Raymond Viladrosa

Study of a fast ablative capillary discharge dedicated to soft x-ray production

A capillary discharge has been developed to produce pulses of intense soft x-ray radiation of tens of nanoseconds duration. The soft x-ray photons were emitted in a plasma column resulting from polyethylene in capillary wall ablation. The spectrum was dominated by the C IV, C V, and C VI emission lines in the soft x-ray spectral range. The experimental value of the electrical circuit inductance has been measured and compared to the calculated one. The electron temperature was estimated to be higher than 50 eV from the plasma resistivity measurement. The time dependence of the electron density outside the capillary channel has been determined using spectroscopic measurement in the visible range. The time dependence of the electron temperature has been determined from the intensity ratio of C V and C VI emission lines, using a collisional radiative equilibrium code.

High repetition rate compact source of nanosecond pulses of 5–100 keV x-ray photons

A powerful, compact, and repetitive flash x-ray system based on a cable transformer technology powered by ceramic capacitors in a Blumlein-like configuration has been developed. Open circuit voltages in excess of 100 kV can be achieved while commutation occurs at low voltage (<20 kV). The x-ray emission from a low impedance x-ray diode with a hollow cathode configuration was observed under a wide range of experimental conditions. The critical parameters limiting the flash x-ray performances are mainly the pressure in the x-ray diode and the anode–cathode space. This true table top device is able to produce doses up to 1 R per shot, measured at the output window, of x-rays between 5 and 100 keV. The pulse widths were about 20 ns and the maximum repetition rate was about 60 Hz. Operation is possible in air or in other gases (He, Ne, Ar, Kr, Xe, H2, N2) at pressures varying from 10−3 mbar for xenon to about 1 mbar for helium.

Flash x-ray radiography of argon jets in ambient air

This paper describes the development and application of a soft x-ray flash radiography technique. A very compact soft x-ray flash source has been specially designed for these studies. The table-top x-ray source developed in this work emits strong doses, up to one roentgen at the output window, of x-ray photons, with most of them in the characteristic lines of the anode material (photon energy in the energy range 5-10 keV), in pulse of 20 ns FWHM with an x-ray emission zone smaller than . All these characteristics make this source attractive for the x-ray radiography of high-speed phenomena, down to ten nanoseconds duration and/or for the media presenting weak absorption for the harder x-ray photons emitted by more conventional flash x-ray systems. Argon streams in ambient air were chosen as a typical case to enlighten the potentialities of this method. Single-shot radiographs of such an argon jet through rectangular nozzles were obtained. No attempt of quantitative measurement of local density in the argon stream has yet been performed, only the qualitative structure of the jet has been investigated. Nevertheless, these preliminary results enable us to state that the diagnostics of gaseous or plasma media, even at rather low pressures, can proceed using soft x-ray flash radiography.

Study of pulsed neon–xenon VUV radiating low pressure plasmas for mercury free fluorescent sign optimization

This work deals with the study and optimization of mercury free fluorescent discharge tubes for publicity lighting applications. The experimental set-up allows for time resolved spectroscopy from 110 up to 900 nm, photometric characterization in a large volume integrating sphere and the current and voltage measurement of microsecond duration signals delivered by lab-developed pulsed drivers. The glow and afterglow radiative process analysis indicates that the best performance measured with the pulsed excitation of rare gas plasma, in comparison with the conventional ac excitation, essentially originates from the efficient plasma relaxation during the afterglow at the benefit of the vacuum ultraviolet (VUV) resonance line radiated at 146.9 nm for xenon. The fit of the VUV time resolved experimental measurements, with the results issued from a simplified kinetic model of neon–xenon plasmas, evidences the crucial role of production of molecular ions during the glow phase and of their radiative recombination during the afterglow. The pulse duration and the gas mixture pressure appear as two experimental parameters whose influence, studied over an extended range, has been demonstrated to bring about a significant sign performance enhancement. There exists an optimum pulse duration range, which results in the appearance of limited stepwise excitation and ionization processes, favourable for an intense afterglow VUV production. The pressure dependence study shows that the best performance for pulsed excitation is obtained in Ne/Xe (100/1) mixtures around 50 mbar, at the difference of an ac driven Ne/Xe plasma for which the best conditions were reported to be of a few millibars. This pressure increase results both in the VUV and sign light output enhancement and the successful continuous operation of pulsed mercury free signs for time as long as 4000 h with neither electrode erosion, nor glass or phosphor degradation nor chromatic coordinate variation. For the green phosphor covered, 65 cm long and 13 mm inner diameter signs, the efficiency of a pulsed neon–xenon discharge likely to be operated for a few thousand hours reaches 50% of that of the same tube filled with mercury based mixtures.

Simultaneous flash x-ray induced fluorescence imaging and radiography of argon jets in ambient air

This paper describes the development and application of a new x-ray based diagnostic technique. A compact flash x-ray source emitting 20 ns pulses of around 8 to 10 keV x-ray photons is used to induce the fluorescence of argon jets in ambient air. This x-ray induced fluorescence (XIF) is then collected for imaging experiments. In these preliminary studies, the feasibility of the XIF imaging technique is demonstrated by the characterization of argon flows in ambient air. The fluorescence of nitrogen molecules, resulting from the x-ray excitation of argon jets and successive energy transfers, is imaged on an intensified linear array CCD detector. The recording of the UV nitrogen fluorescence in our experimental conditions, pulsed excitation and atmospheric pressure, is shown to be an efficient and conclusive method for the characterization of argon flow in ambient air. The propagation paths of two converging argon jets are described and the divergence of each of the two jets expanding through a rectangular nozzle is measured. The reliability of the XIF imaging technique is successfully confirmed by a comparison with data obtained using the x-ray radiography technique. The simultaneous performance of XIF imaging and radiography for the diagnostics of one gaseous flow is achieved in this work. Some future improvements of both techniques are proposed for their application in other experiments.

Energetic Photons From Transient Plasma Discharges

An overview of the plasma based sources of energetic photons, ranging from UV to hard X‐rays, developed in GREMI is proposed. Each source principle is shortly described and applications of these specially designed sources are documented. The possibility of producing energetic photons over a very broad wavelength domain, together with the versatility of the mode of operations allow for a very large range of applications. The matching of the photon energy, the pulse repetition rate, the short duration, of a few nanosecond, of photon pulses offer for instance unique possibility for fast dynamic study, low Z element spray characterization, X‐ray fluorescence of dense targets, lithography issues, and UV VUV radiating plasma optimization.