The Experts below are selected from a list of 192 Experts worldwide ranked by ideXlab platform
Stefano Coda - One of the best experts on this subject based on the ideXlab platform.
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A Variable Structure Control Scheme Proposal for the Tokamak à Configuration Variable
Complexity, 2019Co-Authors: Aitor Marco, Stefano Coda, Izaskun Garrido, Aitor J. Garrido, Tcv TeamAbstract:Fusion power is the most significant prospects in the long-term future of energy in the sense that it composes a potentially clean, cheap, and unlimited power source that would substitute the widespread traditional nonrenewable energies, reducing the geographical dependence on their sources as well as avoiding collateral environmental impacts. Although the nuclear fusion research started in the earlier part of century and the fusion reactors have been developed since the 1950s, the fusion reaction processes achieved have not yet obtained net power, since the generated plasma requires more energy to achieve and remain in necessary particular pressure and temperature conditions than the produced profitable energy. For this purpose, the plasma has to be confined inside a vacuum vessel, as it is the case of the Tokamak reactor, which consists of a device that generates magnetic fields within a toroidal chamber, being one of the most promising solutions nowadays. However, the Tokamak reactors still have several issues such as the presence of plasma instabilities that provokes a decay of the fusion reaction and, consequently, a reduction in the pulse duration. In this sense, since long pulse reactions are the key to produce net power, the use of robust and fast controllers arises as a useful tool to deal with the unpredictability and the small time constant of the plasma behavior. In this context, this article focuses on the application of robust control laws to improve the controllability of the plasma current, a crucial parameter during the plasma heating and confinement processes. In particular, a Variable structure control scheme based on sliding surfaces, namely, a sliding mode controller (SMC) is presented and applied to the plasma current control problem. In order to test the validity and goodness of the proposed controller, its behavior is compared to that of the traditional PID schemes applied in these systems, using the RZIp model for the Tokamak a Configuration Variable (TCV) reactor. The obtained results are very promising, leading to consider this controller as a strong candidate to enhance the performance of the PID-based controllers usually employed in this kind of systems.
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WAC - Sliding Surface Based Schemes for the Tokamak à Configuration Variable
2018 World Automation Congress (WAC), 2018Co-Authors: Aitor Marco, Stefano Coda, Izaskun Garrido, Aitor J. Garrido, Tcv TeamAbstract:Fusion power may be seen as the energy of the future in the sense that it composes a potentially clean, cheap and unlimited power source that would reduce the worldwide dependency on non-renewable energies. Nevertheless, while nowadays the fusion reaction process itself has been achieved, significant net power has not yet been obtained, since the generated plasma needs to remain in particular pressure and temperature conditions. For this purpose, the plasma has to be confined. To do so, one of the solutions is to use a fusion reactor device that creates magnetic fields in a toroidal chamber, called Tokamak reactor. The main issue of Tokamak reactors is the presence of plasma instabilities, which provoke the fusion reaction decay and, in consequence, a reduction in the pulse duration. To maintain this pulse duration as long as possible, the use of robust and fast controllers is mandatory due to the unpredictability and the small time constant of the plasma behavior. In this context, this article focuses on improving the controllability of the plasma current, a relevant control Variable, crucial during the plasma heating and confinement processes. In particular, two new robust control schemes based on sliding surfaces, namely, a Sliding Mode Controller (SMC) and a Supertwisting Controller (STC) are presented and applied to the plasma current control problem. In order to test the validity and goodness of the proposed controllers, their behavior is compared to that of the traditional PID schemes applied in these systems, using the RZIp model for the TCV (Tokamak a Configuration Variable) reactor. The obtained results are very promising, leading to consider these controllers as strong candidates to improve the performance of the PID-based controllers usually employed in this kind of systems.
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Real Time Hybrid Model Predictive Control for the Current Profile of the Tokamak à Configuration Variable (TCV)
Energies, 2016Co-Authors: Izaskun Garrido, Stefano Coda, Aitor J. Garrido, J.-m. MoretAbstract:Plasma stability is one of the obstacles in the path to the successful operation of fusion devices. Numerical control-oriented codes as it is the case of the widely accepted RZIp may be used within Tokamak simulations. The novelty of this article relies in the hierarchical development of a dynamic control loop. It is based on a current profile Model Predictive Control (MPC) algorithm within a multiloop structure, where a MPC is developed at each step so as to improve the Proportional Integral Derivative (PID) global scheme. The inner control loop is composed of a PID-based controller that acts over the Multiple Input Multiple Output (MIMO) system resulting from the RZIp plasma model of the Tokamak a Configuration Variable (TCV). The coefficients of this PID controller are initially tuned using an eigenmode reduction over the passive structure model. The control action corresponding to the state of interest is then optimized in the outer MPC loop. For the sake of comparison, both the traditionally used PID global controller as well as the multiloop enhanced MPC are applied to the same TCV shot. The results show that the proposed control algorithm presents a superior performance over the conventional PID algorithm in terms of convergence. Furthermore, this enhanced MPC algorithm contributes to extend the discharge length and to overcome the limited power availability restrictions that hinder the performance of advanced tokamaks.
