REVIEW OF SCIENTIFIC INSTRUMENTS 86, 035116 (2015)
High average power, highly brilliant laser-produced plasma source for soft X-ray spectroscopy Ioanna Mantouvalou,1 Katharina Witte,1,2 Daniel Grötzsch,1 Michael Neitzel,1 Sabrina Günther,1 Jonas Baumann,1 Robert Jung,2 Holger Stiel,2 Birgit Kanngießer,1 and Wolfgang Sandner2,a) 1 2
Institute for Optics and Atomic Physics, Technical University of Berlin, 10623 Berlin, Germany Max-Born-Institute, 12489 Berlin, Germany
(Received 27 October 2014; accepted 12 March 2015; published online 30 March 2015) In this work, a novel laser-produced plasma source is presented which delivers pulsed broadband soft X-radiation in the range between 100 and 1200 eV. The source was designed in view of long operating hours, high stability, and cost effectiveness. It relies on a rotating and translating metal target and achieves high stability through an on-line monitoring device using a four quadrant extreme ultraviolet diode in a pinhole camera arrangement. The source can be operated with three different laser pulse durations and various target materials and is equipped with two beamlines for simultaneous experiments. Characterization measurements are presented with special emphasis on the source position and emission stability of the source. As a first application, a near edge X-ray absorption fine structure measurement on a thin polyimide foil shows the potential of the source for soft X-ray spectroscopy. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4916193]
I. INTRODUCTION
In recent years, soft X-ray spectroscopic techniques have gained increasing interest due to the possibility for absorption spectroscopy of biological samples in their native environment1–3 as well as for emission spectroscopy under grazing incidence or grazing exit conditions for nm-depth resolution.4,5 From the application point of view, there are two regions of interest: in the water window between 280 and 500 eV, the penetration depth of the soft X-ray radiation in water is much larger than for carbon or nitrogen containing (bio)molecules. The region above 500 eV up to 1200 eV covers the energies of the transition metal (Sc . . . Zn) L-edges facilitating elemental and chemical analysis of a large variety of samples such as functional nanometer-structured material. Up until now, soft X-ray spectroscopy is mainly carried out using synchrotron facilities, as laboratory soft Xray sources are not readily available. Mainly due to heat dissipation problems, the brilliance of X-ray tubes in the soft X-ray range remains dissatisfactory. Therefore, new source schemes have emerged in the last couple of decades such as high harmonics generation6,7 and emission from dense plasmas.8–10 High harmonic generation produces ultra-short coherent radiation with sub fs pulse duration and photon energies up to 2 keV.11 However, the average photon flux of these sources in the water window is rather low6 and in the 1 keV region with a flux of 105 ph/s in 0.1% bandwidth11 still far from a level suitable for fluorescence applications. A rather simple mechanism for incoherent broadband soft X-ray generation is the formation of a hot, dense plasma, which emits the desired radiation. Both discharge-plasmas12,13 as well as laser-produced plasmas (LPPs)8,10,14,15 are already a)Present address: ELI-DC AISBL, 23 Rue Montoyer, 1000 Brussels, Belgium.
