Capillary discharge x-ray laser

A compact, high repetition rate, extreme ultraviolet/soft x-ray laser and method for generating such radiation are described. Excitation of the gaseous or vaporous lasing medium is achieved by discharging energy stored in a solid-dielectric capacitive device through a capillary channel containing the medium. By reducing the inductance of the discharge apparatus, excitation of the laser medium can be achieved without the use of Marx generators. Neon-like Ar atom laser pulses at 46.9 nm having energies of about 13 μJ are generated at repetition rates up to 12 Hz. Between 2 and 3×104 laser shots can be generated using a single capillary. Such a source of intense, short-wavelength radiation can be used for applications which include surface characterization of materials, high resolution imaging and printing, photochemistry and photophysics, laser ablation, characterization of x-ray optics, and dense plasma diagnostics.

FIELD OF THE INVENTION

The present invention relates generally to an efficient source of extreme ultraviolet/soft x-ray laser radiation having increased simplicity and reduced size, driven by a electrical discharge powered by a solid dielectric energy storage medium.

BACKGROUND OF THE INVENTION

Radiation in the extreme ultraviolet (EUV) and soft x-ray (SXR) regions of the electromagnetic spectrum finds use in applications that cannot be performed using visible light, such as imaging and printing of small features, diagnosis of dense large-scale plasmas, and the study of material surfaces by interaction with inner-shell electrons, to name a few. Many of these applications (for example, high-precision measurement techniques based on interferometry) further require radiation having substantial coherence which both allows radiation waves to interfere, and allows radiation to be focused onto small areas, thereby permitting high intensities to be achieved.

Synchrotron sources, high-order harmonic sources, and soft x-ray lasers have been used for generating coherent EUV/SXR radiation. Synchrotrons produce high average powers of EUV/SXR radiation that can be filtered in space and frequency to obtain a substantial coherence. Typically, these sources are multi-user facilities requiring their own buildings and costing hundreds of millions of dollars.

Direct amplification of EUV/SXR radiation in a suitable laser medium is an attractive method for generating intense beams of such radiation from a compact device. However, in order for a laser to operate efficiently and to produce significant amounts of laser output energy, the amplified beam must reach saturation intensity, whereby the amplified radiation extracts the majority of the energy stored in the population inversion. An obstacle for the development of such saturated lasers has been the large excitation power required to produce a sufficiently large population inversion in a volume having sufficient length. Successfully demonstrated EUV/SXR laser media include plasmas sustained using a powerful source of energy; for example, by focusing a high-power infrared, visible or ultraviolet laser radiation into the lasing media. Electrical energy is used to excite such a laser. To date, most saturated EUV/SXR lasers excited by this method have required large and costly pump lasers. In all cases average power output is low.

An alternative excitation method uses an electrical discharge as the energy source, thereby creating a plasma and exciting the laser upper level. In U.S. Pat. No. 4,937,832 which issued to Jorge J. Rocca on. Jun. 26, 1990, an electrical discharge through a capillary channel was demonstrated to produce a plasma column in which large EUV/SXR laser amplification was obtained. The lasing portion of the apparatus occupied a volume of about 1 m×1 m×2.5 m. A liquid-dielectric (de-ionized water or ethylene glycol, as examples), high-voltage capacitor was charged to a chosen high voltage, and discharged through the capillary channel. A high-voltage Marx generator which included a voltage multiplication circuit having at least two series connected stages, each stage containing a capacitor and a high-voltage switch, was used. The capacitors were charged in parallel to a selected voltage using a high-voltage power supply, while the switches remained open. When the switches are simultaneously closed, a voltage that is of the order of the charging voltage times the number of stages is produced. A more compact configuration was achieved by replacing the liquid dielectric capacitor with a liquid-dielectric Blumlein transmission line. This latter device occupied a volume of about 0.4 m×0.4 m×1 m, a significant reduction in size respect to laser-excited EUV/SXR lasers. A Marx generator (typical volume of about 1 m3), which includes a de-ionizing unit for achieving and maintaining sufficiently high resistivity for the liquid dielectric is generally still required.

Accordingly, it is an object of the present invention to provide coherent EUV/SXR radiation from a compact, high average power laser.

Another object of the present invention is to provide coherent EUV/SXR radiation at sufficiently low applied voltages that a Marx generator is not required.

Still another object of the present invention is to provide coherent EUV/SXR radiation at high repetition rates, and high average power.

