Physics considerations for the ITER first wall lifetime

Richard Pitts

ITER

Friday, April 1, 2022

10:00am

Virtual

PSFC Seminars

Abstract: The decision to armor the ~600 m² ITER First Wall (FW) with beryllium (Be) is very largely driven by risk mitigation:  to optimize the development path to the earliest possible achievement of burning plasma operation by minimizing the core impurity contamination from high Z species (e.g. Mo, W) that could be an issue if such materials were instead chosen to face the main plasma. There is a price to pay, however. The low Be melting point means that transient damage, particularly due to the halo current and runaway electron (RE) heat loads during the current quench (CQ) phase of unmitigated disruptions, can be significant even quite early on in the non-nuclear ITER operation phases. In addition, the use of toroidal shaping for leading edge shadowing of the actively cooled first wall panels (FWP) means that stationary plasma loads will be very focused on only ~10% of the full FW surface area. Coupled with the low Be sputtering threshold and the long ITER pulse lengths, long term net erosion can easily reach very high fractions of the Be armour thickness in certain critical regions. In turn, this can lead to a substantial Be migration flux, thick deposited layers, a potentially significant dust source and a high tritium inventory growth rate.

In view of the imminent procurement of the 440 ITER FWPs, decisions are required soon for the required spares allocation to allow for panel replacement during the required Be FW lifetime. These panels are highly complex, costly and difficult to manufacture, meaning that production of the spares must occur as part of the series production for the entire ensemble. Expected lifetimes must be therefore be estimated now, many years before any FWP replacements will be required, and before any experience of ITER operation is acquired. Beginning with a brief update of the ITER construction status, this presentation will describe some of the recent physics efforts which have been made at the ITER Organization and with R&D collaborators to provide the FW lifetime estimates which are being used now to guide decisions on the procurement of FWP spares.

Bio: Richard Pitts earned his Phd in 1991 from the University of London, UK, in collaboration with UKAEA Culham Laboratories, for experimental research with electrostatic probes in the plasma boundary of the DITE, TEXTOR and COMPASS tokamaks. Following a year working on a task agreement at JET, he moved to a postdoctoral position at the CRPP Lausanne (now SPC) and established a plasma boundary research programme on the then new tokamak TCV, which began operation in 1993. A two year postdoc turned into a 16 year stay in Switzerland, but included a 7 year period between 1999 – 2007 with regular short term secondments back to JET, acting first as Deputy, then Leader of the Exhaust Physics Task Force under the European Fusion Development Agreement. In 2008 Richard moved to the ITER Organization (IO) to lead the Plasma-Wall Interactions Section and then, from 2019 the newly formed Experiments and Plasma Operation Section within the Science Division. He is the author/co-author of over 400 journal papers and, since arriving at the IO, has collaborated with experimental and modelling teams in almost all of the ITER Member fusion institutes.