# Hollow pellet injection for magnetic fusion

## TL;DR

This paper explores the use of hollow pellet injection as a novel method for precise mass delivery in magnetic fusion, aiming to improve ELM control and disruption mitigation with reduced mass and targeted plasma region impact.

## Contribution

It introduces the concept of hollow pellets for mass injection, combining empirical ablation models and simulations to demonstrate advantages over solid spheres for plasma control.

## Key findings

- Hollow pellets can effectively trigger ELMs with less mass.
- Hollow pellets allow targeted edge plasma delivery, reducing core contamination.
- Experimental progress includes fabrication, diagnostics, and in-situ testing.

## Abstract

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.