Joseph Ziegel, UT-Austin

"Improving ECE Detection Methods for Controlling Magnetic Islands and Neoclassical Tearing Modes"

Abstract: Within tokamaks, small rope-like defects can develop inside the magnetic field in the plasma. These ropes, called magnetic seed islands, can reduce the bootstrap current and result in a feedback that grows the magnetic island via neoclassical tearing. When the island is large enough this causes catastrophic plasma disruption. Methods to detect and control neoclassical tearing modes (NTMs) are crucial for fusion goals in ITER, etc. By using electron cyclotron emission (ECE) diagnostics, ITER hopes to detect magnetic islands before they reach critical size, and feed information to electron cyclotron current drive (ECCD) systems to suppress the NTM. Efficient and accurate detection of the magnetic island is therefore a priority. This work shows how plasma profiles with magnetic islands can be simulated and synthetic measurements taken to provide computational data to test detection algorithms. Theoretical and experimental understanding of magnetic island seeding is insufficient to predict device susceptibility to NTMs. ITER plans only include one ECCD beam, thus multiple NTMs could result in unavoidable plasma disruption. An diagnostic test using the preferred algorithm found in this work will provide information about magnetic island location and growth. This information could further understanding of initial development of magnetic islands. The diagnostic could also test device susceptibility to NTMs, and determine discharge scenarios that are stable or avoid multiple NTMs.