Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J65/00—Lamps without any electrode inside the vessel; Lamps with at least one main electrode outside the vessel
  • H01J65/04—Lamps in which a gas filling is excited to luminesce by an external electromagnetic field or by external corpuscular radiation, e.g. for indicating plasma display panels
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
  • F02P—IGNITION, OTHER THAN COMPRESSION IGNITION, FOR INTERNAL-COMBUSTION ENGINES; TESTING OF IGNITION TIMING IN COMPRESSION-IGNITION ENGINES
  • F02P9/00—Electric spark ignition control, not otherwise provided for
  • F02P9/002—Control of spark intensity, intensifying, lengthening, suppression
  • F02P9/007—Control of spark intensity, intensifying, lengthening, suppression by supplementary electrical discharge in the pre-ionised electrode interspace of the sparking plug, e.g. plasma jet ignition

Definitions

• the present invention relates to electrodeless laser-pumped plasma light sources producing high-brightness light in the ultra-violet (UV), visible and near infrared (NIR) spectral bands and to methods for starting plasma ignition.
• UV ultra-violet
• NIR near infrared
• Continuous optical discharge is a stationary gas discharge sustained by laser radiation in pre-created relatively dense plasma.
• a COD sustained by a focused beam of a continuous wave (CW) laser, is realized in various gases, in particular, in Xe at a high gas pressure of 10-200 atm (Carlhoff et al., “Continuous Optical Discharges at Very High Pressure,” Physica 103C, 1981, pp. 439-447). Due to a high plasma temperature of about 20,000 K (Raizer, “Optical Discharges,” Sov. Phys. Usp. 23 (11), Nov. 1980, pp.
• COD- based light sources are among the highest brightness continuous light sources in a wide spectral range between about 0.1 pm and 1 pm. Compared to arc lamps, such laser-pumped plasma light sources not only have a higher brightness, but also a longer lifetime, making them preferable for a variety of applications.
• the relatively cold electrodes located near the high-temperature plasma region produce disturbances of convective gas flows in the chamber and, as a result, impair spatial and energetic stability of the laser-pumped plasma light source.
• the presence of electrodes near the radiating plasma region is characterized by “dead” spatial angles restricting the exit of plasma radiation.
• electrode material sputtering may result in decreased transparency of the bulb walls and, correspondingly, to the light source degradation over time.
• the laser beam focus area and, correspondingly, the radiating plasma region are moved from the gap between the igniting electrodes towards the chamber wall.
• an electrode-containing chamber also include the complex technology for sealing the metal / glass joint and the complex chamber shape producing a concentration of stresses which result in lower strength of the chamber when operating at high gas pressures.
• the threshold power of laser radiation required for plasma ignition is usually from about ten to several hundreds of kilowatts or higher, while the laser radiation intensity sufficient for COD sustenance is typically just a few tens of Watts.
• using the same laser with a high output power both for plasma ignition and COD sustenance either results in reduced lifetime of the light source (when the full laser power is used for COD sustenance), or is redundant, expensive and, therefore, impractical if only a fraction of the full laser power is used to sustain the COD.
• Patent U.S. 10057973, issued on August 21, 2018, proposes to overcome this challenge by using a single CW laser with the power of less than 250 Watts and a wavelength of less than 1.1 pm. It is suggested that COD ignition and sustenance is provided by means of sharp focusing of CW laser beam with a focal area cross size of less than 1-15 microns, and a focal area length of 6 microns or lower.
• the beams of one or several igniting lasers and beams of one or several CW lasers, intended for COD sustenance are introduced into an optical fiber used for delivering the radiation of the said lasers to a condensing or focusing optical system.
• the said device superposition of the focusing areas of the pulsed lasers and of CW lasers is achieved, if the wavelengths of the said lasers are similar.
• the technical problem to be solved by the invention relates to the creation of methods and devices for highly reliable laser ignition of continuous optical discharge and to develop high brightness highly-stable laser-pumped plasma light sources on the basis thereof.
