ITER divertor cassette body.

For efficient fusion, the plasma core must contain mainly deuterium and tritium, while helium ash and material impurities must be continuously removed because they dilute the fuel and can increase radiative energy losses. Particles and heat escaping from the confined plasma enter the scrape-off layer and are guided by magnetic field lines towards the divertor, where they can be cooled and pumped out. Unburned deuterium and tritium recovered from the exhaust are processed and recycled through the fuel cycle.

Impurities Collector

To collect and remove these particles, the ITER divertor is located at the bottom of the vacuum vessel. The plasma boundary is shaped so that a magnetic X-point and a separatrix are formed below the confined plasma. Outside the separatrix, open magnetic field lines form divertor legs that guide particles and heat towards the divertor target plates. Pumping systems beneath the divertor remove neutralised gases from this region.

Structure

Tungsten-armoured surface of the inner vertical target.

The ITER divertor consists of 54 cassette assemblies arranged in a toroidal ring at the bottom of the vacuum vessel. Each cassette has a structural body made from austenitic stainless steel and copper alloy and supports tungsten-armoured plasma-facing components: the dome, inner vertical target and outer vertical target. The divertor must withstand some of the highest heat loads in the machine, with local surface heat fluxes reaching up to about 20 MW/m² on the most heavily loaded target regions.

Tungsten Armour

The divertor armour consists of around 300,000 individual tungsten elements that are actively cooled by pressurised water flowing through channels beneath them at about 4 MPa, from roughly 70 °C at the inlet to 120 °C at the outlet. Because local boiling could reduce heat-removal capability and damage the components, acoustic monitoring is used to detect the formation and collapse of vapour bubbles in the cooling channels.

Temperature

The tungsten surface of the inner vertical target is expected to reach about 1,000 °C during normal operation and up to around 2,000 °C under off-normal conditions. These temperatures remain well below tungsten’s melting point of approximately 3,420 °C.

Exact Positioning

Each divertor cassette weighs about 8 tonnes, with the dome alone weighing about one tonne. The cassettes will be installed remotely through the lower ports and positioned with sub-millimetre accuracy; at the narrowest point, the clearance during insertion is only about 12 mm. Precise alignment is essential because the scrape-off-layer field lines must intersect the plasma-facing targets at the intended locations and angles to avoid excessive local heat loads. The cassettes move into position on two concentric toroidal rails, which can be finely adjusted after the magnetic geometry of the assembled machine has been precisely characterised.

Diagnostics

Some divertor cassettes incorporate diagnostics systems for monitoring plasma and divertor performance. For example, selected cassettes will contain fibre-optic sensors capable of measuring temperature, strain, displacement and vibration under intense gamma and neutron radiation. The divertor is designed for remote maintenance so that individual cassettes can be removed and replaced if required during ITER operation.