Experimental observation and theoretical analysis are presented for the nonlinear mode couplings between shear Alfvén wave and magnetic island. In the sub-Alfvén frequency range, two kind axi-symmetry magnetic activities with n = 0 have been observed during NBI on HL-2A. One kind has been identified, and belongs to EGAM. Another kind is found for the first time, its frequency lies in the range of TAE frequency. The Fourier bicoherence analysis suggests these axi-symmetry modes are generated by the nonlinear mode coupling via the decay process between Alfvén eigenmodes and low-frequency MHD modes. The experimental results indicate that the nonlinear mode coupling is one of mechanisms of the energy cascade in energetic-particle turbulence or Alfvén turbulence.
Using the profile analysis, the density perturbation transport analysis, and the Doppler reflectometry measurement, for the first time a spontaneous and steady-state particle-transport barrier has been evidenced in the Ohmic plasmas in the HL-2A tokamak with no externally applied momentum or particle input except the gas puffing. A threshold in density has been found for the observation of the barrier. The particle diffusivity is well-like, and the convection is found to be inward outside the well and outward inside the well. The formation of the barrier coincides with the transition between the trapped electron mode and the ion temperature gradient driven mode.
We report the first experimental evidence of Alfvénic ion temperature gradient (AITG) modes in HL-2A Ohmic plasmas. A group of oscillations with f = 15 − 40 kHz and n = 3 − 6 is detected by various diagnostics in high-density Ohmic regimes. They appear in the plasmas with peaked density profiles and weak magnetic shear, which indicates that corresponding instabilities are excited by pressure gradients. The time trace of the fluctuation spectrogram can be either a frequency staircase, with different modes excited at different times or multiple modes may simultaneously coexist. Theoretical analyses by the extended generalized fishbone-like dispersion relation (GFLDR-E) reveal that mode frequencies scale with ion diamagnetic drift frequency and ηi, and they lie in KBM-AITG-BAE frequency ranges. AITG modes are most unstable when the magnetic shear is small in low pressure gradient regions. Numerical solutions of the AITG/KBM equation also illuminate why AITG modes can be unstable for weak shear and low pressure gradients. It is worth emphasizing that these instabilities may be linked to the internal transport barrier (ITB) and H-mode pedestal physics for weak magnetic shear. Kinetic Alfvén and pressure gradient driven instabilities are very common in magnetized plasmas both in space and laboratory[1][2][3]. In present-day fusion and future burning plasmas, they are easily excited by energetic particles (EPs) and/or pressure gradients. They can not only cause the loss and redistribution of EPs but also affect plasma confinement and transport[4][5]. The physics associated with them is an intriguing but complex area of research. For weak magnetic shear (s = (r/q)(dq/dr) ∼ 0) and low pressure gradients (α = −R 0 q 2 dβ/dr < 1; with β the ratio of kinetic to magnetic pressures.), the stability and effect of them, such as Alfvénic ion temperature gradient (AITG) mode[6][7]/kinetic ballooning mode (KBM)[8], have not been hitherto unrecognized. At weak magnetic shear, the first pressure gradient threshold becomes very small or vanishes and the AITG/KBM spectrum is unstable in the very low pressure gradient region[9][10]. For equilibria with reverse shear where q min is off axis and α max near q min , there exists an unstable low-n global branch of AITG and trapped electron dynamics can further destabilize it[11].The AITG/KBM modes, on the one hand, can cause cross-field plasma transport that set an upper limit on the arXiv:1611.05538v1 [physics.plasm-ph]
A new four-chord Michelson-type formic acid (HCOOH, λ = 432.5 μm) laser interferometer has been successfully commissioned on the HL-2A tokamak to measure the electron density and density fluctuations. Due to the employment of the two-laser heterodyne technique, the time resolution of the interferometer reached 1.0 microseconds (μs). Four chords of line electron densities with a line-averaged density resolution 2 × 10/m were obtained in a recent HL-2A experimental campaign, and detailed electron density fluctuations, caused by events such as edge localized mode, sawtooth precursor-oscillations, and energetic particle driven instabilities, were distinctly measured. In particular, the high-frequency electron density fluctuations (up to 500 kHz) caused by the reversed shear Alfvénic eigenmode were observed by the internal two interferometry channels, and their fluctuation location could be approximately identified from the spectra characteristics of multi-chord line electron densities.
In order to avoid a fringe jump caused by high plasma density and pellet injection [Y. Zhou et al., Rev. Sci. Instrum. 87, 11E107 (2016)], a new CO dispersion interferometer is designed and commissioned on HL-2A for average line-density measurement and density feedback control. The second harmonic technology in this system eliminates the phase shift caused by mechanical vibration. Signals are processed by a digital phase comparator and can be monitored in real time. A series of experiments are conducted to study the characteristics of the system such as a second harmonic coefficient and long-term stability. The resolution of density measurement is less than 8 × 10/m, and the experiment result on HL-2A demonstrates the interferometer's capability to track plasma density evolution with rapid change. The system also shows good stability against mechanical vibrations.
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