Development of a long life, 2 kHz repetition rate X-ray preionizer

A simple and rugged X-ray preionizer system has been developed that is capable of sustained operation at the 2 kHz level with a demonstrated lifetime of2.5× 10⁸ shots and a potential lifetime of >10⁹ shots. At the heart of the system is an advanced form of corona plasma cathode. Cathode wear is minimised by utilising adsorbates from the dielectric surface and not cathode material to form the surface discharge from which the electron beam is extracted. The preionizer chamber background pressure (which is directly proportional to the sticking rate of adsorbates) is found to be of crucial importance in maintaining good cathode emission at high repetition rates. The device emits a rectangular X-ray beam, 5 cm×40 cm, with an X-ray risetime of 20 ns, a FWHM of 60 ns, and a peak energy of 95 keV. Ionization levels of >10⁷ cm^–3 in 1 atm of Ne could be generated after the X-rays had passed through 2 mm of aluminium and 1 mm of steel. Good beam uniformity and reproducibility at 2 kHz were shown to be maintained with the aid of a segmented electrode charge collection chamber. The system should be especially suitable for preionizing lasers containing a strongly electron attaching gas mixture.     

Experimental observation of strong coupling effects on the dispersion of dust acoustic waves in a plasma

The dispersion properties of low frequency dust acoustic waves in the strong coupling regime are investigated experimentally in an argon plasma embedded with a mixture of kaolin and MnO₂ dust particles. The neutral pressure is varied over a wide range to change the collisional properties of the dusty plasma. In the low collisional regime the turnover of the dispersion curve at higher wave numbers and the resultant region of ∂ω/∂k<0$\partial \omega /\partial k<0$ are identified as signatures of dust–dust correlations. In the high collisional regime dust neutral collisions produce a similar effect and prevent an unambiguous identification of strong coupling effects.     

A simple kinetic model of a Ne-H2 Penning-plasma laser

A simple kinetic model of the Ne-H₂ Penning-Plasma Laser (PPL) (NeI 585.3 nm) is proposed. The negative glow of a hollow cathode discharge at intermediate pressures is considered as the active medium. The balance equations for the upper and lower laser levels, electrons, ions and electron energy are solved. The dependences of the laser gain on the discharge conditions (Ne and H₂ partial pressures, discharge current) are calculated and measured. The calculated values are in a good agreement with the experimental data.     

Numerical modeling of a He-Kr coil hollow cathode laser

A theoretical study of a He-Kr laser (KrII, 469.4 nm line) pumped by a helical hollow cathode discharge is presented. A detailed kinetic model of plasmachemical reactions and radiative processes including the influence of gas heating on the active medium is developed. In the process of numerical calculations the dependence of the plasma parameters (electron and gas temperature, electron and metastable density, upper and lower laser level density) and laser characteristics (output power and gain coefficient) on the variations of discharge pumping, gas-mixture partial pressures and geometry of the coil hollow cathode, are investigated. The results of the numerical calculations are in a good agreement with the experimental data.     

Propagation of Ion-Acoustic Wave in an Inhomogeneous Dusty Plasma with. Dust Charge Fluctuation

The propagation of dust ion-acoustic wave in an inhomogeneous dusty plasma is studied by taking the dust charge fluctuation and collisions into account. It is shown that the dust charge fluctuation brings a phase shift to the wave. Furthermore, because of the presence of dust charge fluctuation, a new damping term rises, which makes the damping more sharply.     

Measurement of atom and molecule number densities in hydrogen arcs by means of VUV spectroscopy

The number density of ground state atoms in the 2mm hydrogen arc (T≈20000 K) and the number density of molecules in the 5 mm hydrogen arc (T≈12000 K) have been measured by means of vuv spectroscopy. These species' are particularly likely to deviate from the equilibrium population and may therefore falsify the hydrogen transport coefficients measured in arc experiments. In the present investigation the optically thick line wings of Ly-α and the H₂ molecular spectrum around 1600 Å have been analyzed. The results demonstrate that the former assumptions of LTE and PLTE, respectively, are completely justified on and near the arc axis. However, in the outer zones of these arcs a considerable overpopulation of the molecules must be expected as a consequence of radial diffusion.     

Control of parameters of micropinches formed in current-carrying plasma jet

The temporal evolution and spatial pattern of X-ray emission from a laser-induced vacuum discharge of moderate power has been investigated. It was found that micropinches in the initial stage of the cathode jet expansion into the vacuum ambient were formed. They generated a soft X-ray radiation and beams of accelerated electrons; therewith these phenomena occurred just when both amplitude of the discharge current and energy of the initiating laser pulse lied in the specified ranges of values. Parameters of the micropinch, namely, its position within the interelectrode gap and also, intensity of the X-ray radiation and beams of the accelerated electrons emitted from the micropinch are variable over a wide range of values through changes of energy of the laser pulse and/or amplitude of the discharge current.     

