Concerning the procedure of recovering the electron energy distribution function from the soft X-ray spectrum

In the course of experiments on electron-cyclotron resonance heating of plasma in the L-2M stellarator, it was found that, at high specific heating powers, the SXR spectrum was distorted in the entire photon energy range under study (from 1.5 to 15 keV). To determine the character of this distortion, a procedure was proposed for recovering the electron energy distribution function (EEDF) from soft X-ray (SXR) measurements. The SXR spectra were analyzed for different values of the specific heating power. Numerical calculations show that the edge plasma contributes insignificantly to the chord-averaged SXR spectrum. A conclusion is drawn on the applicability and accuracy of the procedure for recovering the EEDF at high specific heating powers.     

Effect of the magnetic field structure on the intensity of electron cyclotron emission from the plasma of the L-2M stellarator

Results are presented from experimental studies of the influence of the stellarator magnetic field structure on the plasma behavior in electron-cyclotron resonance regimes with a high heating power per electron. The magnetic field structure was changed by varying the induction current I _p from ?14 to +14 kA. The plasma electrons were heated at the second harmonic of the electron gyrofrequency by an X-mode microwave beam with a power of P ～ 200 kW, the average plasma density being in the range n _e = (0.5–2) × 10¹³ cm^?3. At I _p = 0, the rotational transform varies from $\rlap{--} \iota $ (0) = 0.2 on the magnetic axis to 0.8 at the plasma boundary. At a positive current of I _p = 13.5 kA, the rotational transform was $\rlap{--} \iota $ (0) = 0.8 on the axis and $\rlap{--} \iota $ (a _p) = 0.9 at the plasma boundary. Experiments with a positive current have shown that the radiative temperature first increases with current. When the current increases to I _p = 11–14 kA, strong modulation appears in the electron cyclotron emission signals received from all the plasma radii, the emission spectrum changes, and the emission intensity decreases. At a negative current of I _p = ?(6.5–13.5) kA, the rotational transform vanishes at r/a _p = 0.4–0.6. In this regime, the number of suprathermal electrons is reduced substantially and the emission intensity decreases at both low and high plasma densities.     

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.     

Formation of self-consistent pressure profiles in simulation of turbulent convection in tokamak plasmas

The formation of pressure profiles in turbulent tokamak plasmas in ohmic heating regimes and transient regimes induced by turning-on of electron-cyclotron resonance (ECR) heating is investigated. The study is based on self-consistent modeling of low-frequency turbulent plasma convection described by an adiabatically reduced set of hydrodynamic-type equations. The simulations show that, in the ohmic heating stage, turbulence forms and maintains profiles of the total plasma pressure corresponding to turbulentrelaxed states. These profiles are close to self-consistent profiles of the total plasma pressure experimentally observed on the T-10 tokamak in ohmic regimes with different values of the safety factor q _L at the limiter. Simulations of nonstationary regimes induced by turning-on of on- and off-axis ECR heating show that the total plasma pressure profiles in the transient regimes remain close to those in the turbulent-relaxed state, as well as to the profiles experimentally observed on T-10.     

Plasma energy balance in the L-2M stellarator

Results are presented from studies of the effect of the discharge parameters (in particular, plasma density and heating power) and the characteristics of the magnetic configuration (e.g., rotational transform) on the confinement of a low-pressure plasma during electron-cyclotron resonance heating in the L-2M stellarator. An analysis shows that the plasma energy in the steady-state phase of a discharge is fairly well described by the product of power functions of the plasma density, heating power, and rotational transform: $W = W_0 n_e^{\alpha _n } P^{\alpha _p } \iota ^{\alpha _\iota } $ . The energy scalings constructed in terms of the parameters in the initial stage of free plasma decay and those in the steady-state phase are close to one another. The dynamic analysis of the plasma energy decay is now under way.     

Modified Canonical Profile Transport Model for Description of On-Axis Electron-Cyclotron Heating of Tokamak Plasma

Plasma Physics Reports - The specific power deposited in plasma under on-axis electron-cyclotron resonance heating (ECRH) is characterized by strong peaking, resulting in large-amplitude sawtooth...     

Investigation of the H-mode during ECRH in the T-10 tokamak

An improved confinement regime with an external transport barrier (H-mode) is obtained during electron-cyclotron resonance heating of a plasma in the T-10 tokamak. A characteristic feature of this regime is a spontaneous density growth accompanied by a drop in the intensity of D_α line and an increase in β_p by a factor of ～1.6. The threshold power for the L-H transition is close to that predicted by the ITER scaling. The best characteristics of the H-mode are achieved with decreasing q _L to 2.2. It is shown that the external transport barrier arises for electrons, whereas the heat transport barrier insignificantly contributes to improved confinement.     

