Two Heating Neutral Beam Injectors (H-NBI) are planned to be installed in ITER with a total delivered heating power of 33 MW. The main parameters are: 870 kV acceleration voltage with 46 A beam current for Hydrogen beam, and 1 MV voltage with 40 A current for Deuterium beam. The required pulse duration is 3600 s. The combination of these design requirements constitutes a large leap with respect to the performances of the Heating NBI systems presently in operation. For this reason the construction of a specific test facility (PRIMA) has been launched in order to provide the design and operational experience necessary for the successful implementation of the ITER H-NBI system. PRIMA will include a full-size Negative Ion Source with 100 keV extractor and accelerator (SPIDER) and a complete prototype of the ITER 1 MeV Injector accelerator and neutralizer (MITICA). The voltage holding in the 1 MV ITER Neutral Beam Accelerator is recognized to be one of the most critical issues for long pulse (3600s) beam operation, due to the complex electrostatic structure formed by differently shaped electrodes polarized at different potentials. Furthermore, the system works in the p.d range at the left of the Paschen curve where the classical Townsend breakdown criterion is no longer valid. The voltage holding is governed by the mechanism of the long gap insulation in high vacuum, not yet well understood and consolidated. This paper is aimed to describe the optimization of the voltage holding capability for MITICA electrostatic accelerator. As a first step an accurate 3D solution is obtained by a pair of complementary formulations which provide rigorous upper and lower bounds of the numerical solution (fundamental especially in problems that are big enough to prevent error convergence study. The results of this analysis will constitute the input for the Probabilistic Model which is adopted to predict the breakdown probability by means of 2D analyses of the multi electrode – multi voltage system
Voltage holding optimization of the MITICA electrostatic accelerator
BETTINI, PAOLO;
2012
Abstract
Two Heating Neutral Beam Injectors (H-NBI) are planned to be installed in ITER with a total delivered heating power of 33 MW. The main parameters are: 870 kV acceleration voltage with 46 A beam current for Hydrogen beam, and 1 MV voltage with 40 A current for Deuterium beam. The required pulse duration is 3600 s. The combination of these design requirements constitutes a large leap with respect to the performances of the Heating NBI systems presently in operation. For this reason the construction of a specific test facility (PRIMA) has been launched in order to provide the design and operational experience necessary for the successful implementation of the ITER H-NBI system. PRIMA will include a full-size Negative Ion Source with 100 keV extractor and accelerator (SPIDER) and a complete prototype of the ITER 1 MeV Injector accelerator and neutralizer (MITICA). The voltage holding in the 1 MV ITER Neutral Beam Accelerator is recognized to be one of the most critical issues for long pulse (3600s) beam operation, due to the complex electrostatic structure formed by differently shaped electrodes polarized at different potentials. Furthermore, the system works in the p.d range at the left of the Paschen curve where the classical Townsend breakdown criterion is no longer valid. The voltage holding is governed by the mechanism of the long gap insulation in high vacuum, not yet well understood and consolidated. This paper is aimed to describe the optimization of the voltage holding capability for MITICA electrostatic accelerator. As a first step an accurate 3D solution is obtained by a pair of complementary formulations which provide rigorous upper and lower bounds of the numerical solution (fundamental especially in problems that are big enough to prevent error convergence study. The results of this analysis will constitute the input for the Probabilistic Model which is adopted to predict the breakdown probability by means of 2D analyses of the multi electrode – multi voltage systemPubblicazioni consigliate
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