• the technical result of the invention consists in ensuring high reliability of igniting the plasma sustained by a CW laser, and in creating electrode-free high-brightness broadband light sources with the high spatial and power stability on that basis.
• the proposed laser-pumped plasma light source comprising: a high-pressure gas filled chamber, at least a part of which is optically transparent; a region of radiating plasma sustained in the chamber by a focused beam of a continuous wave (CW) laser; at least one output beam (which may also be termed useful beam) of plasma radiation exiting the chamber, and a means for plasma ignition.
• CW continuous wave
• the light source is characterized in that the means for plasma ignition is a pulsed laser system generating a first and a second laser beams focused in the chamber, whereas said first laser beam is arranged for gas optical breakdown, and said second laser beam is arranged for plasma ignition after optical breakdown.
• the first laser beam has a peak radiation power of more than 104 Watts and a pulse length of less than 0.1 microseconds.
• the second laser beam has at least three times more laser pulse energy and at least an order of magnitude lower laser peak power compared to the first laser beam.
• the volume of the plasma ignited by the second laser beam many times, by an order of magnitude or more, exceeds the volume of the plasma created during optical breakdown by the first laser.
• the volume and density of the plasma ignited by the second laser beam are sufficient for stationary sustenance of the plasma by the focused beam of the CW laser.
• the second laser beam provides a plasma size of up to approximately 1 mm (measured as the FWHM of the free electron density or the FWHM of the brightness profile of the light emitting plasma region), and a plasma density of up to 10 18 cm 3 or more (measured as free electrons per volume).
• the output power of the CW laser does not exceed 300 Watts.
• the radiation pulse of the second laser beam ends no earlier than 50 ps after the end of the radiation pulse of the first laser beam.
• the focusing areas of the first and the second laser beams at least partially overlap or are superposed.
• the pulsed laser system comprises two lasers with common cavity mirrors, the first and the second laser beams are parallel and are introduced into the chamber through one common focusing optical system.
• the pulsed laser system is a solid-state laser system.
• the pulsed laser system generates the first laser beam in Q-switching mode or in giant-pulse generation mode.
• the pulsed laser system generates the second laser beam in free running mode.
• the CW laser has a fiber-optic output.
• the wavelength of the CW laser is different from the wavelengths of radiation of the first and the second laser beams.
• the focused beam of the CW laser is directed vertically upwards or close to vertical.
• the external surface and the internal surface of the chamber’s transparent parts are shaped as concentric spheres or parts thereof, and the region of radiating plasma is located in the center of the said concentric spheres.
• the output beam of plasma radiation exits the chamber in all azimuths.
• the output beam of plasma radiation exits the chamber in in a solid angle of not less than 9 sr.
• the laser-pumped plasma light source has three or more output beams of plasma radiation.
• the invention relates to a method for igniting plasma in the laser- pumped plasma light source comprising: direction of a focused beam of a CW laser into a chamber with high-pressure gas, starting plasma ignition and stationary sustenance of a radiating plasma by the focused beam of the CW laser.
• the method is characterized in that the plasma ignition is provided by a pulsed laser system generating a first and a second laser beams focused in the chamber, whereas the first laser beam is used to provide an optical breakdown, after which the second laser beam is used to ignite a plasma, whose volume and density are sufficient for stationary plasma sustenance by the focused beam of the CW laser.
• the pulsed laser system is a solid-state laser system which generates the first laser beam in Q-switching mode and generates the second laser beam in free-running mode.
• Explosive processes may result in suppression of the optical discharge sustained by a CW laser with a low power which is typically not more than 300 Watts.
• this problem is solved by using the second pulsed laser beam to provide plasma ignition after an optical breakdown.
• the pulsed optical discharge sustained by the second laser beam is itself free of explosive phenomena, and the plasma ignited by the second laser beam, is resistant to disturbances caused by the optical breakdown.
• the second laser beam ensures a plasma volume and density sufficient for reliable stationary plasma sustenance by a focused beam of a CW laser with a relatively small output power. This way a reliable COD ignition is achieved.
