Alexander Karpushov
EPFL, Swiss Plasma Center, Faculty Member
In TCV, unstable modes excited by resonant interaction between the shear Alfvèn waves in con- tinuum gaps and energetic particles have been observed in scenarios with Neutral Beam Injection (NBI). TCV is a middle-size device (R 0 /a =... more
In TCV, unstable modes excited by resonant interaction between the shear Alfvèn waves in con- tinuum gaps and energetic particles have been observed in scenarios with Neutral Beam Injection (NBI). TCV is a middle-size device (R 0 /a = 0.88/0.25) equipped with a 1 MW, 25 keV tangential neutral beam injector. In this paper the phenomenology of modes excited with on-axis NBI is presented. The Alfvènic nature of the modes has been confirmed investigating their sensitivity against plasma parameters such as NBI energy, toroidal magnetic field, and cross-checking with the predictions from linear kinetic stability code. The mode radial profile is estimated using Electron Cyclotron Emission mea- surement and agrees well with modelling results. In addition, the fast particle distribution function has been modeled using TRANSP/NUBEAM code. Even with counter-current NBI (leading to higher losses), the drive from the resonant particles is sufficient for the mode excitation. An ad-hoc additional ...
Research Interests:
Research Interests:
Experiments with 3 MW D0 injection have been carried out in the Gas Dynamic Trap (GDT) to simulate the axial profile of the fusion reaction intensity in the projecting neutron source based on the GDT1. Quite narrow angular distribution... more
Experiments with 3 MW D0 injection have been carried out in the Gas Dynamic Trap (GDT) to simulate the axial profile of the fusion reaction intensity in the projecting neutron source based on the GDT1. Quite narrow angular distribution function of the fast ions produced by an oblique neutral beam injection results in a peaked axial profile of the fusion yield. This strong peaking is essential to produce intense neutron flux in the testing zones of the GDT–based neutron source. The scintillation counters were installed in the central cell of the device to monitor the DD fusion reactions products: neutrons (2.45 MeV) and protons (3.02 MeV). Scintillation detectors were located closely to the plasma column inside of the vacuum vessel to avoid contribution from the scattered neutrons and to improve spatial resolution of the measurements. Longitudinal profiles of 2.45 MeV neutrons and 3.02 MeV protons have been measured in the high-beta regime of the GDT operation. In the paper the experimental data are compared with the results of numerical simulations 2. The conclusion is drawn that the kinetics of the fast ion relaxation and scattering is determined by classical Coulomb collisions 3.
Research Interests:
Research Interests:
Research Interests:
Research Interests:
An injector of fast deuterium atoms for plasma heating was designed and installed at the Tokamak à Configuration Variable (TCV). The neutral beam can deliver 1 MW power to the plasma in 2 s pulses. An ion beam of the injector is formed by... more
An injector of fast deuterium atoms for plasma heating was designed and installed at the Tokamak à Configuration Variable (TCV). The neutral beam can deliver 1 MW power to the plasma in 2 s pulses. An ion beam of the injector is formed by a triode multislit ion-optical system with spherical electrodes which provide ballistic focusing. Tests at TCV revealed that the total angular divergence of the neutral beam across the slits exceeded the expected value more than twice. It was finally established that this increase in divergence was caused by the asymmetry of the chamfers at the slit edges of the plasma electrode. The redesigned shape of the slits of the plasma grid together with precise machining significantly improved the beam quality. Experimental testing proved that the neutral beam profile in the direction across the grid slits became very close to the expected value.
Research Interests:
Research Interests:
The paper reports on the results of neutral beam heated high-β two-component plasma confinement studies in Gas-Dynamic Trap experiment 1,2 . The diagnostic set enabled us to characterize the different energy losses channels in the GDT 3 .... more
The paper reports on the results of neutral beam heated high-β two-component plasma confinement studies in Gas-Dynamic Trap experiment 1,2 . The diagnostic set enabled us to characterize the different energy losses channels in the GDT 3 . The energy balance models were applied for analysis of plasma heating 4 . The experimental results on the measurements of the local energy and angular distributions of the fast ions minority are described. These measurements were done by injection of focused diagnostic neutral beam served to produce local charge-exchange target for fast ions 5 . Charge-exchange atoms were registered by an electrostatic energy analyzer. The energy distribution function obtained with this diagnostic was compared in detail with the results of Monte-Carlo 6 and Fokker-Planck simulations 7 .
