Helium exhaust studies at DIII-D utilizing new experimental diagnostics

2025 Research Campaign, Divertor Science and Innovation

Purpose of Experiment

Future fusion reactors using D-T fuel will need to sufficiently pump out helium ash biproduct created from the reaction to maintain burning conditions. Helium, or alpha particles, are created in the plasma core and transported to the edge. Once the particles escape the last closed flux surface and travel to the divertor, they need to be pumped out. Particles which aren’t pumped from the divertor can be recycled back into and cool the plasma. Maintaining a Helium concentration below 10% is needed to satisfy the D-T burn condition. Additional challenges for future fusion reactors will be maintaining low heat deposition on the divertor target while having high core plasma performance. This can be achieved through a detachment state, though ELM mitigation and suppression is critical in protecting the divertor. Simulations based on DIII-D shot have shown RMP ELM suppression for ITER like shape for high impurity puffing rates and various RMP configurations. Reactors running attached or detached states will still need to meet sufficient impurity exhaust. The purpose of this experiment is to study impurity confinement times, fueling and exhaust for RMP ELM suppressed regimes at varying impurity densities for an ITER similar shape. The goal is to use enhanced diagnostic capabilities to measure the effective helium, neon, and nitrogen confinement time τ∗p with and without full RMP ELM suppression while impurity density is varied.

Experimental Approach

The experiment will utilize the extensive suite of diagnostics at DIII-D to study how the confinement time and pump out of the impurity is affected by using RMPs for ELM suppression. A diagnostics system of focus will be the experimental Wisconsin In Situ Penning (WISP) gauge; newly installed under the DIIID’s shelf just a few inches from the lower divertor pumping gap. The WISP gauge is a novel high brightness penning gauge that uses the magnetic field of the fusion device to create a penning trap to measure neutral pressure. Augmenting the WISP gauge with spectroscopy seeks to provide partial neutral pressure measurements of the impurity species being pump out of the lower divertor. This experiment will create a baseline scenario for an ITER like shape plasma with RMP ELM suppression. This can be achieved quickly by referencing past experiments at DIII-D that used the RMP coils to get ELM suppression in similar shapes and power. After a baseline is established small amounts of impurity will be puffed into the vessel; first a shot with an impurity and the RMP coils off followed by a second shot with the respective impurity puff and the RMP coils on. This alternating pattern will first begin for helium, then neon, and finally nitrogen. This will be attempted at a plasma power of 6MW and if time permits repeated and stepped up to 8MW.

See more details, including project leads, at U.S. Department of Energy, Office of Scientific and Technical Information (OSTI).