Long pulse D₂ and N₂ seeded discharges on the upper actively cooled tungsten divertor of WEST

Abstract

A series of L-mode discharges in upper single null conguration with more than 30 s duration were performed in the WEST tokamak with the strike line on actively cooled W-coated divertor components. This series of discharges accumulated 20 min of plasma and a strike point uence of ≈ 2 × 10 25 m −2 in 2 days of operation in attached conditions. The discharges showed good density control, no tungsten accumulation and an overall very benign thermal behaviour of the upper divertor. A subset of the discharges were seeded with nitrogen either in the divertor or midplane region to study plasma-wall interaction and associated ammonia formation in this weakly pumped scenario with all tungsten plasma facing components. While ammonia signals remained too weak for useful analysis, a strong dependence of nitrogen plasma penetration eciency and residence time regarding the injection location was found. This hints towards a currently underestimated, plasma near nitrogen reservoir in these tokamak discharges in divertor conguration.

Long discharges in a steady state with D2 and N2 on the actively cooled tungsten upper divertor in WEST

“…These results also support the fact that the N2 reservoir is primarily located close to the OSP region where the N2 is injected. Indeed, when compared to long discharges performed in the same conditions with N2 injected through the mid-plane, N2 was detected by the RGA during the plasma meaning that all this N2 is not fully ionised and a part can be directly pumped by the main pumping system as suggested in [10]. As a consequence, since no legacy is observed over this series of experiments, these measurements by the RGA support the fact that nearly no (totally negligible) N2 is exhausted during the discharge and that a significant part (~35%) of the retained nitrogen over the discharge is released after the discharge.…”

“…For the 4 seeded pulses, the total amount of N2 injected through the OSP region is 18.65 Pam 3 . This is a significant amount of N2 that can be compared to experiments reported in [10] where a total of 5.4 Pam 3 were injected over 6 pulses with injection for a maximum of 6s @0.15 Pam 3 s -1 . The higher rate and amount of N2 has been obtained for the discharge # 55792, in which a total of 9.49 Except for the discharge # 55790, N2 injection is also performed in the early phase of the plasma from 0 to 3s.…”

“…Also, as the N2 injection is turned on, the electron flux increases up to a steady state whilst it recovers its initial value (prior to the injection) as the seeding is stopped. Several plunges performed with the reciprocating Langmuir probe in similar plasma pulses (see [10]) allow for both Te and ne evaluation up to a normalised radius of =r/a~1.05 showing about Te~75eV and ne~1.6x10 19 m -3 at the separatrix. Except a small increase of the density in the SOL during the injection, no effect is observed.…”

Core tungsten transport in WEST long pulse L-mode plasmas

“…In this context, the tokamak WEST, which operates in a full W environment [6,7], has achieved 32 s L-mode pulses without any sign of W accumulation. These pulses were performed on the upper actively cooled W divertor [8] with nitrogen puffing phases in order to explore the ammonia formation [9]. Observations of the absence of W accumulation with only radio-frequency heating systems (no input torque) are not new, e.g.…”

Healing plasma current ramp-up by nitrogen seeding in the full tungsten environment of WEST

“…This issue of sputtering also applies when the mass of the main ion species is increased, as demonstrated on JET for example when varying the Hydrogen/Deuterium fraction, which anticipates experiments with a mixed Deuterium/Tritium content [22]. The formation of Ammonia is also an issue to be considered when Nitrogen is to be used in a regular way [23,24,25,26]. Evaluating how these beneficial and deleterious effects balance in the ramp-up experiments of WEST gives indications about the possible extension of Nitrogen seeding to future experiments.…”