• FIG. 1 Schematic diagram of laser-pumped plasma light source with the pulsed laser system for plasma ignition in accordance to the present invention
• FIG. 2 Diagram of radiation power of laser beams according to the embodiment of invention
• FIG. 3 Schematic of laser-pumped plasma light source with the solid-state laser system for plasma ignition
• FIG. 4 Schematic view of the light source with the three-channel output of useful plasma radiation
• Fig. 5 Diagram of radiation power of laser beams according to one of the embodiments of the invention.
• the laser-pumped plasma light source comprises the high-pressure gas filled chamber 1, typically 10 atm or higher. At least a part of the chamber 1 is optically transparent.
• Fig. 1 shows an embodiment with a completely transparent chamber manufactured from an optically transparent material, e.g. fused quartz.
• the chamber 1 contains the radiating plasma region 2 sustained in the chamber by the focused beam 3 of the CW laser 4.
• the optical collector 6 forms the radiation beam 7 transmitted, for example, via an optical fiber and/or a system of mirrors to one or more optical consumer systems 8 which uses broadband radiation emitted by plasma.
• the light source also comprises a means for plasma ignition.
• the light source is characterized in that the means for plasma ignition is a pulsed laser system 9 generating the first laser beam 10 and the second laser beam 11 focused in the chamber 1, namely into the region intended for sustaining the radiating plasma 2.
• the first laser beam 10 is intended for starting plasma ignition or for optical breakdown in the chamber 1.
• the second laser beam 11 is intended for plasma ignition after an optical breakdown provided by the first laser beam 10.
• Designing the light source in the proposed way allows to achieve reliable ignition of the continuous optical discharge by choosing the appropriate energy, duration and, correspondingly, pulse power of the two laser beams. This allows to create electrodeless high-brightness broadband laser-pumped plasma light sources characterized by the highest possible spatial and energetic stability.
• the plasma radiation is collecting by the optical collector 6 in a solid angle of 9 sr or more.
• a cost-effective near-infrared diode laser with a fiber-optic output is used as the CW laser 4.
• the expanding laser beam is directed to the collimator 13, for example, in the form of a collecting lens.
• the expanded parallel beam 14 of the CW laser is directed to the focusing optical element 15, for example, in the form of an aspherical condenser lens.
• the focusing optical element 15 ensures sharp focusing of the beam 3 of the CW laser 4 required to achieve a high brightness of the light source.
• the power of the CW laser 4 does not exceed 300 Watts, which is quite enough for a broad range of applications, but is not sufficient for igniting the continuous optical discharge without special means for plasma ignition.
• the pulsed laser system 9 comprises the first laser 16 for generating the first laser beam 10, and the second laser 17 for generating the second laser beam 11, Fig. 1.
• Optical elements for example, in the form of condenser lenses, may be used to focus the first and the second laser beams, without restriction to this option only.
• the focusing areas of the first and the second laser beams are at least partially superposed or overlap.
• the first laser beam 10 is characterized by a high (at least 10 4 W) pulse radiation power. In this case it is sufficient that the laser pulse full width at half maximum does not exceed 0.1 ps.
• the second laser beam has many times lower pulse power, for example, 10 3 Watts, and many times higher laser pulse length and energy, as compared to the first laser beam. This allows, after exposure to the first laser beam, use the second laser beam to create a volume of plasma which is many times, by an order of magnitude or more, larger than the volume of plasma produced by the first laser beam. At the same time, radiation power in the second laser beam is more than by an order of magnitude higher than the CW laser power, Fig. 2. [0064] The second laser beam is intended for creating plasma, the volume and density of which are sufficient for stationary plasma sustenance by the focused beam of the CW laser.
• the generating of the second laser beam begins before the generating of the first laser beam and ends not earlier than 50 ps after the end of the first laser pulse, Fig. 2.
• it makes synchronizing the first and second laser beams easier, on the other hand, it provides sufficient time for plasma evolution under the influence of the second laser beam.
