ISP Lab Graduate Research

Advances in satellite computational and communications systems have allowed smaller satellites to perform missions which could previously only be accomplished with much larger and more expensive spacecraft. One enabling technology for these satellites is a scalable and efficient method of maneuvering the tiny vehicles in space. Existing propulsion technologies, which are deployed on larger satellites, cannot be scaled down in size while maintaining their performance. One promising solution for micropropulsion is called electrospray. This method of propulsion uses electrical energy to accelerate an atomized liquid propellant to produce thrust. Traditionally, this method of propulsion requires a needle-like structure to generate the electric stresses needed to spray the fluid. In 2012, members of the Ion Space Propulsion Lab demonstrated a novel method of electrospray using ferrofluids. Ferrofluids are magnetic liquids that can be manipulated with the use of magnets. When these fluids are in the presence of a magnetic field, the fluid interface naturally deforms into a series of peaks and valleys, referred to as the Rosensweig instability. At the tip of these peaks, the electric field is amplified and electrospray emission can be achieved. The result is a thruster comprising many small peaks—each producing a jet of ions—that assembles itself out of the propellant. Michigan Tech has a pending patent for this ionic liquid ferrofluid propulsion technology. Although ferrofluids are not a new technology, the development of a ferrofluid that has all of the properties required for use as a spacecraft propellant is new. Such a fluid was first developed by Brian Hawkett and Nirmesh Jain at the University of Sydney in Australia. Michigan Tech and the University of Sydney are collaborating on research to produce a marketable thruster from this technology.

Very small satellites require very small thrusters. Designing such thrusters requires detailed understanding of fluid mechanics at the micro scale. The ISP Lab maintains imaging and visualization facilities dedicated to exploring surface instabilities and interfacial dynamics of complex liquids at the micro scale. We employ collimated blue laser backlighting and stereo microscopy to visualize effects such as magnetic and electric liquid surface instabilities and micro-jet breakup instabilities. These optical diagnostics can be used to determine the balance of electric, magnetic, and capillary stresses in the deformed fluid interface at the tip of an electrospray thruster. In order to understand fluid physics on the atomic scale, we employ imaging techniques much more powerful than optical instruments can achieve. We can insert operating electrospray microthrusters into the specimen imaging chamber of an electron microscope to observe the nano-scale physics within the liquid propellant. We can visualize the molecular motion at the base of the ion jet with detail down to a few nanometers. Using this technique, we observe never-before-seen behavior when extreme electric fields as high as 10 Gigavolts per meter are applied to conducting liquid interfaces.

Plasma thrusters are famous for their fuel efficiency. When compared with a chemical rocket, a typical plasma thruster may only require a tenth of the amount of propellant to accomplish a given mission. However, because the electric power available on a spacecraft is limited to a few 10s of kiloWatts, plasma thrusters are limited in the amount of instantaneous thrust they can provide. The result is that, while a plasma thruster requires less fuel, it will take longer to accomplish a given mission than a high-thrust chemical system. For certain missions, it is more important to get the spacecraft to its destination in a short time than it is to conserve propellant. These types of missions require more thrust than is currently available using state-of-the-art Hall thrusters.

Work at the ISP Lab is focusing on increasing the amount of thrust available from a Hall thruster for a given input electric power. These so-called "high thrust-to-power ratio" devices would be an enabling technology for many missions, civilian as well as military, requiring rapid re-positioning of space assets. Researchers are examining the various thrust and power loss mechanisms of current Hall thrusters with the intention of identifying means of improvement. A subtle but critical related area of research involves understanding the coupling voltage between the thermionic hollow-cathode/neutralizer and the thruster plasma.

Hall-effect thrusters (named after an electromagnetic effect discovered by Edwin Hall in 1879) are plasma rockets used to maneuver satellites in space. Hall-effect thrusters can change a satellite’s orbit, maintain a specific orbit in the presence of perturbing effects like drag, or propel a satellite from Earth to another location in the solar system. State-of-the-art Hall thrusters use gaseous xenon for their propellant. Of all the elements that are gaseous at ambient conditions, xenon possesses the best mix of chemical properties for use as a plasma fuel. It is possible, however, to realize a Hall thruster with superior performance if other elements that are not gases could be used in place of xenon. Many candidate elements are condensed (solids) at room temperature, and thus are referred to as "condensable" propellants to distinguish them from gases like xenon.

