R. Ganesh

Gyrokinetic simulations of turbulent transport: size scaling and chaotic behaviour

Important steps towards the understanding of turbulent transport have been made with the development of the gyrokinetic framework for describing turbulence and with the emergence of numerical codes able to solve the set of gyrokinetic equations. This paper presents some of the main recent advances in gyrokinetic theory and computing of turbulence. Solving 5D gyrokinetic equations requires state-of-the-art high performance computing techniques involving massively parallel computers and parallel scalable algorithms. The various numerical schemes that have been explored until now, Lagrangian, Eulerian and semi-Lagrangian, each have their advantages and drawbacks. A past controversy regarding the finite size effect (finite ρ∗) in ITG turbulence has now been resolved. It has triggered an intensive benchmarking effort and careful examination of the convergence properties of the different numerical approaches. Now, both Eulerian and Lagrangian global codes are shown to agree and to converge to the flux-tube result in the ρ∗ → 0 limit. It is found, however, that an appropriate treatment of geometrical terms is necessary: inconsistent approximations that are sometimes used can lead to important discrepancies. Turbulent processes are characterized by a chaotic behaviour, often accompanied by bursts and avalanches. Performing ensemble averages of statistically independent simulations, starting from different initial conditions, is presented as a way to assess the intrinsic variability of turbulent fluxes and obtain reliable estimates of the standard deviation. Further developments

Intrinsic toroidal and poloidal flow generation in the background of ion temperature gradient turbulence

The generation of intrinsic toroidal and poloidal flows in the background of ion temperature gradient (ITG) driven microturbulence has been studied. It is shown that the dynamics of mean toroidal and poloidal flows is coupled. The radial fluxes of toroidal and poloidal momentum have been derived. It is shown that the polarization drift driven toroidal momentum flux is independent of mean flow shear and hence complements the shear driven k∥ symmetry breaking mechanism (Gurcan et al 2007 Phys. Plasmas 14 042306) of intrinsic rotation. The radial flux of poloidal momentum due to polarization drift is found to vanish at the steady state. Comparison of residual toroidal and poloidal momentum fluxes, in the absence of seed flows, shows that toroidal flow dominates over poloidal flow.

Global gyrokinetic stability of collisionless microtearing modes in large aspect ratio tokamaks

Linear full radius gyrokinetic calculations show the existence of unstable microtearing modes (MTMs) in purely collisionless, high temperature, large aspect ratio tokamak plasmas. The present study takes into account fully gyrokinetic highly passing ions and electrons. The global 2-D structures of the collisionless mode with full radius coupling of the poloidal modes is obtained and compared with another electromagnetic mode, namely, the Alfven Ion Temperature Gradient (AITG) mode (or Kinetic Ballooning Mode, KBM) for the same equilibrium profile. Several important characteristics of the modes are brought out and compared, such as a clear signature in the symmetry properties of the two modes, the plasma–β dependence, and radial and poloidal length scales of the electrostatic and magnetic vector potential fluctuations. Extensive parameter scans for this collisionless microtearing mode reveal the scaling of the growth rate with β and the electron temperature gradient ηe . Scans at different β values show an inverse relationship between the ηe threshold and β, leading to a stability diagram, and implying that the mode might exist at moderate to strong temperature gradients for finite β plasmas in large aspect ratio tokamaks. In contrast to small aspect ratio tokamaks where the trapped electron magnetic drift resonance is found to be important, in large aspect ratio tokamaks, a strong destabilization due to the magnetic drift resonance of passing electrons is observed and is identified as a possible collisionless drive mechanism for the collisionless MTM.

A comprehensive gyrokinetic description of global electrostatic microinstabilities in a tokamak

It is believed that low frequency microinstabilities such as ion temperature gradient (ITG) driven modes and trapped electron modes (TEMs) are largely responsible for the experimentally observed anomalous transport via the ion and electron channels in a tokamak. In the present work, a comprehensive global linear gyrokinetic model incorporating fully kinetic (trapped and passing) electrons and ions, actual ion to electron mass ratio, radial coupling, and profile variation is used to investigate the ITG driven modes and pure TEMs. These modes are found to exhibit multiscale structures in the presence of nonadiabatic passing electrons. The multiscale structure is related to the large nonadiabaticity of electrons in the vicinity of mode rational magnetic surfaces and leads to reduced mixing length estimates of transport compared to those obtained from adiabatic electron models.