0034-6748/2015/86(3)/035116/8/$30.00
used in various generation schemes. LPP using a liquid jet or a dense gas medium as target system is widely used if a low debris, high brightness source is required. However, due to the specific properties of the target material (low Z liquids or cryogenic gases), the emission characteristic of those sources is restricted to a limited number of lines. This is of great advantage if the LPP is used, e.g., for imaging like in xray microscopy.16 Application of LPP in x-ray spectroscopy requires certainly a continuous emission spectrum. High Z materials as target systems such like gold, tin, or copper meet this requirement. Whereas reliable LPP sources for extreme ultraviolet (EUV) metrology based on a broad EUV emission of a gold target are well-established,17 there is a lack of such sources for the 1 keV region. In order to get a maximum soft x-ray output in this region, pump intensities in the order of 1014 W/cm2 and sub-ns pulse durations are required. Most of LPP sources for the soft x-ray region rely on low repetition rate flash lamp pumped solid state lasers limiting the average photon flux.9 As it was shown in Refs. 15 and 18, diode pumped slab or thin disk laser systems with pulse durations of less than 1 ns and single pulse energies of more than 100 mJ are well suited as pumping sources for LPP in the soft xray region. Because the average power of diode pumped solid state lasers is easily scalable up to a 200 W level, the relatively small photon flux as characteristic of low repetition rate LPP sources can be overcome. There are only three target concepts using high Z materials allowing long-term continuous source operation: jet target, tape target, and a rotating cylinder. Whereas a commercially available tape target system allows a operation for 24 h a day and 7 days a week,19 the lifetime of a (metal) jet target is limited to some hours if no recycling system is used. Furthermore, both target systems are restricted to a limited number of materials. The most flexible target system is a metal disk or cylinder
86, 035116-1
© 2015 AIP Publishing LLC
035116-2
Mantouvalou et al.
which can be easily manufactured from a large variety of materials. Nevertheless, a flexible, reliable and most of all cost effective instrument with high brilliance in the whole soft Xray region is still not commercially available. We present a novel LPP source relying on a high average power thin disk laser system as pump and a rotating cylinder target system adapted to the high repetition rate of the laser. The LPP source delivers a high photon flux in the soft X-ray and the EUV region between 100 eV and 1200 eV. Design goals for the source were the possibility for routine operation with long operating hours (8 h/day and 5 days/week), cost effectiveness, and maximal flexibility for applications in X-ray spectroscopy.
II. THE SOURCE DESIGN
The source is based on a rotating solid cylinder target system using a thin disk laser for soft X-ray generation up to the 1 keV regime. In the following the laser system, the source and the available diagnostics will be described. A. Driving laser and beam propagation
The laser was originally manufactured by Trumpf, Inc. In order to shorten its pulse duration, the laser system was equipped with a distributed Bragg reflector diode developed by the Ferdinand-Braun Institute, Berlin, and Picolas, Inc. The laser system uses a regenerative amplifier scheme with a thin Yb:YAG-disk as gain medium, which functions as one of the end-mirrors of the resonator. The laser delivers pulses with durations of 240 ps, 1.2 ns, and 12 ns at 1030 nm, with repetition rates of 100–200 Hz and pulse energies are up to 280 mJ. By changing the current of the pump modules, the laser energy can be changed gradually. A micro-Joule energy sensor (J10MB-HE, Coherent) which is placed behind a mirror in the beam propagation path is used for monitoring the laser stability during the experiment. This micro-Joule detector is calibrated against a milli-Joule energy sensor (J-50MB-HE, Coherent), which also acts as a beam dump during measurement breaks. The laser pulses are focused with a biconvex-meniscus lens system (f/4) into a specially designed high-vacuum interaction chamber where the plasma formation takes place. This lens system has a large focusing length (250 mm) so that it
Rev. Sci. Instrum. 86, 035116 (2015)
can be placed outside of the interaction chamber to prevent contamination of debris produced in the plasma ignition and heating process. To realize a small spot size, the laser beam (about 5 mm in output diameter) is expanded beforehand by a telescope to a diameter of about 35 mm.
