Nonmonotonic Spatial Distributions of Charge-Exchange Neutral Fluxes and Optical Radiation from Plasma in the DAMAVAND Tokamak

Results are presented from investigations of the nonmonotonic spatial distributions of charge-exchange neutral fluxes and optical radiation from plasma in the DAMAVAND tokamak. It is shown that, during ohmic heating of the plasma, the regions with enhanced confinement of both the background plasma particles and heavy impurity ions arise near rational magnetic surfaces with q = 1 and 2. These regions are characterized by enhanced emission of accelerated charge-exchange neutrals and optical radiation from impurity ions.     

Particle diagnostics of ion cyclotron resonance plasma heating in the Globus-M tokamak

Basic experimental results on cyclotron heating of the ion plasma component in the Globus-M spherical tokamak obtained by means of the ACORD-12 charge-exchange ion analyzer are presented. A procedure for determining the maximum energy of fast ions confined in the plasma is described. The procedure was applied to estimate the limiting energy of hydrogen minority ions accelerated during ion cyclotron heating in the Globus-M tokamak. The experimental evaluation of the maximum hydrogen ion energy is confirmed by simulations of ion orbits. Recommendations for optimizing experiments on ion cyclotron heating in the Globus-M tokamak are formulated.     

Development of the concept of charge-exchange recombination spectroscopy for ITER

Charge-exchange recombination spectroscopy under ITER conditions is numerically simulated using the DINA code. The code allows one to calculate the attenuation of neutral beams in plasma, the intensity of atomic emission and emission from the ions produced by charge exchange with the neutral beam, and the intensity ratio between the emission related to charge exchange and the background continuum plasma emission. The cross sections for atom-ion interactions in plasma are calculated, and the excitation rates of spectral transitions in hydrogen-like impurity ions are determined. The measurement scheme and the main parameters of the spectrometer proposed for charge-exchange recombination diagnostics in ITER are described.     

The effect of charge exchange with neutrals on the saturation of the spectral line intensities of multicharged ions in plasma

The influence of charge-exchange processes on the spectral line intensities of impurity ions in the edge and core plasmas of fusion devices is considered. It is found that, at a sufficiently high density of neutrals, the rate at which the atomic states are populated through charge exchange becomes independent of the neutral density, which results in the saturation of the spectral line intensities. This effect can substantially limit the efficiency of impurity-ion spectroscopy. Conditions under which the saturation effect manifests itself are examined for both the edge and core plasma regions in the presence of fast neutral beams. The results of calculations for the edge plasma are used to interpret the experimental data from the TORE SUPRA tokamak. It is shown that, in the central plasma region, the intensities of the visible spectral lines associated with the charge exchange of impurity ions in the course of neutral beam injection decrease (rather than increase, as was expected earlier) with increasing ion charge.     

Monitoring of the plasma isotope composition by charge-exchange neutral fluxes in a rippled toroidal magnetic field

To determine the hydrogen isotope ratio in plasma from charge-exchange neutral fluxes, certain assumptions are traditionally adopted, the most restrictive of which concerns the form of the ion distribution function, which is usually assumed to be Maxwellian. For large tokamaks, however, additional analysis is required in order to determine the energy range in which distortions of the distribution function will not lead to errors in isotope ratio measurements. The possible influence of drift motion on the ion distribution function is considered. Experimental results obtained in the ASDEX-Upgrade tokamak are presented. The role this mechanism plays during the transition to the H-mode in the auxiliary heating regime is compared to that in the ohmic heating regime.     

CXSFIT Code Application to Process Charge-Exchange Recombination Spectroscopy Data at the T-10 Tokamak

The applicability of the CXSFIT code to process experimental data from Charge-eXchange Recombination Spectroscopy (CXRS) diagnostics at the T-10 tokamak is studied with a view to its further use for processing experimental data at the ITER facility. The design and operating principle of the CXRS diagnostics are described. The main methods for processing the CXRS spectra of the 5291-Å line of C⁵⁺ ions at the T-10 tokamak (with and without subtraction of parasitic emission from the edge plasma) are analyzed. The method of averaging the CXRS spectra over several shots, which is used at the T-10 tokamak to increase the signal-to-noise ratio, is described. The approximation of the spectrum by a set of Gaussian components is used to identify the active CXRS line in the measured spectrum. Using the CXSFIT code, the ion temperature in ohmic discharges and discharges with auxiliary electron cyclotron resonance heating (ECRH) at the T-10 tokamak is calculated from the CXRS spectra of the 5291-Å line. The time behavior of the ion temperature profile in different ohmic heating modes is studied. The temperature profile dependence on the ECRH power is measured, and the dynamics of ECR removal of carbon nuclei from the T-10 plasma is described. Experimental data from the CXRS diagnostics at T-10 substantially contribute to the implementation of physical programs of studies on heat and particle transport in tokamak plasmas and investigation of geodesic acoustic mode properties.     

