Die Elektronenlawine als Sonde zur Ausmessung von Raumladungsfeldern

The dependence of the carrier amplification of an electron avalanche on the electric field allows to observe space charge fields in the discharge gap by measurements of carrier amplifications using the oscilloscopic method. This procedure is demonstrated by investigations of discharge processes in atmospheric air in a homogeneous field atE/p=34 V/cm Torr with an electrode distanced=15 cm. A test-avalanche was started to run through a positive space charge cloud produced by previous ionisation processes. The increase of the electron number was observed by a photomultiplier (optical method). There was found a strong modification of the time constant 1/αv _?. From this modification there was deduced a field distribution along the gap. The observed space charge was found moving towards the cathode with a velocity, which is equal to the drift velocity of the positive ions.     

Repetitive low-pressure volume discharge induced by attachment instability in an Ar/Cl<Subscript>2</Subscript> mixture

Ignition conditions and the characteristics of a repetitive volume discharge with a spherical anode and plane cathode are investigated. The discharge was ignited in Ar/Cl₂ mixtures (P≤2.0 kPa) used in excimer halogen lamps operating on the ArCl (B-X) 175-nm, Cl₂(D′-A′) 257-nm, and Cl ₂ ^** 195-to 200-nm molecular bands. At an interelectrode distance of 3 cm and a dc anode voltage of U _ch ≤1 kV, a stable repetitive pulsed discharge with a repetition rate of 1–50 kHz was ignited in chlorine or (0.1–2.0)/(0.04–0.12)-kPa Ar/Cl₂ mixtures. The development of attachment instability in the discharge plasma, in which the processes of the formation, decay, and diffusion of the Cl ₂ ⁻ and Cl⁻ negative ions play an important role, leads to the formation of a solitary pearlike plasma domain with an average diameter of 0.2–3.5 cm.     

Lokale Messung von Driftgeschwindigkeit und Temperatur der Ionen einer Argon-Niederdruckentladung mit resonanter Laserstreuung

The radial ion drift velocity and temperature in an argon low-pressure discharge have been measured by a resonant laser scattering method with high spatial resolution. The detection limit for the drift velocities was 2 · 10⁴ cm · s^?1. In the vicinity of the discharge axis the drift velocity grows linear with the distance from the axis. The measurements have shown the existence of a radial dependence of the ion temperature T¹(r). Comparisons of axial ion temperature T_? with T_‖(r = 0) are an experimental demonstration of the anisotropy of the ion velocity distribution in this kind of discharges.     

Formation of Discharges between Glass Insulated Electrodes in Ne + 1% N2 Mixtures. II. Frequency Dependence and Discharge Modes

Discharges between glazed electrodes for p = 2.67 kPa Ne + 1% N₂ and pd = 1.3 kPa cm (p = gas pressure, d = electrode distance) are investigated. They are sustained by a sine wave voltage of variable frequency f. Measurements of the space time structure of the radiation emission provide information of the build up and quenching of discharges and of the structure of the fully developed ac-discharge. It is shown that the discharge types for the different values of frequency and amplitude of the sine wave sustaining voltage are dynamically developed Townsend and glow discharges. For f ≦ 4 kHz the optical structure and its temporal behaviour largely depends on the increment of the sustaining voltage. For f ≧ 20 kHz the axial structure of radiation emission is modified by the influence of consecutive discharges due to residual ions (f ≌ 20 kHz) and metastable Ne(³P₂)-atoms (f > 20 kHz) existing for several periods of sine wave. The Penning effect of metastable Ne-atoms with N₂ molecules then produces start electrons for each discharge in a sequence which improve the initial conditions for the breakdown. Relations between space time structure and breakdown voltage as a function of frequency f are discussed.     

Influence of the cathode and anode jets on the properties of a high-current electric arc

A study is made of the effects related to the formation of electrode jets in discharges in hydrogen and air at a current of 10⁵–10⁶ A, a current growth rate of 10¹⁰ A/s, an initial pressure of 0.1–4.0 MPa, and a discharge gap length of 5–40 mm. After secondary breakdown, jets are observed in a semitransparent discharge channel expanding with a velocity of (4–7)×10² m/s. The formation of shock waves in the interaction of the jets with the ambient gas and the opposite electrode is observed by the shadowgraphy method. Seventy microseconds after the beginning of the discharge, the pressure of the metal vapor plasma near the end of the tungsten cathode amounts to 177 MPa. The brightness temperature in this case is T=59×10³ K, the average ion charge number is [`(m)] = 3.1\overline m = 3.1 , and the metal vapor density is n=5.3×10<sup>19</sup> cm<sup>−3</sup>. After 90 μs, the average ion charge number and the metal vapor density near the anode end are [`(m)] = 2.6\overline m = 2.6 and n=7.4×10¹⁹ cm⁻³, respectively. Based on the experimental data, possible reasons for the abnormally high values of the total voltage drop near the electrodes (up to ∼1 kV) are discussed.     

