Toroidicity-induced Shear Alfven Eigenmodes Research Articles

Dynamic evolutions of multiple toroidal Alfvén eigenmodes with energetic particles

Kinetic simulations based on a reduced model are carried out to study dynamic evolutions of multiple toroidicity-induced shear Alfven eigenmodes driven by energetic particles. It is found that passing and trapped particles play quite different roles on the mode evolution. For cases without background damping, passing particles lead to a longer linear growth phase and subsequent enhancement of the nonlinear saturation level when resonant regions with two modes are closely overlapped. On the other hand, trapped particles are mainly responsible for the persistent mode growth in the nonlinear phase. By comparing the results from anisotropic and isotropic simulations, it is suggested that passing particles have a negative impact on the mode development in the nonlinear phase. For the near marginal stability cases with large background damping, the upward and downward frequency chirping in multiple branches are significantly affected by the multi-mode interaction due to overlapping of hole and clump in the distribution function.

Alfven Eigenmodes in Toroidal Magnetic Confinement. Linear Theories of Alfven Eigenmodes in a Helical Plasma.

Shear Alfven continuum and Toroidicity-induced shear Alfven Eigenmodes (TAE) are analyzed in regard to possible three-dimensional equilibrium in CHS experiments and theoretical expected LHD equilibria. In the analysis of CHS plasma, theoretical results consistent with experimental ones are obtained. Wide spectral gaps are suggested for LHD plasmas.

Alpha particle driven TAE instability suppression by a resonant RF field

The toroidicity induced shear Alfven eigenmode (TAE) instability driven by the fusion alpha particle pressure gradient is found to be stabilized in the presence of a resonant RF field with Ω = k0⊥. vα⊥, where Ω is the frequency, k0 the wavenumber and vα⊥ is the alpha particle velocity component perpendicular to the magnetic field. This kind of resonance could occur during lower hybrid (LH) current drive in an ignited tokamak plasma. The RF electric field strength that is needed for stabilization is available from modern RF power sources

Excitation of high frequency pressure driven modes in non-axisymmetric equilibrium at high βpol in PBX-M

High frequency pressure driven modes have been observed in high poloidal beta discharges in the Princeton Beta Experiment Modification (PBX-M). These modes are excited in a non-axisymmetric equilibrium characterized by a large, low frequency mt = 1/nt = 1 island, and they are capable of expelling fast ions. The modes reside on or very close to the q = 1 surface and have mode numbers with either mh = nh or (less probably) mh/nh = mh/(mh-1), with mh varying between 3 and 10. Occasionally these modes are simultaneously localized in the vicinity of the ml = 2/nl = 1 island. The high frequency modes near the q = 1 surface also exhibit a ballooning character, being significantly stronger on the large major radius side of the plasma. When a large mt = 1/nt = 1 island is present, the mode is poloidally localized in the immediate vicinity of the X point of the island. The modes occur exclusively in high beta beam heated discharges and are likely to be driven by the beam ions. They can thus be a manifestation of either a toroidicity induced shear Alfven eigenmode (TAE) at q = (2mh+1)/2nh, a kinetic ballooning mode, or some other type of pressure driven (high β) mode. Most of the data are consistent with the theoretical predictions for the TAE gap mode. Since the high frequency modes in PBX-M, however, are found exclusively on or in the immediate neighbourhood of magnetic surfaces with low rational numbers (q = 1, 2,...), other possibilities are not excluded