Measurements of CAE mode structure in DIII-D and identification of intermediate frequency AEs*

New measurements are reported of fast-ion driven compressional Alfvén eigenmodes (CAE, ω ≲ ω_ci­) and intermediate frequency AEs (v_A/R ≪ ω ≪ ω_ci­). These modes are of interest because they potentially can cause electron energy transport; additionally, these measurements advance the development of AE spectroscopy as a tool for non-invasive diagnosis of fast-ions in DIII-D and burning plasmas. Measurements of ñ mode structure for the CAEs are obtained with an array of eight fixed-frequency reflectometers (55–75 GHz) offering full coverage of the plasma minor radius under the experimental conditions (B_T=1.3 T, n_eL=2.8 x 10¹⁹ m^-3). Preliminary analysis of some CAEs (with f≈5.5 MHz=0.55 f_ci­) shows them to be core localized. Intermediate frequency modes were observed in beam-heated ELMing H-mode plasmas (B_T=2.1 T, n_eL=5 x 10¹⁹ m^-3) with f∼1–2 MHz∼2–5 v_A/2πR (where v_A/R∼0.03 ω_ci­). The mode frequencies sweep downward as plasma density rises during the inter-ELM period by an amount Δf/f ∼ −Δn_eL/2n_eL, consistent with their identification as AEs. However, the frequencies are too low for CAEs and too high for toroidicity-induced or reverse shear AEs. Two-point toroidal mode number measurements yield |n|=3–6.