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On the non-stiffness of edge transport in L-mode tokamak plasmasa)
Physics of Plasmas, 2014Co-Authors: O. Sauter, B. P. Duval, Stefano Coda, R. Behn, Y. Camenen, Stephan Brunner, D. Kim, G. Merlo, L. Federspiel, Timothy GoodmanAbstract:Transport analyses using first-principle turbulence codes and 11/2-D transport codes usually study radial transport properties between the tokamak plasma magnetic axis and a normalized minor radius around 0.8. In this region, heat transport shows significantly stiff properties resulting in temperature scalelength values (R/L-T) that are relatively independent of the level of the radial heat flux. We have studied experimentally in the tokamak a Configuration Variable [F. Hofmann et al., Plasma Phys. Controlled Fusion 36, B277 (1994)] the radial electron transport properties of the edge region, close to the last closed flux surface, namely, between rho(V) = root V/V-edge = 0.8 to 1. It is shown that electron transport is not stiff in this region and high R/L-Te values (similar to 20-40) can be attained even for L-mode confinement. We can define a "pedestal" location, already in L-mode regimes, where the transport characteristics change from constant logarithmic gradient, inside rho(V) = 0.8, to constant gradient between 0.8 and 1.0. In particular, we demonstrate, with well resolved T-e and n(e) profiles, that the confinement improvement with plasma current I-p, with or without auxiliary heating, is due to this non-stiff edge region. This new result is used to explain the significant confinement improvement observed with negative triangularity, which could not be explained by theory to date. Preliminary local gyrokinetic simulations are now consistent with an edge, less stiff, region that is more sensitive to triangularity than further inside. We also show that increasing the electron cyclotron heating power increases the edge temperature inverse scalelength, in contrast to the value in the main plasma region. The dependence of confinement on density in ohmic plasmas is also studied and brings new insight in the understanding of the transition between linear and saturated confinement regimes, as well as of the density limit and appearance of a 2/1 tearing mode. The results presented in this paper provide an important new perspective with regards to radial transport in tokamak plasmas which goes beyond L-mode plasmas and explains some previous puzzling results. It is proposed that understanding the transport properties in this edge non-stiff region will also help in understanding the improved and high confinement edge properties. (C) 2014 AIP Publishing LLC.
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Current density evolution in electron internal transport barrier discharges in TCV
Plasma Physics and Controlled Fusion, 2008Co-Authors: C. Zucca, E. Fable, O. Sauter, Stefano Coda, Timothy Goodman, E. Asp, M. A. HendersonAbstract:Simulations of the plasma current density evolution in electron internal transport barrier discharges on the Tokamak a Configuration Variable (TCV) have been performed, in order to determine the relationship between the safety-factor profile and the electron transport improvement. The results show that the formation of the transport barrier is correlated with the shear reversal in all cases studied, regardless of the different heating and current drive schemes. No indications were found of discrete effects related to low-order rational q surfaces. On the contrary, the increase in confinement along with the negative shear is gradual, but constant, indicating that the transition is smooth, although it can be very fast.
Tcv Team - One of the best experts on this subject based on the ideXlab platform.
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Manufacturing, installation, commissioning, and first results with the 3D low-temperature co-fired ceramic high-frequency magnetic sensors on the Tokamak à Configuration Variable.
The Review of scientific instruments, 2020Co-Authors: Duccio Testa, Eurofusion Mst Team, Tcv TeamAbstract:Innovative high-frequency magnetic sensors have been designed and manufactured in-house for installation on the Tokamak a Configuration Variable (TCV), which are now routinely operational during the TCV experimental campaigns. These sensors combine the Low Temperature Co-fired Ceramic (LTCC) and the classical thick-film technologies and are in various aspects similar to the majority of the in-vessel inductive magnetic sensors foreseen for ITER (around 450 out of the 505 currently being procured are of the LTCC-1D type). The TCV LTCC-3D magnetic sensors provide measurements in the frequency range up to 1 MHz of the perturbations to the wall-aligned toroidal (δBTOR), vertical (δBVER), and radial (δBRAD) magnetic field components. Knowledge of the equilibrium at the last closed flux-surface allows us to then obtain the field-aligned parallel (δBPAR ∼ δBTOR), poloidal (δBPOL), and normal (δBNOR) components, the latter being in most cases rather different from the vertical and radial components, respectively. The main design principles were aimed at increasing the effective area and reducing the self-inductance of the sensor in each of the three measurement axes, which are centered at the same position on each sensor, while reducing the mutual and parasitic coupling between them by optimizing the on-board wiring. The physics requirements are set by the installation of two high-power/high-energy neutral beam injection systems on TCV, i.e., studying fast ions physics, coherent instabilities, and turbulence in the (super-)Alfvenic frequency range. In this paper, we report the manufacturing, installation, and commissioning