Publications by Stephan Brunner

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The fluctuation short pulse reflectometry diagnostic is simulated with the full-wave CUWA code for ad-hoc turbulence and simplified geometry. The nonlinear effects on the measurements are investigated while accounting for the plasma curvature, probing angl ...

We present an extension of a linearized Coulomb collision operator, previously used in several Eulerian kinetic codes for like-species collisions and unlike-species collisions in the case where the backgrounds about which the linearization is made all are ...

Self-interaction is the process by which a microinstability eigenmode that is extended along the direction parallel to the magnetic field interacts non-linearly with itself. This effect is particularly significant in gyrokinetic simulations accounting for ...

Collisional dissipation is expected to be the primary damping mechanism for helicon waves in low-temperature, high-density, moderate magnetic field plasmas. Combining a normal-mode analysis with high-resolution magnetic field measurements, we validate this expectation across a broad parameter space. However, we identify a distinct regime where the measured helicon damping rate is reduced by nearly a factor of 2 compared with collisional predictions. In this regime, a second harmonic at 2ω is observed, contr

The dispersion properties of ion acoustic waves and their nonlinear coupling to light waves through ponderomotive and thermal forces are sensitive to the strength of electron-ion collisions. Here, with the neglect of electron-electron collisions, the growt ...

TCV experiments and novel theoretical studies have addressed key questions regarding the operational space of negative triangularity (NT) plasmas in view of an L-mode NT reactor, in terms of stability, performance properties and core-edge integration. We show that reducing the top triangularity (δtop) to more negative values induces an H-L back transition, confirming the direct dependence between H-mode existence and access to 2nd ballooning stability region. We also show that the X-point δ can prohibit H-m

Linearized operators describing inter-species and like-species collisions have been discretized and implemented in the gyrokinetic Particle-In-Cell (PIC) code ORB5 [S. Jolliet, Comp. Phys. Comm. 177, 409 ( 2007)] based on the delta-f approach. Simulation r ...

In this paper, we calculate the available energy, an upper bound on the thermal energy released that may in turn drive turbulence, due to an ion-temperature-gradient instability driven by curvature in the presence of adiabatic electrons. This is done by choosing an appropriate set of invariants that neglect parallel dynamics, whilst keeping the density profile fixed. Conditions for vanishing available energy are derived and are found to be qualitatively similar to conditions for stability derived from gyrok

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 ...

Alfven waves in fusion plasmas play an important role in a number of situations. First, in Alfven Wave Heating (AWH) schemes. Second, both theory and experiment have demonstrated the existence of Global Alfven Eigenmodes (GAEs). GAEs have been observed in different tokamaks (PRETEXT, TCA, TEXTOR, etc.) and, more recently, in a stellarator (Wendelstein 7-AS) where they were shown to become unstable under intense Neutral Beam injection. Third, the existence and possible destabilization by fast ions of Toroidi

Using the Eulerian code GENE [1], gyrokinetic simulations of microturbulence were carried out under conditions relevant to electron-Internal Transport Barriers (eITB) in the TCV tokmak [2], generated under conditions of low or negative shear. For typical density and temperature gradients measured in such barriers, the corresponding simulated fluctuation spectra appears to simultaneously contain longer wavelength Trapped Electron Modes (TEM, for typically k_perp*rho_i < 0.5, k_perp being the characteristic p

Low frequency electrostatic instabilities are investigated on TORPEX [Fasoli, Labit, McGrath, Muller, Podesta, and Poli, Bull. Am. Phys. Soc. 48, 119 (2003)], a toroidal device for basic plasma physics experiments with a toroidal magnetic field 100 mT and ...

Charge-exchange collisions and radio frequency (RF) excitation combine to give peaks in the ion energy distribution measured at the ground electrode of an argon plasma in a capacitive reactor. These peaks are used as a diagnostic to reconstruct the profile of the time-averaged potential in the sheath. Particle-in-cell (PIC) simulations show that the method is accurate. The method is applied to investigate the sheath thickness as a function of excitation frequency at constant plasma power. The time-averaged

In this work, we explore the effects of collisions on turbulent transport, focusing on two different TCV magnetic equilibria featuring positive and negative triangularity. The investigation is conducted using gyrokinetic modeling. Both gradient-driven and ...

Local nonlinear gyrokinetic simulations of tokamak plasmas demonstrate that turbulent eddies can extend along magnetic field lines for hundreds of poloidal turns when the magnetic shear is very small. By accurately modeling different field line topologies ...

Gyrokinetic codes in plasma physics need outstanding computational resources to solve increasingly complex problems, requiring the effective exploitation of cutting-edge HPC architectures. This paper focuses on the enabling of ORB5, a state-of-the-art, fir ...

We demonstrate that a symmetry of the local gyrokinetic model is broken when the safety factor q is almost (but not exactly) a rational number and magnetic shear is ŝ ≈ 0. Tokamaks with such a q profile will spontaneously rotate due to turbulent momentum transport. Nonlinear gyrokinetic simulations indicate this mechanism is significantly stronger than all other drives of intrinsic rotation. It also intrinsically transports electric current, pulling q towards rational values and potentially aiding non-induc

In a previous paper, a new gyrokinetic quasilinear model was proposed to study turbulent momentum transport in tokamaks with up-down asymmetric plasma shaping and toroidal flow shear (Sun et al 2024 Nuclear Fusion 64 036026). In this work, we start by appl ...

The particle-in-cell code ORB5 is a global gyrokinetic turbulence simulation code in tokamak geometry. It has been developed at CRPP, Lausanne, Switzerland, with major contributions from IPP, Garching, Germany, and IPP, Greifswald, Germany, under a long-st ...

Stephan Brunner

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Nonlinear Effects in Fluctuation Short Pulse Reflectometry

Linearized Coulomb Collision Operator for Simulation of Interpenetrating Plasma Streams

How eigenmode self-interaction affects zonal flows and convergence of tokamak core turbulence with toroidal system size

Impact of resonant second-harmonic generation on helicon-wave damping

The transition from thermally driven to ponderomotively driven stimulated Brillouin scattering and filamentation of light in plasma

NEGATIVE TRIANGULARITY TOKAMAK OPERATION IN TCV

Negative Triangularity Tokamak Operation in TCV

Global collisional gyrokinetic simulations of ITG microturbulence starting from a neoclassical equilibrium

On the curvature-driven ion-temperature-gradient instability and its available energy

Linear theory of electron-plasma waves at arbitrary collisionality

Alfven wave heating and stability

Gyrokinetic Simulations of Microturbulence in TCV Electron Internal Transport Barriers

Experimental characterization of drift-interchange instabilities in a simple toroidal plasma

Reconstruction of the Time-Averaged Sheath Potential Profile in an Argon Radiofrequency Plasma Using the Ion Energy-Distribution

Collisional effects on positive and negative triangularity through local and global temperature gradient-driven and flux-driven gyrokinetic simulations

Ultra long turbulent eddies, magnetic topology, and the triggering of internal transport barriers in tokamaks

Gyrokinetic simulations on many- and multi-core architectures with the global electromagnetic Particle-In-Cell Code ORB5

Intrinsic momentum transport driven by almost-rational surfaces in tokamak plasmas

A Quasilinear Model for Momentum Transport Including Rotation and Electron-scale Turbulence

Global Nonlinear Electromagnetic Simulations of Tokamak Turbulence