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Extreme ultraviolet spectrometer for the Shenguang III laser facility GANG XIONG,1 GUOHONG YANG,1,* JIYAN ZHANG,1 MINXI WEI,1 YANG ZHAO,1 BO QING,1 MIN LV,1 ZHENGHUA YANG,1 FENG WANG,1 SHENYE LIU,1 HOUZHI CAI,2 AND JINYUAN LIU2 1
Research Center of Laser Fusion, China Academy of Engineering Physics, P. O. Box 919-986, Mianyang 621900, China Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, Institute of Optoelectronics, Shenzhen University, Shenzhen 518060, China *Corresponding author: [email protected]
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Received 18 February 2015; revised 10 May 2015; accepted 12 May 2015; posted 12 May 2015 (Doc. ID 234756); published 4 June 2015
An extreme ultraviolet spectrometer has been developed for high-energy density physics experiments at the Shenguang-III (SG-III) laser facility. Alternative use of two different varied-line-spacing gratings covers a wavelength range of 10–260 Å. A newly developed x-ray framing camera with single wide strip line is designed to record time-gated spectra with ∼70 ps temporal resolution and 20 lp/mm spatial resolution. The width of the strip line is up to 20 mm, enhancing the capability of the spatial resolving measurements. All components of the x-ray framing camera are roomed in an aluminum air box. The whole spectrometer is mounted on a diagnostic instrument manipulator at the SG-III laser facility for the first time. A new alignment method for the spectrometer based on the superimposition of two laser focal spots is developed. The approaches of the alignment including offline and online two steps are described. A carbon spectrum and an aluminum spectrum have been successfully recorded by the spectrometer using 2400 l/mm and 1200 l/mm gratings, respectively. The experimental spectral lines show that the spectral resolution of the spectrometer is about 0.2 Å and 1 Å for the 2400 l/mm and 1200 l/mm gratings, respectively. A theoretical calculation was carried out to estimate the maximum resolving power of the spectrometer. © 2015 Optical Society of America OCIS codes: (050.1950) Diffraction gratings; (110.2970) Image detection systems; (120.4640) Optical instruments; (300.6360) Spectroscopy, laser; (340.7480) X-rays, soft x-rays, extreme ultraviolet (EUV). http://dx.doi.org/10.1364/AO.54.005339
1. INTRODUCTION Extreme ultraviolet (EUV) spectrum plays an important role in laser-produced plasmas [1–7], electron beam ion trap (EBIT) [8–10], tokamak [11,12] and astronomy [13–16]. The EUV range contains lots of interesting spectral lines, such as the K -shell lines of carbon and oxygen, the L-shell lines of argon and aluminum, the M -shell lines of gold, and so on. The EUV spectral lines provide us lots of important information without perturbing the host plasma. The line intensity ratios, linewidths, and bound-free continuum emission can be used for the temperature and density diagnostics. The measurement of EUV x-rays also enables a identification of the bound-bound emission lines from different ion species. Concerning the inertial confinement fusions (ICF) plasmas, a most important application of the EUV spectrum is the measurement of the radiation opacity, which is crucial to energy transport in hightemperature plasmas [17–23]. The EUV spectrum is important in radiation opacity measurements because the radiative energy flow around the maximum of the Planck distribution lies in this 1559-128X/15/175339-07$15/0$15.00 © 2015 Optical Society of America
spectral range [17]. The measurement of opacity [22,23] is usually achieved by measuring the transmission spectrum in a short time duration (∼200 ps) when the status of the plasma is almost unchanged. The transmission spectrum is obtained by comparing the absorption and the backlight spectra which are needed to measure in one shot simultaneously. Hence, a precise opacity measurement requires that the spectrometer must be spectrally, temporally, and spatially resolved. A varied line-space flat-field grating is the key component for a EUV spectrometer to provide the spectral resolution. The early studies of the varied line-space flat-field grating can be ascended to the 1980s [24–27]. In recent decades, great effort has been made to develop EUV spectrometers based on the grating. Beiersdorfer et al. [8] described a EUV spectrometer for the EBIT-I in the spectral region from below 10 up to 50 Å using a 2400 l/mm grating. Later, they [11] developed a new spectrometer in the 6–65 Å spectral region for use on the National Spherical Torus Experiment. Blagojević et al. [9] built a spectrometer in the spectral range from 40 Å to 400 Å for use with the National Institute of Standards and Technology
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(NIST) EBIT by using a 1200 l/mm grating. Chowdhuri et al. reported the characteristics of the spectrometers for the large helical device plasmas in the spectral range of 10–130 Å [10] and 50–500 Å [12] by using the 2400 l/mm and 1200 l/mm gratings, respectively. All the spectrometers described above were coupled to a charge-coupled device (CCD) to complete a time integral measurement. In [11], a shutter was placed in front of the entrance slit to provide a time resolution of about 30 ms. As for laser-produced plasmas, the spectrometer often needs to couple with a framing camera or a stream camera to obtain temporal resolution in the picosecond range [4,21]. We have carried out several opacity measurements on iron and gold plasmas at the Shenguang-II (SG-II) laser facility in recent years by using a spectrometer that consists of a 2400 l/mm flat-field grating, an x-ray framing camera, and an imaging slit [21–23]. Besides the construction of spectrometers, several works concerned the absolute calibrations. Saemann and Edimann [3], Schwanda et al. [4], and Fujikawa et al. [6] calibrated their spectrometers by means of a laser-generated plasma. The spectrum emitted by a laser-irradiated target was measured by the flat-field grating spectrometer (FFS) and a transmission grating spectrometer (TGS) with known efficiency simultaneously. The absolute efficiency of the FFS was obtained by normalized its intensity to the TGS. Park et al. [5] calibrated their spectrometer in a similar way, but normalized the spectrometer to an x-ray microcalorimeter spectrometer at a EBIT. Chowdhuri et al. first normalized their 1200 l/mm flat-field grating spectrometer to an absolutely calibrated visible line [12], and then calibrated their 2400 l/mm grating spectrometer by using the 1200 l/mm grating spectrometer [10]. Besides experimental measurements, there were also some theoretical calculations concerning the grating efficiency [4,28]. A common characteristic of these spectrometers introduced above, including our spectrometer for the SG-II laser facility, is that they are fixed-port diagnostics which mount directly to the experimental target chamber. It