On the nature of global plasma oscillations in a tokamak. Part II: Spatial structure and propagation of perturbations observed at the T-10 tokamak

Large-scale plasma oscillations (so-called MHD oscillations) observed at the T-10 tokamak are investigated. The central electron cyclotron heating was used to enhance oscillations at the m/n = 1/1 mode with the goal of determining the internal characteristics of the process. The spatially resolved electron cyclotron emission diagnostics allowed analyzing the propagation characteristics of plasma perturbations. The experiments have revealed that excitation of oscillations in a particular mode occur simultaneously in the entire area located within the corresponding rational magnetic surface. The propagation of plasma perturbations along the torus is found to be inhomogeneous. The electron cyclotron emission diagnostics allowed finding eigen (resonance) frequencies of plasma oscillations from the parameters of their inhomogeneous propagation in the plasma core and comparing them with spectra of oscillations of the magnetic field induced by the plasma current in the edge plasma, which were recorded by magnetic probes. It is established that the frequencies of eigenmodes are independent of the electron temperature, plasma density, and auxiliary heating power. Even spatial harmonics of the principal magnetic surface are observed under strong excitation of oscillations. The rational magnetic surfaces that determine oscillation harmonics retain their position during the entire steady-state phase of the total plasma current in spite of the strong sharpening of the temperature profile due to central heating.     

Behavior of small-scale density fluctuations in discharges with off-axis electron-cyclotron resonance heating in the T-10 tokamak

In experiments on off-axis electron-cyclotron resonance heating in the T-10 tokamak, a steep gradient of the electron temperature was observed to form for a short time at a relative radius of ρ ≈ 0.25 after the heating power was switched off. Small-scale fluctuations of the electron density were studied with the help of correlation reflectometry. It was found that, in a narrow region near ρ ≈ 0.25, the amplitude of the density fluctuations was two times lower than that in the ohmic heating phase. Quasi-coherent fluctuations were suppressed over a period of time during which the steep temperature gradient existed. Measurements of the poloidal rotation velocity of turbulent fluctuations show that there is no velocity shear after the heating is switched off. An analysis of the linear growth rates of instabilities shows that the ion-temperature-gradient mode is unstable at ρ ≈ 0.25 throughout the entire discharge phase. The effect observed can be explained by an increase in the distance between the rational surfaces near the radius at which the safety factor is q = 1 due to the temporary flattening of the q profile after the off-axis electron-cyclotron resonance heating is switched off.     

Spectroscopic and probe measurements of the electron temperature in the plasma of a pulse-periodic microwave discharge in argon

A pulse-periodic 2.45-GHz electron-cyclotron resonance plasma source on the basis of a permanent- magnet mirror trap has been constructed and tested. Variations in the discharge parameters and the electron temperature of argon plasma have been investigated in the argon pressure range of 1 × 10^–4 to 4 × 10^–3 Torr at a net pulsed input microwave power of up to 600 W. The plasma electron temperature in the above ranges of gas pressures and input powers has been measured by a Langmuir probe and determined using optical emission spectroscopy (OES) from the intensity ratios of spectral lines. The OES results agree qualitatively and quantitatively with the data obtained using the double probe.     

Fast ion loss-driven H-mode transition in RF discharge plasmas of the Uragan-3M torsatron

In the l = 3 Uragan-3M (U-3M) torsatron with a helical divertor the plasma is produced and heated by RF field in the ω ≤ ω _ci range of frequencies. A two-temperature ion perpendicular energy distribution with a suprathermal tail sets in with heating. If the heating power is high enough, a spontaneous transition to an H-like confinement mode is observed. Recently, it has been supposed that the transition is connected with hotter and suprathermal ions (common name “fast ions“, FI) loss. The objective of this work is an experimental elucidation of the real link between the H-transition and FI loss. To do this, a transient regime of the RF discharge with two H-mode states is chosen, and the evolution is followed of electron density, FI content in the confinement volume, FI outflow to the divertor and edge potential. On the basis of juxtaposing of these processes, a conclusion is made that the H-mode transition in U-3M is really driven by FI loss. Possible mechanisms resulting in the transition are discussed, among them the ion orbit loss and the radial drift of helically-trapped ion orbits seem most probable.     

Alfvén resonance heating in Uragan-2M torsatron

Uragan-2M is a medium-size torsatron with reduced helical ripples. This machine has the major plasma radius R = 1.7 m, the average minor plasma radius r _p ≤ 0.24 m and the toroidal magnetic field B ₀ ≤ 2.4 T. The Alfvén resonance heating in a high k _‖ regime is advantageous for small size machines since it can be realized at smaller plasma densities than the minority and second harmonic heating. The Alfvén resonance heating is examined numerically in the approximation of radially non-uniform plasma cylinder with identical ends. The numerical model for wave excitation and propagation accounts for the longitudinal electron thermal motion and the finite ion gyroradius which allow the model to treat correctly the propagation and damping of the kinetic Alfvén wave in hot plasma. A compact antenna consisting of four loop elements is chosen to provide operation in a high k _‖ regime. The major drawback of such an approach is the presence of plasma peripher y heating owing to unavoidable excitation of low k _‖ Alfvén resonances. Calculations show that, with the proper choice of heating regime, the periphery heating has an acceptable level and the major part of the power is deposited inside plasma column.     

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.     

