Core-localized Alfvenic modes driven by energetic ions in HL-2A NBI plasmas with weak magnetic shears

摘要

Recent experimental results that are associated with the core-localized (i.e. normalized radius ρ = r/a＜ 0.5) Alfvenic modes in HL-2A neutral beam injection (NBI) plasmas with weak magnetic shears are reported. In the different plasma parameter regions, the energetic ions produced by the NBI drive multiple Alfvenic instabilities, such as the toroidal Alfven eigenmode (TAE), beta-induced Alfven eigenmode, reversed shear Alfven eigenmode (RSAE) and fishbone and energetic particle mode (EPM). Here, we focus on the high-frequency RSAE (HFRSAE) and resonant kinetic ballooning mode (rKBM). A group of downward-sweeping frequency coherent modes (HFRSAEs) with 100 ＜f＜ 500kHz and n = 3-7 are often observed with an increase in the edge safety factor, q_a. Their measured frequency is more than that of the TAEs, and f_(min) ～f_(TAE). The analysis suggests that these modes localize inside the high-order Alfv6n eigenmode (AE) gap of the Alfvenic continuum, and their eigenfrequency and eigenfunction depend on the q_(min) and q-profile. When the core plasma density is more than n_(e0) ＞ 3.0 × 10~(19) m~(-3) and the impurity or supersonic molecular beam enters the bulk plasma, the profiles of the plasma density/pressure peak, and the magnetic shear is weak or negative. In this case, a group of multi-harmonic coherent modes (rKBMs) with 30 ＜f＜ 150kHz and n = 2-9 are observed through multiple diagnostic techniques, and f_(*Pi)/2 ＜f_(MHD) =f_(lab) - nf_(vφ) ＜f_(*pi), where f_(*pi) = ω_(*pi)/2π is the diamagnetic drift frequency of the thermal ion. It is found that the HFRSAEs can transit into the rKBMs when the density profile suddenly peaks. Neutron monitoring outside the vacuum chamber demonstrates that the HFRSAE and rKBM both degrade the confinement of the energetic ions. The rKBM instabilities also affect the bulk plasma performance.