SUMMARY OF THE INVENTION

To achieve the foregoing and other objects of the present invention, and in accordance with its purposes, as embodied and broadly described herein, the pulsed x-ray laser hereof includes: A pulsed x-ray laser comprising in combination: a capillary tube having a chosen length and a chosen bore, the capillary tube having a first end and a second end; an electrode disposed in the vicinity of the first end of the capillary tube; an electrode in electrical connection with ground potential disposed in the vicinity of the second end of the capillary tube and having an opening adapted so as not to occlude the bore thereof; means for introducing a chosen gas into the bore of the capillary tube; means for preionizing the gas; at least one capacitor having a chosen capacitance and having a first electrode and a second electrode, the first electrode of the at least one capacitor being in electrical contact with ground potential; means for charging the at least one capacitor through the second electrode thereof to a chosen voltage such that an electrical discharge occurs in the gas between the electrode disposed in the vicinity of the first end of said capillary tube and the ground electrode thereof when the gas is preionized, the discharge having a current effective for generating a population inversion in the preionized gas suitable for producing lasing therein; and means for enclosing the capillary tube, the enclosing means being in electrical connection with the grounded electrode, and being adapted such that the return path of the discharge current to ground potential is minimized.

In another aspect of the present invention and in accordance with its objects and purposes, the method for generating pulsed x-ray laser radiation in a capillary discharge hereof includes the steps of: introducing a chosen gas into a capillary tube having a chosen diameter, an electrode disposed in the vicinity of a first end, and an electrode in electrical connection with ground potential disposed in the vicinity of a second end, the grounded electrode having an opening adapted so as not to occlude the bore of the capillary tube, the capillary tube being enclosed by an electrically conductive material in electrical contact with the grounded electrode, such that the return path of discharge current to ground is minimized; charging at least one capacitor having a chosen capacitance a first electrode and a second electrode, the first electrode being in electrical contact with ground potential, to a chosen voltage through the second electrode thereof, such that an electrical discharge occurs in the gas between the electrode disposed in the vicinity of the first end of the capillary tube and the ground electrode thereof when the gas is preionized, the discharge having a current effective for generating a population inversion in the preionized gas suitable for producing lasing therein; and preionizing the chosen gas.

Benefits and advantages of the present method include a reliable, portable, compact EUV/SXR laser capable of operating at modest voltages, typically less than about 100 kilovolts, without the necessity of a Marx generator and an accompanying water de-ionizing unit, while exceeding the saturation intensity of the laser media, thereby allowing high peak-power laser beam generation and efficient laser operation.

DETAILED DESCRIPTION

Briefly, the present invention includes an apparatus and method for generating laser radiation in the extreme ultraviolet (EUV) electromagnetic spectral region (defined for the purposes hereof as the spectral region corresponding to wavelengths between about 55 nm and 150 nm) and in a portion of the soft x-ray (SXR) electromagnetic spectral region (defined for the purposes hereof as the spectral region between about 1 nm and 55 nm). Low-inductance electrical components suitable for generating an electric discharge in a capillary tube containing the lasing medium and providing excitation thereto are employed. Solid dielectric capacitors (typically a ceramic or another type of solid insulator) are pulse charged using electrical circuitry described hereinbelow, and commercially available high-voltage power supplies and pulse generators, to provide fast electrical current pulses through the capillary structure. The apparatus does not require the Marx generator and the accompanying, generally required water de-ionizer utilized in earlier apparatus, and can generate plasma columns having sufficient length to produce a saturated laser beam, while fitting on a small desk.

The capillary tube includes an elongated hollow structure having a large length-to-diameter ratio and insulating walls that include at least two conducting electrodes for injecting fast current pulse into the capillary channel. At least one of these electrodes is adapted to permit laser radiation and gases to exit the capillary structure. The lasing material is introduced into the capillary in the form of a gas or a vapor, and may be flowed therethrough. The current pulse is provided by at least one charged, solid-dielectric capacitor or a transmission line. An electrical switch is used for rapidly directing the high voltage from the capacitor or transmission line to the capillary channel.

The lasing material is preionized to create a uniform axial plasma throughout the capillary channel prior to the arrival of the high-energy, fast high-current pulse. At least one reflecting mirror can be aligned with the axis of the capillary structure to permit more than one pass of the laser radiation through the capillary structure, thereby increasing the laser output energy. Additional capillary channels placed in tandem may be used to further increase laser output energy; that is, laser radiation exiting one of the capillaries would be injected into and along the axis of another capillary.

