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Helicon-injected IEC for High Thrust-to-Weight Electric Propulsion 

George H. Miley, Univ. of Illinois

9/16/09 

Introduction 

Advanced satellite and spacecraft orbital maneuvering requires a propulsion system that can provide high thrust while also maintaining a high ISP. The inertial electrostatic confinement (IEC) thruster offers a very promising candidate for this mission. IEC is a means of confining plasma using a series of spherical electrostatic grids. By designing an asymmetry into the grids the device can produce a beam of high energy plasma that can be used to provide thrust. This is illustrated in the schematic and photo of the U of Illinois jet thruster experiment shown in Figure 1. An IEC electric thruster is capable of an ISP of several thousand seconds while operating at high power (>kW). The design inherently offers a greatly increased 

thrust-to-weight ratio, giving it a significant advantage over other electric thruster designs. This advantage is due to the relative simplicity of the design structure where the primary acceleration stage of the thruster is purely electrostatic requiring only lightweight metallic grids. In this proposal we address a new type of ìtwo-stageî (helicon injected) IEC that incorporates these advantages into an overall system platform. 

Background 

The goal of this proposed work is to build and study a helicon-injected IEC electric thruster. This type of thruster is a two stage device. In the first stage, the primary ionization occurs. We propose to use a helicon discharge as the first stage ion injector into the IEC second stage. A helicon-produced ionization stage provides important improvement in plasma density, hence performance. This high density source will allow for large ion currents for high thrust and central-peaked density profile is ideal for injection in the spherical IEC potential well. The helicon discharge also provides low energy-per-ion cost and an ionization fraction of nearly unity that will give the thruster high efficiency. 

The IEC second stage provides the primary acceleration and formation of the thrust-producing plasma jet. The voltage in this stage is variable with typical values on the order of a few kilovolts. This control along with the appropriate gas choice allows for tuning of the ISP to suit the mission parameters. In addition to the high thrust-to-weight, the IEC second stage provides several advantages as the acceleration stage of the thruster. Because the design is electrostatic, detachment of the plasma from the thruster becomes a simple task compared to magnetic nozzle-based designs. 

Additionally, the plasma jet itself should be essentially a net neutral-charge beam and thus additional neutralization (which decreases overall efficiency) is not required. The thruster nozzle can be made in a small diameter, minimizing fuel leakage from neutral atoms. The asymmetry in the grids allows for rotation of the grid which means that the thrust can be directed in various directions. If desired, further control is possible by multiple independently controlled jets. 

Figure 1. Left ñ schematic of IEC thruster experiment. Right ñ Photo of IEC-produced plasma jet propagated through a port in the vacuum vessel. The grid size is around 15 cm while the beam diameter is less than 1 cm. The grid guide is removed to allow access for photo. 

Proposal 

We propose a one year proof-of-principle study of the helicon-injected IEC electric thruster. If successful, a Phase II proposal would be submitted to construct such a system in the EAFB test stand for thrust verification measurements. 

During this period, several crucial aspects of the two-stage device will be studied. First, the plasma jet from the IEC will be studied using the electrostatic analyzer (ESA) that has been provided by the EAFB EP lab. The helicon source will then be 

characterized and modes will be selected suitable for injection into the IEC second stage. The process of coupling the two stages will be analyzed, followed by a preliminary study of the helicon injection into the IEC second stage. The entire two-stage device can then be run as a thruster to provide preliminary data on thrust, efficiency, ion energy and ISP. Throughout the experiment, the theory of operation will be updated to allow for the optimization and scaling of the device. 

Corresponding computational simulations will provide further insight into the design. 

Deliverable 

At the end of the contract, a report will be prepared detailing the results of the energy measurement of the IEC jet ions and extrapolating the scaling of the critical parameters of the thruster. Assuming the results are encouraging as expected, a follow on two-year project would be proposed for the design and construction of a unit for thrust testing inside the EAFB thrust testing chamber. This unit would have a more flight-like design that would maximize the thrust-to-weight ratio by shedding the large vacuum chamber. This would result in a unit suitable for flight qualification to begin. 

Benefits to EAFB EP program 

The proposed project offers multiple benefits to the AFRL EP program at Edwards AFB. First, by extensive use of existing equipment, it provides a low cost route to the development of a potentially important high thrust-to-weight ratio electric propulsion system for next step application beyond present Hall thruster designs. In addition, it extends present EAFB work on helicon plasmas by development of the technology as a plasma source for injection into other devices such as field-reverse configurations. Another important aspect of this project is the use of the ESA provided by EAFB for the characterization of the IEC plasma jet. The techniques developed would enable effective use of this diagnostic tool on other systems of interest to EAFB. Finally, this project would endow students with the opportunity to spend time at EAFB helping with projects and adding to their experience on electric thruster development. 

Staff 

Professor G. H. Miley, U of Illinois, would serve as the PI on the project. He has interacted with staff at EAFB as a consultant on advanced plasma propulsion for a number of years, most recently supervising Mike Reillyís Ph.D thesis work on helicon plasmas. 

Ben Ulmen and Guilherme Amadio would be the primary graduate students dedicated to the project. Ben has a strong interest in space propulsion and inertial electrostatic confinement. He has a background in physics with an M.S from Michigan Technological University and is current working on a Ph.D in the nuclear, plasma and radiological engineering program. Guilherme has an M.S in nuclear astrophysics from the University of Tokyo. He is working on a Ph.D in the Aerospace 

Engineering program and holds a fellowship. Hugo Leon has worked for Professor Miley for several years as research support staff. He has extensive fabrication experience and knowledge of vacuum systems and his role would be to facilitate the construction and operation of the vacuum and electrical systems as well as providing various technical support. 

Several undergrad students will provide lab assistance and also learn more about the exciting field of EP. 

Estimated Budget 

A cost of about $174k is requested to support this work. This could be reduced somewhat if some further items of equipment can be obtained from EAFB. See following budget proposal. 

Equipment budget(Note:AnyequipmentborrowedfromordonatedbytheAFRLwouldresultinasignificantreductionin the projected equipment costs.)

ComponentEstimated CostVacuum pump8000Power supplies (3)15000Miscellaneous vacuum supplies7000Miscellaneous structural materials2000Miscellaneous RF components2000Diagnostic plasma probes2000Monthly discretionary supplies - 200/mon2400Subtotal38400Personnel budgetStaff (PI 2 weeks summer;1/2 &1/4 53320Personnel Benefits4362T&F for RA grad students18763IDC =58.50% MTDC54453Subtotal130898Travel budget (to EAFB):3 trips4200Total funds requested173498

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