The Monte-Carlo neutral transport code 3D-GAPS is described. The code models impurity transport a... more The Monte-Carlo neutral transport code 3D-GAPS is described. The code models impurity transport and deposition in remote areas, such as gaps between cells of castellated plasma-facing surfaces. A step-by-step investigation of the interplay of different processes that may influence the deposition inside gaps, namely particle reflection, elastic neutral collisions, different particle sources, chemical erosion and plasma penetration into gaps, is presented. Examples of modelling results in application to the TEXTOR experiment with a castellated test limiter are provided. It is shown that only with the assumption of the presence of species with different reflection probabilities, do simulated carbon deposition profiles agree with experimental observations for side surfaces of the gaps. These species can be attributed to different particle sources, e.g. carbon atoms and hydrocarbon radicals. Background carbon ions and atoms have low and moderate values of the reflection coefficient (R <= 0.6), while some of the hydrocarbon radicals produced by chemical erosion of redeposited carbon layers have high reflection probability (R >= 0.9). Deposition at the bottom of the gaps cannot be adequately reproduced unless extreme assumptions on particle sources and reflection properties are imposed. Elastic neutral collisions and ionization of neutrals escaping the gaps have no significant influence on the results. Nevertheless, particle-in-cell simulations of plasma penetration into gaps are essential for estimating the incoming ion flux and leading to a better quantitative agreement with experimental observations.
2D Particle-In-Cell simulations are used to predict the ion current density on the surface of a c... more 2D Particle-In-Cell simulations are used to predict the ion current density on the surface of a cylinder whose top is perpendicular to the magnetic field in a magnetized nonthermal plasma. This current density decreases with increasing distance to the probe top, and the rate of decrease is predicted to be a sensitive function of the ion temperature. We consider the possibility of a new kind of probe to use this effect to measure the ion temperature with a relatively high temporal resolution.
Using the retarding field analyzer technique, ion energies carried by edge localized mode (ELM) f... more Using the retarding field analyzer technique, ion energies carried by edge localized mode (ELM) filaments have been measured for the first time in the far scrape-off layer (SOL) of the ASDEX Upgrade tokamak. Energies, Ei ELM, exceeding 160 eV have been found, 5-6 cm outside the separatrix, with a decay length of about 2 cm. The measured ELM particle ion temperature in the far SOL is in the range Ti ELM ≈ 50-80 eV, in good agreement with the predictions from two simple collisionless models of ELM parallel transport. In between ELMs, Ti ≈ Te ≈ 10 eV is observed in the far SOL, consistent with relatively strong ion-electron thermal coupling in this region.
JET and ITER plasma facing components (PFC) are segmented into cells to minimise the mechanical s... more JET and ITER plasma facing components (PFC) are segmented into cells to minimise the mechanical stress that is produced by thermal expansion of the components under plasma/radiative loads. This complex geometry introduces a significant complication in the estimation of the power load of these components, as the gap width can be comparable to the ion Larmor radius. We present results of two-dimensional, self-consistent kinetic simulations of this problem using the SPICE2 code based on a particle-in-cell technique. It has been developed for edge plasma simulations using arbitrary velocity distribution functions. The code is highly optimized and is designed for computationally demanding simulations. The code follows the charged particle trajectories on their way to the PFCs and estimates the plasma deposition in a non-symmetric electric potential. The results of power and particle loads calculations are presented for JET and ITER conditions.
Ion sensitive probes serve to measure the ion temperature in magnetized plasma. Such a probe typi... more Ion sensitive probes serve to measure the ion temperature in magnetized plasma. Such a probe typically consists of a collector submerged inside a hollow tube, which is oriented perpendicularly to the magnetic field. The principle of the probe is based on geometrical shielding of the ion collector from plasma electrons. According to the basic theory, when the collector is retracted
In this paper we present a new Particle-In-Cell code Spice2. This code operates in two spatial an... more In this paper we present a new Particle-In-Cell code Spice2. This code operates in two spatial and three velocity dimensions. Thanks to a number of enhancements the code allows to calculate scenarios, which would be otherwise dicult to investigate due to time constrains. Spice2 brings several algorithmical novelties such as parallelization by groups of particles and fast Poisson solver based on LU decomposition. Originally, the code was developed to simulate tokamak edge plasma.