is relatively convenient to operate our spectrometer at the SG-II laser facility for the smaller size of the target room compared to the Shenguang-III (SG-III) laser facility, which will be the largest laser facility for ICF research in China [29]. A red laser beam transmitting against the incident x-ray path was used for the alignment of the spectrometer for the SG-II laser facility. One can open a flange window and leave the spectrometer in the atmosphere during the alignment, and can even monitor the alignment online and adjust the pose of the spectrometer just standing nearby the target room. Furthermore, the microchannel plate (MCP) and CCD of the framing camera are located outside the target room. So one can control the camera and acquire experimental data conveniently, without considering the impact of the vacuum. However, the fixed-port EUV spectrometer is not convenient any more at the SG-III laser facility [29]. The SG-III laser facility will be a 48-beam frequency-tripled (λ 0.35 μm) laser system. The total energy SG-III laser facility is designed as 0.2 MJ in 3 ns pulse. The target chamber of the SG-III laser facility is as large as about 6 m in diameter. The construction of the SG-III laser facility will be completed soon and several bundles of the laser beams are operational. The associated diagnostic instruments and techniques are under development
simultaneously. The EUV spectrometer, which is aimed at the opacity measurements primarily, is one of the core instruments for the SG-III laser facility. There are numerous ports in the SG-III chamber wall for laser beams entrance and fixed diagnostic installations. However, one has to wait for a long time to ensure a high vacuum degree of the SG-III target chamber when changing a fixed diagnostic. In fact, opening a flange window or destroying the vacuum of the chamber target are usually not permitted. Therefore, the EUV spectrometer for the SG-III laser facility requires operating in a vacuum environment. It is also demands a quick and convenient change, adjusting or repairing the spectrometer without disturbing the undergoing experiment. As a result, instead of mounting at a fix-port, a EUV spectrometer with insert diagnostics is desired for the SG-III laser facility. Recently, several diagnostic instrument manipulators (DIMs) [30], which are similar with the design at the National Ignition Facility (NIF) (DIM [31]) and Laser Mégajoule (SID [32]), have been mounted at the SG-III laser facility. The DIM at the SG-III laser facility can be operated without destroying the vacuum of the target chamber and provides the capability to place an instrument in a precise position [30]. According to the characteristics of the SG-III target chamber and taking advantage of the DIM, we designed a EUV spectrometer in the spectral range of 10–260 Å for the SG-III laser facility. The spectrometer is designed to ride on the DIM and consists of the existent 1200 l/mm and 2400 l/gratings, but a newly developed x-ray framing camera in an air box, the support and adjustment framework, and the alignment process. This paper will describe the EUV spectrometer in detail. Sample spectra recorded by the spectrometer are presented. The spectral resolving power of the spectrometer is discussed in the end. 2. SPECTROMETER DESIGN A. Appearance
The spectrometer consists of flat-field gratings, an x-ray framing camera in an air box, a set of alignment systems, and some accessories, such as slit, filter, and so on. The schematic of the spectrometer is shown in Fig. 1. All the parts in front of the air box are named as the support and adjustment framework (SAF) for a convenient description. Since the DIM cart cannot reach the target very closely, the SAF is designed as suspended. The air box is heavy enough to support the SAF. The SAF is designed to be slim to reduce its weight, but with thick walls
DIM Air Box
Creeping flange Grating encloser Extension tube Spatial resolving slit Optical Pointers 1# & 2#
Optical pointer 0# (enclosed) Entrance slit (enclosed)
Grating DIM cart
Fig. 1. Schematic of the spectrometer.
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to keep its rigidity. The air box is then fixed on the DIM cart to hold the whole spectrometer stable. The grating is located at a grating encloser. A 1200 l/mm and a 2400 l/mm holographic flat-field gratings are used to disperse the 10–80 Å and 40–260 Å spectral lines, respectively. Two separate grating enclosers are designed for the two gratings, respectively. These gratings have successfully applied on the opacity measurements at the SG-II laser facility. The absolute efficiency of the spectrometer at difference photon energy is not the key issue for the opacity measurement since the efficiency has been eliminated in the transmission spectrum. However, it is useful to know the grating efficiency for preparing an experiment, i.e., choosing the proper gain of the camera and configuring the filters. The works of the pioneers have shown that the order of magnitude of the first-order diffraction efficiency is approximately 10−2 [3,4]. The higher-order diffraction will disturb the spectrum measurement when overlapping with the first-order diffraction. The holographic flat-field gratings employed in the present spectrometer have the ability to suppress the higher-order diffraction, as will be demonstrated in the sample spectrum in Section 3. An entrance slit of about 30 μm is mounted in the front of the grating encloser. The incident angle is 1.35 0.05° for the 2400 l/mm grating and 3 0.05° for the 1200 l/mm grating. The positions of the slit and the grating [the geometrical relationship of r and h labeled in Fig. 2(a)] are finely set via precision machining to assure the accuracy of the incident angle for the two gratings. The distance between the center of the grating and the recorder [l in Fig. 2(a)] can be adjusted by a creeping flange, as shown in Fig. 1. The designed value of this distance is 235 mm and the accuracy of the creeping flange is 0.1 mm. The relative position in the vertical direction between the air box and the creeping flange is carefully designed to assure that the diffract x-ray can reach the recorder (strip line). An imaging slit can be placed at the most front of the spectrometer if we need spatial resolving measurements. An extension tube is used to deliver the imaging slit at a desired position according to the magnification. Two optical pointers used for alignment are mounted at the front of the spectrometer.
Fig. 2. Alignment of the spectrometer. (a) The sketch of the grating encloser and the offline alignment. (b) The superimposition of the two laser spots from the optical pointers (made of a semiconductor laser [not shown] and focusing lenses imaging the end of an optical fiber). (c) The target used for alignment and the focal spots of two optical pointers as seen in the direction of a target positioning video camera.
