Integration concept of an Electron Cyclotron System in DEMO

The pre-conceptual layout for an electron cyclotron system (ECS) in DEMO is described. The present DEMO ECS considers only equatorial ports for both plasma heating and neoclassical tearing mode (NTM) control. This differs from ITER, where four launchers in upper oblique ports are dedicated to NTM control and one equatorial EC port for heating and current drive (H&CD) purposes as basic configuration. Rather than upper oblique ports, DEMO has upper vertical ports to allow the vertical removal of the large breeding blanket segments. While ITER is using front steering antennas for NTM control, in DEMO the antennas are recessed behind the breeding blanket and called mid-steering antennas, referred to the radially recessed position to the breeding blanket. In the DEMO pre-conceptual design phase two variants are studied to integrate the ECS in equatorial ports. The first option integrates waveguide bundles at four vertical levels inside EC port plugs with antennas with fixed and movable mid-steering mirrors that are powered by gyrotrons, operating at minimum two different multiples of the fundamental resonance frequency of the microwave output window. Alternatively, the second option integrates fixed antenna launchers connected to frequency step-tunable gyrotrons. The first variant is described in this paper, introducing the design and functional requirements, presenting the equatorial port allocation, the port plug design including its maintenance concept, the basic port cell layout, the transmission line system with diamond windows from the tokamak up to the RF building and the gyrotron sources. The ECS design studies are supported by neutronic and tokamak integration studies, quasi-optical and plasma physics studies, which will be summarized. Physics and technological gaps will be discussed and an outlook to future work will be given.

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

Physics Insight from Plasma Shaping of TCV Tokamak Plasmas - focus on Electron Heat Transport

Antoine Pochelon

In the search of a fusion reactor using magnetic confinement of toroidal plasmas, many important plasma performance parameters directly depend on the shape of the plasma cross-section. The unique shaping capability of the TCV tokamak ("Tokamak à Configuration Variable") has been exploited to study different aspects of tokamak transport and stability, in which the plasma shape may play a role. This presentation will focus on the effects of plasma shape on transport. The TCV tokamak has produced a host of physics results on diverse topics such as core sawtooth instabilities, edge localised modes (ELMs), electron cyclotron heating (ECH) and current drive (CD), plasma control, MHD stability, internal transport barriers (ITB), innovative “snowflake” divertors. TCV can produce plasmas with extreme shapes, elongation κ up to 2.8 and both negative and positive triangularity δ from –0.8 to +1, and is equipped with 4.5MW of localized Electron Cyclotron Heating (ECH). A significant fraction of TCV results has benefited from plasma shaping. The characterization of the various effects of shaping serves essentially two purposes: 1) it provides stringent tests for theory, being thus a valuable tool for model validation, and 2) it is crucial for the design of a fusion reactor, since plasma shape has a marked influence on confinement, MHD stability and performance. As an example, confinement in TCV has been demonstrated to depend strongly on triangularity in low-collisionality L-mode plasmas, improving towards negative triangularity. For illustration, the heat flux necessary to sustain the same profiles and stored energy in a discharge with δ=-0.4 is only half of that required for a discharge with δ=+0.4. The observed dependences of the electron thermal diffusivity on triangularity (direct) and collisionality (inverse) are qualitatively reproduced by nonlinear gyro-kinetic simulations and shown to be governed by Trapped Electron Mode (TEM) turbulence. Both in the linear and non-linear phases, negative triangularity is found to have a stabilizing influence on the TEM by modifying the toroidal precessional drift of trapped electrons that resonantly drive the modes. Negative triangularity also enhances the local shear, which in turn increases the perpendicular wave number, thus reducing the mixing length transport estimate. Both heat transport measurements and gyro-kinetic simulations demonstrate the stabilising effect of electron-ion collisions, which is now further sustained by recent electron temperature turbulence measurements using correlation-ECE.

2009

Collisions in Global Gyrokinetic Simulations of Tokamak Plasmas using the Delta-f Particle-In-Cell Approach

Thibaut Vernay

The present work takes place within the general context of research related to the development of nuclear fusion energy. More specifically, this thesis is mainly a numerical and physical contribution to the understanding of turbulence and associated transport phenomena occuring in tokamak plasmas, the most advanced and promising form of magnetically confined plasmas. The complexity of tokamak plasma phenomena and related physical models, either fluid or kinetic, requires the development of numerical codes to perform simulations of the plasma behaviour under given conditions defined by the magnetic geometry as well as density and temperature profiles. The studies presented in this work are based on electrostatic kinetic simulations, taking advantage of a reduced kinetic model (the gyrokinetic model) which is particularly suitable for studying turbulent transport in magnetically confined plasmas, in effect solving an approximate form of the Vlasov equation for the distribution function of each species (electrons, ions) along with a reduced form of the Poisson equation providing the self-consistent electric fields. The main tool of this work, the gyrokinetic ORB5 code making use of numerical particles according to the Particle-In-Cell (PIC) method, has been upgraded during this thesis with different linearized collision operators related to both ions and electrons. The BIRDIE code, enabling to study collisional effects on the evolution of Langmuir waves in an unmagnetized plasma, has been written in order to serve as a test-bed for the collision operators ultimately implemented in ORB5. Some essential algorithms related to collisional simulations have been jointly implemented, such as the two-weight scheme which is extensively described in this work. The collision operators in ORB5 have been further carefully tested through neoclassical simu- lations and benchmarked against other codes, providing reliable levels of collisional transport. Together with different procedures controlling the numerical noise, the collision operators have then been applied to the study of collisional turbulent transport in two different regimes, the Ion-Temperature-Gradient (ITG) regime and the Trapped-Electron-Mode (TEM) regime re- quiring a trapped electron kinetic response. Although not dominant in core tokamak plasmas, collisional effects nevertheless lead to interesting modifications in the turbulence behaviour which are not captured by the often considered collisionless gyrokinetic models. The so-called coarse-graining procedure, a noise-control algorithm which is suitable for collisional gyrokinetic simulations with particles, is shown to enable carrying out relevant simulations over many col- lision times. Consequently, reliable conclusions regarding turbulent transport in the presence of collisions could be drawn in this thesis. Namely, the turbulent transport in the ITG regime is found to be enhanced by ion collisions through interactions with so-called zonal flows as- sociated to axisymmetric modes, while it is reduced by electron collisions in the TEM regime through electron detrapping processes. The zonal flow dynamics in collisionless and collisional ITG turbulence simulations is studied, emphasizing the limitation of the zonal flow level due to Kelvin-Helmoltz-type instabilities. Additionally, some purely collisionless issues related to tokamak physics are discussed, such as the finite plasma size effects in TEM-dominated regime which are found to be important in non-linear simulations but unimportant in linear simu- lations. The role of zonal flows in temperature-gradient-driven TEM turbulence saturation is confirmed to be weak, in agreement with previous studies. Finally, a realistic global gy- rokinetic simulation, accounting for a proper TCV tokamak magnetic equilibrium and related experimental profiles, has been successfully carried out thus demonstrating the relevance of the ORB5 code for predictions related to physics of real tokamaks. A good agreement with GAM experimental measurements is indeed obtained.