Engineering the Alcator C-Mod MSE Diagnostic: Solutions to Reactor-Relevant Diagnostic Challenges

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1 PSFC/JA Engineering the Alcator C-Mod MSE Diagnostic: Solutions to Reactor-Relevant Diagnostic Challenges Robert Thomas Mumgaard and Steven D. Scott * * Princeton Plasma Physics Laboratory June, 2015 Plasma Science and Fusion Center Massachusetts Institute of Technology Cambridge MA USA This work was supported by the U.S. Department of Energy, Grant No. DE-FC02-99ER54512 and DE-AC02-09CH Reproduction, translation, publication, use and disposal, in whole or in part, by or for the United States government is permitted.

2 ENGINEERING THE ALCATOR C-MOD MSE DIAGNOSTIC: SOLUTIONS TO REACTOR- RELEVANT DIAGNOSTIC CHALLENGES R. T. Mumgaard MIT Plasma Science and Fusion Center Cambridge, MA S. D. Scott Princeton Plasma Physics Laboratory Princeton, NJ Abstract Deploying precise optical diagnostics in harsh reactor environments poses significant engineering challenges including limited access, large forces, time-varying systematic errors, reflections, low signal-to-background levels and a need for in-situ calibration. Fielding a polarization-based Motional Stark Effect (MSE) diagnostic on Alcator C-Mod has proven to be difficult due to similar challenges, which has led to the development of solutions that enable polarization angle measurements with an accuracy of 0.2 degrees. The limited viewing access on the high-field C-Mod tokamak requires a large, complicated polarization-preserving optical periscope that can survive strong disruption forces while maintaining precise alignment. The non-ideal properties of the many optical elements located in vacuum induce non-linear systematic polarization errors. These errors are further compounded by the time-varying cryogenic environment (>50 deg C/min), which creates polarization drifts of several degrees due to stress-induced birefringence. A robotic calibration system was fielded to characterize these errors and to aid the design of systems to minimize them. Novel low-conduction mounts, radiative heat shields, and active cooling control the thermal environment, while highly-optimized mirrors minimize polarization aberrations. Immediately after every plasma discharge, a fully illuminated in-situ calibration system mechanically inputs polarized light with four known polarization angles into the diagnostic objective lens. This system is used to calibrate any residual temporally varying systematic error sources. It has repeatability better than 0.05 degrees and has operated for >8000 cycles in vacuum to date. The MSE system views a weak diagnostic beam, and the measurement is contaminated by bright polarized light due to reflections from the metallic internal surfaces. Therefore, the system operates at a signal-to-background level an order of magnitude lower than existing systems but similar to what is anticipated in ITER. The polarization angle and fraction are used to identify the reflected background sources that include bremsstrahlung, molecular emission and hot glowing components. Multi-spectral detectors were developed to simultaneously measure the polarization at multiple wavelengths on the same MSE sightline to enable wavelength-interpolation of the background, improving the accuracy of the diagnostic by up to an order of magnitude. These detectors also simultaneously measure the beam s orthogonally polarized sigma and pi emission, providing further checks on systematic errors. Keywords diagnostics, motional Stark effect, spectroscopy, polarization This work was supported by the US D.O.E. contracts DE-FC02-99ER54512 and DE-AC02-76CH I. INTRODUCTION Future burning plasma experiments and reactors present large challenges for optical plasma diagnostics. In contrast to most existing experiments, these devices will require diagnostics with complex and robust neutron-shielding mirror labyrinths. Furthermore, these diagnostics must be wellunderstood and must use in-situ calibration to interpret results in the absence of frequent manned-access for diagnostic investigations. The development of these reactor-relevant diagnostics presents substantial risk to the deployment of future experiments and reactors [1].