Research Interests:
Research Interests:
Neutral beam (NB) injector for the TCV tokamak has been designed to produce a deuterium beam with energy 30 keV, equivalent current up to 50 A, and pulse duration 2 s. The injector operation is accompanied by generation of fast neutrons... more
Neutral beam (NB) injector for the TCV tokamak has been designed to produce a deuterium beam with energy 30 keV, equivalent current up to 50 A, and pulse duration 2 s. The injector operation is accompanied by generation of fast neutrons produced in deuterium-deuterium collisions via a nuclear fusion reaction D(D,n)3He. Main sources of the neutrons are a beam neutralizer and a deuterium-saturated surface of beam dump. Measurements of the neutron yields from the both sources were produced on the prototype of TCV injector in the Budker Institute of Nuclear Physics. Neutron yields from neutralizer and beam dump are equal to 9.5x108 s−1 and 2.3 × 109 s−1 for the nominal parameters of the injector (30 kV, 50 A).
Research Interests:
Experiments in the TCV tokamak show that high power central electron cyclotron heating (ECH) and current drive (ECCD) produce significant direct modification of the plasma rotation profile, as well as an effect on the equilibrium current... more
Experiments in the TCV tokamak show that high power central electron cyclotron heating (ECH) and current drive (ECCD) produce significant direct modification of the plasma rotation profile, as well as an effect on the equilibrium current density profile. In a regime of unsteady rotation, these effects contribute to the onset of neoclassical tearing instabilities, in the absence of triggers such as sawteeth, edge localised modes (ELMS) or relevant ‘error’ fields. In turn the growing tearing modes' breaking axisymmetry provides a nonlinear magnetic torque which converts the power absorption in a co-directed rotation with a flattening of the profile at the rational surfaces. The experimental results are presented and discussed in the context of theoretical models of neoclassical toroidal viscosity and ion inertial effects.
Research Interests:
1. Experimental setup. These experiments on the TCV Tokamak (R=0.88 m, a=0.25 m, Ip < 1 MA, BT < 1.54 T) em ployed the X2 (82.7 GHz) EC system , with a total delivered power of up to 2.4 MW in X-mode, for a wide variety of plasma... more
1. Experimental setup. These experiments on the TCV Tokamak (R=0.88 m, a=0.25 m, Ip < 1 MA, BT < 1.54 T) em ployed the X2 (82.7 GHz) EC system , with a total delivered power of up to 2.4 MW in X-mode, for a wide variety of plasma shapes and a broad range of heating locations. The central electron density was 1-2·10 19 m -3 , the electron temperature reached 7 keV at full X2 EC power, the plasma elongation was 1.15-1.6 and the triangularity –0.3-+0.3. Modulation of the EC power was used to study the dynamics of the fast particle population (Fig.1). A 5-channel Neutral Particle Analyzer [9] with electrostatic discrimination on TCV views the plasma centre along a vertical chord. The NPA voltage sweeps the energy channels to measure neutral particle energies in the energy range of (0.6›6.5 keV), with a time resolution of 13 ms. A 14 vertical channel FIR interferometer is used to m easure the electron density profile. Plasma electron temperature profiles were derived from the soft X-ray radiation spectrum measurements (XTemeasurements). The high energy electron population created by ECCD is diagnosed primarily with a hard X-ray (HXR) pinhole camera (on loan from Tore Supra) and with a high field side electron cyclotron emission (ECE) system. The EC power deposition and current drive profiles (Fig.2) are calculated by the TORAY ray-tracing code with magnetic equilibrium reconstruction from the LIUQE code and Thomson scattering electron temperature and density profiles.
Research Interests:
The diagnostic apparatus used for measurement of plasma parameters in a gasodynamical confinement system with atomic beam injection is described. The used diagnostics made it possible to find the local energy balance (along a given field... more
The diagnostic apparatus used for measurement of plasma parameters in a gasodynamical confinement system with atomic beam injection is described. The used diagnostics made it possible to find the local energy balance (along a given field line) of the background plasma and the fast ion fraction. For the regimes with MHD-stable energy confinement, it was shown that the main energy losses from the plasma bulk are due to the particle flow through the end mirrors. 15 refs., 4 figs.