The present laser was found to emit>10 μJ pulses at approximately λ=46.9 nm at an about 12 Hz repetition rate with argon gas as the lasing medium, and occupies a table area of approximately 0.4 m×0.4 m (0.4 m×0.8 m with the vacuum pump included), which is smaller than that occupied by many widely used ultraviolet gas lasers. The reduced size of the present capillary discharge device is achieved by utilizing a low-inductance, coaxial-discharge configuration that substantially decreases the voltage necessary to generate the peak current required for laser excitation. That is, excitation of the capillary discharge channel utilizes ceramic capacitors which are pulse charged to voltages <90 kV. This reduced voltage permits the use of readily available high voltage power supplies. In actuality, because of the pulse charging procedure, dc voltages of about 65 kV are employed to generate the approximately 90 kV pulsed applied voltage. Consequently, the volume of the pulsed power unit is about 1/9 as large as that of previous capillary discharge lasers [B. R. Benware et al., Rocca, Phys. Rev. Lett. 81, 5804 (1998), supra.] and can be accommodated in a small rack under a regular optical table.

Reference will now be made in detail to the present preferred embodiments of the invention examples of which are illustrated in the accompanying drawings. Turning now toFIG. 1Ahereof, schematic representation of one embodiment of the EUV/SXR laser,10, of the present invention is illustrated which has been successfully tested. In one embodiment of the invention, at least one charged, ceramic capacitor, (shown as two capacitors12aand12b), disposed such that the overall inductance is minimized, is discharged through bore,13, of capillary,14. One electrical contact,16a(16b), of capacitor12a(12b) is placed in electrical connection to grounded, electrical conductor,18, which forms part of an enclosure for the electrical components, while the other output thereof,20a(20b), is placed in electrical connection with electrical conductor,22a(22b). Conductors18and22may be fabricated from metal, such as aluminum, brass or stainless steel. Conductor22a(22b) is pulse charged using pulsed, high-voltage pulsed power source,23(not shown connected to conductor22b), in electrical connection therewith through cable,24. Conductor22a(22b) is also placed in electrical connection with electrode,25a(25b), of high-voltage switch,26, which for the embodiment tested, was a triggerable high-voltage, gas-filled spark gap switch. When capacitors12aand12bare charged to sufficiently high voltage (depending on the characteristics of gas-filled spark gap switch26), the gas in spark-gap26will break down. Gas breakdown can also be achieved as the result of the arrival of an external trigger pulse from pulse source,27, to be described hereinbelow. The charge stored in capacitors12aand12bflows through capillary bore13, thereby generating a fast excitation, high-current pulse.

Electrode,29, of spark gap26is electrically connected to electrode,30, which substantially surrounds one end of capillary tube14. A second electrode,32, substantially surrounds the other end of the capillary. Electrode32has an orifice,34, along the longitudinal axis of bore13of capillary14to allow the exit of laser radiation which may be detected and/or directed to its intended application. In the situation where more than one capillary structure is used, electrode30would similarly be provided with an orifice located in line with bore13of the capillary. In such situation, the lasing gas or vapor could be introduced into either end of capillary14. Additionally, such gas or vapor could be flowed through capillary bore13. Stainless steel has been effectively used for electrodes30and32; however, other metals may be used. In situations where it is desirable to minimize electrode erosion due to electron bombardment, molybdenum or other refractory metals might be employed.

Capillary14was fabricated from aluminum oxide (high purity alumina) in the embodiment tested, but other materials can be used, such as diamond-coated ceramics, other ceramics, and sapphire, as examples. Cylindrical conducting metallic tube,36, closely surrounds capillary14, and is in electrical connection with grounded electrode32to provide a current return path. The close proximity of the metallic tube to the capillary tube permits the minimization of the volume enclosed by the return discharge current path, thereby reducing the inductance of the discharge apparatus, and permitting lower voltages to be used to generate an effective gas discharge. Together with grounded metallic enclosure18, tube36forms a sealed enclosure for EUV/SXR laser10of the present invention. The enclosure substantially shields the electromagnetic noise produced by the fast discharge.

Capillary14, capacitors12aand12b, and spark gap26are immersed in a high dielectric strength fluid,38, such as transformer oil which provides voltage isolation and thermal cooling. The high dielectric strength of this fluid permits the diameter of tube36to be made sufficiently small that the inductance of the discharge apparatus is minimized. Other desirable characteristics of such fluids include high heat conductivity and high decomposition temperature. The transformer oil was circulated using a commercially available chiller unit (not shown inFIG. 1).

Capillary tube14is evacuated through orifice34in electrode32, using roughing vacuum pump,40, which is connected to volume42, through tube,44. Gaseous lasing materials,46, are introduced into volume42and bore13through tube,48, using needle valve,50, to regulate the gas pressure and flow rate. For argon, filling pressures between 0.1 to 1 Torr were found to be effective, depending on the capillary diameter. The lasing gas was not flowed through the capillary bore; however, it is anticipated that “gas renewal” in the capillary would be beneficial for high repetition-rate operation of laser10, subject to the constraint that the pressure must remain approximately constant along capillary bore13. In order to reduce absorption of output laser radiation by the chosen gas, volume42was kept small by means of insert,52, having hole,54, which permits passage of EUV/SXR radiation,55.

