Surface heat loads on the ITER divertor vertical targets

Abstract

The heating of tungsten monoblocks at the ITER divertor vertical targets is calculated using the heat flux predicted by three-dimensional ion orbit modelling. The monoblocks are beveled to a depth of 0.5 mm in the toroidal direction to provide magnetic shadowing of the poloidal leading edges within the range of specified assembly tolerances, but this increases the magnetic field incidence angle resulting in a reduction of toroidal wetted fraction and concentration of the local heat flux to the unshadowed surfaces. This shaping solution successfully protects the leading edges from inter-ELM heat loads, but at the expense of (1) temperatures on the main loaded surface that could exceed the tungsten recrystallization temperature in the nominal partially detached regime, and (2) melting and loss of margin against critical heat flux during transient loss of detachment control. During ELMs, the risk of monoblock edge melting is found to be greater than the risk of full surface melting on the plasma-wetted zone. Full surface and edge melting will be triggered by uncontrolled ELMs in the burning plasma phase of ITER operation if current models of the likely ELM ion impact energies at the divertor targets are correct. During uncontrolled ELMs in pre-nuclear deuterium or helium plasmas at half the nominal plasma current and magnetic field, full surface melting should be avoided, but edge melting is predicted.

A study of planar toroidal–poloidal beveling of monoblocks on the ITER divertor outer vertical target

“…Kinetic modelling including the self-consistent sheath electric field [3] and measurements [4] in the COMPASS tokamak have confirmed the physics of LE heating, and provide further justification for the decision to include a toroidal bevel at the high heat flux areas of the divertor vertical targets [5]. An unexpected prediction of [2] is that toroidal MB edges that are nearly parallel to the magnetic field could exceed allowable limits due to steady state heat loads and mitigated ELMs, even if those should avoid full melting of the principal plasmafacing surface. The heat deposition mechanism in intra-PFU TGs has been experimentally confirmed in the COMPASS tokamak [6] in a dedicated experiment designed to test the predictions.…”

“…The heat deposition mechanism in intra-PFU TGs has been experimentally confirmed in the COMPASS tokamak [6] in a dedicated experiment designed to test the predictions. Furthermore, despite the toroidal bevel, tiny regions of the poloidal LEs known as "optical hot spots" (OHS), accessible along magnetic field lines through TGs, were identified [2]. The intense parallel heat flux from ELMs onto those optical hot spots could be large enough to trigger tungsten boiling.…”

The very high spatial resolution infrared thermography on ITER-like tungsten monoblocks in WEST Tokamak

“…1) [2]. The heat loads deposited per unit of area can then lead to a local melting of components and irreversible damages [3], [4]. Designs are proposed to reduce the risk induced by misalignments.…”

Application of the parareal algorithm to simulations of ELMs in ITER plasma

“…Depending on their amplitudes and frequencies, ELMs are classified into types I (large amplitudes -hence often termed as 'giant' ELMs) and II and III (smaller, hence often known as 'grassy' ELMs). Current predictions estimate [21,40,41] the heat flux due to ELMs on the divertor plates at ITER (the world's biggest experimental tokamak) will be up to 20 times larger than what can be tolerated for a reasonable lifetime of the target materials. This makes research into ELM control and mitigation of primary importance for making fusion a viable source of energy.…”