The modeling of a Tokamak plasma discharge, from first principles to a flight simulator

A newly developed tool to simulate a tokamak full--discharge is presented. The tokamak "flight--simulator" Fenix couples the tokamak control system with a fast and reduced plasma model, yet realistic enough to take into account several of the plasma non--linearities. Distinguishing feature of this modeling tool is that it only requires the Pulse Schedule (PS) as input to the simulator. The output is a virtual realization of the full discharge, which time traces can then be used to judge if the PS satisfies control/physics goals or needs to be revised. This tool is thought for routine use in the control--room before each pulse is performed, but can also be used off--line to correct PS in advance, or to develop and validate reduced models, control schemes, and in general the simulation framework.

Studies on impurity seeding and transport in edge and SOL of tokamak plasma

We present numerical simulation studies on impurity seeding using Nitrogen, Neon, and Argon gases. These impurity gases are ionized by the electron impact ionization. These ions can be at multiply ionized states, recombine again with the plasma electrons, and radiate energy. The radiation losses are estimated using a non-coronal equilibrium model. A set of 2D model equations to describe their self-consistent evolution are derived using interchange plasma turbulence in the edge and SOL regions and solved using BOUT++. It is found that impurity ions (with single or double-positive charges) move in the inward direction with a velocity ∼ 0.02cs so that these fluxes are negative. These fluxes are analyzed for different strengths of an effective gravity that help to understand the impurity ion dynamics. Increased gravity shows an accumulation of certain charged species in the edge region. The radiation loss is seen to have a fluctuation in time with frequency 5-20 kHz that closely follows the behavior of the interchange plasma turbulence. The simulation results on the radiated power and its frequency spectrum compare favourably with observations on the Aditya-U tokamak. The negative fluxes of the impurity ions, their dynamics in the edge region, and the fluctuating nature of the radiation loss are the most important results of this work.

Halo current rotation scaling in post-disruption plasmas

Halo current (HC) rotation during disruptions can be potentially dangerous if resonant with the structures surrounding a tokamak plasma. We propose a drift-frequency-based scaling law for the rotation frequency of the asymmetric component of the HC as a function of toroidal field strength and plasma minor radius (frot ∝ 1/BT a2 ). This scaling law is consistent with results reported for many tokamaks and is motivated by the faster HC rotation observed in the HBT-EP tokamak. Projection of the rotation frequency to ITER and SPARC parameters suggest the asymmetric HC rotation will be on the order of 10 Hz and 60 Hz, respectively.

Interaction between energetic-ions and internal kink modes in a weak shear tokamak plasma

Based on the conventional tokamak HL-2A-like parameters and profiles, the linear properties and the nonlinear dynamics of non-resonant kink mode (NRK) and non-resonant fishbone instability (NRFB) in reversed shear tokamak plasmas are investigated by using the global hybrid kinetic-magnetohydrodynamic (MHD) nonlinear code M3D-K. This work mainly focuses on the effect of passing energetic-ions on the NRK and NRFB instabilities, which is different from the previous works. It is demonstrated that the NRFB can be destabilized by the passing energetic-ions when the energetic-ion beta $\beta_h$ exceeds a critical value. The transition from NRK to NRFB occurs when the energetic-ion beta $\beta_h$ increases to above a critical value. The resonance condition responsible for the excitation of NRFB is interestingly found to be satisfied at $\omega_t+\omega_p\approx\omega$, where $\omega_t$ is the toroidal motion frequency, $\omega_p$ is the poloidal motion frequency and $\omega$ is the mode frequency. The nonlinear evolutions of NRFB's mode structures and Poincar\'{e} plots are also analyzed in this work and it is found that the NRFB can induce evident energetic-ion loss/redistribution, which can degrade the performance of the plasmas. These findings are conducive to understanding the mechanisms of NRFB-induced energetic-ion loss/redistribution through nonlinear wave-particle interaction.