Pressures below about 1×10−4Torr were maintained in chamber,56, to permit radiation propagation with low attenuation through volume,58, to target,60. Target60may include a radiation detector or a material to be irradiated, as examples. Such pressures are readily attainable by differential pumping using turbomolecular pump,62, backed by mechanical pump,64, through tube,66, and are effective for the operation of laser10. Before filling, the apparatus was pumped to a vacuum of approximately 1×10−5Torr.

High-voltage pulsed power source,23, also provides a preionization current to the gases contained in bore13of capillary14through cable67. A combination of a DC current between about 50 and 100 mA followed by a smooth current “ramp” of about 1 μs-duration pre-pulse prior to the firing of the short time duration, high-current discharge pulse. Larger values for the DC preionization current can be used, but heat dissipation may become a problem. The short time duration, high-current pulse ionizes and excites the plasma, creating a population inversion by collisional electron impact excitation. Radiation spontaneously emitted by the excited laser medium is amplified to form a directed high power laser beam. Pinhole,54, having a chosen diameter (about 1 mm for the lasing apparatus described), is placed in close proximity to electrode32, in order to achieve a short optical path for laser radiation55between exit34of electrode32and volume58, thereby reducing absorption of the laser radiation by photoionization of the residual gases.

Length-to-bore diameter ratios between about 20 and 1000 are suitable for capillary14, with bore13diameters ranging between 0.5 mm and 10 mm. Diameters between about 1.5 mm and 6 mm have been observed to provide the best results. The length of the lasing channel is selected to be sufficiently long for the laser intensity to reach the value at which the amplification saturates due to depletion of the population inversion. For example, in the case of operation with Ar gas as the lasing medium, (46.9 nm laser radiation) a capillary channel length of between about 15 cm and 20 cm was found to produce saturated amplification. Capillaries between about 10 cm and 40 cm are expected to generate efficient laser operation. Operation using shorter capillary lengths, for example 11 cm, is readily implemented, but laser output efficiency will be reduced. Thus, in general, laser output efficiency (defined as the ratio between the laser output energy and the excitation energy) will also significantly decrease if the capillary length is reduced below that required to reach saturation.

In actual operation, laser amplification was obtained in a Ne-like Ar plasma column generated in aluminum-oxide capillary14having a 3.2 mm inside diameter and a 21 cm length and filled with pre-ionized Ar at a pressure of about 700 mTorr. The plasma column was excited by current pulses of about 22 kA peak amplitude monitored with a Rogowski coil (not shown inFIG. 1). The excitation current pulse was produced by discharging ceramic capacitors12having a combined capacitance of 27 nF through pressurized air, high-voltage spark-gap switch26that was connected in series with the capillary. Capacitors12, were placed in a ring configuration surrounding the spark-gap, and pulse-charged to 80-90 kV by single-stage, pulsed power unit23connected to the capacitors using coaxial cable24. This configuration further reduces the discharge current return pathway to ground, thereby further reducing the inductance of the discharge apparatus and permitting lower charging voltages to be employed. It should be mentioned that ring-shaped capacitors having an axis approximately collinear with the bore of the capillary tube may also be used. The main current pulse through the capillary was initiated by triggering the spark-gap26with an approximately 50 kV pulse of opposite polarity to that used to charge the capacitors provided by pulse generator27through cable,67. Synchronization of the laser output with external events was observed to have a jitter of several nanoseconds. Sub-nanosecond jitter can be obtained using laser triggering of the spark-gap [See, e.g., B. Luther et al., Appl. Phys. Lett. 79, 3248-3250 (2001).].

FIG. 1Bshows a schematic representation of the components of high-voltage pulsed power source23. Commercial, high-voltage power supply,68, supplies high voltage effective for preionizing the laser gas and for charging capacitor,70, which when switched through spark gap,72, by voltage trigger device,74, provides pulse charging to capacitors12aand12b. As stated hereinabove, voltages of about 65 kV generate the approximately 90 kV pulsed charging voltage.

FIG. 1Cshows an exploded schematic representation of electrode29of spark gap26, showing, in particular, insulating material,76, which is inserted through a hole in electrode29, and conductor,74, which breaks down,78, to electrode29when a trigger voltage from pulsed high-voltage source27is directed through cable28, thereby permitting the short-duration, high current pulse to pass between electrode25aand electrode29.

