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J. J. Rasmussen

Publications and source records attributed to J. J. Rasmussen.

7 recordsLinked to original sources
arXiv2022

Simulations of scrape-off layer power width for EAST H-mode plasma and ITER 15 MA baseline scenario by 2D electrostatic turbulence code

The scrape-off layer power width (λ_q) is an important parameter for characterizing the divertor heat loads. Many experimental, theoretical, and numerical studies have been performed in recent years. In this paper, a 2D electrostatic turbulence code, BOUT-HESEL, has been upgraded to simulate H-mode plasmas for the first time. The code is validated against the previous implementation and the experiments. The simulated λ_q is found to agree quite well with the Eich scaling for the EAST H-mode discharge. The code is utilized to simulate the ITER 15MA baseline scenario. The ITER simulation reveals that the radial particle/heat transports are dominated by blobby transports, and predicts λ_{q,ITER} = 9.6 mm, which is much larger than the prediction by the Eich scaling. Based on the EAST modified cases, an estimated HESEL H-mode scaling, λ_q=0.51R_c^1.1B_t^(-0.3)q_95^1.1 is proposed. This scaling predicts λ_{q,ITER} = 9.3 mm, which agrees surprisingly well with that for the ITER case. A further investigation combined with the basic parameters in the database of the Eich scaling shows that the missing positive scaling dependence on the machine size (Rc) in the Eich scaling appears to be shaded by the negative scaling dependence on the toroidal magnetic field (Bt) for current devices. This is however not the case for ITER, explaining why simulations in recent studies and in this paper can reproduce the Eich scaling for current devices, but predict a much larger λ_q for ITER. According to the simulation results, the strong positive scaling dependence of λ_q on Rc is due to a combination of slowing down the parallel heat transports by increasing the parallel connection length and the enhancement of the radial ExB turbulent heat transports when the machine size is increased.

physics.plasm-ph↗
arXiv2011

Direct observation of current in type I ELM filaments on Asdex Upgrade

Magnetically confined plasmas are often subject to relaxation oscillations accompanied by large transport events. This is particularly the case for the high confinement regime of tokamaks where these events are termed edge localized modes (ELMs). They result in the temporary breakdown of the high confinement and lead to high power loads on plasma facing components. Present theories of ELM generation rely on a combined effect of edge currents and the edge pressure gradient exceeding critical values which result in intermediate mode number ($n \cong 10-15$) structures (\emph{filaments}) localized in the perpendicular plane and extended along the field line. It is shown here by detailed localized measurements of the three components of the magnetic field perturbation carried by an individual type I ELM filament that these filaments carry a substantial current. The observations provide essential information for the understanding of ELM dynamics.

physics.plasm-ph↗
arXiv2007

E-pile model of self-organized criticality

The concept of percolation is combined with a self-consistent treatment of the interaction between the dynamics on a lattice and the external drive. Such a treatment can provide a mechanism by which the system evolves to criticality without fine tuning, thus offering a route to self-organized criticality (SOC) which in many cases is more natural than the weak random drive combined with boundary loss/dissipation as used in standard sand-pile formulations. We introduce a new metaphor, the e-pile model, and a formalism for electric conduction in random media to compute critical exponents for such a system. Variations of the model apply to a number of other physical problems, such as electric plasma discharges, dielectric relaxation, and the dynamics of the Earth's magnetotail.

cond-mat.stat-mech↗
arXiv2003

Toroidal bubbles with circulation in ideal hydrodynamics. A variational approach

Incompressible, inviscid, irrotational, and unsteady flows with circulation $Γ$ around a distorted toroidal bubble are considered. A general variational principle that determines the evolution of the bubble shape is formulated. For a two-dimensional (2D) cavity with a constant area $A$, exact pseudo-differential equations of motion are derived, based on variables that determine a conformal mapping of the unit circle exterior into the region occupied by the fluid. A closed expression for the Hamiltonian of the 2D system in terms of canonical variables is obtained. Stability of a stationary drifting 2D hollow vortex is demonstrated, when the circulation is relatively large, $gA^{3/2}/Γ^2\ll 1$. For a circulation-dominated regime of three-dimensional flows a simplified Lagrangian is suggested, inasmuch as the bubble shape is well described by the center-line $\g{R}(ξ,t)$ and by an approximately circular cross-section with relatively small area, $A(ξ,t)\ll (\oint |\g{R}'|dξ)^2$. In particular, a finite-dimensional dynamical system is derived and approximately solved for a vertically moving axisymmetric vortex ring bubble with a compressed gas inside.

physics.flu-dyn↗
arXiv2001

Finite time singularities in a class of hydrodynamic models

Models of inviscid incompressible fluid are considered, with the kinetic energy (i.e., the Lagrangian functional) taking the form ${\cal L}\sim\int k^α|{\bf v_k}|^2d^3{\bf k}$ in 3D Fourier representation, where $α$ is a constant, $0<α< 1$. Unlike the case $α=0$ (the usual Eulerian hydrodynamics), a finite value of $α$ results in a finite energy for a singular, frozen-in vortex filament. This property allows us to study the dynamics of such filaments without the necessity of a regularization procedure for short length scales. The linear analysis of small symmetrical deviations from a stationary solution is performed for a pair of anti-parallel vortex filaments and an analog of the Crow instability is found at small wave-numbers. A local approximate Hamiltonian is obtained for the nonlinear long-scale dynamics of this system. Self-similar solutions of the corresponding equations are found analytically. They describe the formation of a finite time singularity, with all length scales decreasing like $(t^*-t)^{1/(2-α)}$, where $t^*$ is the singularity time.

physics.flu-dyn↗
arXiv2001

Dynamics of Solitons and Quasisolitons of Cubic Third-Order Nonlinear Schrödinger Equation

The dynamics of soliton and quasisoliton solutions of cubic third order nonlinear Schrödinger equation is studied. The regular solitons exist due to a balance between the nonlinear terms and (linear) third order dispersion; they are not important at small $α_3$ ($α_3$ is the coefficient in the third derivative term) and vanish at $α_3 \to 0$. The most essential, at small $α_3$, is a quasisoliton emitting resonant radiation (resonantly radiating soliton). Its relationship with the other (steady) quasisoliton, called embedded soliton, is studied analytically and in numerical experiments. It is demonstrated that the resonantly radiating solitons emerge in the course of nonlinear evolution, which shows their physical significance.

nlin.SI↗