L. Krupnik - Academia.edu (original) (raw)

Papers by L. Krupnik

Research paper thumbnail of Electron internal transport barriers, rationals and fluctuations in the stellarator TJ-II

The quasicoherent oscillations were observed in TJ-II plasma with different diagnostic: Heavy Ion... more The quasicoherent oscillations were observed in TJ-II plasma with different diagnostic: Heavy Ion Beam Probe (HIBP), 2 ω ECR emission, Langmuir and Mirnov probes.

Research paper thumbnail of Transport, stability and plasma control studies in the TJ-II stellarator

Nuclear Fusion, 2015

The main TJ-II results since 2012 are presented in this overview. Impurity confinement is studied... more The main TJ-II results since 2012 are presented in this overview. Impurity confinement is studied showing an isotopic dependence of impurity confinement time, asymmetries in parallel impurity flows in TJ-II ion-root plasmas and impurity density asymmetries within a flux surface. In addition, first observations of electrostatic potential variations within the same magnetic flux surface are presented. Evidence of the impact of three-dimensional magnetic structures on plasma confinement and L-H transitions is also presented. The leading role of the plasma turbulence is emphasized by the observed temporal ordering of the limit cycle oscillations at the L-I-H transition. Comparative studies between tokamaks and stellarators have provided direct experimental evidence for the importance of multi-scale physics to unravel the impact of the isotope effect on transport. Novel solutions for plasma facing components based on the recently installed Li-liquid limiters (LLLs) have been developed on TJ-II, showing the self-screening effect of evaporating liquid lithium, protecting plasma-facing components against heat loads, and tritium inventory control. Regarding plasma stability, magnetic well scan experiments show that traditional stability criteria, on which 0029-5515/15/104014+09$33.00 1 © 2015 EURATOM Printed in the UK Nucl. Fusion 55 (2015) 104014 J. Sánchez et al the optimization of stellarator configurations is based, may miss some stabilization mechanisms. Further effects of ECRH on Alfvénic instabilities are investigated, showing that moderate off-axis ECH power deposition modifies the continuous nature of the Alfvén eigenmodes, and frequency chirping sets in. This result shows that ECH can be a tool for AE control that might be ITER and reactor-relevant.

Research paper thumbnail of First HIBP results on the WEGA Stellarator

The heavy ion beam probe (HIBP) is an established non perturbing diagnostic for measuring the spa... more The heavy ion beam probe (HIBP) is an established non perturbing diagnostic for measuring the spatial distributions of plasma potential, density, temperature and poloidal magnetic field (axial current) of magnetically confined fusion plasma. These are determined from the change in the primary ion beam parameters (charge, intensity and trajectory) passing through a plasma volume due to collisions with electrons and interactions with the confining magnetic field. A heavy ion beam probe plasma diagnostic system has been installed and tested on the WEGA stellarator in Greifswald, Germany in 2006–2007. The WEGA HIBP operates with a beam of singly charged sodium ions with an energy of up to 60 keV, ion current up to 100 μA, and beam diameter of 5–6 mm in the confined plasma region. Plasma experiments with the HIBP diagnostic system were carried out at a magnetic field strength of B0 = 0.489 T. In the experiments, an argon plasma was heated with ECRH at 28 GHz. In this work the first plasm...

Research paper thumbnail of Test-bench calibration of the Double-Slit Ion Energy Analyzer for Heavy Ion Beam Probing Diagnostics

This work presents the results of the design study, investigations of the features and a calibrat... more This work presents the results of the design study, investigations of the features and a calibration of double-slit ion energy analyzer prototype at the test-bench in IPP NSC KIPT, Kharkov, Ukraine, with beam energy up to 10 keV. Hitherto obtained accuracy for both analyzer slits is DeltaG-2×10-3 and DeltaF-3×10-3, which is limited by the existing step motor accuracy 0.1 mm for entrance slits moving. The presented study work is considered as the first step to the multi-slit analyzer elaboration.

Research paper thumbnail of Development of the HIBP Diagnostic for Uragan-2M Stellarator

The applicability of the HIBP diagnostics for the URAGAN-2M stellarator is described in this arti... more The applicability of the HIBP diagnostics for the URAGAN-2M stellarator is described in this article. It is possible to use such diagnostics for local plasma electric field potential measurements. The necessary voltages on the primary ion beam accelerator, energy analyzer and deflecting plates are acceptable.

