Paulo Lozano

Efficiency Estimation of EMI-BF4 Ionic Liquid Electrospray Thrusters

Ionic liquid-based Electrospray Thrusters offer an attractive alternative for flexible, high specific impulse micropropulsion systems. Among the most promising attributes of these thrusters is their high efficiency as previously estimated by mass spectrometric and time-of-flight techniques. The energetic structure was indirectly revealed by these studies, suggesting that charged particles are emitted with a narrow energy spread centered close to the applied potential with just a small differential probably due to the energy invested in extracting charged species directly from the liquid surface. A direct characterization of the beam energy distribution from a single tungsten emitter externally wetted with the ionic liquid EMI-BF 4 using a retarding potential analyzer complements original estimations allowing a more reliable determination of the thruster overall efficiency.

EXPERIMENTAL MEASUREMENTS OF COLLOID THRUSTER PLUMES IN THE ION-DROPLET MIXED REGIME

Under the right conditions, colloid thruster plumes may be composed of a mixture of ions and droplets. Experimental techniques, including time-of-flight and stopping potential measurements are used to characterize plumes with large ion fraction. The energy distribution of these beams is very rich, indicating that individual components have different energies. Electrostatic focusing with an einzel lens is used to selectively observe charged particles that share a range of energies. Formamide doped with NaI is used as propellant. Measurements show that Na+ with 7 formamide molecules solvated on it is likely to be the ion emitted from the cone-jet structure. The remainder of the current in the beam is taken by droplets with a relatively high dispersion in specific charge.

Magnetic Field Effects on the Plume of a Diverging Cusped-Field Thruster

The Diverging Cusped-Field Thruster (DCFT) uses three permanent ring magnets of alternating polarity to create a unique magnetic topology intended to reduce plasma losses to the discharge chamber surfaces. The magnetic eld strength within the DCFT discharge chamber (up to 4 kG on axis) is much higher than in thrusters of similar geometry, which is believed to be a driving factor in the high measured anode e ciencies. The eld strength in the near plume region is large as well, which may bear on the high beam divergences measured, with peaks in ion current found at angles of around 30-35 from the thruster axis. Characterization of the DCFT has heretofore involved only one magnetic topology. It is then the purpose of this study to investigate changes to the neareld plume caused by altering the shape and strength of the magnetic eld. A thick magnetic collar, encircling the thruster body, is used to lower the eld strength outside of the discharge chamber and thus lessen any e ects caused by the external eld. Changes in the thruster plume with eld topology are monitored by the use of normal Langmuir and emissive probes interrogating the neareld plasma. Results are related to other observations that suggest a uni ed conceptual framework for the important near-exit region of the thruster.

Spectroscopic and Electrostatic Investigation of the Diverging Cusped-Field Thruster

A magnetized plasma thruster variant with a divergent chamber and strong magnetic cusps has been been prototyped and preliminarily characterized at the MIT Space Propulsion Laboratory. This paper expands upon the previous work with a non-invasive, optical examination of the near-field plume and discharge chamber phenomena. Calibrated broadband spectra reveal changes in Xe I and II level populations with operational mode, while high resolution narrow bandwidth scans are used to examine trends in multiply charged ion fractions and Doppler shifted profiles. An internal electrostatic probe apparatus is also decribed which has been installed to observe ion flux to diverging channel surfaces in order to determine the efficacy with which the magnetic topology is used to reduce ion bombardment energies.

Progress Toward a Variable Specific Impulse Electrospray Propulsion System

The ion Electrospray Propulsion System (iEPS) is a microelectromechanical systems (MEMS)-based electrostatic thruster for space propulsion applications. This device leverages the purely ionic electrospray emission regime, as well as a porous emitter substrate, to eliminate the need for cumbersome ancillary components and achieve small mass, volume, and area footprints that are favorable to nano/pico-satellite integration. In this paper, we review the early maturation of the iEPS concept and a first generation device. We outline the fabrication process, including a unique electrochemical etching technique for the porous metal substrate, and highlight the salient problems and conclusions. Potentials solutions to these issues are posed and incorporated within the framework of a second generation design that expands capability to variable specific impulse, constant power (multi-modal) operation using a dual-grid scheme. The merits of multi-modal operation are explored, along with a preliminary study of the ion optics problem governing the achievable performance.

