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Divertor Thomson scattering on DIII-D
Fusion Engineering and Design 34-35 (1997) 609-612 ELSEVIER
Fusion Engineer!ng and Design
Divertor Thomson scattering on DIII-D D.G. Nilson a,* T.N. Carlstrom b D.N. Hill a C.L. Hsieh b G.D. Porter a, R.E. Stockdale b, J.C. Evans a a Lawrence Livet~ore National Laboratory, P.O. Box 808, Livermore, Liverrnore, CA 94550, USA b General Atomics, P.O. Box 85608, San Diego, CA 92186-9784, USA
Abstract
In this paper we describe the newly installed divertor Thomson scattering system for the DIII-D tokamak and present initial results from plasma discharges. Measured plasma densities have ranged from 5 x 10~8 to 5 x 102o m - 3 and divertor plasma temperatures from 1 to 500 eV. These data are compared with earlier Langmuir probe data and qualitatively compared with UEDGE computer simulations. The divertor Thomson system uses one of the eight existing core Thomson scattering lasers (1 J, 20 Hz) which has been re-directed to probe the divertor region of the DIII-D vessel. Scattered light from this multipulse Nd:Yag laser is viewed with an f/6.8 collection optics system which provides eight spatial channels from 1 21 cm above the vessel floor (divertor target), each with 1.5 cm vertical resolution. Translating the plasma across the vessel floor using position controls provides a full scan of the divertor plasma. © 1997 Elsevier Science S.A.
1. Introduction
The divertor power handling and particle control are critical issues in the design of the International Thermonuclear Experimental Reactor (ITER). F o r the past several years, divertor experiments on D I I I - D have been directed at understanding the underlying physical processes associated with these issues. Simulation codes such as U E D G E and D E G A S use these experimental results to scale the D I I I - D plasma to that of ITER. Until now, the only divertor plasma parameters available for these codes was the electron density and temperature obtained with Lang-
* Corresponding author.
muir probes mounted on the vessel floor. We have recently added a new divertor T h o m s o n scattering system which measures the plasma/'/e and T e from the X-point to the divertor target plate. By sweeping the plasma across the floor, we obtain a two-dimensional profile of these parameters. To simplify this diagnostic, we either share or have duplicated much of the existing core T h o m s o n system [1]. Due to the difficulty of designing an insertable vacuum alignment target, we rely on the signal from the linear fiber optic arrays in the image plane to monitor the relative alignment of the laser beam image to the collection fiber optics. During initial density calibrations we discovered excessive stray laser light, which we reduced to acceptable levels by improving the transmission of the exit baffles and vacuum window.
0920-3796/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S0920-3796(96)00616-3
D.G. Nilson et al./Fusion Engineering and Design 34-35 (1997) 609 612
610
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Fig. 1. The new divertor Thomson scattering hardware redirects a core Thomson scattering laser into a nearby vertical port where it is viewed with a two-element f/6.8 collection lens.
2. System description The general method and layout of the divertor Thomson system is an extension of the existing core system. One of the eight core laser beams is separated into its own beamline and directed into a nearby vertical port where it probes the divertor region of the plasma (see Fig. 1). Two close coupled device (CCD) cameras (COHU # 48125000/AL09) are positioned behind the beam steering mirrors to monitor the beam alignment before it reaches the vessel. A third CCD camera positioned in the exit dump box monitors the beam alignment through the vessel. The 5.6 m focal length lens with a 20 mm aperture focuses the beam through a series of 12 light baffles and produces a 3.5 mm diameter spot in the scattering region. A 3 mm radial clearance is maintained between the beam and the baffle cones in the vertical entrance port. At the top of the vessel, the laser beam passes through six large aperture
baffles (39 mm diameter), an exit vacuum window tilted at Brewster's angle, a beamsplitter for the alignment camera, and finally a beam dump constructed of NG-4 glass. A two-element f/6.8 collection lens images the 20 cm vertical scattering region onto 12 rectangular fiber arrays (1.5 mm x 3 mm) with a system demagnification of 0.29. The lens structure and fiber arrays are mounted to a carriage which translates outward with the vessel as it expands during high temperature baking. The collection optics view through a new 20 cm diameter bakeable vacuum window which is an extension of a 15 cm diameter commercial glass/kovar window design (Larson Electric Glass [2]). A 1 cm thick debris shield protects this window from the plasma radiation, while a thin clam-shell shutter covers both of these elements during boronization and discharge cleaning. Eight of the 12 fiber optic bundles are selected to transport the light to interference filter polychromator boxes which
611
D,G. Nilson et al./Fusion Engineering and Design 34-35 (1997) 609-612
estimated photon signal levels associated with this image identified it as the major source of stray light. We mitigated this source by tilting the exit window at Brewster's angle, installing a polarizer to improve the exit window transmission, and enlarging the exit baffles beyond the halo beam diameter. The current stray light level is now sufficiently low to allow for accurate Rayleigh density calibrations to be made on most channels. When normalized by the fiber and polychromator box transmissions and detector sensitivity, these calibrations are essentially constant. We therefore extrapolated the calibration to those channels whose excessive stray light levels prevent direct calibration. In a recent attempt to determine the source of the halo beam we measured the light scatter from two spare beam steering mirrors and found it to be equivalent to the halo beam photon level estimates. The seven mirrors which direct the laser beam into the vessel will now be replaced with more efficient reflectors which will hopefully eliminate the need for extrapolating the calibration.
