Measurements of the Plasma Parameters in a Mirror Trap by Means of Laser-Induced Fluorescence

Results of measurements of the absolute density of ArII ions in a mirror trap with a toroidal divertor are presented. The ion density was measured by the method of laser-induced fluorescence with the use of ArII metastable states. A technique is proposed to estimate the plasma electron temperature from the ratio of the fluorescence signals obtained by comparing the intensities of different optical transitions of ArII.     

Modeling of electron heating and the spatiotemporal evolution of emission in a current-sheet plasma

Results are presented from a detailed study of the behavior of the electron temperature during the evolution of a current sheet by comparing the data from spectral measurements with the spatiotemporal evolution of the emission intensities of the atomic and ionic lines of the working gas (He) and impurities (C, O) calculated in the collisional-radiative model. It is shown that the electron temperature in the center of the sheet attains a value of T _e =110±40 eV; under these conditions, taking into account metastable states affects the calculated results only slightly. The spatial profiles of the electron temperature and the plasma emission in the spectral lines of various atoms and ions across the plasma sheet are calculated as functions of time. It is shown that as the electron temperature grows most of the spectral lines of atoms and ions of the working gas and impurities are depleted in the center of the sheet and the emission region shifts toward the periphery of the sheet. The results obtained confirm the previous conclusion that, in this regime, a hot plasma is formed in the center of the sheet.     

Emission spectroscopy diagnostics of rare gases in the PNX-U facility

Results are presented from measurements of the electron temperature and neutral atom density in a low-temperature microwave plasma by the method of emission spectroscopy. The measurements were conducted in the PNX-U facility—a magnetic confinement system with a “magnetic wall.” Multichord measurements of plasma radiation at a wavelength of 750.37 nm were performed with the help of an absolutely calibrated monochromator. The neutral atom density was calculated using the collisional-radiative model. The degree of plasma ionization near the axis of the facility was found to be close to unity. The electron temperature of the argon plasma was measured from the relative intensities of the spectral lines of neutral helium injected in small amounts into the plasma (the so-called helium thermometer method). At a low microwave heating power, the results of these measurements agree well with the results of probe measurements.     

Boundary conditions on the plasma emitter surface in the presence of a particle counter flow: I. Ion emitter

Emission of positively charged ions from a plasma emitter irradiated by a counterpropagating electron beam is studied theoretically. A bipolar diode with a plasma emitter in which the ion temperature is lower than the electron temperature and the counter electron flow is extracted from the ion collector is calculated in the one-dimensional model. An analog of Bohm’s criterion for ion emission in the presence of a counterpropagating electron beam is derived. The limiting density of the counterpropagating beam in a bipolar diode operating in the space-charge-limited-emission regime is calculated. The full set of boundary conditions on the plasma emitter surface that are required for operation of the high-current optics module in numerical codes used to simulate charged particle sources is formulated.     

Electron temperature in a decaying molecular nitrogen plasma in the presence of weak electric fields

The electron energy distribution function in an afterglow molecular nitrogen plasma is studied both experimentally and theoretically under the conditions of weak electric fields such that the electron gas is heated by superelastic collisions of electrons with vibrationally excited molecules. Based on the mean electron energy balance, it is established that, depending on the degree of plasma ionization and the vibrational temperature of nitrogen molecules, an afterglow plasma may evolve into two states, differing in electron temperature. This kind of bistability is found to stem from the difference in the main mechanisms for electron energy losses in the two stable states. The prediction that the shape of the electron energy distribution function should change in a jumplike manner when a weak electric field is imposed has been confirmed experimentally.     