Research Interests:
Research Interests:
Research Interests:
ABSTRACT Experiments on TCV are designed to complement the work at large integrated tokamak facilities (such as JET) to provide a stepwise approach to extrapolation to ITER and DEMO in areas where medium-size tokamaks can often exploit... more
ABSTRACT Experiments on TCV are designed to complement the work at large integrated tokamak facilities (such as JET) to provide a stepwise approach to extrapolation to ITER and DEMO in areas where medium-size tokamaks can often exploit their experimental capabilities and flexibility. Improving the understanding and control requirements of burning plasmas is a major scientific challenge, requiring access to plasma regimes and configurations with high normalized plasma pressure and a wide range of ion to electron temperature ratios, including Te/Ti ∼ 1. These conditions will be explored by adding a 1 MW neutral heating beam to TCV&#39;s auxiliary for direct ion heating (2015) and increasing the ECH power injected in X-mode at the third harmonic (2 MW in 2015–2016). The manufacturing of the neutral beam injector was launched in 2014.
Research Interests:
Direct and indirect poloidal rotation measurements with improved accuracy were performed and compared in the TCV tokamak. The indirect measurement argues that, provided the plasma flow is divergence free on a flux surface, poloidal... more
Direct and indirect poloidal rotation measurements with improved accuracy were performed and compared in the TCV tokamak. The indirect measurement argues that, provided the plasma flow is divergence free on a flux surface, poloidal rotation can be inferred from the toroidal rotation at the high and low field sides of a flux surface. The key advantage of the method is an intrinsic amplification factor: instead of measuring poloidal rotation directly (typically few km/s i.e. of the order of the measurement accuracy), a difference in toroidal rotation is measured that is 4 to 10 times larger. Here, the main uncertainties arise from the flux surface mapping that are, however, largely compensated by this amplification factor. In TCV, the C^6+ toroidal rotation was measured across the whole plasma diameter by charge exchange (CX) spectroscopy for a series of low collisionality (0.1<&*circ;<1.5) OH and ECH L-mode plasmas, including positive and negative plasma current and toroidal ma...
Research Interests:
The results of extensive experimental investigations of a plasma magnetohydrodynamic (MHD) stability in gas-dynamic trap GDT, which were carried out over the past 3 years at Novosibirsk are summarized. The objective of the experiments... more
The results of extensive experimental investigations of a plasma magnetohydrodynamic (MHD) stability in gas-dynamic trap GDT, which were carried out over the past 3 years at Novosibirsk are summarized. The objective of the experiments reported was to give experimental evidence of plasma stability in GDT and compare the conditions under which it would be achieved, with the theoretical predictions. It was found that theoretical predictions generally agree with the measurements with only one notable exception, namely the limit in the mirror ratio for the plasma stability. This observation is discussed in detail.
Research Interests:
Research Interests:
Abstract. The effect of titanium conditioning of the chamber wall on the plasma confinement in a gas-dynamic confinement system is studied. After depositing titanium on the inner surface of the vacuum chamber, the charge-exchange loss of... more
Abstract. The effect of titanium conditioning of the chamber wall on the plasma confinement in a gas-dynamic confinement system is studied. After depositing titanium on the inner surface of the vacuum chamber, the charge-exchange loss of fast ions turns out to be ...
Research Interests:
For the titanium coating of the GDT first wall we used arc- type evaporators developed at the Budker INP. Highly reliable trigger devices based on electrical breakdown over the surface of the insulator were used to initiate the vacuum... more
For the titanium coating of the GDT first wall we used arc- type evaporators developed at the Budker INP. Highly reliable trigger devices based on electrical breakdown over the surface of the insulator were used to initiate the vacuum arc. The application of the trigger devices allowed one to use a compact pulsed-power supply and coat the first wall a few seconds before the operating cycle of the experimental apparatus. It has been shown in experiments with NB heating that the wall recycling coefficient does not exceed 0.7 for 8 keV mean energy of the neutral hydrogen atoms striking the wall. The use of the titanium coating of the GDT first wall allowed an increase in charge-exchange lifetime of hot ions by a factor of 10.
Research Interests:
The tokamak à configuration variable (TCV) continues to leverage its unique shaping capabilities, flexible heating systems and modern control system to address critical issues in preparation for ITER and a fusion power plant. For the... more
The tokamak à configuration variable (TCV) continues to leverage its unique shaping capabilities, flexible heating systems and modern control system to address critical issues in preparation for ITER and a fusion power plant. For the 2019–20 campaign its configurational flexibility has been enhanced with the installation of removable divertor gas baffles, its diagnostic capabilities with an extensive set of upgrades and its heating systems with new dual frequency gyrotrons. The gas baffles reduce coupling between the divertor and the main chamber and allow for detailed investigations on the role of fuelling in general and, together with upgraded boundary diagnostics, test divertor and edge models in particular. The increased heating capabilities broaden the operational regime to include T e/T i ∼ 1 and have stimulated refocussing studies from L-mode to H-mode across a range of research topics. ITER baseline parameters were reached in type-I ELMy H-modes and alternative regimes with ...
