In-situ wavelength calibration and temperature control for the C-Mod high-resolution X-ray crystal imaging spectrometer

In-situ wavelength calibration and temperature control for the C-Mod high-resolution X-ray crystal imaging spectrometer Luis F. Delgado-Aparicio 1, Y. Podpaly 2, J. Rice 2, M. Bitter 1, K. Hill 1, C. Gao 2, S. Scott 1, R. Bell 1, N. A. Pablant 1, M. L. Reinke 2, D. Johnson 1, P. Beiersdorfer 3, D. Gates 1 and J. R. Wilson 1 1 Princeton Plasma Physics Laboratory (PPPL) 2 Massachusetts Institute of Technology (MIT-PSFC) 3 Lawrence Livermore National Laboratory (LLNL) 52 nd Annual Meeting of the American Physical Society Division of Plasma Physics (APS-DPP) November, 8-12, 2010, Chicago, Illinois, USA

Motivation 1) Prototyping ITER- core imaging x-ray spectrometer (CIXS) diagnostic 2) Understand and overcome the limitations of the present prototype system. 3) Address some of the engineering and physics concerns raised by the US-ITER team aiming at having reliable velocity measurements (e.g. crystal temperature AND spectral calibration). 1) Do some physics

ITER x-ray imaging crystal spectrometers (XICS)have been allocated to USA & INDIA 1 XICS measure profiles of ion temperature and plasma flow velocity by Doppler spectrometry. 2 Radial/tilted view for poloidal-v θ /toroidal-v φ rotation and T i measurements [0.5<Ti<40 kev, 1<v θ,v φ <200 km/s, r/a<0.9]. 3 Two spectrometers cover most of plasma core (USA-team) while edge spectrometer slightly overlaps core sightlines (INDIA-team). 4 Detectors near back of port plug for maximum neutron shielding. 5 0.2 stability for dv φ ~1km/s for Ge crystals?

Issues on prototyping the ITER-XICS diagnostics 1) X-ray crystal spectroscopy In-situ calibration Pd filter has absorption edges that can be used as wavelength markers 2) X-ray crystal spectroscopy Plasma rotation measurement Crystal temperature can affect interplanar spacing introducing apparent velocity offset (0.2 stability for dv φ ~1km/s for Ge crystals?)

Wavelength calibrations used on C-Mod for v φ =0 1 (m,n)=(1,1) sawthooth precursor has f~v φ /R: Useful at the core. No information about the mid-radius outwards. 2 Another spectrometer (single chord: HireX-Jr ) Aimed at the core (R tg0 ) using a single sightline. Records H-like Ar spectra. 3 Locked-modes: Useful at the q=2 surface where mode-locking occurs. After long time after locking we assume that plasma locking is global. 4 New calibration method using 1 µm Pd filter used as natural wavelength-markers (being tested).

1 µm Pd filter has transmission (absorption) edges that can be used as λ-markers Transmission 0.7 0.6 0.5 0.4 0.3 0.2 0.1 H-like Ar spectrum λ~70 ma L-II edge λ~0.3 ma @ 3722.9 ma L-III edge, λ~0.3 ma @ 3907.1 ma w-line He-like Ar spectrum λ~55 ma 0 3700 3750 3800 3850 3900 3950 4000 Wavelength (ma)

Spectrometer & crystal temperature experience temperature swings/drifts > 1 2-ch. thermocouple inside the HiReX-Sr housing 3.5-4 ~1 week

Crystal temperature affect 2d interplanar spacing 1 According to Bragg diffraction law: 2 Regarding thermal expansion: 3 Any change in the interplanar spacing will result in an apparent measurement of a non-zero velocity: 4 ITER specifications indicate that the crystal should be kept at a constant temperature within a fraction of a degree.

Determining the quartz 2d interplanar spacing 1 The d-spacing of a hexagonal quartz crystal is obtained from: where, (h,k,l) are the Miller indices, and c 0 =5.40463A and a 0 =4.91304A are the lattice constants for hexagonal quartz crystals. 2 Considering thermal expansion coefficients in the parallel and perpendicular direction: 3 The temperature-dependent d-spacing can thus be written as:

Effective thermal expansion coefficients of Quartz crystals with different Miller indices

ITER X-ray Imagining Crystal Spectrometer (XICS) will require temperature control 1 The 2d-spacing of a cubic Ge crystal is: 2 a 0 (T=18 )=5.65695 A a 0 (T=20 )=5.65735 A α=35.35 10-6 / 3 The effective thermal expansion coefficient for the ITER crystal is ~4 bigger than the one for C-Mod! 4 ITER specifications indicate that the crystal should be kept at a constant temperature; to WHAT fraction of a degree?

First experimental assessment of crystal temperature sensitivity using thermal-pad Silicon thermal pad attached to HiReX-Sr outer Al wall NOTES 1 Use fiber-based insulating foam on top of the thermal pad. 2 Temperature of the pad was adjusted with a microprocessor-based temperature controller using a 0-5 V range that regulates the temperature within 38 and 260 C. 3 The power to the microprocessor controller was setup by an Ethernet-based power-strip. 4 The thermal-pad temperature rises within 2-4 minutes while the drop from 80 C has a 1/e fold time of ~12-15 mins. 5 Remote control is possible using temperature controllers and/or remotecontrolled RS232/USB-analog output boards.

