF Annular Chamber for Tracking and Identification of Charged particles (TACTIC) is designed to detect low-energy charged particles from inverse kinematics reaction studies performed at the relevant astrophysical energies. TACTIC is a cylindrical ionisation/time-projection chamber with segmented anode strips, which allow the d E/d x of the particle to be determined along with the total energy. Information from drift times allows the particle trajectory to be reconstructed. This in turn identifies the interaction point along the beam axis and hence the centre of mass energy of the reaction. To amplify the expected weak signals, a gas electron multiplier (GEM) will be used in place of the usual Frisch grid. Full digital readout of the charge and timing of each anode strip will be achieved with flash ADC cards allowing pulse shape analysis of the signals.

Experimental investigations of the anode boundary layer in high intensity arcs with cross flow

The anode boundary layer in high intensity arcs determines the anode lifetime and processing efficiency in many industrial applications and can be controlled by cold lateral gas flows (LGFs) parallel to the anode. In this paper, properties of the anode boundary layer in a high intensity argon arc with argon and nitrogen LGFs have been studied. Anode heat and current transfer and restrike frequency and restrike length have been measured using a segmented anode and a high speed camera. Electron temperature, electron density and current density have been measured by a Langmuir probe setup. The correlation between these properties and the previously defined arc attachment mode has been described and explained.

An integrated readout system for drift chambers: the application of monolithic CMOS pixel sensors as segmented direct anode

A small TPC has been read out by means of a MediPix2 readout chip as direct anode. A Micromegas foil was placed 50 μm above the chip, and electron multiplication occurred in the gap. With a He/Isobutane 80/20 mixture, gas multiplication factors up to tens of thousands were achieved, resulting in an efficiency for detecting single electrons of better than 90%. We recorded many frames containing 2D images with tracks from cosmic muons. Along these tracks, electron clusters were observed, as well as δ-rays.

A PEM fuel cell for combined measurement of current and temperature distribution, and flow field flooding

A fuel cell has been designed that combines three methods of spatially resolved measurements. The experimental setup allows to perform a simultaneous evaluation of current, temperature and water distribution in a polymer electrolyte fuel cell (PEFC) under operation. The test cell has a segmented anode flow field for current distribution measurement. The back plate of the cathode flow field is made of an optical window, which is transparent for infrared (IR) as well as for visible wavelengths. This allows infrared thermography and optical surveillance of water droplets and flow field flooding.

Current distribution measurements in a PEFC with net flow geometry

A measurement system for current distribution mapping for a PEFC has been developed. The segmented anode is constructed so as to have high thermal conductivity in order to prevent the formation of large temperature gradients between the electrodes. The construction is therefore feasible for use at high current densities. Both segmented and unsegmented gas diffusion layers are used. The effect of inlet humidification and gas composition at the cathode side is studied. In addition, two different flow geometries are studied. The results show that the measurement system is able to distinguish between current distribution originating from differences in proton conductivity, species concentration and gas diffusion layer properties.

Parallel Ionization Multiplier (PIM): application of a new concept of gaseous structure to tracking detectors

A new amplifying structure has been developed for beta imaging detectors: the Parallel Ionization Multiplier (PIM). PIM is made of a thin sandwich of two metallic micromeshes separated by a new laser machined insulating spacer. This structure allows high gain (10 5) with a good efficiency and a two-dimensional spatial resolution below 50 μm without strong discharge limitation. The capabilities of the PIM are now investigated in order to be used by high-energy physics experiments. A prototype of a tracking detector for charged particles using the PIM structure will be described. Thanks to the very small avalanche extension, the use of a highly segmented anode is possible. This detector would support high rate running conditions of hadron experiment.

A fast switching segmented anode NPN controlled LIGBT

An ultrafast low energy loss lateral insulated gate bipolar transistor (LIGBT) with a novel segmented anode structure is demonstrated. The anode comprises segments of p/sup +/ and n/sup +//p (n/sup +/ region formed within a p-type region) along the width of the device. By simply varying the ratio of these segments the tradeoff between conduction and switching losses can be varied. Unlike an anode shorted structure this does not exhibit an undesirable snapback in its on-state characteristics. This structure is simple to realize in a CDMOS process without the need for any additional process steps.

Imaging micro-well proportional counters fabricated with masked UV laser ablation

The micro-well detector is a gas-proportional counter similar to the CAT (Bartol et al., J. Phys. III 6 (1996) 337) and WELL detectors (Bellazzini et al., Nucl. Instr. and Meth. A 423 (1999) 125). The micro-well is a cylindrical hole formed in the polymer substrate of commercially fabricated copper-clad flexible printed circuit board by UV laser ablation. The micro-wells are drilled at GSFC's UV laser-ablation facility. The cathode is a metal annulus that surrounds the opening of the well. The anode is a metal pad that fills the bottom of the well. Advantages of this topology include intrinsic two-dimensional sensing, thick robust electrodes, and large localized image charge on the cathodes. We have fabricated 5 cm×5 cm micro-well detectors with segmented anodes (1-d) and with both anodes and cathodes segmented (2-d), and have demonstrated: • stable, proportional operation at gas gains in excess of 30,000 in Ar- and Xe-based gases; • FWHM energy resolution of 20% at 6 keV in P-10; • preliminary 1-d spatial resolution of ⩽150 μm (rms) in P-10; and • the capability of MWDs to produce 2-dimensional images. We report on the design, fabrication, and testing of 1-d, 2-d, and pixelized micro-well detectors.