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B. Alignment
The alignment of the spectrometer is based on the superimposition of two continuous laser spots at the center of a target. This method has been successfully applied at the NIF [33] and Ligne d’Intégration Laser facility [34]. Two steps (offline and online) are needed for the entire procedure of the present alignment. The purpose of the offline alignment is to adjust the light paths of optical pointers 1# and 2# (see Fig. 1) to assure the two lights are all aimed at an imitation target. Each of the optical pointers consists of a semiconductor laser (λ 635 nm), a single-mode optical fiber, and a combination of focusing lenses. The position of the imitation target is dependent on the incident x-ray light. A backward laser against the path of the incident x-ray is emitted by another optical pointer (0# in Fig. 1) to assure that the imitation target is on the path of the incident x-ray. The optical pointer 0# is very small and will not stop the diffracted x-rays reaching the recorder. The grating is replaced with a collimation block in the alignment procedure. The alignment block is designed to assure a hole (1 mm in diameter) is located at the center of the grating when the block is pressed into the grating encloser. We adjust the posture of optical pointer 0# to assure the laser transmits through the hole and the center of the entrance slit simultaneously. Then we place the imitation target in the light path. The distance of the target to the entrance slit is designed as 600 and 1000 mm for 2400 l/mm and 1200 l/mm gratings, respectively, according to the spatial magnification of the spectrometer and the limit of the DIM. It is not necessary to control these distances exactly the same as the designed values as long as the imitation target is in the light path. An image slit can be inserted into the light path for spatial resolving measurement. Then adjust optical pointers 1# and 2# to assure the lasers reach the target. The optical pointer 0# can be turned off when finishing the offline alignment. A telescope is employed to monitor the laser spot, the target, the image slit, and so on, to improve the alignment precision. In the online alignment procedure, a plane target [Fig. 2(c)] with concentric circles is placed at the source point in the chamber of the SG-III laser facility. We move the end of the DIM in x, y and forward or backward the car to make sure the laser spots move to the center of the circles. The position of two laser focal spots is viewed by the target positioning video cameras. When the two laser focal spots superimpose on the center of the target, the alignment is finished. The spectrometer was put out and drawn back several times with the help of the DIM during the several days of experimental time. We checked the laser focal spots every time when putting out the spectrometer and found the spots always superimpose on the center of the target. Thus the two optical pointers can be moved out when the alignment is finished if there is a requirement to reduce the solid angle of the spectrometer. The precision of the DIM motions and the rigidity of the SAF are sufficient to reproduce the position of the spectrometer without the assistance of the optical pointers. C. X-Ray Framing Camera
The recorder of the spectrometer is an x-ray framing camera, which consists of an air box, a MCP imager, an electric control system, an optical CCD camera system, and an embedded
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computer. In order to prevent the x-ray framing camera from being damaged at high vacuum where the DIM is operated, an air box is used to contain all of the electronic components at an atmospheric pressure. The air box also acts as a Faraday shield to prevent the electronics inside the box from electromagnetic interference. The air box is made of aluminum material to reduce weight and neutron activation. The size of the air box of about 200 mm × 200 mm × 1500 mm is designed to fit the DIM cart. In order to satisfy the requirement of spatial resolving measurement, the MCP module contains only a single gating strip line but with large area, especially the width, which is up to 20 mm. The length of the strip line is 36 mm. The phosphor screen can be active by using either dc voltages of 3.5 and 4.0 kV or pulses voltages of 3.5 and 4.5 kV. Owing to the wide width (20 mm) of the strip line, four gating electrical pulses were employed to drive the whole strip line simultaneously to obtain a uniform gain. A high voltage ramp pulse generated by avalanche transistor circuit was used to drive two sharpening avalanche diode circuits [35]. Each of the sharpening avalanche diode circuits can produce two picosecond gating pulses. As a result, we finally obtained four gating electrical pulses. Four coaxial cables were used to deliver the four pulses to the MCP via SMA (subminiature version A)-type miniature connectors. The times of the four pulses arriving at the strip line are required to be almost the same to drive the strip line simultaneously. An oscilloscope was employed to monitor the arriving times of the four gating pulses. By way of adjusting the lengths of the coaxial cables, we finally controlled the difference between the arriving times within 20 ps. An electrical trigger provided by the laser facility is used to activate the camera and start the experimental measurement. The original trigger signal combining with the output of the trigger from the camera are fed to a high-bandwidth oscilloscope for monitoring. The four gating pulses can be delayed relative to the trigger up to 25.55 ns in 50 ps steps to record the experimental image at a desired time. A liquid-cooled CCD camera is coupled to the back of the MCP to capture an image electronically. The size of the CCD is 4100 × 4100 pixels, and each pixel size is 9 μm. A computer (PC104) is embedded in the air box to control most of the electrical functions, including the phosphor screen voltage, gating pulses, trigger, data acquisition and save, the environment monitoring (gas pressure, temperature, and humidity), and so on. A remote computer in the control room is employed to communicate with this embedded computer via fiber-optic ethernet. The control code lying in the embedded computer was developed using the C++ language. The temporal and spatial resolutions of the camera have been tested experimentally. The exposure time measurement was similar to [35]. An ultraviolet laser (266 nm) with 30 ps duration was used as the calibration light source. A fiber bunch that contained 30 fibers with gradual increase lengths was used to transmit the ultraviolet laser. The lengths of the fibers were controlled carefully to make sure that the laser transmission time between two neighbor fibers was increased by 30 ps. The 30 laser points transmitted through the fiber bunch were imaged on the strip line photocathode of the MCP. The camera was triggered by another 532 nm laser beam to obtain a
dynamical image. The intensities of the laser points were fitted with a Gaussian profile and the full width at half-maximum (FWHM) was taken as the temporal resolution of the camera. A static image was also obtained to eliminate the impact of the spatial nonuniformity of the MCP. The fitting result shows that the temporal resolution ∼70 ps. The uncertainty is roughly estimated as 17%, containing the contributions of the arriving time of laser point (less than 15%) and the fitting procedure (9%). The spatial resolution was measured with a quartz resolution test target, which was illuminated by an ultraviolet planar lamp and imaged on the camera via a collimator tube. The spatial resolution was obtained by observing and determining the maximum number of the clear line pairs on the image. The result indicates that spatial resolution is 20 lp/mm. The minimum change of line pairs of the quartz resolution test target can be taken as the accuracy of the spatial resolution, which is 1 lp/mm. 3. SAMPLE SPECTRUM The spectrometer has been evaluated at the SG-III laser facility. The image slit was removed in the evaluation for simply. A CH and an aluminum (Al) planar targets were used for 2400 l/mm and 1200 l/mm flat-field gratings, respectively. The eight frequency-tripled beams (λ 0.35 μm) were focused on the targets to create x-ray radiations. The energies were 4 × 100 J in 1 ns for the CH target and 8 × 1000 J in 1 ns for the Al target. The spectrum measured with the 2400 l/mm grating is shown in Fig. 3. The spectrum is dominated by the K -shell transmissions of the Ly-like and He-like C ions. The Ly-α, Ly-β, Ly-γ, He-α, and He-β of the C ions are successfully observed, as labeled in the figure. The higher-order lines are well suppressed. Spectral lines at 56.9 Å, 67.5 Å, and 80.5 Å are the second-order lines of the Ly-β, Ly-α, and He-α lines, respectively. The spectrum shows that the contribution of second-order lights can almost be neglected since they are just above the continuous spectrum. The spectral resolving power derived from λ∕Δλ of the Ly-β line is about 180. The Δλ is the FWHM of a spectral line. Figure 4 shows the Al spectrum measured with the 1200 l/mm grating. Lots of Al spectral lines are clearly observed in the figure. Similarly, obvious higher-order diffractions are not found in the spectrum. A few works concerning on the L–shell Al spectrum were reported [36,37]. We referred to the atomic spectra database provided by the
Fig. 3. Carbon spectrum recorded by the spectrometer using a 2400 l/mm grating.
