Discharge Vessel Research Articles

Optogalvanic detection of acoustic resonances in a high-pressure sodium discharge

In high-pressure discharges, arc instabilities may occur due to acoustic resonances inside the discharge vessel. We have observed experimentally the presence of longitudinal acoustic resonances in a high-pressure sodium discharge well below the threshold above which the arc is unstable. A dye laser, tuned to about 17 140 cm−1, proved to be able to excite the acoustic standing waves, and their galvanic effects have been measured. A local thermodynamic equilibrium discharge model is presented, which qualitatively describes the measurements. It is shown that the instability threshold is correlated to an acoustic pressure amplitude in the ventral segments of about 6 kPa.

Proposal for a time-of-flight Thomson backscattering technique for large fusion devices

The application of 180° Thomson scattering using ultrashort laser pulses for measuring electron temperature and density profiles in large fusion devices is proposed. Spatial resolution along the laser beam is achieved by high-speed detection allowing time-of-flight measurements. This LIDAR (light detection and ranging) technique uses a minimum number of window ports and reduces the number of optical components in the vicinity of the discharge vessel. As an example, the performance of such a system for the JET tokamak geometry is discussed on the basis of available laser and detection technology.

Gaseous products created by electrical discharges in the atmosphere and condensation nuclei resulting from gaseous phase reactions

Our study has a double aim. Firstly, to study the production of NO, NO x, O 3 and condensation nuclei by electrical discharges. Secondly, to attempt to make an estimation of the natural production for the whole earth of these products by atmospheric discharges. We deal firstly with atmospheric electrical discharges which are simulated in a point-to-plane discharge vessel in which we can alter the mean current, the point polarity and the relative humidity (R.H.). We used natural or bottled air or bottled nitrogen with a constant gas flow of 55 f h −1. We noted that the production of O 3 takes place mainly during predischarge regimes, whilst spark discharges are mainly productive of NO and NO x. Condensation nuclei (C.N.) seem to be produced by gaseous phase reactions between NO x and O 3 and depend on R.H. Secondly, we calculate the global production of these species by atmospheric electrical discharges. For the NO x we take account of our experimental results and of the hypothesis used elsewhere by Chameides et al. (1977) and for the O 3 Griffing's hypothesis (1977) as well as thunderstorm statistics given by Schonland (1953). Our estimation for NO x is of 25Tgy −1 and for O 3 our calculations give a lower limit of about 90Tgy −1 which seems to indicate that production of O 3 by atmospheric electrical discharges (corona or point discharges and lightning) is more important than usually assumed. In the actual state of the measurements, it seems unreasonable to attempt to give an estimation of production of the C.N. Complementary experiments with bottled nitrogen seem to indicate that atomic oxygen and OH radicals do not play an initial role in C.N. creation. Our results lead us to believe that these C.N. may be HNO 3 products.

Dependence of light-ion sputtering yields of iron on ion fluence and oxygen partial pressure

Laser excited fluorescence spectroscopy (LEFS) has been used to measure sputtering yields of iron bombarded with 2 keV hydrogen and 6 keV helium ions as a function of the bombarding fluence and the ratio of oxygen gas pressure to the current density. For a low oxygen partial pressure corresponding to a low ratio of the arrival rate of oxygen molecules to ions on the surface the Fe sputtering yields for a new target are initially low but increase by a factor 10–15 after about 20 monolayers of the surface are sputtered away. This increase in the sputtering yield is also correlated with the appearance of a rough surface structure. At high ratios of the oxygen molecule to ion arrival rates at the surface corresponding to a continuous reoxidation of the surface, the sputtering yields stay low, the surface stays flat during the bombardment, and initially rough surfaces become rounded. Transferring these results to fusion research means that the plasma contamination with metal atoms is becoming a larger problem if the surfaces of the discharge vessel are well cleaned.

Activated reactive evaporation of TiO 2 layers and their absorption indices

The discharge vessel used in this work is half as large as and is of greater mechanical stability than that of Heitmann. The absorption measurements prove that negative oxygen ions and electrons are the best oxidants for TiO and Ti 2O 3 used as starting materials for the deposition of TiO 2 layers. The partial ionization of the oxygen reduces the absorption by a factor of 10 (TiO) or 100 (Ti 2O 3) from that found with neutral oxygen. At lower substrate temperatures Ti 2O 3 is a better starting material than TiO because a higher evaporation rate is possible and the refractive index is higher. At higher substrate temperatures there is a strong increase in the absorption. In this case TiO is preferable because the absorption is smaller as a result of the lower probability of dissociation. The lower limit of the absorption index k was found to be 2 × 10 −5 (514 nm).