B. Soft X-ray source
The interaction chamber designed together with the company Bestec GmbH is shown in Figure 1. A more detailed scheme of the layout is depicted in Figure 2. It houses a cylindrical metal target, which is composed of twelve metal facets that are screwed onto a stainless steel housing and turned for optimized surface roughness and roundness. This system offers the possibility to use different metals as target material for the plasma production.15 To ensure a clean surface for each laser shot, the cylinder is rotated and translated during operation with 100 µm distances between each laser shot. For source position stability, a target correction is performed with a feedback routine using an on-line monitoring system. The signal from a permanently attached pinhole camera using a four quadrant diode (4QD, AXUVPS6, IRD) as detector is used to correct for source position instabilities. These arise due to the out-of-roundness of the target cylinder, due to a displacement of rotation axis to cylinder axis and due to temperature changes or mechanical instabilities. The 4Q diode delivers besides signals determining the source position (x- and y-direction values, see Figure 6) additionally information on the overall intensity of the source. These data can be used for subsequent intensity normalization. The y-direction position of the source is stabilized with an on-line feedback routine by displacing a target correction motor, see Sec. III C. The laser hits the target under an angle of 63◦, see Figure 1, right, and Figure 2, and creates a plasma. The chamber has two beam exits (50◦ polar, 11◦ azimuthal and 50◦ polar, 30◦ azimuthal) which can be used for two independent experimental beamlines. Both angle of laser incidence and the exit angles were chosen as a compromise between efficient plasma heating and debris minimization.15,17 The laser entrance window is protected through a thin glass plate from debris, see Figure 1, right. This plate can be changed manually during operation without the necessity of
FIG. 1. Photographs of the LPP source. Left: overview over the laser; the plasma interaction chamber, the two beamlines equipped with the reflection grating (VLS) spectrograph, and the pinhole camera. Right: view inside the plasma interaction chamber; the laser path and the two beamline directions of the soft X-rays are marked.
035116-3
Mantouvalou et al.
Rev. Sci. Instrum. 86, 035116 (2015)
FIG. 2. Schematic of the interaction chamber from two different observation angles: The laser is steered into the interaction high vacuum chamber (B) with two motorized laser mirrors (E) and focused onto the Cu target (A) with a specially designed lens system (D). A permanently attached pinhole camera using a four quadrant detector (C) monitors the plasma emission. The soft X-rays exit the chamber through two beamlines (G). An observation window (F) placed normal to the target surface enables the view of the target surface with the plasma interaction point.
breaking the vacuum. The soft X-ray exits can be protected through thin plastic foils from debris if necessary. An additional XUV-diode (AXUV 100GX, IRD) attached to a goose neck with a 15 µm thick Al-filter to block the laser light can be utilized for the overall monitoring of the plasma emission. An observation window enables the view of the target surface with the plasma interaction point. The position of this point is controlled by an alignment laser and a long working distance microscope equipped with a CCD camera. The window is protected with a motorized shutter during operation of the source. C. Diagnostics for the soft X-ray source
A pinhole-camera can be utilized for the monitoring of the X-ray source position, intensity, and source size. It consists of a 30 µm pinhole (Edmund Optics), a 200 nm thick Al-filter (Lebow) and a CCD camera (GE 2048 512 BI, Greateyes) with variable distances for different magnifications. Typically, a 6-fold magnification is used and images are collected with 100 ms exposure time and at the least 4 s between images due to the data processing time of the CCD. Two grating spectrometers are available for the monitoring of the soft X-ray emission. A transmission grating spectrograph20 is available consisting of a 50 µm entrance slit, a 200 nm Al filter (Lebow), a 10.000 lines/mm Gold on Si3N4 transmission grating, and a CCD camera (Princeton Instruments) for the monitoring of the X-ray emission. This spectrograph is highly compact, easy to align, and has a moderate resolution of λ/∆λ = 50 at the 1 keV region up to λ/∆λ = 250 at 250 eV. A reflection grating spectrograph consisting of a 100 µm entrance slit (Edmund Optics), a 200 nm Al filter (Lebow), a variable line-spaced reflection grating (VLS, Hitachi 0010450), and a CCD camera (DO420A-BN, 1024 × 256 pixel, Andor) was designed to collect the energy range of 250–1200 eV in one shot. This spectrograph has an energy resolution of λ/∆λ = 250–450. Both the reflection grating and the CCD
were calibrated at the PTB (Physikalisch-Technische Bundesanstalt) at the metrology light source (MLS). Herewith, absolute photon intensities can be obtained. III. CHARACTERIZATION RESULTS
In this section, characterization measurements concerning the driving laser performance, the soft X-ray emission, and the stability of the source are described and the results are presented. If not otherwise indicated, the typical parameters for the source operation were 1.2 ns pulse duration, 220 mJ of laser energy, 100 Hz repetition rate, and copper as target material. A. Laser stability, M2, and focus