Investigation of XeCl vibrational and quenching kinetics: Measurements of gain and laser spectra in a discharge-pumped oscillator module

The small signal gain, amplified spontaneous emission and laser spectra of a transverse discharge-excited XeCl laser have been measured. Several gas mixtures, total gas pressures and electron densities have been investigated. From these measurements it is concluded, that bound-free transitions and transitions to high-lying vibrational levels of the ground state contribute significantly to the gain and laser emission. For the upper laser level a vibrational population ratio [XeCl(B, =1)]/[XeCl(B, =0)] corresponding to a temperature of about 370 K has been determined. The intensity ratio of about 1 observed for the two laser lines may be explained by the vibrational and quenching kinetics of the lower laser level together with the upper state kinetics. A table summarizing the result of the small gain measurement has been included for model comparison.     

Effect of Discharge Voltage on an Ion Sheath Formed at a Grid in a Multi-Dipole Discharge Plasma

It is experimentally demonstrated that a relatively strong ion-rich sheath formed at a fixed negative bias of the grid can be changed to a rather weak ion sheath （sheath potential weakly retards electrons） only by increasing the discharge voltage in the system. At sufficiently high negative grid bias, an increase of discharge voltage enhances the ion collection current at the grid. An explanation is put forward in support of this experimental observation. A slight density enhancement with a fall in plasma electron temperature is also observed with the increasing negative grid bias.     

Large area X-ray preionizer for electric discharge lasers

A pulsed X-ray source with a pulse duration of 250 ns (FWHM) which uses plasma cathodes to supply sufficient electrons for emission is described. It is simple to construct and requires low energy input. Less than 14 J of energy can produce up to 10⁷ electron-ion pairs/cm³ in a 10l volume of neon at 1 atm.ENEA Guest on leave from the Shanghai Institute of Optics and Fine Mechanics the Chinese Academy of Sciences, China     

Population densities and optical gain of VUV transitions of Cu II in Cu−He hollow cathode discharges

Population densities of excited Cu II-levels between 16 and 25 eV in Cu–He hollow cathode discharges were determined by emission spectroscopy. Population inversion was detected for several 6s-4p transitions. Investigation of the enhancement of Cu II vuv lines in He compared to Ne discharges showed that the excitation of the 6s levels by charge transfer is up to 100 times higher in He than in pure Ne discharges. Using the population densities and known transition probabilities, a single pass gain of 55% m^–1 at 780.8 nm and 1.2% m^–1 at 154.17 nm at a current density of 0.4 A cm^–2 was calculated.This paper is dedicated to Prof. Dr. H. Gobrecht on the occasion of his 75th birthday     

Particle densities in a decaying SF6 plasma

Particle densities are determined for a decaying axially blown SF₆ are between current zero and brekdown, some 100 μs later. During the time of interest, primarily only atoms, diatomic molecules and ions can be built up by reactions. Therefore, in a first approach equilibrium densities are derived for SF₆ which has decomposed into its considemic and diatomic components. In a second approach reaction kinetics are considered. It turns out that the development of densities occurs in two steps. During the first approx. 100 μs there is a strong deviation from equilibrium. Afterwards the most abundant particles remain near their partial, diatomic equilibrium values. Near breakdown the degree of ionization is of the order of 10¹³ cm⁻³ with the most abundant ions S ₂ ⁺ and F⁻. This density is sufficient to distort the applied electric field. Future theories of breakdown in hot gases must take into account this principal difference as compared with the conditions in cold gases.     

Optogalvanic spectra of calcium in the 6090–6760 Å region

Optogalvanic spectra of some 30 transitions of calcium originating from the ground state as well as from several excited states (4s4p ^1,3P, 4s3d ^1,3D, 4s5p ³P, 3d4p ³D, ³F) have been investigated using laser-irradiation of a low-current glow discharge in a thermionic diode. The influence of various experimental parameters such as bias field, oven temperature and buffer gas pressure on the spectra are reported. Seven previously unreported transitions are recorded including four forbidden transitions which violate the J selection rules. An interesting anomaly observed in the non-statistical population of the 4s3d ³D_3,2,1 multiplet is discussed where the populations of ³D₃:³D₂:³D₁ are in the ratio of 94:5:1. A possible explanation is based on avoided crossing between potential energy curves of the Ca-Ar molecule.     

Power Consideration for Pulsed Discharges in Potassium Seeded Argon

Minimization of energy consumed in plasma generation is critical for applications, in which a large volume of plasmas is needed. We suggest that a high electron density atmospheric pressure plasmas can be generated by pulsed discharges in potassium seeded argon at an elevated temperature with a very small power input. The ionization efficiency and power budget of pulsed discharges in such plasmas are analytically studied. The results show that ionization efficiency of argon, especially at small reduced electric field E/N （the ratio of the electric field to the gas number density）, is improved effectively in the presence of small amount of potassium additives. Power input of pulsed discharge to sustain a prescribed average level of ionization in potassium seeded argon is three orders of magnitude lower than that in pure argon. Further, unlike in pure argon, it is found that very short high-voltage pulses with very high repetition rates are unnecessary in potassium seeded argon. A pulse with lOOns of pulse duration, 5kHz of repetition rate, and 2Td （1 Td = 1 × 10^-21 Vm^2） of E/N is enough to sustain an electron density of 10^19 m^-3 in 1 arm 1500K Ar＋0.1% K mixture, with a very small power input of about 0.08 × 10^4 W/m^3.     