A comparative study of the radio frequency discharge in gas mixtures of helium with flourine, oxygen, nitrogen, and argon

Mass spectrometric sampling of the positive ions from the rf discharge in an 80 mole % helium-20 mole % flourine mixture showed the principal ions to be F+2 and F+. Impurity ions observed in the specially constructed discharge vessel were O+2 and CF+3 accounting for 10 –15% of the total ions. Very high ion flux was obtained from the discharge in the helium-flourine mixture, relative to discharge in pure gases. This effect was also found for mixtures of helium with oxygen, nitrogen, and argon. Floating double probe measurements showed that the average electron energy is higher in the helium mixtures than in the pure gases for comparable pressure and power input. Probe measurements also showed that the degree of ionization was higher in the helium mixtures, but the variations were not as large as the mass spectrometric measurements indicated. Some evidence was found to indicate that F− modifies the sheath, but retardation measurements clearly show the charged particle loss from helium-flourine plasma to be controlled by positive ion-electron ambipolar diffusion.

Substrate floating potential characteristics in planar magnetron and ht sputtering systems

The floating potential to ground Vfg and the ground to plasma potential Vgp have been measured in a magnetron and ht (non-magnetic field) sputtering system. A disc cathode (75 mm dia), made of Cu and water-cooled, was used for the ht arrangement and adapted to provide a magnetic tunnel field (200 gauss) of 52 mm mean dia and 10 mm width for the magnetron. The anode was grounded in both systems. The discharge vessel was evacuated by a silicone oil diffusion pump with an LN2 trap and an LN2 cold finger in the vessel; gas was admitted via an LN2 trap. Experiments were made at three pressures, 5, 50 and WO mtorr in Ar and air with two cathode/receiver gaps of 30 and 60 mm. Wire probes were placed at the work plane centre and edge of the electrode gap. The former was used to determine Vgp from V/I plots and the latter Vfg. The value of Vfg at the receiver centre was measured with a Cu-plate probe resting on a glass slide which simulated the conditions at the surface of a non-conducting substrate or thin film. The substrate floating potential to plasma Vfg was found from the addition of Vgp and Vfg both of which were negative. The difference between Vfg and Vgp is the voltage drop Vfg across an insulator or dielectric film resting on the anode support. For the applied voltage and current regions of 300–600 V and 11 mA cm−2 for the magnetron and 1–2 kV and 1–2 mAcm−2 for the ht system, Vgp was < −10 V and Vfg < −25 V. As the applied voltage was raised ⩾2 kV in the ht system and the pressure reduced < 50 mtorr Vfg rose sharply becoming proportional to the applied voltage and attaining nearly 1 kV as discharge blackout was approached. The negative bias was highest in air and at a small cathode/receiver gap. The high negative values of Vfg obtained in the ht system were attributed to negative charge accumulation from oxygen ions released from the cathode and accelerated by the electric field. Oxygen was present in the air discharge and probably as an impurity in the Cu-cathode. The effects are discussed of receiver bombardment by secondary electrons released from the cathode and accelerated in the cathode dark space. It is shown that negative bias from this source could not exceed −200 V, even in the complete absence of positive ion extraction.

The influence of additives and contaminants in TEA CO2 laser discharges evaluated by electrical measurements

Observation of the electrical characteristic of the glow discharge allows easy study of the influence of additives and contaminants. Moreover, when arcing occurs, interesting information is obtained by observing the damping of the arc current. The efficacy of the low-ionization organic additive to prevent arcing is confirmed, and the deleterious influence of oxygen contamination is demonstrated. This result accounts for the technical rule of preventing air from entering the discharge vessel. Increased output peak power is observed, resulting from the use of a suitable additive.

Thermal Instabilities in Discharging Gas Reservoirs

The flow structure in a discharging gas reservoir has been studied using schlieren cinematography. Effects due to discharge orientation, gas species, and vessel wall vibration have been observed. Initially, the internal flow structure has been found to be controlled by the mass sink, with streamlines going in approximate straight lines from the vessel wall to the exit orifice. At later time, free convection dominates and recirculation patterns are established. Discharge orientation can have a dramatic effect on the free convection flow field as well as on the growth and stability of the wall thermal diffusion layer. A jet-like structure (initiated at very early time and believed to be caused by wave effects) was observed to extend from the vessel wall diametrically opposite the exit orifice. When the discharge was parallel to but in the opposite direction of the gravitational vector, this jet-like structure was found to cause an instability in the lower wall diffusion layer. Vessel wall vibration resulted in a standing cellular acoustic wave pattern in the gas which, depending on discharge orientation, caused a violent instability. A model for temperature variation in the thermal diffusion layer is discussed and a numerical solution is given. Results of the predicted thermal diffusion layer histories are compared to data. A discussion of vessel orientation effects on critical Rayleigh number is given.