[2]. Therefore, it is prudent to develop and demonstrate reactor-relevant diagnostic techniques on existing experiments so lessons learned can be incorporated into the design of diagnostics for future devices. The Motional Stark Effect (MSE) [3] diagnostic is a relatively mature technique utilized on modern tokamaks and is planned for implementation on ITER. The development of the MSE diagnostic on Alcator C-Mod presented many challenges that are similar to that foreseen in future devices. This paper examines the challenges to deploying a MSE diagnostic on Alcator C-Mod, details the solutions developed, and discusses the applicability to future devices. II. THE MOTIONAL STARK EFFECT DIAGNOSTIC A high velocity neutral beam, typically hydrogen or deuterium, is injected into the plasma where the atomic line emission is split due to the motional Stark effect. The resulting multiplets are linearly polarized with a polarization angle geometrically related to the angle of the local magnetic field. There are two types of emission, π and σ, which are orthogonally polarized. Thus, measuring the polarization angle of a sub-multiplet provides information about the local magnetic field. This technique is therefore called the MSE line-polarization (MSE-LP) approach. In practice, a periscope collects the light from the beam and transports it out of the vessel while preserving the polarization information. Once out of the vessel, the polarization information is encoded into amplitude modulations by a set of vibrating crystals called photo-elastic modulators (PEMs). This allows the light to be transmitted to remote detectors via optical fibers. One set of the multiplet, either the π or σ, is then isolated using narrow-bandpass optical filters, detected, and digitized. The polarization information is then demodulated using computer algorithms or lock-in amplifiers and the polarization angle is used to constrain the local magnetic pitch 1

3 angle in numerical plasma equilibrium reconstructions, yielding the plasma current density and safety factor profiles. The MSE-LP approach has been the most successful use of the MSE emission to date, constraining magnetic reconstructions on TFTR [4], JET [5], JT-60U [6], DIII-D [7]. Unlike most plasma diagnostics, MSE-LP measures a polarization angle. Realizing the required polarization angle accuracy of 0.2 is a challenge even in bench measurements, let alone inside a tokamak where the environment is harsh and changing. Not all of the MSE systems that have been fielded have produced physics results and several important diagnostic challenges are anticipated for MSE implementations in future devices such as ITER. The primary challenges to operating the diagnostic are the collection of the light and its transport out of the vacuum chamber while preserving its polarization. A. The Alcator C-Mod diagnostic Alcator C-Mod is a compact, high-field tokamak at MIT with a ten sightline MSE-LP diagnostic that is used to study the current profile during lower hybrid current drive [8]. The diagnostic has undergone a long development with several significant upgrades to enable accurate measurements [9]. These upgrades solved similar problems as those foreseen on next-step devices. The problems, solutions, and future applicability is discussed in the next sections. III. ROBUST PERISCOPE DESIGN Unlike most MSE diagnostics, which have only a few optical elements prior to the PEMs, the C-Mod periscope has thirteen elements that must be polarization preserving as shown in Fig. 1. Due to the compact nature of the device and the required viewing geometry, the majority of the elements are installed internal to the vessel including five SFL6 lenses and three glass-substrate dielectric-coated mirrors. All of these elements are highly optimized to preserve polarization, particularly the mirrors which have s-p reflection ratios of >0.99 and phase shifts less than waves [10]. The large (200 G) disruption forces required developing novel lens and mirror mounting mechanisms. The lenses and mirrors are held in pocket mounts surrounded by cushioning materials (Teflon or Viton) in compression only [11]. Importantly, the cushioning material is fully captured by the mount that prevents them from working their way out of position. Strong thermally and electrically isolating periscope mounts were also developed. A. Laser tracking system The large periscope presents problems with creating and maintaining optical alignment, particularly across an experimental campaign. Therefore, a laser-tracking system was developed as shown in Fig. 2. Two laser diodes are mounted collinear with the MSE fiber bundles in the fiber dissector. The laser beams traverse the periscope and terminate on the interior of the tokamak where they are imaged using a machine protection camera. The initial alignment and any deformation of the MSE periscope can then be monitored by tracking the spots on the presumably fixed outer wall components. Fortunately no change in the viewing geometry across a runday or campaign has been observed. Such a system could be added to optical diagnostics on ITER or other future devices. Fig. 1: The C-Mod MSE periscope optical layout (top) has components inside and outside the vacuum (bottom). The complexity of the periscope is due to the compact nature of C-Mod and the required viewing geometry. Such complex optics present challenges to survivability and polarization preservation. Fig. 2: Laser tracking system enables monitoring of periscope alignment. The laser diodes are mounted rigidly in the 3-D printed fiber dissector (lower left) and traverse the long MSE periscope (upper), terminating on the interior ICRF antenna where the spots are monitored with a machine protection camera (bottom right). 2

