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.     

Coefficient of the Extraction of the Target Isotope and Optimum Parameters of a Collector of Heated Ions in the Context of the ICR Method of Isotope Separation

The separation parameters of a collector of heated ions are estimated in the context of the ion cyclotron resonance method of isotope separation. The separation power dU, the coefficient Γ_C of the extraction of the target isotope, and the collector efficiency η are calculated. These parameters are investigated as functions of the repulsive potential U of the collector plates, the half-height a of the front screen, and the distance b between the plates. It is shown that the dependence of the collector efficiency η on the distance b between the plates has a pronounced maximum at b ≈ 2r _L ^* , where r _L ^* is the mean ion gyroradius.     

Distribution of the lithium deposition on the collector during separation of lithium isotopes by the plasma ICR method

Results of experiments on isotope separation by the ion cyclotron resonance (ICR) method in plasma are presented. The deposition of hot lithium ions on a plane collector was studied. The density distribution and isotope composition of lithium deposited on the surface of the plane collector were measured. It is found that, without applying a positive potential to the collector plate, the deposition density is nonuniform in the direction perpendicular to the external magnetic field, whereas in the presence of a retarding potential (+20 V), the deposited layer becomes almost uniform. It is shown that the asymmetry of deposition is not caused by the rotating RF field selectively heating ions of the extracted isotope. Possible reasons for the nonuniform deposition of lithium on the collector are discussed.     

Simulations of a longitudinal glow discharge in a hot air flow at atmospheric pressure

A one-dimensional axisymmetric time-dependent model of a discharge in a gas flow is developed. The model includes a fairly complete set of plasmochemical reactions and describes the heating and gasdynamic expansion of a plasma channel in air. The processes governing the distribution of nitrogen molecules in the N₂(C ³Π_u, v) state over vibrational levels are considered. The parameters of a longitudinal glow discharge in a hot (T ₀ = 1500?3000 K) air flow at atmospheric pressure are calculated. It is found that gas preheating considerably influences the parameters of the discharge channel. The results of calculations are compared with the experimental data.     

Testing of the spectroscopic method for location of water microleakages in ITER at the L-2M stellarator

Results of testing of a possible method for location of water microleakages in the cooling system of the first wall and vacuum chamber of ITER are presented. The method consists in spectroscopic detection of the emission lines of atoms and ions of the Xe additive dissolved in water. These lines are excited when the water with dissolved Xe contacts the plasma. The high electron cyclotron resonance heating (ECRH) power deposited in a relatively small plasma volume in the L-2M stellarator (P = 0.5 MW, V = 0.24 m³, and the specific heating power ??2 MW/m³) makes it possible to achieve plasma parameters close to those in the edge plasma of ITER for different operating modes, including the H-mode with an edge transport barrier. In test experiments, several lines of Xe ions were revealed suitable for detection of xenon in plasma with parameters close to those in the edge plasma of ITER at leakage rates at a level of ??10^?6 Pa m³ s^?1 and spatial resolution of ??0.5 cm.     

Acceleration of ions in a beam-plasma discharge in a low magnetic field: Interrelation between the electron and ion energy distributions

Previous experiments revealed the effect of stable acceleration of ions in a plasma-beam discharge in a low magnetic field to energies one order of magnitude higher than the electron thermal energy. To verify the previously proposed mechanisms for this effect, the velocity distribution function of the electrons arriving at the collector and the energy distribution of the ions escaping from the discharge transversely to the axis were measured. It is found that ion acceleration is accompanied by significant electron heating near the discharge axis. The time behavior and longitudinal profile of the intensity of the excited high-frequency oscillations in the frequency range ω ～ ω _pe were studied. The accumulation of regular oscillations in the beam-injection region and their stochastization during the propagation along the system axis were observed. The experimental results correlate qualitatively with the data of previous numerical simulations.     

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.     

