The scaling up of conventional distributed electron cyclotron resonance plasmas presents limitations in terms of plasma density, limited to the critical density, and of uniformity, due to the difficulty of achieving constant amplitude standing wave patterns along linear microwave applicators in the metre range. The alternative solution presented in this study is the extension of the concept of distribution from one-to two-dimensional networks of elementary plasma sources sustained at electron cyclotron resonance (ECR). With the so-called multi-dipolar plasmas, large size and uniform low-pressure plasmas are produced from a two-dimensional network of elementary, independent plasma sources sustained at ECR. Each elementary plasma source consists of a permanent magnet on which microwaves are applied via an independent coaxial line. The plasma is produced by the electrons accelerated at ECR and trapped in the dipolar magnetic field of the magnet acting as a tri-dimensional magnetron structure. Large-size uniform plasmas can be obtained by assembling as many such elementary plasma sources as necessary, without any physical or technical limitations. Examples of two-dimensional networks are described and the performances in terms of density and uniformity of such plasma sources are presented. The interesting characteristics and advantages of multi-dipolar plasmas over distributed ECR plasmas are listed and the perspectives for plasma processing emphasized.
Since the signature of the ITER treaty in 2006, a new research programme targeting the emergence of a new generation of Neutral Beam (NB) system for the future fusion reactor (DEMO Tokamak) has been underway between several laboratories in Europe. The specifications required to operate a NB system on DEMO are very demanding: the system has to provide plasma heating, current drive and plasma control at a very high level of power (up to 150 MW) and energy (1 or 2 MeV), including high performances in term of wall-plug efficiency (η > 60%), high availability and reliability. To this aim, a novel NB concept based on the photodetachment of the energetic negative ion beam is under study. The keystone of this new concept is the achievement of a photoneutralizer where a high power photon flux (~3 MW) generated within a Fabry Perot cavity will overlap, cross and partially photodetach the intense negative ion beam accelerated at high energy (1 or 2 MeV). The aspect ratio of the beam-line (source, accelerator, etc.) is specifically designed to maximize the overlap of the photon beam with the ion beam. It is shown that such a photoneutralized based NB system would have the capability to provide several tens of MW of D 0 per beam line with a wall-plug efficiency higher than 60%. A feasibility study of the concept has been launched between different laboratories to address the different physics aspects, i.e., negative ion source, plasma modelling, ion accelerator simulation, photoneutralization and high voltage holding under vacuum. The paper describes the present status of the project and the main achievements of the developments in laboratories.
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