Counting the number of neutrons produced by fusion reactions inside a burning plasma gives a precise indication of how much fusion power is being generated.

But how do you count neutrons?

Previously, fusion installations such as TFTR in the US, JET in Europe and JT-60 in Japan have relied on a device based on the physics of fission: the fission chamber.

A fission chamber is a gas-filled container, whose walls are coated with a thin layer of fissile material such as uranium 235 (U235). When a neutron hits an atom of fissile material the atom splits and a fixed amount of energy is released. Measuring the total energy that is released within the device gives a clear indication of how many neutrons have hit the U235 layer. The number of neutrons, in turn, reveals the quantity of fusion power produced.

In previous tokamaks, fission chambers were rather large and installed outside the vacuum vessel. In ITER, their size will not exceed that of a pencil—hence the term "micro fission chamber."

Four units—containing three micro fission chambers each—will be fitted into the gap between the blanket shield modules and the vacuum vessel. The units' locations have been carefully chosen so that the incoming neutrons can be monitored in all circumstances, whatever the plasma position.

Operating in a burning plasma environment, the ITER micro fission chambers will be necessarily different from those installed in previous fusion machines. Vacuum is one the design challenges: since the argon-filled "pencils" will be placed inside the vacuum vessel, they must be perfectly leak-proof. Temperature must also be taken into consideration, as the vacuum vessel will be "baked" to a temperature of 200 °C prior to plasma shots.

Previous tokamaks did not operate with the fusion fuels deuterium and tritium (or only very briefly). ITER will produce long-duration plasma discharges, submitting the micro fission chambers, like all in-vessel components, to an intense neutron flux.

"The technology is challenging but feasible," says Luciano Bertalot, technical responsible officer for the Micro Fission Chamber Procurement Arrangement signed last week. "The system is designed to hold for the twenty years of ITER operation time. It is 'redundant,' which means that even if one of micro fission chamber units fails, we will still be able to measure neutron emission and evaluate the fusion power that ITER produces."

The Procurement Arrangement documents for the ITER micro fission chambers were signed last Wednesday 28 March by the ITER Organization and countersigned by the Japan Atomic Energy Agency (JAEA) on 4 April. "We are looking forward to making this project a reality," said Yoshinori Kusama, head of diagnostics at the Japanese Domestic Agency (JA-DA) on this occasion.

Staff members from the JA-DA took an active part in the Concept Design phase that preceded the signature. Now, the R&D and prototype development phase will begin in Japan.

An international working group coordinated by Forschungszentrum Jülich, Germany, has completed a new mirror system for ITER ... and for its successors. The system—referred to as a "mirror station"—has shutters that open and close automatically to protect optical components from being contaminated by particle flows in the vacuum vessel. The researchers have been testing the practical applicability of the module at the US research reactor DIII-D in San Diego since mid-March.

Optical diagnostics are indispensable for nuclear fusion experiments. The light produced in a plasma speaks volumes about its properties, such as its composition and the concentration of various isotopes and elements. Due to the intense neutron radiation, it will only be possible to observe the light indirectly, using mirror systems positioned at the plasma edge. In this zone, however, the mirrors are exposed to contamination from beryllium and tungsten particles removed from the wall materials during contact with the hot plasma.

The new mirror system for ITER has fast shutters made of monocrystalline molybdenum, which only uncover the mirror during the main phase of the plasma pulse. The shutters thus protect the sensitive optical components when the plasma is ignited, as the risk of contamination is at its highest during this phase. Since the very strong magnetic fields in the vacuum vessel interfere with electrical circuits, Jülich's mirror station relies entirely on passive control. An additional magnetic field component is utilized for this purpose. It emerges as soon as the tokamak plasma ignites and it acts on a magnetic ferrite core in the "mirror station" which passively opens the protective shutters.

"We have already tested electromagnetic loading of the system in a tokamak environment and used software codes developed at Jülich to minimize the release of contaminating atoms and their redeposition on the mirror surfaces. We believe that our development will make a very substantial contribution to making optical measurements possible at ITER ," says project head Dr. Andrey Litnovsky at Jülich's Institute of Energy and Climate Research. After DIII-D, the practical applicability of Jülich's "mirror station" will be put to the test at the Chinese fusion experiment EAST in Hefei, at the ASDEX Upgrade operated by the Max Planck Institute for Plasma Physics in Garching near Munich, Germany, and at Jülich's TEXTOR Tokamak.

There was a definite classroom atmosphere last week in meeting room 519/110, as young engineers from the Indian and Korean Domestic Agencies, along with staff from a Chinese company involved in the ITER coil power supply, were getting acquainted with the CODAC Core System, the software toolkit supplied by the ITER Organization for creating plant system instrumentation and control (I&C).

As hands were politely raised, instructors went from one student to the other explaining functions and detailing the software's options. The ten students who participated in this four-day training session will soon be using the software "for real" as they will all be directly involved in the procurement of I&C equipment for the ITER plant systems.

The CODAC Core System is a software package that is made available to all users who contribute to the development of the various ITER instrumentation and control systems. It is based on the widely-used EPICS open-source software—a favourite among large research installations such as the Korean tokamak KSTAR, the German synchrotron DESY and the Spallation Neutron Source at Oak Ridge National Laboratory in the US.

Also included in the package (and in the training program) are ITER-developed configuration tools and software components to support hardware standards.

In order to make the training as realistic as possible the students' laptops were connected to the technical room one floor below, where electronic controllers and "looped signals" mimicked the behaviour of actual I&C equipment.

Training sessions began in 2011 and will be organized on a monthly basis throughout the coming years. Franck Di Maio, the CODAC Core System integrator and session organizer, expects to train 70 to 80 people from the ITER Domestic Agencies and industry this year.

At the time when cherry blossoms are announcing spring in Japan, the ITER Organization celebrated in its own way. For the Japanese breakfast organized by the Welcome Office, our Japanese colleagues and their spouses had obviously spent much time to prepare wonderfully refined dishes: Miso soup, salmon sushi, red bean paste, green beans with sesame seeds, onigiri and more were shared among 80 ITER employees who gathered at 8:00 a.m. in the lobby of Agence Iter France for this month's intercultural breakfast.