arXiv subjects

Ramesh Sasmal

Publications and source records attributed to Ramesh Sasmal.

3 recordsLinked to original sources

arXiv2026

Determination of turbulent heating rate and relaxed states in finite Larmor radius magnetohydrodynamic turbulence with helicity barrier

Finite Larmor radius magnetohydrodynamics (FLR-MHD) provides a hybrid model of plasma that explains how turbulent energy cascade extends to sufficiently small parallel length scales, potentially leading to perpendicular heating of the ions in the solar corona and the solar wind. In this work, we derive exact laws for the cascades of energy and generalized helicity in fully developed FLR-MHD turbulence. In large and small scale limits, we obtain the exact laws for reduced MHD and electron reduced MHD turbulence respectively. Unlike ordinary or reduced MHD turbulence, a global stationary state is shown to be absent in the case of a strong imbalance between the Elsasser variables. This is due to the so-called helicity barrier, which leads to two separate stationary energy cascades with different cascade rates. Our derived exact laws enable us to calculate these two cascade rates and therefore their difference, which effectively provides the heating rate of the ions. In addition, we also derive alternative Banerjee-Galtier forms for the exact laws and hence obtain the relaxed states of FLR-MHD turbulence using the framework of recently proposed principle of vanishing nonlinear transfer. The relaxed states show alignment between the velocity and magnetic field fluctuations. However, due to strong anisotropy, no Beltrami alignment is possible for velocity and magnetic fields. Similarly to the exact laws, the relaxed states of reduced and electron reduced MHD emerge in the large and small scale limits, respectively.

physics.plasm-ph↗

arXiv2025

Universal relations between parallel and perpendicular spectral power law exponents in non-axisymmetric magnetohydrodynamic turbulence

Following a general heuristic approach, algebraic constraints are established between the parallel and perpendicular power-law exponents of non-axisymmetric, highly aligned magnetohydrodynamic turbulence, both with and without strong imbalance between the Els\"asser variables. Such relations are universal both for the regimes of weak and strong turbulence and are useful to predict the corresponding turbulent power spectra. For scale-dependent alignment, a Boldyrev-type $k^{-3/2}$ perpendicular spectrum emerges transverse to the direction of alignment whereas a $k^{-5/3}$ spectrum is obtained for the same if the alignment becomes scale-independent. However, regardless of the nature of alignment, our analysis consistently yields a $k_{\parallel}^{-2}$ spectrum - commonly observed in both numerical simulations and in-situ data of solar wind. In appropriate limit, previously obtained algebraic relations and power spectra for axisymmetric MHD turbulence (Galtier, Pouquet and Mangeney, Physics of Plasmas, 2005) are successfully recovered. Finally, more realistic relations capturing weak Alfv\'enic turbulence (with constant $k_{\parallel}$) and the transition to strong turbulence are derived along with their corresponding power spectra.

physics.plasm-ph↗

arXiv2024

Universal cascade and relaxation of strong anisotropic turbulence in fusion plasmas

Starting from the governing equations, exact relations have been derived for three-dimensional reduced magnetohydrodynamic turbulence corresponding to the inertial range cascade of energy and cross-helicity. Justifications are provided for not attempting to recover the said exact relations as a limit of the exact relations previously derived for incompressible magnetohydrodynamic turbulence. Assuming axial symmetry, anisotropic energy spectrum has been predicted from the exact relation and is found to be consistent with the critical balance thus leading to a -5/3 perpendicular energy spectrum. In the case of a strong alignment between the velocity and the magnetic field fluctuations, the derived exact relation implies a generalized anisotropic spectrum with a -3/2 power-law dependence in the direction of alignment. Using the alternative form of the exact relations, it is shown that the flow naturally relaxes towards a state of dynamic alignment in the limit of negligible kinetic and magnetic pressure. Finally, despite having different equations of dynamics, the exact relations for energy and cross-helicity and the relaxed states of a two-dimensional MHD are found to be identical to those in reduced magnetohydrodynamic flow.

physics.plasm-ph↗