UV excimer radiation from dielectric-barrier discharges

Dielectric-barrier (silent) discharges are ideally suited for efficient excitation of high-intensity UV radiation from excimers. The molecular continuum of xenon at 172 nm could be obtained with an efficiency of close to 10%. Model calculations for excimer formation and UV efficiencies in such discharges are presented. The possibility of obtaining many other wavelengths (e.g. KrF^* at 248 nm and XeCl^* at 308 nm) and the variety of conceivable geometries (plane or cylindrical) makes this new UV source an attractive choice for many photophysical and photochemical applications.     

X-ray radiation due to nanosecond volume discharges in air under atmospheric pressure

The formation of nanosecond discharges in atmospheric-pressure air versus the applied pulse polarity and discharge gap geometry is studied. It is shown that the polarity of high-voltage nanosecond pulses and the electrode configuration have a minor effect on the volume discharges under a variety of experimental conditions. When the spacing between needle-like electrodes is large, the discharge is asymmetric and its glow is weakly dependent on the sign of the potential applied to the electrode. Negative voltage pulses applied to the potential electrode generate X-ray radiation from both the surface and volume. For a subnanosecond rise time of the voltage pulse and diffusion character of the discharge, the X-ray radiation comes from the brightly glowing region of a corona discharge. The average values of the fast electron velocity and energy in nitrogen are calculated. At field strengths E/p < 170 kV/cm atm, the average velocity of a fast electron bunch is constant because of central collisions. At field strengths E/p > 170 kV/cm atm, fast electrons run away. Central collisions are the reason for X-ray radiation from the volume.     

Hollow cathode effects in charge development processes in transienthollow cathode discharges

A detailed experimental study of space charge formation and ionization growth in transient hollow cathode discharges (THCD) is presented. The experiment was performed with an applied step voltage up to 30 kV, with rise time less than 50 ns. The discharge was operated in different gases, at pressures in the range 50-750 mTorr, with cathode apertures ranging from 1 to 5 mm diameter and 5 to 20 mm long, with 10 cm electrode separation. Spatial charge formation, both in the hollow cathode region (HCR) and inter electrode space, has been studied with a capacitive probe array. Properties of high energy electron beams have been measured with a beam-target scintillator-photomultiplier arrangement. Detailed correlations of the electron beam evolution with the charge probe signals inside and outside the HCR clearly demonstrate the role of the electron beam in the initial formation and late evolution of a virtual anode and, in turn, the field enhanced ionization when the anode potential is brought close to the HCR. These results clearly identify the different regimes in which the Hollow Cathode plays a significant role in ionization growth in the inter electrode space and in the processes which eventually lead to electric breakdown     

Spatial characteristics of radiation from lightning discharges

The results of measurements of levels of electric and magnetic radiation fields from lightning discharges are presented. Lightning discharges occurred in the region of the terrestrial surface bounded by a sector of ∼90° with a radius of ∼30–300 km. A power spatial dependence of the electromagnetic radiation field components of lightning discharges is established. The results of measurements are interpreted using fractal geometry methods.     

Observation of chaotic oscillations in hollow cathode discharges

We report detailed observations on self-sustained oscillations observed by varying the dynamical resistance in a Ne hollow cathode discharge. The periodicity (T) of these oscillations transform into a state with 2T irrespective of ballast resistance (R_b) at larger driving parameter values. For the intermediate values of driving parameter (4.38-5.06 mA), a window appears for R_b⩾5 kΩ. In the window regime the oscillations are chaotic and the behavior is strongly dependent on R_b     

X-ray generation in low-voltage vacuum discharges

The dynamics of x-ray emission from a low-voltage laser-induced discharge was studied with the aid of a picosecond x-ray streak camera. Directed x-ray emission in the spectral range from 100 eV to 10 keV in the form of point sources and thin layers with lifetimes ranging from 30 ps to 1 ns was observed in a low-voltage vacuum discharge (U=150 V) initiated by a picosecond laser beam. X-ray emission from a discharge was detected with a time delay (1–20 ns) relative to ignition by the laser beam in order to prevent the radiation of the laser plasma from entering the detector. Detection of directed x-ray emission in a low-voltage vacuum discharge is demonstrated. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 9, 622–627 (10 May 1998)     

