Following the completion in May 2011 of the shutdown for the installation of the beryllium wall a... more Following the completion in May 2011 of the shutdown for the installation of the beryllium wall and the tungsten divertor, the first set of JET Campaigns have addressed the investigation of the retention properties and the development of operational scenarios with the new plasma facing materials. The large reduction of the carbon content (more than a factor ten) led to a much lower Z eff (1.2-1.4) during L-and H-mode plasmas, and radiation during the burn-through phase of the plasma initiation with the consequence that breakdown failures are almost absent. Gas balance experiments have shown that fuel retention rates with the new wall are in line with the ITER needs. The re-establishment of high-confinement scenarios compatible with the new wall has required an optimization of the control of metallic impurity sources and heat loads. Stable type I ELMy H-mode regimes with H 98,y2 close to 1 and b N~1 .6 have been achieved in high triangularity plasmas. The ELM frequency is the main factor for the control of the metallic impurities accumulation. Pedestal temperatures tend to be lower with the new wall, leading to somewhat reduced confinement, but nitrogen seeding restores high pedestal temperatures and high confinement. Compared with the carbon wall, major disruptions with the new wall show a lower radiated power and a slower current quench. The higher heat loads on plasma-facing components due to lower radiation, made the routine use of massive gas injection for disruption mitigation essential.
ABSTRACT The increasing importance of providing reliable polarimetric measurements in real time, ... more ABSTRACT The increasing importance of providing reliable polarimetric measurements in real time, for both machine protection and plasma control, has motivated the development of a quick version of the calibration algorithms for JET polarimeter. This new code, which interprets the calibration procedure performed before each shot, is based on a physical equivalent model of the diagnostic optical path and is valid for any operational regime of JET. It provides results before the plasma breakdown, and, with its estimates of the optical paths parameters, the polarimetric measurements have an accuracy more than sufficient for real-time purposes. New approximate equations have been validated in order to obtain the line integrated density from the newly calibrated horizontal chords, so that also these polarimetric measurements can also be used for density feedback and machine protection. The availability of reliable polarimetric measurements in real time opens new perspectives also to the determination of the plasma boundary, the magnetic equilibrium, and their use in advanced feedback control schemes.
ABSTRACT Internal magnetic measurements are essential to obtaining reliable and accurate magnetic... more ABSTRACT Internal magnetic measurements are essential to obtaining reliable and accurate magnetic reconstructions in the interior of the plasma column in Tokamaks. In the last years, polarimetry has been increasingly used to provide global constraints to equilibrium codes. Joint European Torus (JET) polarimeter has four lateral channels, whose arrangement is similar to the topology of the diagnostic foreseen in ITER. A statistical analysis of JET polarimetry measurements have been provided in the past for only one vertical channel (CH3) using a polarimetry propagation code based on the Stokes vector formalism. A new propagation code has therefore been developed for the lateral channels to simulate and interpret the measurements of the Faraday rotation (FR) and Cotton-Mouton (CM) phase shift in JET. In this paper, a complete analysis of the integral form of the Stokes equation for the lateral channels is presented. In particular, the analysis shows that there is a strong interaction between the FR and CM effect even at low currents and low magnetic fields. The code results of both FR and CM have been used to estimate the line-integrated density and a proper benchmarking with experimental data has been performed. Two approximations to obtain the electron density in real time, using only plasma current in case of FR measurements and also toroidal magnetic fields in case of CM measurements, are proposed.
ABSTRACT. Real-time simultaneous control of several radially distributed magnetic and kinetic pla... more ABSTRACT. Real-time simultaneous control of several radially distributed magnetic and kinetic plasma parameters (such as the safety factor, q (x), and gyro-normalized temperature gradient, ρTe*(x), respectively) is being investigated on JET, in view of developing integrated control of advanced tokamak scenarios and internal transport barriers suitable for ITER. This paper describes the new model-based optimal profile controller which has been tested during the last experimental campaign. The controller aims to use the combination ...
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2010
The present electronics of JET Far Infrared polarimeter was commissioned in 2002. Due to the fact... more The present electronics of JET Far Infrared polarimeter was commissioned in 2002. Due to the fact that in the last years the diagnostic has been configured to measure routinely both the Faraday Rotation angle and the Cotton-Mouton angle with a different optical set-up, it becomes important to check the performance of the analog phase sensitive electronics that is the core of the polarimeter. Therefore a simulator for the JET FIR polarimeter analog electronics has been developed with Simulink and with Matlab in order to completely characterize its behaviour, assess its performance and determine its impact on the absolute calibration of the diagnostic. The presented results indicate the electronic devices are working within their design specifications.
