New world record: ELISE experimental testing facility achieves target values for ITER Neutral Beam Heating

An ion source for neutral particle heating is being developed at the Max Planck Institute for Plasma Physics, which is intended to heat the plasma of the world’s largest fusion experiment ITER to many millions of degrees Celsius in the future. Researchers have now been able to generate the ion current densities required for ITER Neutral Beam Injector in the ELISE experimental testing facility for the first time.

April 29, 2024

Fig. 1: Dr. Dirk Wünderlich and Prof. Ursel Fantz at the IPP experimental testing facility ELISE in Garching — *Fig. 1: Dr. Dirk Wünderlich and Prof. Ursel Fantz at the IPP* experimental testing facility *ELISE in Garching*

Photo: MPI for Plasma Physics / Frank Fleschner

*Fig. 1: Dr. Dirk Wünderlich and Prof. Ursel Fantz at the IPP* experimental testing facility *ELISE in Garching*

Photo: MPI for Plasma Physics / Frank Fleschner

Nuclear fusion is the process that powers the sun and stars: light atomic nuclei are fused into heavier ones, releasing a large amount of energy. The international fusion experiment ITER is intended to demonstrate that this principle can also be utilised on Earth. The aim is to achieve a fusion power of 500 megawatts - ten times as much as needs to be invested to heat the fusion plasma. In the facility currently being built in the south of France, the plasma must be heated to a temperature of 150 million degrees Celsius. Around half of the required heating power is to come from a so-called neutral beam injection system (NBI). It shoots fast hydrogen particles into the plasma, which then release their energy in the form of heat through collisions. To do this, hydrogen ions are first generated, extremely accelerated in an electric field and then neutralised to enter in the magnetic cage of the ITER tokamak where the plasma is confined. For ITER, such an ion source is based on negative hydrogen ions.
Such a powerful NBI heating - two particle beams are to deliver 16.5 megawatts each - has never been built before. At the Max Planck Institute for Plasma Physics (IPP) in Garching, a team is testing the scenarios for ITER heating operation at the ELISE (Extraction from a Large Ion Source Experiment) experimental testing facility. The project is financially supported by the European fusion consortium EUROfusion. The aim is to generate a hydrogen ion beam with a reliably high current density and demonstrate quasi-continuous operation. The ion source of ELISE is half the size of the ion source for ITER - in other words, it serves as a scaled-down prototype (see further technical details).

Fig. 2: Image of the ELISE calorimeter taken with an infrared camera, measuring the energy content of the particle beams generated. The square cross-sectional area of the ion beam (1 x 1 metre) can be seen. The brighter the colour, the greater the incident power. ITER requires a homogeneity of 90% over the entire area, which was achieved in the ELISE experiments.

Graphic: MPI for Plasma Physics

Fig. 2: Image of the ELISE calorimeter taken with an infrared camera, measuring the energy content of the particle beams generated. The square cross-sectional area of the ion beam (1 x 1 metre) can be seen. The brighter the colour, the greater the incident power. ITER requires a homogeneity of 90% over the entire area, which was achieved in the ELISE experiments.

Graphic: MPI for Plasma Physics

Pulse length increased by more than tenfold

Researchers at IPP have now achieved a new world record: on 28 March 2024, they succeeded in extracting a negative hydrogen ion current density of almost 300 amperes per square metre from ELISE for ten minutes (600 seconds). This enabled them to increase the previously possible pulse length for such current densities by more than tenfold. For shorter pulses lasting ten seconds, 330 amperes per square metre were generated - which is also a world record. This means that ELISE has already achieved the ITER target, even though only a maximum of 75 per cent of the high-frequency power available at ITER is available to generate the ion source plasma at the experimental testing facility. "Both values represent a real breakthrough for the development of the ITER NBI system," says IPP scientist Dr Dirk Wünderlich. "We are now achieving the target values that are required for the first ITER operating phase with deuterium-tritium fuel."

Fig. 3: Ion current densities from earlier hydrogen experiments in ELISE for stationary (green) and short (blue) beam pulses as a function of the normalised radio frequency (RF) power available for the ITER NBI system. The new world record results are represented by the large stars.

Graphic: MPI for Plasma Physics

Fig. 3: Ion current densities from earlier hydrogen experiments in ELISE for stationary (green) and short (blue) beam pulses as a function of the normalised radio frequency (RF) power available for the ITER NBI system. The new world record results are represented by the large stars.

Graphic: MPI for Plasma Physics

A central component of the ITER NBI system are ion sources in which negative hydrogen or deuterium ions are produced and then extracted and accelerated (deuterium is also known as heavy hydrogen because a neutron is bound in its atomic nucleus in addition to a proton). These ion sources are huge and are roughly the size of a door: at ITER, the cross-section of such an ion beam should have a rectangular area of two times one metre. At ELISE, it is half the size (a square with an edge length of one metre).

The extraction of ITER-relevant negative ion beams from such ion sources is an extremely challenging task, as the ion beam must remain homogeneous over the entire surface and temporarily stable during pulses lasting several hundred seconds. The quantity of electrons inevitably extracted during the process must remain as small as possible, as these would cause damage to the extraction grid in the NBI system. The current pulse lengths of ten minutes were only possible because the researchers have recently been able to make significant progress in controlling these electrons. This now prevents the grids from overheating.

Further world records are planned for ELISE

"These requirements were consistently met during the world record experiments in ELISE," explains IPP Division Head Prof Dr Ursel Fantz. "The next step will be to develop operating scenarios with which the ITER values can be achieved quickly and reliably." Another goal of the researchers: Following the successes with hydrogen, they also want to achieve the values with deuterium ions - a requirement for later operating phases of ITER. The ion current densities now achieved should then be maintained for up to 1 hour (60 minutes). "ELISE is technically designed to achieve these goals," says Ursel Fantz. Further world records are therefore firmly planned for the coming years at the IPP experimental testing facility.