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Breakthrough of Inertial Electrostatic Confinement Concept for Advanced Space Propulsion

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Breakthrough of Inertial Electrostatic Confinement Concept for Advanced Space Propulsion 69th International Astronautical Congress (IAC), Bremen, Germany, 1-5 October 2018. Copyright ©2018 by the International Astronautical Federation (IAF). All rights reserved. IAC-18,C4,7-C3.5,12,x47993 Breakthrough of Inertial Electrostatic Confinement Concept for Advanced Space Propulsion Y.-A. Chana*, G. Herdricha a Institute of Space Systems, University of Stuttgart, Pfaffenwaldring 29, 70569 Stuttgart, Baden-Wüttemberg, Germany, [email protected] * Corresponding Author Abstract The activities of Inertial electrostatic confinement (IEC) research for propulsion application in IRS started from 2009. A breakthrough on the theoretical explanation of working principle in 2017 IEPC enabled the proof-of-concept of IEC thruster for next generation space exploration. [1] Several advanced IEC propulsion concepts have been proposed accompanying with the working principle which demonstrated a wide-spectrum application from atmosphere-breathing electric propulsion for very-low-earth orbit to fusion propulsion for deep-space manned mission. The SDL can offer both intensive ionization and confinement of ions at the same time which provide the advantage to suppress erosion from ion bombardment. Furthermore, distortion of the SDL by manipulating the applied electric field gradient are the key to achieve plasma extraction from core of IEC. This permits the applications of IEC device goes from neutron generation to electron/ion source, which opens the door to space propulsion. Keywords: (maximum 6 keywords) Nomenclature SDL Spherical double layer – Mass SIE Secondary ion emission particle density SSE Secondary electron emission – Temperature UV Ultra-violet – Velocity VELARC Very-low-power arcjet VLEO Very low earth orbit Acronyms/Abbreviations ABEP Atmosphere-breathing electric 1. Introduction propulsion Inertial electrostatic confinement (IEC) is a device EB Electron beam which was originally designed for fusion application. The EP Electric propulsion basic concept is to heat up and to confine plasma by ERG Electron retarding grid strong electric fields in order to achieve fusion reaction ESG Electron suppress grid in the core of IEC. Research on IEC is being performed FG Floating grid since the 1950’s, but only limited in the studies for FP Faraday probe neutron source application, mainly exists in USA [2], [3] FT Feedthrough and Japan [4], [5]. Other applications of IEC are seldom GEO Geostationary earth orbit mentioned. HET Hall effect thruster From 2009, Institute of Space System (IRS) started a HV High voltage project with the purpose to understand the jet extraction IEC Inertial electric confinement from IEC devices as well as to evaluate the applicability IEPC International electric propulsion for space propulsion systems. The idea behind it is to conference establish a stable and constant plasma extraction from the IRG Ion retarding grid plasma core in order to provide thrust force. Several IRS Institute of Space Systems researches discussed about the operation modes and JPL Joint propulsion lab plasma extraction have been published. [6], [7] In LEO low earth orbit addition, a model for estimation of plasma composition LIF Laser induced fluorescence and loss mechanism basic on Maxwellian assumption LP Langmuir probe were developed, which served as the preliminary NIR- Near-infarad understanding of IEC plasma composition. [] In addition, OES Optical emission spectroscopy a numerical model based on Direct Simulate Monte- PPT Pulsed plasma thruster Carlo method and Particle-In-Cell method was applied in TDLAS Tunable diode laser absorption the research of IEC under support from ESA Ariadna spectroscopy study. [8] Summary of activities before 2016 can be TRL Technical readiness level found in Ref. [6] IAC-18,C4,7-C3.5,12,x47993 Page 1 of 9 69th International Astronautical Congress (IAC), Bremen, Germany, 1-5 October 2018. Copyright ©2018 by the International Astronautical Federation (IAF). All rights reserved. In this paper, the author intends to provide a summary forms a non-neutral plasma floating within cathode grid, of breakthrough of IEC development and scientific as known as virtual anode. investigation in IRS from 2016. In addition, several Four different modes have been identified from propulsion concept and their application based on IEC several research, naming the spot mode, star mode, tight- technologies was categorized in this paper as well. More jet mode, and spray-jet mode, respectively, see Fig. 2.