Thermal control of x-ray crystals and detectors for ITER CXIS L. Delgado-Aparicio 1 and P. Beiersdorfer 2 1 Princeton Plasma Physics Laboratory (PPPL) 2 Lawrence Livermore National Laboratory (LLNL) Conceptual design review of ITER CORE X-RAY CRYSTAL IMAGING SPECTROMETER June 4-5 th, 2013

Motivation/outline ② In-situ wavelength calibration Needed for calibrated measurements of the plasma rotation velocity and spectrometer instrumental function. Reliable measurements of plasma emissivity, ion temperature and toroidal flow velocity profiles, requires: ① In-situ uniformity calibration of detectors Needed for calibrated measurements of the local plasma emissivity and estimates of impurity density and its gradients. 2 ③ Crystal/detector temperature monitoring & control Ambient temperature excursions can affect interplanar spacing introducing apparent velocity offsets

Temperature monitoring & control of crystal and detector is crucial for their proper functioning ① ITER specifications indicate that the crystal should be kept at a constant temperature within a fraction of a degree. ② Temperature excursions would likely arise from the changes in the ambient temperature and possibly also from neutron/gamma/x-ray flux. Need to quantify temperature gradients within the spectrometer housing. ④ Baseline scenarios considered for ITER – CXIS components: Crystal: 22±3 o C during bakeout & 22±0.1 o C during operation Detector: 22±3 o C during bakeout & 22±0.5 o C during operation ⑤ Cooling may be accomplished by flowing helium throughout the enclosures, water through the walls of the enclosures, or by electrical cooling (e.g. Peltier coolers). ⑥ Combinations of these techniques can be conceived if its desired to reduced the amount of gas cooling 3

Reminder: C-Mod spectrometer uses an atmosphere of He for x-ray energies of 3-4 keV H-like Ar Crystal H-like Ar Detector He-like Ar Crystal He-like Ar detectors Ne-like Mo 32+ line falls into H-like Ar spectrum (Similar imaging systems in NSTX, KSTAR, EAST and LHD operate in vacuum) 4

Changes of the interplanar 2d-spacing could be misinterpreted as Doppler shifts ① The relationship between the Doppler shift (DS) and the v i : ③ Assuming a constant “d”, the observed relative wavelength shift is given by: ④ For a constant “ ”, a change “  d” leads to a change “  ”. 5 ② Bragg diffraction: ⑤ For small changes:

Crystal temperature affect 2d interplanar spacing 6 Experiments at Alcator C-Mod (MIT-PSFC)

External heat-loads can be mitigated using a Dewar concept for crystal and detector housings 7 ① Not only double-walled but each wall is coated with a highly reflected material (typically Ag). ② Vacuum of at least Torr to keep the conduction below acceptable values. ③ Estimates also assumed a Dewar-type Be window arrangement (+coatings Å ).

Use of ultra-high purity beryllium is a MUST 8 ① The margin of error in Be window transmissivity is considerable when overall transmission is small to begin with. ② T(Fe 24+, 6mm)=14%; T(W 64+, 6mm)=40%. ③ Thinner windows are of course, desirable. Kr

9 ④ Effects of larger absorption due to the presence of impurities with high- atomic numbers is significant when compared standard vs ultra-high purity grades of beryllium. Reduction can be as high as 60%. ⑤ Be window thickness at C-Mod is 0.1 mm. ⑥ Add ribs for thinner windows. STANDARDULTRA-HIGH Use of ultra-high purity beryllium is a MUST

Internal heat-loads can be mitigated using a He- flow entering through bottom walls 10 ① Crystal and detectors are mounted on two translation and one rotation stages. ② 20 W in the 2-crystal enclosure (10 W per arrangement of three stages) while 140 W in the 2-detector housing (15 W per Pilatus detector). ③ Enclosure cooling by He-gas represents a baseline concept for thermal control. He cooling with T-20 o C

Internal heat-loads can be mitigated using a He- flow entering through bottom walls 11 ④ Gas flow (T He-gas is 20 o C lower than the desired enclosure temperature): Crystal enclosure: 11.8m 3 /hr and 5.8m 3 /hr during bakeout & operation. Detector enclosure: 27m 3 /hr and 12m 3 /hr during bakeout & operation. ⑤ Flow rate can be cut in half if temperature difference were doubled to 40 o C (30 o C for the enclosure with T He-gas =-10 o C). ⑥ Crystals should be tested for being able to withstand thermal cycling. He cooling with T-20 o C

Water and Peltier-cooling are options for dissipating internal heat-loads 12 ① Considered cooling the enclosures by cooling the inner wall by water and equilibrating by means of a fan that stirs the He-atm. ② Calculations show that this approach is viable, but He convection coefficients are still required. ③ Pelier-cooling on the crystal mount may provide additional temperature control. Inner wall with embedded water pipes ④ Pilatus-II manufacturers deliver now water cooled detectors (experience at LHD). ⑤ Sensors placed on the crystal mount and other locations throughout the enclosures will provide input for adjusting the flow rates and coolant temperature.

New sensors in MIT-PPPL spectrometer could enable real-time monitoring & feedback RTDs installed on the crystal mounts Four RTDs on the optical table Gas RTDs next to He-inlet 19-pin KF50 adapter carrying 5 RTD channels Be window IR temperature sensor 13

Summary ① Our calculations and simulations show that the goal of maintaining the appropriate temperature within a tightly controlled range can be achieved. ② Baseline scenarios considered for ITER – CXIS components: Crystal: 22±3 o C during bakeout & 22±0.1 o C during operation Detector: 22±3 o C during bakeout & 22±0.5 o C during operation ③ The helium flow serves two roles: in the first role it is a coolant, and in the second role it is a medium that equilibrates the temperature. ④ We also recommend employing water cooling of the inner wall to remove some of its heat load as well as Peltier coolers added to the detectors, crystals and motional stages to remove their waste heat. ⑤ Pilatus-II manufacturer is supplying detectors which are water cooled. ③ Sensors (thermocouples and/or RTDs) placed on the crystal mount and other locations throughout the enclosure will provide input for adjusting the flow rate and coolant temperature. 14

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Spectrometer temperature excursions have been correlated with test cell temperature swings Worked at o C for nearly week. Cell cooled down to ~26 o C in a day after AC was fixed, and even further to ~22 o C after LN 2 cooled the TF magnets. Spectrometer and crystal temperature experience temperature swings/drifts ~ 1 ℃. Gradients between the front and back of spectrometer ~ 4-5 ℃ 16