4 Fig. 3: Robotic calibration system precisely calibrates individual sightlines (top) and installed and operating (bottom). The automation enables polarization aberrations to be quantified in a short operational window. IV. ROBOTIC CALIBRATION Due to the viewing geometry, C-Mod cannot use the typical calibration technique wherein the neutral beam is fired into a gas-filled torus with known vacuum magnetic fields. Therefore, a four-axis robotic calibration system was developed to comprehensively calibrate the diagnostic during manned-access periods [12]. This system traverses along the beam trajectory and inputs many polarization states into each MSE sightline. It is shown calibrating a MSE sightline in Fig. 3. The system can input arbitrary polarization states with an accuracy better than The system operates autonomously for long periods of time and has performed over 50,000 polarization states to date. A similar comprehensive calibration methodology will be required on future devices since the diagnostics need to be well-characterized and the high-energy beams preclude beam-into-gas calibration due to shine-through issues. A. Systematic errors Despite optimization, the many optical elements introduce systematic errors in the polarization angle the measured angle differs non-linearly from the input polarization angle. These polarization aberrations were generally poorly understood at the required accuracy level prior to this work. In addition to calibrating these effects in the range of polarization angles observed by the diagnostic, the robotic calibration system can accurately capture systematic errors using a wide variety of input polarization angles including elliptically polarized light. The system can also alter the diagnostic state by changing the temperature of the optics or the detected wavelength or other parameters. This enables sensitivities in the diagnostic response to be explored and characterized. Knowledge of the systematic errors can then inform design trade-offs for the ITER diagnostic. Fig. 4: Thermal stress-induced birefringence causes linearly polarized light to become elliptically polarized (a) and the polarization angle to change (b). Note: This test case had significantly more birefringence than typically observed during operation. The most important systematic errors are time-varying and thus cannot be properly captured with a static calibration. On C-Mod the most important such error is thermal-stress-induced birefringence: stresses due to thermal gradients cause the glass to alter the polarization angle of the light transmitted through it, converting it from linearly polarized to elliptically polarized with a different polarization angle. An example measurement is shown in Fig. 4 where the output angle differs from the input angle by more than 10 degrees due to heating the vacuum window. The degree of circular polarization also increases due to birefringence.. Note the periodic structure as a function of input angle. Polarization aberrations typically display such period structure. Fig. 5: The robotic calibration system can perform polarized ray-tracing (left) which shows that the birefringence is ray-dependent with significant variation among the rays that comprise a single diagnostic sightline. 3

5 The calibration robot can also perform polarized ray-tracing through the diagnostic as shown in Fig. 5 which indicates that the polarization aberration is ray-dependent. Each ray follows a different path through the diagnostic and thus experiences a different diagnostic response. The total response for a sightline is then the average over all the rays using Stokes calculus. Burning plasma devices will not utilize glass due to neutron darkening and thus birefringence is not expected to be an important polarization aberration. However, these devices will have analogous polarization aberration due to the coating and erosion of the plasma-facing first mirror. Surface composition modifications will change its polarization properties during a campaign, and possibly even during a discharge [13]. These effects are also likely to be ray-dependent due to non-uniform coating and erosion of the mirrors. V. IN-SITU CALIBRATION This time-varying systematic error thus required the development of an in-situ calibration (ISC) system to calibrate the diagnostic at arbitrary times during a campaign. This system has similar accuracy to the robotic calibration system, operates in all conditions, and has sufficient input polarization granularity to determine the diagnostic calibration. A. Design and Operation The ISC system has a single moving part, a carousel, that contains four wire-grid polarizers (WGPs) operating in transmission, each with a backlight scatterer. The carousel rotates around the MSE periscope on precision-machined Vespel