Batch-microfabricated miniaturized planar arrays of Langmuir probes for reentry plasma diagnostics and nanosatellites

Abstract

One of the most important technical goals in spacecraft design is maintaining the vehicle's integrity under the extreme conditions encountered during reentry to the Earth's atmosphere. When a hypersonic vehicle travels through the atmosphere, a high-density and low-temperature plasma sheath forms around it due to shock heating of the surrounding air and ablation of the heat shield material, which leads to the dissociation and ionization of the background atmosphere. The plasma sheath that surrounds the spacecraft affects the heat transfer to the spacecraft, its aerodynamics, and its capability to communicate. A thorough knowledge of reentry plasma sheath properties is needed to effectively develop reentry vehicles capable of maintaining structural integrity, aerodynamic stability, and communications during reentry. This thesis reports the preliminary development of a novel plasma diagnostics technology that is modular and that can be used to both monitor the reentry of a spacecraft and serve as a scientific payload in a miniaturized satellite. The technology utilizes planar arrays of batch-fabricated micro Langmuir probes that can be surface-mounted on a reentry vehicle or miniaturized satellite as a sensorial skin to perform time-resolved measurements of the electron temperature and number density of the surrounding plasma sheath. These low-cost, miniaturized plasma sensors align with the paradigm shift in space technology, where missions are visibly smaller, inexpensive, and high performance. A rough analysis of the reentry heat transfer and plasma data, and our own microfabrication capabilities, led to the design and batch-fabrication process of the micro Langmuir probes. The micro Langmuir probes were constructed by filling-in with electroless nickel 100 pm-diameter tapered vias machined into a Pyrex substrate, resulting in individually addressable probes having 600 ptm-diameter tips. The highest density arrays that were fabricated consist of 25 probes per 1 cm-square tile, having 1.6 mm separation between probes and square packing, though up to 39 probes per 1 cm-square are possible. The MEMS Langmuir probes were preliminarily tested in a plasma environment similar to atmospheric reentry at MIT's Versatile Toroidal Facility (VTF). The MEMS Langmuir probes were operated as a current-mode triple probe to obtain real-time estimates of the electron temperature, number density, and Debye length. The performance of the MEMS Langmuir probes as a triple probe was benchmarked using a homemade conventional triple Langmuir probe. The plasma parameters measured by the MEMS Langmuir probe were within the range of VTF's reported plasma parameters, but the estimates of the electron density and Debye length were not within the range of the estimates from the conventional triple Langmuir probe using identical plasma. Therefore, we believe that better driving circuitry is needed to increase the signal-to-noise ratio in the MEMS probe data. Nonetheless, these preliminary results suggest that the MEMS Langmuir probe technology has a promising role for conducting reentry plasma diagnostics and serving as nanosatellite scientific payload.