Hollow pellet injection for magnetic fusion

Abstract

The precise delivery of mass to burning plasmas is an area of growing interest in magnetic fusion (MF). The amount of mass that is necessary and sufficient can vary depending on such parameters as the type of atoms involved, the type of applications, plasma conditions, mass injector, and injection timing. Motivated by edge localized mode (ELM) control in H-mode plasmas, disruption mitigation and other applications in MF, we report the progress and new possibilities in mass delivery based on hollow pellets. Here, a hollow pellet refers to a spherical shell mass structure with a hollow core. Based on an empirical model of pellet ablation, coupled with BOUT++ simulations of the ELM triggering threshold, hollow pellets are found to be attractive in comparison with solid spheres for ELM control. By using hollow pellets, it is possible to tailor mass delivery to certain regions of edge plasmas while minimizing core contamination and reducing the total amount of mass needed. We also include the experimental progress in mass delivery experiments, in situ diagnostics and hollow pellet fabrication, and emphasize new experimental possibilities for ELM control based on hollow pellets. A related application is the disruption mitigation scheme using powder encapsulated inside hollow shells. Further experiments will also help to resolve known discrepancies between theoretical predictions and experiments in using mass injection for ELM control and leading to better predictive models for ELM stability and triggering.

Achievements of actively controlled divertor detachment compatible with sustained high confinement core in DIII-D and EAST

“…Therefore, the development of new detachment feedback controllers in high β P scenario is needed. In the DIII-D 2018 campaign, the feedback control of radiation was utilized and demonstrated in the high β P scenario [10,16], which accessed partial divertor detachment successfully in LSN configuration. In 2019, a more precise controller utilizing the divertor Langmuir probe measured particle flux (J sat ) to characterize the degree of detachment (DoD) was successfully developed and demonstrated in the high β P scenario, similar to the jointly developed J sat controller in the EAST tokamak [14] and JET [8].…”

“…In Alcator C-Mod, realtime control of heat flux was obtained using surface thermocouple diagnostic [9], with T et,div at the strike point reduced from ∼20 eV to below 10 eV in N 2 seeding discharges. Previous DIII-D divertor radiation power feedback control with N 2 seeding in the high β P scenario was performed without accessing detachment [10], in addition to detachment feedback control via divertor Thomson scattering measured T et, div controller in L-mode [11] and H-mode [12] which accessed marginal detachment between ELMs. In the EAST superconducting tokamak, total plasma radiation feedback control was achieved with the divertor plasma still being attached [13], while H-mode detachment was achieved via the divertor particle flux controller [10,14] with neon seeding mixed with deuterium.…”

“…Previous DIII-D divertor radiation power feedback control with N 2 seeding in the high β P scenario was performed without accessing detachment [10], in addition to detachment feedback control via divertor Thomson scattering measured T et, div controller in L-mode [11] and H-mode [12] which accessed marginal detachment between ELMs. In the EAST superconducting tokamak, total plasma radiation feedback control was achieved with the divertor plasma still being attached [13], while H-mode detachment was achieved via the divertor particle flux controller [10,14] with neon seeding mixed with deuterium. Very recently in TCV, C-III emission front was successfully feedback controlled, while no clear signature of detachment was observed [15].…”

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

“…EAST has developed ITER-like heating systems, active impurity and recycling control systems, an active water-cooled ITER-like tungsten (W) divertor, various ELM mitigation technologies, and modern diagnostics. The combination of these capabilities resulted in H-mode discharges with durations over 100 s [50,51]. To achieve long pulse recycling and impurity control, Li wall conditioning was employed, via a combination of overnight evaporation and periodic use of real-time Li aerosol or granule injection [51,52].…”