Deviation of the electron energy distribution function from maxwellian during ECR heating in the L-2M stellarator in regimes with a high specific heating power

In the previous experiments on ECR heating of a low-density plasma with n _e =(0.3?0.5)×10¹⁹ m^?3 in the L-2M stellarator, the electron temperature profile measured from the intensity of electron cyclotron emission was found to be asymmetric about the magnetic axis and the electron temperature measured by this diagnostics turned out to be higher than that expected from diamagnetic measurements. To find out the character of distortion of the electron energy distribution function, the soft X-ray spectrum was measured in regimes with large values of the specific heating power η (1.5 MW per 10¹⁹ particles). Under these conditions, the X-ray spectrum plotted on a semilogarithmic scale has no linear segments in the photon energy range from 1.5 to 12 keV. This indicates that the electron distribution function is non-Maxwellian over the entire energy range under study.     

Reflection and backscattering of microwaves under doubling of the plasma density and displacement of the gyroresonance region during electron cyclotron resonance heating of plasma in the l-2M stellarator

Reflection and backscattering of high-power (400 kW) gyrotron radiation creating and heating plasma at the second harmonic of the electronic cyclotron frequency in the L-2M stellarator have been investigated experimentally. The effect of the displacement of the gyroresonance region from the axis of the plasma column under doubling of the plasma density on the processes of reflection and backscattering of microwave radiation has been examined. A near doubling of short-wavelength (k _⊥ ≈ 30 cm^–1) turbulent density fluctuations squared is observed. The change in the energy confinement time under variations of plasma parameters and characteristics of short-wavelength turbulence is discussed. A discrepancy between the measured values of the reflection coefficient from the electron cyclotron resonance heating region and predictions of the one-dimensional model is revealed.     

Formation of a charge-exchange target for fast ions in the plasma of large-scale toroidal devices under NBI conditions

High-energy (E>0.2 MeV) charge-exchange diagnostics allow the determination of the distribution function of fast atoms produced via the neutralization of hydrogen isotope ions by target hydrogen-like impurity ions. To derive the distribution function from the experimental data requires knowledge of the composition and spatial distribution of the target ions in a tokamak plasma. A charge-exchange target forms as a result of the interaction between the main impurity nuclei and the heating neutral beams. In different devices, the heating beams are arranged in different ways with respect to the diagnostics; hence, in order to accurately estimate the contribution of the secondary ions to the detected signal, it is necessary to calculate their trajectories for every particular case. A model is proposed that takes into account elementary processes resulting in the ionization equilibrium of the ions of different impurities with allowance for ion motion in a specific tokamak configuration. As an example, the model is applied to the plasma of the JT-60U tokamak. Mechanisms for the formation of charge-exchange atomic flows in various energy ranges are considered. The relative contributions of different heating injectors to the charge-exchange flow are estimated. Based on the calculated results, a method is proposed for local measurements of the ion distribution function with the help of a stationary analyzer.     

Analysis of the efficiency of lower hybrid current drive in the FT-2 tokamak

Results are presented from experimental studies of the efficiency of lower hybrid current drive (LHCD) in the FT-2 tokamak. The dependence of the LHCD efficiency on the grill phasing Δφ and RF oscillator power was determined experimentally in a wide range of plasma densities. It is shown that, at high plasma currents (i.e., at sufficiently high electron temperatures), current drive is suppressed when the plasma density reaches its resonance value n _LH for the pumping wave frequency, rather than when parametric decay comes into play (as was observed in regimes with lower plasma currents and, accordingly, lower electron temperatures T _e ). In order to analyze the experimentally observed effect of LHCD and its dependence on the value and sign of the antenna phasing, the spectra of the excited LH waves, P(N _z ), were calculated. Simulations using the FRTC code with allowance for the P(N _z ) spectrum and the measured plasma parameters made it possible to calculate the value and direction of the LH-driven current, which are determined by the spectrum of the excited LH waves. It is shown that the synergetic effect caused by the interaction between different spectral components of the excited RF wave plays a decisive role in the bridging of the gap in the wave spectrum.     

