Characteristic radiation of nitrogen under subnanosecond breakdown in a highly nonuniform electric field near the positive-polarity electrode

Results are presented from experimental studies of elevated-pressure diffuse discharge in a highly nonuniform electric field. It is shown that, in discharges in nitrogen and air, the characteristic radiation (the K-line of nitrogen) is generated at the positive polarity of the electrode with a small curvature radius. X-ray bremsstrahlung from the anode was detected in a discharge in atmospheric-pressure air at a 12-kV amplitude of the incident voltage pulse.     

The calculation of optical properties of cesium plasma under conditions of a pulse-periodic discharge

The calculations of the radiation spectrum, the color rendering index R _a , the color temperature T _c , and the color coordinates X _c and Y _c of cesium plasma radiation under the conditions characteristic for a pulse-periodic discharge in cesium are presented. The plasma pressure range of 30–1500 Torr and the range of temperatures on the axis of 3200–6000 K are considered. The color rendering index is shown to be R _a > 90 in the entire analyzed plasma parameter ranges. The color temperature of plasma radiation varies within a broad range of 2300–5500 K. The color coordinates of the cesium plasma radiation are close to the X _c and Y _c coordinates of a black body.     

Effect of the high-voltage electrode polarity and wire preheating on the energy characteristics of electric explosion of fine tungsten wires in vacuum

Results are presented from experiments on the explosion of 30.5-μm tungsten wires at a current density of up to 140 MA/cm² and resistive-heating time of 40–100 ns. The experiments were performed both with and without preheating of wires and at different polarities of the high-voltage electrode. The effect of plasma production at the electrodes on the initiation of breakdown along the exploding wire was investigated by using a frame camera. It is shown that, when the polarity of the high-voltage electrode is positive, breakdown begins with the formation of a bright spot on the wire surface near the cathode, whereas at the negative polarity, breakdown begins with the formation of bright spots on the cathode surface. A comparative analysis of the main characteristics of wire explosions is performed. It is shown that preheating of the conductor increases the resistive-heating time and, accordingly, the energy deposited in the wire core. This effect takes place during explosions of both single wires and wire arrays. The evolution of the state of a metal during the explosion (including melting and evaporation) is studied by one-dimensional simulations by using a semiempirical equation of state describing the properties of tungsten over a wide range of parameters.     

Multichannel vacuum arc discharge used for Z-pinch formation

Results are presented from experimental studies of the implosion dynamics and radiative characteristics of an aluminum Z-pinch formed from a plasma shell (PS). The PS with an initial diameter of 4 cm was produced with the help of a multichannel vacuum arc discharge and formed due to the evaporation of the electrode material in ten parallel arc discharges. The PS composition depended on the electrode material in the arc discharge. The described experiments were performed with aluminum electrodes. The total arc current was 80 kA. The PS implosion was provided by an IMRI-5 high-current generator with a current amplitude of 450 kA and rise time of 500 ns. The PS implosion resulted in the formation of a 0.2-cm-diameter plasma column with an electron temperature of 700?C900 eV and average ion density of (5?C8) × 10¹⁷ cm^?3. The maximum radiation power per unit length in aluminum K-lines reached 300 MW/cm, the duration of the radiation pulse being 20 ns.     

The enzymatic binding of aminoacyl-tRNA to reticulocyte ribosomes: The stimulatory effect of donor site bound peptidyl-tRNA

The affinity of ribosomes for the elongation factors EF₁ and EF₂ changes while the ribosome is going through the different steps of the elongation cycle. In this communication we provide evidence that the affinity of the EF₁-aa-tRNA-GTP complex for the ribosomal acceptor site differs for ribosomes having their donor site either vacant or occupied by peptidyl-tRNA or by uncharged tRNA. Ribosomes having peptidyl-tRNA at their donor site bind the EF₁ complex with the highest affinity.Results are discussed in light of recent findings that the two elongation factors are not bound to the ribosome simultaneously.     

Dynamics of the spatial structure of pulsed discharges in dense gases in point cathode−plane anode gaps and their erosion effect on the plane electrode surface

The dynamics of the spatial structure of the plasma of pulsed discharges in air and nitrogen in a nonuniform electric field and their erosion effect on the plane anode surface were studied experimentally. It is established that, at a nanosecond front of the voltage pulse, a diffuse discharge forms in the point cathode–plane anode gap due to the ionization wave propagating from the cathode. As the gap length decreases, the diffuse discharge transforms into a spark. A bright spot on the anode appears during the diffuse discharge, while the spark channel forms in the later discharge stage. The microstructure of autographs of anode spots and spark channels in discharges with durations of several nanoseconds is revealed. The autographs consist of up to 100 and more microcraters 5–100 μm in diameter. It is shown that, due to the short duration of the voltage pulse, a diffuse discharge can be implemented, several pulses of which do not produce appreciable erosion on the plane anode or the soot coating deposited on it.     

Characteristics of a Pulse-Periodic Corona Discharge in Atmospheric Air

Pulse-periodic corona discharge in atmospheric air excited by applying a voltage pulse with a subnanosecond or microsecond rise time to a point electrode is studied experimentally. It is shown that, at a voltage rise rate of dU/dt ~10¹⁴ V/s, positive and negative ball-shaped streamers with a front velocity of ≥2 mm/ns form near the point electrode. As dU/dt is reduced to 10¹⁰?10¹¹ V/s, the streamer shape changes and becomes close to cylindrical. The propagation velocity of cylindrical streamers is found to be ~0.1 mm/ns at dU/dt ~ 2 × 10¹⁰ V/s. It is shown that the propagation direction of a cylindrical streamer can be changed by tilting the point electrode, on the axis of which the electric field strength reaches its maximum value. It is established that, for the negative polarity of the point electrode and a microsecond rise time of the voltage pulse, a higher voltage is required to form a cylindrical streamer than for the positive polarity of the point electrode.     

Energy of electrons generated during a subnanosecond breakdown in atmospheric-pressure air

The influence of the cathode design on the energy of the main group of electrons generated during a subnanosecond breakdown in atmospheric-pressure air was studied experimentally. The electron energy was measured using a time-of-flight spectrometer with a picosecond time resolution. It is shown that the energy of the main group of electrons increases with increasing cathode curvature radius. It is established using 400- to 650-μm-thick aluminum foils that the electron energy reaches its maximum value in voltage pulses with abrupt trailing edges and amplitudes below the maximum amplitude. Electrons with maximum energies are generated with a stronger spatial and amplitude scatter than those with average energies.     

Efficient generation of negative hydrogen ions in a low-voltage cesium-hydrogen discharge

The possibility of achieving the high density of negative hydrogen ions \(N_{H^ - } \) in a low-voltage cesium-hydrogen discharge is investigated. The \(N_{H^ - } \) density is determined experimentally from the absorption of laser radiation due to the photodetachment of electrons from H^? ions. The discharge plasma is investigated by the probe technique. The populations of the excited states of Cs atoms are determined from their emission intensities. With an input power of W≈(15–25) W/cm² in the discharge, densities of \(N_{H^ - } \sim (10^{12} - 10^{13} )cm^{ - 3} \) are achieved. The self-consistent calculations of the plasma parameters in the discharge gap agree well with the experimental results. The absorption of laser radiation due to the photoionization of Cs atoms is investigated. It is shown that the role of this absorption mechanism is negligible.