JINR: J. Budagov, V. Glagolev, M. Lyablin, G. Shirkov CERN: H. Mainaud Durand, G. Stern A laser based fiducial line for high precision multipoint alignment system 1

Precision Laser Fiducial line in air Introduce The next generation of linear colliders is very demanding concerning the alignment tolerances of their components. For the CLIC project, the reference axis of the components will have to be pre-aligned within 10 µm at 1 sigma with respect to a straight line in a sliding window of 200 m A new proposal is using a laser beam over 150 m as a straight alignment reference. The method is based on the laser beam space stabilization effect when a beam propagates in atmospheric air inside a pipe with standing acoustic wave. 2

Effect of laser beam stabilization when it propagates inside an atmospheric air filled pipe with standing acoustic waves σ rms (L)(μm) L(m) The σ rms (L) uncertainty in laser ray space coordinate with respect to propagation distance L inside the atmospheric air filled tube with standing acoustic wave By extrapolation, the expected σ rms for L=150m could be on the 3-4 μm level 3

Laser Fiducial line: operation and design QPr e AeAe AbAb A m δ Laser spot Laser Colimator QPr m The Laser Fiducial Line (LFL) measuring system uses a laser beam as the reference of alignment, with its beginning and end points A b and A e determined in the global reference frame of the tunnel. 4

Beginning point LFL Optical fiber Laser beam Collimator Concrete basis Support Laser Angular positioner The fiber-optic input of the laser radiation into the LFL 5

Optimal collimation laser beam Z max D0D0 The telescopic collimator Z max √2 D 0 =D max The profile of collimated laser beam √2 D 0 The laser beam profile with optimal collimation 6

Dependence of collimation of the maximum diameter of the laser beam The collimation length L k of single mode laser beam in function of beam starting diameter: (A) for D by 10mm ;(B) for D by 50 mm Laser beam diameter (mm) L k (m) A B 7

End point fixation Final QPR Two-axis linear positioner Concrete basis Support Laser beam AeAe 2-axis positioners are needed for precise alignment center of the quadrant photodetector with the laser beam axis 8

Intermediate points of measurements Plane-parallel plate(PP) Measuring station with measurd point-O Measured point-O Two-axis linear positioners Space stabilized laser beam (fiducial line) Measuring QPR Not destroying control system position measured point O 9

Stability of laser ray position Tube Laser beam Transparent glass The short term laser ray stabilization scheme Use effect of laser beam stabilization when it propagates inside an atmospheric air filled pipe with standing acoustic waves 10

Long term stability d A B δL α Δn= ·(Δt/273°C) The long term stability of the temperature is Δt=0.1°C, the refraction index change is Δn=10 -7 where d is the hot area thickness, α is the plate inclination angle with respect to the laser ray, n=1 + Δn is the hot area refractive index. Considering α=45°, d=10 m and Δn=10 -7 : δL ~1 µ m The estimate given above is obtained for a thermal stability of Δt=0.1°C for the LFL location region 11

Long term stability Thermostabilizedair flux Laser beam Outer tube with a heat insulation coating System of short-term stabilization Long term stability system 12

Seismic stability of the LFL laser source Dinner time During lunch time, a significant decrease of industrial noise is observed down to the level of rad - the Dev spread on the CD interval. During the working hours, the earth angular oscillations are characterized by the rad of Dev spread on the AB part. 13

Seismic stability of the LFL laser source The concrete floor deformation –tilt angle – as a result of weight of man presence in 3m distance from detecting device The registered tilt angle was rad 14

Length of LFL With a single mode laser emission, the maximum length of the LFL may be L kmax ~ 2 km. The laser ray propagation length could be increased using the so-called Laguerre-Gaussian modes 15

Conclusion As a summary, different technical issues of a 150 m LFL in atmospheric air were discussed, showing that the combination of:  Single mode laser associated with a fiber beam coupler for the light emitting point  An optimal laser ray focusing collimation  An intermediate sensor having no impact on the straightness of the beam  The calibration of each sensor  The suppression of air media refraction index and long term variation of temperature  The isolation of the laser source from industrial noise can lead to an alignment system providing the location of points within 10µm.  The maximum distance 2km is limited to for a maximal ray diameter of 30 mm. The use of Laguerre-Gaussian modes “10”, “20”, “30” is an interesting possibility to double this length.  Such an alignment system opens new perspectives to reach a new precision level of alignment for projects like CLIC, ILC, and XFEL 16