EGAM induced by energetic electrons and nonlinear interactions among EGAM, BAEs and tearing modes in a toroidal plasma

Abstract

In this letter, it is reported that the first experimental results are associated with the GAM induced by energetic electrons (eEGAM) in HL-2A Ohmic plasma. The energetic-electrons are generated by parallel electric fields during magnetic reconnection associated with tearing mode (TM). The eEGAM localizes in the core plasma, i.e. in the vicinity of q=2 surface, and is very different from one excited by the drift-wave turbulence in the edge plasma. The analysis indicated that the eEGAM is provided with the magnetic components, whose intensities depend on the poloidal angles, and its mode numbers are |m/n|=2/0. Further, there exist intense nonlinear interactions among eEGAM, BAEs and strong tearing modes (TMs). These new findings shed light on the underlying physics mechanism for the excitation of the low frequency (LF) Alfvénic and acoustic fluctuations. Introduction-The very low-frequency (LF) Alfvénic and acoustic fluctuations, such as beta-induced Alfvén eigenmode (BAE), and geodesic acoustic mode (GAM), are presently of considerable interest in the present-day fusion and future burning plasmas [1], e.g. ITER. The low-frequency waves can significantly affect the plasma performance, and induce the particle losses and reduce the plasma self-heating. These LF instabilities can play an important role in turbulence and anomalous transport regulation, especially, while there is significant fraction of high energy particles in plasma [2][3]. They can be used as energy channels to transfer the fusion-bornalpha-particle energy to the thermonuclear plasma, i.e. GAM/BAE channeling [4].The GAM with toroidal mode number n=0 is an eigenmode sustained by the coupling of radial electrostatic field and the poloidal variational density perturbations, and is usually taken to be electrostatic oscillation. The GAM is excited via modulation instability and pumped by the nonlinear interaction of drift wave turbulence [2], and also driven by fast ions [5][6] [7]. The GAM was investigated both using gyro-kinetic simulations and analytical methods in toroidal and slab geometry, and observed extensively in torus plasma [2][3]. Meanwhile, the BAE with n = 0 is also a low frequency mode with parallel wave number k = (n − m/q)/R 0 = 0 , which is due to the plasma finite beta effect under the geodesic curvature, and usually believed to be electromagnetic oscillation, and created by the coupling between the shear Alfvén continuum with the poloidal mode number m and the sound continuum with the mode numbers m-1 and m+1, and driven by fast particles or large magnetic is-

Numerical modelling of geodesic acoustic mode relaxation in a tokamak edge

“…We now need to evaluate the rhs of equation ( 6), where the summation indices (n, l) are constrained by equations ( 7) and (8). To separate the pole contributions, it is convenient to represent the integrals in equation ( 6)…”

“…Finally, note that the case corresponding to |l| = |L| = N and l = n is prohibited by the selection rule in equation (8).…”

“…Driven by turbulence, GAMs are universally observed in the edge of a tokamak under L-mode conditions [2][3][4][5][6]. They can also be excited by energetic particles, either externally injected (e.g., neutral beam injection [7]), or produced as the result of collective instabilities (e.g., tearing mode reconnection [8]). Furthermore, it has been recently proposed to employ external magnetic fields to drive GAMs either for diagnostic purposes, or to modify the turbulent transport in case a sufficiently large amplitude is achievable [9,10].…”

Effect of elongation on energetic particle-induced geodesic acoustic mode

“…Furthermore, Ref. [29] contains some first evidence for EGAM excitation by energetic electrons in experiments at HL-2A. The different fast ion effects on plasma turbulence are strongly nonlinearly connected.…”

Observation of subcritical geodesic acoustic mode excitation in the large helical device

“…This occurs in the presence of an energetic particle-driven GAM (EGAM) [15] with nonlinear temporal evolution of the frequency. EGAMs have been widely observed in toroidal plasmas such as JET [16,17], DIII-D [18], LHD [19,20], JT-60U [21], ASDEX-Upgrade [22] and HL-2A [23]. The frequency of the EGAM often chirps up quickly, and the timescale (~several ms) is much faster than the timescale of change in the global equilibrium parameters.…”