ECH physics and new operational regimes on TCV

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.

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

An overview of results from the TCV tokamak

Stefano Alberti, Yanis Andrebe, Kurt Appert, Gilles Arnoux, Patrick Blanchard, Paolo Bosshard, Yann Camenen, René Chavan, Stefano Coda, Basil Duval, Pierre Etienne, Damien Fasel, Ambrogio Fasoli, Jean-Yves Favez, Ivo Furno, Timothy Goodman, Jean-Philippe Hogge, Jan Horacek, Pierre-François Isoz, Alexander Karpushov, Igor Klimanov, Pierre Lavanchy, 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, Holger Reimerdes, Olivier Sauter, Andrea Scarabosio, Edgar Scavino, Ugo Siravo, Gilbert Tonetti, Minh Quang Tran, Henri Weisen, Marco Wischmeier

The Tokamak Configuration Variable (TCV) tokamak (R = 0.88 m, a < 0.25 m, B < 1.54 T) programme is based on flexible plasma shaping and heating for studies of confinement, transport, control and power exhaust. Recent advances in fully sustained off-axis electron cyclotron current drive (ECCD) scenarios have allowed the creation of plasmas with high bootstrap fraction, steady-state reversed central shear and an electron internal transport barrier. High elongation plasmas, kappa = 2.5, are produced at low normalized current using far off-axis electron cyclotron heating and ECCD to broaden the current profile. Third harmonic heating is used to heat the plasma centre where the second harmonic is in cut-off. Both second and third harmonic heating are used to heat H-mode plasmas, at the edge and centre, respectively. The ELM frequency is decreased by the additional power. In separate experiments, the ELM frequency can be affected by locking to an external perturbation current in the internal coils of TCV. Spatially resolved current profiles are measured at the inner and outer divertor targets by Langmuir probe arrays during ELMs. The strong, reasonably balanced currents are thought to be thermoelectric in origin.

2003

Overview of TCV Results

Ambrogio Fasoli

The Tokamak `a Configuration Variable , TCV, addresses scientific questions to improve our understanding of magnetically confined plasmas and our ability to control them in ITER relevant scenarios, and explores avenues to improve the plasma performance on the way to a conceptual fusion power plant that cannot necessarily be investigated in ITER. The unique flexibility of its shaping and control systems is matched by that of its Electron Cyclotron Heating (ECH) and current drive (ECCD) systems. These include 3 MW from six 82.7 GHz gyrotrons used at the second harmonic in X-mode (X2), and 1.5 MW from three gyrotrons at 118 GHz (X3). This overview highlights the progress accomplished on TCV during the 2004–2006 campaigns, focussed on five main themes: 1) particle, energy and momentum transport in shaped plasmas, investigated over a large range of normalized temperature gradients and including peaked density profiles measured even in the absence of a Ware pinch or a core particle source; 2) plasma edge physics, addressing the question of the origin of anomalous cross-field transport in the SOL; 3) H-mode physics under strong electron heating at reactor relevant beta, e.g. using third harmonic X3 heating (1.5 MW); 4) ECH and ECCD physics, including phase space fast electron transport and electron Bernstein wave heating, demonstrated in the O-X-B scheme; 5) physics of improved steady-state tokamak regimes with internal transport barriers, with or without inductive currents, and with large (over 70%) bootstrap current fractions, confirming the key role of the current profile in the transition to improved confinement and the necessity of a negative core magnetic shear for obtaining eITBs.

2006

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

Petri Nikkola

EPFL2004