Research Interests:
dated 11th of October, 2006, revised following the advice of the referees, resubmitted Abstract. The flux of charge exchange (CX) neutrals measured by Neutral Particle Analyzers (NPA) is the line integral along the view line of the NPA... more
dated 11th of October, 2006, revised following the advice of the referees, resubmitted Abstract. The flux of charge exchange (CX) neutrals measured by Neutral Particle Analyzers (NPA) is the line integral along the view line of the NPA and contains information about the ion energy distribution of the observed plasma. On the Tokamak a ̀ Configuration Variable (TCV) a single chord NPA is used to scan the plasma cross section by vertically displacing a reproducible discharge across its fixed line of sight. The ion temperature inferred from the passive CX flux as a function of the distance of the NPA chord to the magnetic axis is used to obtain an ion temperature profile Ti(ρ). To model the neutral source, simulations of neutral particle penetration from the edge and the neutralization processes are reported. In plasmas with thermalized ion populations, the NPA hydrogen or deuterium temperature profiles agree with the carbon ion temperature profile measured by Charge eXchange Recombinat...
Experiments on the TCV tokamak show that central electron cyclotron heating (ECH) and current drive (ECCD) can modify the rotation profile and lead to the onset of neoclassical tearing instabilities, in absence of triggers such as... more
Experiments on the TCV tokamak show that central electron cyclotron heating (ECH) and current drive (ECCD) can modify the rotation profile and lead to the onset of neoclassical tearing instabilities, in absence of triggers such as sawteeth, ELMS or relevant ”error” field, but in a regime of unsteady rotation. In turn the growing tearing modes provide a nonlinear magnetic braking that flattens the rotation profile. The experimental results are presented and discussed in the frame of the theoretical models of neoclassical toroidal viscosity and ion inertial effects.
Reference EPFL-CONF-218581 URL: http://www.ipfn.ist.utl.pt/EPS2015/ URL: https://crpplocal.epfl.ch/pinboard/papers/154805601.pdf Record created on 2016-05-29, modified on 2017-12-20
Research Interests:
Powerful ECH with an adaptable launching geometry and plasma shaping capability are exploited on TCV to create and control high performance regimes, with 3MW of 2nd and 1.5MW of 3rd harmonic (X3) and real time optimisation of the... more
Powerful ECH with an adaptable launching geometry and plasma shaping capability are exploited on TCV to create and control high performance regimes, with 3MW of 2nd and 1.5MW of 3rd harmonic (X3) and real time optimisation of the absorption by mirror feedback. Full X3 absorption with launching parallel to the resonant surface was obtained. Electron temperature profile stiffness was measured as a function of the shape up for a large range of temperature gradients and confirmed that the diffusivity is lower at negative triangularity and high elongation. The link between shear and transport was verified by interleaved modulation of coand counterECCD. ECCD efficiency and fast electron generation and transport measurements demonstrate the role of transport on the driven current profile. Stationary electron ITBs were created and the role of the current profile in transport reduction was clarified by improving or destroying the barrier with a small induced electric field.
Research Interests:
A new improved L-mode has been found in TCV with high confinement properties, high edge and global density and no edge temperature barrier. The ohmic IN-modes are presented. They have been obtained in two ways. One thanks to a transient... more
A new improved L-mode has been found in TCV with high confinement properties, high edge and global density and no edge temperature barrier. The ohmic IN-modes are presented. They have been obtained in two ways. One thanks to a transient H-mode, even though the plasma is limited, and another with a high reference density at t=0. H98y2 1, H89P=1.6 has been obtained with q95=2.7-3.5, kappa 1.5 and betaN 1.4.