On the Validation of Porous Nickel as Substrate Material for Electrospray Ion Propulsion

The use of nickel as a substrate when creating porous metal Ionic Liquid Ion Source (ILIS) arrays for spacecraft propulsion has been explored and deemed viable. Nickel has been selected as a candidate material due to its compatibility with an electrochemical etching process specifically developed to yield surface micromachining of bulk porous metals without significant pore degradation. The investigation of nickel ILIS includes externally wetted emitters which were fabricated and compared with more common tungsten emitters. Using externally wetted emitters, beam currents measuring 100’s of nA were detected for both positive and negative emission from two ionic liquids, EMI-BF 4 and EMI-Im. The emission from EMI-Im was tested by time of flight spectrometry where it was confirmed that, under most conditions, the emission is purely ionic. In addition, the ionic liquid to nickel interface potential was probed during emission from externally wetted emitters. Here it was found that by alternating the emission polarity at frequencies on the order of 1 Hz , complete charging of the electrochemical double layer can be prevented. Combined with similar results from tungsten emitters it is motivated that detrimental electrochemical effects can therefore be suppressed through voltage alternation. The process implemented for electrochemically fabricating porous nickel ILIS arrays is presented along with a process for housing the array within a silicon enclosure which includes an aligned electrostatic grid. Finally, emission characteristics from a sample array of porous nickel ILIS are presented. Between 100 and 150 emitters fabricated on a 10 x 10 x 1 mm porous nickel wafer were fired yielding current levels up to 200 µA .

Capillary vs. externally wetted ionic liquid ion sources

Recent studies on the use of Taylor cones of ionic liquids as ion sources have demonstrated a considerable effect of source geometry on the characteristics of the resulting ion beam. Of particular interest is the recent discovery by Lozano and Martinez-Sanchez of the excellent performance of externally wetted and electrochemically etched and sharpened tungsten tips. These sources are termed ionic liquid ion sources (ILIS), in analogy to conventional liquid metal ion sources (LMIS). Even more noteworthy is the fact that certain ionic liquids that had in a variety of prior reports always operated in the mixed ion-drop regime from capillary tips, do run exclusively in the purely ionic regime (PIR) on ILIS type sources. The present study uses time of flight mass spectrometry to compare full ion beams emitted from the two types of sources for a diversity of ionic liquids. We confirm earlier reports that internally fed capillary sources achieve purely ionic emissions more readily for liquids having high surface tension (γ > 40 dyn/cm) and high electrical conductivity (K ~ 1 S/m), and produce ion beam currents of several hundred nA with non-negligible energy spread. In contrast, externally wetted ILIS sources produce purely ionic emissions at tens of nA with liquids with substantially smaller γ and K, and much narrower energy distributions.

Low Frequency Oscillations in the Diverging Cusped-Field Thruster

thruster is markedly bi-modal, with a sharp transition to a visually distinct, low anode current mode at high applied voltages. Time resolved measurements of the anode current in the high current, low voltage mode revealed extreme oscillations at 3.5 kHz with the current nearly extinguishing during each period. Discharge current oscillations during high voltage, low current operation are minimal in contrast, though the dominant frequency remains the same. A high speed camera is used to discern the evolution of plasma luminosity through a typical cycle, with the geometry of the discharge chamber allowing shallow views of the ionization region to augment the typical axial imaging. Anode current is measured simultaneously and matched to representative cycles of image intensity inside the discharge chamber. The time resolved thruster dynamics are then compared for a range of operating conditions under each of the observed modes. Trends in oscillation frequency, magnitude, growth rates and decay rates are presented. A strong correlation between luminous plasma regions and magnetic cusp locations is observed.

Characterization of a CubeSat compatible magnetically levitated thrust balance for electrospray propulsion systems

Thrust estimates from an ion Electrospray Propulsion System [1] can be inferred by measuring the electrical current consumed by the device. However, it is very challenging to directly measure the microNewton thrust these thrusters generate. A thrust balance was designed, constructed, and characterized to tackle this difficult problem and measure thrust directly. A complete electrospray power processing unit used for the thrust stand will also be described.

Progress Toward Demonstration of Remote, Autonomous Attitude Control of a CubeSat Using Ion Electrospray Propulsion Systems

As a solution to the problem of scalable propulsion in small satellite architectures, the microfabricated ion Electrospray Propulsion System (iEPS) developed by MIT’s Space Propulsion Laboratory has progressed to a point where it is ready to be characterized on a realistic testbed. In this paper, developments in the iEPS thruster design and testing equipment are outlined. Design changes address the performance and testing issues encountered with the rst version. These changes include features to mitigate the formation of liquid current paths and ease grid-to-tip alignment and grid removal. Additionally, a 1-DOF free-oating CubeSat testbed with an integrated autonomous remote control system and high-voltage PPU has been developed and tested for use as a thrust balance and attitude control demonstrator.