work for these high-frequency LTCC-3D magnetic sensors and conclude with an overview of illustrative experimental results obtained with this system. The LTCC-3D data provide new insights into the δBPOL coherent (eigenmodes, up to ∼400 kHz) and in-coherent background turbulent fluctuations in the higher frequency range up to ∼1 MHz, which were not previously available with the TCV Mirnov sensors. Furthermore, the LTCC-3D δBPOL measurements allow us to cross-check the data obtained with the standard Mirnov coils and have led to the identification of largeelectromagnetic (EM) noise pick-up for the Mirnov data acquisition (DAQ). When the sources of EM noise pick-up on the Mirnov DAQ are removed, the LTCC-3D data for δBPOL are in good overall agreement, i.e., within the expected measurement uncertainties, with those obtained with the standard Mirnov sensors located at the same poloidal position in the frequency range where the respective data acquisition overlap, routinely up to 125 kHz and up to 250 kHz in some discharges. The LTCC-3D δBPAR measurements (not previously available in TCV or elsewhere) provide evidence that certain instabilities have a finite parallel δB at the wall, hence at the LCFS, consistent with the recent theoretical results for pressure-driven modes. The LTCC-3D δBNOR measurements improve significantly on the corresponding measurements with the saddle loops, which are mounted onto the wall and have a bandwidth of ∼3 kHz (due to the wall penetration time). A detailed end-to-end system modeling tool has been developed and applied to test on the simulated data the actual measurement capabilities of this new diagnostic system and obtain the ensuing estimates of the intrinsic measurement uncertainties. A detailed error analysis is then performed so that, finally, fully calibrated, absolute measurements of the frequency-dependent amplitude and spectral breaks of coherent eigenmodes and in-coherent broadband magnetic fluctuations are provided for the first time in physical units with quantitative uncertainties.
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A Variable Structure Control Scheme Proposal for the Tokamak à Configuration Variable
Complexity, 2019Co-Authors: Aitor Marco, Stefano Coda, Izaskun Garrido, Aitor J. Garrido, Tcv TeamAbstract:Fusion power is the most significant prospects in the long-term future of energy in the sense that it composes a potentially clean, cheap, and unlimited power source that would substitute the widespread traditional nonrenewable energies, reducing the geographical dependence on their sources as well as avoiding collateral environmental impacts. Although the nuclear fusion research started in the earlier part of century and the fusion reactors have been developed since the 1950s, the fusion reaction processes achieved have not yet obtained net power, since the generated plasma requires more energy to achieve and remain in necessary particular pressure and temperature conditions than the produced profitable energy. For this purpose, the plasma has to be confined inside a vacuum vessel, as it is the case of the Tokamak reactor, which consists of a device that generates magnetic fields within a toroidal chamber, being one of the most promising solutions nowadays. However, the Tokamak reactors still have several issues such as the presence of plasma instabilities that provokes a decay of the fusion reaction and, consequently, a reduction in the pulse duration. In this sense, since long pulse reactions are the key to produce net power, the use of robust and fast controllers arises as a useful tool to deal with the unpredictability and the small time constant of the plasma behavior. In this context, this article focuses on the application of robust control laws to improve the controllability of the plasma current, a crucial parameter during the plasma heating and confinement processes. In particular, a Variable structure control scheme based on sliding surfaces, namely, a sliding mode controller (SMC) is presented and applied to the plasma current control problem. In order to test the validity and goodness of the proposed controller, its behavior is compared to that of the traditional PID schemes applied in these systems, using the RZIp model for the Tokamak a Configuration Variable (TCV) reactor. The obtained results are very promising, leading to consider this controller as a strong candidate to enhance the performance of the PID-based controllers usually employed in this kind of systems.
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Langmuir probe electronics upgrade on the tokamak à Configuration Variable
The Review of scientific instruments, 2019Co-Authors: H. De Oliveira, B. Marletaz, P. Marmillod, Christian Theiler, René Chavan, Olivier Février, Benoit Labit, P. Lavanchy, R.a. Pitts, Tcv TeamAbstract:A detailed description of the Langmuir probe electronics upgrade for TCV (Tokamak a Configuration Variable) is presented. The number of amplifiers and corresponding electronics has been increased from 48 to 120 in order to simultaneously connect all of the 114 Langmuir probes currently mounted in the TCV divertor and main-wall tiles. Another set of 108 amplifiers is ready to be installed in order to connect 80 new probes, built in the frame of the TCV divertor upgrade. Technical details of the amplifier circuitry are discussed as well as improvements over the first generation of amplifiers developed at SPC (formerly CRPP) in 1993/1994 and over the second generation developed in 2012/2013. While the new amplifiers have been operated successfully for over a year, it was found that their silicon power transistors can be damaged during some off-normal plasma events. Possible solutions are discussed.