Research Interests:
N. VIANELLO1, D. CARRALERO2, 3, C. K. TSUI4, 5, V. NAULIN6, M. AGOSTINI1, J. BOEDO4, B. LABIT5, C. THEILER5, D. AGUIAM7, S. ALLAN8, M. BERNERT3, S. COSTEA9, I. CZIEGLER10, H. DE OLIVEIRA5, J. GALDON-QUIROGA11, G. GRENFELL1, A. HAKOLA12,... more
N. VIANELLO1, D. CARRALERO2, 3, C. K. TSUI4, 5, V. NAULIN6, M. AGOSTINI1, J. BOEDO4, B. LABIT5, C. THEILER5, D. AGUIAM7, S. ALLAN8, M. BERNERT3, S. COSTEA9, I. CZIEGLER10, H. DE OLIVEIRA5, J. GALDON-QUIROGA11, G. GRENFELL1, A. HAKOLA12, C. IONITA9, H. ISLIKER13, A. KARPUSHOV5, J. KOVACIC14, B. LIPSCHULTZ10, R. MAURIZIO4, K. MCCLEMENTS8, F. MILITELLO8, J. OLSEN6, J. J. RASMUSSEN6, T. RAVENSBERGEN16, H. REIMERDES5, B. SCHNEIDER9, R. SCHRITTWIESER9, M. SPOLAORE1, K. VERHAEGH10, J. VICENTE7, N. WALKDEN8, W. ZHANG3, E. WOLFRUM3, the ASDEXUpgrade Team3, the TCV team17 and and the EUROfusion MST1 Team18 1 Consorzio RFX, Padova,Italy, CIEMAT Laboratorio Nacional de Fusión, Madrid, Spain, Max-Planck-Institut für Plasmaphysik, Garching, Germany, EPFL-SPC, Switzerland, UCSD, La Jolla, USA, DTU, Lyngby, Denmark, IPFN, Instituto Superior Técnico, Lisboa, Portugal, CCFE, Culham, UK, Institute for Ion Physics and Applied Physics, Innsbruck, Austria, York Plasma Institute, University of York, UK, U...
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests: Engineering and Physics
Research Interests: Engineering and Tearing
ABSTRACT TCV has the highest power (3MW in X2 and 1.5MW in X3, 2.0s) and most ITER-relevant ECH system in the world today, owing to its real-time poloidal and toroidal steerable mirrors. This paper summarizes recent TCV discharges using... more
ABSTRACT TCV has the highest power (3MW in X2 and 1.5MW in X3, 2.0s) and most ITER-relevant ECH system in the world today, owing to its real-time poloidal and toroidal steerable mirrors. This paper summarizes recent TCV discharges using ECH. In non-inductive discharges, the co-ECCD deposition was placed off-axis creating a large bootstrap fraction and a flat or slightly reversed shear current profile, even though the EC driven current is peaked or only slightly hollow due to radial diffusion. After reaching equilibrium, on-axis counter-ECCD was injected enhancing the reversed shear by increasing the bootstrap fraction ( 80%) and rapidly increasing the confinement time (tau_e), a factor of 1.6 over ITER L-mode scaling. The ITB is generated in stationary conditions and is sustained for 10 times the current redistribution time. Experimental results have shown that the improved taue is directly related to the degree of reverse shear in the center. In particular, the electron temperature and pressure profiles are controlled by the amount of counter-ECCD on axis, and in particular the gradients can be tuned to be marginally MHD stable. No power threshold for the ITB formation is observed. In other discharges the sawteeth are de-stabilized (stabilized) when heating just inside (outside) the q=1 surface. Control of the sawteeth allows the avoidance of NTMs that are otherwise seeded by the sawtooth instability. A transport code linked to a sawtooth model reproduces the variation in sawtooth period with the EC deposition location. Finally, preliminary results with the X3 top-launch have shown 70% single-pass absorption with 0.9MW and densities up to 0.5 10^20m-3 in good agreement with the absorption calculated by a linear ray tracing code.
Research Interests:
Research Interests:
Experimental apparatus and data analysis techniques used in neutral particle analyzer (NPA) diagnostics on the Tokamak à Configuration Variable (TCV) are described. Two NPAs are used on TCV to measure the energy spectrum of neutral... more
Experimental apparatus and data analysis techniques used in neutral particle analyzer (NPA) diagnostics on the Tokamak à Configuration Variable (TCV) are described. Two NPAs are used on TCV to measure the energy spectrum of neutral particle fluxes from the plasma. The “five-channel energy analyzer of atomic particles” used in double electrical analysis mode with fast voltage sweeping detect particles without atomic mass discrimination in the energy range of 0.6–8.0keV with a time resolution of 0.5–2.0ms and an energy resolution of 7%–20%. The 28-channel “compact neutral particle analyzer” (CNPA) is an EIIB spectrometer with mass and energy separations designed for medium sized fusion machines featuring a carbon neutral stripping foil, a permanent magnet for dispersion, and channel-electron multiplier detectors. The CNPA simultaneously detects two mass species [hydrogen (H) and deuterium (D) or D and helium (He)] in the 0.5–50keV energy range with a resolution of 60%–10% and a time r...