Papers by Sergei Sharapov
JET Joint Undertaking, Abingdon, Oxon., OX14 3EA, UK Euratom/UKAEA Fusion Association, Culham Sci... more JET Joint Undertaking, Abingdon, Oxon., OX14 3EA, UK Euratom/UKAEA Fusion Association, Culham Science Centre, Abingdon, Oxon., OX14 3DB, UK Centro Ricerche Energia, Associazione Euratom/ENEA/CNR sulla Fusione, CP 65, 00044 Frascati, Rome, Italy Association Euratom-CEA, CEA-Cadarache, F-13108, St. Paul lez Durance, France Princeton Plasma Physics Laboratory, PO Box 451, Princeton, NJ 08543, USA Plasma Science and Fusion Centre, Massachusetts Institute of Technology, Cambridge, USA
Energetic Particles in Tokamak Plasmas, 2021
Bulletin of the American Physical Society, 2018
Alfvén eigenmodes (AEs) driven unstable by energetic particles are routinely observed in axisymme... more Alfvén eigenmodes (AEs) driven unstable by energetic particles are routinely observed in axisymmetric tokamak and 3D stellerator plasmas. The most frequently observed Alfvén eigenmodes are weakly damped (and easily excited) gap modes, where poloidal harmonics are coupled by inhomogeneity in the magnetic field. For example, the toroidal Alfvén eigenmode (TAE) is caused by toroidicity-induced coupling of poloidal harmonics in tokamaks. In 3D magnetic configurations, toroidal harmonics are also coupled, producing helicity-induced gaps and helical Alfvén eigenmodes.
The behavior of 1 MeV triton in KSTAR deuterium plasma is studied using triton burnup neutron (TB... more The behavior of 1 MeV triton in KSTAR deuterium plasma is studied using triton burnup neutron (TBN) diagnostics and classical burnup calculation code. In KSTAR, TBN is measured by neutron activation system (NAS) and scintillation detectors. Shot-integrated TBN yield is evaluated by NAS with silicon sample. Two kinds of scintillation detectors, stilbene and scintillating-fiber detectors, provide TBN emission rate. Measured TBN emission is analyzed using classical burnup calculation code. The code evaluates expected TBN emission by considering prompt loss rate and Coulomb drag in certain plasma condition. The amount of prompt loss is statistically evaluated using full orbit following code LORBIT. In addition to prompt loss and Coulomb drag, finite confinement time effect can be considered by volume averaged effective diffusion coefficient. Measured and calculated TBN emission are compared in two timing of Alfvén eigenmodes control experiment, with and without Alfvénic activity. Without Alfvénic activity, calculated TBN generally matched with measured value within experimental error. During the Alfvénic activity however, measured value is about half of the calculated value. The amount of confinement degradation due to Alfvénic activity is estimated in terms of volume averaged effective diffusion coefficient.
Bulletin of the American Physical Society, 2005
Submitted for the DPP05 Meeting of The American Physical Society Modelling nonperturbative freque... more Submitted for the DPP05 Meeting of The American Physical Society Modelling nonperturbative frequency sweeping of MHD modes in magnetic confinement fusion plasmas RODERICK VANN, University of York, U.K., RICHARD DENDY, MIKHAIL GRYAZNEVICH, SERGEI SHARAPOV, UKAEA — Frequency sweeping of magnetohydrodynamic modes for which the sweeping rate is approximately constant have been observed at the Mega-Amp Spherical Tokamak in the presence of heating by energetic particles. A fully nonlinear self-consistent numerical implementation of the Berk-Breizman augmentation of the Vlasov-Maxwell system is applied in the nonperturbative regime; frequency sweeping events phenomenologically comparable to experiment are observed. Experimental observations and simulation results are presented; the existence of a congruence between them is discussed. Roderick Vann University of York, U.K. Date submitted: 22 Jul 2005 Electronic form version 1.4
Fusion Science and Technology, 2008
Nuclear Fusion, 2015
Observations made in a JET experiment aimed at accelerating deuterons to the MeV range by third h... more Observations made in a JET experiment aimed at accelerating deuterons to the MeV range by third harmonic radiofrequency (RF) heating coupled into a deuterium beam are reported. Measurements are based on a set of advanced neutron and gamma-ray spectrometers that, for the first time, observe the plasma simultaneously along vertical and oblique lines of sight. Parameters of the fast ion energy distribution, such as the high energy cutoff of the deuteron distribution function and the RF coupling constant, are determined from data within a uniform analysis framework for neutron and gamma-ray spectroscopy based on an one dimensional model and by a consistency check among the individual measurement techniques. A systematic difference is seen between the two lines of sight and is interpreted to originate from the sensitivity of the oblique detectors to the pitch-angle structure of the distribution around the resonance, which is not correctly portrayed within the adopted one dimensional model. A framework to calculate neutron and gamma-ray emission from a spatially resolved, two dimensional deuteron distribution specified by energy/pitch is thus developed and used for a first comparison with predictions from ab initio models of RF heating at multiple harmonics. The results presented in this paper are of relevance for the development of advanced diagnostic techniques for MeV range ions in high performance fusion plasmas, with applications to the experimental validation of RF heating codes and, more generally, to studies of the energy distribution of ions in the MeV range in high performance deuterium and deuterium-tritium plasmas.