bushings with tight (0.2mm), temperature-independent clearances. This allows each polarizer to be positioned in front Fig. 8: ISC uses scattering to fully illuminate the objective lens of the objective lens with an accuracy of better than The carousel contains a hole for the objective lens to view the plasma and doubles as a shutter. The system is shown in Fig. 6. Each polarizer fully illuminates the objective lens using light transmitted into the vessel via fiber optics from remote high-power LEDs. A fixed fiber ferrule uses prisms to direct the light into the side of each backlight scatterer when a WGP is placed in front of the objective lens. The backlight scatterer, which consists of a piece of sand-blasted BK7 glass scatters the light through the polarizer and into the diagnostic in a manner similar to the operation of a thin LCD screen. Thus all the MSE sightlines simultaneously observe a uniformly illuminated polarizer which fills the entire objective lens as shown in Fig. 8. Fig. 6: ISC system design contains four polarizers mounted on a single moving part which rotates around the MSE periscope on precision bushings. Fig. 7: Effect of illumination uniformity on ISC accuracy. A uniform illumination source is required to properly capture changes in polarization response. 4

6 A uniform source is required to properly calibrate the diagnostic because polarization aberrations are ray-dependent. The first generation of backlight scatterers were a simple uniformly sand-blasted pieces of glass. When the vacuum window was heated, causing thermal stress-induced birefringence, the ISC system measured the wrong polarization change as shown in Fig. 7. This was because the illumination had bright spots leading to the wrong sightline averaging. The backlight scatterers were replaced with carefully prescribed sandblasting patterns yielding very uniform illumination after which the system properly tracks the changing diagnostic response. The carousel is rotated around the periscope via three meters of cable-in-conduit and linear bellows vacuum feedthrus with external stepper-motor driven linear actuators. The system actuates a few seconds after the end of each plasma discharge. The carousel rotates to the first polarizer and stops for 0.5 seconds and then sequentially rotates to the other three polarizers, pausing each time in a sequence that takes less than ten seconds. A feedback sensor system is used to determine when the polarizer is in the proper place by detecting when the light is scattered into and out of the backlight diffuser. The system can operate at any time including in magnetic fields and during the plasma and has operated for over 8500 times to date. This system allows the diagnostic to self-monitor once every half-hour 24/7. The WGP polarization angles are chosen so that three angles overlap the polarization angles typically observed from the plasma as shown in Fig. 9. The fourth WGP angle is orthogonal allowing the structure of the polarization aberration to be determined. In fact, to fully capture the diagnostic calibration only a few angles are required. Though the details will vary, a mechanical system with a few, fully-illuminated, polarization angles will be required to calibrate the changing diagnostic response on ITER due to changes in the plasma-facing mirrors. The combination of the different methods to calibrate the diagnostic creates a very powerful toolset for diagnostic understanding as shown in Fig. 10. The high-resolution, high fidelity robotic calibration system can operate only during manned-access while the lower granularity ISC can operate at any time and the beam emission can be used to check the calibration in dedicated calibration discharges. Fig. 10: Alcator C-Mod fields three distinct calibration methodologies, the beam with calibration discharges, the ISC, and the robotic calibration source. These span the operational space of the diagnostic. B. Determination of cause of polarization drift The ISC system is used to quantify the effects of possible errors such as Faraday rotation in the optics and interferences from power systems. The ISC system is also used to determine which optical elements are responsible for the polarization calibration drift. Fig. 11 shows an example where the diagnostic response is monitored every two minutes over 36 hours. The vacuum window changes temperature by Fig. 9: ISC calibration angles are chosen to span the polarization angles from the plasma (top) and to provide information about the structure of the polarization aberration (bottom). Fig. 11: Monitoring the diagnostic response across an experimental runday using the ISC shows that temperature changes of the vacuum window (a) causes stress-induced birefringence (b) leading to changes in polarization angle (c). 5

