A Report on H mode in Magnetic Pole Enhanced Inductively Coupled Nitrogen Plasmas

The properties of low pressure magnetic pole enhanced, inductively coupled nitrogen plasma were studied by using electrical probe (Langmuir probe) under the conditions of RF powers in the range of 50‐220 W and pressures of 15‐75 mTorr. The electron energy probability function (EEPF) and electron density (n_e) obtained from the RF compensated Langmuir probe was compared with the theoretical results. The theoretical fits of the EEPF shows that the shapes of EEPF are evolved from generalised distribution to Maxwellian distribution function. It was also observed that at a low power (50 W) the discharge remains in inductive (H‐mode) mode for all the pressures (15‐75 mTorr). At a higher pressure and relatively low RF power, the measured EEPF show a hole near 3eV of energy. The intensities of the emission lines at 337.1nm (Second Positive System) and 391.4 nm (First Negative System) due to C³Π_u → B³Π_g and B²$\textstyle \sum_u^+$ → X² $\textstyle \sum_g^+$ transitions respectively, closely follows the variation of n_e with RF power and filling gas pressure. The stability of the H mode was also investigated using skin depth. Electron temperature and plasma potential indicate that the discharge at higher power (above 50 W) almost remain in H mode. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical Emission Spectroscopy Used to Study the Characteristics of the Capacitive Radio-Frequency Discharge in Argon

Capacitively coupled radio-frequency (13.56 MHz) glow discharge in argon at low pressure (200 mTorr) has been studied by optical emission spectroscopy (OES) and electrical probe. Increasing power density resulted in lowering of the electron temperature T _e and increase of the electron density n _e. The to transition in this case takes place smoothly. The intensity (I _750.4) of the emission line at 750.4 nm due to 2p₁1s₂ transition closely follows the variation of n _e with power density. Calculation of the electron-energy probability function shows that electron occupation mainly changes in the high-energy tail, which explains close resemblance of I _750.4 to n _e. At moderate pressures, only OES method was applied to observe - transition which is abrupt in this pressure range.     

Dynamics of VUV spectra in fast capillary discharge

The temporal behavior of VUV spectra of capillary discharge with a rate of a current rise on the order of 10¹² A/s is studied. The current is generated using an inductive storage unit with a plasma-erosion opening switch. This discharge is accompanied by the generation of a shock wave on an inner wall of a capillary and by its subsequent cumulation on the discharge axis. The capillary is prefilled with argon at a pressure of 80 Pa. Radiation spectra are obtained using an off-Rowland spectrograph based on a concave grating with optimal focusing in the wavelength range of 20 nm. The radiation is recorded with a detector based on a microchannel plate with a time resolution of 20 ns, which makes it possible to separate the spectra of two phases of the discharge, i.e., the cumulation phase of the shock wave in argon that fills the capillary (T _e ∼ 20–30 eV) and the phase of subsequent discharge in a substance desorbed from the capillary wall T _e ∼ 50 eV).     

Number density and temperature measurements obtained using sensitive diode laser spectroscopy in an argon plasma

The absolute concentration and translational temperature of the 2p₁₀ and 2p₇ excited states of argon have been measured in an inductively coupled plasma chamber under a variety of operating conditions using both calibrated diode laser frequency modulation spectroscopy and cavity enhanced absorption spectroscopy. Accurate lineshape analysis of frequency modulation signals has been employed to extract the desired information, and is corroborated by cavity enhanced measurements. Temperatures are found to vary linearly with pressure from ∼400 K at 20 mTorr (2.7 Pa) to ∼510 K at 90 mTorr (12 Pa) in a 200 W discharge while concentrations peak at 3.25×10⁸ cm^-3 at 30 mTorr (4 Pa) (also in a 200 W discharge). The uncertainty in the recovered temperature is 7%, dominated by uncertainties in the calibration of the frequency scale. PACS 42.62Fi; 52.70kz     

Plasma parameters in the upgraded Trimyx-M Galathea

Results are presented from measurements of the plasma parameters in the upgraded Trimyx-M Galathea. After the barrier magnetic field and the energy of the injected hydrogen plasma bunch were increased to B _bar ∼ 0.1 T and W ₀ ≈ 200 J, respectively, the following plasma parameters were achieved: the density n ∼ 5 × 10¹³ cm⁻³, the plasma confinement time τ* = 800–900 μs, the elergy of the confined plasma W ₁ ∼ 100 J, the ratio of the plasma pressure to the barrier magnetic pressure β ₀ ∼ 0.2, the electron temperature T _e ∼ 20 eV, and the ion temperature T _i ∼ 2T _e . The maximum time during which the plasma density decreased e-fold, τ _p , was found to be 300 μs at B _bar = 0.1 T, which agrees with the classical transport model.     