| openaire: EC/H2020/633053/EU//EUROfusionPeer reviewe
We report on recent experiments on the ASDEX Upgrade (AUG) tokamak (major radius R ≈1.65 m, minor... more We report on recent experiments on the ASDEX Upgrade (AUG) tokamak (major radius R ≈1.65 m, minor radius a ≈ 0.5 m) with third harmonic ICRF heating of deuterium beam ions. Prior to this work, the scheme has been developed and applied on the JET tokamak, the largest currently operating tokamak (R ≈ 3 m, a ≈ 1 m), for fusion product studies and for testing alpha particle diagnostics in preparation of ITER [1]. The experiments reported here demonstrate that this scheme can also be used in medium size tokamaks such as AUG despite their reduced fast ion confinement.
Nuclear Fusion, 2019
The JET exploitation plan foresees D-T operations in 2020 (DTE2). With respect to the first D-T c... more The JET exploitation plan foresees D-T operations in 2020 (DTE2). With respect to the first D-T campaign in 1997, when JET was equipped with a carbon wall, the experiments will be conducted in presence of a beryllium-tungsten ITER-like wall (ILW) and will benefit from an extended and improved set of diagnostics and higher additional heating power (34MW NBI + 8MW ICRH). Among the challenges presented by operations with the new wall, there are a general deterioration of the pedestal confinement (not completely explained yet), the risk of heavy impurity accumulation in the core, which, if not controlled, can cause the radiative collapse of the discharge, and the requirement to protect the divertor from excessive heat loads, which may damage it permanently. Therefore, an intense activity of scenario development has been undertaken at JET during the last three years to overcome these difficulties and prepare the plasmas needed to demonstrate stationary high fusion performance and clear alpha-particle effects. The paper describes the main achievements of the scenario developed, both from an operational and plasma physics point of view.
Energetic Particles in Tokamak Plasmas
Nuclear Fusion, 2007
This chapter reviews the progress accomplished since the redaction of the first ITER Physics Basi... more This chapter reviews the progress accomplished since the redaction of the first ITER Physics Basis (1999 Nucl. Fusion 39 2137-664) in the field of energetic ion physics and its possible impact on burning plasma regimes. New schemes to create energetic ions simulating the fusion-produced alphas are introduced, accessing experimental conditions of direct relevance for burning plasmas, in terms of the Alfvénic Mach number and of the normalised pressure gradient of the energetic ions, though orbit characteristics and size cannot always match those of ITER. Based on the experimental and theoretical knowledge of the effects of the toroidal magnetic field ripple on direct fast ion losses, ferritic inserts in ITER are expected to provide a significant reduction of ripple alpha losses in reversed shear configurations. The nonlinear fast ion interaction with kink and tearing modes is qualitatively understood, but quantitative predictions are missing, particularly for the stabilisation of sawteeth by fast particles that can trigger neoclassical tearing modes. A large database on the linear stability properties of the modes interacting with energetic ions, such as the Alfvén eigenmode has been constructed. Comparisons between theoretical predictions and experimental measurements of mode structures and drive/damping rates approach a satisfactory degree of consistency, though systematic measurements and theory comparisons of damping and drive of intermediate and high mode numbers, the most relevant for ITER, still need to be performed. The nonlinear behaviour of Alfvén eigenmodes close to marginal stability is well characterized theoretically and experimentally, which gives the opportunity to extract some information on the particle phase space distribution from the measured instability spectral features. Much less data exists for strongly unstable scenarios, characterised by nonlinear dynamical processes leading to energetic ion redistribution and losses, and identified in nonlinear numerical simulations of Alfvén eigenmodes and energetic particle modes. Comparisons with theoretical and numerical analyses are needed to assess the potential implications of these regimes on burning plasma scenarios, including in the presence of a large number of modes simultaneously driven unstable by the fast ions.
