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### Ion-temperature-gradient stability near the magnetic axis of quasisymmetric stellarators

The stability of the ion-temperature gradient mode in quasisymmetric stellarators is assessed. This is performed using a set of analytical estimates together with linear gyrokinetic simulations. The peak growth rates, their corresponding real frequencies and wave-vectors are identified. A comparison is made between a first-order near-axis expansion model and eleven realistic designs obtained using numerical optimization methods. It is found that while the near-axis expansion is able to replicate the growth rates, real frequencies and perpendicular wave-vector at the inner core (both using simplified dispersion relations and first-principle gyrokinetic simulations), it leads to an overestimation of the growth rate at larger radii. An approximate analytic solution of the ITG dispersion relation for the non-resonant limit suggests growth rates could be systematically higher in quasi-axisymmetric (QA) configurations compared to quasi-helically (QH) symmetric ones. However except for very close to the axis, linear gyrokinetic simulations do not show systematic differences between QA and QH configurations.

June 2021

Plasma Physics and Controlled Fusion (link)

ArXiv ePrint 2102.12390

### The On-Axis Magnetic Well and Mercier’s Criterion for Arbitrary Stellarator Geometries

A simplified analytical form of the on-axis magnetic well and Mercier’s criterion for interchange instabilities for arbitrary three-dimensional magnetic field geometries is derived. For this purpose, a near-axis expansion based on a direct coordinate approach is used by expressing the toroidal magnetic flux in terms of powers of the radial distance to the magnetic axis. The magnetic well and Mercier’s criterion are then written as a one-dimensional integral with respect to the axis arclength. When compared with the original work of Mercier, the derivation here is presented using modern notation and in a more streamlined manner that highlights essential steps, especially for the case of vacuum fields. Finally, for the first time, this expression is verified numerically using several stellarator configurations including Wendelstein 7-X.

May 2021

Journal of Plasma Physics (link)

ArXiv ePrint 2011.07416

### The Use of Near-Axis Magnetic Fields for Stellarator Turbulence Simulations

The design of turbulence optimized stellarators has so far relied on three-dimensional equilibrium codes such as VMEC in order to find the minimum of a given objective function. In this work, we propose a complimentary approach based on the near-axis expansion to compute the geometry parameters of neoclassicaly optimized stellarators used in turbulence studies. As shown here, the near-axis expansion can be a reasonable approximation of the geometric parameters relevant for turbulence and stability simulations of the core of existing optimized stellarator designs. In particular, we examine the geometry coefficients that appear in the gyrokinetic equation, the drift-reduced fluid equations and the ideal ballooning equation. This approach may allow for the development of new stellarator optimization techniques significantly faster than conventional methods.

November 2020

Plasma Physics and Controlled Fusion (link)

ArXiv ePrint 2008.09057

### Four-dimensional drift-kinetic model for scrape-off layer plasmas

A four-dimensional plasma model able to describe the scrape-off layer region of tokamak devices at arbitrary collisionality is derived in the drift-reduced limit. The basis of the model is provided by a drift-kinetic equation that retains the full nonlinear Coulomb collision operator and describes arbitrarily far from equilibrium distribution functions. By expanding the dependence of the distribution function over the perpendicular velocity in a Laguerre polynomial basis and integrating over the perpendicular velocity, a set of four-dimensional moment equations for the expansion coefficients of the distribution function is obtained. The Coulomb collision operator as well as Poisson’s equation are evaluated explicitly in terms of perpendicular velocity moments of the distribution function.

November 2020

Physics of Plasmas (link)

ArXiv ePrint 2008.09401

### Magnetic well and Mercier stability of stellarators near the magnetic axis

We have recently demonstrated that by expanding in small distance from the magnetic axis compared to the major radius, stellarator shapes with low neoclassical transport can be generated efficiently. To extend the utility of this new design approach, here we evaluate measures of magnetohydrodynamic interchange stability within the same expansion. In particular, we evaluate magnetic well, Mercier’s criterion, and resistive interchange stability near a magnetic axis of arbitrary shape. In contrast to previous work on interchange stability near the magnetic axis, which used an expansion of the flux coordinates, here we use the inverse expansion in which the flux coordinates are the independent variables. Reduced expressions are presented for the magnetic well and stability criterion in the case of quasisymmetry. The analytic results are shown to agree with calculations from the VMEC equilibrium code. Finally, we show that near the axis, Glasser, Greene, & Johnson’s stability criterion for resistive modes approximately coincides with Mercier’s ideal condition.