• a large plasma volume of up to around 1 mm, and a plasma density of up to 1018 cm3, are provided, which are sufficient for reliable stationary plasma sustenance by the focused CW laser beam.
• a plasma density of 10 18 cm 3 corresponds to a gas with a temperature of 18,000 K and 10% ionization in the region of radiating plasma at initial gas pressure in the chamber of about 16 atm.
• the laser-pumped plasma light source operates as follows.
• the focused beam 3 of the CW laser 4 is directed into the at least partially transparent high-pressure gas chamber 1, Fig. 1.
• Xenon, other inert gases and their mixtures, including metal vapor, for example, mercury, and/or a variety of gas mixtures, including gas halides, may be contained in the chamber as a high-efficient plasma fuel.
• the focused second laser beam 11 of the second laser 17 is directed into the region intended for sustaining the radiating plasma 2.
• the maximum radiation power in the second laser beam 11 may have a value of around 10 3 Watts, while the laser pulse length may be around 10 4 s.
• the first laser beam 10 is generated, the focusing area of which at least partially superposes on the focusing area of the second laser beam.
• the second laser beam 11, whose energy and laser pulse length are many times higher than those of the first laser beam 10, is used to sustain the optical discharge at the power of laser radiation (in the order of 10 3 W or more) which is many times higher than the radiation power in the beam 3 of the CW laser.
• the pulse length of which is around 100 ps or more the plasma volume is increased due to its moving towards the laser beam 11 along the caustic and its radial expansion. So the plasma size of up to 1 mm can be achieved. Due to the sufficiently high (in the order of 0.1 J/pulse or higher) radiation pulse energy of the second laser beam 11, in the increased plasma volume a level of electron density is provided which is sufficient for reliable sustenance of the radiating plasma by the focused beam 3 of the CW laser 4 with a relatively small power not exceeding 300 Watts. Thus, the second laser beam provides a plasma density that is higher than the threshold plasma density of a continuous optical discharge having a value in the order of 10 18 electrons/cm 3 or higher. In stationary mode, broadband radiation is output from the radiating plasma region 2 by at least one output beam 5 of the plasma radiation exiting through optically transparent part of the chamber 1 and intended for subsequent use.
• Designing the light source as proposed above achieves reliable ignition of the continuous optical discharge without the use of igniting electrodes. This allows to significantly improve the chamber’s design by simplifying its shape and eliminating mechanical stresses in the points where metal is hermetically introduced into the chamber, increasing the light source reliability and lifetime. Design simplification allows to use a chamber shape that reduces aberrations introduced into the output beam of plasma radiation exiting the chamber, and thereby increase the light source brightness. Also, it provides for the possibility to use chamber material with a higher transparency in the UV spectral range. Electromagnetic noise when starting the light source is reduced. Chamber lifetime is increased because metallization of its optically transparent parts is eliminated.
• absence of electrodes allows to significantly increase the spatial angle of radiation output and to raise the power in the output beam of plasma radiation.
• elimination of igniting electrodes significantly reduces turbulence of convective flows inside the chamber, and, thereby, significantly increases the spatial and power stability of the laser-pumped plasma light source. Further improvement of stability is achieved due to the possibility of optimizing the dimensions of the electrodeless chamber. In general, an increase of the light source brightness and stability is achieved, the possibility of raising its optical output in the UV range is realized, reliability and lifetime are increased, convenience of its operation is improved, and the operating costs are reduced.
• the pulsed laser system 9 is a solid-state one, Fig. 3.
• the pulsed laser system 9 comprises two optically pumped solid-state lasers 16, 17.
• flash lamps 18, 19 with reflectors can be used as sources of optical pumping. Lamps are switched on with an optimized delay relative to each other.
• Rods made of a transparent base material, for example, yttrium-aluminum garnet (YAG), doped with metal ions, for example, neodymium (Nd), can be used as the active elements 20, 21.