Controlled crystal temperature scan of C-Mod spectra using reproducible ohmic plasmas 2-ch. thermocouple inside the HiReX-Sr housing Attached to the crystal mount ~15 o C Constant temperature during the first 10 shots In the center of the housing Cooling Cooling Heating Heating

Multi-gauss fitting confirm reproducible conditions before crystal heating & cooling experiment

Spectral features during heating and cooling experiment 1 Regarding thermal expansion: 2 Raw data indicate that a ~10 degree swing produced an apparent Doppler shift of the order of 1-2 pixels. 3 Non-linear multi-gaussian least square fit routine needed for high accuracy. 4 Moly 32+ contribution is different on a shot-to-shot basis.

Crystal heating and cooling provides apparent Doppler shift on both He- and H-like systems 1 Plasma conditions were not changed during 30-shot experiment. 2 The width of the He-like w- line and H-like Ly α1 -line did not on a shot-to-shot basis indicating reproducible plasmas. 3 4 Change in the position of the He-like w-line and H-like Ly α1 -line indicate the small apparent Doppler-shift changes on a shot-to-shot basis. Crystal heating/cooling is shown as a shift to smaller/longer wavelengths.

Apparent Doppler-shift from changes in 2dspacing is characterized by hysteresis-like curve A ~10 C degree excursion is equivalent to a ~1.56-pixel shift of the w-line in He-like spectra; this shift represents a false velocity measurement of approximately 75-80 km/s!

Inferred quartz thermal expansion coefficient in good agreement with expected values 1 The C-Mod (1,0,2) 2d-spacing at 25 C is 4562.25 ma. 2 A ~1.56 pixel shift for a crystal temperature excursion of 10.91 C on the Pilatus detector represents a displacement dx of the order of 0.26832 mm. 3 The change of angle for line w is 0.0123 deg from the original (unperturbed T=25 C) 59.9592 deg. 4 The new 2d-spacing for a temperature excursion of 10.91 C is 4562.81 ma. 5 Using: 6 The theoretically derived effective thermal expansion coefficient is ~9.5 x 10-6 / C and is valid in a temperature range from 0 C to 80 C. 7 For Alcator C-Mod:

Spectrometer housing has been dressed up to allow temperature monitoring & feedback control 2 1 May 2008 October 2008 1 2 3 Installation of 50 copper tubing and individual thermal tape that regulates the He-gas temperature flowing to the housing. Installation of silicon rubber thermal pads [~5W/in 2 ] and 3/8 insulating Polyamide foam as primary tools for active feedback and control of housing. Initial thermocouple measurements indicate a temperature difference of ~3-5 o C between the front and back of spectrometer.

New in-situ temperature sensing & feedback systems can be controlled/adjusted remotely Elements 1 8-channel ethernetbased power strip. 3 2 1 2 Remotely controlled 5- element temperaturecontrol-unit capable of sensing and tuning the individual temperatures of the thermal pads according to programmable setpoints. 3 Data acquisition switch/multiplexer unit allowing control of five 4-wire flexible-molded resistive temperature devices (RTDs) monitors placed inside HiRex-Sr. Real-time data will also be available in the C-Mod data archive tree.

New off-the-shelf electronics allows heating and control of spectrometer housing at 30 ±0.1 2 3 3 1 2 5 1 4 a) b) c) d) e) 1 Local area network. 2 Individual K-type thermocouples sense the heating-pad temperatures and activate the feedback process. 3 CNi16D44-C24 controller with C4EI option for slaving up to 32 devices through MOD- BUS RS-485 protocol. Pulse width output (duty cycle: 0 0.99) 4 Solid state relays (duty cycle control: capable up to 25 Amps). 5 Diode limiting the current and power dissipated (no full use of ~5W/in 2 capability).

Installation of new RTDs inside HiReX-Sr will enable real-time feedback algorithm RTDs installed on the crystal mounts Be window Gas RTDs next to He-inlet Four RTDs on the optical table 19-pin KF50 adapter carrying 5 RTD channels

Summary 1 New in-situ wavelength calibration method - using a 1 µm Pd filter - has been proposed as natural wavelength-markers (currently under tests). 2 Excursions in the spectrometer temperature have been correlated with test cell temperature swings, while crystal temperature can affect interplanar spacing introducing apparent velocity offset to the HiReX spectra. 3 The effective thermal expansion coefficient (α eff ) for the LHD, EAST and KSTAR crystals is 50% higher than that of C-Mod. The ITER crystal has a α eff ~4 bigger 0.1 stability for dv φ ~1 km/s for Ge crystal. 4 Experimentally inferred quartz thermal expansion coefficient for HiReX-Sr @ MIT Alcator-C-Mod is in good agreement with expected values. 5 Spectrometer housing has been dressed-up to allow temperature monitoring and feedback control. 6 New off-the-shelf electronics allows heating, RTD monitoring and feedback-control of spectrometer housing at an elevated 30 ±0.1.

Prints

Tracking locked-modes in Alcator C-mod 1 Drop in reflectometer and Mirnov (densityand magnetic) fluctuations generally indicate the mode locking-time. 2 After sawtooth disappear we assume that plasma locking is global. 3 The additional assumption is that the remaining 1 khz MHD is not rotating.

Temperature excursions on heating-pads during feedback process are not bigger that 0.1 C