Real-time observation of dynamic floor motion of the KEKB injector linac with a laser-based alignment system

A new remote-controllable sensing system for a laser-based alignment system is in development to measure slow dynamic displacements of the tunnel floor in real time at the KEKB injector linac. Although the injector linac had initially been aligned with a laser-based alignment system, we have found that, based on recent real-time observations, the transverse displacements of the tunnel floor to the laser axis caused by dynamic ground motion are not negligibly small compared with the required alignment tolerance, in terms of the straightness of the beam line. Based on spatial and temporal cross-correlation analyses performed using displacement vectors from data recorded during a nearly eight-month period, it was found that the tunnel floor moves coherently in space and time domains over the entire length of the linac on average over long periods of time, while the cross-correlation function varies irregularly and complexly over shorter periods of time. This report describes in detail the basic design, experimental results, and present status of the new remote-sensing system for real-time alignment observations at the KEKB injector linac.

Figure 1

Schematic layout of the KEKB injector linac and laser fiducials for two long straight sections. The abbreviation e.j. indicates an expansion joint between the building blocks of the linac. The two arrows (red) show the laser-based fiducials for two long straight sections. The solid circles (blue) represent the remote-controllable QPDs fixed to the accelerator unit and the open circles (blue) represent the reference remote-controllable QPDs fixed on the tunnel floor.

Figure 2

Mechanical jigs for mounting the tracker target. The QPD is fixed at the end face of the accelerator unit. The jig and QPD are connected by high-precision flange-flange mechanical joining.

Figure 3

Target base on the accelerating structure in the alignment measurement with a laser tracker.

Figure 4

Remote-controllable QPD.

Figure 5

Remote-controllable QPDs installed at SY2.

Figure 6

Schematic block diagram of the local control system.

Figure 7

Input optical system with a 10-mW He-Ne laser for the C5 straight section. The laser beam is delivered from the input optical system to the end point of the linac through metallic vacuum pipes (to top direction). M: reflecting mirror, BSP: beam splitter, ST: linear stage, PP: parallel plate, f: focusing lens. It can be seen that the floor for the input optical system is structurally separated from the tunnel floor.

Figure 8

Schematic block diagram of the detector circuit in the local control system.

Figure 9

Feedback control system for stabilizing the laser axis at the last QPD.

Figure 10

Distribution plots for the pointing stabilities of the laser beam in the $x$ axis.

Figure 11

Results from the horizontal calibration procedure for all the remote-controllable and reference QPDs. It should be noted that a laser source and the fiducial QPD are located at $z=0$ and $z=500\mathrm{m}$, respectively. The target positions controlled with the feedback control are defined by $x_0$ at $z=500\mathrm{m}$.

Figure 12

Time traces in the (a) norm and (b) phase of the displacement vector for the accelerator units (or tunnel floor) measured by the remote-controllable QPDs.

Figure 13

Variations of the (a) maximum norm and (b) maximum growth rate in the displacement vector measurements as a function of the QPD location.

Figure 14

Time traces in the spatial cross-correlation function of the displacement vectors, for comparisons between the reference QPD (QPD_REF28DA) and other remote-controllable QPDs for the accelerator units during the same term.

Figure 15

Variations of (a) ${\widehat{R}}_0$ and (b) $T_{\text{cor}}$ as a function of the QPD location.

Figure 16

Time traces in the temporal cross-correlation function of the displacement vectors, for comparisons between the reference QPD and other remote-controllable QPDs for the accelerator units during the same term.

Figure 17

Intensity distributions in the spatial cross-correlation function between the remote-controllable QPDs in the displacement vector measurements of the accelerator units along the C5 straight section, obtained on (a) 15 Feb 2016 at 0:00 am and (b) 27 Feb 2016 at 8:00 am. The intensity distribution pattern in the cross-correlation function is linearly encoded by the color scale, where bright yellow (dark blue) represents ${\widehat{R}}_0$ with the maximum (minimum) value.