The injection of neutral particles from negative ions (Neutral Beam Injection) is one of the principle heating methods of the plasma fuel inside magnetic confinement fusion devices.
What is Neutral Beam Injection and how can we reach the super high temperatures needed for fusion reactions?
Let’s ask Vanno Toigo, engineer and principal CNR researcher at Consorzio RFX, Project Manager of the NBTF Project.
In fusion reactors, the ohmic effect due to the high currents circulating in the plasma is not sufficient to bring the gas to the temperatures necessary for fusion reactions to take place. It is therefore necessary to use additional heating systems. Among them, the most important is the neutral particle injector.
“To give an idea, let’s think of a large hot-water tap capable of producing a fast stream of “hot” particles accelerated to the energy of 1 megaelectronvolt (MeV). This particle beam injected into the plasma mixes with the “lukewarm” plasma particles, whose energy is about 100 times lower, initiating an enormous number of very intense collisions. In this way it transfers its kinetic heat energy to the matter inside the reactor to reach the hottest temperatures ever measured in the known Universe”.
Additional heating systems in fusion reactors
To generate neutral particles at high energies, negative hydrogen or deuterium ions are used which are extracted from a “plasma source” and accelerated by means of electrostatic fields.
The plasma source is a container into which the operational gas (hydrogen or deuterium) is injected, which, due to the effect of radiofrequency electric fields, is transformed into a low-energy plasma from which negative ions are then extracted.
Electrostatic field accelerators consist of a system of perforated plates (grids) to which an electrical voltage is applied and located on the front side of the source. Through the holes, the negatively charged ions are extracted and accelerated by the attraction caused by these very strong positive electric fields.
The accelerated charged particles are subsequently neutralized (i.e. have their negative charge removed) to allow the beam to pass freely through the magnetic field which, in fusion experiments, confine the plasma inside the reactor.
The neutralizer is a compartment in which neutral gas of the same kind is present at low pressure. High-energy negative ions passing through this component collide with the gas molecules. Due to the shocks, a portion of the ions lose their electrical charge and become neutral, without however losing their energy, and thus continue undisturbed until they penetrate the reactor plasma. The system has an average efficiency of 50% which means that only 50% of the ions become neutral and can pass through to the plasma, while the rest remain ions and get deflected away by the magnetic fields.
The residual ions are subsequently eliminated from the beam by effect of transverse electric fields; they are deflected laterally until they intercept the side walls where they remain trapped. Given the high associated energy, the walls are water-cooled. This system is called the “Residual Ion Dump“.
The last component in line with the beam is the calorimiter. It is made up of two walls of water cooled pipes which, with its articulation system, can assume the so-called “V” configuration and intercept the neutral beam, or remain parallel to allow the beam exit undisturbed from the injector.
When the calorimeter is closed, it absorbes all the beam and allows for power measurements. When open, the injector is delivering its beam power.
In this case, the beam of neutral particles exiting the injector transfers its kinetic energy to the plasma particles inside the Tokamak, raising the temperature to the point where the fusion reactions are able to start.
ITER is the first fusion reactor prototype and is now under construction in France as part of a worldwide collaboration between 7 partners: Europe, China, Korea, Japan, India, Russia and the United States of America.
Injection of neutral particles from negative ions is a widely and long-used system for heating plasma in fusion experiments already in operation. However, the dimensions of ITER requires energy, power and continuity at parameters never reached before.
ITER requires denser particle beams and much faster particles able to penetrate to the heart of the reactor and guarantee ITER the extreme temperature conditions, 150 million degrees, necessary to ignite the fusion reactions.
Compared to the systems already developed, the scientific and technological leap is enormous.
The technological challenges to be overcome to achieve the performance levels required by ITER made it necessary to create the NBTF Project, a dedicated laboratory
for the development of the neutral injection system.
The NBTF plant was entrusted to Italy by the international scientific community
For this reason it was decided to build a research and development plant for the NBI system, called the Neutral Beam Test Facility – NBTF. The NBTF facility will offer scientists the opportunity to investigate very challenging physics and engineering aspects and to validate the concepts behind the operation of the system, before it is installed on ITER.
The design, construction and operation of the NBTF plant was entrusted to Italy, in collaboration with European, Japanese and Indian laboratories.
A first agreement, in force from 2012 to 2019, for the construction of the plant defined the partners: the ITER Organization (IO), Europe, through the European Agency Fusion for Energy (F4E) and Italy, through Consorzio RFX.
During this phase, several European laboratories contributed: such as IPP-Garching, KIT-Karlsruhe, CCFE-Culham, CEA-Cadarache, Italian research bodies, through some of their institutes: CNR, ENEA, INAIL, University of Padua and Milan Bicocca , and international laboratories: QST (Japan), IPR (India), NIFS (Japan).
During this first agreement, the components and scientific equipment of the plant were designed for which all the supply contracts were launched, and the construction of the plant was completed.
The buildings and infrastructures were financed by the Italian government for around €25 M, while the components and experimental plants were supplied by the three ITER partners: Europe, Japan and India, through their Domestic agencies: F4E, JADA, INDA for approximately €250 M.
Since 2020 a new agreement is in place for the operation of the experiments, with a ten-year duration, between ITER and Consorzio RFX, for a total funding of approximately 150 million euros.
Europe has joined this agreement, this time through EUROfusion, making scientists available from various European laboratories.