DISCOVER MITICA IN DETAIL

DISCOVER MITICA

MITICA is the prototype of the neutral beam injector for hydrogen or deuterium particles and is an integral part of the NBTF (Neutral Beam Test Facility) project.

The injector will be used to heat the plasma of ITER, THE FIRST NUCLEAR FUSION REACTOR currently under construction in Cadarache, France.

To achieve this, it must be capable of producing a beam of neutral hydrogen or deuterium particles with sufficient speed and energy to penetrate the reactor.

NBI heating system of ITER

To accelerate the hydrogen or deuterium atoms so they can reach the required speed and energy levels, MITICA must first ionize them — that is, give them a negative charge.

Once accelerated, it must then neutralize them (i.e., remove the negative charge) so they can reach the core of ITER without interference.

MITICA is located in Building 1, the large experimental hall of the NBTF facility, alongside SPIDER, the prototype of the negative ion source.

Like SPIDER, MITICA is also placed inside a reinforced concrete bunker with 180 cm thick walls which, once sealed, protects against neutron radiation and X-rays produced during operation.

This ensures safe access to the experimental area.

Building 1 of the NBTF facility – on the left, from above, MITICA open bunker

On the left, MITICA – In the center, MITICA from above – On the right, MITICA open bunker under construction

The heart of MITICA — known by researchers as the “beam line” — is housed inside a large stainless steel box measuring 15m x 5m x 5m, called the vacuum vessel, where ultra-high vacuum can be generated.

It consists of four main components (see figure below):

the Beam Source, which generates the beam of negative ions

the Neutraliser and Electron Dump, which neutralizes most of the negative ions, making them neutral

the Residual Ion Dump (RID), which completes the neutralization of any remaining charged ions

the Calorimeter, which intercepts and analyzes the neutral particle beam, destined to heat the core of the fusion reactor once operational.

Diagram of MITICA’s beam line with its components. From right to left: source, neutralizer, RID, calorimeter

The Vacuum Vessel is a stainless steel parallelepiped measuring 15m x 5m x 5m, made of two modules in 304L stainless steel: the Beam Source Vessel (BSV) and the Beam Line Vessel (BLV), described below.

In-vessel view

The first module, the Beam Source Vessel (BSV), houses the beam source. It has the shape of a cube with 5-meter edges and weighs 47 tons.

The second module is the so-called Beam Line Vessel (BLV).

With its impressive dimensions — 4.5 meters in height, 4.5 meters in width, 11 meters in length — and a mass of 49 tons, it contains all the components through which the beam passes.

The interior of MITICA’s vessel

On the side walls of the vacuum vessel, there are openings. Some allow the insertion of MITICA diagnostic instruments, while others provide connections to three other essential systems for the injector’s operation:

To maintain the vacuum inside the vessel, cryopumps are used—pumps that operate at extremely low temperatures.

They measure 8 meters in length, 2.5 meters in width, and 40 centimeters in depth.

They are mounted inside the vessel along the two vertical side walls, and due to their size, they cover almost the entire area.

They are made of steel and aluminum and are partially coated with an absorbent material derived from coconut shells.

When cooled to about –268°C (approximately 5 K), this material is able to absorb hydrogen molecules present inside the vessel, helping to achieve the desired vacuum.

Once saturated, it will be necessary to remove the trapped hydrogen to make it operational again. This will be possible by raising the temperature to around 100 K and extracting the released gas


Inside the Beam Source Vessel (BSV) is MITICA’s Beam Source, the most critical component of the injector.

It measures 3m x 3m x 4.5m and weighs about 15 tons

The source consists of two parts, each dedicated to different functions:

  • the PLASMA SOURCE, where the plasma is generated
  • the EXTRACTOR and ACCELERATOR, where the negative ion beam is extracted and accelerated.

The Plasma Source (also called the RF Source) is equipped with 8 drivers, which are cylindrical chambers containing the gas (hydrogen or deuterium) that will be transformed into plasma.

Around each driver are copper tubes (coils) through which alternating current flows at a suitable frequency.

The alternating current flowing in the coils generates radiofrequencies capable of exciting the gas molecules inside the drivers, ionizing them and thus transforming the gas into a low-energy plasma mainly composed of negative ions, positive ions, and electrons.

The Plasma Source is electrically powered by the ISEPS system.


(Discover later what alternating current is)

The second part of the source, dedicated to extracting and accelerating the negative ions that form the beam, consists of 7 perforated copper plates (grids) with 1280 precisely calibrated and properly aligned holes.

To increase the efficiency of the extraction operation, cesium is applied to the surface of the grids. (Discover later what cesium is used for)

The acceleration grids are electrically powered by the AGPS system.

The grids are arranged parallel to each other and perpendicular to the beam.

In sequence, they are:

  • the plasma grid
  • the extraction grid
  • the 4 acceleration grids
  • the grounded grid

In detail:

Of all the charges present in the plasma produced by the eight drivers in the space before the plasma grid, only the negative ions are “channeled” by the extraction grid towards the following grids up to the grounded grid, generating the negative ion beam.

What accelerates these electrically charged atoms is the potential difference between the grids. Each grid is charged at an increasing electric potential provided by the direct current power supply line. The potential difference for each stage is 200 kV; it goes from the grid at -800 kV to the next at -600 kV, then -400 kV, -200 kV, and finally to the grounded grid at 0 V.

(Discover later what potential difference is)

By summing these potential differences, the total voltage applied by the source reaches 1 million volts, and the generated negative ion beam gains enough energy to reach the core of the fusion reactor plasma, heating it.

This system produces a hydrogen ion beam with a current of 49 A and/or a deuterium ion beam at 40 A.