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WAC - Sliding Surface Based Schemes for the Tokamak à Configuration Variable
2018 World Automation Congress (WAC), 2018Co-Authors: Aitor Marco, Stefano Coda, Izaskun Garrido, Aitor J. Garrido, Tcv TeamAbstract:Fusion power may be seen as the energy of the future in the sense that it composes a potentially clean, cheap and unlimited power source that would reduce the worldwide dependency on non-renewable energies. Nevertheless, while nowadays the fusion reaction process itself has been achieved, significant net power has not yet been obtained, since the generated plasma needs to remain in particular pressure and temperature conditions. For this purpose, the plasma has to be confined. To do so, one of the solutions is to use a fusion reactor device that creates magnetic fields in a toroidal chamber, called Tokamak reactor. The main issue of Tokamak reactors is the presence of plasma instabilities, which provoke the fusion reaction decay and, in consequence, a reduction in the pulse duration. To maintain this pulse duration as long as possible, the use of robust and fast controllers is mandatory due to the unpredictability and the small time constant of the plasma behavior. In this context, this article focuses on improving the controllability of the plasma current, a relevant control Variable, crucial during the plasma heating and confinement processes. In particular, two new robust control schemes based on sliding surfaces, namely, a Sliding Mode Controller (SMC) and a Supertwisting Controller (STC) are presented and applied to the plasma current control problem. In order to test the validity and goodness of the proposed controllers, their behavior is compared to that of the traditional PID schemes applied in these systems, using the RZIp model for the TCV (Tokamak a Configuration Variable) reactor. The obtained results are very promising, leading to consider these controllers as strong candidates to improve the performance of the PID-based controllers usually employed in this kind of systems.
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X-Point-Position-Dependent Intrinsic Toroidal Rotation in the Edge of ă the TCV Tokamak
Physical Review Letters, 2015Co-Authors: T. Stoltzfus-dueck, A. N. Karpushov, Olivier Sauter, B. P. ă Duval, B. Labit, Holger Reimerdes, W. A. J. Vijvers, Y. ă Camenen, Tcv TeamAbstract:Edge intrinsic rotation was investigated in Ohmic L-mode discharges on ă the Tokamak a Configuration Variable, scanning the major radial position ă of the X point, R-X. Edge rotation decreased linearly with increasing ă RX, vanishing or becoming countercurrent for an outboard X point, in ă agreement with theoretical expectations. The core rotation profile ă shifted fairly rigidly with the edge rotation, changing the central ă rotation speed by more than a factor of two. Core rotation reversals had ă little effect on the edge rotation velocity. Edge rotation was modestly ă more countercurrent in unfavorable than favorable del B shots.
M. A. Henderson - One of the best experts on this subject based on the ideXlab platform.
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Current density evolution in electron internal transport barrier discharges in TCV
Plasma Physics and Controlled Fusion, 2008Co-Authors: C. Zucca, E. Fable, O. Sauter, Stefano Coda, Timothy Goodman, E. Asp, M. A. HendersonAbstract:Simulations of the plasma current density evolution in electron internal transport barrier discharges on the Tokamak a Configuration Variable (TCV) have been performed, in order to determine the relationship between the safety-factor profile and the electron transport improvement. The results show that the formation of the transport barrier is correlated with the shear reversal in all cases studied, regardless of the different heating and current drive schemes. No indications were found of discrete effects related to low-order rational q surfaces. On the contrary, the increase in confinement along with the negative shear is gradual, but constant, indicating that the transition is smooth, although it can be very fast.
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High-bootstrap, noninductively sustained electron internal transport barriers in the Tokamak a Configuration Variable
Physics of Plasmas, 2005Co-Authors: Stefano Coda, E. Fable, O. Sauter, M. A. Henderson, R. Behn, T. P. Goodman, Alberto Bottino, Y. Camenen, An. Martynov, P. NikkolaAbstract:Important ingredients of the advanced-tokamak route to fusion have been explored in depth in the Tokamak a Configuration Variable [F. Hofmann, J. B. Lister, M. Anton , Plasma Phys. Controlled Fusion 36, B277 (1994)] over the past two years. Using a uniquely powerful and flexible electron-cyclotron resonance heating (ECRH) system as the primary actuator, fully noninductive, steady-state electron internal transport barrier discharges have been generated with an electron-energy confinement time up to five times longer than in L mode, poloidal beta up to 2.4, and bootstrap fraction up to 75%. Interpretative transport modeling confirms that the safety factor profile is nonmonotonic in these discharges. The formation of the barrier is a discrete event resulting in rapid and localized confinement improvement consistent with the time and location of magnetic-shear reversal. In steady state, however, the confinement quality appears to depend on the current gradient in a broader negative-shear region enclosed by the barrier, improving with increasing shear: in particular, the width and depth of the barrier can be controlled and finely tuned, along a magnetohydrodynamic-stable path, by manipulating the current profile with ECRH (six independently steerable 0.45 MW launchers). The crucial role of the current profile has been clearly demonstrated by applying small Ohmic current perturbations which dramatically alter the properties of the barrier, enhancing or reducing the confinement with negative and positive current, respectively, with negligible Ohmic heating. These results are in agreement with theoretical estimates: first-principle-based numerical simulations of microinstability dynamics and turbulence-driven transport predict a substantial suppression of turbulence and anomalous energy diffusivity near the location of the minimum in the safety factor. (c) 2005 American Institute of Physics.