have been optimized for low Te measurements. The polychromators use silicon avalanche photodiode detectors on five separate wavelength channels, one of which is at the 1064.3 nm laser line. Plasma temperatures of 1 eV are being measured with the addition of a new narrow bandwidth filter (1062 nm, 2 nm BW) which has a laser line rejection ration of 8 x 10 - 5. Further details of the design of the system can be found in Ref. [3].
3. System alignment and calibration The collection optics were first checked with focal plane mapping and aberration tests to verify that the laser beam image blue was acceptable. When allowing for a 1 mm laser misalignment, we found that most channels still collect the entire laser image, but there is a slight loss of signal ( < 10%) at the extreme ends of the viewing region due to off-axis induced blur. We then aligned the collection optics by first positioning 1 tam light emitting diodes (LEDs) at each end of the viewing region while simultaneously aligning them to the vertical laser beam. Fiber optic alignment arrays, consisting of five in-line 400 ~tm diameter fibers, were mounted at each end of the fiber block array in the focal plane to collect the L E D images. This LED signal provided feedback to center the fiber block array. During operation, the L E D light is replaced with the laser scattering signal to provide the in situ alignment monitor. To measure low temperatures, the polychromator channels require an accurate wavelength calibration which we obtained using a computer-calibrated monochromator with 0.2 nm resolution. Relative channel to channel gain was obtained using a calibrated photodiode ( E G & G model 640) traceable to NBS standards. Absolute detector gain was calibrated using Rayleigh scattering in argon. Initial Rayleigh scattering density calibrations indicated that the stray laser light levels were over 500 times higher than the scattered signal. By placing a C O H U CCD alignment camera in the vessel during a laser shot, we found that a 3 cm diameter halo surrounded the laser beam and was striking the exit baffle at the vessel ceiling. The
electron temperature (eV) 0.35
-
0.30 0.25 0.20 --J
0.15
0.10 0.05 -0.00 0.980
0
I0
0.990 1.000 t.010 Normalized flux (Psi)
20
30 40 50 Temp394041
1.020
60
Fig. 2. The plasma temperature measured during H-mode discharges are as low as 1 eV at the target plate under detached conditions.
612
D.G. Nilson et a l . / Fusion Engineering and Design 34-35 (1997) 609-612
4. Results We are now using this instrument to measure the plasma temperature and density from 1 to 21 cm above the target plate using eight data channels. Two-dimensional profiles are obtained by sweeping the plasma with position controls (see Fig. 2). Overall, the plasma models qualitatively reproduce experimentally observed scrape-offlayer plasma modes including M A R F E formation and detachment. From the two-dimensional profiles obtained during D2 gas puffing experiments, we find that both the temperature and pressure drop along the entire scrape-off layer from the X-point to the floor. The initial results show that typical measured electron densities are about 1 x 102o m -3, while the electron temperatures for a detached plasma at the target plate are four to five times lower than anticipated based on earlier Langmuir probe data. This indicates that the divertor is operating in a new regime where ion-neutral interactions such as volume recombination may be important. In summary, we now have a divertor Thomson
scattering system on DIII-D which is obtaining two-dimensional profiles of the plasma temperature and density under detached conditions. An efficient design of the two-element f/6.8 collection optics provides strong signal levels which are well above the plasma background light. We have used an in-vessel CCD camera for stray light detection and an in situ alignment monitor technique to successfully complete this extension of the core Thomson scattering system.
Acknowledgements This work is supported by the US Department of Energy under Contract Nos. DE-AC0389ER51114 and W-7405-ENG-48.
References [1] T.N. Carlstrom et al., Rev. Sci. Instrum., 63 (1992) 4901. [2] Window provided by Larson Electronic Glass, Redwood City, CA, USA. [3] T.N, Carlstrom et al., Rev. Sci. lnstrum., 66 (1995) 495.
