Analysis of damping rate measurements of toroidal Alfven eigenmodes on JET as a function of n: part I

The linear stability of Alfven eigenmodes (AEs) is studied experimentally in the JET tokamak using its active MHD spectroscopy system, the so-called Alfven Eigenmode Active Diagnostic (AEAD). Following the optimization of the AEAD system, AEs with toroidal mode numbers (n) in the low-n and medium-n range were excited systematically. A database was created from the damping rate measurements of toroidal AEs (TAEs), obtained in ohmically heated plasmas with monotonic q-profile. The TAE damping rate measurements were studied as a function of n, focusing on the effects of the edge plasma shape and the q profile and investigating their implications for the importance of the continuum and radiative damping mechanisms. In this paper, the first part of the analysis is presented: a statistical treatment of the damping rate database is performed and the correlations with various plasma parameters and relevant quantities are explored. The analysis shows that medium-n modes tend to be less damped than low-n modes, with the measured damping rates (gamma/omega) of modes with n >= 4 ranging from similar to 0.3% to 4%. In a follow-up paper (part II), the analysis is carried out using a discharge-following approach, by investigating the damping rate variations that are observed in individual discharges.