Research paper thumbnail of Recent results of plasma potential measurements in ECRH NBI plasmas in the TJ-II stellarator

Research paper thumbnail of Recent physical results and diagnostic development of Heavy Ion Beam Probing in TJ-II

The direct measurements of an electric potential and its fluctuations in a core plasma are of a p... more The direct measurements of an electric potential and its fluctuations in a core plasma are of a primary importance for the understanding the confinement improvement mechanisms and the role of electric field E r in toroidal plasma confinement. Heavy Ion Beam Probe diagnostics, which is a unique and powerful tool to study directly the plasma potential  and turbulence characteristic in the core plasma and in the edge as well, is used in TJ-II (fourperiod flexible heliac, B 0 = 1 T, = 1.5 m, = 0.22 m). 150 keV Cs + ions are used to probe ECR and NBI heated plasmas (P ECRH  0.6MW, P NBI  1MW) from the edge to the core [1]. Poloidally resolved potential and density measurements provide short range correlations, poloidal electric field E pol = ( 1 - 2 )/x, x~1 cm. The turbulent particle flux:  r =  EpolxBtor = < n e~ v r~> [2] and poloidal plasma turbulence rotation [3] were retrieved for the broadband turbulence and for the quasicoherent modes as well .

Research paper thumbnail of HIBP diagnostic and radial electric fields in the TJ-II stellarator

Research paper thumbnail of Characterization of the quasi-coherent oscillations by HIBP diagnostic in the TJ-II stellarator

Research paper thumbnail of Kinetic Effects and Outward Particle Flux Induced by Ecrh in the TJ-II Stellarator

Electron Cyclotron Emission and Electron Cyclotron Resonance Heating (EC-15) - Proceedings of the 15th Joint Workshop, 2009

This work deals with the outward particle flux induced by Electron Cyclotron Resonance Heating (E... more This work deals with the outward particle flux induced by Electron Cyclotron Resonance Heating (ECRH) in TJ-II, usually known as pump-out, and with the kinetic effects induced on transport. Heat wave experiments have been used in TJ-II to study electron heat transport and to estimate the power deposition profile&amp;amp;amp;#39; and several effects that cannot be attributed to a diffusive behaviour

Research paper thumbnail of Comparative study design of a heavy ion and neutral beam diagnostic for the international tokamak experiment reactor

Review of Scientific Instruments, 2003

... A. Malaquias, I. Nedzelsky, B. Gonçalves, CAF Varandas, JAC Cabral, A. Melnikov, L. Eliseev .... more ... A. Malaquias, I. Nedzelsky, B. Gonçalves, CAF Varandas, JAC Cabral, A. Melnikov, L. Eliseev ... three-dimensional trajectory nature of the ions requires a curved blanket opening allowing collection ... An indication of the feasibility of these measurements can be given by comparison ...

Research paper thumbnail of Installation of an advanced heavy ion beam diagnostic on the TJ-II stellarator

Review of Scientific Instruments, 2001

An advanced 200 keV heavy ion beam diagnostic has been developed for the TJ-II stellarator based ... more An advanced 200 keV heavy ion beam diagnostic has been developed for the TJ-II stellarator based on the simultaneous utilization of a 30° Proca-Green electrostatic energy analyzer and a multiple cell array detector. This innovative design allows instantaneous measurements of plasma potential and electron density profiles together with their respective fluctuations. In this article we present a description of the

Research paper thumbnail of Plasma Electric Potential Evolution at the Core and Edge of the TJ-II Stellarator and T-10 Tokamak

In this article are presented main results on electric potential investigations in the stellarato... more In this article are presented main results on electric potential investigations in the stellarator/heliac TJ-II and tokamak T-10 in the comparable regimes with ECR heating.