Heat flux effect in ηi-mode driven solitary and shock waves in electron-positron-ion plasma

The specific role of ion heat flux on the characteristics of the linear and nonlinear ion temperature gradient (ηi) driven mode in inhomogeneous electron-positron-ion plasma is presented. Inhomogeneity in density, temperature, and the magnetic field is considered. A modified linear dispersion relation is obtained, and its different limiting cases are when ηi 2/3, ωD(gradient in magnetic field) = 0 and β(density ratio of plasma species) = 1 are discussed. Furthermore, an expression for the anomalous transport coefficient of the present model is obtained. Nonlinear structure solutions in the form of solitons and shocks show that mode dynamics enhance in the presence of ion heat flux in electron-positron-ion plasma. The present study is essential in energy confinement devices such as tokamak because the heat flux observed experimentally in tokamak plasma is much higher than those described by collisions. Further, it could be helpful to understand the nonlinear electrostatic excitations in the interstellar medium.

Low-frequency zonal flow eigen-structure in tokamak plasmas

The nonlocal eigenmode analysis of low-frequency zonal flows in toroidally rotating tokamak plasmas is performed in the framework of the reduced one-fluid ideal MHD-model. It is shown that for typical profiles of plasma parameters toroidal plasma rotation results in the global zonal flow formation on the periphery of plasma column. For some types of equilibria these zonal flows are aperiodically unstable that leads to the excitation of the differential plasma rotation at the tokamak plasma edge.

Tomotok: python package for tomography of tokamak plasma radiation

A python package, called Tomotok, focused on performing tomographic inversion of tokamak plasma radiation is being developed at the Institute of Plasma Physics of the Czech Academy of Sciences. It aims at providing multiple inversion algorithms with an user friendly interface. In order to enable and ease performing tomographic inversion on different devices worldwide, it is planned to publish this software as open source in the near future. In this contribution, the package structure allowing an easy implementation of various tokamak and diagnostic geometries is described and an overview of the package contents is given. Apart from inversion methods, overview of Tomotok auxiliary content is given. The package provides tools for creating simple synthetic diagnostic system. These can be used for testing and benchmarking the code. This includes tools for building geometry matrices that describe the view of detectors using single line of sight approximation and artificial data generators capable of creating simple or hollow Gaussian profiles. The implemented inversion methods cover the minimum Fisher regularisation, biorthogonal decomposition and linear algebraic methods. The implementation of each method is explained, example results obtained by inverting phantom models are presented and discussed. The computation speed of implemented algorithms is benchmarked and compared.

Experimental validation of universal plasma blob formation mechanism

Abstract Anomalous plasma transport in the boundary region of a tokamak plasma is commonly associated with the formation and evolution of coherent density structures known as blobs. Recently, a theory for a universal mechanism of plasma blob formation has been put forward. It is based on a breaking process of a radially elongated streamer due to poloidal and radial velocity shears. The theory is well supported by two-dimensional and three-dimensional numerical simulation results but lacks experimental validation. In this work, we report the first ever experimental validation of this universal criterion by testing it against NSTX data on blobs obtained using the gas-puff imaging (GPI) diagnostic. It is found that the criterion is widely satisfied in most L-mode discharges and may explain the significantly larger number of blob events. We also validate the theoretical criterion against ADITYA Langmuir probe data taken in the scrape-off layer region.

Analytical edge power loss at the lower hybrid resonance: ANTITER IV validation and application to ion cyclotron resonance heating systems

In the ion cyclotron range of frequency (ICRF), the presence of a lower hybrid (LH) resonance can appear in the edge of a tokamak plasma and lead to deleterious edge power depositions. An analytic formula for these losses is derived in the cold plasma approximation and for a slab geometry using an asymptotic approach and an analytical continuation near the LH resonance. The way to minimize these losses in a large machine like ITER is discussed. An internal verification between the power loss computed with the semi-analytical code ANTITER IV for ion cyclotron resonance heating (ICRH) and the analytic result is performed. This allows us to check the precision of the numerical integration of the singular set of cold plasma wave differential equations. The set of cold plasma equations used is general and can be applied in other parameters domain.