ABSTRACT Recently a complete model of JET polarimeter calibration has been developed. A specific ... more ABSTRACT Recently a complete model of JET polarimeter calibration has been developed. A specific propagation code has also been written to simulate the measurements of both the vertical and horizontal chords during plasma operation. In this paper, these two models have been checked with residual analysis methods valid for nonlinear systems. The differences between the estimates of the codes and the experimental measurements, during both calibration and plasma operation, have been calculated and analysed statistically. The distribution of the residuals confirms the quality of the model developed to interpret the calibration of the diagnostic. On the other hand, during plasma operation, the residuals do not present a Gaussian distribution and there is clear evidence of serial correlation at subsequent times. Correlation analysis with the inputs indicates that problems with the magnetic field topology, provided by an independent equilibrium code, are the source of the inadequacies in the results of the propagation model.
ABSTRACT Internal magnetic measurements are essential to obtaining reliable and accurate magnetic... more ABSTRACT Internal magnetic measurements are essential to obtaining reliable and accurate magnetic reconstructions in the interior of the plasma column in Tokamaks. In the last years, polarimetry has been increasingly used to provide global constraints to equilibrium codes. Joint European Torus (JET) polarimeter has four lateral channels, whose arrangement is similar to the topology of the diagnostic foreseen in ITER. A statistical analysis of JET polarimetry measurements have been provided in the past for only one vertical channel (CH3) using a polarimetry propagation code based on the Stokes vector formalism. A new propagation code has therefore been developed for the lateral channels to simulate and interpret the measurements of the Faraday rotation (FR) and Cotton-Mouton (CM) phase shift in JET. In this paper, a complete analysis of the integral form of the Stokes equation for the lateral channels is presented. In particular, the analysis shows that there is a strong interaction between the FR and CM effect even at low currents and low magnetic fields. The code results of both FR and CM have been used to estimate the line-integrated density and a proper benchmarking with experimental data has been performed. Two approximations to obtain the electron density in real time, using only plasma current in case of FR measurements and also toroidal magnetic fields in case of CM measurements, are proposed.
ABSTRACT In the past, the analysis of JET polarimetry measurements were carried out only for the ... more ABSTRACT In the past, the analysis of JET polarimetry measurements were carried out only for the vertical channels using a polarimetry propagation code based on the Stokes vector formalism 1 and 2. A new propagation code has been developed therefore for the horizontal chords to simulate and interpret the measurements of the Faraday rotation and Cotton–Mouton phase shift in JET. The code has been used to develop a theoretical study to the effect of the input polarization on the eventual quality of the measurements. The results allow choosing the best polarization to optimize the polarimetric measurements for the various experiments.
Following the completion in May 2011 of the shutdown for the installation of the beryllium wall a... more Following the completion in May 2011 of the shutdown for the installation of the beryllium wall and the tungsten divertor, the first set of JET Campaigns have addressed the investigation of the retention properties and the development of operational scenarios with the new plasma facing materials. The large reduction of the carbon content (more than a factor ten) led to a much lower Z eff (1.2-1.4) during L-and H-mode plasmas, and radiation during the burn-through phase of the plasma initiation with the consequence that breakdown failures are almost absent. Gas balance experiments have shown that fuel retention rates with the new wall are in line with the ITER needs. The re-establishment of high-confinement scenarios compatible with the new wall has required an optimization of the control of metallic impurity sources and heat loads. Stable type I ELMy H-mode regimes with H 98,y2 close to 1 and b N~1 .6 have been achieved in high triangularity plasmas. The ELM frequency is the main factor for the control of the metallic impurities accumulation. Pedestal temperatures tend to be lower with the new wall, leading to somewhat reduced confinement, but nitrogen seeding restores high pedestal temperatures and high confinement. Compared with the carbon wall, major disruptions with the new wall show a lower radiated power and a slower current quench. The higher heat loads on plasma-facing components due to lower radiation, made the routine use of massive gas injection for disruption mitigation essential.
ABSTRACT The increasing importance of providing reliable polarimetric measurements in real time, ... more ABSTRACT The increasing importance of providing reliable polarimetric measurements in real time, for both machine protection and plasma control, has motivated the development of a quick version of the calibration algorithms for JET polarimeter. This new code, which interprets the calibration procedure performed before each shot, is based on a physical equivalent model of the diagnostic optical path and is valid for any operational regime of JET. It provides results before the plasma breakdown, and, with its estimates of the optical paths parameters, the polarimetric measurements have an accuracy more than sufficient for real-time purposes. New approximate equations have been validated in order to obtain the line integrated density from the newly calibrated horizontal chords, so that also these polarimetric measurements can also be used for density feedback and machine protection. The availability of reliable polarimetric measurements in real time opens new perspectives also to the determination of the plasma boundary, the magnetic equilibrium, and their use in advanced feedback control schemes.
