A. Simonin

R&D around a photoneutralizer-based NBI system (Siphore) in view of a DEMO Tokamak steady state fusion reactor

ince 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 D0 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.

Negative ion source development for a photoneutralization based neutral beam system for future fusion reactors

In parallel to the developments dedicated to the ITER neutral beam (NB) system, CEA-IRFM with laboratories in France and Switzerland are studying the feasibility of a new generation ofNBsystem able to provide heating and current drive for the future DEMOnstration fusion reactor. For the steadystate scenario, theNBsystem will have to provide a highNBpower level with a high wall-plug efficiency (η∼60%). Neutralization of the energetic negative ions by photodetachment (so called photoneutralization), if feasible, appears to be the ideal solution to meet these performances, in the sense that it could offer a high beam neutralization rate (>80%) and a wall-plug efficiency higher than 60%. The main challenge of this new injector concept is the achievement of a very high power photon flux which could be provided by 3MWFabry–Perot optical cavities implanted along the 1 MeVD− beam in the neutralizer stage. The beamline topology is tall and narrow to provide laminar ion beam sheets, which will be entirely illuminated by the intra-cavity photon beams propagating along the vertical axis. The paper describes the presentR&D(experiments and modelling) addressing the development of a new ion source concept (Cybele source) which is based on a magnetized plasma column. Parametric studies of the source are performed using Langmuir probes in order to characterize and compare the plasma parameters in the source column with different plasma generators, such as filamented cathodes, radio-frequency driver and a helicon antenna specifically developed at SPC-EPFL satisfying the requirements for the Cybele (axial magnetic field of 10 mT, source operating pressure: 0.3 Pa in hydrogen or deuterium). The paper compares the performances of the three plasma generators. It is shown that the helicon plasma generator is a very promising candidate to provide an intense and uniform negative ion beam sheet.

Conceptual design of the beam source for the DEMO Neutral Beam Injectors

DEMO (DEMOnstration Fusion Power Plant) is a proposed nuclear fusion power plant that is intended to follow the ITER experimental reactor. The main goal of DEMO will be to demonstrate the possibility to produce electric energy from the fusion reaction. The injection of high energy neutral beams is one of the main tools to heat the plasma up to fusion conditions. A conceptual design of the Neutral Beam Injector (NBI) for the DEMO fusion reactor, is currently being developed by Consorzio RFX in collaboration with other European research institutes. High efficiency and low recirculating power, which are fundamental requirements for the success of DEMO, have been taken into special consideration for the DEMO NBI. Moreover, particular attention has been paid to the issues related to reliability, availability, maintainability and inspectability. A conceptual design of the beam source for the DEMO NBI is here presented featuring 20 sub-sources (two adjacent columns of 10 sub-sources each), following a modular design concept, with each sub-source featuring its radio frequency driver, capable of increasing the reliability and availability of the DEMO NBI. Copper grids with increasing size of the apertures have been adopted in the accelerator, with three main layouts of the apertures (circular apertures, slotted apertures and frame-like apertures for each sub-source). This design, permitting to significantly decrease the stripping losses in the accelerator without spoiling the beam optics, has been investigated with a self-consistent model able to study at the same time the magnetic field, the electrostatic field and the trajectory of the negative ions. Moreover, the status on the R&D carried out in Europe on the ion sources is presented.

SIPHORE: Conceptual Study of a High Efficiency Neutral Beam Injector Based on Photo-detachment for Future Fusion Reactors

An innovative high efficiency neutral beam injector concept for future fusion reactors is under investigation (simulation and RD the main feature of SIPHORE being the relevance for the future Fusion reactors (DEMO), where high injector efficiency (up to 70–80%), technological simplicity and cost reduction are key issues to be addressed.The paper presents the on‐going developments and simulations around this project, such as, a new concept ...

European negative ion based neutral beam developments

Abstract The European negative ion based neutral beam development programme consists presently of experiments on high energy electrostatic accelerators and high current D − sources, and of neutral beam physics and system design studies. At CEA-Cadarache the source test bed MANTIS started operation last June, and the 1 MV, 0.1 A, D − acceleration project SINGAP will be operational by the end of 1994. Additionally, negative ion source physics-oriented experiments will be conducted at Dublin City University. A Pagoda source, developed for the SINGAP experiment, recently produced more than 1 A D − beams and j (D − ) of 10 mA cm −2 , with Cs seeding, at 0.6 Pa operating pressure. 1.3 A H − and 0.5 A D − beams have also been obtained without the use of Cs during the first experiments on MANTIS. Studies on neutral beam heating (NBH) for a next step machine have been conducted during 1993, mainly at JET and CEA-Cadarache. NBH physics studies based on the RLW model indicate that ignition on ITER-EDA should be reached with about 40 MW of 0.4–1 MeV D 0 beams. The outlines of an NBH system for ITER-EDA, based on SINGAP, have been studied.