Research paper thumbnail of Quasi-coherent Oscillations in the TJ-II Stellarator

Quasi-coherent oscillations have been observed in TJ-II plasma with different diagnostics: Heavy ... more Quasi-coherent oscillations have been observed in TJ-II plasma with different diagnostics: Heavy Ion Beam Probe (HIBP), ECE, Langmuir probes and Mirnov coils. A recent improvement in the signal to noise ratio of the HIBP allowed us to observe the radial structure of these oscillations from the edge to the plasma core. The edge quasi-coherent fluctuations have been observed in some

Research paper thumbnail of Improved 20 keV injection system for the heavy-ion-beam diagnostic of the tokamak ISTTOK

Review of Scientific Instruments, 2003

ABSTRACT The initial injection system of the heavy-ion-beam diagnostic (HIBD) of the tokamak ISTT... more ABSTRACT The initial injection system of the heavy-ion-beam diagnostic (HIBD) of the tokamak ISTTOK provides a 2 mm diameter and 3.6 mrad divergence beam of Cs + or Xe + ions, after extraction from a plasma ion source and acceleration up to 20 keV by a gap lens and diameter definition by a set of apertures. However, the beam current is significantly reduced to 1.5 μA, therefore, complicating the HIBD measurements of the plasma parameters. Creation of the beam with a higher current and the same, or close, geometrical parameters requires a significant improvement of the ion optics of the injection system. This article describes an injection system built in the acceleration tube, and presents results of test experiments with a solid-state thermionic source (SSTS) and a monocusp plasma ion source (MPIS). The improved injection system is characterized by a good match to both ion sources. The use of the long focus ion optics of the acceleration tube allows one to obtain an output beam current up to 11 μA of Cs + ions with SSTS and 39 μA of Xe + ions with MPIS, and dimension preservation at the focus point. Operating on the tokamak ISTTOK, the injection system significantly increased the signal-to-noise ratio and improved the measurements of the plasma parameters. © 2003 American Institute of Physics.

Research paper thumbnail of Evaluation of the incident angle of the beam into 30° electrostatic energy analyzer directly during HIBP operation in plasma experiments

Review of Scientific Instruments, 2003

ABSTRACT The resolution of the plasma potential measurements by a 30° electrostatic energy analyz... more ABSTRACT The resolution of the plasma potential measurements by a 30° electrostatic energy analyzer of a heavy-ion beam probe (HIBP) is strongly related to the accuracy of knowledge of the incident angle of the beam (theta). Initially it is determined by the calculations of the beam trajectories in the confining magnetic field of the plasma device. However, because the magnetic-field simulation codes are not exact, an uncertainty in theta always exists. A common practice to reduce the influence of this uncertainty consists in tuning the analyzer geometry parameters in such a way as to minimize the dependence on the variation of theta that results from a scan of the beam across the plasma. Still, the attachment of the energy analyzer to the diagnostic port cannot be controlled with enough precision and, therefore, an unknown shift out from the chosen optimal range of theta is predictable. This article proposes a method for the direct evaluation of the incident angles during operation of HIBP in plasma experiments. It is based on the addition of a second split-plate detector along a 30° exit axis of the analyzer, and on the comparison of the respective normalized beam currents. The experimental verification of the method on a test facility has shown that an accuracy better than Deltatheta=0.1° can be attained.

Research paper thumbnail of Formation of electron transport barriers under ECR control of the q(r) profile in the T-10 tokamak

Plasma Physics Reports, 2001

Abstract-the formation of transport barriers under electron cyclotron resonance heating and curre... more Abstract-the formation of transport barriers under electron cyclotron resonance heating and current drive in the t-10 tokamak is studied. in regimes with off-axis co-eccd and q L &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;4 at the limiter, a spontaneous transition to improved confinement accompanied by the formation of two electron transport barriers is observed. the improvement resembles an L-H transition. It manifests itself as density growth,

Research paper thumbnail of Electron internal transport barrier formation and dynamics in the plasma core of the TJ-II stellarator

Plasma Physics and Controlled Fusion, 2004

The influence of magnetic topology on the formation of electron internal transport barriers (e-IT... more The influence of magnetic topology on the formation of electron internal transport barriers (e-ITBs) has been studied experimentally in electron cyclotron heated plasmas in the stellarator TJ-II. e-ITB formation is characterized by an increase in core electron temperature and plasma potential. The positive radial electric field increases by a factor of 3 in the central plasma region when an e-ITB forms. The experiments reported demonstrate that the formation of an e-ITB depends on the magnetic configuration. Calculations of the modification of the rotational transform due to plasma current lead to the interpretation that the formation of an e-ITB can be triggered by positioning a low order rational surface close to the plasma core region. In configurations without any central low order rational, no barrier is formed for any accessible value of heating power. Different mechanisms associated with neoclassical/turbulent bifurcations and kinetic effects are put forward to explain the impact of magnetic topology on radial electric fields and confinement.

