Overview of the design of the ITER heating neutral beam injectors

Abstract

The heating neutral beam injectors (HNBs) of ITER are designed to deliver 16.7 MW of 1 MeV D 0 or 0.87 MeV H 0 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 SF 6 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: negative ion sources to tune fusion plasmas

“…HNBs have been described in detail in another contribution to this issue [15]. Table 2 summarises the beam parameter requirements for the HNB and DNB systems at ITER.…”

“…The 1 MV D or the 870 keV H beams are produced by eight driver RF based negative beam sources which use a seven grid five stage accelerator system whereas the 100 keV H beams for the DNB use a source similar to the HNB but with a three grid system with one acceleration stage. The details of the source design and the accelerators can be found in [15,17,19,20]. Depending on the beam aiming requirements which are dependent on the plasma parameters for shine-through reasons and to mitigate Alfven eigenmodes (TAEs) [9] the HNB source can be tilted in the vertical direction with respect to the neutral beam axis within ±X mrad with an angular accuracy of ±1 mrad.…”

“…This paper is divided into two major sections. The first section is dedicated to a brief description of the important parameters required for neutral beams at ITER, and are covered in more detail in the paper by Hemsworth et al [15] on this issue. A comparison of heating and diagnostic beam parameters is given in this section only to provide the reader with an idea of the varied beam requirements for the two systems.…”

The PRIMA Test Facility: SPIDER and MITICA test-beds for ITER neutral beam injectors

“…Since these operating conditions have never been reached jointly before, it was recognized that a dedicated Neutral Beam Test Facility (NBTF) is needed to carry out an international R&D programme aimed at constructing, testing and optimizing the full-scale prototype of the HNBs and at assisting ITER during its operation [3,4].…”

Acquisition, data retrieval, interlock and control systems for the negative ion source NIO1

“…Negative ion sources are a key component of the neutral beam injectors (NBIs) to be installed in the ITER experiment [1] and in successive fusion devices [2] to heat the plasma: the challenging specifications of such NBIs (40A of D -ion current and 1MV acceleration voltage required) call for careful engineering design and for detailed numerical modelling. To support these activities, providing both a benchmark for codes and a facility for quick and simple manufacturing tests, a versatile negative ion source, named NIO1 (Negative Ion Optimization), is in operation at Consorzio RFX in Padua (Italy).…”