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Register a new userSimulating Gyrokinetic Microinstabilities in Stellarator Geometry with GS2
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Added by
Jessica Baumgaertel
Publication date
2011
fields
Physics
and research's language is
English
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The nonlinear gyrokinetic code GS2 has been extended to treat non-axisymmetric stellarator geometry. Electromagnetic perturbations and multiple trapped particle regions are allowed. Here, linear, collisionless, electrostatic simulations of the quasi-axisymmetric, three-field period National Compact Stellarator Experiment (NCSX) design QAS3-C82 have been successfully benchmarked against the eigenvalue code FULL. Quantitatively, the linear stability calculations of GS2 and FULL agree to within ~10%.
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An analytic equilibrium, the Toroidal Bessel Function Model, is used in conjunction with the gyrokinetic code GYRO to investigate the nature of microinstabilities in a reversed field pinch (RFP) plasma. The effect of the normalized electron plasma pressure ({beta}) on the characteristics of the microinstabilities is studied. A transition between an ion temperature gradient (ITG) driven mode and a microtearing mode as the dominant instability is found to occur at a {beta} value of approximately 4.5%. Suppression of the ITG mode occurs as in the tokamak, through coupling to shear Alfven waves, with a critical {beta} for stability higher than its tokamak equivalent due to a shorter parallel connection length. There is a steep dependence of the microtearing growth rate on temperature gradient suggesting high profile stiffness. There is evidence for a collisionless microtearing mode. The properties of this mode are investigated, and it is found that curvature drift plays an important role in the instability.
A linear gyrokinetic particle-in-cell scheme, which is valid for arbitrary perpendicular wavelength $k_perprho_i$ and includes the parallel dynamic along the field line, is developed to study the local electrostatic drift modes in point and ring dipole plasmas. We find the most unstable mode in this system can be either electron mode or ion mode. The properties and relations of these modes are studied in detail as a function of $k_perprho_i$, the density gradient $kappa_n$, the temperature gradient $kappa_T$, electron to ion temperature ratio $tau=T_e/T_i$, and mass ratio $m_i/m_e$. For conventional weak gradient parameters, the mode is on ground state (with eigenstate number $l=0$) and especially $k_parallelsim0$ for small $k_perprho_i$. Thus, bounce averaged dispersion relation is also derived for comparison. For strong gradient and large $k_perprho_i$, most interestingly, higher order eigenstate modes with even (e.g., $l=2,4$) or odd (e.g., $l=1$) parity can be most unstable, which is not expected by previous studies. High order eigenstate can also easily be most unstable at weak gradient when $tau>10$. This work can be particularly important to understand the turbulent transport in laboratory and space magnetosphere.
The capability to model the nonlinear magnetohydrodynamic (MHD) evolution of stellarator plasmas is developed by extending the M3D-$C^1$ code to allow non-axisymmetric domain geometry. We introduce a set of logical coordinates, in which the computational domain is axisymmetric, to utilize the existing finite-element framework of M3D-$C^1$. A $C^1$ coordinate mapping connects the logical domain to the non-axisymmetric physical domain, where we use the M3D-$C^1$ extended MHD models essentially without modifications. We present several numerical verifications on the implementation of this approach, including simulations of the heating, destabilization, and equilibration of stellarator plasmas with strongly anisotropic thermal conductivity, and of the relaxation of stellarator equilibria to integrable and non-integrable magnetic field configurations in realistic geometries.
We study the effect of turbulent transport in different magnetic configurations of the Weldenstein 7-X stellarator. In particular, we performed direct numerical simulations with the global gyrokinetic code GENE-3D, modeling the behavior of Ion Temperature Gradient turbulence in the Standard, High-Mirror, and Low-Mirror configurations of W7-X. We found that the Low-Mirror configuration produces more transport than both the High-Mirror and the Standard configurations. By comparison with radially local simulations, we have demonstrated the importance of performing global nonlinear simulations to predict the turbulent fluxes quantitatively.
The quasilinear particle flux arising from gyrokinetic instabilities is calculated in the electrostatic and collisionless approximation, keeping the geometry of the magnetic field arbitrary. In particular, the flux of electrons and heavy impurity ions is studied in the limit where the former move quickly, and the latter slowly, along the field compared with the mode frequency. Conclusions are drawn about how the particle fluxes of these species depend on the magnetic-field geometry, mode structure and frequency of the instability. Under some conditions, such as everywhere favourable or unfavourable magnetic curvature and modest temperature gradients, it is possible to make general statements about the fluxes independently of the details of the instability. In quasi-isodynamic stellarators with favourable bounce-averaged curvature for most particles, the particle flux is always outward if the temperature gradient is not too large, suggesting that it might be difficult to fuel such devices with gas puffing from the wall. In devices with predominantly unfavourable magnetic curvature, the particle flux can be inward, resulting in spontaneous density peaking in the centre of the plasma. In the limit of highly charged impurities, ordinary diffusion (proportional to the density gradient) dominates over other transport channels and the diffusion coefficient becomes independent of mass and charge. An estimate for the level of transport caused by magnetic-field fluctuations arising from ion-temperature-gradient instabilities is also given and is shown to be small compared with the electrostatic component.
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