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Fig. 4. Aluminum spectrum recorded by the spectrometer using a 1200 l/mm grating.
NIST [38], a theoretical calculation [36], and a experimental result from an EBIT [37] to analyze the Al spectrum. Most of the lines are emitted from the L-shell transmissions (2p − 3d , 2p − 3s, 2s − 3p, etc.) of the Be-like to Ne-like Al ions. In addition, serval lines emitted from K -shell transitions (2p − 4d ) of Be-like O ions [38] are observed (labeled with 1 and 2 in Fig. 4), which are caused by the aluminum oxide. In some cases, there is only one transmission accounting for an experimental line. For example, the neon-like Al IV has a fairly simple spectrum, dominated by the well-known lines commonly labeled 3F and 3G. The Al IV lines 3F and 3G refer to (2p1∕2 3s1∕2 J 1 → 2p43∕2 J 0) and (2p31∕2 3s 1∕2 J 1 → 2p43∕2 J 0) transitions at 160.07 Å and 161.69 Å, respectively [37]. However, in most instances, the lines observed in Fig. 4 are a blend of several transitions. More detailed analysis of the spectral lines is ongoing. The spectral resolving power given by λ∕Δλ is shown in Figs. 5(a) and 5(b) for the 2400 l/mm and 1200 l/mm gratings, respectively. The open circles stand for the experimental values. The uncertainty contains the impact of the continuum spectrum and error of the linewidth fit procedure. As shown in the figure, the spectral resolving powers obtained from the experimental lines are around 200 and 150 for the 2400 l/mm and 1200 l/mm gratings, respectively. The limit of the spectral
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resolving power in the first diffraction is given by the number of total grooves [4]. However, the practical spectral resolving power is quite lower due to several limitations, such as the width of the entrance slit, the size of the x-ray source, the imaging aberration of the grating, and the resolution limit of the x-ray framing camera. Another important truth of the spectra emitted from the laser plasmas with high density and temperature is that the broaden and blend of transitions will increase the width of the observed lines significantly. As a result, the experimental spectral resolving power obtained from the ratio of the wavelength and the linewidth will be lower than the actual value. In order to estimate the actual resolving power, a theoretical calculation was carried out. The wavelength resolution of the spectrometer can be written as [4] dλ Δx ; (1) d x min where Δx min is the minimum width of a spectral line on the recorder, d λ∕d x is the inverse linear dispersion, which can be deduced from the grating equation Δλ
mλ d 0 sin α − sin β;
(2)
where m is the diffraction order, λ is the wavelength, d 0 is the nominal groove spacing, α is the incident angle, and β is the diffraction angle, which is given by β arctanl ∕x;
(3)
where l 235 mm is the distance between the center of the grating and the recorder and x is the position of a spectral line on the recorder [l and x are labeled in Fig. 2(a)]. The differential equation can be derived from Eqs. (2) and (3) as dλ
d 0 T λ d x; l
(4)
where T λ ≡
t ; 1 − t1 t∕1 − t3∕2
and t ≡ sin β2
λ 2 sin α − : d0
(5)
(6)
Substituting Eq. (4) into Eq. (1), we can obtain the spectral resolving power as λ λl 1 : Δλ d 0 T λ Δx min
Fig. 5. Experimentally obtained spectral resolving power λ∕Δλ (Δλ, full width at half-maximum) accompanying with the theoretical calculations.
(7)
Then the calculation of resolving power λ∕Δλ becomes to the estimation of the Δx min . The width of the entrance slit constrain the Δx min will not be smaller than 30 μm. The theoretical calculations carried out by Nakano et al. [24] have shown that a point source contributes a width Δx min 7 μm. The imaging aberration of the grating is relatively small and will not be taken into account here. Owning to the finite spatial resolution of 20 lp/mm, the x-ray framing camera contributes a maximum of Δx min 50 μm, which is finally taken as the resolution limitation of the spectrometer. The linewidth of 50 μm corresponds to about 5.5 pixels of the CCD imager. The Δx min is taken as a constant in the present wavelength range 10 –260 Å for simplicity. This simplify is reasonable since
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Chowdhuri et al. found the Δx was almost the same at different wavelengths for their spectrometers [10,12]. The wavelength ranges were 10–130 Å [10] and 150–300 Å [12] for their 2400 l/mm and 1200 l/mm grating spectrometers, respectively. The Δx was about four channels of the CCD detector (full width at foot position) for both of their ruled gratings. The holographic gratings showed similar results as Chowdhuri’s works [10,12]. The theoretical results of the resolving power limitation are plotted with red dashed lines in Figs. 5(a) and 5(b) for 2400 l/mm and 1200 l/mm gratings, respectively. The average experimental widths of the spectral lines are about 9 pixels (81 μm) and 15 pixels (135 μm) for 2400 l/mm and 1200 l/mm gratings, respectively. Taking these values as the Δx min in Eq. (7), we obtain the average experimental resolving powers of the spectrometer in the present experimental conditions, as shown with blue solid lines in Fig. 5. As discussed above, the lower experimental resolving power is mostly attributed to the blend and broadening of the transitional spectral lines. The blend is more severe for the L-shell aluminum spectrum (Fig. 4) and the strong continuum spectrum also enlarges the uncertainty of the linewidth. The average experimental resolving power given in Fig. 5 proved us a helpful reference value when designing an experiment where the spectral resolution is important. 4. SUMMARY AND OUTLOOK A newly designed extreme ultraviolet spectrometer with spectral range of 10–260 Å for the SG-III laser facility has been presented. The spectrometer is designed to be fielded in a DIM. A new x-ray framing camera roomed in an air box and a new alignment procedure are developed for the spectrometer. The temporal and spatial resolutions of the framing camera are ∼70 ps and 20 lp/mm. The experimental spectra measured by the spectrometer show that the spectral resolving power is about 200 and 150 in the present experimental conditions for 2400 l/mm and 1200 l/mm gratings, respectively. The capability of the spectrometer will be especially valuable for the radiation opacity experiments in the extreme ultraviolet band after the construction of the SG-III laser facility is completed. It is noted that substitution of the laser pointers (1# and 2#, see Fig. 2) for CCD cameras with high spatial resolution will improve the accuracy of the alignment significantly [39], and this will be our next work. Although the relative measurement is used in the opacity experiment and the efficiency of the whole system is removed in the transmission spectrum, the response of the spectrometer as a function of the x-ray intensity is interesting since strong x-ray intensity may drive the spectrometer into nonlinear response [40]. The calibration concerning the response will be our future work. China Academy of Engineering Physics (2014B0102011, 2014B0102012); National Natural Science Foundation of China (11404303). The authors would like to thank the target fabrication, the laser operation, and the experiment diagnostics staffs for co-operation.