• the first and the second laser beams 10, 11 are preferably parallel and introduced into the chamber 1 via one common focusing optical system 22, for example, in the form of an aspherical condenser lens.
• the first and the second solid-state lasers 16, 17 preferably have common cavity mirrors 23, 24. This provides superposition of the focusing areas of the first and the second laser beams 10, 11, required for plasma ignition.
• the pulsed laser system 9 generates the first laser beam 10 in Q-switching mode or in giant-pulse generation mode, and the second laser beam 11 in free-running mode.
• the first laser is equipped with the Q-switch 25, for example, a passive one made of phototropic material.
• an active Q-switching may be used.
• the additional deflecting mirror 27 can be used in it, Fig. 3, or several such mirrors.
• additional optical elements in the pathway of the CW laser beam 14, or in the pulsed laser system 9, additional optical elements (not shown) can be installed to offset chromatic aberrations and more accurately align the focusing areas of the CW and pulsed laser beams.
• additional optical elements for example, polarizers, filters, diaphragms, can be installed to control parameters of the first and the second laser beams.
• the axis of the CW laser focused beam 3 is directed vertically upwards, i.e. against the force of gravity 28, Fig. 3, or close to vertical.
• the proposed design achieves the highest stability of the light source radiation power. This is due to the fact that usually the region of radiating plasma 2 is slightly moved from the focus towards the focused beam 3 of the CW laser up to the focused laser beam cross-section where the intensity of the focused beam 3 of the CW laser is still enough to sustain the radiating plasma region 2.
• the focused beam 3 of the CW laser is directed from the bottom upwards, the radiating plasma region 2 that contains the hottest plasma with the lowest mass density, tends to float under the influence of the buoyant force.
• the rising region of radiating plasma 2 ends up in the location closest to the focus where the cross-section of the focused beam 3 of the CW laser is smaller, and the laser radiation intensity is higher. On the one hand, this increases the plasma radiation brightness, and on the other hand, it equalizes the forces acting on the radiating plasma region, which ensures high stability of the radiation power of the high-brightness laser-pumped plasma light source.
• the chamber 1 must be axisymmetric, and the axis of the focused beam 3 of the CW laser must be aligned with the chamber’s axis of symmetry.
• this invention realizes the possibility of achieving the highest brightness of laser-pumped broadband light sources, in particular, by means of optimizing the shape and dimensions of the electrode-free chamber.
• the external surface and the internal surface either of the chamber, or of its transparent parts are shaped as concentric spheres, and the region of radiating plasma 2 is located in the center of the said concentric spheres, Fig. 3.
• aberrations introduced by the chamber walls are eliminated, making it possible to achieve a sharper focusing of the beam 3 of the CW laser and to increase the light source brightness.
• aberrations that distort the path of rays in the beam 5 of useful plasma radiation are eliminated, increasing its brightness.
• Another positive outcome of the invention is the possibility of minimizing the chamber’s dimensions.
• This increases the focusing sharpness of the CW laser beam 3 due to moving the focusing optical system 22 closer to the region of radiating plasma 2.
• the closer the region of radiating plasma to the walls of the chamber 1, in particular to the top chamber wall the smaller the pulse acquired under the action of the buoyant force, by the gas heated in the region of radiating plasma 2. Consequently, the speed and turbulence of gas convective flows are the smaller, the smaller the distance from the plasma to the chamber wall.
• the possibility is provided to further increase the brightness and stability of the laser-pumped plasma light source designed according to present invention.
• the optically transparent parts of the chamber are preferably made of a material belonging to the group consisting of: crystalline magnesium fluoride (MgF 2 ), crystalline calcium fluoride (CaF 2 ), crystalline sapphire or leucosapphire (A1 2 0 2 ), fused or crystalline quartz.
• MgF 2 crystalline magnesium fluoride
• CaF 2 crystalline calcium fluoride
• A1 2 0 2 leucosapphire
• the chamber ensures that the output beam of plasma radiation 5 exits the chamber in a planar angle of 2p radians, without restriction to this option only Fig. 1, Fig. 3.
