An extreme ultraviolet and plasma spectrometer scientific payload for a cubesat
Ayeleso, Ayokunle Oluwaseun
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In the Earth’s upper atmosphere, damage to spacecraft electronics is caused by the exposure to extreme ultraviolet (EUV) radiation, solar protons and cosmic rays emanating from the Sun. One particular region where these types of radiation occur is the South Atlantic Magnetic Anomaly (SAMA) region. Therefore, there is a need to design sensors which could be used to investigate the flux and energy levels of radiation in this region. In order to do so, the present study focuses on the numerical simulations of plasma (atomic oxygen ion) in the ionosphere’s F-layer region using the principle of Langmuir single probe theory. Another goal of this study is to investigate the behaviour of a copper plate sensor under exposure to ultraviolet (UV) radiation through simulation and experimentation in a laboratory setting in terms of the photoelectric effect principle. Simulation results of the plasma model of the ionosphere’s F-layer region using an ideal single spherical probe showed that ionospheric plasma parameters such as electron temperature and electron density can be determined using the I−V characteristic curve obtained from the probe. Similarly, the simulation results of the behaviour of the copper plate under exposure to UV radiation showed that UV light with wavelengths shorter than 237 nm produced photocurrents. After calculations and simulations, the stopping potentials that could decelerate electrons emitted with UV radiation wavelengths between 100 nm and 237 nm are, respectively, between -7.6 V and -0.2 V. When comparing the calculated copper stopping potentials with the simulated copper stopping potentials, the results converge for shorter wavelengths. These short wavelengths are relevant in this study because they are comparable to EUV wavelengths (10 nm to 121 nm) in the SAMA region. Therefore, the stopping potential results were used in the experimental design and VUV radiation testing of the sensor prototypes. Two different copper sensor prototypes, namely a spherical extreme ultraviolet and plasma spectrometer (SEPS) sensor and a planar sensor were designed and constructed for the realisation of this project. The planar sensor, with dimensions that fit on the sides of a one unit (1U) CubeSat was developed to minimise the experimental uncertainty observed with the SEPS sensor. The aim of this study is to characterise the sensors and their responses to typical plasma and EUV radiation levels using the facilities available in the Western Cape region (South Africa). Plasma testing was not investigated due to lack of test facilities. Therefore, the investigation into plasma measurements was simulation-based. Experimental EUV radiation testing was conducted. Extreme ultraviolet radiation measurements were performed with different vacuum ultraviolet (VUV) sources, namely a pulsed laser and a deuterium lamp (steady) radiation. These measurements posed challenges due to the significant ambient noise present in the laboratory. Consequently, the study looked at both time and frequency domain analyses as avenues to mitigate the effect of noise on the measurements. The initial VUV pulsed laser radiation experiments, with the SEPS sensor used as a two plate spherical capacitor by connecting its outer grids and positively biased, produced no voltage pulses. Only the significant ambient noise levels were observed. The absence of voltage pulses can be attributed to the fact that electrons escaped the metal sphere too energetically, and may fly through the outer combined grids under the additional acceleration of the positive bias and are not collected. A triple probe structure with a negative inner grid is necessary to slow down electrons enough to be captured by the positive outer grid. Based on this principle of operation, the simpler planar sensor was used for further experimentation. In that way, the experimental uncertainties that are compounded by the concentric spherical construction of the SEPS sensors would be avoided. In the VUV pulsed laser radiation experiment, the time-averaged measured terminal voltages were obtained. The frequency content of the measured terminal voltages was also observed through performing a Fast Fourier Transform (FFT) on the time signals. Significant harmonics were observed at frequencies ranging from 210 MHz to 250 MHz. In addition, these results showed that output voltage pulses were detected through both time and frequency domain analyses. In other experiments with VUV deuterium lamp (steady) radiation, the measured time-averaged terminal voltages obtained from the planar sensor showed significant generation of currents (0 μA, -0.7 μA and 1.0 μA) which compared well with the theoretical calculation (0.69 μA). Conclusively, these results validate the measurement approach and operation of the planar sensor and could be used to design a one unit (1U) CubeSat sensor that measures EUV radiation when launched into space. The study further highlights the challenges that future sensor development will face in terms of the facilities available in the region, due to the very small signals that have to be measured in laboratory environments with high levels of ambient noise.