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Extreme ultraviolet spectrometer for the Shenguang III laser facility GANG XIONG,1 GUOHONG YANG,1,* JIYAN ZHANG,1 MINXI WEI,1 YANG ZHAO,1 BO QING,1 MIN LV,1 ZHENGHUA YANG,1 FENG WANG,1 SHENYE LIU,1 HOUZHI CAI,2 AND JINYUAN LIU2 1
Research Center of Laser Fusion, China Academy of Engineering Physics, P. O. Box 919-986, Mianyang 621900, China Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, Institute of Optoelectronics, Shenzhen University, Shenzhen 518060, China *Corresponding author: [email protected]
2
Received 18 February 2015; revised 10 May 2015; accepted 12 May 2015; posted 12 May 2015 (Doc. ID 234756); published 4 June 2015
An extreme ultraviolet spectrometer has been developed for high-energy density physics experiments at the Shenguang-III (SG-III) laser facility. Alternative use of two different varied-line-spacing gratings covers a wavelength range of 10–260 Å. A newly developed x-ray framing camera with single wide strip line is designed to record time-gated spectra with ∼70 ps temporal resolution and 20 lp/mm spatial resolution. The width of the strip line is up to 20 mm, enhancing the capability of the spatial resolving measurements. All components of the x-ray framing camera are roomed in an aluminum air box. The whole spectrometer is mounted on a diagnostic instrument manipulator at the SG-III laser facility for the first time. A new alignment method for the spectrometer based on the superimposition of two laser focal spots is developed. The approaches of the alignment including offline and online two steps are described. A carbon spectrum and an aluminum spectrum have been successfully recorded by the spectrometer using 2400 l/mm and 1200 l/mm gratings, respectively. The experimental spectral lines show that the spectral resolution of the spectrometer is about 0.2 Å and 1 Å for the 2400 l/mm and 1200 l/mm gratings, respectively. A theoretical calculation was carried out to estimate the maximum resolving power of the spectrometer. © 2015 Optical Society of America OCIS codes: (050.1950) Diffraction gratings; (110.2970) Image detection systems; (120.4640) Optical instruments; (300.6360) Spectroscopy, laser; (340.7480) X-rays, soft x-rays, extreme ultraviolet (EUV). http://dx.doi.org/10.1364/AO.54.005339
1. INTRODUCTION Extreme ultraviolet (EUV) spectrum plays an important role in laser-produced plasmas [1–7], electron beam ion trap (EBIT) [8–10], tokamak [11,12] and astronomy [13–16]. The EUV range contains lots of interesting spectral lines, such as the K -shell lines of carbon and oxygen, the L-shell lines of argon and aluminum, the M -shell lines of gold, and so on. The EUV spectral lines provide us lots of important information without perturbing the host plasma. The line intensity ratios, linewidths, and bound-free continuum emission can be used for the temperature and density diagnostics. The measurement of EUV x-rays also enables a identification of the bound-bound emission lines from different ion species. Concerning the inertial confinement fusions (ICF) plasmas, a most important application of the EUV spectrum is the measurement of the radiation opacity, which is crucial to energy transport in hightemperature plasmas [17–23]. The EUV spectrum is important in radiation opacity measurements because the radiative energy flow around the maximum of the Planck distribution lies in this 1559-128X/15/175339-07$15/0$15.00 © 2015 Optical Society of America
spectral range [17]. The measurement of opacity [22,23] is usually achieved by measuring the transmission spectrum in a short time duration (∼200 ps) when the status of the plasma is almost unchanged. The transmission spectrum is obtained by comparing the absorption and the backlight spectra which are needed to measure in one shot simultaneously. Hence, a precise opacity measurement requires that the spectrometer must be spectrally, temporally, and spatially resolved. A varied line-space flat-field grating is the key component for a EUV spectrometer to provide the spectral resolution. The early studies of the varied line-space flat-field grating can be ascended to the 1980s [24–27]. In recent decades, great effort has been made to develop EUV spectrometers based on the grating. Beiersdorfer et al. [8] described a EUV spectrometer for the EBIT-I in the spectral region from below 10 up to 50 Å using a 2400 l/mm grating. Later, they [11] developed a new spectrometer in the 6–65 Å spectral region for use on the National Spherical Torus Experiment. Blagojević et al. [9] built a spectrometer in the spectral range from 40 Å to 400 Å for use with the National Institute of Standards and Technology
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(NIST) EBIT by using a 1200 l/mm grating. Chowdhuri et al. reported the characteristics of the spectrometers for the large helical device plasmas in the spectral range of 10–130 Å [10] and 50–500 Å [12] by using the 2400 l/mm and 1200 l/mm gratings, respectively. All the spectrometers described above were coupled to a charge-coupled device (CCD) to complete a time integral measurement. In [11], a shutter was placed in front of the entrance slit to provide a time resolution of about 30 ms. As for laser-produced plasmas, the spectrometer often needs to couple with a framing camera or a stream camera to obtain temporal resolution in the picosecond range [4,21]. We have carried out several opacity measurements on iron and gold plasmas at the Shenguang-II (SG-II) laser facility in recent years by using a spectrometer that consists of a 2400 l/mm flat-field grating, an x-ray framing camera, and an imaging slit [21–23]. Besides the construction of spectrometers, several works concerned the absolute calibrations. Saemann and Edimann [3], Schwanda et al. [4], and Fujikawa et al. [6] calibrated their spectrometers by means of a laser-generated plasma. The spectrum emitted by a laser-irradiated target was measured by the flat-field grating spectrometer (FFS) and a transmission grating spectrometer (TGS) with known efficiency simultaneously. The absolute efficiency of the FFS was obtained by normalized its intensity to the TGS. Park et al. [5] calibrated their spectrometer in a similar way, but normalized the spectrometer to an x-ray microcalorimeter spectrometer at a EBIT. Chowdhuri et al. first normalized their 1200 l/mm flat-field grating spectrometer to an absolutely calibrated visible line [12], and then calibrated their 2400 l/mm grating spectrometer by using the 1200 l/mm grating spectrometer [10]. Besides experimental measurements, there were also some theoretical calculations concerning the grating efficiency [4,28]. A common characteristic of these spectrometers introduced above, including our spectrometer for the SG-II laser facility, is that they are fixed-port diagnostics which mount directly to the experimental target chamber. It is relatively convenient