Electrostatic Sensors for SPIDER Experiment: Design, Manufacture of Prototypes and First Tests

A system of electrostatic sensors capable of operating in RF negative ion H /D source has been designed for the SPIDER experiment [1], prototype source (100 keV, 50 A) of the ITER neutral beam injector [2]. This system allows the investigation of the homogeneity of plasma parameters, such as plasma density, electron temperature, plasma potential, in the extraction region of the ion source, where mostof the extracted negative ions are produced. In order to experimentally test the system a prototype was manufactured and tested in the BATMAN [3] experiment, where a plasma environment most similar to that of SPIDER is available. In plasmas generated by radio frequency discharge, RF fluctuations of plasma potential lead to distortion of I-V characteristic curve obtained from the probe and hence to incorrect values of the plasma density and temperature. Special attention was devoted to this aspect and a RF passive compensation circuit has been used to effectively reduce the RF effects, by means of a floating electrode exposed to the plasma coupled to a Langmuir probe through a shunting capacitor.The circuit has been designed according to the results obtained with software simulations in which the effects of real components impedance and their different configurations were reproduced. According to the particular constraints for the installation on BATMAN, a special holder has been designed and manufactured to house two prototypes Langmuir probes with the respective reference electrode and two thermocouples to measure the temperature of the probe support. The entire electrostatic probe system, from probes through RF compensation circuit to acquisition and signal conditioning device, has been tested. Different plasma conditions expected in the BATMAN experiment were explored in order to check the reliability of the sensor to provide different ion density at the probe location. Ion saturation current values obtained during these tests are in the range assumed for the design, so the plasma density estimate deduced from the collected data is consistent with the expected values. [1] P. Sonato et al., Fusion Eng. Des. 84, 269 (2009). [2] M. Spolaore, G. Serianni et al. J. Phys. D: Appl. Phys. 43 (2010) 124018. [3] U. Fantz et al., Plasma Phys. Control. Fusion 49 (2007) B563-B580.