7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

First results from operating an ITER-grade divertor in the full tungsten actively
cooled tokamak WEST
A Ekedahl1 and the WEST Team*

1 CEA, IRFM, F-13108 Saint Paul-lez-Durance, France, *http://west.cea.fr/WESTteam

e-mail (speaker): annika.ekedahl@cea.fr
The mission of WEST (tungsten-W Environment in
Steady-state Tokamak) is to explore long pulse operation
in a full W environment for preparing next-step fusion
devices (ITER and DEMO) with a focus on testing theITER actively cooled W divertor in tokamak conditions.
After successfully having completed phase 1 of WEST
(2016-2021) [1], phase 2 started in December 2022, withthe lower divertor entirely composed of ITER-grade
actively cooled tungsten mono-blocks [2]. Fast progress
towards long pulse operation was achieved, resulting indischarges of 100 s pulse length and injected energy of~300 MJ per discharge (Figure 1). In total, the cumulated
injected energy during the campaign reached 43 GJ andthe cumulated plasma time exceeded 5 hours.
Tungsten contamination can lead to 50% radiated power
fraction in standard attached WEST plasmas, making thetransitions into H-mode challenging. Conventionalboronisation can decrease this level to about 20-30%, but
only for a few discharges. Two alternative routes areexplored. Firstly, encouraging results have been obtainedwith boron powder injection during the plasma discharges,
with beneficial effects on both machine conditioning andcore plasma performance. Secondly, experiments withnitrogen seeding have demonstrated that the W divertor
sources can be extinguished and that a transition to an Xpoint Radiator (XPR) regime is obtained, similar to earlier
results from AUG [3]. A stable radiating regime has been
maintained for 19 s in WEST, with real-time control using
nitrogen seeding as an actuator for 12 s (Figure 2). An
increase in core performance by 20% was observed during
the XPR phase.
Efforts to develop robust plasma scenarios, conciliatingedge and core plasma constraints in a full W environment,
have been pursued. Plasma start-up assisted by Ion
Cyclotron Resonant Heating (ICRH) was successfully
performed to assess the conditions for breakdown at lowelectric field in ITER. The analysis of W accumulation
events [4] confirms the need for a large core electron heatsource in a tungsten environment. This allows maximising
ion temperature screening effect by equipartition, thus
limiting the W peaking [5]. The installation of a 3 MWElectron Cyclotron Resonant Heating (ECRH) willconsolidate plasma scenarios in this respect [6].
The last part of the experimental campaign in 2023 wasdevoted to a high fluence campaign in deuterium, with thegoal to reach a cumulated particle fluence exceeding anITER discharge in the Pre-Fusion Plasma Operation phase.
Repetitive discharges of ~60 s pulse length were carriedout during one month of operation, with a plasma scenario
giving a divertor heat load of 4 MW/m2 and electron
temperature on the target of ~20 eV, leading to a
cumulated particle fluence in the range 5×1026 D/m2.

No boronisation or boron powder injection was carried out
during the last 7 weeks of operation. Only glow dischargecleaning was performed once or twice per week. An
increasing number of radiative events appeared towards
the end of the campaign, becoming significant once thecumulated injected energy had reached ~20 GJ. Theseradiative events could be related to dust from thin,redeposited layers near the inner strike point on thedivertor. Post-mortem analyses are now being carried out
on selected tungsten plasma facing units to quantify thecomposition of the redeposited layers.
References[1] J Bucalossi et al., Nucl. Fusion 62 (2022) 042007[2] M Missirlian et al., Fus. Eng. Des. 193 (2023)
113683[3] M Bernert et al., Nucl. Fusion 61 (2021) 024001[4] M Goniche et al., Nucl. Fusion 62 (2022) 126058[5] V Ostuni et al., Nucl. Fusion 62 (2022) 106034[6] L Delpech et al., Fus. Eng. Des. 186 (2023) 113360

Figure 1: Main parameters of a 100 s long discharge with~300 MJ injected energy, performed in WEST phase 2.

Figure 2: X-point radiator experiment in WEST, withfeedback control using nitrogen injection.