Experimental Studies of Neutral Particle Effects on Edge Transport Barriers in Tokamaks Using the Lyman-alpha Measurement Apparatus

Abstract

Prediction capabilities remain limited for the tokamak edge density profile due to small spatial scales, complex plasma transport, and non-negligible effects of neutral particles. To better quantify the hydrogenic neutral particle source, a one-dimensional, absolutely calibrated pinhole camera system was installed on the DIII-D tokamak to measure edge Lyman-alpha (Ly-𝛼) emission. Ly-𝛼 emission from hydrogenic isotopes can be used to infer hydrogenic neutral density and ionization rate profiles. To provide a high spatial resolution measurement in a compact footprint, the camera utilizes advanced engineering and manufacturing techniques including 3D printing, high-stability mirror mounts, improved filtering components, and a novel alignment procedure. Absolutely calibrated, spatially resolved Ly-𝛼 brightness measurements utilize a bright, isolated line with low parasitic surface reflections and enable quantitative comparison to the edge density profile to study radial particle transport.

Ly-𝛼 Measurements coupled to advances in inference techniques from the Aurora code, a 1.5-dimensional flux surface average particle transport and radiation forward model, allow calculation of diffusion and convection profiles in the steep density gradient region. Quantitative edge ionization source measurements and plasma profiles during H-mode steady-state and dynamic events, such as the edge localized mode (ELM) cycle and edge gas puff modulation, are investigated. Experiments show quantitative evidence of a 1 m s⁻¹ inward particle pinch and diffusion of 0.05 m s⁻² in the steep gradient region of the density pedestal. Near the last-closed flux surface, there is evidence of saturated gradient behavior which suggests limits to the 1.5-dimensional diffusive-convective model. Inboard and outboard ionization source measurements show a significant asymmetry confirming strong poloidal variation of the ionization source. Main ion transport profiles and experimental neutral particle measurements allow quantitative evaluation of the processes forming the density pedestal. Furthermore, experimental main ion transport profiles offer the opportunity to constrain modern 2D edge codes, benchmarking their modeling of current devices, thereby improving their predictive capabilities for future burning plasma devices.