Anne-Marie Pointu

Production of active species in N2–O2 flowing post-discharges at atmospheric pressure for sterilization

A flowing afterglow of very pure molecular nitrogen at atmospheric pressure with admixture of controlled amounts (from some tens to some thousands ppm) of molecular oxygen is studied. For flows of 40 slm, spectroscopic measurements down the discharge allow us to estimate concentrations in atomic nitrogen and in singlet-S metastable oxygen atoms. With UV emission due to nitrogen oxide, all three reactive agents exhibit sporicidal effects, and their relative role is estimated.

Pulsed intense electron beams generated in transient hollow cathode discharges: fundamentals and applications

For the commercial application of pulsed power, material processing with intense pulsed particle beams is a very interesting subject. Recently, high-voltage (1-70 kV), low-pressure (1-100 Pa) transient hollow-cathode discharges turned out to be sources for pulsed intense electron beam generation suitable for this application. The remarkable parameters of these electron beams-beam currents of 50-1000 A (10-30% of the maximum discharge current) with a high energy component (mean energy of about 0.25-0.75 of maximum applied voltage) of 20-70% of the maximum beam current, power density up to 10 W/cm/sup 2/, beam diameters of 0.1-3 mm, beam charge efficiency of 3-5%-captured the attention not only of the scientific community in the last decade. The electron beam is emitted during the early phases of the discharge, and only weak dependence of the high energetic peak of the beam current was found on the external capacity, which determine the development of the later high-current phases. However, the beam parameters depend on the breakdown voltage, gas pressure, and discharge geometry (including self-capacity). In this paper, the characteristics of the pulsed intense electron beams generated in two configurations-multigap pseudosparks and preionization-controlled open-ended hollow-cathode transient discharges (PCOHC)-are described. Such electron beams already were used successfully in a variety of pulsed power applications in material processing, deposition of superconducting (YBaCuO) and diamond-like thin films, microlithography, electron sources for accelerators, and intense point-like X-ray sources, and some preliminary experiments revealed new potential applications such as pumping of short-wavelength laser active media. These pulsed electron beams could be used further in any kind of pulsed power applications that require high-power density, small or high electron energy, and small-beam diameters.

The characterization of pre-ionization-controlled electron beams produced in open-ended hollow-cathode transient discharges

We measured the parameters of the electron beam produced in a hollow-cathode transient discharge with an open-ended cathode but using a proper pre-ionization. The mean energy of the energetic electron spectrum is 0.6 - 0.76 of the breakdown voltage (18 - 23 kV) and the energetic beam duration is less than 10 ns. The electron beam current is approximately 0.1 of the maximum discharge current (600 - 700 A). The integrated diameter of the x-ray emission spot at beam interaction with Al foils for several tens of thousands of shots was estimated to . The beam parameters are similar to those obtained in a pseudo-spark configuration; however, the FWHM of the beam energy distribution seems smaller for the same breakdown voltages.

The role of multielectrode geometry in the generation of pulsed intense electron beams in preionization-controlled open-ended hollow-cathode transient discharges

High-voltage hollow-cathode glow discharges are used more and more to generate intense, pulsed electron beams. Such intense electron beams can be produced with high efficiency in preionization-controlled open-ended hollow-cathode transient discharges (PCOHC). This novel discharge is initiated by a low-current dc preionization discharge. The beam parameters are similar to those of the electron beam generated in pseudospark discharges. In this work, we present some measurements of the parameters for the electron beam generated by using a multielectrode (multigap) system instead of the single-gap device in this PCOHC configuration. This kind of multielectrode device was already used in pseudosparks to improve the intensity and collimation of the extracted beam. By using the multigap instead of the single gap, the total beam current (100-120 A) and the energetic part of the beam current (peak current 60-90 A and electron energies higher than approximately 3 keV) were substantially increased. However, the energy spectrum of the fast component has a large fraction of electrons at lower energies (4-10 keV for 26 kV breakdown voltage) when a multigap device is used instead of the single-gap configuration. A comparison between the single-gap and multigap PCOHC-produced pulsed intense electron beam is made too. The differences between the high-power pulsed electron beams produced in single-gap and multigap PCOHC configurations seem to be due to different developments of beam generation phases.