Physical Parameters of a Reactor-Stellarator with Small Ripples of the Helical Magnetic Field

The paper describes the calculation data on the physical parameters of a reactor-stellarator, where the nonuniformities of the helical field are smaller than the toroidal magnetic field nonuniformities: ε_h < ε_t. Unlike the previous studies, where the ion-component transport coefficients had the collision frequency dependence proportional to ν^1/2, this being equivalent to the ε_h > ε_t case, in the present calculations, these coefficients were assumed to be in proportion to the first power of the collision frequency, D_i ∝ ν for ν_eff < 2ω_E, and to D_i ∝ ν^?1 for the inverse inequality. Here, ω_E is the rotation frequency of plasma in the radial electric field. As before, the plasma electrons corresponded to the mode of D_e ∝ ν^?1. As initial parameters for numerical calculations, a reactor with R = 8 m, r_p = 2 m, and B₀ = 5 Т was taken. A numerical code was used to solve the set of equations that describes the plasma space?time behavior in the reactor-stellarator under the conditions of equal diffusion fluxes. The start of reactor operation in the mode of thermonuclear burning was provided by heating sources with a power of several tens of megawatts. Steady-state operating conditions of a self-sustained thermonuclear reaction were attained by maintaining the plasma density through DT fuel pellet injection into the plasma.     

Displacement of the electron cyclotron resonance heating region and time evolution of the characteristics of short-wavelength turbulence in the 3D magnetic configuration of the L-2M stellarator

Reflection of the heating extraordinary microwave incident obliquely onto the surface of the electron cyclotron resonance (ECR) at the second harmonic of the electron gyrofrequency in the 3D magnetic configuration of the L-2M stellarator was studied experimentally. The plasma was heated using two gyrotrons with a total power of 600–700 kW, the specific heating power being 2.4–2.8 MW/m³. The displacement of the ECR region in the course of heating was monitored by measuring the phase of the reflected extraordinary wave. It is found that the growth of the plasma density is accompanied by the displacement of the ECR heating region from the center of the plasma column toward its periphery. The coefficient of reflection of the heating microwave beam from the ECR region was measured. The spectra of short-wavelength (k _s ≈ 30 cm^?1) plasma density fluctuations were explored by analyzing backscattered microwave radiation. A tenfold increase in the energy of short-wavelength density fluctuations and the growth of the spectral density of fluctuations in the frequency range of 0.3–1.5 MHz were observed.     

Laser spectroscopy measurements of the effective temperature of argon ions in the PNX-U plasma neutralizer

Laser spectroscopy measurements of the effective temperature of Ar¹⁺ ions in the PNX-U multipole trap, in which argon plasma is ionized and heated by microwaves under electron-cyclotron-resonance conditions, are performed using a narrow-band tunable dye laser. The absorption profile of the 611.5-nm line is examined. In a microwave power range of 5–50 kW, the observed behavior of the effective temperature of argon ions T_{i, eff} indicates an anomalous mechanism for ion heating. It is shown that certain information about the electron temperature can be deduced from measurements by the laser-induced fluorescence (LIF) technique. The measurements performed also serve to test the laser technique and apparatus that is presently being developed for diagnosing additives to the ITER divertor plasma by the LIF method.     

Spectroscopic measurements of the parameters of the helium plasma jets generated in the plasma focus discharge at the PF-3 facility

The spectroscopic technique used to measure the parameters of the plasma jets generated in the plasma focus discharge and those of the plasma of the immobile gas through which these jets propagate is described. The time evolution of the intensities and shapes of spectral lines in experiments carried out with helium at the PF-3 facility was studied by means of electron-optical streak cameras. The plasma electron temperature, T ≈ 4–5 eV, was determined from the intensity ratio of two spectral lines, one of which (λ₁ = 5876 Å) belongs to neutral helium, while the other (λ₂ = 4686 Å), to hydrogen-like helium ions. The plasma density at different time instants was determined from the Stark broadening of these lines in the electric fields of different nature. The plasma density is found to vary from 4 × 10¹⁴ to 2 × 10¹⁷ cm^?3.     

Plasma diagnostics from intensities of resonance line series of He-like ions

The possibility of using the relative intensities of the 1snp ¹P₁–1s ^{2 1}S0 transitions with n = 3–6 in He-like multicharged ions to diagnose plasma in a nonstationary ionization state is considered. The calculations performed for F VIII ions show that, at electron temperatures of T _e = 10–100 eV, the intensity ratios are sensitive to the plasma electron density in the range of N _e = 10¹⁶–10²⁰ cm^–3. The universal calculated dependences can be used to diagnose various kinds of recombining or ionizing plasmas containing such ions.     