Analysis of the formation of ordered dust-grain structures in a thermal plasma

The possibility is studied of the formation of ordered dust-grain structures in a low-temperature thermal plasma consisting of electrons, ions, and micron-sized charged dust grains. The range of the required values of the coupling parameter Γ defining the degree to which the plasma is nonideal is calculated using the results of diagnostic measurements carried out in a plasma consisting of combustion products of propane in air with grains of different materials. The results obtained show that the most favorable conditions for the formation of strongly correlated grain structures (for both positively and negatively charged grains) take place at the maximum grain number density and a plasma temperature close to the minimum flame temperature (∼1600 K). In this case, the optimum grain radii lie in the range 4–10 μm and the maximum value of the parameter Γ is less than 200. Since the calculated values of Γ give an upper estimate, liquidlike ordered structures are most likely to form in a thermal plasma. Based on the results of the analysis, it is stated that an increase in the parameter Γ and, accordingly, the formation of plasma-crystal structures in a thermal plasma can only occur for positively charged grains. __________ Translated from Fizika Plazmy, Vol. 26, No. 7, 2000, pp. 626–632. Original Russian Text Copyright ? 2000 by Samarian, Vaulina, Nefedov, Petrov.     

Diagnostics of plasma oscillations by the field of surface waves guided by a dielectric waveguide

The potentialities of the diagnostic method for determining the plasma parameters by recording the surface waves guided by a dielectric waveguide and scattered by plasma oscillations are discussed. The use of surface (slowed) waves makes it possible to improve both the sensitivity and spatial resolution of measurements. The scattering is the most intense near the waveguide cutoff, at which the dependence of the wave propagation constant on the plasma density is the steepest. It is shown experimentally that the method proposed makes it possible to determine the discharge plasma density and electron energy and to estimate the amplitude of the RF field of the plasma waves forming the discharge and the amplitude of plasma density oscillations in these waves. The data obtained from the measurements of the amplitudes of both high-and low-frequency plasma density oscillations by the proposed method agree satisfactorily with theoretical predictions. The experimental data on the plasma density are confirmed by other diagnostic measurements. The ways of reducing measurement errors are proposed.     

Quasi-optics near the plasma frequency of a CuBr vapor laser

The kinetic parameters of the plasma of a CuBr vapor laser with a molecular hydrogen additive are determined by time measurements of the plasma transmittance at probing frequencies close to the plasma frequency. The measurements were performed across the discharge at frequencies of 94 and 142 GHz within the time interval 2–85 μs. The measurement results are used to calculate the time dependences of the electron density, electron temperature, and electron momentum relaxation time.     

Measurement and Analysis of Atomic Hydrogen and Diatomic Molecular AlO,C2, CN,and TiO Spectra Following Laser-induced Optical Breakdown

In this work, we present time-resolved measurements of atomic and diatomic spectra following laser-induced optical breakdown. A typical LIBS arrangement is used. Here we operate a Nd:YAG laser at a frequency of 10 Hz at the fundamental wavelength of 1,064 nm. The 14 nsec pulses with anenergy of 190 mJ/pulse are focused to a 50 µm spot size to generate a plasma from optical breakdown or laser ablation in air. The microplasma is imaged onto the entrance slit of a 0.6 m spectrometer, and spectra are recorded using an 1,800 grooves/mm grating an intensified linear diode array and optical multichannel analyzer (OMA) or an ICCD. Of interest are Stark-broadened atomic lines of the hydrogen Balmer series to infer electron density. We also elaborate on temperature measurements from diatomic emission spectra of aluminum monoxide (AlO), carbon (C₂), cyanogen (CN), and titanium monoxide (TiO).The experimental procedures include wavelength and sensitivity calibrations. Analysis of the recorded molecular spectra is accomplished by the fitting of data with tabulated line strengths. Furthermore, Monte-Carlo type simulations are performed to estimate the error margins. Time-resolved measurements are essential for the transient plasma commonly encountered in LIBS.     

Optical characteristics of a gallium laser plasma

Results are presented from studies of the emission from an erosion gallium laser plasma at a moderate intensity (W=(1–5)×10⁸ W/cm²) of a 1.06-μm laser radiation. It is shown that, under these conditions, the lower excited states of gallium atoms are populated most efficiently. Among the ions, only the most intense GaII lines are observed in the emission spectrum. The populations of GaI and GaII excited states are not related to direct electron excitation, but are determined by the recombination of gallium ions with slow electrons. The recombination times of GaIII and GaII ions in the core of the plasma jet are determined from the waveforms of emission in the GaII and GaI spectral lines and are equal to 10 and 140 ns, respectively. The results obtained are of interest for spectroscopic diagnostics of an erosion plasma produced from gallium-containing layered crystals during the laser deposition of thin films. __________ Translated from Fizika Plazmy, Vol. 27, No. 1, 2001, pp. 85–88. Original Russian Text Copyright ? 2001 by Shuaibov, Shimon, Dashchenko, Shevera, Chuchman.     

