Neutral-beam injection

Résumé

Neutral-beam injection (NBI) is one method used to heat plasma inside a fusion device consisting in a beam of high-energy neutral particles that can enter the magnetic confinement field. When these neutral particles are ionized by collision with the plasma particles, they are kept in the plasma by the confining magnetic field and can transfer most of their energy by further collisions with the plasma. By tangential injection in the torus, neutral beams also provide momentum to the plasma and current drive, one essential feature for long pulses of burning plasmas. Neutral-beam injection is a flexible and reliable technique, which has been the main heating system on a large variety of fusion devices. To date, all NBI systems were based on positive precursor ion beams. In the 1990s there has been impressive progress in negative ion sources and accelerators with the construction of multi-megawatt negative-ion-based NBI systems at LHD (H0, 180 keV) and JT-60U (D0, 500 keV). The NB

Source officielle

https://en.wikipedia.org/wiki/Neutral-beam_injection

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Cours associés (2)

This course completes the knowledge in plasma physics that students have acquired in the previous two courses, with a discussion of different applications, in the fields of magnetic confinement and controlled fusion, astrophysical and space plasmas, and societal and industrial applications.

The goal of the course is to provide the physics and technology basis for controlled fusion research, from the main elements of plasma physics to the reactor concepts.

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Recently, a novelmulti-turn injection technique was proposed. It is based on beam merging via resonance crossing. The various beamlets are successively injected and merged back by crossing a stable resonance generated by non-linear magnetic fields. Space charge is usually a crucial effect at injection in a circular machine and it could have an adverse impact on the phase space topology required for merging the various beamlets. Numerical simulations were performed to assess the stability of the merging process as a function of injected beam charge. The results are presented and discussed in this paper.

2010

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Negative Hydrogen Ions in a Helicon Plasma Source

Riccardo Agnello

There is nowadays a large interest to employ helicon plasmas as sources of negative ions for Neutral Beam Injectors (NBIs). Helicon plasma sources offer a number of advantages compared to current Inductive Couple Plasma (ICP) negative ions source, such as high energy efficiency, high plasma density production, low electron temperature and a considerable amount of H- and D- produced through volumetric processes, without caesium. Therefore, helicon plasma sources are promising candidates to be employed as a negative ion source for future NBIs, such as in DEMO, a next generation tokamak closer to commercial devices. However, many physics and technical aspects of helicon plasmas have to be deeply investigated, before their possible integration in future NBIs. The goal of this thesis is to advance the understanding of the physics of helicon-based negative ion sources in the helicon plasma source RAID (Resonant Antenna Ion Device) at the Swiss Plasma Center of Ecole Polytéchnique Fédérale de Lausanne, Switzerland. The investigations are carried out by employing a variety of plasma diagnostics, and the results are interpreted and compared to numerical models.

EPFL2020

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Negative ion density in the ion source SPIDER in Cs free conditions

Riccardo Agnello

The SPIDER experiment, operated at the Neutral Beam Test Facility of Consorzio RFX, Padua, hosts the prototype of the H-/D- ion source for the ITER neutral beam injectors. The maximization of the ion current extracted from the source and the minimization of the amount of co-extracted electrons are among the most relevant targets to accomplish. The Cavity Ring-Down Spectroscopy diagnostic measures the negative ion density in the source close to the plasma grid (the plasma-facing grid of the ion acceleration system), so to identify the source operational parameters that maximize the amount of negative ions which can be extracted. In this study SPIDER was operated in hydrogen and deuterium in Cs-free conditions, therefore negative ions were mostly produced by reactions in the plasma volume. This work shows how the magnetic filter field and the bias currents, present in SPIDER to limit the amount of co-extracted electrons, affect the density of negative ions available for extraction. The results indicate that the magnetic filter field in front of the acceleration system should be set between about 1.6 mT, condition that maximizes the density of available negative ions, and about 3.2 mT, condition that minimizes the ratio of electron current to ion current. The negative ion density also resulted to be maximized when the plasma grid and its surrounding bias plate was positively biased against the source body with a total current in the range 0-100 A. The paper shows also how much, in Cs-free conditions, the electric fields in the acceleration system can affect the density of negative ions in the source, close to the plasma grid apertures.

2022

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