(Discover later what electric current is)


Continuing along the beam line, there is the NEUTRALIZER, the component responsible for neutralizing the negative ions produced by the source, that is, making sure they can give up the electron they previously acquired.

If the particle beam were not neutral, it would be deflected by the intense magnetic fields that confine the plasma inside the ITER reactor. Therefore, the neutralizer plays a fundamental role.

The neutralizer consists of four channels made from five copper panels, containing hydrogen or deuterium gas, through which the particle beam passes.

It works by charge exchange with gas molecules of the same type as the beam.

High-speed negative ions, colliding with the rarefied hydrogen or deuterium molecules inside the channels, lose the electron they had acquired in the source (but not their speed or energy) and become neutral.

One of the most critical aspects of the neutralizer is the cooling circuits of the copper panels, which must withstand high thermal power (up to a total of 6 MW). Its construction allows it to resist power densities up to 5 MW/m².


The neutralizer has an expected efficiency of about 50-60%, meaning only 50-60% of the ions are neutralized. The remaining 40-50% of negative ions are subsequently removed by a third component called the RID or “RESIDUAL ION DUMP”

Inside the RID, transverse electric fields are generated that can deflect the remaining negative ions after they pass through the neutralizer and attract them to the device’s lateral walls, where they remain trapped.

(Discover later below what an electric field is)

The instrument is made up of five longitudinal panels. Between each pair of panels, a constant direct voltage of up to 30 kV maximum is applied.

Given the high energy deposited, the walls must necessarily be cooled with water.

The RID is designed to absorb and dissipate a total power of about 19 MW and to withstand power densities up to 8 MW/m².


The MITICA calorimeter is designed to intercept the high-energy beam produced by the injector, absorb all its power, and measure its parameters.

It consists of two walls of water-cooled tubes that, in ITER, through a suitable movement system, can assume two different configurations:

  • “V” configuration: which allows interception of the neutral beam. In this case, the calorimeter is closed, absorbs all the beam energy, and enables power measurements;
  • Open configuration: aligned with the beam. In this case, the beam can exit the injector undisturbed, delivering all its power to the ITER plasma.

The calorimeter is designed to absorb beam energy up to 18 MW.

-1MV

MITICA uses a 1 million Volt (1 MVdc) direct current power supply system, which is the world’s first prototype at this voltage, power (56 MW), and size.

The system was designed and produced in collaboration with the Japanese laboratory QST and the European agency F4E.

It consists of several parts (see figure below):

  • the Acceleration Grid Power Supply (AGPS) , which provides the -1 MVdc voltage to the MITICA negative ion electrostatic accelerator
  • the AGPS control system (AGPS-CS)
  • the High Voltage Deck1 (HVD1) , which houses the power supplies for the Negative Ion Source (called ISEPS, Ion Source and Extraction Power Supply)
  • the Transmission Line, (TL)

The AGPS supplies voltage to the acceleration grids. It is composed of five -200 kV DC Generators (DCGs) connected in series.

Each DCG contains a three-phase step-up transformer and a three-phase diode bridge.

  • The oil-insulated transformer is responsible for stepping up the voltage.
  • The diode bridge, housed inside a container pressurized with insulating gas (sulfur hexafluoride, SF₆), serves to convert the alternating voltages coming from the transformer into direct voltages.

The output conductors from the five DCGs pass, through SF₆-insulated lines, into an RC filter unit that filters and selects the voltages to the desired values.

To power the AGPS transformer and diode system, the AGPS control system (AGPS-CS) is used, which includes AC/DC converters and inverters.

  • The AC/DC converters draw alternating current from the power grid and convert it into direct current.
  • The inverters convert this direct current back into alternating current (DC/AC).

The use of inverters at this stage allows automatic intervention in case of detection of electrical discharges between the acceleration grids (which occur during normal operation of the accelerator), by suspending the current supply within very short timescales (on the order of microseconds)

In summary:


The High Voltage Deck 1 (HVD1) is an imposing Faraday cage insulated in air at -1 MV DC.

It contains the power supplies for the Negative Ion Source (ISEPS).

Measuring 12.5 meters long, 9.6 meters high, and 8.4 meters wide, it rests on insulators 6.5 meters tall that support a significant weight (about 100 tons of structure and contained equipment), ensuring electrical insulation.

-1MV

High Voltage Deck 1

An Insulation Transformer (1 MV Insulating Transformer, see figure) powers the ISEPS inside it. Between the transformer and HVD1, a device called LCR (Inductor L, Capacitor C, and Resistor R) is connected in series to protect the transformer against overvoltages in the system (see figure).

The LCR device – The isolation transformer

The Transmission Line (TL) connects the AGPS and ISEPS to the Beam Source installed inside MITICA’s Vessel. It is a tube about 100 meters long, with a final diameter of 3 meters, insulated with sulfur hexafluoride (SF6) gas at 6 bar pressure.

The conductors exiting the ISEPS reach the Transmission Line through an air-gas SF6 electrical feedthrough.

All this equipment, insulated in air, is installed inside a High Voltage Room with controlled environmental conditions to avoid contamination from dust

La TRANSMISSION LINE

 

The COOLING SYSTEM of the entire facility (MITICA and SPIDER) consists of a series of 10 primary circuits capable of dissipating the heat produced by the whole plant. The power values involved at full operation of both prototypes are enormous, around 70 MW nominal. This requires a complex cooling system.

To ensure heat exchange under very high voltage conditions (up to 1 MV) and to prevent the formation of damaging electrical discharges, a specific part of the system called the Chemical Control System is used. This consists of a series of treatment processes aimed at obtaining ultrapure water that resists electrical current flow.

The cooling system of MITICA and SPIDER