Abstract
Inductive coupling of radiofrequency power to plasma is a complicate process, since it depends from the density of plasma itself, because ionization is a chain reaction process, and, at low ...density a capacitive coupling may mix with inductive coupling (with no Faraday screen).
Plasma temperature
T
e
, density
n
e
and vector potential are closely coupled, giving nonlinear and singular systems of Partial Differential equations, which require slow iterative solutions, motivating the consideration of a 2D model, also as rapid design and first approximation tool. Plasma conductivity and heating depend on collision rate, which includes also the so-called stochastic collisions (mainly electron collisions with walls), proportionally more important at low gas density
n
g
. Conductivity is also affected by static and radiofrequency magnetic fields; results for skin depth and stochastic collision estimates are reported. The transport of
T
e
and
n
e
inside source can be controlled by a magnetic filter
B
f
. Considering a 5 cm radius H
-
ion source as example, solution of
n
e
and
T
e
are reported as a function of filter strength
B
fa
, applied rf power and wall status; solver convergence methods and
key plasma observables are briefly discussed. Due to small dimension, a filter strength
B
fa
in the order of 8 mT is needed to achieve electron temperature lower than 2 eV (for negative ion production) at extraction.
The ITER Neutral Beam Test Facility (NBTF), called PRIMA (Padova Research on ITER Megavolt Accelerator), is hosted in Padova, Italy and includes two experiments: MITICA, the full-scale prototype of ...the ITER heating neutral beam injector, and SPIDER, the full-size radio frequency negative-ions source. The NBTF realization and the exploitation of SPIDER and MITICA have been recognized as necessary to make the future operation of the ITER heating neutral beam injectors efficient and reliable, fundamental to the achievement of thermonuclear-relevant plasma parameters in ITER. This paper reports on design and R&D carried out to construct PRIMA, SPIDER and MITICA, and highlights the huge progress made in just a few years, from the signature of the agreement for the NBTF realization in 2011, up to now-when the buildings and relevant infrastructures have been completed, SPIDER is entering the integrated commissioning phase and the procurements of several MITICA components are at a well advanced stage.
The ITER Neutral Beam Test Facility (PRIMA) is presently under construction at Consorzio RFX (Padova, Italy). PRIMA includes two experimental devices: an ITER-size ion source with low voltage ...extraction, called SPIDER, and the full prototype of the whole ITER Heating Neutral Beams (HNBs), called MITICA. The purpose of MITICA is to demonstrate that all operational parameters of the ITER HNB accelerator can be experimentally achieved, thus establishing a large step forward in the performances of neutral beam injectors in comparison with the present experimental devices. The design of the MITICA extractor and accelerator grids, here described in detail, was developed using an integrated approach, taking into consideration at the same time all the relevant physics and engineering aspects. Particular care was taken also to support and validate the design on the basis of the expertise and experimental data made available by the collaborating neutral beam laboratories of CEA, IPP, CCFE, NIFS and JAEA. Considering the operational requirements and the other physics constraints of the ITER HNBs, the whole design has been thoroughly optimized and improved. Furthermore, specific innovative concepts have been introduced.
Deflection of negative ion beamlets due to the magnets embedded in the first extraction electrode for the purpose of dumping the co-extracted electrons is a serious issue for multiaperture ion ...accelerators of neutral beam injectors. Several kinds of magnet arrays which offer the possibility of cancelling ion deflection, employing crossed rows of magnets or even more compact parallel row arrangements, are discussed. A general equation for beamlet deflection is presented here, and the interference of the magnetic deflection and the electrostatic lens steering is carefully calculated; this equation may also include beamlet-beamlet interactions and image charge effects. Analytical expressions are given for the field and the line integrals for the magnet arrays, and these are simplified for beam optics calculations, but still retain an excellent agreement with numerical values. Optimization formulas for the filling fraction xy of the magnets are given, for cancellation of deflection both after the first electrode or after the second accelerating electrode. The latter case is of direct interest for the design of small accelerators (e.g., NIO1), for which compact solutions are proposed, while the former case may approximate well the design of a large accelerator such as MITICA, with a predicted xy = 0.1015 against a numerical optimized value of 0.0975 ± 0.005 in normal conditions. The detailed comparison between simulation results and theory shows that thin lens models are suitable approximations for calculating beam steering. Stability of optimal xy prediction with respect to the first accelerating gap length is shown, and the variation of xy with the voltage is discussed.
Abstract
Energy recovery of residual ions may be needed to increase the energy efficiency of Neutral Beam (NB) injectors for fusion plants as DEMO while a deflection-based system has been proposed ...until now to dump residual ions. As an alternative, a compact beam energy recovery system, based on space charge effects due to the residual ion deceleration into 2 Farady Cups (FC) with holes for D
0
passage, can replace the Electrostatic Residual Ion Dump (ERID) designed for ITER to stop the residual D
-
and D
+
before the NB injection in the tokamak plasma. All parameter tunings and preliminary simulations are here described, also providing the suppression of back streaming to the ion source. Ion energy spread and rectangular geometry are considered. Collection of ions at low energy (a few percent of the full neutral beam energy Ek
i
) instead of Ek
i
as in ERID gives advantages that will be mentioned.
Neutral beam injection is one of the most important methods of plasma heating in thermonuclear fusion experiments, allowing the attainment of fusion conditions as well as driving the plasma current. ...Neutral beams are generally produced by electrostatically accelerating ions, which are neutralised before injection into the magnetised plasma. At the particle energy required for the most advanced thermonuclear devices and particularly for ITER, neutralisation of positive ions is very inefficient so that negative ions are used. The present paper is devoted to the description of the phenomena occurring when a high-power multi-ampere negative ion beam travels from the beam source towards the plasma. Simulation of the trajectory of the beam and of its features requires various numerical codes, which must take into account all relevant phenomena. The leitmotiv is represented by the interaction of the beam with the background gas. The main outcome is the partial neutralisation of the beam particles, but ionisation of the background gas also occurs, with several physical and technological consequences. Diagnostic methods capable of investigating the beam properties and of assessing the relevance of the various phenomena will be discussed. Examples will be given regarding the measurements collected in the small flexible NIO1 source and regarding the expected results of the prototype of the neutral beam injectors for ITER. The tight connection between measurements and simulations in view of the operation of the beam is highlighted.
This topical review gathers the last updates concerning caesium-free negative ion sources presented during the
63
rd
Course of the International school of Quantum Electronics of the Ettore Majorana ...Foundation and European collaborative works related to these lectures. Hence, beyond the frame of this course this topical review addresses both theoretical and experimental work performed during these last few years and complexities represented by the conception of a negative ion source ranging from the creation of negative ions to their neutralization.
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