Formation of dust-acoustic shock waves in a strongly coupled cryogenic dusty plasma

The nonlinear propagation of ultra-low-frequency dust-acoustic (DA) waves in a strongly coupled cryogenic dusty plasma has been investigated, by using the Boltzmann distributed electrons and ions, as well as modified hydrodynamic equations for strongly coupled charged dust grains. The reductive perturbation technique is used to derive the Burger equation. It is shown that strong correlations among negatively charged dust particles acts like a dissipation, which is responsible for the formation of the DA shock waves. The latter are associated with the negative potential, i.e. with the compression of negatively charged cryogenic dust particle density. It is also found that the effective dust-temperature, which arises from electrostatic interactions among negatively charged dust particles, significantly affects the height of the DA shock structures. New laboratory experiments at cryogenic temperature should be conducted to verify our theoretical prediction.     

Observation of Periodic Multiplication and Chaotic Phenomena in Atmospheric Cold Plasma Jets

We investigate the temporal evolution of the current pulses from an ac Fie cold plasma jet at atmospheric pressure and with driving frequency in the range 14.76-15.30 kHz. The driving frequency is used as the plasma system＇s bifurcation parameter in analogy with the evolution in which the current pulses undergoes multiplication and chaos. Such time-domain nonlineaxity is important for controlling instabilities in atmospheric glow discharges. In addition, the observation can provide some data to support the simulation results reported previously [Appl. Phys. Lett. 90 （2007） 071501].     

Rotating high current arc

A high current arc (1–16 kA) of 5–20 ms duration is investigated, which rotates between two annular electrodes of variable distance (5–16 mm) at elevated pressures (0.1–1 MPa) in SF₆. The are is driven by the magnetic field of two ring-shaped coils in cusp geometry, one on either side of the electrodes. Both experiments and theoretical models concentrate on the investigation of the hot wake. The temperature, velocity, and size of the wake are determined by schlieren photography, streak records, and shock waves. A simple model is developed which yields a quantitative picture. The immediate surroundings of the transversely blown are can be described in terms of sucking in cold gas with associated heating and expansion, whereas the far-field region is governed by turbulent entrainment. The model involves only one empirical parameter: the entrainment coefficient α, which turns out to have a value of about 10⁻³.     

The influence of Exciting Frequency on N2 and N2+ Vibrational Temperature of Nitrogen Capacitively Coupled Plasma

By using optical emission spectroscopy （OES）, N2 and N2^＋ vibrational temperatures in capacitively coupled plasma discharges with different exciting frequencies are investigated. The vibrational temperatures are acquired by comparing the measured and calculated spectra of selected transitions with a least-square procedure. It is found that N2 and N2^＋ vibrational temperatures almost increase linearly with increasing exciting frequency up to 23 MHz, then increase slowly or even decrease. The pressure corresponding to the maximum point of N2 vibrational temperature decreases with the increasing exciting frequency. These experimental phenomena are attributed to the increasing electron density, whereas the electron temperature decreases with exciting frequency rising.     

The influence of vaporization upon the roots of a high current arc. II

The root of an argon are was stabilized by strong cooling of the graphite cathode; both the composition and the temperature of the plasma at the arc root were determined spectroscopically. The measurements of the absolute intensities of two CI and two CII lines revealed that the plasma is composed almost entirely of atomic carbon. Immediately in front of the cathode a temperature of 12000 K was measured and the degree to which the gas was ionized was found to be 30%. The velocity of the plasma was 300 m/s. This work is based upon material presented in the final report “Basic research programme for plasma technology, high-pressure arcs in SF₆” to the Federal Department of Research and Technology, Fed. Rep. Germany.     

Effect of overlapping Debye spheres on structures of 2D dusty plasmas

In dusty plasmas, overlapping Debye spheres around dust grains could produce an attractive force between them. Its effects on static structures of two-dimensional (2D) dusty plasmas are studied here by using molecular dynamics simulations. Results, in terms of the equilibrium radial distribution function, are compared with those deduced from purely repulsive Debye-Hückel or Yukawa potential for different Coulomb-coupling and screening parameters. The effect of the attractive force is found quite noticeable for usual experimental conditions, and becomes more pronounced for larger screening parameter κ. In particular, it is observed that for large κ the attractive force is dominant, and dust grains tend to aggregate and form patterns with scattering voids.