Design and Preliminary Testing Plan of Electronegative Ion Thruster

Electronegative ion thrusters are a new iteration of existing gridded ion thruster technology differentiated by their ability to produce and accelerate both positive and negative ions. The primary motivations for electronegative ion thruster development include the elimination of lifetime-limiting cathodes from a thruster system and the ability to generate appreciable thrust through the acceleration of both positive or negative-charged ions. Proof-of-concept testing of the PEGASES (Plasma Propulsion with Electronegative GASES) thruster demonstrated the production of positively and negatively-charged ions (argon and sulfur hexafluoride, respectively) in an RF discharge and the subsequent acceleration of each charge species through the application of a time-varying electric field to a pair of metallic grids similar to those found in gridded ion thrusters. Leveraging the knowledge gained through experiments with the PEGASES I and II prototypes, the MINT (Marshall's Ion-ioN Thruster) is being developed to provide a platform for additional electronegative thruster proof-of-concept validation testing including direct thrust measurements. The design criteria used in designing the MINT are outlined and the planned tests that will be used to characterize the performance of the prototype are described

Direction for the Future - Successive Acceleration of Positive and Negative Ions Applied to Space Propulsion

Electrical space thrusters show important advantages for applications in outer space compared to chemical thrusters, as they allow a longer mission lifetime with lower weight and propellant consumption. Mature technologies on the market today accelerate positive ions to generate thrust. The ion beam is neutralized by electrons downstream, and this need for an additional neutralization system has some drawbacks related to stability, lifetime and total weight and power consumption. Many new concepts, to get rid of the neutralizer, have been proposed, and the PEGASES ion-ion thruster is one of them. This new thruster concept aims at accelerating both positive and negative ions to generate thrust, such that additional neutralization is redundant. This chapter gives an overview of the concept of electric propulsion and the state of the development of this new ion-ion thruster.Comment: 10 pages, contribution to the CAS-CERN Accelerator School: Ion Sources, Senec, Slovakia, 29 May - 8 June 2012, edited by R. Bailey. appears in CERN Yellow Report CERN-2013-007, pp.575-58

Grounded radio-frequency electrodes in contact with high density plasmas

An analytical model is developed of an asymmetric electrode system immersed in a plasma, consisting of two dc-grounded electrodes, where the smaller one is biased at 13.56MHz. The model is compared with a set of experiments performed in a high density low pressure plasma source (an electron cyclotron resonance source) where a second electrode is immersed into the plasma and powered by radio frequency. Excellent agreement is obtained between the analytical model and the experimental results. It is found that the time average plasma potential and the direct current(dc) flowing in the system during steady state are strongly dependent on both the rf voltage (or power) and the area ratio between the larger and smaller electrodes. For area ratios larger than 80, the dc current is large and the plasma potential is constant with respect to the applied rf voltage. For area ratios smaller than 80 but larger than unity, the plasma potential increases linearly with the applied rf voltage, and the dc current is reduced compared to the large area ratio case

Experiments and theory of an upstream ionization instability excited by an accelerated electron beam through a current-free double layer

A low-frequency instability varying from 10 to 20kHz has been discovered in the presence of a current-free double layer (DL) in a low-pressure expanding helicon plasma. The instability is observed using various electrostatic probes, such as Langmuir probes floating or biased to ion saturation and emissive probes measuring the plasma potential. A retarding field energy analyzer measuring the ion energy distribution function downstream of the double layer is used together with the LP to simultaneously observe the DL and the instability, confirming their coexistence. The frequency of the instability decreases with increasing neutral pressure, increases with increasing magnetic field in the source and increases with increasing rf power. A theory for an upstream ionizationinstability has been developed, in which electrons accelerated through the DL increase the ionization upstream and are responsible for the observed instability. The theory is in good agreement with the experimental results and shows that the frequency increases with the potential drop of the double layer and with decreasing chamber radius

Flow dynamics and magnetic induction in the von-Karman plasma experiment

The von-Karman plasma experiment is a novel versatile experimental device designed to explore the dynamics of basic magnetic induction processes and the dynamics of flows driven in weakly magnetized plasmas. A high-density plasma column (10^16 - 10^19 particles.m^-3) is created by two radio-frequency plasma sources located at each end of a 1 m long linear device. Flows are driven through JxB azimuthal torques created from independently controlled emissive cathodes. The device has been designed such that magnetic induction processes and turbulent plasma dynamics can be studied from a variety of time-averaged axisymmetric flows in a cylinder. MHD simulations implementing volume-penalization support the experimental development to design the most efficient flow-driving schemes and understand the flow dynamics. Preliminary experimental results show that a rotating motion of up to nearly 1 km/s is controlled by the JxB azimuthal torque

Comparative study of positive and negative ion flows extracted from downstream plasmas beyond magnetic and electrostatic electron filters