to operate our spectrometer at the SG-II laser facility for the smaller size of the target room compared to the Shenguang-III (SG-III) laser facility, which will be the largest laser facility for ICF research in China [29]. A red laser beam transmitting against the incident x-ray path was used for the alignment of the spectrometer for the SG-II laser facility. One can open a flange window and leave the spectrometer in the atmosphere during the alignment, and can even monitor the alignment online and adjust the pose of the spectrometer just standing nearby the target room. Furthermore, the microchannel plate (MCP) and CCD of the framing camera are located outside the target room. So one can control the camera and acquire experimental data conveniently, without considering the impact of the vacuum. However, the fixed-port EUV spectrometer is not convenient any more at the SG-III laser facility [29]. The SG-III laser facility will be a 48-beam frequency-tripled (λ 0.35 μm) laser system. The total energy SG-III laser facility is designed as 0.2 MJ in 3 ns pulse. The target chamber of the SG-III laser facility is as large as about 6 m in diameter. The construction of the SG-III laser facility will be completed soon and several bundles of the laser beams are operational. The associated diagnostic instruments and techniques are under development
simultaneously. The EUV spectrometer, which is aimed at the opacity measurements primarily, is one of the core instruments for the SG-III laser facility. There are numerous ports in the SG-III chamber wall for laser beams entrance and fixed diagnostic installations. However, one has to wait for a long time to ensure a high vacuum degree of the SG-III target chamber when changing a fixed diagnostic. In fact, opening a flange window or destroying the vacuum of the chamber target are usually not permitted. Therefore, the EUV spectrometer for the SG-III laser facility requires operating in a vacuum environment. It is also demands a quick and convenient change, adjusting or repairing the spectrometer without disturbing the undergoing experiment. As a result, instead of mounting at a fix-port, a EUV spectrometer with insert diagnostics is desired for the SG-III laser facility. Recently, several diagnostic instrument manipulators (DIMs) [30], which are similar with the design at the National Ignition Facility (NIF) (DIM [31]) and Laser Mégajoule (SID [32]), have been mounted at the SG-III laser facility. The DIM at the SG-III laser facility can be operated without destroying the vacuum of the target chamber and provides the capability to place an instrument in a precise position [30]. According to the characteristics of the SG-III target chamber and taking advantage of the DIM, we designed a EUV spectrometer in the spectral range of 10–260 Å for the SG-III laser facility. The spectrometer is designed to ride on the DIM and consists of the existent 1200 l/mm and 2400 l/gratings, but a newly developed x-ray framing camera in an air box, the support and adjustment framework, and the alignment process. This paper will describe the EUV spectrometer in detail. Sample spectra recorded by the spectrometer are presented. The spectral resolving power of the spectrometer is discussed in the end. 2. SPECTROMETER DESIGN A. Appearance
The spectrometer consists of flat-field gratings, an x-ray framing camera in an air box, a set of alignment systems, and some accessories, such as slit, filter, and so on. The schematic of the spectrometer is shown in Fig. 1. All the parts in front of the air box are named as the support and adjustment framework (SAF) for a convenient description. Since the DIM cart cannot reach the target very closely, the SAF is designed as suspended. The air box is heavy enough to support the SAF. The SAF is designed to be slim to reduce its weight, but with thick walls
DIM Air Box
Creeping flange Grating encloser Extension tube Spatial resolving slit Optical Pointers 1# & 2#
Optical pointer 0# (enclosed) Entrance slit (enclosed)
Grating DIM cart
Fig. 1. Schematic of the spectrometer.
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to keep its rigidity. The air box is then fixed on the DIM cart to hold the whole spectrometer stable. The grating is located at a grating encloser. A 1200 l/mm and a 2400 l/mm holographic flat-field gratings are used to disperse the 10–80 Å and 40–260 Å spectral lines, respectively. Two separate grating enclosers are designed for the two gratings, respectively. These gratings have successfully applied on the opacity measurements at the SG-II laser facility. The absolute efficiency of the spectrometer at difference photon energy is not the key issue for the opacity measurement since the efficiency has been eliminated in the transmission spectrum. However, it is useful to know the grating efficiency for preparing an experiment, i.e., choosing the proper gain of the camera and configuring the filters. The works of the pioneers have shown that the order of magnitude of the first-order diffraction efficiency is approximately 10−2 [3,4]. The higher-order diffraction will disturb the spectrum measurement when overlapping with the first-order diffraction. The holographic flat-field gratings employed in the present spectrometer have the ability to suppress the higher-order diffraction, as will be demonstrated in the sample spectrum in Section 3. An entrance slit of about 30 μm is mounted in the front of the grating encloser. The incident angle is 1.35 0.05° for the 2400 l/mm grating and 3 0.05° for the 1200 l/mm grating. The positions of the slit and the grating [the geometrical relationship of r and h labeled in Fig. 2(a)] are finely set via precision machining to assure the accuracy of the incident angle for the two gratings. The distance between the center of the grating and the recorder [l in Fig. 2(a)] can be adjusted by a creeping flange, as shown in Fig. 1. The designed value of this distance is 235 mm and the accuracy of the creeping flange is 0.1 mm. The relative position in the vertical direction between the air box and the creeping flange is carefully designed to assure that the diffract x-ray can reach the recorder (strip line). An imaging slit can be placed at the most front of the spectrometer if we need spatial resolving measurements. An extension tube is used to deliver the imaging slit at a desired position according to the magnification. Two optical pointers used for alignment are mounted at the front of the spectrometer.
Fig. 2. Alignment of the spectrometer. (a) The sketch of the grating encloser and the offline alignment. (b) The superimposition of the two laser spots from the optical pointers (made of a semiconductor laser [not shown] and focusing lenses imaging the end of an optical fiber). (c) The target used for alignment and the focal spots of two optical pointers as seen in the direction of a target positioning video camera.
