11D605-2
Baronova, Stepanenko, and Pereira
Rev. Sci. Instrum. 85, 11D605 (2014)
FIG. 3. Quartz Cauchois crystal on a standardized substrate.
FIG. 1. Geometry of the spectrometer.
vacuum chamber’s only useful part: in the visible the vacuum is useful too, avoiding refraction by any windows that might otherwise be needed, and of course vacuum is essential for the VUV and for soft x-rays. The inlet pipe with its KF-40 flange in Figure 2 is at a slightly larger angle and longer than shown in Figure 1. This illustrates that each diffractive element may need its own inlet, depending on the diffraction angle: slits may be needed too. The different pipes are easily exchanged, with the 4 wing nuts shown. To the right side is a vacuum port, for when it is necessary to evacuate the spectrometer before connecting it to the vacuum in the plasma device, and a flange that can be opened to insert a film strip, convenient when it is difficult to take off the top cover. The spectrometer’s costliest parts are the gratings and crystals that disperse the radiation of interest. The standard gratings have 2n multiples of 300 lines per mm (300 –2400 l/mm), all on a 305 mm spherical radius. The standard crystals listed in the x-ray data booklet4 can be spherically bent to this same radius, for use with in the standard reflection (Johann) geometry, and cylindrically bent for use in the Cauchois (transmission) geometry. Figure 3 shows a Cauchois crystal, glued to a mount that matches the spectrometer. It attaches to the spectrometer housing as in a kinematic mount: magnets on the back keep the crystal properly oriented in fitting slots. The quartz crystal in Figure 3 is transparent
to visible light, a convenient feature that makes it easy to check out alignment: a visible laser beam traces out the path of diffracted x-rays, at any angle, by partial reflection from the surface of the quartz; such easy alignment is not possible with silicon or germanium.
III. SPECTROMETER OPERATION
We illustrate the spectrometer’s operation with a visible and near-UV spectrum from a vacuum spark. Figure 4 sketches the relevant radiation paths in the spectrometer. In zero order the grating reflects all the light, which in this geometry comes at an angle of 36◦ through a slit on the Rowland circle, along the path suggested by the dashed arrow: this path is the same as that of x-rays diffracting from a Johann crystal at the Bragg condition. However, in first order a grating with 600 lines per mm would diffract a particular line along the solid arrow, while a grating with 1200 lines per mm diffracts at a larger angle, as shown conceptually in Figure 4. Figure 5 is the result. The strip in the middle is the exposed film. This is placed along the Rowland circle as schematically shown in Figure 4, from the zero-order reflection seen in the dark lines to the left to where the first-order diffracted spectrum is for the two gratings. The top spectrum is from diffraction off a 600 lines/mm grating, the bottom spectrum from a 1200 l/mm grating with the correspondingly higher dispersion. Above and below the film strip in the center are the two spectra. For the 600 l/mm grating on top the spectrum stretches over the part called out by the box: the densitometer scan above it has the wavelength scale, from 230 nm to 450 nm; the line identifications are in the table. For the bottom Rowland circle 36
(Johann) crystal grating(s)
o
Bragg 0−order slit 600 l/mm 1200 l/mm FIG. 2. Photograph of the spectrometer.
FIG. 4. Radiation paths for the spectra of Figure 5.
11D605-3
Baronova, Stepanenko, and Pereira
Rev. Sci. Instrum. 85, 11D605 (2014)
FIG. 5. Spectra from a vacuum spark.
film the spectrum is shown over the same wavelength range, starting where the box of the top spectrum ends. Software written for this spectrometer, which includes the dispersion data for the different diffractive elements and the line identifications from NIST, makes it easy to identify the lines. As expected, the lines are stronger for the 600 l/mm grating and narrower for the 1200 l/mm grating; comparing the two spectra facilitates identifying the lines that may come from a different order of diffraction. IV. SUMMARY
The compact spectrometer we present in this paper has shown its utility in the niche it is intended for, in education and for use in small laboratories. It accommodates many different diffractive elements that cover the wavelength or photon energy ranges of interest in many plasma physics experiments, and is convenient to connect to typical hardware used in pulsed plasma research. When the spectrometer is matched to the appropriate diffractive element, with the corresponding radiation inlet and a slit in the right position, the instrument should give reasonable data when it is used properly.