Tetraf luoromethane Injected into a dc Argon Glow Discharge

Abstract Low partial pressures (10−5 to 10−3 Torr) of tetrafluoro-methane have been injected into a dc argon glow discharge (85 × 10−3 Torr, 1000 V) and the discharge-induced removal rate of the CF4 molecules from the vapor phase has been studied with a mass spectrometer as a function of the discharge current and the CF4 concentration. The ultimate disposition of the CF4 is believed to be in the form of polymeric deposit on the walls of the discharge vessel. The removal rate of CF4 is proportional to the discharge current and also proportional to the concentration of CF4in the low concentration regime. At higher partial pressures of CF4 (∼1 × 10−3 Torr) the removal rate tends to saturate, becoming independent of the CF4 concentration.

A method for the accurate measurement of pulse height distribution in AC discharge pulses

A method of complete separation of superimposed discharge current pulses from the capacitance current through the discharge vessel is described. The potentiality of the method for studying pulse height distribution of discharge current pulses generated with low or high frequency fields is demonstrated.

Microwave plasma generation by end-on and side-on irradiation in the pressure range 0.1–10 mTorr of hydrogen

This paper describes investigations of hydrogen plasmas produced by mean of linearly polarized microwaves (f=2.45 GHz,P 0=2.2–5.0 kW). A superimposed uniform and stationary magnetic field $$\vec B_z $$ is tuned in most of the experiments to the electron cyclotron resonance. The RF is coupled to the plasma by different antennae. A conical horn is used for end-on irradiation (wave vector $$\vec k\parallel \vec B_z $$ ). With regard to toroidal discharge geometries the side-on irradiation $$\vec k \bot \vec B_z $$ is of particular interest. For this purpose two newT-shaped antennae were developed. In the pressure range investigated the plasma produced by the microwaves fills up the whole discharge vessel (l=200 cm,V≈104cm3). The electron densities are in the range between 2.5 · 1010 and 1.5 · 1011 cm−3 and the electron temperatures between 1.2 and 5.0 eV. The plasma produced by side-on RF irradiation $$(\vec k \bot \vec B_z ,\vec E\parallel \vec B_z )$$ is used as a pre-ionization plasma for aZ-pinch discharge. The degree of ionization obtained in this way is ≳5%. The optimum conditions for this type of preionization are evaluated from density measurements at higher cyclotron harmonics.

On the breakdown phenomena of capacitive discharges

The breakdown phenomena, in argon and mercury vapour in a glass vessel placed between metal plates, to which a 50 Hz voltage was applied, are interpreted by means of a statistical description of a model. The pressure of the gas in the discharge vessel in our experiments was 2·5 Torr of argon and 6 mTorr of mercury vapour. The distance between the external electrodes, 30 mm in diameter, was 20 mm.

Proposed New Method for Separating Isotopes

RECENTLY1 a double mercury discharge vessel was used (Fig. 1), in which a large current running in the outer vessel irradiated the small current discharge in the inner vessel with 2537 A resonance radiation. This irradiation caused the running voltage and electron temperature in the inner discharge to be reduced appreciably over the non-irradiated mode of operation. The explanation offered was that the neutral Hg atoms in the inner vessel absorbed the resonance radiation and entered the 63P1 state (see Fig. 2), and from this level the atom was ionized by electron impact. In addition the 63P0 and 63P2 metastable levels are populated from the 63P1 level by electron collisions and by cumulative absorption and radiative decay of higher levels. Because the radiation field is doing approximately half the ionization work, the electron temperature drops correspondingly.