The retarding field analyzer (RFA) is one of the only widely accepted diagnostics for measurement... more The retarding field analyzer (RFA) is one of the only widely accepted diagnostics for measurements of ion temperature Ti in the tokamak scrape-off layer. In this paper we analyze some instrumental effects of the RFA and their influence on Ti measurements. It is shown that selective ion transmission through the RFA slit is responsible for an overestimation of Ti by less than 14%, even for a relatively thick slit plate. Therefore, thicker slit plates are preferable, since they reduce, e.g., the risk of melting during off-normal events, and the effect of positive space charge inside the cavity. The influence of the electron repelling grid, as well as misalignment of the slit with respect to the magnetic field on Ti measurements are negligible.
ABSTRACT Plasma deposition in narrow gaps between tiles of ITER plasma-facing components during E... more ABSTRACT Plasma deposition in narrow gaps between tiles of ITER plasma-facing components during ELMs is simulated by means of particle-in-cell technique. The particle and power loads onto and between the divertor tiles are estimated for a multispecies plasma. We simulate an equal mixture of deuterium and tritium (T) for the main plasma with one impurity, the carbon (C). The aim of this paper is to know whether the C ions can enter the narrow gaps between tiles and with which energy. Due to its radioactivity, the T deposition in the gaps is also of high interest and is estimated. The two basic orientations of the gap with respect to the magnetic field lines, poloidal and toroidal, have been studied. The global penetration of the two hydrogen species into the gap is on the order of the gap width ( ~ 0.5 mm), and their own contribution on the total heat flux to the gap is given. The main impurity penetrates into the gap in between 50% to 70% of the total plasma-deposition length but with a power deposited representing less than 1% of the total deposited power.
This paper presents particle-in-cell simulations of the plasma behaviour in the vicinity of gaps ... more This paper presents particle-in-cell simulations of the plasma behaviour in the vicinity of gaps in castellated plasma-facing components (PFCs). The point of interest was the test limiter of the TEXTOR tokamak, a PFC designed for studies of plasma-wall interactions, in particular, related to impurity transport and fuel retention. Simulations were performed for various plasma conditions in the vicinity of the castellated surface, where the gaps can be either shaped or unshaped. It was observed that depending on plasma parameters the transport of plasma particles inside the gap can be either in potential- or geometry-dominated regimes. The mechanisms responsible for the formation of a potential peak inside the poloidal gap and its consequences on plasma deposition profiles are discussed. A study of gap shaping was performed in order to validate its effectiveness.
The Monte-Carlo neutral transport code 3D-GAPS is described. The code models impurity transport a... more The Monte-Carlo neutral transport code 3D-GAPS is described. The code models impurity transport and deposition in remote areas, such as gaps between cells of castellated plasma-facing surfaces. A step-by-step investigation of the interplay of different processes that may influence the deposition inside gaps, namely particle reflection, elastic neutral collisions, different particle sources, chemical erosion and plasma penetration into gaps, is presented. Examples of modelling results in application to the TEXTOR experiment with a castellated test limiter are provided. It is shown that only with the assumption of the presence of species with different reflection probabilities, do simulated carbon deposition profiles agree with experimental observations for side surfaces of the gaps. These species can be attributed to different particle sources, e.g. carbon atoms and hydrocarbon radicals. Background carbon ions and atoms have low and moderate values of the reflection coefficient (R <= 0.6), while some of the hydrocarbon radicals produced by chemical erosion of redeposited carbon layers have high reflection probability (R >= 0.9). Deposition at the bottom of the gaps cannot be adequately reproduced unless extreme assumptions on particle sources and reflection properties are imposed. Elastic neutral collisions and ionization of neutrals escaping the gaps have no significant influence on the results. Nevertheless, particle-in-cell simulations of plasma penetration into gaps are essential for estimating the incoming ion flux and leading to a better quantitative agreement with experimental observations.
2D Particle-In-Cell simulations are used to predict the ion current density on the surface of a c... more 2D Particle-In-Cell simulations are used to predict the ion current density on the surface of a cylinder whose top is perpendicular to the magnetic field in a magnetized nonthermal plasma. This current density decreases with increasing distance to the probe top, and the rate of decrease is predicted to be a sensitive function of the ion temperature. We consider the possibility of a new kind of probe to use this effect to measure the ion temperature with a relatively high temporal resolution.
Using the retarding field analyzer technique, ion energies carried by edge localized mode (ELM) f... more Using the retarding field analyzer technique, ion energies carried by edge localized mode (ELM) filaments have been measured for the first time in the far scrape-off layer (SOL) of the ASDEX Upgrade tokamak. Energies, Ei ELM, exceeding 160 eV have been found, 5-6 cm outside the separatrix, with a decay length of about 2 cm. The measured ELM particle ion temperature in the far SOL is in the range Ti ELM ≈ 50-80 eV, in good agreement with the predictions from two simple collisionless models of ELM parallel transport. In between ELMs, Ti ≈ Te ≈ 10 eV is observed in the far SOL, consistent with relatively strong ion-electron thermal coupling in this region.