Impurity radiation from a tokamak plasma

In tokamak operating modes, energy balance is often governed by impurity radiation. This is the case near the divertor plates, during impurity pellet injection, during controlled discharge disruptions, etc. The calculation of impurity radiation is a fairly involved task (it is sometimes the most difficult part of the general problem) because the radiation power is determined by the distribution of ions over the excited states and by the rate constants of elementary processes of radiation and absorption. The objective of this paper is to summarize in one place all the approximate formulas that would help investigators to describe radiation from the most often encountered impurities in a fairly simple way in their calculations accounting for plasma radiation, without reference to special literature. Simple approximating formulas describing ionization, recombination, and charge-exchange processes, as well as radiative losses from ions with a given charge, are presented for five impurity species: beryllium, carbon, oxygen, neon, and argon. Estimating formulas that allow one to take into account plasma opacity for resonant photons in line impurity radiation are also presented.     

Kinetics of impurity charge-state distributions in tokamak plasmas

An analysis of impurity behavior in tokamak plasmas with the use of the observation results on impurity emission shows that it is necessary to distinguish between the ion dynamics (for example, ion transport) and ion kinetics, i.e., the processes related to the motion of ions on the charge states and/or excited states due to atomic processes in plasma. This paper presents a systematic analysis of the kinetics of impurity chargestate distributions and the related effects, as well as their typical scales and conditions for their observation. The quantitative analysis is performed in terms of the lowest moments of charge-state distributions such as the average charge m and dispersion D. Analytic approaches to solving charge-state kinetic equations are considered. An approach based on the symmetry properties of the kinetic matrix is proposed for the first time. The simplest types of impurity charge-state kinetics and the most important limiting cases are considered. A detailed analysis of the nonstationary behavior of the function of the moments D(m) of the charge-state distribution is presented. A quantitative analysis of the available experimental and model charge-state distributions of C, O, Ne, and Ar impurities in the JET, DIII-D, TORE SUPRA, ALCATOR-C, TEXTOR, PLT, TFR, and DAMAVAND tokamaks is performed in terms of the moments D(m). It is shown that the moments D(m)of the model charge-state distributions of the above impurities in the plasma core are essentially insensitive to the empirical diffusion coefficient. The equivalent curves D(m) obtained for the plasma periphery can be attributed to the convective fluxes of ionizing and/or recombining impurity ions.     

Experimental study and numerical simulation of ion cyclotron heating of a hydrogen plasma in the T-11M tokamak

Results are presented from investigations of the possibility of heating a hydrogen plasma at the fundamental harmonic of the ion cyclotron frequency in the T-11M tokamak. The fluxes of charge-exchange atoms that escape from the plasma in the radial direction and across the toroidal magnetic field (transverse neutrals) were recorded by a Lakmus neutral particle analyzer. Measurements by the analyzer show that, during an RF pulse, the ion temperature increases by approximately 50–100 eV. Such plasma parameters as the ion temperature, rotation velocity, and isotopic composition were measured by a high-resolution spectrometer. According to the data from high-resolution spectroscopy, the ion temperature increases by approximately 150 eV. Results from numerical simulations of the ion cyclotron resonance heating of a hydrogen plasma in the T-11M tokamak are also given.     

Spatial distribution of the plasma temperature under ion-beam fast ignition

Results are presented from theoretical studies of the formation of the spatial temperature distribution in plasma heated by a high-energy ion beam under the conditions in which the free path lengths of ions of different parts of the beam in plasma varies in the course of its heating. Special attention is paid to ionbeam heating of deuterium-tritium (DT) plasma under the conditions of fast ignition of inertial confinement fusion (ICF) targets. The influence of the initial energy spectrum of the heating beam ions on the spatial temperature distribution is investigated. For beams with different ion charges, masses, and initial energy spectra, criteria are determined for the formation of different types of spatial temperature distributions, namely, a distribution with a negative temperature gradient and a quasi-uniform distribution, which correspond to the edge ignition of a precompressed ICF target, as well as a distribution with a temperature peak, which corresponds to the ignition in the inner region of the target.     

