English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Zendy Open

Presents the pdf analysis as premium feature

PDF Analysis

Insights

Presents the summarisation as premium feature

Summarisation

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the zaia usage as premium feature

ZAIA

Zendy Tools

Zendy Plus

Presents the access of premium content as premium feature

Premium Content

As part of an effort to develop efficient hollow cathodes for the International Space Station and low power electric propulsion systems, the effects of orifice length-to-diameter aspect-ratio on the performance of 0.5 to 2 A hollow cathodes were examined to compare with previous experiments and a recently developed model. Cathodes were constructed with nominally identical orifice diameters and lengths that varied from 1 to 6 diameters. The performance of the cathodes was evaluated at the beginning of life and after at least 50 hours of operation. The data generally followed the trends predicted by the model with a few exceptions which appear to be related to the thermal environment of the cathode. Power consumption scaled with the orifice aspect-ratio, while the minimum spot mode flow rate was inversely related to the length-to-diameter ratio. At currents below 1.0 A, the cathode operation became more complex than that which is assumed in the orifice model; low operating temperature caused instabilities in several of the devices. Nomenclature AR = aspect-ratio of the orifice c = speed of light, 3 x 10 m/s h = Planck's Constant, 6.63 x 10' J-s k = Boltzmann's Constant, 1.38 x 10' J/K n = index of refraction T = temperature, K e = emissivity X = wavelength, m Introduction A parametric investigation has been conducted at the NASA Lewis Research Center in order to evaluate the effect of orifice aspect-ratio on the performance of low current hollow cathodes. The aspect-ratio is defined as the orifice length divided by its diameter. The NSTAR derivative ion thruster for the Deep Space 4 mission, the plasma contactor for the International Space Station, and the NASA Lewis 8 cm ion thruster program all have requirements for low flow rate, low power cathodes.' Experimental data' exist both to support and to refute the hypothesis that cathode performance is inversely proportional to the orifice aspect-ratio. A recent hollow cathode model developed by Mandell and Katz describes the physics leading to the conclusion that cathode performance scales inversely with orifice aspect-ratio. Ions entering a finite length orifice will undergo collisions as they traverse the channel. Ions lost to collisions with the walls must be balanced by ions created in the orifice in order to satisfy the condition of a quasineutral plasma at the exit of the orifice. As orifice aspect-ratio increases, the number of ionizations occurring increases, and the conductivity of the plasma in the orifice decreases. The energy transport within the orifice also varies inversely with the aspect-ratio, and modeling the orifice processes becomes increasingly important to making an accurate prediction about the performance of the cathode. The physical arguments for the importance of orifice aspect-ratio to cathode performance are sufficiently compelling to warrant a new evaluation of the experimentally observed effects, particularly when trying to minimize the power consumption during low current operation. This report summarizes the experimental findings concerning the impact of * Graduate Student Researcher, Student Member AIAA 1 Associate Professor, Senior Member AIAA * Research Engineer, Member AIAA Copyright © 1998 by Matthew T. Domonkos. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. 1 American Institute of Aeronautics and Astronautics orifice aspect-ratio on the power consumption of low current hollow cathodes. Although the current density in the orifice far exceeds that found anywhere else in a plasma contactor, ion thruster, or Hall thruster, previous modeling efforts' have ignored the effects of the orifice processes on the overall performance. Mandell and Katz have proposed a model which conserves both energy and ions necessary for quasineutrality. The model assumes the plasma properties at the entrance to the orifice are known. The ion loss to the walls of the orifice is balanced by electron-neutral collisional ionization. The Ohmic heating in the plasma is balanced by the ionization, radiative, and convective losses. With the orifice region taken as a control volume, the flux of ions incident at the entrance will either collide with the walls or be transmitted through the orifice. Since ion-wall collisions represent a loss in the energy balance, optimization of power consumption requires minimization of the orifice aspect-ratio. Typically in orificed hollow cathodes, the maximum required current determines the orifice diameter. While the theoretical picture indicates that orifice length should then be minimized, practical considerations force a finite length orifice. The machining and welding methods currently employed to fabricate the cathode tubes require a certain minimum overall thickness of the orifice plate. Rawlin and Kerslake9 were among the first investigators to observe the erosion of the orifice plates on hollow cathodes, and increasing the thickness of the orifice plate would, to first order, mitigate the problem of erosion. The goal of the investigation reported here was to provide experimental data for comparison with Mandell and Katz's5 model while meeting the practical considerations for cathode design. This paper presents the results of a parametric experimental investigation of the effects of the orifice aspect-ratio on the power consumption of 3.2 mm diameter orificed hollow cathodes. The experimental apparatus and techniques are described first. Next, the performance data are presented. The results are then analyzed with an emphasis on the performance, and the implications for cathode lifetime are also discussed. Finally, the major conclusions of this investigation are summarized. Experimental Apparatus Four 3.2 mm outer-diameter hollow cathodes with different orifice aspect-ratios were assembled as shown in Figure 1. The construction of the hollow cathodes closely followed the methods developed previously at the NASA Lewis Research Center.10-11 The orifice plates were machined on a lathe. Figure 2 illustrates the variation in the aspect-ratios for three of the cathodes tested. Although some dimensions of this schematic were exaggerated for clarity, the orifice plate dimensions, the cathode-keeper gap (Lck), and the keeper orifice diameter (dj are all drawn to scale. For convenience, the cathodes are referred to in this paper as AR1, AR3, and AR6 for the orifices with aspect-ratios of 1, 3, and 6, respectively. The fourth cathode had an orifice diameter forty percent smaller than the other cathodes and an aspect-ratio of approximately 3; it will be referred to as AR3' here. The orifice plates were electron beam welded to a refractory metal alloy tube with a 3.2 mm outer diameter. The electron emitter was a cylinder of sintered porous tungsten impregnated with 4BaOCaO-Al2O3 and was located at the downstream end of the tube. The emitter outer diameter was nominally 0.38 mm smaller than the inner diameter of the tube. Cathode Tube with Insert Propellant Isolator Keeper Thermocouple — Heater Electrical Stand-Offs Mounting Bracket a) Schematic for the Hollow Cathode Assemblies b) IPhotbgraph Figure 1 The Hollow Cathode Assembly Refractory metal electrical leads attached to the upstream end of the emitter were swaged into electrical contact the with cathode tube. This also fixed the position of the emitter within the cathode American Institute of Aeronautics and Astronautics tube. A sheathed heater was wound and friction fit to the end of the cathode tube. Metal foil was wrapped tightly around the outside of the heater to function as a radiation shield.

Parametric investigation of orifice aspect-ratio on low current hollow cathode power consumption