The ISP Lab has been conducting research into condensable-propellant-fueled Hall thrusters since 2004. Bismuth, zinc, and magnesium thrusters have been successfully tested using the condensable propellant test facility. These metal propellants have superior performance to xenon for many missions and are also significantly cheaper than xenon. The most challenging aspect when using metallic propellants is that they need to be vaporized before being supplied to the thruster. This vaporization process can consume large amounts of electrical power—resulting in an intolerably low system efficiency. Michigan Tech has patented and developed a unique process to vaporize metallic propellants within the Hall thruster by utilizing waste heat from the plasma discharge.

An additional potential benefit of condensable propellant Hall thrusters is that planetary research has shown that zinc and magnesium can be found in high concentrations throughout the solar system. A Hall-effect thruster running off one of these propellants could potentially refuel itself during a mission by harvesting these sources from Martian soil or asteroid regolith.

Plasma thrusters operate by expelling a high-velocity stream of positively charged ions to create reactive thrust. However, an isolated spacecraft that continually emits positive ions will very quickly develop a large imbalance in electrical charge due to an excess of negative electrons onboard. Plasma thrusters therefore require a device known as a cathode that is capable of expelling electrons into space at the same rate that the thruster emits ions.

Typical cathodes operate by using a small amount of propellant gas to create a plasma discharge—which exhausts the excess electrons into space. Since this gas does not effectively create any thrust, it is effectively a waste of valuable propellant. Overall thruster performance could be drastically improved by a technology capable of emitting electrons without using any gas. Technology known as a field-emission cathode can create beams of electrons without any external gas feed or electrical heater. Field emission cathodes rely on quantum mechanical emission from very sharp (10-nanometer-radius) electrode tips. Because of the fragility of the nanometer-sized tips, the structures are susceptible to damage that blunts the tip and destroys the device functionality. This type of failure in space would render the propulsion system useless and leave the spacecraft stranded.

At Michigan Tech, we have developed and patented a field-emission cathode for use with plasma thrusters that has the ability to be regenerated when the emitter tip becomes damaged. The cathode is made by using a strong electric field to shape a small volume of liquid metal (such as indium or gallium). The metal is pulled into a nano-sharp tip by the electric field, and then the resulting structure is frozen into permanence by removing electrical heat. The nano-tip then acts as an effective electron source. The tip still experiences erosion and blunting as do conventional field-emitters; however, the sharpening process takes only seconds and can be repeated an unlimited number of times, continually refreshing the tip to like-new condition and rendering the lifetime unlimited.

The measurement of plasma properties, such as electron temperature and density, can be obtained by inserting small electrically biased probes into the plasma region of interest. The voltage of the probe is varied while the current hitting the probe from the plasma is measured. Using these techniques, we can ascertain how well our thrusters are utilizing electromagnetic energy and also how the exhausted plasma might interact with the spacecraft in orbit.

In some instances, the physical presence of these probes disrupts the plasma in the area being measured, ultimately changing the properties that are being measured. Laser Thomson scattering (LTS) is a noninvasive measurement technique employed in the ISP Lab which can be used to determine the temperature and density of free electrons in a plasma using only light. We use high-power lasers to shoot a burst of photons of known frequency (color) into the plasma. These photons collide with free electrons in the plasma and are elastically scattered in all directions. We use a sensitive spectrograph to compare the color of the scattered photons to their original color coming out of the laser—small differences can tell us how fast the free electrons were moving and, from this, the plasma temperature.

When a strong electric field is applied to a fluid surface, the fluid will stretch. As the electric field strength is continually increased, a point can be reached at which the fluid can no longer resist the electrical pull. At this point a jet, or spray, of fluid will result from the emission site—a phenomenon known as electrospray. The emission of this fluid results in a minute amount of thrust. By packing tens to hundreds of these emission sites in an array, a scalable thruster can be built that is well suited for small satellite (typically 10s of kilograms or less). As a result of ongoing research into ionic liquid ferrofluid microthrusters, the ISP Lab has developed a series of computer simulations to model the complex interactive ferro- and electro-hydrodynamics of these fluids.

Utilizing the High Magnification Imaging Setup, researchers are able to image the surface deformation of a fluid in controllable electric and magnetic fields. These images serve as a verification of the simulation performance.