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ITER's most powerful external heating system—neutral beam injection—will be tested in advance of operation at the Neutral Beam Test Facility (NBTF) in Padua, Italy.
Although neutral beam injection is routinely used for plasma heating in fusion devices, the size of ITER imposes enhanced requirements: particle beams have to be much thicker, for example, and individual particles have to be much faster in order to travel far into the core of the plasma.
The Neutral Beam Test Facility will offer scientists the possibility to investigate challenging physics and technology issues and validate concepts before the neutral beam system is installed on ITER.
Challenging negative ion technology
Neutral beam injection relies on beams of high-speed, high-energy atoms that penetrate into the plasma, initiating collisions and transferring their energy. ITER will be equipped with two heating neutral beam injectors (with a provision of a third injector) and a neutral beam line for diagnostic purposes. The injectors will be capable of operating for up to 3600 seconds, each delivering deuterium beams of 16.5 MW with particle energies of 1 MeV. Their combined heating power—33 MW—represents over half of the external heating required by ITER.
Neutral beams are formed through the acceleration and subsequent neutralization of ions—typically positive ions—but at ITER power levels and speeds the positive ions become difficult to neutralize. The negative ion technology chosen for ITER neutral beam injection offers much higher neutralization efficiency, however the negative ions are significantly harder to handle during production and acceleration.
Creating reliable high energy neutral beams at ITER parameters, from a negative ion source, will require a large jump in technology. Some of the critical challenges include:
Extracting a 40A negative ion beam from a large-size radiofrequency source;
Accelerating the negative ion beam to 1 MeV with accurate beam optics;
Developing high-voltage, gas-insulated transmission lines to connect the power supply to the beam source;
Holding high voltage (1 MV) over pulses of 3600 seconds;
Developing diagnostics to monitor the source.
R&D programs in Japan and Europe have been instrumental over the last decade in helping to close the gap between current technology and what is needed for the ITER neutral beam program. The ITER source is based on a radio-frequency-driven negative ion source that has been evolving over several generations of prototypes at the Max Planck Institute for Plasma Physics (IPP) in Garching, Germany.
Since 2009 IPP's negative source ELISE—half the size of what is projected for ITER—has been a valuable source of experimental data as technological solutions have been tried, tested and advanced. (The ELISE R&D program is financed by the European Domestic Agency as a voluntary contribution to the ITER neutral beam program.)
The next step is a facility that can achieve the challenging requirements for the ITER neutral beam system in terms of power, energy, and pulse length simultaneously. The Neutral Beam Test Facility—launched in 2012 after the approval of the ITER Council—has been strongly endorsed by the ITER Organization and the ITER parties involved in the development of the neutral beam injectors, namely Europe and Japan for the in-kind contribution of the heating neutral beam injectors, and India for the diagnostic neutral beam.
A centre of excellence in Italy
In a 17,500 m² building on the premises of Consorzio RFX, in Padua, two neutral beam test stands are under construction:
SPIDER (Source for the Production of Ions of Deuterium Extracted from a Radio frequency plasma) is an ITER-scale negative ion source designed to achieve all ion source requirements;
MITICA (Megavolt ITER Injector and Concept Advancement) is a full-size prototype of the 1 MV heating neutral beam injectors, capable of full acceleration voltage and power.
The components designed for the Neutral Beam Test Facility are the same components that ITER will be using on its heating neutral beams in ITER, which will allow the neutral beam teams to acquire valuable information about neutral beam manufacturability and operation.
Europe is contributing all SPIDER and MITICA mechanical components (including beam sources and accelerators); the SPIDER power supply (excluding the acceleration grid and power supply provided by India); the MITICA low-voltage power supply and high-voltage deck; plant auxiliaries (cooling, vacuum and gas injection systems, insulating SF6 gas handling and storage plant, cryogenic plant); instrumentation and control; and finally diagnostics;
India is providing the calorimeter and the acceleration grid power supply for SPIDER;
Japan is providing high voltage components for the 1 MV power supply system of MITICA (including the megavolt bushing, the megavolt transmission line and the high voltage part of the megavolt power supply).
Italy is providing buildings, auxiliaries and the power grid.
The Italian government has provided important financial support by building the facility on a two-hectare site at Consorzio RFX, an Italian research laboratory that has been investigating plasma physics and controlled nuclear fusion since 1996. Consorzio RFX, as host, is providing expertise through a large contribution to manpower.
All contributions to the Neutral Beam Test Facility are voluntary (i.e., outside the scope of in-kind contributions to the ITER Project).
The schedule of planned experiments is strictly aligned with the general timeline of ITER so as to guarantee that sufficient experience on the operation of the prototype source (SPIDER, see a tour of the experiment at left) and of the full injector (MITICA) will be available before installation of the neutral beam system at ITER.
The realization of the two test stands is proceeding in parallel at Consorzio RFX. SPIDER entered operation in June 2018.
Purpose: A joint international effort to develop the neutral beam injector prototypes for ITER.
Funding (construction phase): The European Domestic Agency, the ITER Organization and Italy. The Domestic Agencies of Japan and India are contributing some components.
Construction timeline: Decision made by the ITER Council to build the facility (2010); construction begins at Consorzio RFX (2012); delivery of all buildings, including the 17,500 m² main building (2015); component installation underway.
SPIDER timeline: Design of all components completed (2011); manufacturing of beam source (November 2012-November 2017); overall commissioning (March 2018); start of operation (June 2018); first beam (June 2019).
MITICA timeline: Design of all components concluded (early 2016); installation of power supply and tests completed (early 2018); manufacturing of beam source in Europe is about to start (mid 2018); experimental phase begins (2022 planned).
Labs with a role in the neutral beam development program: Europe—the French Alternative Energies and Atomic Energy Commission (CEA); the Max Planck Institute of Plasma Physics (IPP); the Karlsruhe Institute of Technology (KIT); the Culham Centre for Fusion Energy (CCFE). Japan—National Institutes for Quantum and Radiological Science and Technology. India—ITER-India, Institute for Plasma Research.
NBTF = Neutral Beam Test Facility
SPIDER = Source for the Production of Ions of Deuterium Extracted from a Radio frequency plasma
MITICA = Megavolt ITER Injector and Concept Advancement
PRIMA = Padova Research on ITER Megavolt Accelerator (another name for the Neutral Beam Test Facility)
Consorzio RFX = A research organization promoted by ENEA (Italian National Agency for New Technologies, Energy and Sustainable Economic Development); CNR (National Research Council); INFN (National Institute for Nuclear Physics); University of Padova; and steel producer Acciaierie Venete SpA.