Studying the Generation Stage of a Plasma Jet in a Plasma Focus Discharge

A dense compact plasmoid generated at the pinch collapse stage is revealed in a plasma focus discharge by laser optical methods. The initial size of the plasmoid is ~1 mm, its electron density is more than 2 × 10¹⁹ cm^–3, and the plasmoid propagates along the axis from the anode at an average velocity of more than 10⁷ cm/s. A shock wave is generated in the residual argon plasma during the motion of the bunch, its density decreases to 10¹⁸ cm^–3 at a distance of 3 cm from its place of generation, and the plasmoid expands by 3–5 times and almost merges together with the leading edge of the shock wave.     

Time resolved spectroscopic studies of a FXR discharge

Time resolved spectroscopic studies of some electrode material lines and an impurity line have been performed in a flash X-ray (FXR) discharge. The results indicate that the breakdown takes place in impurities and adsorbed gases released from the electrodes. Thereafter, a front of anode material propagating with a velocity of (1–2)×10⁶ cm/sec moves towards the cathode. A cloud of cathode material is localized outside the cathode during an early stage of the discharge. The anode front and the cathode cloud penetrate each other after current maximum and a “metallic arc” is formed between the electrodes     

Metal vapour transport in a FXR discharge

Streak photographs of the axial motion of vapourized anode and cathode materials in a flash X-ray (FXR) discharge correlated with the time derivative of the discharge current (dI/dt) indicate that clouds of vapourized metal are formed outside the electrode surfaces during the first significantdI/dt dip, i.e. during the pinching of the discharge column. The anode cloud propagates towards the cathode with a velocity of (1–1.5)×10⁶ cm/sec and reaches the cathode cloud at current zero. This means that the discharge current is not entirely carried by a metallic arc until after the first current zero. The metal vapour propagation is discussed in relation to existing theories.     

Crystallization of a dusty plasma in the positive column of a glow discharge

The formation of macroscopic ordered structures in the standing striations of a stationary glow discharge in Ne is observed. A Coulomb quasicrystal is formed by spherical glass particles with diameters of 50–63 μm and charge Z _p~7·10⁵ e. The interparticle distance is approximately 300 μm. This corresponds to a nonideality parameter Γ~5·10⁴, which leads to crystallization in the Yukawa model. The factors leading to the formation of a quasicrystal in the striations are discussed. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 2, 86–91 (25 July 1996)     

Some Investigations of the Ignition and Development of Breakdown in Cylindrical Hollow Cathode Glow Discharges

The breakdown behaviour of a hollow cathode glow discharge is investigated in a cylindrical, hollow cathode structure having an internal diameter of 2 cm. The anode is a plane electrode across one end of the cathode cylinder. Pressures of argon between 20 and 107 Pa were used (0.15 to 0.80 torr), and applied voltages between 800 and 2500 V. It is shown that the statistical time lag for breakdown is in the range of ～ 1 ms and depends on the applied voltage, the gas pressure, and the history of operation of the discharge tube. The rise time of the discharge current ranges from about 10 ns at high pressure and voltage to about 200 ns at the lowest pressure and voltage used. The discharge propagates along the cathode axis at a speed of about 10⁸ cm s^?1. From the obtained data, a qualitative model of the first stage of the discharge is derived. Based on this model, a simple calculation gives values of Townsend modified first coefficient η at high values of E/N, 10⁴ < E/N < 8 · 10⁴ Td which fit well at the lowest E/N, where they approach the data of PENNING and KRUITHOF in argon. In contrast to the extremely short initial current-rise times, in the submicrosecond range, the discharge currents reach steady-state values only after about 300 μs.     

On distribution over the cathode surface of the current due to positive ions in discharge with oscillating electrons

The mechanisms of processes determining distribution of positive ions on the cathode surface in a discharge with oscillating electrons are studied. At low pressures P≤5×10⁻⁵ torr over the entire range of anode length variation l _a=0.5–11 cm, the ion current distribution over the cathode radius J _i(r) features a single maximum in the center and drops steeply with distance from the axis. At pressures P=1–4×10⁻⁴ torr, the distribution J _i(r) for short anodes (l _a<6 cm) is similar to the previous one but, for long anodes (l _a=6–10 cm), new maxima at higher J _i values have been detected. A physical explanation for the obtained results is proposed.     