Responses of the carbon and oxygen isotope compositions of desert plants to spatial variation in soil salinity in Central Asia

We examined the isotopic parameters in two C₃ species (Artemisia diffusa H. Krasch and Tamarix hispida Willd.) and a C₄ species [Haloxylon aphyllum (Minkw.) Iljin.] growing or planted in soils with different levels of salinity in a Central Asian desert. The oxygen isotope ratios of stem water (δ¹⁸O_stem) in T. hispida and H. aphyllum distributed in high-salinity zones were similar to the δ¹⁸O of artesian water (δ¹⁸O_artesian) and different from that in A. diffusa distributed in lower-salinity zones. This indicates that T. hispida and H. aphyllum depend on water with low salinity in the deeper soil layer, whereas A. diffusa depends on water in the shallower soil layer that would be affected by salt accumulation. The carbon isotope composition of leaf organic matter (δ¹³C_om) and oxygen isotope enrichment in leaf organic matter above stem water (Δ¹⁸O_om) were lower in A. diffusa than in the other species. The responses of δ¹³C_om and Δ¹⁸O_om to soil salinity observed for T. hispida suggest that the species decreased its transpiration rate and increased its intrinsic water-use efficiency in response to increasing soil salinity. The δ¹³C_om and Δ¹⁸O_om of H. aphyllum were higher than those of the C₃ species, and were not correlated with soil salinity, suggesting that H. aphyllum reduced its salt uptake by decreasing transpiration—even though it was able to access less saline water in the deeper soil layer. These results indicate that the water-use strategy of desert plants in high-salinity environments can be assessed based on their carbon and oxygen isotope ratios.     

Formation of multiply charged ions in the interaction of laser radiation with targets of various densities

Mass, charge, and energy spectra of multiply charged ions in a plasma formed by laser heating of Ho₂O₃ and Y₂O₃ targets of various densities are investigated. The features of the formation of multiply charged oxygen and holmium (yttrium) ions at fairly low ion energies (≤50 eV) are examined. It is found that, for oxygen ions, Z _max is achieved at low target densities, whereas, for holmium (yttrium) ions, it is achieved at high target densities. It is suggested that these features are related mainly to nonequilibrium ionization processes in the plasma.     

Pulsed nuclear magnetic resonance studies on 23 Na, 7 Li and 35 Cl binding to human oxy- and carbon monoxyhaemoglobin

Pulsed nuclear magnetic resonance studies of the longitudinal (T₁) and transverse (T₂) quadrupolar relaxation times of ⁷Li, ²³Na, ³⁵Cl ions in the absence and presence of human oxy- and carbon monoxyhaemoglobin have been used to investigate the interaction of the ions and the macromolecule.The relaxation data show that Cl^? interacts strongly with the haemoglobin but provide no evidence for binding of Na⁺ up to concentrations of 0.5 m. In the case of Li⁺, evidence for interaction is obtained at concentrations of about 0.1 m.The dependence of relaxation rate on frequency was followed over a limited frequency range in an attempt to separate the effects of correlation times and exchange rates of the bonded ions on the relaxation. In the case of Cl^?, an upper limit for the mean lifetime divided by the number of sites can be set at about 1 × 10^?6 second, and a lower limit at about 1 × 10^?8 second.     

Influence of the plasma density and heating power on the intensity of electron cyclotron emission in the L-2M stellarator

Results are presented from experiments on studying the plasma behavior in the L-2M stellarator in regimes with a high power deposition in electrons during electron cyclotron heating at the second harmonic of the electron gyrofrequency (X mode) at heating powers of P _in=120–400 kW and average plasma densities from n _e≤3×10¹⁹ to 0.3×10¹⁹ m^?3. It is shown that, as the plasma density decreases and the heating power increases, the electron cyclotron emission spectrum is modified; this may be attributed to a deviation of the electron energy distribution from a Maxwellian and the generation of suprathermal electrons. At low plasma densities, the emission intensity at the second harmonic of the electron gyrofrequency increases, whereas the plasma energy measured by diamagnetic diagnostics does not increase. This poses the question of the correctness of determining the plasma electron temperature by electron cyclotron emission diagnostics under these conditions.