Recombination kinetics of impurities during a major disruption in tokamak plasmas

The investigations of major disruptions in the TVD and DAMAVAND tokamaks showed that, in the rapid phase of disruption, accelerated (～1 keV) ions and charge-exchange neutrals are generated near the rational magnetic surfaces; this is accompanied by the bursts of line emission from light impurities (C, O). In the present paper, an analysis is made of the bursts of the CV triplet emission (2271–2278 Å) observed over all of the viewing chords in high-current discharges and also of a decrease in the spectral line emission below its initial (predisruption) level both in the plasma core and at the plasma edge in low-current discharges. The data from measurements of the spatial and temporal parameters of the CV line emission from the central and peripheral plasma regions in the rapid phase of disruption in the DAMAVAND tokamak are compared to the results from model calculations of the kinetics of the charge-state distribution of carbon impurity ions (during the disruption, their kinetics is governed by the increase in the effective recombination rate). A key result of the kinetic model is an increase in the effective rate of charge exchange of impurity ions by two orders of magnitude. Numerical simulations show that the dispersion of the charge-state distribution increases substantially; this is attributed to the rapid phase of disruption being dominated by the recombination of impurity ions through charge exchange with neutrals rather than by the anomalous transport. In this case, carbon impurities in the plasma are transported to the region of increased radiative losses on a time scale of 50 μs.     

Charge-exchange recombination spectroscopy of the plasma ion temperature at the T-10 tokamak

Charge-exchange recombination spectroscopy (CXRS) based on a diagnostic neutral beam has been developed at the T-10 tokamak. The diagnostics allows one to measure the ion temperature profile in the cross section of the plasma column. In T-10 experiments, the measurement technique was adjusted and the elements of the CXRS diagnostics for ITER were tested. The used spectroscopic equipment makes it possible to reliably determine the ion temperature from the Doppler broadening of impurity lines (helium, carbon), as well as of the spectral lines of the working gas. The profiles of the plasma ion temperature in deuterium and helium discharges were measured at different plasma currents and densities, including with the use of active Doppler measurements of lines of different elements. The validity and reliability of ion temperature measurements performed by means of the developed CXRS diagnostics are analyzed.     

Study of fast-ion losses in experiments on neutral beam injection on the Globus-M spherical tokamak

The paper presents a review of the main results on the heating of plasma ions and behavior of fast ions in experiments on neutral beam injection (NBI) carried out in 2003–2010 on the Globus-M spherical tokamak. It is noticed that, along with significant success achieved in NBI plasma heating, there is experimental evidence indicating significant losses of the power injected into the plasma. Most probably, the power is lost due to so-called first-orbit losses, i.e., losses of fast ions that are produced in plasma after ionization of beam atoms and occur in unconfined trajectories. Until recently, the absence of appropriate diagnostic equipment did not allow one to verify this hypothesis. The use of the ACORD-M charge-exchange analyzer directed tangentially to the plasma column made it possible to measure the spectra of fast ions slowed down in plasma and confirm the assumption on the presence of substantial orbit losses of fast particles (～25–50% of the beam power). In addition to the review of the experimental results, the paper presents analysis of orbit losses on the basis of 3D simulations of fast-ion trajectories in plasma. The results of experiments on studying the influence of the magnitude of the tokamak magnetic field on the confinement of fast ions are also presented. Along with computer simulations, these experiments made it possible to formulate recommendations on the reduction of orbit losses in the Globus-M tokamak.     

Possibility of X-ray spectral diagnostics of a plasma in the tracks of fast multicharged ions

A plasma model of the relaxation of a medium within the tracks of heavy ions in condensed matter is proposed that is based on solving time-dependent radiative collisional kinetic equations with the initial condition corresponding to a medium’s state described by the classical model of multiple ionization of the target atoms by the field of fast multicharged ions. It is shown that the plasma model allows one to describe X-ray spectra recorded in the interaction of ion beams with condensed targets. An X-ray spectral method for plasma diagnostics is proposed that is based on the plasma model. The results obtained can also be used to study the initial stage of the formation of defects in solid bodies under the action of individual fast heavy ions.     

Calculation of the ion distribution function over transverse velocities under ICR heating conditions and separation parameters of a collector of heated ions

The ion distribution function over transverse velocities and the ion heating efficiency (which is defined as the fraction η of ions heated above a certain energy W_min) are calculated in the context of a plasma method for isotope separation on the basis of ion cyclotron resonance heating. The ion distribution function over longitudinal velocities is assumed to be linear in the range of low velocities. It is shown that, when the ions are heated to high energies, the averaged ion distribution function over transverse velocities becomes highly nonequilibrium and has two peaks. Results are presented from calculations of the ion heating efficiency η for W_min=40 eV and for different values of the parameter p that characterizes the ratio of the wavelength λ of the antenna electric field to the length L of the heating region. The relative roles of the time-of-flight and the Doppler broadening are analyzed, and the separation parameters of a collector of heated ions are estimated.     

