  Precise delivery of mass to burning plasmas is a problem of growing interest
in magnetic fusion. The answers to how much mass is necessary and sufficient
can vary depending on parameters such 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 magnetic fusion, we report 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 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
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 lead to
better predictive models for ELM stability and triggering.