Negative ion production with the PAGODA source

Abstract A 1 MV, 0.1 A, 10 mA cm−2 current density, multi-second D− ion beam acceleration project called SINGAP is scheduled to start at the end of 1994 at Cadarache. This experiment will be equipped with a PAGODA ion source for which experimental results are presented here. The negative ion beam was accelerated with a three-electrode system to up to 65 keV and measured calorimetrically on a target located 3 m from the ion source. With the full extraction area (240 cm2) and with caesium seeding in the source, a total D− transmitted current of 1 A was obtained. By masking some parts of the plasma grid consisting of five segments, various ion beam extraction areas were utilized, and higher current densities were obtained from smaller extraction surfaces. With 1 5 th of the extraction area (central segment), a transmitted D− mean current density of 10 mA cm−2 was measured at a filling pressure of 0.65 Pa with caesium seeding in the ion source, which meets the SINGAP requirements. Spectroscopic and caesium deposition measurements were also performed. The energetic electron leaks were measured and compared with the predictions of a Monte Carlo code.

Ion source development for a photoneutralization based NBI system for fusion reactors

The next step after ITER is to demonstrate the viability and generation of electricity by a future fusion reactor (DEMO). The specifications required to operate an NBI system on DEMO are very demanding. The system has to provide a very high level of power and energy, ~100MW of D° beam at 1MeV, including high wall-plug efficiency (η > 60%). For this purpose, a new injector concept, called Siphore, is under investigation between CEA and French universities. Siphore is based on the stripping of the accelerated negative ions by photo-detachment provided by several Fabry-Perot cavities (3.5MW of light power per cavity) implemented along the D− beam. The beamline is designed to be tall and narrow in order that the photon flux overlaps the entire negative ion beam. The paper will describe the present R&D at CEA which addresses the development of an ion source and pre-accelerator prototypes for Siphore, the main goal being to produce an intense negative ion beam sheet. The negative ion source Cybele is based on a...

Experimental study of the characteristics of D− and H− beams produced by a multi‐hole multi‐ampere accelerator at energies up to 105 keV

The acceleration of D− and H− beams has been studied at energies of up to 105 keV, with currents of the order of 2 A, in multi‐second pulses. The optimum extraction voltage has been studied as a function of the beam energy and current. The perveance of the negative ion accelerator has been measured over a wide range of operating conditions with H− and D− beams. The experimental results have been compared to beam optics calculations performed with the SLACAD negative ion code developed at Cadarache: a good agreement has been found. Strong perturbations of negative ion beamlet trajectories have been evidenced in the case of large gap, large aspect ratio accelerators. A comparison between merged beamlets and multi‐hole accelerators has been performed. The observed linear increase of the stray accelerated electron current with the extraction voltage has been explained by simulation with a Monte‐Carlo code developed at Cadarache.

Results of the 1 MV SINGAP experiment

Injection of intense neutral beams based on the neutralisation of negative deuterium ions up to energies of 1 MeV is an important option for plasma heating and non-inductive current drive in future thermonuclear fusion machines. The objective of the SINGAP experiment is to demonstrate the acceleration of 100 mA of D− to 1 MeV in an electrostatic accelerator concept which is substantially simpler than the multi-aperture, multi-grid accelerator system foreseen for the ITER neutral beam injectors. Voltage holding without beam has been demonstrated at above 1 MV after only 35 min of integrated voltage on-time. H− beams have been produced up to 860 kV, 40 mA in 1 s pulses and without caesium seeding of the ion source. With caesium admixture, D− beams of 105 mA have been accelerated to 630 keV for 1 s. These results were obtained at energies lower than 700 keV because of the bushing deterioration. The measurements of the beam profiles are in good agreement with the results obtained from 3D trajectory calculations.

Negative ion based neutral beam development activities at Cadarache

The work in the field of negative ions at Cadarache aims at a coherent D− beam development in view of the needs of future magnetic fusion experiments. An overview of recent activities is first given: (i) during the recent collaboration with JAERI, 2.2 A, 100 keV, D− long pulse beams have been produced and extensively studied at Cadarache; (ii) the production of negative ions from the Pagoda source is now under study; (iii) two important ITER NB related studies have been conducted, the conceptual study of a 1 MV, 15 A power supply and the design of a 1 MV, 4 A D− beamline and test bed. The future experimental studies, which constitute the main elements of the European strategy in neutral beam development for magnetic fusion applications are the following: (i) the application for financial support in EC has been made for a 1 MeV 0.1 A D− beam acceleration experiment, which is projected to be carried out in 1994; (ii) a new test bed MANTIS for multi‐ampere D− source development will also be established, cons...