Neutral Efflux Research Articles

Cytoplasmic Domain Filter Function in the Mechanosensitive Channel of Small Conductance

Mechanosensitive channels, inner membrane proteins of bacteria, open and close in response to mechanical stimuli such as changes in membrane tension during osmotic stress. In bacteria, these channels act as safety valves preventing cell lysis upon hypoosmotic cell swelling: the channels open under membrane tension to release osmolytes along with water. The mechanosensitive channel of small conductance, MscS, consists, in addition to the transmembrane channel, of a large cytoplasmic domain (CD) that features a balloon-like, water filled chamber opening to the cytoplasm through seven side pores and a small distal pore. The CD is apparently a molecular sieve covering the channel that optimizes loss of osmolytes during osmoadaptation. We employ diffusion theory and molecular dynamics simulations to explore the transport kinetics of Glu− and K+ as representative osmolytes. We suggest that the CD indeed acts as a filter that actually balances passage of Glu− and K+, and possibly other positive and negative osmolytes, to yield a largely neutral efflux and, thereby, reduce cell depolarization in the open state and conserve to a large degree the essential metabolite Glu−.

Neutral flux measurements during neutral-beam injection on Doublet III (abstract)

Neutral particle flux parallel and perpendicular to the plasma current and at energies equal to and below the injection energy has been observed with the parallel and perpendicular charge exchange spectrometers on Doublet III. Initial neutral injection is marked by a rapid increase in the high-energy neutral efflux which usually relaxes over tens of milliseconds to a lower equilibrium value. As injected power is increased by the stepped addition of more neutral beams, the measured flux is not linearly additive, often increasing slightly or not at all. For very high values of injected power, in the 5–7-MW range, the neutral particle flux is often observed in bursts correlated with the rapid fall of giant sawteeth on soft x-ray signals. In many of these shots the fishbone instability is present and is observed with the perpendicular spectrometer, however the parallel flux does not correlate with fluctuations observed with magnetic probes unless the instability terminates rapidly, in which case both a soft x-ray drop and a burst of neutrals occurs. Some aspects of these measurements have been modeled by a particle transport code with particular emphasis on changes in the neutral density profile during high-power neutral injection.

Perturbation of tokamak edge plasma by laser blow-off impurity injection

We describe measurements of tokamak edge plasma perturbations caused by rapid injection of impurities such as Sc, Fe, and Mo into Ohmically heated PLT discharges. The temporal evolution of the radiated power, convected power, charge-exchange neutral flux, electron temperature, density, and floating potentials were monitored using recently developed fast bolometers, directional calorimeters, and other diagnostics. The central radiated power can be doubled without plasma disruptions. Radiation from the edge then increases tenfold for ∼2 ms. Neutral efflux at the limiter decreases up to two orders of magnitude for approximately 20 ms after injection. The electrical potential of the limiters increases, approaching the potential of the vacuum vessel. MHD activity in the m=1 (sawtooth) mode tends to increase while that in the m=2 and m=3 modes decreases. The power flowing along field lines in the limiter shadow region sometimes increases or sometimes decreases by more than a factor of 4. The density and electron temperature in the limiter shadow region, generally, does not change more than 50%.

Analysis of Alkali Surface Ionization on Porous Substrates

The evaporation of alkali atoms and ions from porous substrates is analyzed, making the basic assumption that the transport of alkali material occurs only by surface flow of densely packed alkali atoms close to the emitter surface. The alkali efflux velocity to the emitter surface depends on the flow rate per pore and the minimum pore radius within the surface-flow range. A concentration gradient exists radial to the pore exit which causes the neutral efflux and threshold temperature for surface ionization to be larger than the values characteristic of ionizers coated with vapor-deposited alkali. Alkali emission originates from an area surrounding and including the pore exit. The diameter of the emitting region depends on the alkali flow velocity from the pore and the atom and ion desorption energies. The emission region is divided into an inner zone that emits atoms and an outer zone which predominantly emits ions. This analysis predicts that the neutral efflux from the inner zone is inversely proportional to the pore density N and gives numerical values in good agreement with measured data. The neutral efflux from the outer zone follows the Saha-Langmuir equation, the total neutral efflux being the sum of both. The analysis leads to the conclusion that the threshold temperature depends on the minimum pore radius within the surface-flow range. The threshold temperature is also related to the pore density. This analysis may not be applicable to short straight capillaries because it depends on surface flow in the vicinity of the emitter.

Improved Surface Ionization Efficiency by High Work Function Refractory Metals and Alloys

For continuous one-year operation without deterioration of the surface ionization engine characteristic, the neutral efflux should be in the 0.1% range. For the neutral efflux reduction the Saha equation implies the use of high work function metals. Therefore, tungsten-rhenium alloys and iridium filaments were investigated. For completeness, tungsten and rhenium filaments were also studied with regard to their thermionic work function and threshold temperature for cesium surface ionization. The data were measured in an all-metal guard ring diode with directly heated filament under clean surface conditions. The electron work functions and the threshold temperatures of the tungsten-rhenium alloys increase with increasing amount of rhenium in tungsten. For φ-I≤0.5 eV the electron work function governs the surface ionization. One porous tungsten-25% rhenium pellet and two porous tungsten substrates chemically coated with rhenium and iridium yielded work functions below those measured with the corresponding filaments. At 10-mA/cm2 cesium ion current density, the neutral efflux from the rhenium coated pellet was, with 2×106 pores/cm2 (traverse counting technique), 1%±0.1%. Under clean conditions 0.3% neutral efflux may be expected. The increase of the threshold temperature with electron work function leads to increased grain size and lower pore density for long term stability of the porous structure.

Factors in Life Testing Ion Engines

The goal of contemporary ion rocket research and development is the attainment of reliable, long-life ( 10,000 hr) operation. In addition to vacuum test chamber environment, the interaction of electrode thermal characteristics and neutral efflux from the ionizer are dominant in determining surf ace-con tact ion engine lifetime. Several key physical processes are discussed in the light of recent experimental results. Charge exchange erosion is the fundamental limitation to engine lifetime. Although experiment and analysis indicate attainable lifetimes of 5000 to 20,000 hr at 10 ma/cm using the best present-day ionizers, shorter lifetimes result when interrelated secondary physical phenomena within the acceleration region of the engine emerge. Cesium ionization on the focus electrodes is considered in detail, since it is a lifetime barrier in those designs which utilize such beam-forming elements. Tolerable charge exchange erosion of accel electrodes, to assure a given lifetime, is used to provide boundary conditions for ionization efficiency, focus temperature, and focus ion current. The effect of focus electrode roughness on accel erosion patterns is studied in detail. The influence of the ground-based test chamber environment vs a true space vacuum environment is also considered.

371. The influence of the residual gas atmosphere in space chambers on the neutral efflux and critical temperature of tungsten ionizer: O K Husmann et al, AIAA Paper- 64–693, Hughes Research Labs, Malibu, Calif.

371. The influence of the residual gas atmosphere in space chambers on the neutral efflux and critical temperature of tungsten ionizer: O K Husmann et al, AIAA Paper- 64–693, Hughes Research Labs, Malibu, Calif.

Influence of residual gas atmosphere in space chambers on neutral efflux and critical temperature of tungsten ionizers.

Influence of residual gas atmosphere in space chambers on neutral efflux and critical temperature of tungsten ionizers