Investigation of ion acceleration in an expanding laser plasma by using a hybrid Boltzmann-Vlasov-Poisson model

The acceleration of ions of different species from a plasma slab under the action of a charge-separation electric field driven by hot and cold electrons is studied by using a hybrid Boltzmann-Vlasov-Poisson model. The obtained spatial and energy distributions of light and heavy ions in different charge states demonstrate that the model can be efficiently used to study the ion composition in a multispecies expanding laser plasma. The regular features of the acceleration of ions of different species are investigated. The formation of compression and rarefaction waves in the halo of light ion impurity, as well as their effect on the energy spectrum of the accelerated ions, is analyzed. An approach is proposed that makes it possible to describe the production of fast ions by laser pulses of a given shape. It is shown that the energy of fast ions can be increased markedly by appropriately shaping the pulse. The effect of heating of the bulk of the cold target electrons on the ion acceleration is discussed.     

Beams of fast neutral atoms and molecules in low-pressure gas-discharge plasma

Fast neutral atom and molecule beams have been studied, the beams being produced in a vacuum chamber at nitrogen, argon, or helium pressure of 0.1–10 Pa due to charge-exchange collisions of ions accelerated in the sheath between the glow discharge plasma and a negative grid immersed therein. From a flat grid, two broad beams of molecules with continuous distribution of their energy from zero up to e(U + U _c ) (where U is voltage between the grid and the vacuum chamber and U _c is cathode fall of the discharge) are propagating in opposite directions. The beam propagating from the concave surface of a 0.2-m-diameter grid is focused within a 10-mm-diameter spot on the target surface. When a 0.2-m-diameter 0.2-m-high cylindrical grid covered by end disks and composed of parallel 1.5-mm-diameter knitting needles spaced by 4.5 mm is immersed in the plasma, the accelerated ions pass through the gaps between the needles, turn inside the grid into fast atoms or molecules, and escape from the grid through the gaps on its opposite side. The Doppler shift of spectral lines allows for measuring the fast atom energy, which corresponds to the potential difference between the plasma inside the chamber and the plasma produced as a result of charge-exchange collisions inside the cylindrical grid.     

Parameters of an ion beam and characteristic features of its slowing-down in a plasma during fast ignition of an inertial fusion target

The physics of the heating of an inertial fusion target by a high-energy ion beam under the conditions of fast ignition of fusion reactions is studied theoretically. The characteristic features of the formation of the spatial distribution of the energy transferred to the plasma from a beam of ions with different initial energies, masses, and charges under fast ignition conditions are determined. The notion of the Bragg peak is extended with respect to the spatial distribution of the temperature of the ion-beam-heated medium. The parameters of the ion beams are determined with which to initiate different regimes of fast ignition of a thermonuclear fuel precompressed to a density of 300–500 g/cm³—the edge regime, in which the ignition region is formed at the outer boundary of the target, and the internal regime, in which the ignition region is formed within the target and, in particular, in its central parts.     

Study of plasma heating in discharges with neutral beam injection in the Globus-M spherical tokamak

Results from experimental studies on the injection of high-energy neutral hydrogen beams into the plasma of the Globus-M spherical tokamak are reviewed. In the Introduction, the importance of these studies for implementing the controlled fusion research program and constructing the ITER tokamak is proved. Some problems related to the use of neutral beam injection in small and low-aspect-ratio tokamaks is analyzed. Results are presented from numerical simulations of the experiment by using the ASTRA transport code. It is shown that the use of neutral beam injection in the Globus-M tokamak ensures efficient ion heating and increases the plasma stored energy. The greater part of the review is devoted to the survey of experiments on the injection of 22-to 30-keV hydrogen and deuterium beams with a power of 0.4–0.8 MW into the plasma of the Globus-M spherical tokamak in a wide range of plasma currents and densities. The experimental results are analyzed and compared with the results of numerical simulations. The achievement of top plasma parameters is highlighted.     

Turbulent dynamics of helical perturbations in the edge plasma of the T-10 tokamak

Turbulent dynamics of the edge plasma in the T-10 tokamak is simulated numerically by solving nonlinear MHD equations in the framework of the four-field {?, n, p _e , p _i } reduced two-fluid Braginskii hydro-dynamics. It is shown that the transition from ohmic to electron-cyclotron heating is accompanied by a decrease in the amplitudes of turbulent fluctuations in plasma. This is caused by the enhancement of longitudinal dissipation due to the increase in the electron temperature. However, phase relations between potential fluctuations of different modes change in such a way that the Reynolds turbulent force increases, which leads to an increase in the poloidal velocity in the direction of ion diamagnetic drift. Since the poloidal and ion diamagnetic drift velocities enter into the equation of the radial force balance for ions with different signs, the radial electric field decreases. The simulation results agree qualitatively with the results of experiments in the T-10 tokamak. The dependence of the radial electric field on the plasma density, ion pressure, and neutral density is also calculated.