In the present paper we compare the positive and negative ion flows created using a recently developed electrostatic grid-type filter with the flows formed using a magnetic filter. Langmuir probe measurements show electron cooling with both filters, allowing effective formation of negative ions via electron dissociative attachment in the region of low electron temperature. The energy distribution functions of positive and negative ions extracted from the filtered plasmas are measured in both systems showing an almost monoenergetic nature of the ions with the energy corresponding to the imposed extraction potential. It is shown that in both cases strongly electronegative plasmas where the negative ion density is much larger than the electron density can be formed downstream of the filter. Biasing an internal electrode or the electrostatic filter grid allows control of the plasma potential. In the case of the electrostatic filter the plasma could be biased negatively compared to ground and effective extraction of negative ion was achieved.В данной работе проведено сравнение потоков положительных и отрицательных ионов, генерируемых с использованием недавно разработанного электростатического сеточного фильтра с потоками, формируемыми с использованием магнитного фильтра. Измерения ленгмюровскими зондами показали эффективное “охлаждение” электронов при использовании обоих фильтров, обеспечивающее условия для эффективного образования отрицательных ионов в области с низкой электронной температурой в результате диссоциативного прилипания. Функции распределения по энергии положительных и отрицательных ионов, извлекаемых из вторичной плазмы, измеренные в обеих системах, показали моноэнергетичность генерируемых потоков ионов с энергией, соответствующей приложенному извлекающему потенциалу. Показано, что в обоих случаях возможно формирование сильно электроотрицательной плазмы на выходе фильтра с плотностью отрицательных ионов значительно превышающей плотность электронов. Смещение внутреннего электрода либо сетки фильтра позволило достичь управления потенциалом плазмы. В случае электростатического фильтра потенциал плазмы может принимать отрицательные значения по отношению к заземленному электроду, благодаря чему было достигнуто эффективное извлечение отрицательных ионов.У даній роботі проведено порівняння потоків позитивних і негативних іонів, що генеруються з використанням недавно розробленого електростатичного сіткового фільтру з потоками, що формуються з використанням магнітного фільтру. Вимірювання ленгмюрівськими зондами показали ефективне “охолоджування” електронів при використанні обох фільтрів, що забезпечує умови для ефективного утворення негативних іонів в області з низькою електронною температурою в результаті диссоціативного прилипання. Функції розподілу по енергії позитивних і негативних іонів, витягуваних з вторинної плазми, виміряні в обох системах, показали моноенергетичність потоків іонів, що генеруються, з енергією, відповідною до прикладеного витягуючого потенціалу. Показано, що в обох випадках можливе формування сильно електронегативної плазми на виході фільтру з щільністю негативних іонів що значно перевищує щільність електронів. Зсув потенціалу внутрішнього електроду або сітки фільтр дозволив досягти керування потенціалом плазми. У разі електростатичного фільтру потенціал плазми може приймати негативні значення по відношенню до заземленого електроду, завдяки чому було досягнуте ефективне витягання негативних іонів

Design and Preliminary Performance Testing of Electronegative Gas Plasma Thruster

In classical gridded electrostatic ion thrusters, positively charged ions are generated from a plasma discharge of noble gas propellant and accelerated to provide thrust. To maintain overall charge balance on the propulsion system, a separate electron source is required to neutralize the ion beam as it exits the thruster. However, if high-electronegativity propellant gases (e.g., sulfur hexafluoride) are instead used, a plasma discharge can result consisting of both positively and negatively charged ions. Extracting such electronegative plasma species for thrust generation (e.g., with time-varying, bipolar ion optics) would eliminate the need for a separate neutralizer cathode subsystem. In addition for thrusters utilizing a RF plasma discharge, further simplification of the ion thruster power system may be possible by also using the RF power supply to bias the ion optics. Recently, the PEGASES (Plasma propulsion with Electronegative gases) thruster prototype successfully demonstrated proof-of-concept operations in alternatively accelerating positively and negatively charged ions from a RF discharge of a mixture of argon and sulfur hexafluoride.i In collaboration with NASA Marshall Space Flight Center (MSFC), the Georgia Institute of Technology High-Power Electric Propulsion Laboratory (HPEPL) is applying the lessons learned from PEGASES design and testing to develop a new thruster prototype. This prototype will incorporate design improvements and undergo gridless operational testing and diagnostics checkout at HPEPL in April 2014. Performance mapping with ion optics will be conducted at NASA MSFC starting in May 2014. The proposed paper discusses the design and preliminary performance testing of this electronegative gas plasma thruster prototype

Initial Thrust Measurements of Marshall's Ion-ioN Thruster

Electronegative ion thrusters are a variation of tradition gridded ion thruster technology differentiated by the production and acceleration of both positive and negative ions. Benefits of electronegative ion thrusters include the elimination of lifetime-limiting cathodes from the thruster architecture and the ability to generate appreciable thrust from both charge species. Following the continued development of electronegative ion thruster technology as exhibited by the PEGASES (Plasma Propulsion with Electronegative GASES) thruster, direct thrust measurements are required to push interest in electronegative ion thruster technology forward. For this work, direct thrust measurements of the MINT (Marshall's Ion-ioN Thruster) will be taken on a hanging pendulum thrust stand for propellant mixtures of Sulfur Hexafluoride and Argon at volumetric flow rates of 5-25 sccm at radio frequency power levels of 100-600 watts at a radio frequency of 13.56 MHz. Acceleration grid operation is operated using a square waveform bias of +/-300 volts at a frequency of 25 kHz