Research paper thumbnail of Multi-machine studies of the role of turbulence and electric fields in the establishment of improved confinement in tokamak plasmas

Plasma Physics and Controlled Fusion, 2007

An extensive (INTAS) research programme started in 2002 to investigate the correlations between, ... more An extensive (INTAS) research programme started in 2002 to investigate the correlations between, on the one hand, the occurrence of transport barriers and improved confinement in the medium-size tokamaks TEXTOR and T-10 and on the smaller tokamaks FT-2, TUMAN-3M and CASTOR, and on the other hand, electric fields, modified magnetic shear and electrostatic and magnetic turbulence using advanced diagnostics with

Research paper thumbnail of Influence of magnetic topology on transport and stability in stellarators

Plasma Physics and Controlled Fusion, 2005

The influence of the magnetic topology on transport and stability has been investigated in four s... more The influence of the magnetic topology on transport and stability has been investigated in four stellarators: an almost shearless medium size flexible heliac (TJ-II), a medium size and a large heliotron (CHS and LHD) with shear, and a quasihelically symmetric device (HSX) with moderate shear. All of these have variable rotational transform profiles and magnetic ripples. Using these capabilities, bifurcated states can appear and plasma can jump from one to another with subsequent changes in the transport properties. Low rational values of ι/2π can create transport barriers in LHD and TJ-II when they are located close to the plasma core or at the edge. The key ingredient for transport barriers is a positive and sheared electric field. Internal transport barriers also appear in CHS, but the role of rationals is not clear yet in this device. The time evolution of the electric field shows the onset of a bifurcation triggered either by the rational or by the presence of the ion and electron roots. The electric potential inside ITBs follows the ECE-temperature profile in a fast time scale. The plasma stability properties and its effect on the viscosity are also studied in the HSX, and the influence of the dynamics of rational surface is studied in the LHD and TJ-II stellarators.

Research paper thumbnail of Electron internal transport barriers, rationals and fluctuations in the stellarator TJ-II

The quasicoherent oscillations were observed in TJ-II plasma with different diagnostic: Heavy Ion... more The quasicoherent oscillations were observed in TJ-II plasma with different diagnostic: Heavy Ion Beam Probe (HIBP), 2 ω ECR emission, Langmuir and Mirnov probes.

Research paper thumbnail of Transport, stability and plasma control studies in the TJ-II stellarator

Nuclear Fusion, 2015

The main TJ-II results since 2012 are presented in this overview. Impurity confinement is studied... more The main TJ-II results since 2012 are presented in this overview. Impurity confinement is studied showing an isotopic dependence of impurity confinement time, asymmetries in parallel impurity flows in TJ-II ion-root plasmas and impurity density asymmetries within a flux surface. In addition, first observations of electrostatic potential variations within the same magnetic flux surface are presented. Evidence of the impact of three-dimensional magnetic structures on plasma confinement and L-H transitions is also presented. The leading role of the plasma turbulence is emphasized by the observed temporal ordering of the limit cycle oscillations at the L-I-H transition. Comparative studies between tokamaks and stellarators have provided direct experimental evidence for the importance of multi-scale physics to unravel the impact of the isotope effect on transport. Novel solutions for plasma facing components based on the recently installed Li-liquid limiters (LLLs) have been developed on TJ-II, showing the self-screening effect of evaporating liquid lithium, protecting plasma-facing components against heat loads, and tritium inventory control. Regarding plasma stability, magnetic well scan experiments show that traditional stability criteria, on which 0029-5515/15/104014+09$33.00 1 © 2015 EURATOM Printed in the UK Nucl. Fusion 55 (2015) 104014 J. Sánchez et al the optimization of stellarator configurations is based, may miss some stabilization mechanisms. Further effects of ECRH on Alfvénic instabilities are investigated, showing that moderate off-axis ECH power deposition modifies the continuous nature of the Alfvén eigenmodes, and frequency chirping sets in. This result shows that ECH can be a tool for AE control that might be ITER and reactor-relevant.