Thermal ionization of helium in the quantum mechanical representation of Feynman path integrals

The method of Feynman path integrals is used to calculate the degree of ionization in a thermodynamically equilibrium, dense helium plasma. A quantum statistical calculation which is based on the “first principles” and in which the permutation symmetry and electron spin variable are explicitly taken into account is carried out for a helium atom that is in thermal and mechanical contact with a plasma at a temperature from 30000 to 400000 K. Numerical integration over virtual paths is performed by the Monte Carlo method. When the temperature and density are varied, the amount of singly charged ions in a plasma varies nonmonotonically, reaching a maximum at a temperature of about 70 000 K and a pressure of 100 MPa. Radial profiles of the electron density around a helium nucleus are obtained, as well as contour lines of the degree of ionization in the p-T plane of the plasma states. The role of fluctuations is investigated. It is shown that the fluctuations of the local electron density near a helium ion have a radical effect on the values of the ionization-recombination equilibrium constants.     

The temperature dependence of P680(+) reduction in oxygen-evolving photosystem II

The temperature dependence for the reduction of the oxidized primary electron donor P680(+) by the redox active tyrosine Y(Z) has been studied in oxygen-evolving photosystem II preparations from spinach. The observed temperature dependence is found to vary markedly with the S-state of the manganese cluster. In the higher oxidation states, S(2) and S(3), sub-microsecond P680(+) reduction exhibits activation energies of about 260 meV. In contrast, there is only a small temperature dependence for the sub-microsecond reaction in the S(0) and S(1) states (an activation energy of approximately 50 meV). Slower microsecond components of P680(+) reduction show an activation energy of about 250 meV which, within experimental error, is independent of the oxidation state of the Mn cluster. By combining these values with measurements of DeltaG for electron transfer, the reorganization energies for each component of P680(+) reduction have been calculated. High activation and reorganization energies are found for sub-microsecond P680(+) reduction in S(2) and S(3), demonstrating that these electron transfers are coupled to significant reorganization events which do not occur in the presence of the lower S-states. One interpretation of these results is that there is an increase in the net charge on the manganese cluster on the S(1) to S(2) transition which acts as a barrier to electron transfer in the higher S-states. This argues against the electroneutrality requirement for some models of the function of the manganese cluster and hence against a role for Y(Z) as a hydrogen abstractor on all S-state transitions. An alternative or additional possibility is that there are proton (or other ion) motions in the sub-microsecond phases in S(2) and S(3) which contribute to the large reorganization energies observed, these motions being absent in the S(0) and S(1) states. Indeed charge accumulation may directly cause the increased reorganization energy.     

Emission characteristics and properties of a laser-plume germanium plasma

The initial stage of the evolution of a laser-plume germanium plasma under typical deposition conditions of thin germanium films doped with nanoparticles was studied by emission spectroscopy. The velocity of the laser plasma, the ion recombination time, and the electron temperature and density at distances of 1 and 7 mm from the target surface exposed to high-power IR radiation are determined. The measured electron temperature and density are compared with the calculated ones. The differences in the emission characteristics and velocities of laser plasmas obtained by irradiating germanium targets with UV and IR lasers under similar conditions are examined.     