Although laser10, has been demonstrated using argon, with lasing taking place at 46.9 nm, other gases, metal vapors and mixtures of gases may be used. Lasing using chlorine gas at 52.9 nm in the present apparatus is expected [See, e.g., M. Frati et al., “Demonstration of a 10 μJ tabletop laser at 52.9 nm in neon-like chlorine”, Optics Lett. 25, 1022 (2000)]. Using a different (and larger) capillary discharge laser, lasing in Sulfur at 60.8 nm was demonstrated [See, e.g., F. G. Tomasel et al., “Lasing at 60.8 nm in Ne-like Sulfur Ions in Ablated Material Excited by a Capillary Discharge” Phys. Rev. A 55, 1437 (1997)]. The lasing material can also be generated within the bore as a result of ablation from the capillary walls.

FIG. 2shows oscilloscope traces of the approximately 13 μJ laser output pulse having a duration of approximately 1 ns, and the excitation current pulse accompanying laser operation. The pulse has 10% to 90% rise time of approximately 60 ns, and a first half-cycle duration of 165 ns. A pronounced dip may be observed in the current about 40 ns after the beginning of the current pulse. This local minimum of the current occurs at the time the plasma column reaches its minimum diameter of 200-300 μm, and is believed by the inventor to be caused by the significant increase in the plasma column inductance that accompanies the reduction of the plasma column diameter. The resulting laser pulse of 1.5 ns FWHM duration may be seen to occur shortly before the time of maximum plasma compression, which takes place about 35 ns after the initiation of the current pulse.

The laser output pulse energy was measured using a vacuum photodiode placed at 80 cm from the exit of the laser and the data were recorded and stored by a 5 Gs/s digitizing oscilloscope. The quantum efficiency of the Al photocathode used was previously calibrated with respect to a silicon photodiode of known quantum yield [J. J. Rocca et al. Phys. Rev. Lett. 77, 1476 (1996), supra.]. The laser output was attenuated with several stainless steel meshes of measured transmissivity to avoid saturation of the photodiode. The laser was successfully operated at repetition rates up to 12 Hz.FIG. 3ashows the shot to shot variation of the peak of the excitation current pulse for 1500 consecutive shots at 12 Hz repetition rate.FIG. 3bandFIG. 3cillustrate the corresponding shot-to-shot variation of the laser output pulse energy as a function of the shot number and its statistical distribution respectively. The average pulse energy was 13 μJ and the standard deviation is ±1.3 μJ, which corresponds to an average power of about 0.15 mW. The use of external triggering of the spark-gap permitted relatively low jitter operation.FIG. 3dillustrates the statistical distribution of the time delay between the TTL trigger signal into the high voltage trigger device that fires the spark-gap and the laser output pulse for the 1500 laser shots ofFIG. 3a. The standard deviation of the jitter is ±5 ns.

Ablation of the capillary walls by the discharge over a large number of shots was observed to increase their surface roughness, ultimately leading to the deterioration of the uniformity of the plasma column and to a consequent decrease of the laser output energy. Capillary lifetime tests were conducted at 12 Hz repetition rate by recording the laser output energy for a large number of pulses. The laser output energy was observed to decay by a factor of about 2 after between approximately 2 and 3×104pulses as may be observed fromFIG. 4hereof. The full output pulse energy can be recovered by replacing the used capillary discharge tube by an unused tube.

The far field laser output intensity distribution was measured using a microchannel plate/phosphor screen read by a CCD array detector of 1024×1024 pixels placed at 157.5 cm from the exit of the laser. The microchannel plate was gated with an approximately 5 ns voltage pulse to enable the discrimination of the laser light from the spontaneous light emitted by the plasma in hundreds of extreme ultraviolet transitions that are several orders of magnitude less intense than the laser emission, and which produce a significant background when temporally integrated over the duration of the discharge. The beam profile was observed to have an annular shape that is the result of refraction of the amplified rays by radial electron density gradients in the plasma column [See, e.g., C. H. Moreno et al., Phys. Rev. A 58, 1509 (1998); and J. L. A. Chilla and J. J. Rocca, J. Opt. Soc. Am. B 13, 2841 (1996).].

FIG. 5shows a cross section of an output intensity pattern acquired in a single shot at 157.5 cm from the capillary exit. The peak-to-peak divergence was about 5.2 mrad. Although the wavefront has not yet been characterized, previous measurements of similar annular capillary discharge laser beams have shown good focusing properties [See, e.g., S. Le Pape et al., Phys. Rev. Lett. 88, 183901 (2002).].

The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching.

The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.