Radiation induced non-linear oscillations in ITER baseline scenario plasmas in DIII-D

Abstract

This work shows how the radiation brought about by metals or metal-equivalent radiators such as Kr and Xe produces non-linear dynamics on otherwise stationary β N flattops of DIII-D ITER Baseline Scenario demonstration discharges. The Kr and Xe gases are used to reproduce the radiative loss rates of W in present machines that operate at core temperatures much lower than the expected ITER temperature. Experiments on DIII-D with injection of Kr and Xe, as well as with sources of intrinsic metals reach the range of radiated fraction values expected in the ITER core and experience slow oscillations in temperature and radiated power. In many cases of high radiated fraction, the core temperature decreases enough for the safety factor profile to rise above the 1/1 rational surface, naturally eliminating sawteeth and occasionally producing a persistent helical core. The oscillations can be reproduced by a modified Lotka–Volterra system for temperature and radiated fraction if diffusion and noise are included, which indicates that the interplay between temperature and radiation can be the main cause of the cyclic nature of the system. A new physics based model which includes equations for temperature, density and input power can also reproduce the oscillations observed in the experiments. The present results suggest that the non-linearity of the system can be increased by the inclusion of the inherently non-linear alpha heating term, which is proportional to ∼n e 2 T i 2, and obtains oscillations in the model when added to an otherwise more stationary system.

First tungsten radiation studies in DIII-D’s ITER baseline demonstration discharges

“…from 2.5 to 1.5 keV), the non-linear impact of the Lz curves is also observed in the rapid increase in f rad , not unlike Wwall machines that experience a fast increase in core radiation in the same regime when Lz(W) increases from 2 × 10 −31 to 5 × 10 −31 while T e drops from 2.8 to 2 keV. This nonlinearity and its interplay with the NBI feedback can cause the system to bifurcate into a slowly oscillating regime when the radiated fraction is in the range expected for ITER f rad ∼ 30%, and the details of those plasmas are described in a separate publication [5].…”