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Recent results from the electron cyclotron heated plasmas in Tokamak à Configuration Variable (TCV)
Physics of Plasmas, 2003Co-Authors: M. A. Henderson, Stefano Coda, Stefano Alberti, P. Blanchard, R. Behn, C. Angioni, Y. Camenen, G. Arnoux, P. Bosshard, I. CondreaAbstract:In noninductively driven discharges, 0.9 MW second harmonic (X2) off-axis co-electron cyclotron current drive deposition is combined with 0.45 MW X2 central heating to create an electron internal transport barrier (eITB) in steady plasma conditions resulting in a 1.6-fold increase of the confinement time (τEe) over ITER-98L-mode scaling. The eITB is associated with a reversed shear current profile enhanced by a large bootstrap current fraction (up to 80%) and is sustained for up to 10 current redistribution times. A linear dependence of the confinement improvement on the product of the global shear reversal factor (q0/qmin) and the reversed shear volume (ρq-min2) is shown. In other discharges heated with X2 the sawteeth are destabilized (respectively stabilized) when heating just inside (respectively outside) the q=1 surface. Control of the sawteeth may allow the avoidance of neoclassical tearing modes that can be seeded by the sawtooth instability. Results on H-mode and highly elongated plasmas using the...
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Steady-state fully noninductive operation with electron cyclotron current drive and current profile control in the tokamak à Configuration Variable (TCV)
Physics of Plasmas, 2001Co-Authors: O. Sauter, Stefano Coda, M. A. Henderson, F. Hofmann, Jean-marc Moret, C. Angioni, J.-p. Hogge, T. P. Goodman, P. Gomez, P. NikkolaAbstract:Fully noninductive, steady-state electron cyclotron current drive (ECCD) has been demonstrated for the first time in experiments carried out in the tokamak a Configuration Variable (TCV) [O. Sauter et al., Phys. Rev. Lett. 84, 3322 (2000)]. By appropriately distributing six 0.45 MW ECCD sources over the discharge cross section, fully noninductive, stable, and stationary plasmas with Ip up to 210 kA were obtained for the full discharge duration of 1.9 s, corresponding to more than 900 energy confinement times and more than 10 current redistribution times at an average current drive efficiency η20CD=0.01[1020 A W−1 m−2]. These experiments have also demonstrated for the first time the steady recharging of the ohmic transformer using ECCD only. The effect of localized off-axis electron cyclotron heating (ECH) and EC current drive (ECCD) (co- and counter-) is investigated showing that locally driven currents amounting to only 1% of Ip significantly alter sawtooth periods and crash amplitudes. An improved quasi...
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Central electron temperature enhancements due to sawtooth stabilization during counter electron cyclotron current drive in Tokamak à Configuration Variable
Physics of Plasmas, 2000Co-Authors: Z.a. Pietrzyk, Stefano Coda, Timothy Goodman, M. A. Henderson, F. Hofmann, Jean-marc Moret, R. Behn, C. Angioni, J.-p. Hogge, A. PochelonAbstract:In the Tokamak a Configuration Variable (TCV), the central electron temperature obtained in discharges with counter (CNTR) electron cyclotron current drive (ECCD) is larger than with CO-ECCD or electron cyclotron resonance heating (ECRH) alone. Comparison of experimental results with calculations by the transport code PRETOR [IAEA Technical Conference on Advances in Simulation and Models of Thermonuclear Plasmas. Montreal 142 (International Atomic Energy Agency, Vienna 1992)] indicates that sawtooth stabilization is responsible for the increased confinement time and the attendant twofold enhancement of the central temperature. Sawtooth stabilization is caused in turn by the central safety factor q0 rising above 1 for CNTR-ECCD; by contrast, the simulation results show that q0