Obviously, the spectrometer cannot give good data for each and every plasma source. No spectrometer can produce a usable spectrum when the source is too weak for the diagnostics, or when the desired lines cannot be seen above the background. In particular, when a pulsed discharge produces hard bremsstrahlung in addition to the desired softer x-rays, it may be essential to supplement the spectrometer’s aluminum housing with additional shielding. Or, the theory that predicts a certain radiation output to be sufficient for a useful spectrum, given the sensitivity of the diagnostic, may turn out to be wrong. A more sensitive detector, perhaps one that sacrifices spectral resolution for sensitivity (e.g., a filter-fluorescer technique) may then give more useful data than this, or any, wavelength dispersive instrument that by its nature uses only a small fraction of the x-rays. 1 E.
O. Baronova, M. M. Stepanenko, and N. R. Pereira, Rev. Sci. Instrum. 72, 1416 (2001). 2 J. Seely, J. Glover, L. Hudson, Y. Ralchenko, N. R. Pereira, U. Feldman, and C. DiStefano, “Measurement of high-energy (10 keV to 60 keV) x-ray spectral line widths with eV accuracy,” Rev. Sci. Instrum. (these proceedings). 3 A. E. Shumack et al., “X-ray crystal spectrometer upgrade for ITER-like wall experiments on JET,” Rev. Sci. Instrum. (these proceedings). 4 See X-ray data booklet, Table 4.1 (obtainable form http://xdb.lbl.gov).
REVIEW OF SCIENTIFIC INSTRUMENTS 85, 11D606 (2014)
X-ray continuum emission spectroscopy from hot dense matter at Gbar pressuresa) D. Kraus,1,b) T. Döppner,2 A. L. Kritcher,2 B. Bachmann,2 D. A. Chapman,3 G. W. Collins,2 S. H. Glenzer,4 J. A. Hawreliak,2 O. L. Landen,2 T. Ma,2 S. Le Pape,2 P. Neumayer,5 D. C. Swift,2 and R. W. Falcone1 1
Department of Physics, University of California, Berkeley, California 94720, USA Lawrence Livermore National Laboratory, Livermore, California 94550, USA 3 Plasma Physics Group, Radiation Physics Department, AWE plc, Reading RG7 4PR, United Kingdom and Centre for Fusion, Space and Astrophysics, University of Warwick, Coventry CV4 7AL, United Kingdom 4 SLAC National Accelerator Laboratory, Menlo Park, California 94309, USA 5 GSI Helmholtzzentrum für Schwerionenforschung, 64291 Darmstadt, Germany 2
(Presented 2 June 2014; received 31 May 2014; accepted 2 July 2014; published online 22 July 2014) We have measured the time-resolved x-ray continuum emission spectrum of ∼30 times compressed polystyrene created at stagnation of spherically convergent shock waves within the Gbar fundamental science campaign at the National Ignition Facility. From an exponential emission slope between 7.7 keV and 8.1 keV photon energy and using an emission model which accounts for reabsorption, we infer an average electron temperature of 375 ± 21 eV, which is in good agreement with HYDRA-1D simulations. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4890263] I. INTRODUCTION
Precise experimental constraints on the parameter space of hot dense implosion plasmas are a challenging task in order to benchmark models of these extreme states of matter. In particular temperature diagnostics are important to complete or over-constrain measurements of density and pressure to a complete equation of state (EOS) data set. The implosion of solid spheres is an outstanding tool to characterize the shock Hugoniot of materials approaching pressures of 1 Gbar and temperatures of several hundred eV.1 For a complete characterization of these dense matter states, at least some constraint on temperature is urgently needed in addition to the classical Hugoniot observables density and pressure. As the interesting hot dense matter states around convergence of the spherical compression waves are surrounded by large amounts of still inward moving outer material, which is less dense and