Measurements of the turbulent magnetic field in a Hall thruster have been carried out between 1 kHz and 30 MHz with the aim of understanding electron transport through the magnetic field. Small detecting coils at the exit of the accelerating channel and outside of the ionic plume were used to characterize various instabilities. The characteristic frequencies of the observed power spectral densities correspond to known classes of instabilities: low frequency ͑20-40 kHz͒, transit time ͑100-500 kHz͒, and high frequency ͑5-10 MHz͒. A model of the localized electron currents through a magnetic barrier is proposed for the high-frequency instability, and is found to be in good quantitative agreement with the observations. Based on the measured high-frequency turbulent magnetic field, the turbulent electric field is estimated to be about 1 V / cm outside of the plume and ranges from 10 to 10 2 V / cm at the channel midradius at the exit of the thruster. The "anomalous" electron collision frequency, related to the high-frequency instability, is estimated to be Ͻ10 6 s −1 , which largely exceeds the classical frequency in the core of the exit plasma but is lower than the frequency that is generally used in hybrid codes.
After a brief historical review of inductive sensors used for measuring terrestrial magnetic field variations, this article explains the difficulty in extending the working range of the well known search coil toward high frequencies. The theory of the closed loop magnetometer is exposed and practical considerations leading to an optimized sensor are discussed. Two examples of prototypes are then shown, with the simulated and experimental curves of the transfer function and the noise spectral density. Finally, an experimental facility to measure the sensitivity of the sensor to the pure electric field is shown, and the results of one of the prototypes are presented.
A high sensitivity 1D magnetic field sensor is developed for spatial applications, in order to replace the heavy search-coils currently used. This new sensor combines a flux concentrator, biasing coils for field modulation and magnetic tunnel junctions. These three elements are fabricated and independently characterized. Finally, the expected performance of a sensor combining these three elements can be estimated.
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