Research paper thumbnail of First HIBP results on the WEGA Stellarator

The heavy ion beam probe (HIBP) is an established non perturbing diagnostic for measuring the spa... more The heavy ion beam probe (HIBP) is an established non perturbing diagnostic for measuring the spatial distributions of plasma potential, density, temperature and poloidal magnetic field (axial current) of magnetically confined fusion plasma. These are determined from the change in the primary ion beam parameters (charge, intensity and trajectory) passing through a plasma volume due to collisions with electrons and interactions with the confining magnetic field. A heavy ion beam probe plasma diagnostic system has been installed and tested on the WEGA stellarator in Greifswald, Germany in 2006–2007. The WEGA HIBP operates with a beam of singly charged sodium ions with an energy of up to 60 keV, ion current up to 100 μA, and beam diameter of 5–6 mm in the confined plasma region. Plasma experiments with the HIBP diagnostic system were carried out at a magnetic field strength of B0 = 0.489 T. In the experiments, an argon plasma was heated with ECRH at 28 GHz. In this work the first plasm...

Research paper thumbnail of Test-bench calibration of the Double-Slit Ion Energy Analyzer for Heavy Ion Beam Probing Diagnostics

This work presents the results of the design study, investigations of the features and a calibrat... more This work presents the results of the design study, investigations of the features and a calibration of double-slit ion energy analyzer prototype at the test-bench in IPP NSC KIPT, Kharkov, Ukraine, with beam energy up to 10 keV. Hitherto obtained accuracy for both analyzer slits is DeltaG-2×10-3 and DeltaF-3×10-3, which is limited by the existing step motor accuracy 0.1 mm for entrance slits moving. The presented study work is considered as the first step to the multi-slit analyzer elaboration.

Research paper thumbnail of Development of the HIBP Diagnostic for Uragan-2M Stellarator

The applicability of the HIBP diagnostics for the URAGAN-2M stellarator is described in this arti... more The applicability of the HIBP diagnostics for the URAGAN-2M stellarator is described in this article. It is possible to use such diagnostics for local plasma electric field potential measurements. The necessary voltages on the primary ion beam accelerator, energy analyzer and deflecting plates are acceptable.

Research paper thumbnail of Recent results of plasma potential measurements in ECRH NBI plasmas in the TJ-II stellarator

Research paper thumbnail of Recent physical results and diagnostic development of Heavy Ion Beam Probing in TJ-II

The direct measurements of an electric potential and its fluctuations in a core plasma are of a p... more The direct measurements of an electric potential and its fluctuations in a core plasma are of a primary importance for the understanding the confinement improvement mechanisms and the role of electric field E r in toroidal plasma confinement. Heavy Ion Beam Probe diagnostics, which is a unique and powerful tool to study directly the plasma potential  and turbulence characteristic in the core plasma and in the edge as well, is used in TJ-II (fourperiod flexible heliac, B 0 = 1 T, = 1.5 m, = 0.22 m). 150 keV Cs + ions are used to probe ECR and NBI heated plasmas (P ECRH  0.6MW, P NBI  1MW) from the edge to the core [1]. Poloidally resolved potential and density measurements provide short range correlations, poloidal electric field E pol = ( 1 - 2 )/x, x~1 cm. The turbulent particle flux:  r =  EpolxBtor = < n e~ v r~> [2] and poloidal plasma turbulence rotation [3] were retrieved for the broadband turbulence and for the quasicoherent modes as well .

Research paper thumbnail of HIBP diagnostic and radial electric fields in the TJ-II stellarator

Research paper thumbnail of Characterization of the quasi-coherent oscillations by HIBP diagnostic in the TJ-II stellarator

Research paper thumbnail of Kinetic Effects and Outward Particle Flux Induced by Ecrh in the TJ-II Stellarator

Electron Cyclotron Emission and Electron Cyclotron Resonance Heating (EC-15) - Proceedings of the 15th Joint Workshop, 2009

This work deals with the outward particle flux induced by Electron Cyclotron Resonance Heating (E... more This work deals with the outward particle flux induced by Electron Cyclotron Resonance Heating (ECRH) in TJ-II, usually known as pump-out, and with the kinetic effects induced on transport. Heat wave experiments have been used in TJ-II to study electron heat transport and to estimate the power deposition profile&amp;amp;amp;#39; and several effects that cannot be attributed to a diffusive behaviour

