Classifications

• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05G—X-RAY TECHNIQUE
  • H05G2/00—Apparatus or processes specially adapted for producing X-rays, not involving X-ray tubes, e.g. involving generation of a plasma
  • H05G2/001—Production of X-ray radiation generated from plasma
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05H—PLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
  • H05H1/00—Generating plasma; Handling plasma
  • H05H1/0006—Investigating plasma, e.g. measuring the degree of ionisation or the electron temperature
  • H05H1/0012—Investigating plasma, e.g. measuring the degree of ionisation or the electron temperature using electromagnetic or particle radiation, e.g. interferometry
  • H05H1/0031—Investigating plasma, e.g. measuring the degree of ionisation or the electron temperature using electromagnetic or particle radiation, e.g. interferometry by interferrometry

Definitions

• the invention relates to the field of characterization or measurement of media using an interferometric technique of the Martm-Puplett type.
• Such a technique is particularly well suited to the characterization of condensed, or gaseous, ionized media, such as plasmas.
• Such media have sometimes very short lifetimes, and therefore exhibit a highly evolutionary character in a state outside thermodynamic equilibrium. therefore particularly difficult to acquire precise knowledge of the spatio-temporal evolution of the physical characteristics of these environments.
• the invention is applicable not only to the study of plasmas, such as plasmas such as laser welding or arc welding, plasmas with magnetic fusion, or plasmas with fusion by inertial confinement, because it is also applies to the study of condensed matter, ionized gases, breakdowns in gases (in particular for the study of the functioning of candles, spark gaps, lightning)
• plasmas such as laser welding or arc welding
• plasmas with magnetic fusion or plasmas with fusion by inertial confinement
• Another field of application is that of etu ⁇ e ⁇ e the combustion in jet engines (in aeronautics) as well as the study of the isospnere and the mag ⁇ etospnere in space p ⁇ ysi ⁇ ue
• thermo ⁇ ynamic, electronic and onysico-cmmi ⁇ ues properties of ionized media with transient lifespan and of evolving nature such as laser olasmas, -1 is generally implemented means of diagnosis and spectroscopy calling upon the illumination of this medium by a source of electromagnetic radiation (of the X-ray type), this source having an intensity making it possible to illuminate the medium studied during its transitory evolutionary phase .
• the source must also allow the generation of radiation such that the measurements, either of the reflected part or of the transmitted part, of this radiation can reveal the specific temporal physical characteristics of said medium.
• a process is known using a source of pulsed radiation, either conerent (laser) or incoherent (X-rays).
• the source radiation is up to conerent laser radiation
• the characterization of the radiation transmitted by the medium of ionized plasma can be carried out by a measurement of magnetic rotary polarization, from which it is possible to calculate the spatial gradient at density and that of temperature that ionized medium
• the single-shot system requires an additional diagnostic element which can restore the cartographic aspect of the radiation emission: it is necessary to obtain a spatial resolution of the medium studied, which is all the more delicate since said medium can have a very small thickness or spatial extension (of the order of a few tens of micrometers).
• the signal to be detected by the rapid sampling device cannot be temporally modulated, and a high level of system noise affects the sensitivity of the measurement by sampling and limits the application to the detection and measurement of high amplitude signals. , clearly emerging above the noise from the associated electronic system.
• the subject of the invention is therefore a device and a mterferometric method, for characterizing a medium, making it possible to obtain both a temporal resolution and a spatial resolution of the medium
• the invention has for its object an interferometric device for characterizing a medium comprising: - means for generating a train of pulses of radiation, at a repetition rate greater than 10 10 Hz,
• a beam called the pump beam
• the pump beam means for directing, from said pulse train, a beam, called the pump beam, onto the medium to be measured or characterized, the latter producing, in response to the pulse train of the pump beam, a signal beam
• a probe beam means for directing a beam, called a probe beam, from the pulse train towards the detection means
• the pulse train can be obtained by reflection of a single pulse on a multilayer mterferential mirror, each pulse of the pulse train corresponding to the reflection of the initial pulse on each layer of the mirror.
• the modulation of the pumping and probe beams is not of the active type, but of passive type: the mirror plays the role of sequencer, delivering a pulse tram at a repetition rate higher than that of all existing modulating devices.
• the pulses generated can be of the X-ray type. In this case, it is convenient to use a laser emitting laser pulses towards a target of a solid material, the X-rays then being obtained by impact of the laser on the solid target.
• the means for generating a train of pulses can be means for producing pulses of ultraviolet radiation. This can be obtained by multiplying, using non-linear means, the frequency of an ultraviolet, visible or infrared pulse radiation emitted by a pulse laser. Due to the generally very limited lifetimes of the media to be studied, the laser can be a "femtosecond" type laser, the pulses emitted being of a temporal width less than 10 ⁇ 13 s