Polymerization of styrene in an electrodeless glow discharge

AbstractThe polymerization (polymer deposition) rate of styrene in an electrodeless glow discharge from styrene vapor and a mixture of styrene vapor and gas (H2, He, A, and N2) was investigated. The rate of polymerization, R, was found to be independent of the discharge power. The rate of polymerization of the pure monomer was found to be proportional to the square of monomer pressure pM. The addition of gas increased the rate of polymerization depending upon the partial pressure of the gas, px, and R can be generally expressed by R = a[pM]2{1 + b[px]}. The value of b is dependent of the type of gas and follows the order of N2, &gt; A &gt; He &gt; H2. The distribution of polymer deposition was found to be nearly independent of the partial pressure of the gas and of the discharge power with N2 and H2 as plasma gas; however, with He and A, the distribution is highly dependent on the partial pressure of the gas and on the discharge power. The study strongly suggests that polymerization occurs in the vapor phase and that the growing polymer radicals deposit on the surface of the discharge vessel, yielding highly crosslinked polymer deposition.

A high frequency ion source with radial excitation field

A high frequency ion source with radial excitation field is described and compared with a usual axial excitation type. In the former type, the plasma is localized near excitation electrodes. The energy of emitted ions is considerably lower than an applied probe potential. Ionizing electrons were not lost by random scattering at the protruding parts of discharge vessel as in the case of the axial excitation type because these were not included in the ionization region in the radial excitation type. The operation is more stable and the startup is easier than in an ordinary axial excitation type.

Spatial variation of the electron energy in a H. F. ring discharge

The electron temperature has been measured in inductively coupled high frequency gas discharges in Ar, Kr and Xe with the aid of Langmuir probes. The measured temperature shows a rather low radial space dependence up to values ofpR ≈ 30 Torr cm (p=gas pressure,R=radius of the discharge vessel). This behaviour is explained on the basis of the local energy balance of the electrons as a consequence of the energy transport by thermal conduction in the electron gas.

Production of Photons in the Bactericidal Effect of Transient Electric Arcs in Aqueous Systems

The radiation in the visible and ultraviolet regions from submerged, transient electrical arcs was measured with a K(3)Fe(C(2)O(4))(3) chemical actinometer and was compared to the bactericidal effect obtained with the same electrical arrangements. Photon production and bactericidal effect were obtained at lower voltages with a smaller electrode separation than with a wider one. At higher voltages, both increased with wider electrode separations. The voltages at maximal photon production efficiency coincided with those of maximal bactericidal efficiency. However, the same photon radiation produced by different electrical arrangements did not always yield the same bactericidal effect in the small discharge vessel usually employed. In a larger discharge vessel, the bactericidal effect was closely correlated with the photon production. The efficiency of photon production by transient arcs was smaller than that of germicidal mercury lamps, particularly with respect to wavelengths of great bactericidal activity. The mechanisms of inactivation and their use for practical disinfection purposes are discussed.

Production of photons in the bactericidal effect of transient electric arcs in aqueous systems.

The radiation in the visible and ultraviolet regions from submerged, transient electrical arcs was measured with a K(3)Fe(C(2)O(4))(3) chemical actinometer and was compared to the bactericidal effect obtained with the same electrical arrangements. Photon production and bactericidal effect were obtained at lower voltages with a smaller electrode separation than with a wider one. At higher voltages, both increased with wider electrode separations. The voltages at maximal photon production efficiency coincided with those of maximal bactericidal efficiency. However, the same photon radiation produced by different electrical arrangements did not always yield the same bactericidal effect in the small discharge vessel usually employed. In a larger discharge vessel, the bactericidal effect was closely correlated with the photon production. The efficiency of photon production by transient arcs was smaller than that of germicidal mercury lamps, particularly with respect to wavelengths of great bactericidal activity. The mechanisms of inactivation and their use for practical disinfection purposes are discussed.

Investigation of the helium afterglow I. Mass spectrometric observations

Mass spectrometric observations of helium afterglows by ion sampling from the walls of a large discharge vessel are described. It is shown that in very pure cataphoretically-cleaned helium in the pressure range 0?3 to 2 torr the only important processes occurring in the late afterglow are ambipolar diffusion of atomic and molecular ions and electrons, and conversion of atomic to molecular ions by three-body collisions with neutral helium atoms. The atomic and molecular ion mobilities corresponding to the observed diffusion rates are respectively 11?2 ? 0?4 cm2 v-1 s-1 and 15?9 ? 0?4 cm2 v-1 s-1, in acceptable agreement with several previous mobility measurements, and the value obtained for the three-body conversion coefficient, 85 ? 3 torr-2 s-1 (pressure reduced to 0 ?C) is in good agreement with a recent similar measurement by other workers in a higher pressure range. The wall temperature of the discharge vessel during these measurements was 295 ?K. Measurements carried out in helium samples which were not subjected to cataphoretic purification demonstrated the serious effect of small amounts of impurity on the decay rate of the molecular helium ion.