Fusion Engineer!ng and Design
Divertor Thomson scattering on DIII-D D.G. Nilson a,* T.N. Carlstrom b D.N. Hill a C.L. Hsieh b G.D. Porter a, R.E. Stockdale b, J.C. Evans a a Lawrence Livet~ore National Laboratory, P.O. Box 808, Livermore, Liverrnore, CA 94550, USA b General Atomics, P.O. Box 85608, San Diego, CA 92186-9784, USA
Abstract
In this paper we describe the newly installed divertor Thomson scattering system for the DIII-D tokamak and present initial results from plasma discharges. Measured plasma densities have ranged from 5 x 10~8 to 5 x 102o m - 3 and divertor plasma temperatures from 1 to 500 eV. These data are compared with earlier Langmuir probe data and qualitatively compared with UEDGE computer simulations. The divertor Thomson system uses one of the eight existing core Thomson scattering lasers (1 J, 20 Hz) which has been re-directed to probe the divertor region of the DIII-D vessel. Scattered light from this multipulse Nd:Yag laser is viewed with an f/6.8 collection optics system which provides eight spatial channels from 1 21 cm above the vessel floor (divertor target), each with 1.5 cm vertical resolution. Translating the plasma across the vessel floor using position controls provides a full scan of the divertor plasma. © 1997 Elsevier Science S.A.
1. Introduction
The divertor power handling and particle control are critical issues in the design of the International Thermonuclear Experimental Reactor (ITER). F o r the past several years, divertor experiments on D I I I - D have been directed at understanding the underlying physical processes associated with these issues. Simulation codes such as U E D G E and D E G A S use these experimental results to scale the D I I I - D plasma to that of ITER. Until now, the only divertor plasma parameters available for these codes was the electron density and temperature obtained with Lang-
* Corresponding author.
muir probes mounted on the vessel floor. We have recently added a new divertor T h o m s o n scattering system which measures the plasma/'/e and T e from the X-point to the divertor target plate. By sweeping the plasma across the floor, we obtain a two-dimensional profile of these parameters. To simplify this diagnostic, we either share or have duplicated much of the existing core T h o m s o n system [1]. Due to the difficulty of designing an insertable vacuum alignment target, we rely on the signal from the linear fiber optic arrays in the image plane to monitor the relative alignment of the laser beam image to the collection fiber optics. During initial density calibrations we discovered excessive stray laser light, which we reduced to acceptable levels by improving the transmission of the exit baffles and vacuum window.
0920-3796/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S0920-3796(96)00616-3
D.G. Nilson et al./Fusion Engineering and Design 34-35 (1997) 609 612
610
Alignment Camera Window ExitBaffles 12CollectionFibersand2
ifIL ,,,
)~,~ /
To 8 Polychromators
Elementf/6.8 CollectionOptics
~\I "
InputBaffles
P i c k - o ~ A l i g n m e n t
VacuumValve DustShutter
-
VacuumWindow h
J
I
Lensand;perture [ 1
I
111
¢
AlignmentCamera
~
Camera 8 LaserCluster fromCoreSystem
IA Steerina Mirror ' g M'rro
Clan-LaserPolarizer EnergyMonitor
FiberOpticto Detector and D/ASystem
A
~1~GENE.q_i~KIOMICS
Fig. 1. The new divertor Thomson scattering hardware redirects a core Thomson scattering laser into a nearby vertical port where it is viewed with a two-element f/6.8 collection lens.
2. System description The general method and layout of the divertor Thomson system is an extension of the existing core system. One of the eight core laser beams is separated into its own beamline and directed into a nearby vertical port where it probes the divertor region of the plasma (see Fig. 1). Two close coupled device (CCD) cameras (COHU # 48125000/AL09) are positioned behind the beam steering mirrors to monitor the beam alignment before it reaches the vessel. A third CCD camera positioned in the exit dump box monitors the beam alignment through the vessel. The 5.6 m focal length lens with a 20 mm aperture focuses the beam through a series of 12 light baffles and produces a 3.5 mm diameter spot in the scattering region. A 3 mm radial clearance is maintained between the beam and the baffle cones in the vertical entrance port. At the top of the vessel, the laser beam passes through six large aperture
baffles (39 mm diameter), an exit vacuum window tilted at Brewster's angle, a beamsplitter for the alignment camera, and finally a beam dump constructed of NG-4 glass. A two-element f/6.8 collection lens images the 20 cm vertical scattering region onto 12 rectangular fiber arrays (1.5 mm x 3 mm) with a system demagnification of 0.29. The lens structure and fiber arrays are mounted to a carriage which translates outward with the vessel as it expands during high temperature baking. The collection optics view through a new 20 cm diameter bakeable vacuum window which is an extension of a 15 cm diameter commercial glass/kovar window design (Larson Electric Glass [2]). A 1 cm thick debris shield protects this window from the plasma radiation, while a thin clam-shell shutter covers both of these elements during boronization and discharge cleaning. Eight of the 12 fiber optic bundles are selected to transport the light to interference filter polychromator boxes which
611
D,G. Nilson et al./Fusion Engineering and Design 34-35 (1997) 609-612
estimated photon signal levels associated with this image identified it as the major source of stray light. We mitigated this source by tilting the exit window at Brewster's angle, installing a polarizer to improve the exit window transmission, and enlarging the exit baffles beyond the halo beam diameter. The current stray light level is now sufficiently low to allow for accurate Rayleigh density calibrations to be made on most channels. When normalized by the fiber and polychromator box transmissions and detector sensitivity, these calibrations are essentially constant. We therefore extrapolated the calibration to those channels whose excessive stray light levels prevent direct calibration. In a recent attempt to determine the source of the halo beam we measured the light scatter from two spare beam steering mirrors and found it to be equivalent to the halo beam photon level estimates. The seven mirrors which direct the laser beam into the vessel will now be replaced with more efficient reflectors which will hopefully eliminate the need for extrapolating the calibration.
