Prof. Manuel Martinez-Sanchez

Electrospray Propulsion: Electrospray thrusters are electrostatic accelerators of charged liquid droplets, solvated ions or both. Individual emitters yield sub-(N thrust levels, although, with high efficiency and over a wide range of specific impulses. These emitters can be arrayed for higher thrust levels. Current research at SPL focuses on the design of these arrays, produced by micro-fabrication techniques in Silicon. Several other topics under investigation in this area are the physics of cold ion extraction from a liquid, the design of ion lenses to avoid impingement and the capillary-driven liquid flow on an external roughened surface.

Hall Thrusters: Hall thrusters are electrostatic ion accelerators in which the counterflow of electrons is impeded by an interposed magnetic field. This forces electrons to accumulate in the strong field region, leading to efficient ionization. At the same time, no grids are necessary, as in ion engines, because the electrons neutralize the plasma everywhere. We pursue both, modeling and experimentation on these thrusters, as well as on the plasma plumes they generate, and their interaction with the spacecraft surfaces. The modeling is mainly pursued through numerical simulation using variations of the Particle-in-Cell method, in which fictitious aggregates of charged or neutral particles are tracked as they move under the influence of electric and magnetic fields. These fields are themselves affected by the particle locations and velocities, and need to be updated between particle-move cycles. The method yields very detailed space-time histories of plasma parameters, but is still in development due to the multiplicity of complex physical effects involved. The experiments are at this time related to the plume plasma and its properties, of great importance to spacecraft users; in addition, we are also developing miniaturized versions of these thrusters, for use by small satellites.

Radio-Frequency Propulsion: This is a new departure for the SPL. Towards the end of 2005, a new Helicon plasma generator came on line, utilizing our medium-size (1.5x1.5 m) vacuum chamber and a set of magnetic coils and flow and power systems designed and assembled by a student team. Helicon waves are known to be very efficient in producing ionization of many gases, and are already in use by the electronic industry for semiconductor etching. Several other research groups around the world are trying to develop and adapt this technology for space propulsion, and our facility falls in this category. The first test series was successful in generating dense plasmas in Nitrogen and Argon, and in producing electrostatic double layers of the beginning of our magnetic divergent nozzle. Detailed plasma mapping and thrust measurements are planned for the near future.

Space Tethers: Long conducting cables (tethers) can find many interesting applications in space. We have in the past studied their use as either thrusters or generators, utilizing the magnetic forces that are exerted on a current-carrying cable as it crosses the Earth's magnetic field at orbital speed. The main technical and scientific issue here is the collection of electrons in sufficient quantity at the positive end of the tether, since their available population in the ionosphere is small. We have simulated the physics involved using similar numerical codes to those described for Hall thrusters. At this point, the emphasis has shifted to a study of methods that could one day be used to "clean up" the radiation (Van Allen) belts. In principle, a very high-voltage tether (with a lower voltage "grounding" tether) can be used to scatter charged particles into directions close enough to the geomagnetic lines of force that they would penetrate deep into the upper atmosphere and be absorbed there. Other methods, involving wave interactions are under study as well.

Other topics: At various recent times, Prof. Martinez-Sanchez has been involved in research on optimal planning of orbits for satellites using low-thrust engines, studies of potential interference of propulsive plumes with light-gathering instruments on science satellites, and of condensation of engine effluents on supercooled satellite surfaces.