Application of vacuum arc cathode spot model to graphite cathode

A model of near-electrode processes is applied here to describe the behavior of cathode spots on graphite cathode in vacuum arc. The physical model is based on a kinetic treatment of cathode evaporation, electron emission from the cathode, and plasma production. The model consists of physical assumptions and a system of equations that are formulated in the paper. Spot parameters, such as cathode erosion rate, cathode potential drop, cathode surface temperature, current density, electric field, and plasma density, temperature, and velocity in the near-electrode region are calculated numerically. The calculation includes the dependence of spot parameters on spot current and spot lifetime. The variation of spot parameters as a function of spot lifetime are very strong at lifetimes shorter than 10 μs. The calculations indicate that Joule heating in the cathode body is significant, and may exceed cathode heating by the ion heat flux. Calculated spot parameters are compared with the corresponding experimental data for relatively low arc currents (<100 A) and their agreement is discussed     

Parametric X-ray radiation from powders

Parametric X-ray Radiation (PXR) produced in powders has been observed for the first time. PXR spectra were measured under observation angles of 150° and 180° during the interaction of relativistic 7 MeV electrons with a tungsten powder. All the PXR peaks that theoretically can be produced in the studied energy region were registered. The performed absolute comparison of the experiment with the PXR kinematical theory from randomly oriented crystallites showed a good accordance.     

The characteristics of pulsed hollow cathode discharges used forpseudospark switch triggering

The complex interaction between the trigger discharge and the main switch discharge in high-power gas discharge switches influences both the switching characteristics, and the switch and trigger lifetime. Any attempts to improve either of these parameters has to take into account the pressure and geometry dependence of a particular trigger geometry. Yet, although not very intensely investigated in detail for this particular purpose, pulsed hollow cathode discharges are commonly used for low-pressure gas discharge triggering as in pseudospark switches. Measurements of the electron current flowing to the cathode backplane of a pseudospark switch from the pulsed hollow cathode trigger discharge show that maximum current densities are peaked around the symmetry axis of the trigger electrode, an effect which is more pronounced at low pressures. Delayed (and slowed-down) increase of the current density at larger radii leads to increasing delay and jitter, provided the trigger coupling holes in the cathode backplane are located off-axis. The electron current density increases with decreasing diameter of the trigger electrode, and with increasing pressure of the working gas. In addition, it is shown that a preionization (keep-alive) current in the trigger electrode region shows a distinct influence on the trigger current distribution, proving that there exists an optimum keep-alive current depending on the geometry and gas pressure     

X-ray generation in inverse capillary discharges forpumping

An inexpensive, tunable, filamentary X-ray generator, based on the bremsstrahlung of a pulsed, high-power-density electron beam cruising along a dielectric fiber is presented. At small scale, it encodes some of the features of an X-ray machine for triggering the energy stored in the long-lived ¹⁷⁸Hf^m2 nuclear isomer, in particular an optimum coupling to the active medium This revised version was published online in August 2006 with corrections to the Cover Date.     

Light emission during cathode sheath formation in preionizedhigh-pressure glow discharges

The cathode sheath formation in a transient spatially small X-ray preionized high-pressure glow discharge with parameters typical for neon buffered XeCl lasers is investigated experimentally. Pictures of the light emission are taken with high temporal (≈1 ns) and spatial (≈25 μm) resolution using a gated CCD-camera as well as a streak camera. Quantitative data on the spatial and temporal development of the sheath structure are given. The most prominent feature is an extremely fast formation of a thin and very bright cathode sheath at a time which could be time correlated with an accuracy of ±0.2 ns with the discharge current. The results can be used to check numerical model calculations of the sheath formation     

Thermocapillary convection and dynamics of the cathode spot at the liquid cathode

