Supported by. Alignment of the Thomson scattering diagnostic on NSTX. B.P. LeBlanc and A. Diallo Princeton Plasma Physics Laboratory

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1 NSTX-U Supported by Coll of Wm & Mary Columbia U CompX General Atomics FIU INL Johns Hopkins U LANL LLNL Lodestar MIT Lehigh U Nova Photonics ORNL PPPL Princeton U Purdue U SNL Think Tank, Inc. UC Davis UC Irvine UCLA UCSD U Colorado U Illinois U Maryland U Rochester U Tennessee U Tulsa U Washington U Wisconsin X Science LLC Alignment of the Thomson scattering diagnostic on NSTX B.P. LeBlanc and A. Diallo Princeton Plasma Physics Laboratory 16 th International Symposium Laser Aided Plasma Diagnostics September 22-26, 2013 Madison, WI * Work supported by USA DoE contract DE-AC02-09CH11466 Culham Sci Ctr York U Chubu U Fukui U Hiroshima U Hyogo U Kyoto U Kyushu U Kyushu Tokai U NIFS Niigata U U Tokyo JAEA Inst for Nucl Res, Kiev Ioffe Inst TRINITI Chonbuk Natl U NFRI KAIST POSTECH Seoul Natl U ASIPP CIEMAT FOM Inst DIFFER ENEA, Frascati CEA, Cadarache IPP, Jülich IPP, Garching ASCR, Czech Rep

2 Abstract The Thomson scattering diagnostic can provide profile measurement of the electron temperature, T e, and density, n e, in plasmas. Proper laser beam path and optics arrangement permits profiles T e (R) and n e (R) measurement along the major radius R. Keeping proper alignment between the laser beam path and the collection optics is necessary for an accurate determination of the electron density. As time progresses the relative position of the collection optics field of view with respect to the laser beam path will invariably shift. This can be kept to a minimum by proper attention to the physical arrangement of the collection and laserbeam delivery optics. A system has been in place to monitor the relative position between laser beam and collection optics. Variation of the alignment can be detected before it begins to affect the quality of the profile data. This paper discusses details of the instrumentation and techniques used to maintain alignment during NSTX multi-month experimental campaigns. 2

3 Contents 1. Introduction 2. Alignment channels 2.1 Hardware 2.2 Alignment scan 2.3 Experimental example 3. Discussion 3

4 Introduction The Multi-Point Thomson Scattering (MPTS) diagnostic on NSTX measures T e and n e profiles on the horizontal midplane, on both sides of the magnetic axis. The system covers over 90% of the machine aperture, including the scrape-off layer. The diagnostic provides n e, which is absolutely calibrated from first principles[1]. This density data is routinely used by machine operators and needs to remain available throughout the experimental campaign. Hence maintaining alignment is critical [1] B.P. LeBlanc, et. al. Rev. Sci. Instrum., 79, 10E737 (2008) 4

5 Collection Optics Back scattering geometry Vacuum Window Fiber bundle holder 36 bundles in place Laser beam (LB), laser tube (LT), TF coil (TF), focal point (FP), primary mirror (PM), aperture stop (AS), image of laser path (IM), fiber bundle (FB), plasma edge (PE), major radius (R), center stack (CS), vacuum window (VW) 5

6 MPTS Laser System Two 30-Hz Nd:YAG Lasers 1064 nm, 1.5 J/pulse Laser 2 Laser 1 6

7 Laser-Beam Delivery Optics Both laser beams are commonly reflected on M4 and M6 This figure is not to scale B1, B2: laser beams 1 and 2 M1 through M9: beam steering mirrors L1 through L4: lenses W1, W2, W3: laser windows NTC: NSTX test cell COB: Collection optics box CS: center stack BD: Beam dump 7

8 Alignment Channels Even with attention to the design of laser-beam delivery and collection optics for pointing and viewing stability, alignment drift will invariably occur over time The alignment channels constitute an invaluable component of the MPTS system They permit determination of the relative position of the laser beams and the collection -optics field of view Two of the 21 fibers composing each fiber bundle are reserved for alignment These fibers are grouped in bundle pairs viewing the top and the bottom of segments along the laser-beam path Four of possible six alignment channels are currently in operation 8

