29th Symposium on Fusion Technology (SOFT 2016)

I2.1 Plasma operation with full W divertor – experiences from JET equipped with the ITER-Like Wall

6 Sep 2016, 08:30

40m

Congress Hall 2nd floor (Prague Congress Centre)

Board: 1

Oral I2.1 S. Brezinsek

Speaker

S. Brezinsek (EUROfusion Consortium)

Description

Since installation of the JET ITER-Like Wall more than 30h of plasma operation with the inertial cooled full W divertor took place. Successfully, the divertor plasma-facing components PFCs handled harsh tokamak conditions with (i) high surface temperature excursions passing the ductile-to-brittle temperature and re-crystallisation temperature multiple times, (ii) ITER-relevant steady-state and peak power loads due to more than 1.5 million of transients (edge-localised modes or ELMs), (iii) combined impact of deuterium and intrinsic impurities (C, Be, O) as well as extrinsic impurities like He, Ne, Ar, N2 and Xe, and (vi) multiple complex conditioning cycles with baking, deuterium glow discharges and ion-cyclotron-wall conditioning. Routinely, monitoring discharges have been applied to characterise the impurity content in the plasma and the performance of the tungsten divertor. Overall the bulk divertor showed no impact of damage and only moderate damages of the W-coating CFC tiles could be observed. We present an overview of physics findings obtained from the operation with full tungsten divertor including in-situ observations as well as post-mortem analysis of extracted tiles in different interventions. In particular three aspects will be presented in depth: Erosion of the full W divertor: The contribution of fast particle and heat burst (ELMs) to the total W erosion of plasma-facing components as well as the fraction of prompt re-deposition are determined. Revealing that in normal operation, the intra-ELM contribution is governing the total sputtering source and determines the lifetime. The subsequent migration of W within the divertor to remote areas has been studies by spectroscopy, deposition probes and post-mortem analysis of dedicated poloidal sectors showing moderate, but unexpected transport to remote areas. Fuel retention: The retained fuel in the divertor PFCs has been determined and classified as implantation and co-deposition with Be resulting in short and long-term retention. The complex dynamic fuel retention behaviour due to surface temperature excursion will be presented and related to modelling descriptions. The role of impurities on the retention will be outlined. Power handling: The passive cooling of the W PFCs required careful operation in order to ensure integrity of the tiles. Dedicated analysis showed that the bulk W divertor with his segmented modules and lamella fulfilled the predicted behaviour. Operation with plasma seeding N2 or Ne leading to divertor cooling which allowed expanding the operational window to ITER-relevant divertor conditions.