• the detector chosen is preferably an ultra-rapid detector, of CdTe, GaAs, SiO / sapphire, or diamond composition.
• the choice of diamond is generally preferable, since this material is the most efficient from the point of view of resistance to radiation.
• the constituent material of the detector is preferably an ultra-fast photoconductive material, the lifetime of the carriers being less than the picosecond.
• the invention also relates to an interferometric method for characterizing a medium, comprising:
• the pulse train can be obtained from a single pulse reflected on a multilayer interference mirror.
• the pulses can be X-ray pulses, which can be obtained by emission of laser pulses on a target of solid material, the impact of the laser on the target producing the X-rays.
• the pulses can be pulses of ultraviolet radiation, obtained for example by emission of laser pulses in the ultraviolet, visible or infrared range, and by multiplication of the frequency of the laser radiation by a non-linear crystal.
• FIG. 1 represents a block diagram of a device according to the invention
• FIGS. 2A to 2C represent the time evolution of various signals or beams used in the context of a method according to the invention
• FIG. 3 is a block diagram of an autocorrelator with optoelectronic sampling.
• FIG. 1 shows a block diagram of a device for implementing the invention.
• a laser not shown in the figure, emits pulses of radiation reference 2 in the figure. This radiation is focused by a lens 4 on the surface of a solid target 6.
• the pulses emitted by the laser are preferably pulses of the femtosecond type, with a temporal width of the order of] _0-14 _] _ o _ 13 s T
• he target 6 is composed of a metallic material of the titanium, nickel, zinc or tungsten type. Under the effect of the focused laser beam 2, a radiative emission from the surface of this target takes place, and this results in the emission of intense radiation 8 of X-ray radiation, whose energy and spectral characteristics depend on the target material selected. .
• the X-ray beam thus obtained is reflected by a mirror 12, for X-rays, in the direction of a multilayer mirror 14.
• This mirror essentially comprises a series of alternating layers 16, 18, 20, 22.
• these layers may for example be alternately layers of carbon and tungsten, or tungsten and molybdenum.
• This multilayer mirror transforms a single, incident X-ray pulse into a plurality or train of pulses. Indeed, a single X-ray pulse will undergo successive propagation and reflections on the stacked layers. From the absorption and reflection coefficients of the layers of the mirror 14, it is possible to choose the spacing and the number of the reflecting layers of the mirror as a function of the time range over which the pulse tram must be spread. If n is the index of the material constituting a layer of the mirror, at the average wavelength of X-rays, and knowing that a delay of nxlOO additional femtoseconds corresponds to a layer thickness of 30 ⁇ m (for directions of orthogonal incidence and reflection), it is possible to select the time difference between the X pulses of the pulse train.
• the multilayer mirror 14 allows:
• This second pulse is wider than the first: for a laser pulse of the order of IO "* 14 s, the X-ray pulse has a time width of approximately 10 ⁇ 12
• S- Figure 2B represents the time course of the pulse train obtained after reflection of a single X-ray pulse on the multilayer mirror 14.
• the pulse tram has as many peaks or elementary pulses as there are reflections on the mirror 14.
• Each of the two beams, the beam emitted towards the medium 24 to be studied, and the reference beam 30 emitted towards the detector 26, have the same time distribution as that represented e in FIG. 2B
• the interaction of the pulse tram that is to say of the beam 28 with the medium 24, causes the latter to re-emit a signal beam 32 in the direction of the detector 26.
• the temporal evolution of the signal 32 is schematically represented in FIG. 2C.
• the detector is therefore subjected to two beams: the reference beam 30 coming directly from the multilayer mirror 14, and the signal beam 32, re-emitted by the medium 24. These two beams interfere at the level of the detector 26: the field to be taken into account for the d ⁇ ecle ⁇ nchem-ent of the latter is the electric field
• E Eo + E "2, where -E" o is the contribution of the beam 0 to the electric field, at the level of the detector 26, and ⁇ 3 E2 is the contribution of the beam 32 to the electric field, at the level of the detector 26. Consequently, if the fields E> o and ⁇ E2 are, at a certain time t, in phase, the detector is sensitive to the total field and is triggered. On the other hand, if, at an instant t ', the ⁇ ⁇ components Eo and E2 are in phase opposition, the resulting field is zero at the level of the detector 26 and the latter is not triggered. This principle allows
• the fast or ultra-fast detector 26 is sensitive in the range of radiation to be studied. For X-rays, this may for example be a detector based on an ultra-fast photoconductive material, the lifetime of the carriers being less than one picosecond.
• a material can be CdTe, GaAs, oxygen-doped silicon on sapphire, diamond, etc. Diamond is the most efficient material because it is the most resistant to radiation.
• This detector 26 can be coupled to an optoelectronic autocorrelator device 34, with sliding contact.