Experimental study of optical gas-dynamic processes of polymeric material ablation by ultrashort laser pulses

Optical gas-dynamic processes occurring in polymeric targets ((CH₂O) _n , (C₂F₄) _n ) exposed to ultrashort laser pulses (τ _0.5 ∼ 45 − 70 fs; λ _I,II,III = 266, 400, 800 nm; and E/S ∼ 0.1 − 40 J/cm² at r ₀ ∼ 20 μm) were studied under normal conditions and in vacuum (p ∼ 10⁻² Pa). The dynamics of the mass flow from the target surface (m′ ∼ 10⁻⁵ − 10⁻⁴ g/J) was studied and the spectral-energy thresholds of laser ablation, the electron density distribution (n _e ∼ 10¹⁴ − 10¹⁸ cm⁻³), the mass-averaged velocity of the material flow from the target surface (∼ 10³ m/s), and the chemical composition and average temperature in the near-surface plasma formation (T ∼ 5000 K) were determined using interference microscopy, emission spectroscopy, and shadowgraphy.     

Spectroscopic study ofβ-Ni(OH)2 under pressure

Infrared absorption and Raman study ofβ-Ni(OH)₂ has been carried out up to 25 GPa and 33 GPa, respectively. The frequency ofA _2u internal antisymmetric stretching O-H mode decreases linearly with pressure at a rate of −0.7 cm⁻1/GPa. The FWHM of this mode increases continuously with pressure and reaches a value of ∼ 120 cm⁻¹ around 25 GPa. There was no discernible change observed in the frequency and width of the symmetric stretchingA _1g O-H Raman mode up to 33 GPa. The constancy of the Raman mode is taken as a signature of the repulsion produced by H-H contacts in this material under pressure. Lack of any discontinuity in these modes suggests that there is no phase transition in this material in the measured pressure range.     

Spatiotemporal spectroscopic diagnostics of a pulse-periodic strontium vapor laser

Results of spectroscopic investigations into plasma of a pulse-periodic strontium vapor laser operating in the superradiance mode on the infrared transition at λ = 6.45 μm are presented. The method of determining the electron temperature and concentration as well as the gas temperature – T _e , n _e , and T _g – based on measuring the absolute intensities of some SrI and SrII and buffer gas (helium or neon) spectral lines is used. Time dependences of the line intensities during a current pulse (τ = 150 ns) and near afterglow (up to 3 μs) are obtained under conditions of non-equilibrium plasma ionization and recombination. The optical system collects radiation from the entire length of the plasma column by means of separating radial volume zones, includingthe central zone and the zone closer to the walls, with the monochromator slit. The results obtained allow us not only to calculate T _e , n _e , and T _g values, but also to trace the spatiotemporal plasma evolution.     

Low-frequency fluctuations in a pure toroidal magnetized plasma

A magnetized, low-β plasma in pure toroidal configuration is formed and extensively studied with ion mass as control parameter. Xenon, krypton and argon plasmas are formed at a fixed toroidal magnetic field of 0.024 T, with a peak density of ∼10¹¹ cm⁻³, ∼4 × 10¹⁰ cm ⁻³ and ∼2 × 10¹⁰ cm ⁻³ respectively. The experimental investigation of time-averaged plasma parameter reveals that their profiles remain insensitive to ion mass and suggests that saturated slab equilibrium is obtained. Low-frequency (LF) coherent fluctuations (ω < ω _ci) are observed and identified as flute modes. Here ω _ci represents ion cyclotron frequency. Our results indicate that these modes get reduced with ion mass. The frequency of the fluctuating mode decreases with increase in the ion mass. Further, an attempt has been made to discuss the theory of flute modes to understand the relevance of some of our experimental observations.     