October 2020

Journal of Plasma Physics (link)

ArXiv ePrint 2006.14881

### Construction of Quasisymmetric Stellarators Using a Direct Coordinate Approach

Optimized stellarator configurations and their analytical properties are obtained using a near-axis expansion approach. Such configurations are associated with good confinement as the guiding center particle trajectories and neoclassical transport are isomorphic to those in a tokamak. This makes them appealing as fusion reactor candidates. Using a direct coordinate approach, where the magnetic field and flux surface functions are found explicitly in terms of the position vector at successive orders in the distance to the axis, the set of ordinary differential equations for first and second order quasisymmetry is derived. Examples of quasi-axisymmetric shapes are constructed using a pseudospectral numerical method. Finally, the direct coordinate approach is used to independently verify two hypotheses commonly associated with quasisymmetric magnetic fields, namely that the number of equations exceeds the number of parameters at third order in the expansion and that the near-axis expansion does not prohibit exact quasisymmetry from being achieved on a single flux surface.

June 2020

Nuclear Fusion (link)

ArXiv ePrint 2003.06388

### A gyrokinetic model for the plasma periphery of tokamak devices

A gyrokinetic model is presented that can properly describe strong flows, large and small amplitude electromagnetic fluctuations occurring on scale lengths ranging from the electron Larmor radius to the equilibrium perpendicular pressure gradient scale length, and large deviations from thermal equilibrium. The formulation of the gyrokinetic model is based on a second order description of the single charged particle dynamics, derived from Lie perturbation theory, where the fast particle gyromotion is decoupled from the slow drifts, assuming that the ratio of the ion sound Larmor radius to the perpendicular equilibrium pressure scale length is small. The collective behavior of the plasma is obtained by a gyrokinetic Boltzmann equation that describes the evolution of the gyroaveraged distribution function and includes a non-linear gyrokinetic Dougherty collision operator. The gyrokinetic model is then developed into a set of coupled fluid equations referred to as the gyrokinetic moment hierarchy. To obtain this hierarchy, the gyroaveraged distribution function is expanded onto a velocity-space Hermite-Laguerre polynomial basis and the gyrokinetic equation is projected onto the same basis, obtaining the spatial and temporal evolution of the Hermite-Laguerre expansion coefficients. The Hermite-Laguerre projection is performed accurately at arbitrary perpendicular wavenumber values. Finally, the self-consistent evolution of the electromagnetic fields is described by a set of gyrokinetic Maxwell’s equations derived from a variational principle, with the velocity integrals of the gyroaveraged distribution function explicitly evaluated.

March 2020

Journal of Plasma Physics (link)

ArXiv ePrint 1904.06863

### Near-Axis Expansion of Stellarator Equilibrium at Arbitrary Order in the Distance to the Axis

A direct construction of equilibrium magnetic fields with toroidal topology at arbitrary order in the distance from the magnetic axis is carried out, yielding an analytical framework able to explore the landscape of possible magnetic flux surfaces in the vicinity of the axis. This framework can provide meaningful analytical insight on the character of high-aspect-ratio stellarator shapes, such as the dependence of the rotational transform and the plasma beta-limit on geometrical properties of the resulting flux surfaces. The approach developed here is based on an asymptotic expansion on the inverse aspect-ratio of the ideal MHD equation. The analysis is simplified by using an orthogonal coordinate system relative to the Frenet-Serret frame at the magnetic axis. The magnetic field vector, the toroidal magnetic flux, the current density, the field line label and the rotational transform are derived at arbitrary order in the expansion parameter. Moreover, a comparison with a near-axis expansion formalism employing an inverse coordinate method based on Boozer coordinates (the so-called Garren-Boozer construction) is made, where both methods are shown to agree at lowest order. Finally, as a practical example, a numerical solution using a W7-X equilibrium is presented, and a comparison between the lowest order solution and the W7-X magnetic field is performed.

January 2020

Journal of Plasma Physics (link)

ArXiv ePrint 1911.02659

### Nonlinear Gyrokinetic Coulomb Collision Operator

A gyrokinetic Coulomb collision operator is derived, which is particularly useful to describe the plasma dynamics at the periphery region of magnetic confinement fusion devices. The derived operator is able to describe collisions occurring in distribution functions arbitrarily far from equilibrium with variations on spatial scales at and below the particle Larmor radius. A multipole expansion of the Rosenbluth potentials is used in order to derive the dependence of the full Coulomb collision operator on the particle gyroangle. The full Coulomb collision operator is then expressed in gyrocentre phase-space coordinates, and a closed formula for its gyroaverage in terms of the moments of the gyrocenter distribution function in a form ready to be numerically implemented is provided. Furthermore, the collision operator is projected onto a Hermite-Laguerre velocity space polynomial basis and expansions in the small electron-to-ion mass ratio are provided.