• This sampler is a microsystem capable of analyzing pulses up to 50 gigahertz. Schematically, such a device is represented in FIG. 3. It is an integral component, realizes in a microelectronic type technology. It comprises a main propagation line 36 on which the single signal S (t) to be sampled is sent, as well as n sampling lines 38-1, 38-2, 38-3, ..., 38-n. These sampling lines 38- ⁇ (l ⁇ i ⁇ n) are arranged in a "comb" along the main line. Between each of the sampling lines 38- ⁇ and the main line 36 is a pad 40- ⁇ (l ⁇ i ⁇ n) of photoconductive material. Each sampling line is also also connected to a storage capacity 42-1, 42-2, ... 42-n as well as to an acquisition electronics not shown in FIG. 3. Each photoconductive element 40 -1, 40-2, ...
• 40-n is triggered by an ultra-fast laser pulse.
• a convenient means of obtaining, for each photoconductive element such a pulse consists, as illustrated in FIG. 1, of taking a secondary beam 44-1 from the beam 2 of the femtosecond laser, using a mirror 46.
• This secondary beam 44-1 can itself be divided into several sub-beams 44-2, ..., 4 ⁇ -n, using mirrors 43-2, ..., 43-n partially transparent interposed on its way.
• the main sub-beam 44-1 triggers the photoconductive element 40-1.
• a first secondary sub-beam 44-2 obtained using the mirror 43-2 triggers the photoconductive element 40-2 at an instant determined by the length of the delay line defined by the path of the beam between the mirror 43 -2 and the photoconductive element 40-2.
• the third photoconductive element 40-3 is triggered by a second secondary sub-beam 44-3, at an instant itself defined by the length of a second delay line.
• This trigger principle is applied to all photoconductive elements 40- ⁇ (l ⁇ i ⁇ n), this being triggered at an instant t 1 defined by the length of the ith delay line.
• the ultrafast laser pulse incident on a photoconductor 40- ⁇ closes the switch formed by this photoconductor.
• a signal is then taken, which corresponds to the intensity of the signal S opposite the line i at the instant t x of closing of the photoconductor.
• each photoconductive material may for example be gallium arsenide low temperature, while the main and secondary transmission lines can be made of aluminum.
• An electronic circuit for measuring, for example of the charge amplifier type, and for recording the signals delivered by each sampling line 38- ⁇ is also provided, but is not shown in FIG. 3.
• the signal sampling time step delivered by the. photodetector 26 is defined by the spatial distance between two neighboring lines 38- ⁇ , as explained in the publication mentioned above.
• the main trigger beam 44-1 taken from the femtosecond laser beam 2, determining the instant at which the first photoconductor 40-1 is closed, determines the instant at which sampling will start.
• this triggering instant will define the portion of the plasma or of the medium 2 ⁇ being studied from which the analyzed signals will be retransmitted. This means that it is possible to choose to sample signals coming from a peripheral zone 50 of the studied medium 24 or from a zone 52 lying deep in the studied medium 24.
• the device described above corresponds to an analysis of the medium carried out using X-rays.
• the principle of the invention is not limited to the use of X-rays, but can also be applied , for example, the use of ultraviolet rays.
• a femtosecond laser device emitting in the infrared, or the visible or in the ultraviolet is used, the radiation of which then passes through a cell of a non-linear material, for example a frequency doubler or tripler ( material of the KDP or KTP type), adapted on the one hand to the frequency of the laser radiation and on the other hand to the desired investigation frequency.
• a multilayer mirror the layers of which are adapted to the wavelength of the beam to be reflected, allows get pulse tram, and the beam is then directed towards the medium to be analyzed.
• the multilayer mirror can comprise a stack of layers, made of fused silica, whose transparency to ultraviolet radiation is high, but whose index jumps at the level of the diopters between each layer (of silica shade are sufficient to return a separate reflected component.
• a probe or reference beam is emitted directly in the direction of the detector intended to receive the beams re-emitted by the medium to be studied. The same principle of spatio-temporal analysis is then applicable.
• the detector is chosen as a function of the spectral range of the reference beam and of the beam re-emitted by the medium to be studied.
• a trigger beam is taken from the laser beam, and a plurality of delay lines are formed using this beam, in order to trigger the different photoconductive elements of a sampling autocorrelator optoelectronics.

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• Physics & Mathematics (AREA)
• Engineering & Computer Science (AREA)
• Plasma & Fusion (AREA)
• Health & Medical Sciences (AREA)
• Electromagnetism (AREA)
• General Health & Medical Sciences (AREA)
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• Spectroscopy & Molecular Physics (AREA)
• Optics & Photonics (AREA)
• Analysing Materials By The Use Of Radiation (AREA)
• X-Ray Techniques (AREA)
• Investigating, Analyzing Materials By Fluorescence Or Luminescence (AREA)

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• 1995
  • 1995-12-22 FR FR9515393A patent/FR2742867B1/fr not_active Expired - Fee Related
• 1996
  • 1996-12-20 EP EP96943161A patent/EP0868834A1/de not_active Withdrawn
  • 1996-12-20 WO PCT/FR1996/002045 patent/WO1997024019A1/fr not_active Application Discontinuation

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