RF antennas as plasma monitors

Invited tal

Electromagnetic, complex image model of a large area RF resonant antenna as inductive plasma source

A large area antenna generates a plasma by both inductive and capacitive coupling; it is an electromagnetically coupled plasma source. In this work, experiments on a large area planar RF antenna source are interpreted in terms of a multi-conductor transmission line coupled to the plasma. This electromagnetic treatment includes mutual inductive coupling using the complex image method, and capacitive matrix coupling between all elements of the resonant network and the plasma. The model reproduces antenna input impedance measurements, with and without plasma, on a 1.2x1.2 m2 antenna used for large area plasma processing. Analytic expressions are given, and results are obtained by computation of the matrix solution. This method could be used to design planar inductive sources in general, by applying the termination impedances appropriate to each antenna type

Helicon wave-generated plasmas for negative ion beams for fusion

In the next generation of fusion reactors, such as DEMO, neutral beam injectors (NBIs) of high energy (0.8-1 MeV) deuterium atoms with high wall-plug efficiency (>50%) will be required to reach burning plasma conditions and to provide a significant amount of current drive. The present NBI system for DEMO assumes that 50 MW is delivered to the plasma by 3 NBIs. In the Siphore NBI concept, negative deuterium ions are extracted from a long, thin ion source 3 m high and 15 cm wide, accelerated and subsequently photo-neutralized. This requires the development of a new generation of negative ion sources. At the Swiss Plasma Center, a novel radio frequency helicon plasma source, based on a resonant network antenna source delivering up to 10 kW at 13.56 MHz, has been developed and is presently under study on the Resonant Antenna Ion Device (RAID). RAID is a linear device (1.9 m total length, 0.4 m diameter) and is equipped with an extensive set of diagnostics for full plasma characterization. In this work, the principles of operation of resonant antennas as helicon sources are introduced. We present absolute spectroscopy, Langmuir probe, and interferometry measurements on helicon plasmas. We characterize the performance of the source in terms of hydrogen/deuterium dissociation and negative ion production as a function of the input power. Furthermore, first results with the helicon birdcage antenna installed on the Cybele negative ion source at CEA-IRFM are presented, as a first step towards the validation of the Siphore concept

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

International audienceAs 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 D 2 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 H 2 plasmas (0.3 Pa), preliminary results show a helicon wave right-handed polarized with a wavelength of approximately 240 mm

Magnetic field configurational study on A Helicon-Based Plasma Source for Future Neutral Beam Systems

International audienceA helicon-based plasma source is under development and exploration at CEA-IRFM to produce an efficient and dense magnetized plasma column. The final objective of this development is the extraction and acceleration of a blade-like negative ion beam in view of future neutral beam injector systems for fusion reactors. The extraction of a negative ion beam from a magnetized plasma column requires a specific topology of the magnetic confinement, which significantly impacts the plasma parameters and wave propagation along the column. The magnetic confinement under investigation is based on water-cooled internal coils implemented under vacuum along the source (column) axis; these coils are supplied with a high DC current (~1000 A), providing the axial magnetic field (B//~10 mT). Different diagnostics developed in academic laboratories, such as 3D B-dot probes, optical emission spectroscopy and Langmuir probes, have been used for plasma characterization. The paper reports the experimental results obtained under different operating conditions of this particular magnetic confinement