9 Alignment Fibers and Bundle Arrangement Fig.1: Alignment measurement: Each bundle (a) is made of 21 fibers, of which 2 are reserved for alignment measurement (b). The long arrow represents a laser beam (c). 9

10 Alignment Segments along Beam Path Fig.2: Typical H-mode n e (R) profile and location of alignment segments: inner, middle and outer segments. Also shown are the wall of the center stack (CS) and the location of the HHFW antenna limiter (ANT). 10

11 Alignment Channels: Two top-and-bottom pairs Fig.3: The two alignment-channel pairs: (1) inner edge top and bottom and (2) outer edge top and bottom The filters bandpasses have been selected for transmission during Raman scattering in N 2 and also during Thomson scattering operation, even at low T e 11

12 Alignment Figure of Merit A For each of the two alignment detector pairs, we can define a top over bottom ratio, namely: A in = Top_detector_signal[in] Bottom_detector_signal[in] A out = Top_detector_signal[out] Bottom_detector_signal[out] for inner segment for outer segment Parameter A is very sensitive to the relative position of the laser beam and the collection field of view. One can use A to quantify the calibration alignment* One can use A to quantify drift away from calibration alignment *Calibration alignment is discussed next 12

13 Calibration Alignment Fine alignment is done with Raman scattering in nitrogen Typically 50 Torr of N 2 We use the spectral channels that can detect Raman scattering light to maximize signal during alignment This is done by optimizing the positions of the last two mirrors M4 and M6 of the laser delivery optics and the vertical position of the collection optics field of view The optimal alignment is referred to as the calibration alignment, namely the alignment at which the system is Raman/Rayleigh calibrated The calibration alignment is the alignment that we want to maintain during the experimental campaign 13

14 Alignment Scan One way to document the system behavior in the vicinity of the calibration alignment is to proceed to controlled scans of the optical components around the calibration alignment with Raman scattering. In the present case, this would involve scans of the collection optics, or of the last two mirrors of the laser-beam delivery optics. This work is usually done just prior to Raman calibration. This alignment scan data is used to quantify the eventual alignment shift. 14

15 Optimization of alignment Variation of the collection optics vertical setting V Radial pos. 1 to 10 Laser 1 and 2 Calibration alignment set point Radial pos. 11 to 20 Laser 1 and 2 Fig.4: Variation of Raman signal during collection optics vertical scan. The wavelength bandwidth center is at 1058 nm. The calibration alignment is indicated by the vertical dashed line. Data shown from two sets of ten polychromators (PH1 and PH2) for lasers L1 and L2. Radial positions with R > R GM = 85 cm are drawn with dashed lines. 15

16 Scan around Calibration Alignment Variation of the collection optics vertical setting Contour plot view 16

17 Alignment Channel Data Acquired during Alignment Scan Fig. 5: Collection optics vertical scan around calibration alignment at V=0.0. Top panels: alignment channel signals for inner and outer segments. Bottom panels: alignment parameter A. Data shown for lasers 1 and 2. 17

18 Alignment Signals during Plasma Discharge 18

19 Alignment Analysis of a Plasma Discharge Interpolate experimental A[in] and A[out] squares onto the vertical scan data of collection optics solid lines and x symbols. Determine the vertical shift DV for each laser. 19

20 Discussion The alignment channels constitute a powerful tool permitting us to evaluate quantitatively the alignment of the two individual laser beams and the collection optics. It is observed experimentally that alignment drift almost always involves both lasers moving together in the same direction. There are only three optical elements that could be responsible for this effect, namely: the collection optics itself and the last two mirrors of the laser delivery optics, which reflect both beams (M4 and M6). These two mirrors are many meters away from NSTX and usually a correction is applied to the farthest mirror. While an additional goal could have been to recover a measurement made when the system was significantly far from the calibration alignment, it has proven impractical without the data from the middle alignment segment. The difficulty comes from the details of the laser beam profiles convoluted with the profile of the viewing optics vertical extent. As a result, our approach has been to keep the alignment always close to the calibration alignment, in which case there is no need to correct the n e (R) profile for alignment effects. Practically, we keep DV below ± 0.05cm. 20

21 END OF POSTER 21

22 Alignment of the Thomson scattering diagnostic on NSTX B.P. LeBlanc and A. Diallo, PPPL Alignment fibers Alignment drift assessment Alignment channels