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B. Alignment
The alignment of the spectrometer is based on the superimposition of two continuous laser spots at the center of a target. This method has been successfully applied at the NIF [33] and Ligne d’Intégration Laser facility [34]. Two steps (offline and online) are needed for the entire procedure of the present alignment. The purpose of the offline alignment is to adjust the light paths of optical pointers 1# and 2# (see Fig. 1) to assure the two lights are all aimed at an imitation target. Each of the optical pointers consists of a semiconductor laser (λ 635 nm), a single-mode optical fiber, and a combination of focusing lenses. The position of the imitation target is dependent on the incident x-ray light. A backward laser against the path of the incident x-ray is emitted by another optical pointer (0# in Fig. 1) to assure that the imitation target is on the path of the incident x-ray. The optical pointer 0# is very small and will not stop the diffracted x-rays reaching the recorder. The grating is replaced with a collimation block in the alignment procedure. The alignment block is designed to assure a hole (1 mm in diameter) is located at the center of the grating when the block is pressed into the grating encloser. We adjust the posture of optical pointer 0# to assure the laser transmits through the hole and the center of the entrance slit simultaneously. Then we place the imitation target in the light path. The distance of the target to the entrance slit is designed as 600 and 1000 mm for 2400 l/mm and 1200 l/mm gratings, respectively, according to the spatial magnification of the spectrometer and the limit of the DIM. It is not necessary to control these distances exactly the same as the designed values as long as the imitation target is in the light path. An image slit can be inserted into the light path for spatial resolving measurement. Then adjust optical pointers 1# and 2# to assure the lasers reach the target. The optical pointer 0# can be turned off when finishing the offline alignment. A telescope is employed to monitor the laser spot, the target, the image slit, and so on, to improve the alignment precision. In the online alignment procedure, a plane target [Fig. 2(c)] with concentric circles is placed at the source point in the chamber of the SG-III laser facility. We move the end of the DIM in x, y and forward or backward the car to make sure the laser spots move to the center of the circles. The position of two laser focal spots is viewed by the target positioning video cameras. When the two laser focal spots superimpose on the center of the target, the alignment is finished. The spectrometer was put out and drawn back several times with the help of the DIM during the several days of experimental time. We checked the laser focal spots every time when putting out the spectrometer and found the spots always superimpose on the center of the target. Thus the two optical pointers can be moved out when the alignment is finished if there is a requirement to reduce the solid angle of the spectrometer. The precision of the DIM motions and the rigidity of the SAF are sufficient to reproduce the position of the spectrometer without the assistance of the optical pointers. C. X-Ray Framing Camera
The recorder of the spectrometer is an x-ray framing camera, which consists of an air box, a MCP imager, an electric control system, an optical CCD camera system, and an embedded
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computer. In order to prevent the x-ray framing camera from being damaged at high vacuum where the DIM is operated, an air box is used to contain all of the electronic components at an atmospheric pressure. The air box also acts as a Faraday shield to prevent the electronics inside the box from electromagnetic interference. The air box is made of aluminum material to reduce weight and neutron activation. The size of the air box of about 200 mm × 200 mm × 1500 mm is designed to fit the DIM cart. In order to satisfy the requirement of spatial resolving measurement, the MCP module contains only a single gating strip line but with large area, especially the width, which is up to 20 mm. The length of the strip line is 36 mm. The phosphor screen can be active by using either dc voltages of 3.5 and 4.0 kV or pulses voltages of 3.5 and 4.5 kV. Owing to the wide width (20 mm) of the strip line, four gating electrical pulses were employed to drive the whole strip line simultaneously to obtain a uniform gain. A high voltage ramp pulse generated by avalanche transistor circuit was used to drive two sharpening avalanche diode circuits [35]. Each of the sharpening avalanche diode circuits can produce two picosecond gating pulses. As a result, we finally obtained four gating electrical pulses. Four coaxial cables were used to deliver the four pulses to the MCP via SMA (subminiature version A)-type miniature connectors. The times of the four pulses arriving at the strip line are required to be almost the same to drive the strip line simultaneously. An oscilloscope was employed to monitor the arriving times of the four gating pulses. By way of adjusting the lengths of the coaxial cables, we finally controlled the difference between the arriving times within 20 ps. An electrical trigger provided by the laser facility is used to activate the camera and start the experimental measurement. The original trigger signal combining with the output of the trigger from the camera are fed to a high-bandwidth oscilloscope for monitoring. The four gating pulses can be delayed relative to the trigger up to 25.55 ns in 50 ps steps to record the experimental image at a desired time. A liquid-cooled CCD camera is coupled to the back of the MCP to capture an image electronically. The size of the CCD is 4100 × 4100 pixels, and each pixel size is 9 μm. A computer (PC104) is embedded in the air box to control most of the electrical functions, including the phosphor screen voltage, gating pulses, trigger, data acquisition and save, the environment monitoring (gas pressure, temperature, and humidity), and so on. A remote computer in the control room is employed to communicate with this embedded computer via fiber-optic ethernet. The control code lying in the embedded computer was developed using the C++ language. The temporal and spatial resolutions of the camera have been tested experimentally. The exposure time measurement was similar to [35]. An ultraviolet laser (266 nm) with 30 ps duration was used as the calibration light source. A fiber bunch that contained 30 fibers with gradual increase lengths was used to transmit the ultraviolet laser. The lengths of the fibers were controlled carefully to make sure that the laser transmission time between two neighbor fibers was increased by 30 ps. The 30 laser points transmitted through the fiber bunch were imaged on the strip line photocathode of the MCP. The camera was triggered by another 532 nm laser beam to obtain a
dynamical image. The intensities of the laser points were fitted with a Gaussian profile and the full width at half-maximum (FWHM) was taken as the temporal resolution of the camera. A static image was also obtained to eliminate the impact of the spatial nonuniformity of the MCP. The fitting result shows that the temporal resolution ∼70 ps. The uncertainty is roughly estimated as 17%, containing the contributions of the arriving time of laser point (less than 15%) and the fitting procedure (9%). The spatial resolution was measured with a quartz resolution test target, which was illuminated by an ultraviolet planar lamp and imaged on the camera via a collimator tube. The spatial resolution was obtained by observing and determining the maximum number of the clear line pairs on the image. The result indicates that spatial resolution is 20 lp/mm. The minimum change of line pairs of the quartz resolution test target can be taken as the accuracy of the spatial resolution, which is 1 lp/mm. 3. SAMPLE SPECTRUM The spectrometer has been evaluated at the SG-III laser facility. The image slit was removed in the evaluation for simply. A CH and an aluminum (Al) planar targets were used for 2400 l/mm and 1200 l/mm flat-field gratings, respectively. The eight frequency-tripled beams (λ 0.35 μm) were focused on the targets to create x-ray radiations. The energies were 4 × 100 J in 1 ns for the CH target and 8 × 1000 J in 1 ns for the Al target. The spectrum measured with the 2400 l/mm grating is shown in Fig. 3. The spectrum is dominated by the K -shell transmissions of the Ly-like and He-like C ions. The Ly-α, Ly-β, Ly-γ, He-α, and He-β of the C ions are successfully observed, as labeled in the figure. The higher-order lines are well suppressed. Spectral lines at 56.9 Å, 67.5 Å, and 80.5 Å are the second-order lines of the Ly-β, Ly-α, and He-α lines, respectively. The spectrum shows that the contribution of second-order lights can almost be neglected since they are just above the continuous spectrum. The spectral resolving power derived from λ∕Δλ of the Ly-β line is about 180. The Δλ is the FWHM of a spectral line. Figure 4 shows the Al spectrum measured with the 1200 l/mm grating. Lots of Al spectral lines are clearly observed in the figure. Similarly, obvious higher-order diffractions are not found in the spectrum. A few works concerning on the L–shell Al spectrum were reported [36,37]. We referred to the atomic spectra database provided by the
Fig. 3. Carbon spectrum recorded by the spectrometer using a 2400 l/mm grating.