Thomson Scattering Applied to a Noisy Radiative Plasma

Thomson scattering with a 1.5 ms long pulse mode 20 J ruby laser has been applied to a radiative argon plasma with electron densities n_e from 2.5 10¹⁹ m^?3 to 1.5 10²⁰ m^?3 and an electron temperature T_e of about 3 eV. Photon counting techniques have been used. The accuracy of n_e and T_e to be reached is about 5% after 10 shots. The signal to noise ratio S/N has been optimized by the use of optical filters and a special purpose grating. The effects of these elements on S/N have been calculated. The entrance angle, transmission and quantum efficiency have also been optimized. A comparison between 5 possible laser systems, including a normal mode and a Q-switched mode ruby laser, has been carried out.     

Spectral diagnostics of a unipolar high-frequency discharge excited in nitrogen and argon at pressures from 1 to 12 atm

By methods of spectral diagnostics, the temperature of neutral gas and the electron temperature and density have been determined in the channel of a unipolar high-frequency discharge excited at very high pressures. In nitrogen the h.f. discharge was excited at pressures of 1–5 atm, in argon at pressures of 1–12 atm. In the discharge excited in argon, the electron temperature does not change with increasing pressure and isT _e =(6–7)×10³ °K; the electron density increases with increasing pressure. It can be demonstrated that the electron velocity distribution is given by a Maxwellian distribution function although the plasma of a unipolar high-frequency discharge is non-isothermal (T _e –T _n 5×10³ °K).In conclusion, the author thanks Prof. Dr. V. Truneek for stimulating remarks and his kind interest in this work.     

Negative ion effects in CO2 convection laser discharges

Negative ions are computed to be formed on a time scale and in quantities such that they may be a cause of plasma instability observed in low pressure electrical discharge convection CO₂ lasers. In a typical CO₂−N₂−He−H₂O laser mixture the principal ions are CO ₃ ⁻ , CO ₄ ⁻ and H⁻ with the total negative ion densityn ₋ given by 0.1n _e <n ₋<n _e , wheren _e is the electron density: but if the gases are re-cycled or if there is an air leak NO ₂ ⁻ and NO ₃ ⁻ are formed in significant amounts andn ₋ can become greater thann _e in a time considerably less than the gas dwell time in the electrical excitation discharge. CO is effective in reducingn ₋ in a system without re-cycling, but is ineffective in a re-cycled system with the oxides of nitrogen present.     

Cluster ion emission from LiF induced by MeV N<Superscript>q+</Superscript> projectiles and <Superscript>252</Superscript>Cf fission fragments

Ion cluster desorption yields from LiF were measured at PUC-Rio with ≈0.1 MeV/u N ^q+ (q = 2,4,5,6) ion beams by means of a time-of-fight (TOF) mass spectrometer. A ²⁵²Cf source mounted in the irradiation chamber allows immediate comparison of cluster emissions induced by ≈65 MeV fission fragments (FF). Emission of (LiF) _n Li⁺ clusters are observed for both the N beams and the ²⁵²Cf fission fragments. The observed cluster size n varies from 1 to 6 for N ^q+ projectiles and from 1 to ≈40 for the ²⁵²Cf-FF. The size dependence of the Y(n) distributions suggests two cluster formation regimes: (i) recombination process in the outgoing gas phase after impact and (ii) emission of pre-formed clusters from the periphery of the impact site. The corresponding distribution of ejected negative cluster ions (LiF) _n F⁻ closely resembles that of the positive secondary (LiF) _n Li⁺ ions. The desorption yields of positive ions scale as Y(n) ∼ q ⁵. A calculation with the CASP code shows that this corresponds to a cubic scaling ∼S _e ³ with the electronic stopping power S _e , as predicted by collective shock wave models for sputtering and models involving multiple excitons (Frenkel pair sputtering). We discuss possible interpretations of the functional dependence of the evolution of the cluster emission yield Y(n) with cluster size n, fitted by a number of statistical distributions.     

Coherent backscattering of electromagnetic waves in a magnetised plasma

Summary A microwave coherent backscattering experiment has been carried out on Mirabelle, a weakly ionised plasma device, with the objective of measuring the electron density fluctuation level. The experiment is a preliminary step in order to prepare the detection system for a microwave stimulated backscattering experiment. The incident electromagnetic wave is focused in front of a plane grid which excites ion acoustic or electron Bernstein waves inducing fluctuations in the plasma. The backscattering signal is collected by the launching circuit and detected by homodyne mixing. The typical ratio of the scattered power to the incident power is about 10⁻¹² and the relative density fluctuations are of the order of δn _e/n _e=10⁻³ against a background electron density ofn _e=1–5·10⁹ cm⁻³. The backscattering measurement is compared with Langmuir probe measurements. The spectral width of the backscattered signal has also been studied, by taking into account effects due to the incident wave focusing and plasma wave damping. The authors of this paper have agreed to not receive the proofs for correction     