External combustion of high-speed multicomponent hydrocarbon-air flow under conditions of low-temperature plasma

The stabilization of external combustion (at the plate surface) of high-speed multicomponent (air, alcohol, and propane) flows is realized experimentally. It is shown that heat fluxes during alcohol combustion rise by a factor of about 7 and during propane combustion by a factor of 15, in comparison with the heat flux from a discharge in a high-speed air flow. The electron concentration measured at a distance of 10 cm down-stream from the electrode ends is approximately 10⁹ cm^?3 in the case of the discharge in an air flow, whereas during alcohol combustion it attains 2 × 10¹¹ cm^?3 and during propane combustion it is 3 × 10¹¹ cm^?3. The flame temperature in the area of the discharge existence varies from 2000 up to 2500 K and outside the discharge at the distance of z = 20 cm from electrodes is 1800 K, gradually decreasing downstream. It is shown that the combined discharge in the subsonic flow allows for complete combustion of liquid and gaseous hydrocarbons. The completeness of combustion in supersonic flows attains 95%, depending on the flow velocity.     

Ionenschallinstabilitäten in Edelgas-Gleichstromentladungen bei niedrigen Drücken. I. Meßanordnung und experimentelle Ergebnisse

An experimental study of self-excited ion acoustic waves with wavelengths greater than the radius of the discharge tube in a de-low pressure column is presented. The propagation of this type of waves was observed over a pressure range of 5 · 10^?4 torr ≦ p ≦ 10^?1 torr and currents of 0,02 A ≦ i ≦ 0,6 A in various gases (argon, neon, helium, and hydrogen) in cylindrical glass tubes with diameters of d = 2;4 and 6 cm. The Dispersion behaviour and the existence range were measured in dependence of the internal parameters of the discharge (characteristic velocities and collision frequencies) and the geometry of the discharge tube. It is shown, that the existence range depends not only on gas pressure and current but is influenced also by the geometry of the discharge tube. The minimum wavelengths which belong to the upper cut-off frequencies correlate with the radius of the discharge tube at currents higher than i ～ 0,2 A and increasing pressure.     

Die Eigenschaften von Elektronenlawinen bei hohen Verstärkungen in Äther

The oscilloscopic study of avalanches of high carrier numbersn allows to investigate the influence of the space charge on the carrier amplification aboven≈10⁶. The investigation was made with ether in the range ofE/p from 60 to 200 V/Torr·cm, withpd-values from 6 to 500 Torr·cm and with gap widths of 0,3; 0,9 and 2,7 cm. Betweenn=10⁶ and 10⁸ the number of carriers increases less than exp (α₀ x). The space charge field of the positive ions reduces the ionisation effect of the electrons. By comparing the lowering of α(n)/α₀ with the theory one can calculate the diffusion radius of avalanches and thus the mean energy of agitation in ether, which had a nearly constant value of 3 to 4 eV. Aboven≈5·10⁶ to 10⁷ the avalanche diameter increases considerably especially at high pressures. This effect is produced by the electrostatic repulsion of the electrons in the head of the avalanche. If the carrier number reachesn≈10₈ α(n)/α₀ increases. This is apparently due to processes (“Nachstrom”) following the development of an avalanche just after its transit time.     

On Various Kinds of Dielectric Barrier Discharges

Dielectric barrier discharges in Xe and Ne/Xe, He/Xe mixtures are investigated with regard to the ranges of existence of various discharge types over a wide range of parameters (total gas pressure p, electrode distance d, frequency f and mixture ratio X). The discharges were explained in more detail because of the bistable behaviour and the transfered charges, partly of the peak current and the maximum of the visible radiation pulse. The time behaviour of the discharge was observed, too. In this paper a distinction of various discharge types could be achieved by the similarity parameters f/p and pd. The charge carriers remaining in the volume and long-living plasma species could be identified as essential mechanisms for the development of various types of discharges. The influence of space charges on the bistable behaviour could be shown indirectly.     

Investigation of scaling laws as applied to the gas discharge in the case of a barrier-discharge-excited Kr/CCl<Subscript>4</Subscript> mixture

The electrical and luminescent characteristics of a barrier-discharge lamp filled with a Kr/CCl₄ (150: 1) mixture are experimentally studied versus the value of pd, which varies in the range (7.6–14) × 10³ Pa cm. When simulating the gas discharge using similarity parameters, the following relationships are fulfilled: for pd = const (p is the pressure, d is the interelectrode distance), the pulse duration and the mean current density are τ _j ∼ 1/p and 〈j〉 ∼ p; the surface charge density on the electrodes, σ ∼ const; the duration of the UV radiation pulse and the efficiency of UV radiation due to a KrCl* (222 nm) exciplex, τ_rad ∼ 1/p and η ∼ p ². The maximal radiation efficiency achieved in the experiments is about 13%. Deviations from the similarity laws for the gas discharge are related to the filamentary form of the observed discharge. Qualitative analysis indicates that similarity laws may be fulfilled for such a form of discharge as well but locally, within a single filament.     

