M. Urbani

Overview of the design of the ITER heating neutral beam injectors

The heating neutral beam injectors (HNBs) of ITER are designed to deliver 16.7 MW of 1 MeV D0 or 0.87 MeV H0 to the ITER plasma for up to 3600 s. They will be the most powerful neutral beam (NB) injectors ever, delivering higher energy NBs to the plasma in a tokamak for longer than any previous systems have done. The design of the HNBs is based on the acceleration and neutralisation of negative ions as the efficiency of conversion of accelerated positive ions is so low at the required energy that a realistic design is not possible, whereas the neutralisation of H− and D− remains acceptable (≈56%). The design of a long pulse negative ion based injector is inherently more complicated than that of short pulse positive ion based injectors because: • negative ions are harder to create so that they can be extracted and accelerated from the ion source; • electrons can be co-extracted from the ion source along with the negative ions, and their acceleration must be minimised to maintain an acceptable overall accelerator efficiency; • negative ions are easily lost by collisions with the background gas in the accelerator; • electrons created in the extractor and accelerator can impinge on the extraction and acceleration grids, leading to high power loads on the grids; • positive ions are created in the accelerator by ionisation of the background gas by the accelerated negative ions and the positive ions are back-accelerated into the ion source creating a massive power load to the ion source; • electrons that are co-accelerated with the negative ions can exit the accelerator and deposit power on various downstream beamline components. The design of the ITER HNBs is further complicated because ITER is a nuclear installation which will generate very large fluxes of neutrons and gamma rays. Consequently all the injector components have to survive in that harsh environment. Additionally the beamline components and the NB cell, where the beams are housed, will be activated and all maintenance will have to be performed remotely. This paper describes the design of the HNB injectors, but not the associated power supplies, cooling system, cryogenic system etc, or the high voltage bushing which separates the vacuum of the beamline from the high pressure SF6 of the high voltage (1 MV) transmission line, through which the power, gas and cooling water are supplied to the beam source. Also the magnetic field reduction system is not described.

Heating neutral beams for ITER: Present status

The heating neutral beam (HNB) systems at ITER are designed to inject a total of 33 MW of either 1 MeV D0 or 870 keV H0 beams into the ITER plasma using two injectors with a possible addition of a third injector later to increase the injected power to ~50 MW. The injectors become radioactive due to the neutron flux from ITER and, in order to avoid the resulting complex remote maintenance, the design, choice of materials and the manufacturing process of each component of the injector is, wherever possible, such that they survive the life time of ITER. To ensure a smooth operational phase of neutral beams at ITER a neutral beam test facility (NBTF) is under construction at Consorzio RFX, Padova, (hereinafter referred to as RFX), which consists of 2 test beds, the 100 kV “SPIDER”, and a 1 MV “MITICA” facilities, which will be used to optimize the source operation for H and D beams. MITICA is essentially a full scale ITER prototype injector for the ITER beam parameters. The manufacturing and operation of the facility will allow validation of the operational space of the injectors and provide valuable information about the manufacturing processes applicable to HNB components. Operation of the two facilities is expected to begin in 2016 and 2019 respectively. Currently experiments on the ELISE facility with a half ITER sized RF beam source are underway. ITER relevant parameters for the H beams have almost been achieved. Efforts are underway to optimise the same with D beams. The experimental database from ELISE will be an important input for establishing the ITER relevant parameter space on the SPIDER source. This paper discusses the present status of the design and development of the injectors for ITER and the progress on the test facilities.

Heating Neutral Beams for ITER: Present Status

The heating neutral beam (HNB) systems at ITER are designed to inject a total of 33 MW of either 1 MeV D0 or 870 keV H0 beams into the ITER plasma using two injectors with a possible addition of a third injector later to increase the injected power to ~50 MW. The injectors operate in a radioactive environment and should survive the life time of ITER, placing thereby stringent requirements on material and manufacturing choices. To ensure a smooth operational phase of neutral beams at ITER, a neutral beam test facility is under construction at Consorzio RFX, Padova, (hereinafter referred to as RFX), and consists of two test beds. The 100-kV SPIDER test bed will be used to optimize the source operation for H and D beams. The 1-MV MITICA test bed is essentially a full scale ITER prototype injector. The manufacturing and operational experiences at MITICA will not only establish the manufacturing processes of ITER HNB components but will also allow validation of the operational space of the injectors for ITER HNB. Operation of the two facilities is expected to begin in 2016 and 2019, respectively. Currently, the experiments on the ELISE facility, IPP Garching, with a half ITER sized RF beam source are underway. The ITER relevant parameters for the H beams have been achieved. Efforts are underway to optimize the same with D beams. The experimental database from ELISE will be an important input for establishing the SPIDER operation. This paper discusses the present status of the design and development of the injectors for ITER and the progress on the test facilities.

ITER neutral beam Vacuum Vessel design

The Vacuum vessel of the ITER Heating Neutral Beam injector is a key component of the neutral beam system. It supports the injector components and is also an extension of the ITER Vacuum Vessel. This paper presents the status of the design of the vessel. In the first part of this paper we present the main design requirements. Besides the pressure and thermal loads, the various interfaces with other components are important constraints for the design. Some have specific requirements concerning displacement, or require specific features for their maintenance. Indeed all maintenance involving opening of the vessel needs to be performed using remote handling systems. The second part aims at presenting the present design of the NB vessel. Various concepts were studied before coming to the present design. Structures were optimised to satisfy the chosen design code (RCC-MR) under the various loading scenarios applied to the vessel, both for normal and accidental cases. In addition, the vessel incorporates two sets of long lip seals which tolerate limited displacements. Limiting the displacement is a real challenge considering the size of the flange and the forces involved. An innovative solution has been developed to improve the mechanical connection between the lids and the vessel walls in order to fulfil the requirements. The vessel displacement is also important for the alignment of the Beam Line Components with respect to the neutral beam and for the cryopumps interfaces. The geometry of the vessel was optimised to reduce the electric field stresses and to increase the voltage holding of the Beam Source, which is at 1 MV potential.