[6] detail can be found in the following subsection. The spot mode can be considered as the formation of potential well by recirculating ions within IEC, while star 1.1. IEC principle mode is likely to be the combination of potential well and The simplified scheme and working principle of IEC ions/electrons leakage from core. The other two mode are are shown in Fig. 1. The setup consists of two spherical tight-jet mode Fig. 2 (c)) and the spray-jet mode (see grid-like electrodes, both of which were concentrically Fig. 2(d)), respective. These two modes are resulted from aligned. The outer electrode served as the anode while distortion of E-field topography by enlarging one of the the inner one being the cathode. The setup is placed in grid gates on a spherical-symmetric cathode grid vacuum with a controlled particle density environment configuration. Plasma experience the change of E-field (background pressure). Having a strong electric potential and attempt to escape from the weakest point of the field. gradience between the both grids, a spherical, center- Considering the application for space propulsion, these pointing E-field topography is generated. Electrons are the most preferable two modes. The pictures for these driven by field emission on cathode fly toward anode and two operation conditions in IRS-IEC are showing in Fig. cause the electron impact ionization with neutral 4. particles, which provides the production of ions between two spherical grids. Ions get accelerated toward the cathode while electrons are kept directing to the outer grid with the respective electric potential energy they experienced at position they collide. The chance of ion- grid collision is respectively low due to grid-like configuration of cathode which offer high transparency for ions. Most of the inward driven ions can travel through the grid gate and fly toward to opposite side of IEC with their inertia if no further collision happened on their path. When these ions reach to the equal-potential position, they are push inward again by the E-field. This keep ions flying back and forth until their kinetic energy are dissipated by another collision processes. If the kinetic energy of ion is sufficient, fusion relevant processes might occur in either ion-ion or ion-neutral collision within IEC device. These oscillating ions increase the probability of ions appearance at the center Fig. 2 Operation modes in IEC [6] of IEC which accumulates the electrical potential and 1.1.1. Conventional concept and confliction Conventional IEC confinement hypothesis indicates the virtual anode could further form an inner virtual cathode by trapping electrons at center of IEC via a similar formation processes as virtual anode did. [2] Accordingly a nested potential well shells, known as “Poissors”, can be formed as the applied current further increased. The electron’s Poissor shell and ion’s Poissor shell serves as the confinement mechanism for each other due to its potential well gradience. This hypothesis has been tried to verify since 1990s but never been experimental confirmed. On the other hand, several 1D potential models were developed to solve the potential distribution along the radius of a spherical IEC in order the verified the concept. However, none of it can perform the expected Poissor’s structure without impractical assumptions. [10] In addition, the estimated plasma ball Fig. 1 Concept schematic of IEC [9] dimension is the same size as the cathode gird, which is IAC-18,C4,7-C3.5,12,x47993 Page 2 of 9 69th International Astronautical Congress (IAC), Bremen, Germany, 1-5 October 2018. Copyright ©2018 by the International Astronautical Federation (IAF). All rights reserved. Fig. 4 Tight jet (left) and spray jet (right) mode in IRS IEC [1] far from experimental observation. On the other hand, the plasma ball dimension and properties. The schematic of conventional hypothesis can not explain the plasma SDL occurred in IEC device can be seen in Fig. 3 Ions extraction mechanism of from the core of IEC. This leads are created on the layer with approximately same thermal the concerns on the accuracy of the hypothesis. velocity as background gas ( ) and constricted by the center-pointing electron wind which 1.1.2. Spherical double-layer theory has high drifting velocity ( ). The The problem of the conventional hypothesis is the electron energy is usually high enough to trigger electron ignorance of the momentum balance of particles, which impact ionization, accordingly, high ionization efficiency is a dominated factor for the distribution and extraction is expected. The kinetic energy of incoming electrons is of charge particles within IEC. To establish the dissipated by the ionization processes and trapped within momentum balance on ions and electron cloud in a non- the layer. Some electrons can escape the envelop through neutral plasma, double layer theory is needed. the weak point of potential topography due to Double layer is widely observed in most of plasma accumulating of the electron pressure. [1] condition, which is a boundary representing a dynamic In addition, the extraction mechanism, as above equilibrium of non-neutral plasma. The electric potential mentioned, initiated by the non-uniformity of electric gradience between both side of double layer is usually field. This will result the non-uniformity of incoming slightly above the ionization energy of gas. The potential electron’s kinetic energy, and hence, the imbalance of gradience creates an ambipolar field where electrons and momentum between incoming electron cloud and trapped ions are driven toward the thin layer and reach a plasma around the weak point of SDL.
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