Investigation of ion cyclotron range of frequencies mode conversion at the ion–ion hybrid layer in Alcator C-Mod

Abstract

Mode conversion (MC) of long wavelength fast electromagnetic magnetosonic waves (fast wave, or FW) into shorter wavelength electrostatic (ion-Bernstein, or IBW) or slow electromagnetic (ion cyclotron, or ICW) waves is of great interest in laboratory, magnetic fusion and space physics experiments. Such processes are particularly important in multi-ion species plasmas. In this paper we report recent results from high power ion cyclotron range of frequencies (ICRF) heating experiments in the Alcator C-Mod tokamak. Mode converted waves near the He–H3 hybrid layer have been detected by means of phase contrast imaging in H(3He,D) plasmas [E. Nelson-Melby et al., Phys. Rev. Lett. 90, 155004 (2003)]. The measured wave k spectrum and spatial location are in agreement with theoretical predictions [F. W. Perkins, Nucl. Fusion 17, 1197 (1977)], which showed that in a sheared magnetic field, mode-conversion of FW into ICW may dominate over IBW for appropriate ion species (i.e., D–T, or equivalently, H–3He). Recent modeling with full wave codes, as well as solving the hot plasma dispersion equation in the presence of sheared magnetic fields, verifies the interpretation of such a mode conversion process. Thus, the geometry of the magnetic field, as well as the particular ion species mix, influences the physics of ICRF mode conversion. In this paper, we also report recent results on the study of mode conversion electron heating (MCEH) in D(H) plasmas [Y. Lin et al., Plasmas Phys. Controlled Fusion 45, 1013 (2003)]. By comparing the experimentally measured MCEH profile with modeling, the study shows that the MC ICW may make a significant contribution to the direct electron heating when the D–H hybrid layer is off axis on the high field side. Preliminary results of mode conversion poloidal plasma flow drive experiments in D(3He) are also reported.