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Fig. 4. Aluminum spectrum recorded by the spectrometer using a 1200 l/mm grating.
NIST [38], a theoretical calculation [36], and a experimental result from an EBIT [37] to analyze the Al spectrum. Most of the lines are emitted from the L-shell transmissions (2p − 3d , 2p − 3s, 2s − 3p, etc.) of the Be-like to Ne-like Al ions. In addition, serval lines emitted from K -shell transitions (2p − 4d ) of Be-like O ions [38] are observed (labeled with 1 and 2 in Fig. 4), which are caused by the aluminum oxide. In some cases, there is only one transmission accounting for an experimental line. For example, the neon-like Al IV has a fairly simple spectrum, dominated by the well-known lines commonly labeled 3F and 3G. The Al IV lines 3F and 3G refer to (2p1∕2 3s1∕2 J 1 → 2p43∕2 J 0) and (2p31∕2 3s 1∕2 J 1 → 2p43∕2 J 0) transitions at 160.07 Å and 161.69 Å, respectively [37]. However, in most instances, the lines observed in Fig. 4 are a blend of several transitions. More detailed analysis of the spectral lines is ongoing. The spectral resolving power given by λ∕Δλ is shown in Figs. 5(a) and 5(b) for the 2400 l/mm and 1200 l/mm gratings, respectively. The open circles stand for the experimental values. The uncertainty contains the impact of the continuum spectrum and error of the linewidth fit procedure. As shown in the figure, the spectral resolving powers obtained from the experimental lines are around 200 and 150 for the 2400 l/mm and 1200 l/mm gratings, respectively. The limit of the spectral
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resolving power in the first diffraction is given by the number of total grooves [4]. However, the practical spectral resolving power is quite lower due to several limitations, such as the width of the entrance slit, the size of the x-ray source, the imaging aberration of the grating, and the resolution limit of the x-ray framing camera. Another important truth of the spectra emitted from the laser plasmas with high density and temperature is that the broaden and blend of transitions will increase the width of the observed lines significantly. As a result, the experimental spectral resolving power obtained from the ratio of the wavelength and the linewidth will be lower than the actual value. In order to estimate the actual resolving power, a theoretical calculation was carried out. The wavelength resolution of the spectrometer can be written as [4] dλ Δx ; (1) d x min where Δx min is the minimum width of a spectral line on the recorder, d λ∕d x is the inverse linear dispersion, which can be deduced from the grating equation Δλ
mλ d 0 sin α − sin β;
(2)
where m is the diffraction order, λ is the wavelength, d 0 is the nominal groove spacing, α is the incident angle, and β is the diffraction angle, which is given by β arctanl ∕x;
(3)
where l 235 mm is the distance between the center of the grating and the recorder and x is the position of a spectral line on the recorder [l and x are labeled in Fig. 2(a)]. The differential equation can be derived from Eqs. (2) and (3) as dλ
d 0 T λ d x; l
(4)
where T λ ≡
t ; 1 − t1 t∕1 − t3∕2
and t ≡ sin β2
λ 2 sin α − : d0
(5)
(6)
Substituting Eq. (4) into Eq. (1), we can obtain the spectral resolving power as λ λl 1 : Δλ d 0 T λ Δx min
Fig. 5. Experimentally obtained spectral resolving power λ∕Δλ (Δλ, full width at half-maximum) accompanying with the theoretical calculations.
(7)
Then the calculation of resolving power λ∕Δλ becomes to the estimation of the Δx min . The width of the entrance slit constrain the Δx min will not be smaller than 30 μm. The theoretical calculations carried out by Nakano et al. [24] have shown that a point source contributes a width Δx min 7 μm. The imaging aberration of the grating is relatively small and will not be taken into account here. Owning to the finite spatial resolution of 20 lp/mm, the x-ray framing camera contributes a maximum of Δx min 50 μm, which is finally taken as the resolution limitation of the spectrometer. The linewidth of 50 μm corresponds to about 5.5 pixels of the CCD imager. The Δx min is taken as a constant in the present wavelength range 10 –260 Å for simplicity. This simplify is reasonable since
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Chowdhuri et al. found the Δx was almost the same at different wavelengths for their spectrometers [10,12]. The wavelength ranges were 10–130 Å [10] and 150–300 Å [12] for their 2400 l/mm and 1200 l/mm grating spectrometers, respectively. The Δx was about four channels of the CCD detector (full width at foot position) for both of their ruled gratings. The holographic gratings showed similar results as Chowdhuri’s works [10,12]. The theoretical results of the resolving power limitation are plotted with red dashed lines in Figs. 5(a) and 5(b) for 2400 l/mm and 1200 l/mm gratings, respectively. The average experimental widths of the spectral lines are about 9 pixels (81 μm) and 15 pixels (135 μm) for 2400 l/mm and 1200 l/mm gratings, respectively. Taking these values as the Δx min in Eq. (7), we obtain the average experimental resolving powers of the spectrometer in the present experimental conditions, as shown with blue solid lines in Fig. 5. As discussed above, the lower experimental resolving power is mostly attributed to the blend and broadening of the transitional spectral lines. The blend is more severe for the L-shell aluminum spectrum (Fig. 4) and the strong continuum spectrum also enlarges the uncertainty of the linewidth. The average experimental resolving power given in Fig. 5 proved us a helpful reference value when designing an experiment where the spectral resolution is important. 4. SUMMARY AND OUTLOOK A newly designed extreme ultraviolet spectrometer with spectral range of 10–260 Å for the SG-III laser facility has been presented. The spectrometer is designed to be fielded in a DIM. A new x-ray framing camera roomed in an air box and a new alignment procedure are developed for the spectrometer. The temporal and spatial resolutions of the framing camera are ∼70 ps and 20 lp/mm. The experimental spectra measured by the spectrometer show that the spectral resolving power is about 200 and 150 in the present experimental conditions for 2400 l/mm and 1200 l/mm gratings, respectively. The capability of the spectrometer will be especially valuable for the radiation opacity experiments in the extreme ultraviolet band after the construction of the SG-III laser facility is completed. It is noted that substitution of the laser pointers (1# and 2#, see Fig. 2) for CCD cameras with high spatial resolution will improve the accuracy of the alignment significantly [39], and this will be our next work. Although the relative measurement is used in the opacity experiment and the efficiency of the whole system is removed in the transmission spectrum, the response of the spectrometer as a function of the x-ray intensity is interesting since strong x-ray intensity may drive the spectrometer into nonlinear response [40]. The calibration concerning the response will be our future work. China Academy of Engineering Physics (2014B0102011, 2014B0102012); National Natural Science Foundation of China (11404303). The authors would like to thank the target fabrication, the laser operation, and the experiment diagnostics staffs for co-operation.
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