Experimental and theoretical study of quasicoherent fluctuations in enhanced D(alpha) plasmas in the Alcator C-Mod tokamak

A comparison of experimental measurements and theoretical studies of the quasicoherent (QC) mode, observed at high densities during enhanced D(alpha) (EDA) H mode in the Alcator C-Mod tokamak, are reported. The QC mode is a high frequency ( approximately 100 kHz) nearly sinusoidal fluctuation in density and magnetic field, localized in the steep density gradient ("pedestal") at the plasma edge, with typical wave numbers k(R) approximately 3-6 cm(-1), k(theta) approximately 1.3 cm(-1) (midplane). It is proposed here that the QC mode is a form of resistive ballooning mode known as the resistive X-point mode, in reasonable agreement with predictions by the BOUT (boundary-plasma turbulence) code.