Current Projects

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

George H. Miley, Univ. of Illinois



 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. 


 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. 



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. 


 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. 



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