O. Sauter - One of the best experts on this subject based on the ideXlab platform.
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On the non-stiffness of edge transport in L-mode tokamak plasmasa)
Physics of Plasmas, 2014Co-Authors: O. Sauter, B. P. Duval, Stefano Coda, R. Behn, Y. Camenen, Stephan Brunner, D. Kim, G. Merlo, L. Federspiel, Timothy GoodmanAbstract:Transport analyses using first-principle turbulence codes and 11/2-D transport codes usually study radial transport properties between the tokamak plasma magnetic axis and a normalized minor radius around 0.8. In this region, heat transport shows significantly stiff properties resulting in temperature scalelength values (R/L-T) that are relatively independent of the level of the radial heat flux. We have studied experimentally in the tokamak a Configuration Variable [F. Hofmann et al., Plasma Phys. Controlled Fusion 36, B277 (1994)] the radial electron transport properties of the edge region, close to the last closed flux surface, namely, between rho(V) = root V/V-edge = 0.8 to 1. It is shown that electron transport is not stiff in this region and high R/L-Te values (similar to 20-40) can be attained even for L-mode confinement. We can define a "pedestal" location, already in L-mode regimes, where the transport characteristics change from constant logarithmic gradient, inside rho(V) = 0.8, to constant gradient between 0.8 and 1.0. In particular, we demonstrate, with well resolved T-e and n(e) profiles, that the confinement improvement with plasma current I-p, with or without auxiliary heating, is due to this non-stiff edge region. This new result is used to explain the significant confinement improvement observed with negative triangularity, which could not be explained by theory to date. Preliminary local gyrokinetic simulations are now consistent with an edge, less stiff, region that is more sensitive to triangularity than further inside. We also show that increasing the electron cyclotron heating power increases the edge temperature inverse scalelength, in contrast to the value in the main plasma region. The dependence of confinement on density in ohmic plasmas is also studied and brings new insight in the understanding of the transition between linear and saturated confinement regimes, as well as of the density limit and appearance of a 2/1 tearing mode. The results presented in this paper provide an important new perspective with regards to radial transport in tokamak plasmas which goes beyond L-mode plasmas and explains some previous puzzling results. It is proposed that understanding the transport properties in this edge non-stiff region will also help in understanding the improved and high confinement edge properties. (C) 2014 AIP Publishing LLC.
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Current density evolution in electron internal transport barrier discharges in TCV
Plasma Physics and Controlled Fusion, 2008Co-Authors: C. Zucca, E. Fable, O. Sauter, Stefano Coda, Timothy Goodman, E. Asp, M. A. HendersonAbstract:Simulations of the plasma current density evolution in electron internal transport barrier discharges on the Tokamak a Configuration Variable (TCV) have been performed, in order to determine the relationship between the safety-factor profile and the electron transport improvement. The results show that the formation of the transport barrier is correlated with the shear reversal in all cases studied, regardless of the different heating and current drive schemes. No indications were found of discrete effects related to low-order rational q surfaces. On the contrary, the increase in confinement along with the negative shear is gradual, but constant, indicating that the transition is smooth, although it can be very fast.
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Electron thermal transport analysis in Tokamak a Configuration Variable
Physics of Plasmas, 2008Co-Authors: E. Asp, A. N. Karpushov, O. Sauter, Juhyung Kim, Wendell Horton, L. Porte, Stefano Alberti, Y. R. Martin, G. TurriAbstract:A Tokamak a Configuration Variable (TCV) [G. Tonetti, A. Heym, F. Hofmann et al., in Proceedings of the 16th Symposium on Fusion Technology, London, U.K., edited by R. Hemsworth (North-Holland, Amsterdam, 1991), p. 587] plasma with high power density (up to 8MW∕m3) core deposited electron cyclotron resonance heating at significant plasma densities (⩽7×1019m−3) is analyzed for the electron thermal transport. The discharge distinguishes itself as it has four distinct high confinement mode (H-mode) phases. An Ohmic H-mode with type III edge localized modes (ELMs), which turns into a type I ELMy H-mode when the ECRH is switched on. The ELMs then vanish, which gives rise to a quasistationary ELM-free H-mode. This ELM-free phase can be divided into two, one without magnetohydrodynamics (MHD) and one with. The MHD mode in the latter case causes the confinement to drop by ∼15%. For all four phases both large-scale trapped electron (TEM) and ion temperature gradient (ITG) modes and small-scale electron temperature...
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High-bootstrap, noninductively sustained electron internal transport barriers in the Tokamak a Configuration Variable
Physics of Plasmas, 2005Co-Authors: Stefano Coda, E. Fable, O. Sauter, M. A. Henderson, R. Behn, T. P. Goodman, Alberto Bottino, Y. Camenen, An. Martynov, P. NikkolaAbstract:Important ingredients of the advanced-tokamak route to fusion have been explored in depth in the Tokamak a Configuration Variable [F. Hofmann, J. B. Lister, M. Anton , Plasma Phys. Controlled Fusion 36, B277 (1994)] over the past two years. Using a uniquely powerful and flexible electron-cyclotron resonance heating (ECRH) system as the primary actuator, fully noninductive, steady-state electron internal transport barrier discharges have been generated with an electron-energy confinement time up to five times longer than in L mode, poloidal beta up to 2.4, and bootstrap fraction up to 75%. Interpretative transport modeling confirms that the safety factor profile is nonmonotonic in these discharges. The formation of the barrier is a discrete event resulting in rapid and localized confinement improvement consistent with the time and location of magnetic-shear reversal. In steady state, however, the confinement quality appears to depend on the current gradient in a broader negative-shear region enclosed by the barrier, improving with increasing shear: in particular, the width and depth of the barrier can be controlled and finely tuned, along a magnetohydrodynamic-stable path, by manipulating the current profile with ECRH (six independently steerable 0.45 MW launchers). The crucial role of the current profile has been clearly demonstrated by applying small Ohmic current perturbations which dramatically alter the properties of the barrier, enhancing or reducing the confinement with negative and positive current, respectively, with negligible Ohmic heating. These results are in agreement with theoretical estimates: first-principle-based numerical simulations of microinstability dynamics and turbulence-driven transport predict a substantial suppression of turbulence and anomalous energy diffusivity near the location of the minimum in the safety factor. (c) 2005 American Institute of Physics.
