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10-15 May 2021
Nice, France
Europe/Vienna timezone
The Conference will be held virtually from 10-15 May 2021

Termination of discharges in high performance scenarios in JET

12 May 2021, 08:30
4h
Nice, France

Nice, France

Regular Poster Magnetic Fusion Experiments P3 Posters 3

Speaker

Carlo Sozzi (Istituto per la Scienza e Tecnologia dei Plasmi ISTP-CNR Milano Italy)

Description

The termination of high performance plasmas in tokamak devices with high Z metal plasma facing components presents challenges related to the influx of heavy impurities which, if not kept under control, cause an increase of the radiative losses, radiative cooling and high probability of disruption.
A number of key players in these dynamics have been identified by intensive research performed after the first years of operation in tungsten machines as AUG and JET in preparation of ITER operation. Inward neoclassical convection related to the peaking of the density profile, poloidal asymmetries, plasma rotation and centrifugal effects, temperature screening, pedestal temperature, pedestal density and ELMs control are among them [1-3].
The objective of D-T fuelled plasmas with high neutron yield in stationary conditions, foreseen in the near future at JET, focuses the operations towards high performance in terms of thermal energy content and plasma current and consequently with higher disruption risk. The reduction of such risks is being pursued for the specific features of the two plasma scenarios being developed, baseline (β¬N ~1.8, q95 ¬~ 3) and hybrid (β¬N ~2–3, q95~4) [4]. The high plasma current (Ip≥2.5MA) experiments based on the baseline scenario performed in the high power campaign C36b (2016) had 65% overall disruptivity (Ip,disr≥1.0MA) with 49% pulses ending with a disruption at Ip,disr≥2.0MA. The high plasma current (Ip≥2.MA) experiments based on hybrid scenario of the same campaign had 39% overall disruptivity with 21% of the pulses ending with a disruption at Ip,disr≥2.0MA. The inspection of the corresponding databases for the recent C38 campaign (2019) reveals a significant reduction of the disruption rate. The overall disruptivity for baseline has been 34% with 27% at Ip,disr≥2.0MA. For hybrid the overall disruptivity has been ~9% (16% considering only the Ip≥2.MA database) with 5% at Ip,disr≥2.0MA. Such numbers are better understood by taking into account the necessary explorative nature of scenario development, and, in particular, the progressive adaptation towards the target parameters of higher plasma current, 3.5-4 MA for baseline and 2.2-2.8 MA at flat top for hybrid. The key elements developed to obtain a smooth termination of the high performances discharges in JET and their effectiveness in reducing the disruptivity are discussed in this contribution.
The analysis of the previous experimental campaign and the data so far collected in the present campaign indicate that the combination of edge and core W control is needed to obtain a safe plasma termination, with the optimized use of the available actuators: gas and pellet for ELMs control, ramp-down waveform of the NBI heating power while maintaining a relevant ICRH additional power, sweeping of the separatrix hitting point on the divertor to reduce the heat load and to decrease the W source. These elements, common for baseline and hybrids although with a different tuning [5], have been tested in the C38 campaign and some of them are represented in the pair of discharges in Fig.(1) where an impurity influx has been deliberately generated by drastically reducing the gas fuelling during the main heating phase.
With respect to the typical termination scheme previously adopted the gas fuelling has been increased to burst the ELM frequency in order to favor impurities removal at the plasma edge and to reduce the divertor temperature. The slow NBI power ramp-down delays the H-L transition. This needs to be tuned to shorten the free-ELMs phase at the ramp-down starting. At high current and high density the ICRH central heating may be inefficient to counteract the core impurity effect being dominantly ion heating. However, a significant core electron heating is obtained by increasing the H minority fraction during the ramp-down as shown in the figure, where the central electron temperature is recovered and the disruption avoided.
A further adaptation of the termination scheme may be required for the application to the higher plasma current D-T scenarios in development. Moreover, since the termination time cannot be foreseen in case of off-normal events, real-time tools are being implemented in a dedicate event detector to identify the nature of the off normal condition, e.g. core impurity accumulation or edge temperature collapse, and to start the termination phase with a real-time controlled response. The new detectors include real-time tomography which can estimate the amount of radiated power from different region of interest [6] and a Generative Topographic Mapping algorithm [7] aiming to compute the probability of disruptive evolution for core and edge radiative collapse respectively. A termination algorithm aiming to optimize the input power waveform during the ramp-down in order to keep a safety margin to overcome the radiative losses both in H and L mode is also being planned.

Time evolution of two baseline-like JET discharges. From top: Plasma current, NBI and ICRH power and radiative power, H minority ion concentration, gas fueling rate, bulk radiative power, W line intensity, ELMs trough BeII line intensity, electron temperature peaking, N=1 mhd activity, locked mode amplitude.

This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014–2018 and 2019–2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.

  1. C. Angioni, et al. et al 2014 Nucl. Fusion 54 083028
  2. F Köchl et al 2018 Plasma Phys. Control. Fusion 60 074008
  3. E. de la Luna et al. 2018 27th IAEA Fusion Energy Conference – IAEA CN-258, EX/2-1
  4. L. Garzotti et al 2019 Nucl. Fusion 59 076037
  5. J. Hobirk et al 2018 Nucl. Fusion 58 076027
  6. D.D. Carvalho et al 2019 JINST 14 C09011
  7. A. Pau et al 2019 Nucl. Fusion 59 106017
Affiliation ISTP-Consiglio Nazionale delle Ricerche, Milano, Italia
Country or International Organization Italy

Primary author

Carlo Sozzi (Istituto per la Scienza e Tecnologia dei Plasmi ISTP-CNR Milano Italy)

Co-authors

Prof. Alessandra Fanni (Electrical and Electronic Engineering Department,University of Cagliari, Italy) Alessandro Pau (SPC-EPFL) Athina Kappatou (Max-Planck-Institut für Plasmaphysik) Prof. Barbara Cannas (Electrical and Electronic Engineering Department,University of Cagliari, Italy.) Mr Chris Stuart (Culham Centre for Fusion Energy, Abingdon, OX143DB) Clive Challis (Culham Centre for Fusion Energy) Costanza Maggi (CCFE) Dr Daniel Valcarcel (Culham Centre for Fusion Energy, Abingdon, OX143DB, UK) Dr Diogo Ferreira (Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, Portugal.) Dirk Van Eester (LPP-ERM/KMS) Domenico Frigione (ENEA) E Lerche (LPP-ERM/KMS, Association EUROFUSION-Belgian State, TEC partner, Brussels, Belgium;) E E De La Luna (Laboratorio Nacional de Fusión, Ciemat, 28040 Madrid, Spain;) Dr Edoardo Alessi (ISTP-Consiglio Nazionale delle Ricerche, Milano, Italia. ) Emmanuel Joffrin (CEA) Eurofusion JET1 contributors# Dr Fernanda Rimini (UKAEA) Dr Giuliana Sias (Electrical and Electronic Engineering Department,University of Cagliari, Italy.) Dr Ivo Carvalho (Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, Portugal) Dr Joerg Hobirk (Max-Planck Institut für Plasmaphysik Garching, Germany) Lidia Piron (CCFE, Culham Science Centre) Luca Garzotti (United Kingdom Atomic Energy Agency - Culham Centre for Fusion Energy) Morten Lennholm (European Commission) Olivier Sauter Dr Peter Lomas (Culham Centre for Fusion Energy, Abingdon, OX143DB) Sergei Gerasimov (CCFE) Spyridon Aleiferis (National Centre for Scientific Research ‘Demokritos’, Athens, Greece)

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