The stability of the laser system affects directly the stability of the X-ray source and must be therefore monitored carefully. In Figure 3 the laser characteristics for the 1.2 ns pulse duration are presented. The pulse energy for the plasma production is typically set to 220 mJ. A stability measurement over two hours is shown in Figure 3 obtained with the milli-Joule energy meter. The measured single shot intensity is displayed in black, +/−2% statistical deviations are marked as dotted lines, +/−5% as dashed lines. The overall uncertainty amounts to about 2%. The beam profile is almost perfectly Gaussian as measured with an M2 machine (Ophir Laser systems). The M2-values displayed in the left inset lie below 1.3 and do not change significantly for all measured single shot intensities. In the right inset, the focus of the laser derived with the biconvexmeniscus lens system is presented for 220 mJ single shot energy. The focus is highly symmetrical and almost diffraction limited with a full width half maximum (FWHM) of about 10 µm corresponding to 17 µm full width at 1/e2. The M2 value and the focus size for the other two pulse lengths are in the same range as for the 1.2 ns mode, see Table I.
035116-4
Mantouvalou et al.
Rev. Sci. Instrum. 86, 035116 (2015)
FIG. 3. Laser characteristics: the single shot energy shows a pulse-to-pulse stability of 98% at the typical energy of 220 mJ. The left inset shows M2 values for different pulse energies and the right inset shows the focus of the laser at the target surface.
With the 12 ns pulse length, the intensity stability is higher and pulse energies up to 280 mJ can be reached. With the 240 ps pulse, the maximum pulse energy amounts to 60 mJ with a typical fluctuation of about 5%. B. Dependency on laser pulse duration and brilliance
The temporal shape of the three pulses is displayed in Figure 4, graph A. The single shot profiles were collected with a fast InGaAs PIN detector (Newport Type 818-BB-35, rise/fall time
High average power, highly brilliant laser-produced plasma source for soft X-ray spectroscopy Ioanna Mantouvalou,1 Katharina Witte,1,2 Daniel Grötzsch,1 Michael Neitzel,1 Sabrina Günther,1 Jonas Baumann,1 Robert Jung,2 Holger Stiel,2 Birgit Kanngießer,1 and Wolfgang Sandner2,a) 1 2
Institute for Optics and Atomic Physics, Technical University of Berlin, 10623 Berlin, Germany Max-Born-Institute, 12489 Berlin, Germany
(Received 27 October 2014; accepted 12 March 2015; published online 30 March 2015) In this work, a novel laser-produced plasma source is presented which delivers pulsed broadband soft X-radiation in the range between 100 and 1200 eV. The source was designed in view of long operating hours, high stability, and cost effectiveness. It relies on a rotating and translating metal target and achieves high stability through an on-line monitoring device using a four quadrant extreme ultraviolet diode in a pinhole camera arrangement. The source can be operated with three different laser pulse durations and various target materials and is equipped with two beamlines for simultaneous experiments. Characterization measurements are presented with special emphasis on the source position and emission stability of the source. As a first application, a near edge X-ray absorption fine structure measurement on a thin polyimide foil shows the potential of the source for soft X-ray spectroscopy. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4916193]
I. INTRODUCTION
In recent years, soft X-ray spectroscopic techniques have gained increasing interest due to the possibility for absorption spectroscopy of biological samples in their native environment1–3 as well as for emission spectroscopy under grazing incidence or grazing exit conditions for nm-depth resolution.4,5 From the application point of view, there are two regions of interest: in the water window between 280 and 500 eV, the penetration depth of the soft X-ray radiation in water is much larger than for carbon or nitrogen containing (bio)molecules. The region above 500 eV up to 1200 eV covers the energies of the transition metal (Sc . . . Zn) L-edges facilitating elemental and chemical analysis of a large variety of samples such as functional nanometer-structured material. Up until now, soft X-ray spectroscopy is mainly carried out using synchrotron facilities, as laboratory soft Xray sources are not readily available. Mainly due to heat dissipation problems, the brilliance of X-ray tubes in the soft X-ray range remains dissatisfactory. Therefore, new source schemes have emerged in the last couple of decades such as high harmonics generation6,7 and emission from dense plasmas.8–10 High harmonic generation produces ultra-short coherent radiation with sub fs pulse duration and photon energies up to 2 keV.11 However, the average photon flux of these sources in the water window is rather low6 and in the 1 keV region with a flux of 105 ph/s in 0.1% bandwidth11 still far from a level suitable for fluorescence applications. A rather simple mechanism for incoherent broadband soft X-ray generation is the formation of a hot, dense plasma, which emits the desired radiation. Both discharge-plasmas12,13 as well as laser-produced plasmas (LPPs)8,10,14,15 are already a)Present address: ELI-DC AISBL, 23 Rue Montoyer, 1000 Brussels, Belgium.