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Steady-state fully noninductive operation with electron cyclotron current drive and current profile control in the tokamak à Configuration Variable (TCV)
Physics of Plasmas, 2001Co-Authors: O. Sauter, Stefano Coda, M. A. Henderson, F. Hofmann, Jean-marc Moret, C. Angioni, J.-p. Hogge, T. P. Goodman, P. Gomez, P. NikkolaAbstract:Fully noninductive, steady-state electron cyclotron current drive (ECCD) has been demonstrated for the first time in experiments carried out in the tokamak a Configuration Variable (TCV) [O. Sauter et al., Phys. Rev. Lett. 84, 3322 (2000)]. By appropriately distributing six 0.45 MW ECCD sources over the discharge cross section, fully noninductive, stable, and stationary plasmas with Ip up to 210 kA were obtained for the full discharge duration of 1.9 s, corresponding to more than 900 energy confinement times and more than 10 current redistribution times at an average current drive efficiency η20CD=0.01[1020 A W−1 m−2]. These experiments have also demonstrated for the first time the steady recharging of the ohmic transformer using ECCD only. The effect of localized off-axis electron cyclotron heating (ECH) and EC current drive (ECCD) (co- and counter-) is investigated showing that locally driven currents amounting to only 1% of Ip significantly alter sawtooth periods and crash amplitudes. An improved quasi...
R. Behn - One of the best experts on this subject based on the ideXlab platform.
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On the non-stiffness of edge transport in L-mode tokamak plasmasa)
Physics of Plasmas, 2014Co-Authors: O. Sauter, B. P. Duval, Stefano Coda, R. Behn, Y. Camenen, Stephan Brunner, D. Kim, G. Merlo, L. Federspiel, Timothy GoodmanAbstract:Transport analyses using first-principle turbulence codes and 11/2-D transport codes usually study radial transport properties between the tokamak plasma magnetic axis and a normalized minor radius around 0.8. In this region, heat transport shows significantly stiff properties resulting in temperature scalelength values (R/L-T) that are relatively independent of the level of the radial heat flux. We have studied experimentally in the tokamak a Configuration Variable [F. Hofmann et al., Plasma Phys. Controlled Fusion 36, B277 (1994)] the radial electron transport properties of the edge region, close to the last closed flux surface, namely, between rho(V) = root V/V-edge = 0.8 to 1. It is shown that electron transport is not stiff in this region and high R/L-Te values (similar to 20-40) can be attained even for L-mode confinement. We can define a "pedestal" location, already in L-mode regimes, where the transport characteristics change from constant logarithmic gradient, inside rho(V) = 0.8, to constant gradient between 0.8 and 1.0. In particular, we demonstrate, with well resolved T-e and n(e) profiles, that the confinement improvement with plasma current I-p, with or without auxiliary heating, is due to this non-stiff edge region. This new result is used to explain the significant confinement improvement observed with negative triangularity, which could not be explained by theory to date. Preliminary local gyrokinetic simulations are now consistent with an edge, less stiff, region that is more sensitive to triangularity than further inside. We also show that increasing the electron cyclotron heating power increases the edge temperature inverse scalelength, in contrast to the value in the main plasma region. The dependence of confinement on density in ohmic plasmas is also studied and brings new insight in the understanding of the transition between linear and saturated confinement regimes, as well as of the density limit and appearance of a 2/1 tearing mode. The results presented in this paper provide an important new perspective with regards to radial transport in tokamak plasmas which goes beyond L-mode plasmas and explains some previous puzzling results. It is proposed that understanding the transport properties in this edge non-stiff region will also help in understanding the improved and high confinement edge properties. (C) 2014 AIP Publishing LLC.
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A new far-infrared polarimeter on the tokamak à Configuration Variable
2008 IEEE 35th International Conference on Plasma Science, 2008Co-Authors: A. Zhuchkova, H. Weisen, R. Behn, P. BlanchardAbstract:Summary form only given. A new far-infrared polarimeter diagnostic is under installation on the TCV tokamak at CRPP. Its aim is to obtain the poloidal current distribution via measurements of Faraday rotation angles of the beam polarization along 10 different vertical lines. The Faraday rotation angle is proportional to the product of the plasma density and the vertical magnetic field, providing constraints for improving the reconstruction of the internal magnetic field distribution. The setup consists of two FIR lasers operating at a wavelength 432.5e-6 m, optically pumped by a 120 W continuous wave CO2 laser. The diagnostic is based on a method proposed by Dodel and Kunz. The FIR laser beams are detuned by 750 kHz by adjusting the cavity lengths. They are relayed to the tokamak by separate dielectric waveguides and combined into a single beam with a linear polarization rotating at the difference frequency. This probing beam is divided into five vertical beams across a poloidal section of the device. At the top of the tokamak each beam is equipped by 2 polarisation sensitive waveguide Schottky diode detectors. Measurements of Faraday rotation angles are obtained using a heterodyne detection scheme, with a reference signal from a line not passing through the plasma. The main aspects of the system and the use of the Faraday rotation measurements for the equilibrium reconstruction by the LIUQE equilibrium code will be presented.
