Papers by Alexander Karpushov
Nuclear Fusion
In TCV, unstable modes excited by resonant interaction between the shear Alfvèn waves in con- tin... 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 ...
Fusion Engineering and Design
Fusion Technology, 2001
Experiments with 3 MW D0 injection have been carried out in the Gas Dynamic Trap (GDT) to simulat... 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.
Bulletin of the American Physical Society, 2015
Review of Scientific Instruments, 2020
An injector of fast deuterium atoms for plasma heating was designed and installed at the Tokamak ... 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.
Fusion Technology, 2001
The Budker Institute of Nuclear Physics made a proposal for a highly intense neutron source on th... more The Budker Institute of Nuclear Physics made a proposal for a highly intense neutron source on the base of a gas dynamic trap. It is mainly intended for fusion material irradiation. The gas dynamic trap is an axisymmetric open system with a high mirror ratio for the confinement of a collision dominated plasma and a high-energetic ion component which is fed by an oblique neutral beam injection. In addition to research at the experimental facility of the Budker Institute an Integrated Transport Code System is under development in collaboration with the Forschungszentrum Rossendorf. It is to calculate the relevant physical effects connected with the target plasma, fast ions, neutral gas and the neutrons appearing inside the central cell of the device. The paper briefly describes the functions of the main modules and reports on the first exercise devoted to the planned upgrade of the facility.
Fusion Technology, 1999
The paper reports on the results of neutral beam heated high-β two-component plasma confinement s... 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 .
Nuclear Fusion, 2017
A new generation of Neutral Beam (NB) systems will be required in future fusion reactors, such as... more A new generation of Neutral Beam (NB) systems will be required in future fusion reactors, such as DEMO, able to deliver high power, in total up to 50 MW, with high, 800 keV or higher, neutral energy. Only negative ion beams may be able to attain this performance, engendering strong research focus on negative ion production from both surface and volumetric plasma sources. A novel helicon plasma source, based on the resonant birdcage network antenna configuration, is currently under study at the Swiss Plasma Center (SPC), before installation on the Cybele negative ion source at CEA-IRFM. This source is driven by up to 10 kW at 13.56 MHz, and is being tested on a linear Resonant Antenna Ion Device (RAID). Passive spectroscopic measurements of the first three Balmer lines α, β and γ and of the Fulcher-α bands were performed with an f/2 spectrometer, for both hydrogen and deuterium. Multiple viewing lines and an absolute intensity calibration were used to determine the plasma radiance profile, with a spatial resolution < 3 mm. A minimum Fisher regularisation algorithm is applied to obtain the absolute emissivity profile for each emission line, for cylindrical symmetry that was experimentally confirmed. An uncertainty estimate of the inverted profiles was performed using a Monte Carlo approach. Finally, an RFcompensated Langmuir probe was inserted to measured the electron temperature and density profiles. The absolute line emissivities are interpreted using the collisional-radiative code YACORA which estimates the dissociation degree and the distribution of the atomic and molecular species, that includes the negative ion density. This paper reports the results of a power scan up to 5 kW in conditions satisfying Cybele requirements for the plasma source, namely a low neutral pressure, p ≤ 0.3 Pa and magnetic field B ≤ 150 G.
AIP Conference Proceedings, 2016
Neutral beam (NB) injector for the TCV tokamak has been designed to produce a deuterium beam with... 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).
Nuclear Fusion, 2015
Experiments in the TCV tokamak show that high power central electron cyclotron heating (ECH) and ... 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.
1. Experimental setup. These experiments on the TCV Tokamak (R=0.88 m, a=0.25 m, Ip < 1 MA, BT... 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.
Plasma Physics Reports
The diagnostic apparatus used for measurement of plasma parameters in a gasodynamical confinement... 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.
Fusion Technology 1994, 1995
Fusion Engineering and Design, 2015
Fusion Engineering and Design, 2015
ABSTRACT Experiments on TCV are designed to complement the work at large integrated tokamak facil... 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.
Direct and indirect poloidal rotation measurements with improved accuracy were performed and comp... 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...
The results of extensive experimental investigations of a plasma magnetohydrodynamic (MHD) stabil... 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.
Plasma Physics Reports, 2002
The dynamics of the ion temperature of the target plasma in a gas-dynamic trap during high-power ... more The dynamics of the ion temperature of the target plasma in a gas-dynamic trap during high-power neutral beam injection is measured by using the Rutherford scattering technique. A comparison of the experimental results with the results of simulations by a model based on the theory of pair Coulomb collisions indicates no significant anomalous losses from the ion plasma component.
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Papers by Alexander Karpushov