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A new device for the mitigation and control of negative spacecraft charging phenomena has been developed at Thales Alenia Space in Italy. The instrument is based on a very low weight and power consuming cathode, emitting electrons in a diode configuration. The emitting units can be moderately biased with respect to the spacecraft ground in order to guarantee the net current exchange with the environmental plasma and assure the discharge of the spacecraft under negatively charged condition.
The prevention/alleviation of the S/C surface charging is an effective approach to drastically reduce risky situations impacting the S/C health. The charging status of a S/C can be influenced basically with two approaches:
The Active Charging Compensator System (ACCS) is an instrument, proposed and developed by TAS-I, dedicated to the active control of the S/C Charging level (preventing/alleviating the build-up of a surface charging retained dangerous for the S/C safe operation), by a controlled emission of electrons generated and accelerated through a suitable electron generation device.
The ACCS can be considered an actively conditioned device, but its operation, once the cathode has been activated and the device switched on, acts in a passive way ( as the electrons emission can be considered also self-controlled).
The ACCS is suitable for reducing or preventing negative absolute charging of the overall S/C main structure (absolute charging prevention ). Furthermore, a S/C part – charged with respect to another S/C part - can be protected as well (differential charging prevention), provided that the eventual potential build-up around the S/C does not trap the emitted electrons.
The AESD-NU and AESD-PS have been successfully submitted to a sequence of tests. In particular, The AESD-NU has been manufactured at EM level, pre-qualified and characterised in terms of ability to exchange current with a target (acting as a plasma) and to lower the potential difference between the target and the ground (acting as the S/C).
Results of tests performed on the AESD-NU coupled to the AESD-PS ( with a dedicated EGSE ) are considered very promising as the absolute negative charging of a S/C in GEO (but also in LEO) may be effectively controlled through the use of a low electron accelerating e-gun and power emitting cathode.
The proposed instrument takes its advantages with respect to other competing active technologies mainly in view of the light-weight and low power consumption cathode advanced design; developed at TAS-I in the recent past, in the frame of neutraliser applications for Electric Propulsion systems. In addition, the Integration of the basic ACC instrument with an additional sensor, the SPD, would give to the system the maximum degree of flexibility and versatility.
The general architecture of the proposed ACCS instrument includes:
The “basic” ACCS configuration, or ACC, includes the Active Electron Source/generator (AESD-NU) and its relevant power supply electronics (AESD-PS).
The initial phase of the project has been focused on assessing the key aspects for charging prevention. Moving from the study, the ACCS configuration and requirement specifications have been generated along with the detailed design of AESD-NU / PS sub-units. The ACC has been developed and successfully submitted to performance and pre-qualification tests.
The on-ground validation of the instrument concept and its capability to control the charging potential of a S/C have been successfully achieved and completed, in the frame of a dedicated ESA ARTES 5 Contract No. 22176/08/NL/EM.
A complete stand-alone version of the ACCS instrument including the necessary controlling interface and monitoring system electronics, will be proposed in the frame of a call for interest on In-Orbit Validation of payloads hosted on-board Telecommunication satellites, focussed on the EDRS-C hosting opportunity.