SnO2 thin films grown by pulsed Nd:YAG laser deposition

SnO₂ thin films have been deposited on glass substrates by pulsed Nd:YAG laser at different oxygen pressures, and the effects of oxygen pressure on the physical properties of SnO₂ films have been investigated. The films were deposited at substrate temperature of 500°C in oxygen partial pressure between 5.0 and 125 mTorr. The thin films deposited between 5.0 to 50 mTorr showed evidence of diffraction peaks, but increasing the oxygen pressure up to 100 mTorr, three diffraction peaks (110), (101) and (211) were observed containing the SnO₂ tetragonal structure. The electrical resistivity was very sensitive to the oxygen pressure. At 100 mTorr the films showed electrical resistivity of 4×10⁻² Ω cm, free carrier density of 1.03×10¹⁹ cm⁻³, mobility of 10.26 cm² V⁻¹ s⁻¹ with average visible transmittance of ∼87%, and optical band gap of 3.6 eV.     

Ferromagnetic and spin-wave resonance in trilayer exchange-coupled structures NiFe/Cu/NiFe

SWR spectra of exchange-coupled NiFe/Cu/NiFe structures were investigated. We found that the optical satellites of exchange spin-wave modes are characterized not by the standard Kittel dependence but by their own dependence of the resonance field on the mode number: H _r ^opt(n) ∼ n ^5/2.     

Verbreiterung von Hauptserienlinien des Kaliums durch Wechselwirkung mit Kalium- und Cäsiumatomen bei hohen Teilchendichten

For certain conditions the afterglow of low pressure discharges can be dominated by three body recombination. These conditions — electron densities of about 10¹² cm^?3 and electron temperatures of about 500 °K— are realized 20 μsec after switch off of the discharge for gas pressures (hydrogen) between 2 and 6 mTorr. In this pressure range the energy transfer from the electrons via the ions to the neutrals and subsequently to the wall is high enough to permit a decay of the electron temperature to low values before the electron density has decreased appreciably. A theoretical model demonstrates that the electron density decaydn _e/dt～n _e ³ f(T _e) is in good approximation equivalent todn _e/dt=const·n _e ² for a certain range of time and of parameters. This proportionality is observed. The measured values of the constant differ from the calculated ones by about 15%.     

Laser gain measurements at 193 nm in a small discharge cell containing ArF excimer laser gas mixtures

Spatial and temporal gain profiles as well as the peak net gain at 193 nm have been measured in X-ray preionized discharges excited by a single pulse electrical system working in the charge transfer mode. Ar- and F₂-containing laser gas mixtures with He or Ne as a buffer gas have been used. With a pumping pulse duration of ∼ 100 ns (FWHM) and a specific peak power deposition of ∼ 1 MW cm^-3 bar^-1 in a gas mixture containing F₂ : Ar : He (0.1%:5%:94.9%), at 2 bar total pressure, a very high peak net gain coefficient of ∼ 30% cm^-1 was measured in the gas discharge. The FWHM of the gain waveform was ∼ 60 ns. PACS 42.55.Lt; 42.60.Lh; 52.80.-s     

Electron temperature (T e) measurements by Thomson scattering system

Thomson scattering technique based on high power laser has already proved its superoirity in measuring the electron temperature (T _e and density (n _e) in fusion plasma devices like tokamaks. The method is a direct and unambiguous one, widely used for the localised and simultaneous measurements of the above parameters. In Thomson scattering experiment, the light scattered by the plasma electrons is used for the measurements. The plasma electron temperature is measured from the Doppler shifted scattered spectrum and density from the total scattered intensity. A single point Thomson scattering system involving a Q-switched ruby laser and PMTs as the detector is deployed in ADITYA tokamak to give the plasma electron parameters. The system is capable of providing the parameters T _e from 30 eV to 1 keV and n _e from 5 × 10¹²cm⁻³−5 × 10¹³cm⁻³. The system is also able to give the parameter profile from the plasma center (Z=0 cm) to a vertical position of Z=+22 cm to Z=−14 cm, with a spatial resolution of 1 cm on shot to shot basis. This paper discusses the initial measurements of the plasma temperature from ADITYA.