Research paper thumbnail of Comparative study design of a heavy ion and neutral beam diagnostic for the international tokamak experiment reactor

Review of Scientific Instruments, 2003

... A. Malaquias, I. Nedzelsky, B. Gonçalves, CAF Varandas, JAC Cabral, A. Melnikov, L. Eliseev .... more ... A. Malaquias, I. Nedzelsky, B. Gonçalves, CAF Varandas, JAC Cabral, A. Melnikov, L. Eliseev ... three-dimensional trajectory nature of the ions requires a curved blanket opening allowing collection ... An indication of the feasibility of these measurements can be given by comparison ...

Research paper thumbnail of Installation of an advanced heavy ion beam diagnostic on the TJ-II stellarator

Review of Scientific Instruments, 2001

An advanced 200 keV heavy ion beam diagnostic has been developed for the TJ-II stellarator based ... more An advanced 200 keV heavy ion beam diagnostic has been developed for the TJ-II stellarator based on the simultaneous utilization of a 30° Proca-Green electrostatic energy analyzer and a multiple cell array detector. This innovative design allows instantaneous measurements of plasma potential and electron density profiles together with their respective fluctuations. In this article we present a description of the

Research paper thumbnail of Plasma Electric Potential Evolution at the Core and Edge of the TJ-II Stellarator and T-10 Tokamak

In this article are presented main results on electric potential investigations in the stellarato... more In this article are presented main results on electric potential investigations in the stellarator/heliac TJ-II and tokamak T-10 in the comparable regimes with ECR heating.

Research paper thumbnail of Quasi-coherent Oscillations in the TJ-II Stellarator

Quasi-coherent oscillations have been observed in TJ-II plasma with different diagnostics: Heavy ... more Quasi-coherent oscillations have been observed in TJ-II plasma with different diagnostics: Heavy Ion Beam Probe (HIBP), ECE, Langmuir probes and Mirnov coils. A recent improvement in the signal to noise ratio of the HIBP allowed us to observe the radial structure of these oscillations from the edge to the plasma core. The edge quasi-coherent fluctuations have been observed in some

Research paper thumbnail of Improved 20 keV injection system for the heavy-ion-beam diagnostic of the tokamak ISTTOK

Review of Scientific Instruments, 2003

ABSTRACT The initial injection system of the heavy-ion-beam diagnostic (HIBD) of the tokamak ISTT... more ABSTRACT The initial injection system of the heavy-ion-beam diagnostic (HIBD) of the tokamak ISTTOK provides a 2 mm diameter and 3.6 mrad divergence beam of Cs + or Xe + ions, after extraction from a plasma ion source and acceleration up to 20 keV by a gap lens and diameter definition by a set of apertures. However, the beam current is significantly reduced to 1.5 μA, therefore, complicating the HIBD measurements of the plasma parameters. Creation of the beam with a higher current and the same, or close, geometrical parameters requires a significant improvement of the ion optics of the injection system. This article describes an injection system built in the acceleration tube, and presents results of test experiments with a solid-state thermionic source (SSTS) and a monocusp plasma ion source (MPIS). The improved injection system is characterized by a good match to both ion sources. The use of the long focus ion optics of the acceleration tube allows one to obtain an output beam current up to 11 μA of Cs + ions with SSTS and 39 μA of Xe + ions with MPIS, and dimension preservation at the focus point. Operating on the tokamak ISTTOK, the injection system significantly increased the signal-to-noise ratio and improved the measurements of the plasma parameters. © 2003 American Institute of Physics.

