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Register a new userGlobal linear and nonlinear gyrokinetic modelling of Alfven eigenmodes in ITER
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Thomas Hayward-Schneider
Publication date
2020
fields
Physics
and research's language is
English
Authors
T. Hayward-Schneider
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Linear and nonlinear modelling of Alfvenic instabilities, most notably toroidal Alfven eigenmodes (TAEs), obtained by using the global nonlinear electromagnetic gyrokinetic model of the code ORB5 are presented for the 15 MA scenario of the ITER tokamak. Linear simulations show that elliptic Alfven eigenmodes and odd-parity TAEs are only weakly damped but not excited by alpha particles, whose drive favours even-parity TAEs. Low mode number TAEs are found to be global, requiring global treatment. Nonlinearly, even with double the nominal EP density, single mode simulations lead to saturation with negligible EP transport however multi-mode simulations predict that with double the nominal EP density, enhanced saturation and significant EP redistribution will occur.
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Recent upgrades in H-1 power supplies have enabled the operation of the H-1 experiment at higher heating powers than previously attainable. A heating power scan in mixed hydrogen/helium plasmas reveals a change in mode activity with increasing heating power. At low power (<50 kW) modes with beta-induced Alfven eigenmode (BAE) frequency scaling are observed. At higher power modes consistent with an analysis of nonconventional Global Alfven Eigenmodes (GAEs) are observed, the subject of this work. We have computed the mode continuum, and identified GAE structures using the ideal MHD solver CKA and the gyrokinetic code EUTERPE. An analytic model for ICRH-heated minority ions is used to estimate the fast ion temperature from the hydrogen species. Linear growth rate scans using a local flux surface stability calculation, LGRO, are performed. These studies demonstrate growth from circulating particles whose speed is significantly less than the Alfven speed, and are resonant with the mode through harmonics of the Fourier decomposition of the strongly-shaped heliac magnetic field. They reveal drive is possible with a small, hot energetic tail of the hydrogen species. Local linear growth rate scans are also complemented with global calculations from CKA and EUTERPE. These qualitatively confirm the findings from the LGRO study, and show that the inclusion of finite Larmor radius effects can reduce the growth rate by a factor of three, but do not affect marginal stability. Finally, a study of damping of the global mode with the thermal plasma is conducted, computing continuum, and the damping arising from parallel electric fields. We find that continuum damping is of order 0.1% for the configuration studied. The inclusion of resistivity lifts the damping to 19%. Such large damping is consistent with experimental observations that in absence of drive the mode decays rapidly (~0.1 ms).
Gyrokinetic theory of nonlinear mode coupling as a mechanism for toroidal Alfven eigenmode (TAE) saturation in the fusion plasma related parameter regime is presented, including 1) para- metric decay of TAE into lower kinetic TAE (LKTAE) and geodesic acoustic mode (GAM), and 2) enhanced TAE coupling to shear Alfven wave (SAW) continuum via ion induced scattering. Our theory shows that, for TAE saturation in the parameter range of practical interest, several processes with comparable scattering cross sections can be equally important.
Nonlinear saturation of toroidal Alfven eigenmode (TAE) via ion induced scatterings is investigated in the short-wavelength gyrokinetic regime. It is found that the nonlinear evolution depends on the thermal ion b{eta} value. Here, b{eta} is the plasma thermal to magnetic pressure ratio. Both the saturation levels and associated energetic-particle transport coefficients are derived and estimated correspondingly.
Spontaneous nonlinear excitation of geodesic acoustic mode (GAM) by toroidal Alfven eigenmode (TAE) is investigated using nonlinear gyrokinetic theory. It is found that, the nonlinear decay process depends on thermal ion beta value. Here, beta is the plasma thermal to magnetic pressure ratio. In the low-beta limit, TAE decays into a GAM and a lower TAE sideband in the toroidicity induced shear Alfven wave continuous spectrum gap; while in the high-beta limit, TAE decays into a GAM and a propagating kinetic TAE in the continuum. Both cases are investigated for the spontaneous decay conditions. The nonlinear saturation levels of both GAM and daughter wave are derived. The corresponding power balance and wave particle power transfer to thermal plasma are computed. Implications on thermal plasma heating are also discussed.
This paper presents a study of the interaction between Alfven modes and zonal structures, considering a realistic ASDEX Upgrade equilibrium. The results of gyrokinetic simulations with the global, electromagnetic, particle-in-cell code ORB5 are presented, where the modes are driven unstable by energetic particles with a bump-on-tail equilibrium distribution function, with radial density gradient. Two regimes have been observed: at low energetic particles concentration, the Alfven mode saturates at much higher level in presence of zonal structures; on the other hand at high energetic particles concentration the difference is less pronounced. The former regime is characterized by the zonal structure (identified as an energetic particle driven geodesic acoustic mode), being more unstable than the Alfven mode. In the latter regime the Alfven mode is more unstable than the zonal structure. The theoretical explanation is given in terms of a 3-wave coupling of the energetic particle driven geodesic acoustic mode and Alfven mode, mediated by the curvature-pressure coupling term of the energetic particles.
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