Type-I ELM mitigation by continuous lithium granule gravitational injection into the upper tungsten divertor in EAST

Abstract

Large edge-localized modes (ELMs) were mitigated by gravitational injection of lithium granules into the upper X-point region of the experimental advanced superconducting tokamak (EAST) device with tungsten plasma-facing components. The maximum ELM size was reduced by ∼70% in high β N H-mode plasmas. Large ELM stabilization was sustained for up to about 40 energy confinement times, with constant core radiated power and no evidence of high-Z or low-Z impurity accumulation. The lithium granules injection reduced the edge plasma pedestal density and temperature and their gradients, due to increased edge radiation and reduced recycling from the plasma-facing components. Ideal stability calculations using the ELITE code indicate that the stabilization of large ELMs correlates with improved stability of intermediate-n peeling-ballooning modes, due to reduced edge current resulting from the profile changes. The pedestal pressure reduction was partially offset by a core density increase, which resulted in a modest ∼7% drop in core stored energy and normalized energy confinement time. We surmise that the remnant small ELMs are triggered by the penetration of multiple Li granules just past the separatrix, similar to small ELMs triggered by deuterium pellet Futatani et al (2014 Nucl. Fusion 54 073008). This study extends previous ELM elimination with Li powder injection Maingi et al (2018 Nucl. Fusion 58 024003) in EAST because (1) use of small, dust-like powder and the related potential health hazards were eliminated, and (2) use of macroscopic granules should be more applicable to future devices, due to deeper penetration than dust particles, e.g. inside the separatrix with velocities ∼10 m s−1 in EAST.

Developing solid-surface plasma facing components for pilot plants and reactors with replenishable wall claddings and continuous surface conditioning. Part A: concepts and questions

“…Subsequent experiments on DIII-D also showed surface condition effects, and fairly uniform thin-film formation was measured using DiMES with composition very similar to that measured for traditional boronizations [14]. In EAST, Li granule injection has also been successful in edge localized mode mitigation [11,15]. [56]; the internal references are listed in the supplementary document (available online at stacks.iop.org/PPCF/64/055018/mmedia).…”

“…The challenge of managing the low-Z slag resulting from continuous surface conditioning will not only be qualitatively similar to the challenge of managing the low-Z slag from the main wall armor, but it appears that it will be quantitatively similar as well. Powder injection rates employed in present tokamaks [11][12][13][14][15] range up to 500 mg s −1 , i.e. ∼16 000 kg fpy, −1 which is comparable to the rates of net erosion of the main wall armor, table 1.…”

“…precisely the same challenge described above for use of low-Z main-wall armor. Recently, powder droppers used to inject low-Z surface conditioning material ∼continuously into operating discharge plasmas have been demonstrated to be quite effective [11][12][13][14][15] and may be usable in pilot plants and reactors as well. The resulting massive generation of low-Z debris, however, has the same potential to seriously disrupt tokamak operation, as described above.…”

“…Section 2 considers the magnitude, nature and consequences that are likely to attend solid PFC slag generation in pilot plants and reactors. Section 3 describes a separate but closely related matter, which is also of potentially critical importance for high-duty-cycle tokamaks: the option of using powder droppers to surface condition plasma-wetted surfaces by continuously dropping low-Z materials such as B into the operating discharges, whether clad with low-Z or high-Z armor [10][11][12][13][14][15]. Section 4 describes some candidate material choices for PFC low-Z cladding (armor) material together with supporting rationale.…”

Recent progress in Chinese fusion research based on superconducting tokamak configuration

“…Since 2014, the EAST has been investigating ELM control with the most existing methods, including resonant magnetic perturbations (RMPs), 41 pellet pacing, 42 supersonic molecular beam injection, 43 LHWs, 44 and Li pellet injection, 45 allowing it to achieve long-pulse steady-state operation. The nonlinear transition from mitigation to suppression of the ELMs by using RMPs was first observed in the EAST in 2016.…”