Steady-state fully noninductive current driven by electron cyclotron waves in a magnetically confined plasma

A steady-state, fully noninductive plasma current has been sustained for the first time in a tokamak using electron cyclotron current drive only. In this discharge, 123 kA of current have been sustained for the entire gyrotron pulse duration of 2 s. Careful distribution across the plasma miner radius of the power deposited from three 0.5-MW gyrotrons was essential for reaching steady-state conditions. With central current drive, up to 153 kA of current have been fully replaced transiently for 100 ms. The noninductive scenario is confirmed by the ability to recharge the Ohmic transformer The dependence of the current drive efficiency on the minor radius is also demonstrated.

The effect of electron transport on electron cyclotron current drive in tokamak plasmas

Petri Nikkola

The operation of the most promising concept for the future fusion rector, tokamak, relies on the generation of plasma current. Traditionally, this is created by magnetic induction which makes the tokamak operation pulsed. With electro-magnetic waves, in the range of the electron cyclotron (EC) frequency of the plasma electrons, current can be driven non-inductively. This provides the possibility of a continuous tokamak operation. In the TCV tokamak, a fully non-inductive operation is a routine experiment. This allows not only the possibility to study the non-inductive tokamak operation, but also the EC wave driven current is easily measurable. In addition, the plasma confinement has been observed to improve when the EC waves drive current in certain locations inside the plasma. This mode of operation is called the internal transport barrier (ITB) and it has been attainable in TCV since 2002. It is assumed that a specific shape of the current density profile causes the ITB to form. However, in TCV no measurement of the current density profile is available. Theoretically, the linear modeling of EC wave driven current fails in some cases to reproduce the experimentally observed current. In addition, TCV is expected to operate on the quasilinear regime. Yet the predicted quasilinear enhancement in the efficiency of the EC wave driven current is not observed, the simulated current being too large by a factor up to ten. These facts suggest that in traditional modeling some important mechanism is missing. For these reasons, modeling experiments where the plasma current is driven by EC waves is an interesting challenge. In this thesis we assume that radial transport of electrons is the missing part in the models. Transport acts as a partially linearizing mechanism in bringing the theoretical current to the observed level. Radial transport also causes current diffusion, affecting strongly the current density profile. Various experiments including non-inductive operation, plasmas with a varying fraction of inductive current, and the ITB mode have been modeled with this technique. The simulated results are consistent with respect to many measured plasma parameters: the driven current, the total plasma energy, the temperature profile, and the observed hard X-ray spectra. In addition, the transport level is in agreement with the experimental limit. Also, in ITB plasmas, the resulting current density profile is in agreement with the theories of ITB formation. These results show that radial transport has a major role in EC wave driven current experiments in TCV. Therefore, only quasilinear simulations taking into account radial transport effects provide the correct experimental results in all cases in TCV.

EPFL2004

Bootstrap current compensation with electron-cyclotron waves in 3D reactor-size configurations

Sergi Ferrando I Margalet

In magnetic confinement devices, the inhomogeneity of the confining magnetic field along a magnetic field line generates the trapping of particles (with low ratio of parallel to perpendicular velocities) within local magnetic wells. One of the consequences of the trapped particles is the generation of a current, known as the bootstrap current (BC), whose direction depends on the nature of the magnetic trapping. The BC provides an extra contribution to the poloidal component of the confining magnetic field. The variation of the poloidal component produces the alteration of the winding of the magnetic field lines around the flux surfaces quantified by the rotational transform ι. When ι reaches low rational values, it can trigger the generation of ideal MHD instabilities. Therefore, the BC may be responsible for the destabilisation of the configuration. This thesis is divided into two parts. In the first part, we present a self-consistent method to calculate the BC and assess its effect on equilibrium and stability in general 3D configurations. This procedure is applied to two reactor-size prototypes (both with plasma volumes ∼ 1000m3): a quasi-axisymmetric (QAS) system and a quasi helically symmetric (QHS) system with magnetic structures that develop BC in opposite directions. The BC increases with the plasma pressure, therefore its relevance is enhanced when dealing with reactor-level scenarios. The behaviour of both prototypes at reactor level values of β ≡ (kinetic plasma pressure)/(magnetic pressure) is assessed, as well as its alteration of the equilibrium and stability. In the QAS prototype, BC-consistent equilibria have been computed up to β = 6.7% and the configuration is shown to be stable up to β = 6.4%. Convergence of self-consistent BC calculations for the QHS case is achieved only up to β = 3.5%, but the configuration is unstable for β ≥ 0.6%. The relevance of symmetry breaking modes of the Fourier expansion of the confining magnetic field on the generation of BC is studied for each prototype. This proves the close relationship between magnetic structure and BC. Having established the potentially dangerous implication of the BC, principally, in reactor prototypes, a method to compensate its harmful effects is proposed in the second part of the thesis. It consists of the modelling of the current driven by externally launched ECWs within the plasma to compensate the effects of the BC. This method is flexible enough to allow the identification of the appropriate scenarios in which to generate the required CD depending on the nature of the confining magnetic field and the specific plasma parameters of the configuration. Both the BC and the CD calculations are included in a self-consistent scheme which leads to the computation of a stable BC+CD-consistent MHD equilibrium. This procedure is applied in this thesis to simulate the required CD to stabilise the QAS and QHS prototypes introduced in the first part. The estimation of the input power required and the effect of the driven current on the final equilibrium of the system is performed for several relevant scenarios and wave polarisations providing various options of stabilising driven currents. Several scenarios have been devised for each prototype in order to drive current at the appropriate location and with the desired direction. Different polarisations and launching conditions have been employed to this purpose. In particular, a HFS launched X2 ECW with an input power of 1.5MW has been shown to drive sufficient current to maintain the rotational transform below the critical value 2/3 at β = 6.4% for the QAS reactor. Correspondingly, in the QHS reactor, an X3-mode ECW of 100KW was sufficient to drive the current required to push the rotational transform below unity near the magnetic axis at β = 3%. Thus, stabilisation of BC-driven instabilities with externally launched ECWs has been achieved for both contrasting configurations. The method proposed in this thesis allows also the utilisation of EBW in the generation of CD. The possible advantages of EBCD for compensation studies are described as well as their possible application to the two prototypes under consideration. The BC+CD procedure is particularly interesting to investigate new magnetic geometries as potential candidates for fusion reactors. With this numerical tool, it is possible to assess the implications of their consistent BC when operating at reactor level. It also allows to quantify how much power would be required to maintain the system MHD stable in these circumstances. Nevertheless, this method is flexible enough to be applicable to any configuration.

EPFL2006

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.