Nuclear Fusion, 2013
The contents of this preprint and all other JET EFDA Preprints and Conference Papers are availabl... more The contents of this preprint and all other JET EFDA Preprints and Conference Papers are available to view online free at www.iop.org/Jet. This site has full search facilities and e-mail alert options. The diagrams contained within the PDFs on this site are hyperlinked from the year 1996 onwards. 2. operAtIon of Jet wIth the Iter-lIke wAll. 2.1 Plasma imPurity content and imPurity sources from Plasma facing comPonents From the very first JET-ILW plasmas the impurity content has been significantly reduced as compared to JET-C conditions. The carbon content is on average a factor 20 lower than in comparable JET-C plasmas as taken by the density normalised intensity of CIII (97.7nm) line (Fig.2). There is no evidence of any significant increase in residual carbon in time, indicating that no damage of the W-coatings on CFC substrate in the divertor has occurred. Oxygen levels are also lower by roughly one order of magnitude with respect to JET-C with non-optimal wall conditions (although only marginally lower than in JET C-wall plasmas with well conditioned wall and following Be evaporations). This large reduction of residual impurity levels is similar to that found earlier by AUG when going from a all-C to a boronised [4] all-W wall. The line averaged Z eff decreases from ~2 (JET-C) to 1.2 (JET-ILW) for similar values of density and heating power. The lower residual carbon and oxygen level in JET-ILW is thought to be mainly due to gettering of carbon and oxygen by beryllium [5]. Specific effort has been devoted to investigate the impact to the operational space due to W core accumulation [6] and surface melting [7]. Beryllium erosion can shorten component lifetimes, contribute to tritium retention by re-deposition [8] and cause significant sputtering of tungsten. Tungsten sputtering has been studied in Land H-mode discharges. It is found that for the present range of temperatures and impurity content at JET most of the W sputtering in L-mode can be attributed to low Z impurities, more specifically Be ions (Fig.3). Experiments in L-mode have been performed to compare the impurity content under neutral beam (NBI) and Ion Cyclotron Resoance Frequency (ICRF) heating. As shown in Fig.4, the bulk radiation, defined as power radiated from inside the separatrix, is higher with ICRF [9] although significant electron heating is obtained and the increase in plasma energy is similar to C-wall values. It was estimated that around 80% of the radiation comes from W and 20% by Nickel with sources from the divertor entrance and main chamber [10] [11]. The cause for this increased radiation remains the subject of ongoing investigation [9]. It could be due to the generation of fast Be ions in front of the antenna following the field lines to the divertor and giving rise to enhanced W sputtering. In H-mode plasmas, measurements of intra and inter ELM radiation from WI lines [12] indicate that ELM events dominate the sputtering of W (Fig.5). Transient increases have been observed in total radiated power, probably associated with small particles of medium/high Z materials, mostly W, entering the plasma. Since the start of plasma operation with the ILW, the frequency of such events has first increased, with the progressive increase of the additional heating power to the plasma, and then decreased. These sudden influxes are not necessarily fatal for the plasma which often recovers from the increased radiation [13]. 2.2. Plasma Breakdown and current rumP-uP The strong decrease of carbon content in the machine with the ILW has lead to a significant reduction
JET neutron profile monitor is a unique instrument among neutron diagnostics available on large f... more JET neutron profile monitor is a unique instrument among neutron diagnostics available on large fusion research facilities. The plasma coverage of the emissive region enables tomographic reconstruction of the spatial profiles of the γ-ray emission. However, due to the availability of only two projection angles and to the coarse sampling, tomography is a highly limited data set problem. Information on plasma equilibrium from EFIT/EFTM is essential in order to compensate for the lack of experimental data and to obtain reconstruction with physical relevance. Several techniques have been developed to cope with this problem. Among them the method based on the maximum likelihood principle, which incorporates a regularizing procedure that assumes smoothness on magnetic surfaces, given by plasma equilibrium proved to be a robust solution [5]. Recently several improvements have been performed in order to allow routinely analysis during the JET experimental campaigns. The method implementatio...
Experiments have been performed on JET to study in detail the linear stability property and the n... more Experiments have been performed on JET to study in detail the linear stability property and the non-linear evolution of Alfvén modes in conventional and advanced tokamak scenarios with internal transport barriers. For the first time the systematic dependence of the damping rate of low-n AEs on the plasma beta, the normalized Larmor radius, the magnetic shear and safety factor, and
Bulletin of the American Physical Society, 2018
Submitted for the DPP10 Meeting of The American Physical Society Identification of the background... more Submitted for the DPP10 Meeting of The American Physical Society Identification of the background plasma damping mechanisms of antenna-driven toroidal Alfven eigenmodes of medium n on JET THEODOROS PANIS, AMBROGIO FASOLI, DUCCIO TESTA, CRPP-EPFL, Lausanne, NICOLAS MELLET, CEA-Cadarache, SERGEI SHARAPOV, CCFECulham, JET-EFDA CONTRIBUTORS TEAM — In tokamak burning plasma experiments such as ITER, it is expected that Alfvén eigenmode (AE) instabilities of, typically, medium and high toroidal mode number n will be triggered by populations of energetic ions, such as α-particles. The stability of this specific class of AEs is studied experimentally in the Joint European Torus by observing the plasma response to antenna-driven frequency-sweeping perturbations at the plasma edge. During the 2008/9 experimental campaigns, the complete set of the new antennas was operated and medium-n AEs were excited under various plasma conditions. A big collection of damping rate measurements of, mainly, t...
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Papers by Sergei Sharapov