Two-slope soft X-ray spectra observed in experiments on electron cyclotron resonance plasma heating in the L-2M stellarator

In experiments on the generation and electron cyclotron resonance heating (ECRH) of plasma in the L-2M stellarator, non-Maxwellian two-slope soft X-ray (SXR) spectra were observed. The temperatures of the thermal and epithermal components of the spectra were measured as functions of the heating power and plasma density. A hypothesis based on the experimental results is suggested to explain the formation mechanism of two-slope SXR spectra in the ECRH experiments at the L-2M stellarator. The measured SXR spectra are compared with the results of numerical simulations.     

Coupled rotational dynamics of two MHD modes in the T-10 tokamak

The coupled rotational dynamics of the m/n = 2/1 and 3/2 MHD modes in an ohmic discharge and the m/n = 2/1 and 4/1 modes in a regime with on-axis electron-cyclotron resonance heating were studied experimentally. The specific features of rotation of these modes in the presence of an error magnetic field, in particular the dependence of the instantaneous frequency Ω(t) of the MHD mode on its spatial orientation at the current instant, were revealed. In analyzing the results obtained, MHD modes were identified with tearing modes (magnetic islands) having a corresponding spatial structure. A possible mechanism for the mutual influence of magnetic islands resulting in their consistent rotation is discussed.     

Determination of the plasma parameters in the PF-3 facility by the methods of X-ray spectroscopy

NeIX and NeX spectra emitted by the PF-3 high-current (2 MA) plasma focus facility are measured. A numerical model describing the spectral intensities of the emission of helium- and hydrogen-like neon ions from an optically thick plasma is proposed. The electron temperature T _e and electron density n _e in the plasma of the PF-3 facility are determined by comparing the calculated and measured emission spectra of neon.     

Reinvestigation of the visible absorption bands of the 2,2′-bipyrimidine complexes W(CO)4(bpym) and (μ-bpym)[M(CO)4]2 (M=Mo, W) with resonance Raman spectroscopy; the emission spectrum of (μ-bpym)[Mo(CO)4]2

The character of the two lowest energy transitions of W(CO)₄(bpym) and (μ-bpym)[M(CO)₄]₂ (M=Mo, W) were established with resonance Raman spectroscopy. According to these spectra the two bands belong to MLCT transitions to different π* orbitals of the bpym ligand. Contrary to expectations it is not the first (lowest energy) but the second and more intense electronic transition which, according to the resonance Raman spectra, is directed to the lowest lying π* orbital (b_2u*, LUMO) of these complexes. This interpretation explains the different band intensities and the untypically low g values of the ESR signals of corresponding anion radicals. Excitation of (μ-bpym)[Mo(CO)₄]₂ in CH₂Cl₂ at 400 nm produced a weak emission with an onset at 700 nm. According to the excitation spectrum, this emission originates from the lowest MLCT-excited state of the complex.     

Sterilization mechanism of nitrogen gas plasma: induction of secondary structural change in protein

The mechanism of action on biomolecules of N₂ gas plasma, a novel sterilization technique, remains unclear. Here, the effect of N₂ gas plasma on protein structure was investigated. BSA, which was used as the model protein, was exposed to N₂ gas plasma generated by short‐time high voltage pulses from a static induction thyristor power supply. N₂ gas plasma‐treated BSA at 1.5 kilo pulses per second showed evidence of degradation and modification when assessed by Coomassie brilliant blue staining and ultraviolet spectroscopy at 280 nm. Fourier transform infrared spectroscopy analysis was used to determine the protein's secondary structure. When the amide I region was analyzed in the infrared spectra according to curve fitting and Fourier self‐deconvolution, N₂ gas plasma‐treated BSA showed increased α‐helix and decreased β‐turn content. Because heating decreased α‐helix and increased β‐sheet content, the structural changes induced by N₂ gas plasma‐treatment of BSA were not caused by high temperatures. Thus, the present results suggest that conformational changes induced by N₂ gas plasma are mediated by mechanisms distinct from heat denaturation.     

Quasilinear modification of the spectra of cyclotron emission from a toroidal plasma near the ECRH frequency

A study is made of the modification of the spectra of electron cyclotron emission from an ECR heated plasma in a toroidal magnetic confinement system into which the heating radiation is launched from the low-field side. It is shown that, at frequencies close to the heating frequency, cyclotron emission can become more intense because of the deformation of the distribution function of the resonant electrons. This effect can be used to diagnose the slightly pronounced quasilinear perturbations of the electron distribution in the thermal energy range, which are typical of experiments on ECR plasma heating. Results of a qualitative analysis carried out for model electron distribution functions are presented, and examples of three-dimensional numerical simulations of a circular tokamak are described.     

Formation of a Plasma Ring by a Microwave Discharge in a Narrow Coaxial Cavity beyond the ECR Region

Results are presented from the experimental study of conditions for the formation of a plasma ring by a microwave discharge in a narrow coaxial cavity in an axisymmetric magnetic field the magnitude of which is below the electron-cyclotron resonance value. It is established that the necessary condition for this process is the presence of an electrostatic wave propagating in the azimuthal direction, the circumference of the plasma ring being a multiple of the wave half-length.