2024 – Particle transport and fueling in ELM free fusion relevant regimes

Particle transport and fueling in ELM free fusion relevant regimes

2024 Research Campaign, Pedestal and Non/Small ELM Regimes

Purpose of Experiment

Future fusion power plants will need to manage the amount of heat flowing to the walls in order to operate safely. The highest performance operational regime in current devices, “High Confinement Mode,” has the drawback of periodic instabilities which expel large amounts of particles and energy out of the confined region, leading to heat loads on the walls that will not be tolerable for future devices. Several techniques have been developed to suppress these “Edge-Localized Mode” (ELM) instabilities for future reactors. By altering the characteristic particle transport in the edge of the confined region, it is possible to avoid the sharp pressure gradients that play a role in triggering the ELM. One method of doing so is to apply “Resonant Magnetic Perturbations,” (RMPs) to create a ripple in the magnetic field which enhances particle diffusion in the edge region. New diagnostics at DIII-D and advances in analysis methods will allow this experiment to quantify and compare the transport properties of RMP ELM-suppressed discharges, to help assess the viability of the RMP technique for future fusion power plants and predict their performance.

Experimental Approach

In order to investigate particle transport properties in the edge of a fusion plasma, this experiment will modulate the fueling rate by introducing periodic puffs of extra fuel gas. This leads to modulations in the density which propagate inwards from the edge. These density modulations can be tracked and analyzed to determine the underlying rates of diffusion and convection at different positions in the plasma by considering the rate at which the density changes. This technique has been utilized before in fusion experiments, but has previously been most reliable in the core of the plasma where any changes in density are not complicated by large and difficult to measure rates of fueling. DIII-D now has the capacity to measure fueling rates at the edge of the confined region with the LLAMA and ALPACA diagnostics, enabling us to use this gas puff transport analysis in the edge. By applying these gas puffs to RMP discharges that successfully suppress ELMs, we will be able to quantify the degree to which diffusion is enhanced in the edge region by the RMPs. We will then apply varying levels of RMP in order to compare how the transport enhancement in the edge changes as a result of the applied magnetic perturbation. This is a key aspect of the experiment, as previous experiments have been unable to precisely quantify the degree to which transport in the edge is changed due to difficulties in measuring the rate fuel enters the plasma. The lack of ELMs should also make the resulting fueling and density modulations easier to distinguish, enhancing the precision of our analysis over gas puff experiments that do include ELMs.

Interested in a behind-the-scenes look at DIII-D? Join us for a virtual OR in-person tour during Fusion Energy Week (May 5-9)! Sign up for a tour here.

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