Geometry,strength of binding and Br2 charge redistribution in the complex OC ··· Br2 determined by rotational spectroscopy

The ground-state rotational spectra of the six isotopomers ¹⁶O¹²C ··· ⁷⁹Br⁷⁹Br, ¹⁶O¹²C ··· ⁸¹Br⁷⁹Br, ¹⁶O¹²C ··· ⁸¹Br⁸¹Br, ¹⁶O¹²C ··· ⁷⁹Br⁸¹Br, ¹⁶O¹³C ··· ⁷⁹Br⁷⁹Br, ¹⁶O¹³C ··· ⁸¹Br⁷⁹Br, were observed by pulsed-nozzle, Fourier-transform microwave spectroscopy. The spectroscopic constants B _O, D _J, χ _aa (Br_i), χ _aa (Br_o), M_bb (Br_i) and M _bb (Br_o), where i = inner and o = outer, were determined for each isotopomer. The complex is linear, with the weak bond between the C atom of CO and Br_i. The rotational constants were used to determine the distance r(C ··· Br_i) = 3.1058Å and to show that the Br—Br bond lengthens by ～0.005–0.01Å on complex formation. The intermolecular stretching force constant kσ = 5.0 Nm^?1 was obtained from D_J and the Br nuclear quadrupole coupling constants were interpreted to reveal that a fraction δ = 0.02 of an electronic charge is transferred from Br_i to Br_o when Br₂ is subsumed into the complex. Properties of the two series OC ··· XY and H₃N ··· XY, where XY = C1₂, Br₂ and BrC1, are compared.     

Influence of structural dynamics on charge recombination rates in photogenerated radical ion pairs: Evidence from EPR spectroscopy and computation

In order to better understand the dependence of charge recombination rate vs. temperaturek _CR(T) within a linear donor-chromophore-acceptor (D-C-A) molecular triad, the structural dynamics of the cation radical D⁺-C is studied individually using variable-temperature electron paramagnetic resonance (EPR) spectroscopy and electronic structure calculations. Here, the donor D isp-methoxyaniline, the chromophore C is 4-(N-piperidinyl)-naphthalene-1,8-dicarboximide, and the acceptor A is naphthalene-1,8∶4,5-bis(dicarboximide). The EPR spectra of D⁺-C exhibit marked changes in their overall shape throughout the 190–295 K temperature range. These spectra have hyperfine splittings that are strikingly well simulated with a model that includes methoxy group rotation, which occurs at a rate of 2.6 · 10⁴ s^?1 at 210 K and speeds up to 1.25 · 10⁷ s^?1 at 295 K, corresponding to an energy barrier of 38 kJ/mol. This considerable barrier reflects the partial conjugation between MeO and the aromatic ring and is confirmed by the calculated energy of a series of D^{+ ·}-C rotamers. The simulations also reveal that inversion of the anilino N center emerges atT > 250 K and can be represented by a planar and a pyramidal conformation with the equilibrium constantK = [pyramidal]/[planar] increasing from 0.029 at 250 K to 0.56 at 295 K. In the same temperature range, the charge recombination rate of D^{+ ·}-C-A^{? ·} accelerates abruptly and can be separated into two components, according to the above planar/pyramidal equilibrium. Thek _CR (T) of the pyramidal conformation has an activation energy of 41 kJ/mol, virtually the same as the barrier of MeO rotation. These results show that the intramolecular structural dynamics of the radical cation within D^+·-C-A^{? ·} control the overall charge recombination reaction with this radical ion pair.     

Design considerations and scaling laws for high power convective cooled CW CO2 lasers

Various criteria for designing high power convective cooled CO₂ lasers have been discussed. Considering the saturation intensity, optical damage threshold of the optical resonator components and the small-signal gain, the scaling laws for designing high power CW CO₂ lasers have been established. In transverse flow CO₂ lasers having discharge of square cross-section, the discharge lengthL and its widthW for a specific laser powerP (Watt) and gas flow velocityV (cm/s) can be given byL = 1.4 x 10⁴ p ^1/2 V ^-1 cms andW = 0.04P ^1/2 cms. The optimum transmitivity of the output coupler is found to be almost constant (about 60%), independent of the small signal gain and laser power. In fast axial flow CO₂ lasers the gas flow should be divided into several discharge tubes to maintain the flow velocity within sonic limit. The discharge length in this type of laser does not depend explicitly on the laser power, instead it depends on the input power density in the discharge and the gas flow velocity. Various considerations for ensuring better laser beam quality are also discussed.