JET and ITER plasma facing components (PFC) are segmented into cells to minimise the mechanical s... more JET and ITER plasma facing components (PFC) are segmented into cells to minimise the mechanical stress that is produced by thermal expansion of the components under plasma/radiative loads. This complex geometry introduces a significant complication in the estimation of the power load of these components, as the gap width can be comparable to the ion Larmor radius. We present results of two-dimensional, self-consistent kinetic simulations of this problem using the SPICE2 code based on a particle-in-cell technique. It has been developed for edge plasma simulations using arbitrary velocity distribution functions. The code is highly optimized and is designed for computationally demanding simulations. The code follows the charged particle trajectories on their way to the PFCs and estimates the plasma deposition in a non-symmetric electric potential. The results of power and particle loads calculations are presented for JET and ITER conditions.
Ion sensitive probes serve to measure the ion temperature in magnetized plasma. Such a probe typi... more Ion sensitive probes serve to measure the ion temperature in magnetized plasma. Such a probe typically consists of a collector submerged inside a hollow tube, which is oriented perpendicularly to the magnetic field. The principle of the probe is based on geometrical shielding of the ion collector from plasma electrons. According to the basic theory, when the collector is retracted
In this paper we present a new Particle-In-Cell code Spice2. This code operates in two spatial an... more In this paper we present a new Particle-In-Cell code Spice2. This code operates in two spatial and three velocity dimensions. Thanks to a number of enhancements the code allows to calculate scenarios, which would be otherwise dicult to investigate due to time constrains. Spice2 brings several algorithmical novelties such as parallelization by groups of particles and fast Poisson solver based on LU decomposition. Originally, the code was developed to simulate tokamak edge plasma.
The retarding field analyzer (RFA) is one of the only widely accepted diagnostics for measurement... more The retarding field analyzer (RFA) is one of the only widely accepted diagnostics for measurements of ion temperature Ti in the tokamak scrape-off layer. In this paper we analyze some instrumental effects of the RFA and their influence on Ti measurements. It is shown that selective ion transmission through the RFA slit is responsible for an overestimation of Ti by less than 14%, even for a relatively thick slit plate. Therefore, thicker slit plates are preferable, since they reduce, e.g., the risk of melting during off-normal events, and the effect of positive space charge inside the cavity. The influence of the electron repelling grid, as well as misalignment of the slit with respect to the magnetic field on Ti measurements are negligible.
ABSTRACT Plasma deposition in narrow gaps between tiles of ITER plasma-facing components during E... more ABSTRACT Plasma deposition in narrow gaps between tiles of ITER plasma-facing components during ELMs is simulated by means of particle-in-cell technique. The particle and power loads onto and between the divertor tiles are estimated for a multispecies plasma. We simulate an equal mixture of deuterium and tritium (T) for the main plasma with one impurity, the carbon (C). The aim of this paper is to know whether the C ions can enter the narrow gaps between tiles and with which energy. Due to its radioactivity, the T deposition in the gaps is also of high interest and is estimated. The two basic orientations of the gap with respect to the magnetic field lines, poloidal and toroidal, have been studied. The global penetration of the two hydrogen species into the gap is on the order of the gap width ( ~ 0.5 mm), and their own contribution on the total heat flux to the gap is given. The main impurity penetrates into the gap in between 50% to 70% of the total plasma-deposition length but with a power deposited representing less than 1% of the total deposited power.
This paper presents particle-in-cell simulations of the plasma behaviour in the vicinity of gaps ... more This paper presents particle-in-cell simulations of the plasma behaviour in the vicinity of gaps in castellated plasma-facing components (PFCs). The point of interest was the test limiter of the TEXTOR tokamak, a PFC designed for studies of plasma-wall interactions, in particular, related to impurity transport and fuel retention. Simulations were performed for various plasma conditions in the vicinity of the castellated surface, where the gaps can be either shaped or unshaped. It was observed that depending on plasma parameters the transport of plasma particles inside the gap can be either in potential- or geometry-dominated regimes. The mechanisms responsible for the formation of a potential peak inside the poloidal gap and its consequences on plasma deposition profiles are discussed. A study of gap shaping was performed in order to validate its effectiveness.
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Papers by Michael Komm