less hot, it is difficult to extract the temperature of the core.2 In indirect drive experiments the experimental geometry is additionally complicated due to the confining high-Z hohlraum which reduces possible lines-of-sight and creates additional background radiation. Here we report on the application of x-ray continuum emission spectroscopy to determine the core temperature using a gated high efficiency crystal spectrometer which was originally designed for x-ray Thomson scattering.3, 4 While the total x-ray Thomson scattering amplitude is mainly proportional to the number of scattering electrons, and thus nearly exclusively sensitive to the large region of relatively cold plasma surrounding the hot core due to the volumeta) Contributed paper, published as part of the Proceedings of the 20th
Topical Conference on High-Temperature Plasma Diagnostics, Atlanta, Georgia, USA, June 2014. b) Author to whom correspondence should be addressed. Electronic mail: [email protected] 0034-6748/2014/85(11)/11D606/3/$30.00
ric weight,2 the strong energy dependence of high energy bremsstrahlung emission provides a measurement which is very sensitive to the dense and hot core.5, 6 The key of the measurement presented here is that the continuum emission is recorded at photon energies which are ∼20× above the plasma temperature allowing the relevant radiation to escape the massive CH sphere as well as obtaining high sensitivity to temperature.
II. EXPERIMENT
The experiments were performed within the Gbar EOS campaign1 of the fundamental science program at the National Ignition Facility (NIF). The target design is based on the one-dimensional convergent ablator platform,7 which was developed for the National Ignition Campaign (NIC). For temperature measurements, a mono-angle crystal spectrometer (MACS),4 which uses a gated x-ray detector allowing for snapshots with
Baronova, Stepanenko, and Pereira
Rev. Sci. Instrum. 85, 11D605 (2014)
FIG. 3. Quartz Cauchois crystal on a standardized substrate.
FIG. 1. Geometry of the spectrometer.
vacuum chamber’s only useful part: in the visible the vacuum is useful too, avoiding refraction by any windows that might otherwise be needed, and of course vacuum is essential for the VUV and for soft x-rays. The inlet pipe with its KF-40 flange in Figure 2 is at a slightly larger angle and longer than shown in Figure 1. This illustrates that each diffractive element may need its own inlet, depending on the diffraction angle: slits may be needed too. The different pipes are easily exchanged, with the 4 wing nuts shown. To the right side is a vacuum port, for when it is necessary to evacuate the spectrometer before connecting it to the vacuum in the plasma device, and a flange that can be opened to insert a film strip, convenient when it is difficult to take off the top cover. The spectrometer’s costliest parts are the gratings and crystals that disperse the radiation of interest. The standard gratings have 2n multiples of 300 lines per mm (300 –2400 l/mm), all on a 305 mm spherical radius. The standard crystals listed in the x-ray data booklet4 can be spherically bent to this same radius, for use with in the standard reflection (Johann) geometry, and cylindrically bent for use in the Cauchois (transmission) geometry. Figure 3 shows a Cauchois crystal, glued to a mount that matches the spectrometer. It attaches to the spectrometer housing as in a kinematic mount: magnets on the back keep the crystal properly oriented in fitting slots. The quartz crystal in Figure 3 is transparent
to visible light, a convenient feature that makes it easy to check out alignment: a visible laser beam traces out the path of diffracted x-rays, at any angle, by partial reflection from the surface of the quartz; such easy alignment is not possible with silicon or germanium.
