2025 – Improved Understanding of Plasma Fueling Through Measurements of the Neutral Energy Distribution

Improved Understanding of Plasma Fueling Through Measurements of the Neutral Energy Distribution

2025 Research Campaign, Thrust: High Opacity and Density Operation

Purpose of Experiment

The proposed experiment seeks to unravel the way neutrals contribute to plasma fueling, and also provide initial insights into their impact on energy, momentum, impurity transport, and sputtering induced by high-energy neutrals. This is achieved by leveraging the newly installed charge exchange neutral spectroscopy (CENS) diagnostic to obtain the neutral density distribution function above the x-point and compare it to that found at the midplane using passive MICER measurements. Charge exchange between recycled atomic neutrals and ions leads to neutrals with significantly larger energies and mean free paths than would typically be expected. This process can occur multiple times allowing the neutrals to obtain energies that are typical of ions inside the pedestal top, enabling them to fuel deeper in the confined plasma. This is the dominant fueling process within the confined plasma due to recycled neutrals, and can provide a particle source that is significantly larger than expected from the neutral beams inside the pedestal top. Furthermore, this process allows the recycled neutrals to partially overcome the challenges posed by opaque scrape off layers which are expected to hinder plasma fueling in future devices. Codes such as SOLPS/EIRENE, DEGAS2, and FIDASIM include modeling of the atomic physics processes that lead to these higher energy neutrals; however, while these simulations are typically relied on to estimate quantities such as the particle source in ITER and future FPPs, they are largely untested and often poorly constrained (when used interpretively) due to a lack of measurements of the neutral distribution function. Therefore, these experiments will provide an extensive data set that can be used to validate these codes, and also provide improved understanding of how neutrals fuel DIII-D and play a dominant role integrating the core and the edge. An open question is the degree to which fueling happens due to neutrals that originate in the divertor and move up through the x-point region as opposed to those that originate in the main chamber. While more neutrals recycle at the divertor, they have a longer path to make it to the pedestal top due to flux expansion compared with neutrals that originate from the midplane. This is illustrated in the SOLPS/EIRENE simulation. The principal result will be an assessment of the neutral distribution function and associated fueling from the x-point versus the midplane as a variety of parameters that are expected to affect the neutrals mean free path are varied: ● EdgeTi→broader fueling profile due to faster thermal neutrals ● Increase edge ne → narrow edge fueling profile due to reduced mean free path ● Increase Zeff → narrow edge fueling profile due to reduced probability of CX with D+ Here the expected dependencies that are described are based on FIDASIM simulations. While a primary goal of the experiment would be to determine the extent to which x-point vs main chamber/midplane fueling is important, a world first dataset of the neutral distribution function would be acquired allowing for many investigations going forward.

Experimental Approach

The general approach is to vary the main parameters that affect the energy and subsequent mean free path of the neutrals (see bullet points in Section 1) while making measurements of these quantities above the x-point and at the LFS midplane along with turbulent measurements, and standard pedestal profile measurements including direct measurements of the deuterium density profile using active MICER. The L-mode, L-H transition, and H-mode pedestal buildup and inter-ELM behavior will all provide useful information for modeling efforts. The reference plasma will be standard DIII-D configuration, lower single null with ITER similar shape that has good pumping for better density control, such as those in 179444 (or more recently 200666). Scans of the input power (dominantly NBI) and gas puffing will be used to scan through a range of temperatures and densities. This is effectively a collisionality scan which will also be useful for comparing with neoclassical and turbulent transport models. Following this initial dataset, Zeff will be modified using the impurity powder dropper (IPD), which in previous experiments has been successfully utilized to modulate carbon concentration and Zeff in the range 1-4. If this is not available or possible, then Zeff will be increased using Ne gas puffing. This is expected to make it more difficult for the neutrals to travel into the plasma (see discussion and predict first results in Figure 2 of section 1).