November 2019

Journal of Plasma Physics (link)

ArXiv ePrint 1906.03252

### Linear Theory of Electron-Plasma Waves at Arbitrary Collisionality

The dynamics of electron-plasma waves is described at arbitrary collisionality by considering the full Coulomb collision operator. The description is based on a Hermite–Laguerre decomposition of the velocity dependence of the electron distribution function. The damping rate, frequency and eigenmode spectrum of electron-plasma waves are found as functions of the collision frequency and wavelength. A comparison is made between the collisionless Landau damping limit, the Lenard–Bernstein and Dougherty collision operators and the electron–ion collision operator, finding large deviations in the damping rates and eigenmode spectra. A purely damped entropy mode, characteristic of a plasma where pitch-angle scattering effects are dominant with respect to collisionless effects, is shown to emerge numerically, and its dispersion relation is analytically derived. It is shown that such a mode is absent when simplified collision operators are used, and that like-particle collisions strongly influence the damping rate of the entropy mode.

April 2019

Journal of Plasma Physics (link)

ArXiv ePrint 1811.12855

### Theory of the Drift-Wave Instability at Arbitrary Collisionality

We introduce a framework, based on the expansion of the distribution function on a Hermite-Laguerre polynomial basis, to study the effects of collisions on magnetized plasma instabilities at arbitrary mean-free path. Focusing on the drift-wave instability, we show that our framework allows retrieving established collisional and collisionless limits. At the intermediate collisionalities relevant for present and future magnetic nuclear fusion devices, deviations with respect to collision operators used in state-of-the-art turbulence simulation codes show the need for retaining the full Coulomb operator in order to obtain both the correct instability growth rate and eigenmode spectrum, with potentially important implications to the understanding of plasma turbulence. The exponential convergence of the spectral representation that we propose makes the representation of the velocity space dependence, including the full collision operator, optimally efficient.

October 2018

Physical Review letters (link)

ArXiv ePrint 1806.10538

### Quasilinear approach to ray tracing in weakly turbulent, randomly fluctuating media

Ray propagation in weakly turbulent media is described by means of a quasilinear (QL) approach in which the dispersion relation and the ray equations are expanded up to, and including, second-order terms in the medium and ray fluctuations, leading to equations for the ensemble-averaged ray and its root-mean-square (rms) spreading. An important feature of the QL formalism is that the average ray does not coincide with the zero-order, unperturbed ray but may exhibit a drift with respect to the latter that is governed by the mean squared fluctuations. The theory is complete in that equations can be set for all quantities necessary to compute the ray trajectory and the rms spreading along its path, yet they obey an infinite downward recurrence in which equations involving lower-order derivatives of the medium fluctuations are recursively generated by the subsequent higher-order derivative, and which must thus be truncated for practical purposes. Using as examples the propagation of rays in homogeneous media with fluctuations arising from the presence of either a single random mode or a multimode isotropic turbulent spectrum, the QL formalism is validated against Monte Carlo (MC) calculations and, whenever possible, its numerical implementation is verified by comparison with analytical predictions. Choosing 4% both for the level of fluctuations and for the maximum ratio between the wavelengths of the propagating ray and of the turbulent modes, so as to remain within the validity of the second-order expansion in the random perturbations and of the eikonal approximation, the overall agreement between QL and MC results is fairly good, particularly for quantities such as the distance traveled by the average ray, its perpendicular rms spread, and the averages of the wave-vector components.

August 2018

Physical Review A (link)

### A drift-kinetic analytical model for scrape-off layer plasma dynamics at arbitrary collisionality

A drift-kinetic model to describe the plasma dynamics in the scrape-off layer region of tokamak devices at arbitrary collisionality is derived. Our formulation is based on a gyroaveraged Lagrangian description of the charged particle motion, and the corresponding drift-kinetic Boltzmann equation that includes a full Coulomb collision operator. Using a Hermite–Laguerre velocity space decomposition of the gyroaveraged distribution function, a set of equations to evolve the coefficients of the expansion is presented. By evaluating explicitly the moments of the Coulomb collision operator, distribution functions arbitrarily far from equilibrium can be studied at arbitrary collisionalities. A fluid closure in the high-collisionality limit is presented, and the corresponding fluid equations are compared with previously derived fluid models.