Analysis of cylindrical Langmuir probe using experiment and different theories

Cylindrical probe data have been analyzed using different theories in order to determine some plasma parameters (electron temperature and electron and ion densities). Langmuir probe data are obtained in a cylindrical DC glow discharge in the positive column plasma at argon gas pressures varied from 0.5 to 6 Torr and at constant discharge current equal to 10 mA. The electron density has calculated from the electron current at the space potential and from Orbital Motion Limited (OML) collisionless theory. Ion density has obtained from the OML analysis of the ion saturation currents. In addition, the electron temperature has measured by three different methods using probe and electrons currents. The electron temperature T _e , plasma density n _e , and space potential V _s , have been obtained from the measured single cylindrical probe I–V characteristic curves. The radial distribution of the electron temperature and plasma density along the glow discharge are measured and discussed. Using the collisionless theories by Langmuir cylindrical probe and up to several Torr argon gas pressures the differences between the values of electron temperature and electron and ion densities stay within reasonable error limits.     

Development of a self-consistent model of dust grain charging at elevated pressures using the method of moments

A model of dust grain charging is constructed using the method of moments. The dust grain charging process in a weakly ionized helium plasma produced by a 100-keV electron beam at atmospheric pressure is studied theoretically. In simulations, the beam current density was varied from 1 to 10⁶ μA/cm². It is shown that, in a He plasma, dust grains of radius 5 μm and larger perturb the electron temperature only slightly, although the reduced electric field near the grain reaches 8 Td, the beam current density being 10⁶ μA/cm². It is found that, at distances from the grain that are up to several tens or hundreds of times larger than its radius, the electron and ion densities are lower than their equilibrium values. Conditions are determined under which the charging process may be described by a model with constant electron transport coefficients. The dust grain charge is shown to be weakly affected by secondary electron emission. In a beam-produced helium plasma, the dust grain potential calculated in the drift-diffusion model is shown to be close to that calculated in the orbit motion limited model. It is found that, in the vicinity of a body perturbing the plasma, there may be no quasineutral plasma presheath with an ambipolar diffusion of charged particles. The conditions for the onset of this presheath in a beam-produced plasma are determined. __________ Translated from Fizika Plazmy, Vol. 29, No. 3, 2003, pp. 214–226. Original Russian Text Copyright ? 2003 by Filippov, Dyatko, Pal’, Starostin.     

Dynamics of the electron thermal diffusivity at improved energy confinement during lower hybrid plasma heating in the FT-2 tokamak

The dynamics of electron heat transport at improved energy confinement during lower hybrid plasma heating in the FT-2 tokamak was studied experimentally. Evolution of the profiles of the electron temperature and density was thoroughly investigated under conditions of fast variation in the plasma parameters. The energy balance in the electron channel is calculated with the help of the ASTRA code by using the measured plasma parameters. Correlation is revealed between the dynamics of electron heat transport and the behavior of small-scale drift turbulence measured using the enhanced scattering correlation diagnostics. The suppression of heat transfer and turbulence agrees well with the increase in the shear of poloidal plasma rotation calculated from experimental data in the neoclassical approximation.     

Formation of a micropinch and generation of multiply charged ions at the front of a current-carrying plasma jet

The formation of a neck in the cathode plasma jet in the initial stage of a low-voltage vacuum spark is investigated experimentally and theoretically. X-ray bursts corresponding to an electron temperature of 150–300 eV are detected. With the use of a pinhole camera, it is found that an emitting region less than 1 mm in size is located near the cathode. The free expansion of a current-carrying cathode plasma jet with a current growing in accordance with the experimentally observed time dependence is simulated using a hydrodynamic model. It is shown that the neck forms at the front of the plasma jet due to the plasma compression by the magnetic self-field. In the constriction region, the plasma is rapidly heated and multiply charged ions are generated. The calculated spatial and temporal variations in the electron temperature and average ion charge are close to the measured dependences over a wide range of the discharge parameters.     

Application of canonical profiles transport model to the H-mode shots in tokamaks

The linear and nonlinear versions of the Canonical Profiles Transport Model (CPTM), which includes both heat and particle transport equations, are used to simulate core and pedestal plasma for JET, and MAST H-mode shots. Simulations by the nonlinear version show reasonable agreement with experiment for both ELMy and ELM-free shots. RMS deviations of calculated results from the experimental ones are on the level 10–12% in main. The calculated ion and electron temperature profiles are very insensitive to the change of the deposited peaked power profiles. The calculated pedestal temperature rapidly increases with plasma current; density profile peaking increases at low collisionalities.     

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.