ABSTRACT Internal magnetic measurements are essential to obtaining reliable and accurate magnetic... more ABSTRACT Internal magnetic measurements are essential to obtaining reliable and accurate magnetic reconstructions in the interior of the plasma column in Tokamaks. In the last years, polarimetry has been increasingly used to provide global constraints to equilibrium codes. Joint European Torus (JET) polarimeter has four lateral channels, whose arrangement is similar to the topology of the diagnostic foreseen in ITER. A statistical analysis of JET polarimetry measurements have been provided in the past for only one vertical channel (CH3) using a polarimetry propagation code based on the Stokes vector formalism. A new propagation code has therefore been developed for the lateral channels to simulate and interpret the measurements of the Faraday rotation (FR) and Cotton-Mouton (CM) phase shift in JET. In this paper, a complete analysis of the integral form of the Stokes equation for the lateral channels is presented. In particular, the analysis shows that there is a strong interaction between the FR and CM effect even at low currents and low magnetic fields. The code results of both FR and CM have been used to estimate the line-integrated density and a proper benchmarking with experimental data has been performed. Two approximations to obtain the electron density in real time, using only plasma current in case of FR measurements and also toroidal magnetic fields in case of CM measurements, are proposed.
ABSTRACT. Real-time simultaneous control of several radially distributed magnetic and kinetic pla... more ABSTRACT. Real-time simultaneous control of several radially distributed magnetic and kinetic plasma parameters (such as the safety factor, q (x), and gyro-normalized temperature gradient, ρTe*(x), respectively) is being investigated on JET, in view of developing integrated control of advanced tokamak scenarios and internal transport barriers suitable for ITER. This paper describes the new model-based optimal profile controller which has been tested during the last experimental campaign. The controller aims to use the combination ...
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2010
The present electronics of JET Far Infrared polarimeter was commissioned in 2002. Due to the fact... more The present electronics of JET Far Infrared polarimeter was commissioned in 2002. Due to the fact that in the last years the diagnostic has been configured to measure routinely both the Faraday Rotation angle and the Cotton-Mouton angle with a different optical set-up, it becomes important to check the performance of the analog phase sensitive electronics that is the core of the polarimeter. Therefore a simulator for the JET FIR polarimeter analog electronics has been developed with Simulink and with Matlab in order to completely characterize its behaviour, assess its performance and determine its impact on the absolute calibration of the diagnostic. The presented results indicate the electronic devices are working within their design specifications.
ABSTRACT Recently a complete model of JET polarimeter calibration has been developed. A specific ... more ABSTRACT Recently a complete model of JET polarimeter calibration has been developed. A specific propagation code has also been written to simulate the measurements of both the vertical and horizontal chords during plasma operation. In this paper, these two models have been checked with residual analysis methods valid for nonlinear systems. The differences between the estimates of the codes and the experimental measurements, during both calibration and plasma operation, have been calculated and analysed statistically. The distribution of the residuals confirms the quality of the model developed to interpret the calibration of the diagnostic. On the other hand, during plasma operation, the residuals do not present a Gaussian distribution and there is clear evidence of serial correlation at subsequent times. Correlation analysis with the inputs indicates that problems with the magnetic field topology, provided by an independent equilibrium code, are the source of the inadequacies in the results of the propagation model.
ABSTRACT Internal magnetic measurements are essential to obtaining reliable and accurate magnetic... more ABSTRACT Internal magnetic measurements are essential to obtaining reliable and accurate magnetic reconstructions in the interior of the plasma column in Tokamaks. In the last years, polarimetry has been increasingly used to provide global constraints to equilibrium codes. Joint European Torus (JET) polarimeter has four lateral channels, whose arrangement is similar to the topology of the diagnostic foreseen in ITER. A statistical analysis of JET polarimetry measurements have been provided in the past for only one vertical channel (CH3) using a polarimetry propagation code based on the Stokes vector formalism. A new propagation code has therefore been developed for the lateral channels to simulate and interpret the measurements of the Faraday rotation (FR) and Cotton-Mouton (CM) phase shift in JET. In this paper, a complete analysis of the integral form of the Stokes equation for the lateral channels is presented. In particular, the analysis shows that there is a strong interaction between the FR and CM effect even at low currents and low magnetic fields. The code results of both FR and CM have been used to estimate the line-integrated density and a proper benchmarking with experimental data has been performed. Two approximations to obtain the electron density in real time, using only plasma current in case of FR measurements and also toroidal magnetic fields in case of CM measurements, are proposed.
ABSTRACT In the past, the analysis of JET polarimetry measurements were carried out only for the ... more ABSTRACT In the past, the analysis of JET polarimetry measurements were carried out only for the vertical channels using a polarimetry propagation code based on the Stokes vector formalism 1 and 2. A new propagation code has been developed therefore for the horizontal chords to simulate and interpret the measurements of the Faraday rotation and Cotton–Mouton phase shift in JET. The code has been used to develop a theoretical study to the effect of the input polarization on the eventual quality of the measurements. The results allow choosing the best polarization to optimize the polarimetric measurements for the various experiments.
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