• the light source can have at least three diverging output beams 5a, 5b, 5c of plasma radiation, as illustrated in Fig. 4, which shows the light source cross-section in the horizontal plane passing through the region of radiating plasma 2.
• the laser beams in Fig. 4, used for COD ignition and sustenance, are located below the plane of the drawing. Using several, in particular, three beams of plasma radiation from a single light source is required for a variety of industrial applications.
• the chamber 1 of the laser-pumped plasma light source is installed in the housing 29 which is equipped with three optical collectors 6a, 6b, 6c.
• the optical collectors 6a, 6b, 6c form the plasma radiation beams 7a, 7b, 7c, transmitted, for example, via an optical fiber to the optical consumer systems 8a, 8b, 8c, which use broadband plasma radiation.
• This allows to use one light source for three or more optical consumer systems resulting in compact size of the system and identical parameters of broadband radiation in all optical channels.
• the method of plasma ignition in the laser-pumped plasma light source illustrated in Fig.l, Fig. 3, is as follows.
• the focused beam 3 of the CW laser 4 is directed into the high-pressure gas filled chamber 1, typically 10 atm or higher.
• the plasma ignition is provided by the pulsed laser system 9 generating the first and the second laser beams 10, 11 focused in the chamber.
• the first laser beam 10 is arranged to provide the optical breakdown, after which the second laser beam 11 is used to ignite the plasma, whose volume and density are sufficient for stationary plasma sustenance by the focused beam 3 of the CW laser 4.
• the solid-state laser system is used, which generates the first laser beam 10 in Q-switching mode and generates the second laser beam 11 in free-running mode, Fig. 3.
• the pulsed laser system 9 preferably comprises two solid-state lasers 16, 17, for example, Nd:YAG lasers with optical pumping sources 18, 19 in the form of flash lamps.
• the first and the second laser beams 10, 11 are preferably parallel and introduced into the chamber 1 via the focusing optical system 22.
• the solid-state lasers 16, 17 preferably have the common mirrors 23, 24 of the cavity.
• the first laser 16 is equipped with the Q-switch 25.
• Xe gas pressure in the chamber is 30 atm.
• the optical breakdown plasma has a characteristic dimension of 50 to 100 pm.
• the optical breakdown mode does not provide reliable ignition of an optical discharge sustained by the focused beam 3 of the CW laser 4. Therefore, after optical breakdown the second laser beam is used to ignite the plasma, whose volume (up to 1 mm 3 ) and the density (over 10 18 cm 3 ) are sufficient for stationary plasma sustenance by the focused beam 3 of the CW laser 4.
• the energy of the second laser beam is 150 mJ
• the pulse length is 100 ps
• the radiation pulse of the second laser beam ends no earlier than 50 ps after the end of the first laser beam radiation pulse, as illustrated in Fig. 2.
• the time of at least 50 ps is necessary to allow for decay of disturbances from the optical breakdown and for the plasma dimensions and density evolving to the values sufficient for stationary plasma sustenance by the focused beam of the CW laser.
• Generation of the second laser beam can start before the first laser pulse, Fig. 2.
• the invention is not limited only to these embodiments.
• COD ignition is also provided when the second laser beam is generated with a large, up to ten seconds or more, delay after the first laser beam generation, as illustrated in Fig. 5.
• the mechanism of such plasma ignition probably, is connected to the influence of the giant pulse on the chamber walls, when long-lived clusters or solid fine particles are created as a result of this influence.
• the proposed invention allows to ensure high reliability of laser igniting the laser-sustained plasma and to create high-brightness broadband light sources with the highest spatial and power stability on that basis.
• High-brightness high-stability laser-pumped plasma light sources designed according to this invention can be used in a variety of projection systems, for spectrochemical analysis, spectral microanalysis of bioobjects in biology and medicine, microcapillary liquid chromatography, for inspection of the optical lithography process, for spectrophotometry and for other purposes.

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