0034-6748/2015/86(3)/035116/8/$30.00
used in various generation schemes. LPP using a liquid jet or a dense gas medium as target system is widely used if a low debris, high brightness source is required. However, due to the specific properties of the target material (low Z liquids or cryogenic gases), the emission characteristic of those sources is restricted to a limited number of lines. This is of great advantage if the LPP is used, e.g., for imaging like in xray microscopy.16 Application of LPP in x-ray spectroscopy requires certainly a continuous emission spectrum. High Z materials as target systems such like gold, tin, or copper meet this requirement. Whereas reliable LPP sources for extreme ultraviolet (EUV) metrology based on a broad EUV emission of a gold target are well-established,17 there is a lack of such sources for the 1 keV region. In order to get a maximum soft x-ray output in this region, pump intensities in the order of 1014 W/cm2 and sub-ns pulse durations are required. Most of LPP sources for the soft x-ray region rely on low repetition rate flash lamp pumped solid state lasers limiting the average photon flux.9 As it was shown in Refs. 15 and 18, diode pumped slab or thin disk laser systems with pulse durations of less than 1 ns and single pulse energies of more than 100 mJ are well suited as pumping sources for LPP in the soft xray region. Because the average power of diode pumped solid state lasers is easily scalable up to a 200 W level, the relatively small photon flux as characteristic of low repetition rate LPP sources can be overcome. There are only three target concepts using high Z materials allowing long-term continuous source operation: jet target, tape target, and a rotating cylinder. Whereas a commercially available tape target system allows a operation for 24 h a day and 7 days a week,19 the lifetime of a (metal) jet target is limited to some hours if no recycling system is used. Furthermore, both target systems are restricted to a limited number of materials. The most flexible target system is a metal disk or cylinder
86, 035116-1
© 2015 AIP Publishing LLC
035116-2
Mantouvalou et al.
which can be easily manufactured from a large variety of materials. Nevertheless, a flexible, reliable and most of all cost effective instrument with high brilliance in the whole soft Xray region is still not commercially available. We present a novel LPP source relying on a high average power thin disk laser system as pump and a rotating cylinder target system adapted to the high repetition rate of the laser. The LPP source delivers a high photon flux in the soft X-ray and the EUV region between 100 eV and 1200 eV. Design goals for the source were the possibility for routine operation with long operating hours (8 h/day and 5 days/week), cost effectiveness, and maximal flexibility for applications in X-ray spectroscopy.