Research paper thumbnail of Evaluation of the incident angle of the beam into 30° electrostatic energy analyzer directly during HIBP operation in plasma experiments

Review of Scientific Instruments, 2003

ABSTRACT The resolution of the plasma potential measurements by a 30° electrostatic energy analyz... more ABSTRACT The resolution of the plasma potential measurements by a 30° electrostatic energy analyzer of a heavy-ion beam probe (HIBP) is strongly related to the accuracy of knowledge of the incident angle of the beam (theta). Initially it is determined by the calculations of the beam trajectories in the confining magnetic field of the plasma device. However, because the magnetic-field simulation codes are not exact, an uncertainty in theta always exists. A common practice to reduce the influence of this uncertainty consists in tuning the analyzer geometry parameters in such a way as to minimize the dependence on the variation of theta that results from a scan of the beam across the plasma. Still, the attachment of the energy analyzer to the diagnostic port cannot be controlled with enough precision and, therefore, an unknown shift out from the chosen optimal range of theta is predictable. This article proposes a method for the direct evaluation of the incident angles during operation of HIBP in plasma experiments. It is based on the addition of a second split-plate detector along a 30° exit axis of the analyzer, and on the comparison of the respective normalized beam currents. The experimental verification of the method on a test facility has shown that an accuracy better than Deltatheta=0.1° can be attained.

Research paper thumbnail of Formation of electron transport barriers under ECR control of the q(r) profile in the T-10 tokamak

Plasma Physics Reports, 2001

Abstract-the formation of transport barriers under electron cyclotron resonance heating and curre... more Abstract-the formation of transport barriers under electron cyclotron resonance heating and current drive in the t-10 tokamak is studied. in regimes with off-axis co-eccd and q L &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;4 at the limiter, a spontaneous transition to improved confinement accompanied by the formation of two electron transport barriers is observed. the improvement resembles an L-H transition. It manifests itself as density growth,

Research paper thumbnail of Electron internal transport barrier formation and dynamics in the plasma core of the TJ-II stellarator

Plasma Physics and Controlled Fusion, 2004

The influence of magnetic topology on the formation of electron internal transport barriers (e-IT... more The influence of magnetic topology on the formation of electron internal transport barriers (e-ITBs) has been studied experimentally in electron cyclotron heated plasmas in the stellarator TJ-II. e-ITB formation is characterized by an increase in core electron temperature and plasma potential. The positive radial electric field increases by a factor of 3 in the central plasma region when an e-ITB forms. The experiments reported demonstrate that the formation of an e-ITB depends on the magnetic configuration. Calculations of the modification of the rotational transform due to plasma current lead to the interpretation that the formation of an e-ITB can be triggered by positioning a low order rational surface close to the plasma core region. In configurations without any central low order rational, no barrier is formed for any accessible value of heating power. Different mechanisms associated with neoclassical/turbulent bifurcations and kinetic effects are put forward to explain the impact of magnetic topology on radial electric fields and confinement.

Research paper thumbnail of Multi-machine studies of the role of turbulence and electric fields in the establishment of improved confinement in tokamak plasmas

Plasma Physics and Controlled Fusion, 2007

An extensive (INTAS) research programme started in 2002 to investigate the correlations between, ... more An extensive (INTAS) research programme started in 2002 to investigate the correlations between, on the one hand, the occurrence of transport barriers and improved confinement in the medium-size tokamaks TEXTOR and T-10 and on the smaller tokamaks FT-2, TUMAN-3M and CASTOR, and on the other hand, electric fields, modified magnetic shear and electrostatic and magnetic turbulence using advanced diagnostics with

Research paper thumbnail of Influence of magnetic topology on transport and stability in stellarators

Plasma Physics and Controlled Fusion, 2005

The influence of the magnetic topology on transport and stability has been investigated in four s... more The influence of the magnetic topology on transport and stability has been investigated in four stellarators: an almost shearless medium size flexible heliac (TJ-II), a medium size and a large heliotron (CHS and LHD) with shear, and a quasihelically symmetric device (HSX) with moderate shear. All of these have variable rotational transform profiles and magnetic ripples. Using these capabilities, bifurcated states can appear and plasma can jump from one to another with subsequent changes in the transport properties. Low rational values of ι/2π can create transport barriers in LHD and TJ-II when they are located close to the plasma core or at the edge. The key ingredient for transport barriers is a positive and sheared electric field. Internal transport barriers also appear in CHS, but the role of rationals is not clear yet in this device. The time evolution of the electric field shows the onset of a bifurcation triggered either by the rational or by the presence of the ion and electron roots. The electric potential inside ITBs follows the ECE-temperature profile in a fast time scale. The plasma stability properties and its effect on the viscosity are also studied in the HSX, and the influence of the dynamics of rational surface is studied in the LHD and TJ-II stellarators.