III. SPECTROMETER OPERATION
We illustrate the spectrometer’s operation with a visible and near-UV spectrum from a vacuum spark. Figure 4 sketches the relevant radiation paths in the spectrometer. In zero order the grating reflects all the light, which in this geometry comes at an angle of 36◦ through a slit on the Rowland circle, along the path suggested by the dashed arrow: this path is the same as that of x-rays diffracting from a Johann crystal at the Bragg condition. However, in first order a grating with 600 lines per mm would diffract a particular line along the solid arrow, while a grating with 1200 lines per mm diffracts at a larger angle, as shown conceptually in Figure 4. Figure 5 is the result. The strip in the middle is the exposed film. This is placed along the Rowland circle as schematically shown in Figure 4, from the zero-order reflection seen in the dark lines to the left to where the first-order diffracted spectrum is for the two gratings. The top spectrum is from diffraction off a 600 lines/mm grating, the bottom spectrum from a 1200 l/mm grating with the correspondingly higher dispersion. Above and below the film strip in the center are the two spectra. For the 600 l/mm grating on top the spectrum stretches over the part called out by the box: the densitometer scan above it has the wavelength scale, from 230 nm to 450 nm; the line identifications are in the table. For the bottom Rowland circle 36
(Johann) crystal grating(s)
o
Bragg 0−order slit 600 l/mm 1200 l/mm FIG. 2. Photograph of the spectrometer.
FIG. 4. Radiation paths for the spectra of Figure 5.
11D605-3
Baronova, Stepanenko, and Pereira
Rev. Sci. Instrum. 85, 11D605 (2014)
FIG. 5. Spectra from a vacuum spark.
film the spectrum is shown over the same wavelength range, starting where the box of the top spectrum ends. Software written for this spectrometer, which includes the dispersion data for the different diffractive elements and the line identifications from NIST, makes it easy to identify the lines. As expected, the lines are stronger for the 600 l/mm grating and narrower for the 1200 l/mm grating; comparing the two spectra facilitates identifying the lines that may come from a different order of diffraction. IV. SUMMARY
The compact spectrometer we present in this paper has shown its utility in the niche it is intended for, in education and for use in small laboratories. It accommodates many different diffractive elements that cover the wavelength or photon energy ranges of interest in many plasma physics experiments, and is convenient to connect to typical hardware used in pulsed plasma research. When the spectrometer is matched to the appropriate diffractive element, with the corresponding radiation inlet and a slit in the right position, the instrument should give reasonable data when it is used properly.
Obviously, the spectrometer cannot give good data for each and every plasma source. No spectrometer can produce a usable spectrum when the source is too weak for the diagnostics, or when the desired lines cannot be seen above the background. In particular, when a pulsed discharge produces hard bremsstrahlung in addition to the desired softer x-rays, it may be essential to supplement the spectrometer’s aluminum housing with additional shielding. Or, the theory that predicts a certain radiation output to be sufficient for a useful spectrum, given the sensitivity of the diagnostic, may turn out to be wrong. A more sensitive detector, perhaps one that sacrifices spectral resolution for sensitivity (e.g., a filter-fluorescer technique) may then give more useful data than this, or any, wavelength dispersive instrument that by its nature uses only a small fraction of the x-rays. 1 E.
O. Baronova, M. M. Stepanenko, and N. R. Pereira, Rev. Sci. Instrum. 72, 1416 (2001). 2 J. Seely, J. Glover, L. Hudson, Y. Ralchenko, N. R. Pereira, U. Feldman, and C. DiStefano, “Measurement of high-energy (10 keV to 60 keV) x-ray spectral line widths with eV accuracy,” Rev. Sci. Instrum. (these proceedings). 3 A. E. Shumack et al., “X-ray crystal spectrometer upgrade for ITER-like wall experiments on JET,” Rev. Sci. Instrum. (these proceedings). 4 See X-ray data booklet, Table 4.1 (obtainable form http://xdb.lbl.gov).