II. THE SOURCE DESIGN
The source is based on a rotating solid cylinder target system using a thin disk laser for soft X-ray generation up to the 1 keV regime. In the following the laser system, the source and the available diagnostics will be described. A. Driving laser and beam propagation
The laser was originally manufactured by Trumpf, Inc. In order to shorten its pulse duration, the laser system was equipped with a distributed Bragg reflector diode developed by the Ferdinand-Braun Institute, Berlin, and Picolas, Inc. The laser system uses a regenerative amplifier scheme with a thin Yb:YAG-disk as gain medium, which functions as one of the end-mirrors of the resonator. The laser delivers pulses with durations of 240 ps, 1.2 ns, and 12 ns at 1030 nm, with repetition rates of 100–200 Hz and pulse energies are up to 280 mJ. By changing the current of the pump modules, the laser energy can be changed gradually. A micro-Joule energy sensor (J10MB-HE, Coherent) which is placed behind a mirror in the beam propagation path is used for monitoring the laser stability during the experiment. This micro-Joule detector is calibrated against a milli-Joule energy sensor (J-50MB-HE, Coherent), which also acts as a beam dump during measurement breaks. The laser pulses are focused with a biconvex-meniscus lens system (f/4) into a specially designed high-vacuum interaction chamber where the plasma formation takes place. This lens system has a large focusing length (250 mm) so that it
Rev. Sci. Instrum. 86, 035116 (2015)
can be placed outside of the interaction chamber to prevent contamination of debris produced in the plasma ignition and heating process. To realize a small spot size, the laser beam (about 5 mm in output diameter) is expanded beforehand by a telescope to a diameter of about 35 mm.
B. Soft X-ray source
The interaction chamber designed together with the company Bestec GmbH is shown in Figure 1. A more detailed scheme of the layout is depicted in Figure 2. It houses a cylindrical metal target, which is composed of twelve metal facets that are screwed onto a stainless steel housing and turned for optimized surface roughness and roundness. This system offers the possibility to use different metals as target material for the plasma production.15 To ensure a clean surface for each laser shot, the cylinder is rotated and translated during operation with 100 µm distances between each laser shot. For source position stability, a target correction is performed with a feedback routine using an on-line monitoring system. The signal from a permanently attached pinhole camera using a four quadrant diode (4QD, AXUVPS6, IRD) as detector is used to correct for source position instabilities. These arise due to the out-of-roundness of the target cylinder, due to a displacement of rotation axis to cylinder axis and due to temperature changes or mechanical instabilities. The 4Q diode delivers besides signals determining the source position (x- and y-direction values, see Figure 6) additionally information on the overall intensity of the source. These data can be used for subsequent intensity normalization. The y-direction position of the source is stabilized with an on-line feedback routine by displacing a target correction motor, see Sec. III C. The laser hits the target under an angle of 63◦, see Figure 1, right, and Figure 2, and creates a plasma. The chamber has two beam exits (50◦ polar, 11◦ azimuthal and 50◦ polar, 30◦ azimuthal) which can be used for two independent experimental beamlines. Both angle of laser incidence and the exit angles were chosen as a compromise between efficient plasma heating and debris minimization.15,17 The laser entrance window is protected through a thin glass plate from debris, see Figure 1, right. This plate can be changed manually during operation without the necessity of
FIG. 1. Photographs of the LPP source. Left: overview over the laser; the plasma interaction chamber, the two beamlines equipped with the reflection grating (VLS) spectrograph, and the pinhole camera. Right: view inside the plasma interaction chamber; the laser path and the two beamline directions of the soft X-rays are marked.
035116-3
Mantouvalou et al.
Rev. Sci. Instrum. 86, 035116 (2015)
FIG. 2. Schematic of the interaction chamber from two different observation angles: The laser is steered into the interaction high vacuum chamber (B) with two motorized laser mirrors (E) and focused onto the Cu target (A) with a specially designed lens system (D). A permanently attached pinhole camera using a four quadrant detector (C) monitors the plasma emission. The soft X-rays exit the chamber through two beamlines (G). An observation window (F) placed normal to the target surface enables the view of the target surface with the plasma interaction point.