7 approximately 80 C due to the cryogenic magnet cooling during the runday. This causes stress-induced birefringence leading to large and rapid changes in circularly polarization fraction and polarization angle. VI. THERMAL CONTROL OF THE PERISCOPE A thermal management system was therefore installed to prevent thermal stress-induced birefringence. The vacuumwindow and external optics are thermally controlled using high-velocity flow from a thermal reservoir through tubing wrapped around the optical elements as shown in Fig. 12. This ensures a spatially uniform lens temperature and a slowly changing bulk temperature, preventing stress-induced birefringence in these elements. The internal periscope elements are thermally controlled using passive radiation shields and low-conduction periscope and lens mounts. The external surface of the periscope and internal surface of the heat shield are gold-plated with emissivity of Thus the periscope is always at a uniform and slowly changing temperature. These systems have eliminated the polarization drift in the diagnostic. Fig. 13: The diagnostic observes polarized light even in the absence of beam neutrals. VII. POLARIZED BACKGROUND Light from the beam is not the only light collected since there is significant emission in the MSE wavelengths. Fortunately, the diagnostic is only sensitive to polarized light. However, the light from the beam is not the only polarized light inside the vessel. As shown in Fig. 13, when the beam is not firing there is still some polarized background light detected. This polarized background light systematically skews the polarization angle from the beam if not properly subtracted. Accurate polarization angle measurements therefore require very high polarized signal-to-polarized background ratios or very accurate estimates of the polarized background. The later is difficult to accomplish using the usual technique of timeinterpolation across beam pulses. Only C-Mod operates an MSE diagnostic viewing a lowpower diagnostic neutral beam. All other successful MSE systems observe high-power heating beams and thus have polarized signal-to-polarized background ratios an order of magnitude higher than on C-Mod where contamination form polarized background is often the dominant source of error. Fig. 12: The thermal control scheme consists of active thermal control of the external optics using forced flow around the VW (b) from a thermal reservoir (c) which leads to uniform VW temperatures (d). The internal periscope is thermally isolated using low-emissivity coatings on the periscope (e) and the interior of heat shields (f) which protect the periscope from the plasma (g). Fi 14 U l i d li ht b l i d fl ti 6

8 A. Origin of polarized background light To better understand the nature of the polarized background the first study of polarized background light inside a tokamak was conducted. It was found that unpolarized light is partiallypolarized upon reflection from the antennas upon which the MSE sightlines terminate. A novel polarization camera was developed to image the polarization upon reflection. Fig. 14 shows that the reflected light has spatially complex polarization angles and polarization fractions. Thus any light emitted inside the tokamak can become partially-polarized MSE background upon reflection from the interior surfaces of the tokamak and the details of the polarization will depend on the geometry of the source, wall, and sightline. Several sources of partially-polarized light have been identified. The three most dominant sources are (1) visible bremsstrahlung, (2) hot glowing structures, an example of which is shown in Fig. 15 where ICRF power heats a component which then glows and is reflected into only two of the MSE sightlines, and (3) molecular deuterium emission in the divertor which scales as the Dα emission intensity as shown in Fig. 15. This source experiences rapid changes during L to H-mode transitions and back transitions. These three sources are projected to be important for ITER which will likely operate at similar polarized signal-to-background ratios as C- Mod. These sources are too fast to subtract accurately enough using time-interpolation and the spatial complexity requires measuring the background on the same sightline as the MSE measurement. Fig. 16: The MSE-MSLP approaches measures the polarization at multiple wavelengths both within the MSE spectrum and outside of the MSE spectrum on the same sightline (top) using a high-throughput polarization polychromator (bottom). B. MSE Multi-spectral line polarization (MSE-MSLP) The solution developed on C-Mod is based on the observation that all the sources are quasi-broadband in the region of interest. Instead of measuring only a single wavelength, the polarization of the light is measured at multiple wavelengths on the same sightline using a polarization polychromator as shown in Fig. 16. This device, similar to polychromators used for Thomson scattering but much larger and higher resolution, relays the light from the tokamak to different temperature-tuned interference filters which pass specific wavelengths to avalanche photodiode detectors and reflect the remaining light down the optical chain. The polarization at wavelengths to the red and blue of the MSE emission is measured in real-time on the same sightline as the MSE emission. This allows the polarized background to Fig. 15: Polarized background is a problem during high power heating due to hot glowing surfaces (top) and during plasma transitions due to divertor molecular deuterium emission (bottom). Fig. 17: The polarization at different wavelengths shows the same timehistory allowing wavelength interpolation to estimate the MSE polarized background. 7

