Submicron accurate CLIC accelerating structure internal alignment quantification

Submicron accurate CLIC accelerating structure internal alignment quantification Risto Montonen 1, 2, Ivan Kassamakov 1, 2, Edward Hӕggström 2, and Kenneth Österberg 1, 2 5 mm 1 CLIC Workshop 2018 Helsinki Institute of Physics, University of Helsinki 2 Department of Physics, University of Helsinki 1

Introduction • Internal shape and alignment of accelerating structure (AS) discs is critical. ü Fiber-optic Fourier domain short coherence interferometer (FDSCI) ü Submicron accuracy ü 10 mm range 2

Length calibration strategy • Plate transfer standards – Certified geometric thickness H • FDSCI measures optical thickness h • h and H are related to each other by group refractive index In thick standards dispersion is significant and needs to be quantified 3

Group refractive index, ng, quantification using Sagnac interferometer • h. M, measured optical thickness of the coverslip • AM, measured Sagnac beam path difference between beams –– and –– • AM measures dispersion in the sample #00 coverslip: 1. 5426 ± 0. 0042 #0 coverslip: 1. 5434 ± 0. 0046 Schott dispersion data: 1. 5445 R. Montonen, I. Kassamakov, P. Lehmann, K. Österberg, and E. Hæggström, ”Group refractive index quantification using Fourier domain short coherence Sagnac interferometer, ” Opt. Lett. , doc. ID 315401 (posted 17 January 2018, in press) 4

Submicron accuracy achieved • Copper step height analysis (CLIC Workshop 2016) augmented with ng results FDSCI Veeco NT 3300 (at black boxes) Step height quoted at 95% confidence level [µm] 40. 27 ± 0. 49 40. 27 ± 0. 14 60. 05 ± 0. 70 60. 44 ± 0. 22 5

Fiber-optic FDSCI 0. 023 nm linewidth at 1550 central wavelength provides > 10 mm measurement range Required for k-space linearization • • 0. 1 – 10 mm transfer standards, ~0. 1 µm accuracy Accuracy of ng increases with thickness → Submicron accuracy 6

CLIC AS disc measurement 7

CLIC AS internal alignment measurement strategy 1. Integration of fiber-optic FDSCI into cylinder symmetric scanning device - Nonuniform rotational distortion (NURD) foreseen - Normal of the rotation table and the pullback axis need to be aligned parallel to the rotation axis (standard procedure in form measuring machines) 2. Disc stack alignment measurement - Although NURD exists the eccentricity of each disc can be quantified. At two rotational angles the iris is perpendicular to the probe beam. These angles are identified as interference intensity maximums. R, FDSCI measurement γ, rotation angle → Disc stack alignment 8

CLIC AS internal alignment measurement strategy 3. Disc shape measurement - Eccentricity zeroing to minimize NURD - Each disc measured separately 4. Inside – outside referencing - Two fiber-optic probes in Sagnac configuration - Beam coaxiality ensured to better than 1 mrad, interprobe distance D calibrated by transfer standard, H - RI, internal measurement; RE, external measurement 9

Conclusions ü FDSCI provides submicron accurate metrology on hard-to-reach CLIC AS cavity • Length calibration conducted using plate transfer standards in Sagnac interferometer configuration • Disc eccentricity measures the alignment of the disc stack. NURD affecting the disc shape measurement minimized by zeroing eccentricity. 10

Thank You CLIC Workshop 2018 11