breaking the vacuum. The soft X-ray exits can be protected through thin plastic foils from debris if necessary. An additional XUV-diode (AXUV 100GX, IRD) attached to a goose neck with a 15 µm thick Al-filter to block the laser light can be utilized for the overall monitoring of the plasma emission. An observation window enables the view of the target surface with the plasma interaction point. The position of this point is controlled by an alignment laser and a long working distance microscope equipped with a CCD camera. The window is protected with a motorized shutter during operation of the source. C. Diagnostics for the soft X-ray source
A pinhole-camera can be utilized for the monitoring of the X-ray source position, intensity, and source size. It consists of a 30 µm pinhole (Edmund Optics), a 200 nm thick Al-filter (Lebow) and a CCD camera (GE 2048 512 BI, Greateyes) with variable distances for different magnifications. Typically, a 6-fold magnification is used and images are collected with 100 ms exposure time and at the least 4 s between images due to the data processing time of the CCD. Two grating spectrometers are available for the monitoring of the soft X-ray emission. A transmission grating spectrograph20 is available consisting of a 50 µm entrance slit, a 200 nm Al filter (Lebow), a 10.000 lines/mm Gold on Si3N4 transmission grating, and a CCD camera (Princeton Instruments) for the monitoring of the X-ray emission. This spectrograph is highly compact, easy to align, and has a moderate resolution of λ/∆λ = 50 at the 1 keV region up to λ/∆λ = 250 at 250 eV. A reflection grating spectrograph consisting of a 100 µm entrance slit (Edmund Optics), a 200 nm Al filter (Lebow), a variable line-spaced reflection grating (VLS, Hitachi 0010450), and a CCD camera (DO420A-BN, 1024 × 256 pixel, Andor) was designed to collect the energy range of 250–1200 eV in one shot. This spectrograph has an energy resolution of λ/∆λ = 250–450. Both the reflection grating and the CCD
were calibrated at the PTB (Physikalisch-Technische Bundesanstalt) at the metrology light source (MLS). Herewith, absolute photon intensities can be obtained. III. CHARACTERIZATION RESULTS
In this section, characterization measurements concerning the driving laser performance, the soft X-ray emission, and the stability of the source are described and the results are presented. If not otherwise indicated, the typical parameters for the source operation were 1.2 ns pulse duration, 220 mJ of laser energy, 100 Hz repetition rate, and copper as target material. A. Laser stability, M2, and focus
The stability of the laser system affects directly the stability of the X-ray source and must be therefore monitored carefully. In Figure 3 the laser characteristics for the 1.2 ns pulse duration are presented. The pulse energy for the plasma production is typically set to 220 mJ. A stability measurement over two hours is shown in Figure 3 obtained with the milli-Joule energy meter. The measured single shot intensity is displayed in black, +/−2% statistical deviations are marked as dotted lines, +/−5% as dashed lines. The overall uncertainty amounts to about 2%. The beam profile is almost perfectly Gaussian as measured with an M2 machine (Ophir Laser systems). The M2-values displayed in the left inset lie below 1.3 and do not change significantly for all measured single shot intensities. In the right inset, the focus of the laser derived with the biconvexmeniscus lens system is presented for 220 mJ single shot energy. The focus is highly symmetrical and almost diffraction limited with a full width half maximum (FWHM) of about 10 µm corresponding to 17 µm full width at 1/e2. The M2 value and the focus size for the other two pulse lengths are in the same range as for the 1.2 ns mode, see Table I.
035116-4
Mantouvalou et al.
Rev. Sci. Instrum. 86, 035116 (2015)
FIG. 3. Laser characteristics: the single shot energy shows a pulse-to-pulse stability of 98% at the typical energy of 220 mJ. The left inset shows M2 values for different pulse energies and the right inset shows the focus of the laser at the target surface.
With the 12 ns pulse length, the intensity stability is higher and pulse energies up to 280 mJ can be reached. With the 240 ps pulse, the maximum pulse energy amounts to 60 mJ with a typical fluctuation of about 5%. B. Dependency on laser pulse duration and brilliance
The temporal shape of the three pulses is displayed in Figure 4, graph A. The single shot profiles were collected with a fast InGaAs PIN detector (Newport Type 818-BB-35, rise/fall time