November 2017

Journal of Plasma Physics (link)

ArXiv ePrint 1709.01411

### Superradiance of rotating cohomogeneity-1 black holes: Scalar case

Greybody factors of rotating cohomogeneity-1 black holes in higher odd dimensions are studied for the cases in which the cosmological constant is zero, positive, or negative. Attention is given to the main superradiant modes. It is shown that the increase of the intensity of the cosmological constant can have diverse effects on the maximum amplification obtained. In the case of de Sitter (dS) black holes, maximum amplification is enhanced for higher values of the cosmological constant. In the case of Anti-de Sitter (AdS) black holes, the increase of the absolute value of the cosmological constant has the effect of suppressing the maximum amplification initially, but eventually this behavior reverses and we observe growth. This phenomenon can be interpreted as contributions from amplification peaks of distinct origin that become dominant in different regimes.

November 2017

Proceedings of the The Fourteenth Marcel Grossmann Meeting (link)

### Plasma Turbulence in the Scrape-off Layer of the ISTTOK Tokamak

The properties of plasma turbulence in a poloidally limited scrape-off layer (SOL) are addressed, with focus on ISTTOK, a large aspect ratio tokamak with a circular cross section. Theoretical investigations based on the drift-reduced Braginskii equations are carried out through linear calculations and non-linear simulations, in two- and three-dimensional geometries. The linear instabilities driving turbulence and the mechanisms that set the amplitude of turbulence as well as the SOL width are identified. A clear asymmetry is shown to exist between the low-field and the high-field sides of the machine. A comparison between experimental measurements and simulation results is presented.

October 2016

Physics of Plasmas (link)

ArXiv ePrint 1606.09538

### Greybody factors for rotating black holes in higher dimensions

We perform a thorough study of greybody factors for minimally-coupled scalar fields propagating on the background of rotating black holes in higher (odd) dimensions with all angular momenta set equal. For this special case, the solution enjoys an enhanced symmetry, which translates into the advantageous feature of being cohomogeneity-1, i.e., these backgrounds depend on a single radial coordinate. Our analysis contemplates three distinct situations, with the cosmological constant being zero, positive or negative. Using the technique of matched asymptotic expansions we compute analytically the greybody factors in the low-frequency regime, restricting to s-wave scattering. Our formulas generalize those obtained previously in the literature for the static and spherically symmetric case, with corrections arising from the change in the horizon area due to rotation. It is also proven that, for this family of black holes, the horizon area is a decreasing function of the spin parameter, without regard of dimensionality and of cosmological constant. Through an improvement on a calculation specific to the class of small black holes in anti-de Sitter and not restricted to the usual low-frequency regime, we uncover a rich structure of the greybody spectrum, more complex than previously reported but also enjoying a certain degree of universality. We complement our low-frequency analytic results with numerical computations valid over a wide range of frequencies and extend them to higher angular momentum quantum numbers, \ell>0. This allows us to probe the superradiant regime that is observed for corotating wavefunctions. We point out that the maximum amplification factor for intermediate-size black holes in anti-de Sitter can be surprisingly large.

February 2015

Classical and Quantum Gravity IOP (link)

ArXiv ePrint 1410.4590

### Indefinite theta functions and black hole partition functions

We explore various aspects of supersymmetric black hole partition functions in four-dimensional toroidally compactified heterotic string theory. These functions suffer from divergences owing to the hyperbolic nature of the charge lattice in this theory, which prevents them from having well-defined modular transformation properties. In order to rectify this, we regularize these functions by converting the divergent series into indefinite theta functions, thereby obtaining fully regulated single-centered black hole partitions functions.

February 2014

Journal of High Energy Physics (link)

ArXiv ePrint 1309.4428

### Nonlinear Acoustics — Perturbation Theory and Webster’s Equation

Webster’s horn equation (1919) offers a one-dimensional approximation for low-frequency sound waves along a rigid tube with a variable cross-sectional area. It can be thought as a wave equation with a source term that takes into account the nonlinear geometry of the tube. In this document we derive this equation using a simplified fluid model of an ideal gas. By a simple change of variables, we convert it to a Schr\”odinger equation and use the well-known variational and perturbative methods to seek perturbative solutions. As an example, we apply these methods to the Gabriel’s Horn geometry, deriving the first order corrections to the linear frequency. An algorithm to the harmonic modes in any order for a general horn geometry is derived.

November 2013

ArXiv ePrint 1311.4238