Measurement and simulation of small cryogenic neon pellet Ne-I 640 nm photon efficiency during ablation in DIII-D plasma

Abstract

Small (∼1 mm) neon pellet fragments are fired into DIII-D H-mode plasmas, and resulting trajectory-averaged photon efficiencies (neutral neon ionization events for every photon emitted) of [Formula: see text] are estimated for Ne-I 640 nm by dividing the estimated initial pellet fragment mass by the measured number of emitted Ne-I photons. The experiments are modeled by running the Lagrangian particle (LP) fluid/magneto-hydrodynamic pellet code to estimate axial ablation plume neon density profiles and temperature profiles at each pellet position. These solutions are then fed into the PrismSPECT collisional-radiative code, which calculates resulting neon charge states and photon emission rates, giving a profile-average of [Formula: see text]. The burnthrough plasma minor radius predicted by LP ([Formula: see text]) is reasonably close to the experimental observation [Formula: see text]. The modeling indicates that local S/ XB is not constant along the pellet trajectory but tends to increase with increasing ablation rate. Non-equilibrium kinetics are predicted to be very important, while line trapping is predicted to be relatively unimportant (for Ne-I 640 nm S/ XB).

Structure of pellet cloud emission and relation with the local ablation rate

“…Initially, this model was built for interpreting the spectrum of the ablation cloud of a pellet injected in the LHD (shown in figure 3 of [12]). Starting from the fact that the emission lines of neutral Hydrogen exhibit Stark broadening profiles, the electron density was evaluated through comparison with theoretical data, with a high enough value for justifying complete Local Thermodynamic Equilibrium (LTE) in the cloud (this still remains an approximation, as demonstrated in [17], its importance must be determined and taken into account during the analysis of the results). It was also demonstrated that the continuum radiation is dominated by two components, which correspond to radiative recombination and radiative electron attachment, respectively.…”