Summary Consideration is given to the influence of the Marangoni convection, which develops in a layer of liquid metal under the cathode spot, on its properties. The structure and parameters of the thermocapillary cell formed have been determined. It is shown that the convective transfer of heat plays a dominant role in the cathode spot and the thermal model, which ignores it, will lead to highly overestimated temperature values. We describe a dynamic model of equilibrium of the liquid-metal surface in the spot which explains the possibility of appearance of high local pressures in the fluid. We discuss the absolute instability of the cathode spot rest state and the hydrodynamic mechanism of motion, which is determined by breakdown of the symmetry of a convective cell. It appears that such different (at first glance) phenomena as the random motion of the cathode spot, the repulsion of the spots during the approach and division, the detention of the spot on the wetting line and the retrograde motion of the spot in a magnetic field can be explained in unified terms with the aid of the above-mentioned hydrodynamic mechanism. The cited effects are characterized by external actions on the spot of different nature, which give rise to an asymmetry of convection with respect to the heating centre and, consequently, the ordered motion of the cathode spot. The behaviour of the cathode spot on a liquid-metal layer of variable thickness is considered and a new method of detention of the spot is proposed. We propose ideas of experiments which would make it possible to determine the size of a thermocapillary cell and to check the hydrodynamic mechanism of the cathode spot motion. We discuss the relationship of the phenomena considered with the processes of ordering and self-organization in nonequilibrium systems.

---

Riassunto Si dà importanza all’influenza della convezione di Marangoni, che si sviluppa in uno strato di metallo liquido sopra la macchia catodica, sulle sue proprietà. Sono stati determinati la struttura e i parametri della cella termocapillare formata. Si mostra che il trasferimento convettivo di calore assume un ruolo dominante nella macchia catodica e il modello termico, che lo ignora, porta a valori di temperatura altamente sovrastimati. Si descrive un modello dinamico di equilibrio della superficie del metallo liquido nella macchia che spiega la possibilità che compaiano alte pressioni locali nel fluido. Si discute l’instabilità assoluta dello stato di quiete della macchia catodica e il meccanismo idrodinamico di moto che si determina dalla rottura della simmetria di una cella convettiva. Sembra che tali (a prima vista) differenti fenomeni, come il moto casuale della macchia catodica, la repulsione delle macchie durante l’avvicinamento e la divisione, il permanere della macchia sulla linea bagnante e il moto retrogrado della macchia in un campo magnetico possano essere spiegati in termini unificati con l’aiuto del meccanismo idrodinamico sopracitato. Gli effetti citati sono caratterizzati da azioni esterne di diversa natura sulla macchia, che danno origine all’asimmetria della convezione rispetto al centro di riscaldamento e, conseguentemente, al moto ordinato della macchia catodica. Si considera il comportamento della macchia catodica su uno strato di metallo liquido di spessore variabile e si propone un nuovo metodo per trattenere la macchia. Si propongono idee di esperimenti che renderebbero possibile determinare le dimensioni di una cella termocapillare e controllare il meccanismo idrodinamico del moto della macchia catodica. Si discute la relazione dei fenomeni considerati con i processi di ordinamento e autoorganizzazione nei sistemi di non equilibrio.

---

Резюме Рассмотрено влияние конвекции Марангони, развивающейся в слое жидкого металла под катодным пятном, на его свойства. Определены структура и параметры образующейся термокапиллярной ячейки. Показано, что конвективный перенос тепла играет в катодном пятне главную роль и тепловая модель, не учитывающая его, приведет к сильно завышенным значениям температуры. Описана динамическая модель равновесия поверхности жидкого металла в пятне, объясняющая возможность возникновения в жидкости больших локальных давлений. Обсуждаются абсолютная неустойчивость состояния покоя катодного пятна и гидродинамический механизм движения обусловленный нарушением симметрии конвективной ячейки. Представляется, что такие столь различные на первый взгляд явления, как хаотическое движение катодного пятна, расталкивание пятен при сближении и делении, фиксация пятна на линии смачивания и ретрогралное движение пятна в магнитном поле могут быть объяснены с единой точки зрения с помошью указанного гидродинамического механизма. Перечисленные эффекты характеризуются внешними воздействиями на пятно разной природы, вызывающими появление асимметрии конвекции относительно центра нагрева и, следовательно, упорядоченное перемещение катодного пятна. Рассмотрено поведение катодного пятна на слое жидкого металла переменной толщины и предложен новый способ фиксации пятна. Предложены идеи экспериментов, которые позволили бы определить размеры термокапиллярной ячейки и проверить гидродинамический механизм движения катодного пятна. Обсуждаетсяя связь рассмотренных явлений с процессами упорядочения и самоорганизации в неравновесных системах.