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10-channel far-infrared polarimeter for the tokamak à Configuration Variable
Review of Scientific Instruments, 2006Co-Authors: P. Blanchard, R. Behn, H. Weisen, A. ZhuchkovaAbstract:A new far-infrared polarimeter diagnostic for the tokamak a Configuration Variable (TCV) is under construction at CRPP. It uses two FIR lasers at 432.5 mu m, optically pumped by a 120 W continuous wave CO2 laser. The two FIR cavities will be detuned such that the combination of the beams, using a method proposed by Dodel and Kunz [Infrared Phys. 18, 773 (1978)] , produces a single beam with a linear polarization rotating at the difference frequency (set to 750 kHz). For measurements across the minor radius of TCV, this beam will be split into ten beams, each equipped with a Schottky barrier diode as detector. Faraday rotation angles will be measured by coherent detection. In order to optimize the sensitivity of the polarimeter for the parameter range of interest [n(e)(0) < 3 x10(19) m(-3)], we have chosen to keep it separate from the existing 14-channel interferometer operating at 214 mu m. This also leads to substantial simplification of the design and signal processing. The design of the system as well as its expected sensitivity are presented and discussed. The required accuracy to measure profiles of current density and safety factor for typical operating scenarios with internal transport barriers on TCV (including cases with reversed magnetic shear) is assessed by numerical simulations. (c) 2006 American Institute of Physics.
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High-bootstrap, noninductively sustained electron internal transport barriers in the Tokamak a Configuration Variable
Physics of Plasmas, 2005Co-Authors: Stefano Coda, E. Fable, O. Sauter, M. A. Henderson, R. Behn, T. P. Goodman, Alberto Bottino, Y. Camenen, An. Martynov, P. NikkolaAbstract:Important ingredients of the advanced-tokamak route to fusion have been explored in depth in the Tokamak a Configuration Variable [F. Hofmann, J. B. Lister, M. Anton , Plasma Phys. Controlled Fusion 36, B277 (1994)] over the past two years. Using a uniquely powerful and flexible electron-cyclotron resonance heating (ECRH) system as the primary actuator, fully noninductive, steady-state electron internal transport barrier discharges have been generated with an electron-energy confinement time up to five times longer than in L mode, poloidal beta up to 2.4, and bootstrap fraction up to 75%. Interpretative transport modeling confirms that the safety factor profile is nonmonotonic in these discharges. The formation of the barrier is a discrete event resulting in rapid and localized confinement improvement consistent with the time and location of magnetic-shear reversal. In steady state, however, the confinement quality appears to depend on the current gradient in a broader negative-shear region enclosed by the barrier, improving with increasing shear: in particular, the width and depth of the barrier can be controlled and finely tuned, along a magnetohydrodynamic-stable path, by manipulating the current profile with ECRH (six independently steerable 0.45 MW launchers). The crucial role of the current profile has been clearly demonstrated by applying small Ohmic current perturbations which dramatically alter the properties of the barrier, enhancing or reducing the confinement with negative and positive current, respectively, with negligible Ohmic heating. These results are in agreement with theoretical estimates: first-principle-based numerical simulations of microinstability dynamics and turbulence-driven transport predict a substantial suppression of turbulence and anomalous energy diffusivity near the location of the minimum in the safety factor. (c) 2005 American Institute of Physics.
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Recent results from the electron cyclotron heated plasmas in Tokamak à Configuration Variable (TCV)
Physics of Plasmas, 2003Co-Authors: M. A. Henderson, Stefano Coda, Stefano Alberti, P. Blanchard, R. Behn, C. Angioni, Y. Camenen, G. Arnoux, P. Bosshard, I. CondreaAbstract:In noninductively driven discharges, 0.9 MW second harmonic (X2) off-axis co-electron cyclotron current drive deposition is combined with 0.45 MW X2 central heating to create an electron internal transport barrier (eITB) in steady plasma conditions resulting in a 1.6-fold increase of the confinement time (τEe) over ITER-98L-mode scaling. The eITB is associated with a reversed shear current profile enhanced by a large bootstrap current fraction (up to 80%) and is sustained for up to 10 current redistribution times. A linear dependence of the confinement improvement on the product of the global shear reversal factor (q0/qmin) and the reversed shear volume (ρq-min2) is shown. In other discharges heated with X2 the sawteeth are destabilized (respectively stabilized) when heating just inside (respectively outside) the q=1 surface. Control of the sawteeth may allow the avoidance of neoclassical tearing modes that can be seeded by the sawtooth instability. Results on H-mode and highly elongated plasmas using the...