Ion Optics Modeling
Ion thrusters are a type of electric propulsion spacecraft engines. These advanced thrusters operate by ionizing neutral gas inside an ionization chamber. One wall of the chamber is covered by ion optics, set of two or three closely spaced metal electrodes perforated with holes. Strong electric field is generated between the grids. This field accelerates the ions produced inside the chamber out of the device. Proper sizing of the grid holes and the electrode spacing is critical for optimal operation of these thrusters. We have developed a three dimensional particle in cell plasma simulation code to model the plasma flow in ion optics. Our code allowed us to study the motion of ions as they pass through the optics, and the merging of individual beamlets into a single ion beam. By adjusting the grid geometry and applied voltage, we were able to investigate what role these parameters play on the thruster performance.
Kinetic modeling of plume neutralization
The electric field configuration in the ion optics effectively prevents any electrons from leaving the chamber. The beamlets thus constitute only of positively charged ions, and must be neutralized by the electrons in order to generate a quasi-neutral ion beam. Ion thrusters use an externally placed cathode to provide the neutralizing electrons. However, the process by which electrons enter and neutralize the ion beam is not completely understood. In our study, we performed a fully kinetic simulation of the near exit mixing region and found that electrons form an envelope around the ion beam. The exact energy loss mechanism is still under investigation. This work also included basic cathode modeling, and investigation of several clustering options.
Collaborative immersed data visualization
Scientific simulation codes generate huge amounts of data. This data is generally meaningless in its raw form – it consists of a large file full of numbers. Visualization takes this collection of numbers, and transforms it into comprehensible contour plots, isosurfaces, streamlines, and vector fields. It helps the researches make sense of the data. We have developed a simple 3D visualization program, capVTE. capVTE utilizes the VTK and QT architecture. Interesting feature of capVTE was networked collaborative visualization, which allowed multiple parties to share the visualization workspace, and thus explore the data in a collaborative fashion. capVTE also supported export of data in a format compatible with the CAVE immersed reality environment. Note, capVTE is currently being rewritten to make it fully compatible with the latest versions of VTK and QT. Stay tuned for the upcoming release.
3D modeling of electric propulsion plumes and spacecraft interactions
Contamination is a concern to any space mission. Volatile materials can evaporate from one surface and collect on another one. If the target surface is a a sensitive instrument or a radiator, the impact to the mission can be catastrophic. Similarly, microscopic particles can shake off surfaces by launch vibrations and collect on image sensors. Spacecraft that operate plasma thrusters have another source of contamination to worry about: the thruster plume. Although thruster plumes, chemical or ion, are oriented such that they do not directly impinge on any instruments, ion plumes have the tendency to expand into regions with no direct line of sight to the thruster. This is due to so-called charge exchange (CEX) reaction occurring near the thruster exit. No thruster is 100% efficient at ionizing the propellant. The non-ionized propellant diffuses out of the thruster, and due to its low velocity (corresponding to the thermal speed), will accumulate into a dense neutral cloud outside the thruster exit. The ions leaving the thruster have to first penetrate this cloud. In the process, it is possible that an electron jumps from the neutral to the ion without affecting the momentum of either particle. This results in formation of a fast neutral and a slow ion. A radial electric field forms naturally in the plasma plume due to the density decay in that direction. These slowly moving ions will then react to this field and will be accelerated radially out of the plume. This results in the formation of a donut-like structure around the main plume; this structure is called the charge exchange wings. The actual shape of the wings and the plasma environment around the spacecraft will depend on the actual geometry of the satellite, as well the local surface potentials, and cannot be estimated analytically. We have developed a three dimensional electrostatic particle in cell code to model the plume of electric propulsion thrusters and its interaction with spacecraft instruments. This code has been applied to a wide range of research topics.
Spacecraft environmental effects
We have also studied other factors influencing spacecraft lifetime and performance. These include charging and particulate redistribution. We have developed a computer model to simulate surface charging and charge propagation in dielectric materials. We have also performed charging studies using commercially available tools, and performed some integrated charging / plume modeling studies. We have performed kinetic particle tracing analysis to study the transport of dust contaminants under various environments encountered during integration, testing, and launch.
Atmospheric low temperature plasma jets
Low temperature atmospheric plasma jets, in which ions remain near room temperature, have recently become popular in the medical community. Plasma medicine is an exiting new research area involving studying the effect of plasma beams on living tissue. This technology can be used to aid in wound healing, surface disinfection, but also in more exciting areas such as surgery and cancer treatment. One of the big unresolved questions includes understanding what exactly causes the controlled cell death, apoptosis, seen in cells exposed to plasma beams. To help understand this issue, we have developed a diffusion driven model of the atmospheric plasma jet. Our model captures the streamer propagation of the plasma bullet and computes production rates and decay of various species by considering chemical rate equations.
Electron transport in magnetized plasmas
Another type of plasma space propulsion is the Hall thruster. Unlike the ion thruster, the Hall thruster does not use electrode plates to accelerate the beam. Instead, a Hall thruster consists of an annular or cylindrical channel, with the anode located at one end. The other end is open to the ambient environment. A strong magnetic field is applied across a section of the channel. Electrons are generated outside the device and flow towards the anode. When they encounter the magnetic field, electrons become magnetized, and can no longer freely move towards the anode. This effect produces an increased electron density needed to effectively ionize the propellant. It also acts to form an electric field in the direction perpendicular to the magnetic field. In a Hall thruster, the magnetic field generates a sort of a virtual ion optics. However, despite many decades of flight heritage, many unknown still remain in the understanding of these devices. Among the primary unresolved issues is the transport of electrons across the magnetic field. From classical theory, electrons can move across magnetic field only by undergoing collisions. Collisions alone are however not able to explain the anode currents seen experimentally. We have developed a kinetic model that simulates the motion of electrons along the magnetic field line. Using this code we are hoping to predictively determine the transport coefficients that are needed in order to predict thruster lifetime and performance.