Publication
Neutral Beam Injectors (NBI) for post-DEMO-like reactors must achieve a neutralization efficiency (fraction neutrals/ions) above 70%. To reach this, NBIs based on negative ions are necessary and an innovative concept of this type of injector is based on photo-neutralization. In this context, RAID (Resonant Antenna Ion Device), a linear helicon system located at the Swiss Plasma Center (SPC) in the Ecole Polytechnique Fe ' de ' rale de Lausanne (EPFL), aims to study the production of negative ions (D- and H-) in the plasma volume.RAID is routinely operated with a single resonant birdcage antenna, located on one end of the vacuum chamber (1.4 m long and 40 cm diameter) at 0.3 Pa of pressure in D2 and H2 discharges. This antenna, excited at 13.56 MHz with a RF power up to 10 kW, generates helicon driven plasma beams extending all along the chamber when a DC axial magnetic field (standard value 200 G) is applied. Typical electron densities at the center of the column are 1-2.5 x 1018 m - 3 and the electron temperature of 4-6 eV. In order to improve the symmetry as well as the homogeneity of plasma parameters along the axis, a second source was installed at the opposite end of the vacuum vessel and a complementary coil for the DC magnetic field was added. Axial profiles and 2D mappings of plasma density and temperature have been measured with a Double Langmuir Probe (DLP). Interestingly, for most phases, the relative RF phase of the two antennas influences the plasma causing very low frequency (few Hz) beating. In the future, other plasma diagnostics will be developed such as Thomson scattering, LIF (Laser Induced Fluorescence) and B-dot measurements to characterize in detail the wave propagation in RAID with two antennas and to understand better some phenomena observed during the test campaign described in this article.