     

High-resolution X-ray projection radiography of a pin cathode in a high-current vacuum diode using X-pinch radiation

To study processes in a high-current vacuum diode with a cathode in the form of a single pin made of a metallic wire 20–30 μm in diameter, the method of high-resolution projection X-ray radiography with an X-pinch as a source has been used. A strong inhomogeneity of the energy contribution to the wire has been revealed. The smallest energy release has been observed near the end of the pin, where the electric field strength is maximal. Hard X rays, as well as the ejection of matter from the anode, have been observed, indicating the generation of an electron beam with the parameters characteristic of explosive electron emission in the diode with this configuration. The data obtained indicate complex processes occurring in the diode. Possible scenarios of their development have been considered.     

微空心阴极放电的流体模型模拟

采用流体模型研究了微空心阴极放电(MHCD)的特点，对放电中电场的形成，电子和离子的密度分布，电子能量分布进行了数值模拟.该计算是针对高气压，圆筒形阴极结构下的He放电.结果表明放电中存在空心阴极效应，从电子能量分布可以看出，放电中存在高能电子，放电空间的电场分布主要表现为径向电场.此外，通过改变气压，阴极孔径等参数计算出它们对放电的影响.分析表明减小孔径有利于负辉区更充分的重合.提高气压将缩短阴极位降区. 关键词： 微空心阴极放电 流体模型     

Low frequency longitudinal waves in argon hot cathode discharges. II

Taking into account the multistep ionization a simple hydrodynamical model for the low-frequency longitudinal waves in low-current argon discharges with a thermionic cathode at the pressure from 10 Pa to 100 Pa is presented. A conclusion is drawn that the most important instability factor in such discharges is the interaction between the low-energy electron beam (U _b12 eV) and the plasma. The calculated dipersion characteristics show good agreement with the experimental results.     

Coherent electromagnetic radiation from the fragments of nuclear fission

The electromagnetic radiation from two vibrating fission fragments is classically treated. The rotational and translational motion of the fragments during emission and the interference of the radiation from the two individual fragments are taken into account. It is shown that the angular distribution of the radiation contains valuable information on the fission process and that the spectral distribution depends sensitively on the damping of the shape vibration.Dedicated to Professor P. Armbruster and P. Kienle on the occasion of their 60th birthday     

Low frequency longitudinal waves in argon hot cathode discharges. I

Experimental investigation of low-frequency longitudinal waves in argon low-current thermionic cathode discharges at the pressure from 10 Pa to 100 Pa is described. The longitudinal waves are generated in the discharge volume forpd _ac Pa. cm if the thermionic emission-discharge current ratio,I _em/I _d, is greater than 30.     

Measurement of cathode spot parameters with pulsed laserdiagnostics

The cathode spot formation in air within the first 170 ns was investigated by laser absorption photography and ps-pulse interferometry. The discharge was initiated between electrodes made from Ag or Pd with cathode-anode distance below 300 μm, the arc duration was some milliseconds, and the arc current 5-10 A. Picosecond holographic interferometry and momentary absorption photography yielded spatial-temporal density distributions in the ignition phase of the cathode spot. An absolute electron density value on the order of 4×10²⁶ m^-3 has been found. In contrast to vacuum, the cathode spot plasmas broaden little with increasing distance from the cathode, thus narrow plasma channels are observed in the vicinity of the cathode surface having diameters <20 μm