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The large-scale fusion experiment ITER (Latin for "the way") is currently under construction in Cadarache in the south of France as part of an international cooperation

ITER aims to demonstrate the physical and technological feasibility of fusion energy on a power-plant scale and thus pave the way for the commercial exploitation of fusion. By means of the fusion of heavy hydrogen (deuterium) and super-heavy hydrogen (tritium), up to 500 million watts of fusion power will be produced at ITER for the first time ever.

To achieve this, the deuterium-tritium mixture must be heated to temperatures of over 100 million degrees. At ITER, this is accomplished through the injection of energetic neutral particles, radio waves, and microwaves, which typically deliver a total heating power of 50 million watts to the plasma.

ITER is a torus-shaped magnetic confinement device based on the tokamak concept. In a tokamak, the hot plasma is confined by a magnetic field which is produced by superconducting coils and a current flowing through the plasma. The major challenge is to maintain stable control of the hot plasma – at temperatures of over 100 million degrees – and of the heat and particle fluxes escaping from the plasma so that the walls of the vacuum vessel can withstand the interaction with the plasma on a long-term basis.

The following is a summary of ITER's key operating data:

Plasma volume700 m3
Magnetic field strength5.3 Tesla
Plasma current15 Mega-Ampere
Fusion power500 MW
External heating power500 MW
Power gainQ = 10
Start of operationapprox. 2020

Querschnittzeichnung ITER mit Port Plug

ITER is the world’s first fusion reactor, and its aim is to achieve a positive energy balance. However, the duration of the plasma pulses produced will only be between a few minutes (full fusion power) and one hour (at reduced power). In addition, the expected power gain of Q = 10 at a maximum is still too low for the fusion power produced to be used for net electricity generation. ITER concentrates instead on developing and demonstrating solutions on a practical scale for all remaining physical and technical issues on the path towards the commercial exploitation of fusion. Net electricity generation will be the task of a subsequent device – the demonstration reactor DEMO. In comparison to ITER, DEMO will be somewhat larger, have a higher fusion power and power gain, produce considerably longer plasma pulses, and will thus be capable of feeding several hundred million watts of electrical energy into the grid.

The entire body of existing knowledge in fusion research worldwide is being incorporated into the design and construction of ITER. With partners from Europe, the USA, Japan, Russia, China, India, and South Korea, almost all of the world’s major industrial regions are involved in its construction. Jülich also plays a role in ITER’s development with a wide range of research activities and project contributions. Jülich’s studies on plasma-wall interactions, for example, performed essential preliminary work for ITER.

One of Jülich’s specific project contributions to the construction of ITER involves developing an extensive measuring system known as charge exchange spectroscopy, which will be used to measure the helium formed during the fusion reaction at ITER. This measuring technique is based on spectroscopic analysis of the light emitted from the plasma and has already been successfully employed in numerous fusion experiments around the world.

Development of charge exchange spectroscopy at ITER

Charge exchange spectroscopy – also known as CXRS diagnostics – will determine a wide range of important measurements in ITER’s central plasma from a detailed analysis of the plasma’s visible light emission produced by directing a high-energy beam of neutral hydrogen particles into the plasma. This enables important state parameters to be determined for the fusion matter. The first of these is the density of the helium formed as a result of the fusion reaction, which must continuously be removed from the vacuum vessel so that dilution does not cause the fusion fire to extinguish. In addition, this method provides information on plasma temperature and velocity, knowledge that is required for maintaining reliable control of the plasma. Finally, CXRS can also be used to determine the composition of the plasma, i.e. deuterium and tritium densities. The fusion power achieved is dependent on the relation between these densities.

However, the use of this measurement technique at ITER poses a major challenge, particularly for the engineers, as all of the components inside the ITER device are exposed to strong loads during operation: plasma radiation, neutron flux, material ablation and redeposition, temperature differences, and electromagnetic forces all impact on the components. Maintenance and repairs at ITER are greatly complicated by the ionizing radiation and difficulty of access to the device and can only be carried out using remote-controlled tools or robots.

When developing measurement technology and diagnostics systems, therefore, existing concepts cannot be simply applied to ITER directly. Instead, specific, sophisticated developments are needed to ensure that the systems at ITER function with as little maintenance as possible for a long time. Due to the complexity of the technical challenges listed above, before construction begins on these systems, engineers will first develop and calculate concepts in detail, and prototypes will then be developed and eventually tested. On the basis of the test results, the final components will then be developed and manufactured for ITER. Over the course of the step-by-step construction of ITER, some components will be delivered and installed before the first plasma is produced in 2020, while others will be integrated later, by around 2023.

The CXRS diagnostic device is being developed as part of a consortium involving partners from the Netherlands (ITER-NL), the United Kingdom (CCFE), Hungary (HAS), and Karlsruhe (KIT). Jülich’s contribution to CXRS initially involves developing the mechanical components for a mirror labyrinth inside the inset in ITER known as the “port plug”, in which the front part of the CXRS system must be integrated. This mirror labyrinth is responsible for directing the light to be measured from the ITER plasma to the spectrometers and detectors. To this end, we are developing concepts for the best possible mirror assembly while at the same time optimizing the lifetime of the mirrors and their optical performance.

Konstruktionszeichnung Port Plug

Construction sketch of the Upper Port Plug for ITER

Models and prototypes are being developed and tested for the most important components within the CXRS port plug: a pneumatic shutter for protecting the mirror directly facing the plasma (“first mirror”), an ion-beam-assisted cleaning system for removing wall materials deposited on the surface of the first mirror, a calibration system for measuring the transmission of the mirror labyrinth, technical devices for facilitating maintenance and repairs, and mechanical mirror holders that can withstand the forces and thermal loads and can be installed and adjusted using remote-controlled tools. Moreover, we calculate atomic physics data for evaluating the spectra measured and, together with our consortium partners, we are developing and testing a prototype spectrometer for conducting detailed analysis of the light to be measured. Finally, we play a leading role internationally in studies on understanding and optimizing the lifetime of the first mirrors for the CXRS diagnostic device as well as for other first mirrors in ITER.

Finding solutions to the multifaceted problems associated with ITER’s CXRS system calls for close collaboration between physicists and engineers from various disciplines. Subtasks in this project can also be assigned to students in the form of master’s or PhD theses.


Dr. Philippe Mertens

Tel. +49 2461 61-3036
Fax +49 2461 61-3331