Non-Linear Gyrokinetic Simulations of Microturbulence in TCV Electron Internal Transport Barriers

Using the local (flux-tube) version of the Eulerian code GENE (Jenko et al 2000 Phys. Plasmas 7 1904), gyrokinetic simulations of microturbulence were carried out considering parameters relevant to electron-internal transport barriers (e-ITBs) in the TCV tokamak (Sauter et al 2005 Phys. Rev. Lett. 94 105002), 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 (TEMs, for typically k⊥ρi < 0.5, k⊥ being the characteristic perpendicular wavenumber and ρi the ion Larmor radius) and shorter wavelength ion temperature gradient modes (ITG, k⊥ρi > 0.5). The contributions to the electron particle flux from these two types of modes are, respectively, outward/inward and may cancel each other for experimentally realistic gradients. This mechanism may partly explain the feasibility of e-ITBs. The non-linear simulation results confirm the predictions of a previously developed quasi-linear model (Fable et al 2010 Plasma Phys. Control. Fusion 52 015007), namely that the stationary condition of zero particle flux is obtained through the competitive contributions of ITG and TEM. A quantitative comparison of the electron heat flux with experimental estimates is presented as well.

Non-linear gyrokinetic simulations of microturbulence in TCV electron internal transport barriers

Stephan Brunner, Emiliano Fable, Xavier Lapillonne, Olivier Sauter, Laurent Villard

Using the local (flux-tube) version of the Eulerian code GENE (Jenko et al 2000 Phys. Plasmas 7 1904), gyrokinetic simulations of microturbulence were carried out considering parameters relevant to electron-internal transport barriers (e-ITBs) in the TCV tokamak (Sauter et al 2005 Phys. Rev. Lett. 94 105002), 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 (TEMs, for typically k(perpendicular to)rho(i) < 0.5, k(perpendicular to) being the characteristic perpendicular wavenumber and rho(i) the ion Larmor radius) and shorter wavelength ion temperature gradient modes (ITG, k(perpendicular to)rho(i) > 0.5). The contributions to the electron particle flux from these two types of modes are, respectively, outward/inward and may cancel each other for experimentally realistic gradients. This mechanism may partly explain the feasibility of e-ITBs. The non-linear simulation results confirm the predictions of a previously developed quasi-linear model (Fable et al 2010 Plasma Phys. Control. Fusion 52 015007), namely that the stationary condition of zero particle flux is obtained through the competitive contributions of ITG and TEM. A quantitative comparison of the electron heat flux with experimental estimates is presented as well.

2011

Gyrokinetic analysis of radial dependence and global effects on the zero particle flux condition in a TCV plasma

Stephan Brunner, Alberto Mariani, Gabriele Merlo, Olivier Sauter

In small-sized tokamaks, finite Larmor radius effects could lead to a significant discrepancy between gyrokinetic local flux-tube results and global ones. This has been highlighted by previous turbulent transport studies as in McMillan et al (2010 Phys. Rev. Lett.). The impact of such effects on the zero particle flux condition is investigated here. The zero particle flux condition is useful to estimate the density peaking, reducing the uncertainty on physical input parameters derived from experimental measurements, for cases where the particle source is negligible. This constraint has been applied to the analysis of a particular TCV discharge, where a detailed reconstruction of the zero particle flux hyper-surface in the multidimensional physical parameter space at fixed radius had been presented in Mariani et al (2018 Phys. Plasmas). Here, we extend these results, investigating their radial dependence, together with the impact of global effects. These so-called rho* effects are analyzed by simulating a plasma annulus corresponding to the stiff region 0.4

IOP2019

ECH physics and new operational regimes on TCV

Stefano Alberti, Yanis Andrebe, Kurt Appert, Gilles Arnoux, Roland Behn, Patrick Blanchard, Paolo Bosshard, Yann Camenen, René Chavan, Stefano Coda, Basil Duval, Damien Fasel, Ambrogio Fasoli, Jean-Yves Favez, Timothy Goodman, Jean-Philippe Hogge, Jan Horacek, Pierre-François Isoz, Alexander Karpushov, Igor Klimanov, Jonathan Bryan Lister, Xavier Llobet, Jean-Claude Magnin, Adriano Manini, Blaise Marlétaz, Philippe Marmillod, Yves Martin, Andrei Martynov, Jean-Marc Moret, Petri Nikkola, Albert Perez, Richard Pitts, Antoine Pochelon, Laurie Porte, Olivier Sauter, Andrea Scarabosio, Edgar Scavino, Ugo Siravo, Gilbert Tonetti, Minh Quang Tran, Henri Weisen, Marco Wischmeier

The physics of tokamak plasmas, in which electrons are heated by electron cyclotron heating (ECH) and whose current is driven by electron cyclotron current drive (ECCD), is investigated in this paper together with applications on tokamak A configuration variable (TCV) using modifications of the pressure and current profiles to improve the operational regimes. In order to explain the experimentally determined current drive efficiency and hard x-ray and electron cyclotron emission measurements, it is shown that quasi-linear effects and radial transport of the suprathermal electrons are necessary. Plasmas with fully non-inductively driven currents were obtained with 0.9 MW of off-axis ECCD and 0.45 MW of on-axis counter ECCD. The combination of the driven current and the bootstrap current, accounting for 50% of the total current and peaking off-axis, yields a reversed safety factor profile and a wide and stable electron internal transport barrier. This barrier leads to an enhancement in the energy confinement by a factor of 4.5. ECH is also used to broaden the current profile of high elongation, low normalized-current plasmas whose vertical position would otherwise be uncontrollable on TCV, but whose MHD stability properties should allow high beta values. An elongation of 2.47 at a normalized-current of 1.05 MA mT(-) 1 is obtained with off-axis ECH absorbed at an optimized normalized radius between 0.55 and 0.7. Finally, third harmonic ECH is tested in various scenarios, all using vertical beam launching. In particular, high density Ohmic target and preheating with second harmonic ECH are presented. The fraction of third harmonic power absorbed reaches 65% and 85%, respectively.

2002