have been optimized for low Te measurements. The polychromators use silicon avalanche photodiode detectors on five separate wavelength channels, one of which is at the 1064.3 nm laser line. Plasma temperatures of 1 eV are being measured with the addition of a new narrow bandwidth filter (1062 nm, 2 nm BW) which has a laser line rejection ration of 8 x 10 - 5. Further details of the design of the system can be found in Ref. [3].
3. System alignment and calibration The collection optics were first checked with focal plane mapping and aberration tests to verify that the laser beam image blue was acceptable. When allowing for a 1 mm laser misalignment, we found that most channels still collect the entire laser image, but there is a slight loss of signal ( < 10%) at the extreme ends of the viewing region due to off-axis induced blur. We then aligned the collection optics by first positioning 1 tam light emitting diodes (LEDs) at each end of the viewing region while simultaneously aligning them to the vertical laser beam. Fiber optic alignment arrays, consisting of five in-line 400 ~tm diameter fibers, were mounted at each end of the fiber block array in the focal plane to collect the L E D images. This LED signal provided feedback to center the fiber block array. During operation, the L E D light is replaced with the laser scattering signal to provide the in situ alignment monitor. To measure low temperatures, the polychromator channels require an accurate wavelength calibration which we obtained using a computer-calibrated monochromator with 0.2 nm resolution. Relative channel to channel gain was obtained using a calibrated photodiode ( E G & G model 640) traceable to NBS standards. Absolute detector gain was calibrated using Rayleigh scattering in argon. Initial Rayleigh scattering density calibrations indicated that the stray laser light levels were over 500 times higher than the scattered signal. By placing a C O H U CCD alignment camera in the vessel during a laser shot, we found that a 3 cm diameter halo surrounded the laser beam and was striking the exit baffle at the vessel ceiling. The
electron temperature (eV) 0.35
-
0.30 0.25 0.20 --J
0.15
0.10 0.05 -0.00 0.980
0
I0
0.990 1.000 t.010 Normalized flux (Psi)
20
30 40 50 Temp394041
1.020
60
Fig. 2. The plasma temperature measured during H-mode discharges are as low as 1 eV at the target plate under detached conditions.
612
D.G. Nilson et a l . / Fusion Engineering and Design 34-35 (1997) 609-612
4. Results We are now using this instrument to measure the plasma temperature and density from 1 to 21 cm above the target plate using eight data channels. Two-dimensional profiles are obtained by sweeping the plasma with position controls (see Fig. 2). Overall, the plasma models qualitatively reproduce experimentally observed scrape-offlayer plasma modes including M A R F E formation and detachment. From the two-dimensional profiles obtained during D2 gas puffing experiments, we find that both the temperature and pressure drop along the entire scrape-off layer from the X-point to the floor. The initial results show that typical measured electron densities are about 1 x 102o m -3, while the electron temperatures for a detached plasma at the target plate are four to five times lower than anticipated based on earlier Langmuir probe data. This indicates that the divertor is operating in a new regime where ion-neutral interactions such as volume recombination may be important. In summary, we now have a divertor Thomson
scattering system on DIII-D which is obtaining two-dimensional profiles of the plasma temperature and density under detached conditions. An efficient design of the two-element f/6.8 collection optics provides strong signal levels which are well above the plasma background light. We have used an in-vessel CCD camera for stray light detection and an in situ alignment monitor technique to successfully complete this extension of the core Thomson scattering system.
Acknowledgements This work is supported by the US Department of Energy under Contract Nos. DE-AC0389ER51114 and W-7405-ENG-48.
References [1] T.N. Carlstrom et al., Rev. Sci. Instrum., 63 (1992) 4901. [2] Window provided by Larson Electronic Glass, Redwood City, CA, USA. [3] T.N, Carlstrom et al., Rev. Sci. lnstrum., 66 (1995) 495.