First B-dot measurements in the RAID device, an alternative negative ion source for DEMO neutral beams

As a new concept of Neutral Beam Injectors (NBI) for DEMO-like reactors, SIPHORE (IRFM, CEA, in France) expects to extract negative deuterium ions and photo-neutralize the accelerated D-. The Swiss Plasma Center (SPC) of EPFL is involved in this project by developing an innovative helicon source, which could provide the adequate D2 negative ion blade-shaped plasma, in terms of density and homogeneity along the axial direction. In the Resonant Antenna Ion Device (RAID), the test bed, a helicon wave is sustained by a resonant antenna plasma source at 13.56 MHz (input power ≤ 10 kW), connected to a cylindrical vacuum chamber (1.5 m long, 0.4 m diameter) and is surrounded by 6 Helmholtz coils, providing a DC magnetic field up to 800 G on axis. To characterize the helicon wave propagation, RAID has been recently equipped with a three-axis magnetic probe (B-dot). The paper describes the RAID experiment and its helicon source, including a 3D characterization of density and temperature, together with the B-dot design and calibration. It presents measurements of helicon wave propagation; in typical H2 plasmas (0.3 Pa), preliminary results show a helicon wave right-handed polarized with a wavelength of approximately 240 mm.