Time-resolved measurement of the energy distribution function of an electron beam created by a transient hollow cathode discharge

The energy distribution function of the electron beam produced in a transient hollow cathode discharge is measured using a method which is based on the collector self-biasing, allowing an access very close to the extraction hole. The distribution obtained is a mainly polyenergetic one extending over a range of up to some tenths of kiloelectronvolts. It is composed both of runaway electrons, whose energies we assume are equal to the instant discharge voltage, and a main part composed of a monotonically decreasing energy distribution. The ratio between these two populations is around 10% and strongly increases with distance from the extraction hole.

Generation of intense pulsed electron beams by superposition of two discharges

A pulsed self-collimated intense electron beam of diameter less than 1 mm, a few centimetres in length and up to 280 A peak current, 20 ns pulse duration and 50 Hz repetition rate is produced along the axis of a cylindrical discharge tube using the synergy of two discharges. It needs neither bore holes nor internal electrodes and can be obtained in various gases and for a large range of tube diameters.

Transportation of nitrogen atoms in an atmospheric pressure post-discharge of pure nitrogen

In this paper, the transportation of nitrogen atoms in a flowing post-discharge of molecular nitrogen at atmospheric pressure is studied. The axial variation of the density of nitrogen atoms is modelled and compared with measurements. A relatively high initial density, i.e. several 1014 atoms/cm3, is obtained by this approach, showing also that the main losses are caused by volume and surface recombination during transportation. A low value is found for the recombination probability of the used polyamide surface (around some 10−4) allowing therefore quite long transportation times and distances.

Intense electron beams produced in pseudospark and PCOHC for beam–plasma interaction experiments

Abstract High-voltage hollow-cathode glow discharges are used to generate intense pulsed electron beams with remarkable parameters. A comparison of the electron beam characteristics and the development of beam-induced plasma in the drift space is made for two of these configurations: preionization controlled open ended hollow cathode (PCOHC) and pseudospark, both in a multigap geometry. The dependency of electron beam parameters on external capacity, gas pressure and breakdown voltage is discussed.

Study of an atmospheric pressure flowing afterglow in N2/NO mixture and its application to the measurement of N2(A) concentration

The transportation of species in a high velocity (~103 cm s−1) flowing corona afterglow of molecular nitrogen at atmospheric pressure with added NO as impurity (<10−5) is studied. The variation of species densities along 50 cm downstream of the discharge in an 8 mm inner diameter tube is modelled and compared with emission spectroscopy measurements. It is shown that, according to a quite simple O and NO kinetic mechanism for such a gas composition in the post discharge, N2(A) concentration can be obtained from the emissions of the NOγ band at 247.9 nm and the NOβ band at 320.7 nm using a low resolution spectrometer (1 nm). The validity of both the measurement technique and of the assumed creation and loss mechanisms of N2(A) are demonstrated. An empirical relationship using the ratios of two line intensities has been plotted for several hundred points. The N2(A) concentration thus obtained, around 1011 cm−3, depending on the experimental conditions, is consistent with the results of direct measurements using the N2 Vegard–Kaplan system at 260.4 nm.

HIGH-REPETITION-RATE OPERATION OF A DOUBLE-DISCHARGE ELECTRON BEAM GENERATOR

We report on the construction and study of a high-repetitionrate double-discharge electron beam generator. A synergetic superposition of a pulsed discharge on a continuous one is used to generate an intense electron beam (tens of kilo-electron-volts and hundreds of amperes) that might be used as a useful and inexpensive tool for many applications such as laser generation, x-ray generation, photochemistry, material deposition, and others. To increase the average power the repetition rate is increased in different discharge configurations. Two types of high-voltage switches are used: an electromechanical switch working up to 100 Hz and a thyratron-based switch operating up to 10 kHz. A limit of 714 Hz is found for generator operation at 0.1 l/min flow speed and pulse voltage of about 12 kV in air.