9 Fig. 18: MSE-MSLP allows simultaneous measurement of the MSE π and σ emission and a real-time estimate of the background enables long pulses beam operation. be wavelength-interpolated instead of time-interpolated. This is very effective since, as shown in Fig. 17, the polarization at different wavelengths is very similar the proxy wavelengths can clearly be used to estimate the MSE background. This wavelength interpolation allows the C-Mod MSE system to perform 5-10 better in situations with strong background emission. Fig. 18 shows an example where the background and beam emission are measured simultaneously. The MSE-MSLP technique also measures both the MSE π and σ emission simultaneously. This increases the signal and, because the emissions are orthogonally polarized, allows checks on the polarization calibration. Additionally, since the background is estimated in real-time, the beam no longer needs to be modulated as frequently which is an improvement for diagnostics viewing heating beams. All ten MSE sightlines on C-Mod have been converted to MSE-MSLP using ten polarization polychromators. This technique is also attractive for ITER were it is easily applicable to the high-energy heating beams which have large Stark splits as shown in the simulation in Fig. 19. VIII. CONCLUSIONS The MSE diagnostic on C-Mod has now become productive and reliable through a comprehensive assessment of sources of error and their remediation and now contributes to important physics studies. In this process key advances to the MSE technique have been demonstrated using the Alcator C- Mod diagnostic. This includes experience with a large manyelement periscope and the associated problems with polarization aberration and robustness; the development of novel calibration systems that span the operational space of the diagnostic including an in-situ system which was used to identify the causes of time-varying systematic errors which were then eliminated; a better understanding of polarized background; and new ways to subtract it using the MSE-MSLP technique with polarization polychromaotrs. This is an example of retiring diagnostic risks for future devices using current devices. Fig. 19: The C-Mod detectors have been converted to MSE-MSLP (left). High energy beams on ITER make MSE-MSLP attractive as shown in the simulation (right). REFERENCES [1] R. Hazeltine et al, Research needs for magnetic fusion energy sciences. Report of the Research Needs Workshop (ReNeW), Betheda, MD 8-12 June 2009, U.S. Depoartment of Energy. [2] M. Greenwald, et. al, Priorities, Gaps and Opportunities:Towards A Long-Range Strategic Plan For Magnetic Fusion Energy, FESAC Report 2007 [3] F. Levinton, et al, Magnetic field pitch-angle measurments in the PBX- M tokamak using the motional Stark effect, Phys. Rev. Let., vol. 63, pp. 2060, [4] F. Levinton, The multichannel motional Stark effect diagnostic on TFTR, Rev. Sci. Ins. vol 63 (10), pp. 5157, 1992 [5] N. Hawkes, K. Blackler, B. Viaccoz, C. Wilson, J. Migozzi, B. Stratton, Design of the Joint European Torus motional stark effect diagnostic, Rev. Sci. Ins., vol 70 (1), pp. 894, 1999 [6] T. Fujita, H. Kuko, T. Sugie, N. Isei, K. Ushigusa, Current profile measurements with motional Stark effect polarimeter in the JT-60U tokamak, Fusion Engineering and Design, vol 34-35, pp , 1997 [7] D. Wroblewski, K. Burrell, L. Lao, P. Politzer, W. West, Motional Stark effect polarimetry for a current profile diagnostic in DIII-D, Rev. Sci. Inst., vol 61 (11), pp. 3552, 1990 [8] N. Bretz, et al, A motional stark effect instrument to measure q(r) on the C-mod tokamak, Rev. Sci. Inst., vol 72 (10), pp. 1012, 2001 [9] R. Mumgaard, Engineering upgrades to the motional Stark effect diagnostic on Alcator C-Mod, Masters Thesis, MIT, 2015 [10] J. Ko, Current profile measurements using motional Stark effect on Alcator C-Mod, PhD Thesis, MIT, 2009 [11] H. Yuh, The motional Stark effect diagnostic on Alcator C-Mod, PhD Thesis, MIT, 2005 [12] R. Mumgaard, S. Scott, J. Ko, Robotic calibration of the motional Stark effect diagnostic on Alcator C-Mod, Rev. Sci. Inst., vol 85 (5), pp , 2014 [13] M. Kuldkepp, E. Rachlew, N. Hawkes, B. Schunke, First mirror contamination studies for polarimetry motional Stark effect measurements for ITER, Rev. Sci. Instr., vol 75 (10), pp ,