A Time-of-Flight Mass Spectrometer for Upper Atmospheric Measurements

The mesosphere-lower thermosphere (MLT) is perhaps the least understood region of the earth’s atmosphere due to the difficulty of obtaining in-situ measurements. Access to the MLT is limited to high-speed sounding rockets for brief periods of at most a few minutes. Because of its wide mass range and high scan rate, Time-of-flight mass spectrometry (TOF-MS) has potential to resolve thin layers of diverse species in the MLT. However, because ambient pressures can reach into the millitorr range, TOF-MS has rarely been applied in the MLT due to its dependence on high voltages and microchannel plate (MCP) detectors. A novel dual mode, compact axial TOF-MS suitable for deployment aboard a sounding rocket for measurements in the MLT is presented. This TOF-MS is capable of operating in either a standard TOF mode or in a multiplexing mode to achieve high measurement duty cycles with a theoretically unlimited mass range. Experimental data is presented demonstrating successful MCP operation in a variety of environments including O2, N2, and air at pressures into the low millitor range. Also presented are results from extensive simulation and modeling efforts to approximate the in-flight operating environment of the TOF-MS. Gas flow modeling in a typical MLT environment is performed using the Direct Simulation Monte Carlo (DSMC) method. Standard gas flow equations are combined with DSMC results to estimate pressures inside the TOF-MS. Modeling, simulations and experimental work combine to demonstrate the potential of the new dual mode TOF-MS for deployment in the upper atmosphere