Finite ballooning angle effects on ion temperature gradient driven mode in gyrokinetic flux tube simulations

This paper presents effects of finite ballooning angles on linear ion temperature gradient (ITG) driven mode and associated heat and momentum flux in Gyrokinetic flux tube simulation GENE. It is found that zero ballooning angle is not always the one at which the linear growth rate is maximum. The ITG mode acquires a short wavelength (SW) branch (k⊥ρi > 1) when growth rates maximized over all ballooning angles are considered. However, the SW branch disappears on reducing temperature gradient showing characteristics of zero ballooning angle SWITG in case of extremely high temperature gradient. Associated heat flux is even with respect to ballooning angle and maximizes at nonzero ballooning angle while the parallel momentum flux is odd with respect to the ballooning angle.

Nature of energetic ion transport by ion temperature gradient driven turbulence and size scaling

Energetic ion transport has been studied using a global gyrokinetic nonlinear simulation in the presence of ion temperature gradient (ITG) driven turbulence. The measured transport and its nature show dependence on the system size of the tokamak expressed as the ratio of plasma minor radius (a) to the thermal ion Larmor radius (ρi). It increases with system size initially and then tends to saturate at larger system size. The nature of transport, on the other hand, exhibits nondiffusive character for smaller system size which eventually becomes diffusive one as the system size becomes larger.

Radial transport of energetic ions in the presence of trapped electron mode turbulence

The nature of transport of hot ions is studied in the presence of microturbulence generated by the trapped electron mode in a Tokamak using massively parallel, first principle based global nonlinear gyrokinetic simulation, and with the help of a passive tracer method. Passing and trapped hot ions are observed to exhibit inverse and inverse square scaling with energy, while those with isotropic pitch distribution are found to exhibit inverse dependence on energy. For all types of hot ions, namely, isotropic, passing, and trapped, the radial transport appears to be subdiffusive for the parameters considered.

Toroidal universal drift instability: A global gyrokinetic study

An electron density gradient driven instability identified as the toroidal branch of the universal drift instability is studied using a global gyrokinetic model treating both electrons and ions fully nonadiabatically and valid at all orders in the ratio of the Larmor radius to the wavelength. The physics of the magnetic drift resonance, Landau resonance and transit resonance, which are considered to be important for the toroidal universal mode, are kept for both species. A systematic parametric study is carried out for the mode. The toroidal universal drift mode is observed to sustain finite temperature gradient and can thus coexist with the temperature gradient driven modes and may contribute to the observed particle transport along with other drift modes. Especially at intermediate scales between the ion temperature gradient driven mode and electron temperature gradient driven mode, this branch of the drift instability can also be a plausible candidate for the observed particle loss. The effect of magne...

Short wavelength ion temperature gradient mode and coupling with trapped electrons

The effect of trapped electrons on the ion temperature gradient (ITG) mode in a regime where its wavelength is shorter than the conventional ITG mode (k⊥ρLi≤1) has been studied. Such a mode propagates in the ion diamagnetic direction with a typical scale length k⊥ρLi⪢1 and is termed as the short wavelength ITG (SWITG) mode. The effect of the trapped electrons on this SWITG mode is investigated, for the first time, using a global and local linear gyrokinetic model. The trapped electrons are observed to destabilize the mode strongly. Comparison of the various parameter scans for the SWITG mode with and without the trapped electrons is presented. One important result obtained is that, while in the absence of the trapped electrons the mode was found to subside with increasing value of ϵn=Ln/R exhibiting the character of a slablike mode, the presence of the trapped electrons has been observed to enhance the ϵn=Ln/R window of the existence of the SWITG mode making the mode more toroidal like.

A simple experimental method to determine magnetic field topology in toroidal plasma devices.

Estimation of the parallel wavenumber in plasma devices finds wide applications such as determining the nature of instabilities. This task is often challenging, especially in toroidal magnetic configurations. In the present work, a simple yet effective method of achieving accurate probe-alignment along the magnetic field lines is demonstrated in a simple magnetized toroidal device BETA (Basic Experiments in Toroidal Assembly). The alignment was achieved by aligning each probe to a tiny localized plasma source. Such an alignment is necessary for determining the parallel wavenumber precisely. The probe-alignment was confirmed further from the measurements in the plasma and the corresponding parallel wavenumber was found to be in good agreement with the analytical predictions.