A segmented anode, npn controlled lateral insulated gate bipolar transistor

A fast switching area efficient segmented npn anode lateral insulated gate bipolar transistor is proposed. Segments of p+ and npn are incorporated at the anode along the width in the third dimension instead of placing them linearly along the drift length of the device. The p-base parameters of the npn segment and the ratio of the p+ and npn segments are found to strongly influence the switching behaviour.

A position-sensitive ionization chamber for thermal neutrons

A position-sensitive neutron detector based on /sup 3/He is being developed for a national crystal backscattering spectrometer (CBS) at a pulsed neutron source. It comprises a single gas volume, of constant depth, in the form of an annulus that is also part of a truncated cone. Charge from each event is collected on one or two anode strips, out of a total 336 around the annulus, yielding an angular resolution of about 1/spl deg/. Low-noise signal processing, in the form of a monolithic preamplifier/shaping amplifier connected to each anode strip, provides electronic noise that is small enough to permit detector operation in an ionization mode. In addition, the segmented anodes ensure near complete induced charge collection. This new detector therefore has significant advantages over previous detectors used in CBS instruments, which primarily consisted of arrays of individual /sup 3/He filled cylindrical counters with dead regions and nonuniform detection efficiency. The optimization of gas mixture/pressure, drift field, and shaping time is significant in achieving successful performance in the new detector.

A Self-Consistent Computational Model of Electrodynamic and Thermogasdynamic Processes in Electric-Discharge Lasers

The paper deals with a self-consistent computational electrodynamic and thermogasdynamic model of the working chamber of a gas-discharge laser with a segmented cathode. The computational model is based on a set of Navier–Stokes equations, with the effective turbulent viscosity described within the k–e model of turbulence, energy equation for partly ionized gas, equations of motion of charged particles (electrons and ions) in a diffusion-drift approximation, and continuity equation for electric current. The model is used to investigate the processes in the gas-discharge chamber of a laser with a segmented cathode and continuous anode. The calculations are performed for the real parameters of technological CO2 lasers.

Bistability and hysteresis in the characteristics of segmented-anode lateral IGBTs

The voltage and current controlled bistability of segmented anode lateral insulated gate bipolar transistors (SA-LIGBTs) containing more than one p-anode segment has been investigated experimentally by means of monitoring the infrared recombination radiation, as well as theoretically by two-dimensional (2-D) device simulation. We have found out that the instabilities are caused by the switching of single p-anode segments from the MOS-mode into the conductivity modulated state and vice-versa. The switching current and voltage show pronounced memory effects, if the single cell structure exhibits S-type negative differential conductivity (SNDC) because of conductivity modulation. The simulation of a simplified vertical hybrid diode/pnp-transistor structure shows very good qualitative agreement with measured characteristics and carrier distributions in the SA-LIGBT. Generally, such hysteresis effects may occur in all electronic devices containing SNDC-elements switched in parallel.

Compact thermal ion detector for space and laboratory applications

We have developed a new compact thermal ion sensor suitable for spaceflight or laboratory applications. The device is highly sensitive and has a wide dynamic range due to its use of a microchannel plate charge multiplier. It allows spatial variations in velocity and anisotropic temperatures to be measured in a flowing plasma stream using a segmented anode. It has a high rejection to solar UV due to the trajectories of the particles within the device. The device has been flight tested as part of the Charging Hazards and Wake Studies flight experiment and sample data from the flight are presented. We discuss interpretation of the output of the device including extracting the parallel and perpendicular temperatures and the flow energy of a plasma stream.

A large-acceptance Bragg curve spectrometer with a longitudinal electric field and a segmented anode

A large-acceptance Bragg curve spectrometer with a longitudinal electron collection field and a segmented anode has been constructed and tested. The effects on the charge resolution of the entrance angle and entrance position of the incident particle have been studied. Simulations have been done in order to isolate the contribution to the overall detector performance of the signal-shaping electronics from that of the intrinsic design of the detector.

Anode power deposition in an applied-field segmented anode MPD thruster

Anode heat flux measurements of a water cooled segmented anode applied-field MPD thruster were made to investigate anode heat transfer phenomena. Pure argon and argon-hydrogen mixtures were used as propellants for a variety of thruster currents, propellant mass flow rates, and axial applied magnetic field strengths. The thruster was operated in two modes; with all four segments active, and with two of the segments floating. In addition, thrust and specific impulse were determined for each operating condition. The results show that the heat flux to the anode increases monotonically with axial magnetic field strength and thruster current. Between 50 and 75 percent of the anode heat flux is transported by the current carrying electrons. Convective and radiative heat transfer account for the remaining portion of the power deposited in the anode. The addition of hydrogen to the argon propellant results in the reduction of the fraction of anode power deposited by the anode fall to a level equivalent to that deposited by convection and radiation.

Analysis and characterization of the segmented anode LIGBT

A LIGBT (lateral insulated gate bipolar transistor) structure, called the segmented-anode LIGBT, is presented and analyzed. The anode structure responsible for the injection of minority carriers for conductivity modulation is implemented using segments of p/sup +/ and n/sup +/ diffusion along the device width. This minimizes the forward bias of the p/sup +/ injector during device turn-off, resulting in higher switching speed compared to the shorted-anode LIGBT. Since the n/sup +/ region required for electron extraction is implemented along the device width and consumes only a small amount of area, a reduction in device size is also achieved. The switching speed and reduced device size result in better tradeoff between on-resistance and turn-off time compared with other LIGBTs. Two-dimensional numerical simulations are carried out to demonstrate the operation and characteristics of the structure, and the experimental inductive and resistive switching characteristics of the structure are discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>