REVIEW OF SCIENTIFIC INSTRUMENTS 85, 11D606 (2014)
X-ray continuum emission spectroscopy from hot dense matter at Gbar pressuresa) D. Kraus,1,b) T. Döppner,2 A. L. Kritcher,2 B. Bachmann,2 D. A. Chapman,3 G. W. Collins,2 S. H. Glenzer,4 J. A. Hawreliak,2 O. L. Landen,2 T. Ma,2 S. Le Pape,2 P. Neumayer,5 D. C. Swift,2 and R. W. Falcone1 1
Department of Physics, University of California, Berkeley, California 94720, USA Lawrence Livermore National Laboratory, Livermore, California 94550, USA 3 Plasma Physics Group, Radiation Physics Department, AWE plc, Reading RG7 4PR, United Kingdom and Centre for Fusion, Space and Astrophysics, University of Warwick, Coventry CV4 7AL, United Kingdom 4 SLAC National Accelerator Laboratory, Menlo Park, California 94309, USA 5 GSI Helmholtzzentrum für Schwerionenforschung, 64291 Darmstadt, Germany 2
(Presented 2 June 2014; received 31 May 2014; accepted 2 July 2014; published online 22 July 2014) We have measured the time-resolved x-ray continuum emission spectrum of ∼30 times compressed polystyrene created at stagnation of spherically convergent shock waves within the Gbar fundamental science campaign at the National Ignition Facility. From an exponential emission slope between 7.7 keV and 8.1 keV photon energy and using an emission model which accounts for reabsorption, we infer an average electron temperature of 375 ± 21 eV, which is in good agreement with HYDRA-1D simulations. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4890263] I. INTRODUCTION
Precise experimental constraints on the parameter space of hot dense implosion plasmas are a challenging task in order to benchmark models of these extreme states of matter. In particular temperature diagnostics are important to complete or over-constrain measurements of density and pressure to a complete equation of state (EOS) data set. The implosion of solid spheres is an outstanding tool to characterize the shock Hugoniot of materials approaching pressures of 1 Gbar and temperatures of several hundred eV.1 For a complete characterization of these dense matter states, at least some constraint on temperature is urgently needed in addition to the classical Hugoniot observables density and pressure. As the interesting hot dense matter states around convergence of the spherical compression waves are surrounded by large amounts of still inward moving outer material, which is less dense and less hot, it is difficult to extract the temperature of the core.2 In indirect drive experiments the experimental geometry is additionally complicated due to the confining high-Z hohlraum which reduces possible lines-of-sight and creates additional background radiation. Here we report on the application of x-ray continuum emission spectroscopy to determine the core temperature using a gated high efficiency crystal spectrometer which was originally designed for x-ray Thomson scattering.3, 4 While the total x-ray Thomson scattering amplitude is mainly proportional to the number of scattering electrons, and thus nearly exclusively sensitive to the large region of relatively cold plasma surrounding the hot core due to the volumeta) Contributed paper, published as part of the Proceedings of the 20th
Topical Conference on High-Temperature Plasma Diagnostics, Atlanta, Georgia, USA, June 2014. b) Author to whom correspondence should be addressed. Electronic mail: [email protected] 0034-6748/2014/85(11)/11D606/3/$30.00
ric weight,2 the strong energy dependence of high energy bremsstrahlung emission provides a measurement which is very sensitive to the dense and hot core.5, 6 The key of the measurement presented here is that the continuum emission is recorded at photon energies which are ∼20× above the plasma temperature allowing the relevant radiation to escape the massive CH sphere as well as obtaining high sensitivity to temperature.
II. EXPERIMENT
The experiments were performed within the Gbar EOS campaign1 of the fundamental science program at the National Ignition Facility (NIF). The target design is based on the one-dimensional convergent ablator platform,7 which was developed for the National Ignition Campaign (NIC). For temperature measurements, a mono-angle crystal spectrometer (MACS),4 which uses a gated x-ray detector allowing for snapshots with
