Final implementation, commissioning, and performance of embedded collimator beam position monitors in the Large Hadron Collider

During Long Shutdown 1, 18 Large Hadron Collider (LHC) collimators were replaced with a new design, in which beam position monitor (BPM) pick-up buttons are embedded in the collimator jaws. The BPMs provide a direct measurement of the beam orbit at the collimators, and therefore can be used to align the collimators more quickly than using the standard technique which relies on feedback from beam losses. Online orbit measurements also allow for reducing operational margins in the collimation hierarchy placed specifically to cater for unknown orbit drifts, therefore decreasing the ${\beta }^{*}$ and increasing the luminosity reach of the LHC. In this paper, the results from the commissioning of the embedded BPMs in the LHC are presented. The data acquisition and control software architectures are reviewed. A comparison with the standard alignment technique is provided, together with a fill-to-fill analysis of the measured orbit in different machine modes, which will also be used to determine suitable beam interlocks for a tighter collimation hierarchy.

Figure 1

View of one end of a TCSP collimator jaw showing the circular BPM pick-up button embedded in the taper.

Figure 2

Software architecture developed for the data acquisition and operation of the embedded collimator BPMs for collimator alignment and beam orbit monitoring.

Figure 3

BPM electrode signals during the electronics calibration test ($\mathrm{LU}=\mathrm{left}\text{−}\mathrm{upstream}$, $\mathrm{LD}=\text{left}\text{−}\text{downstream}$, $\mathrm{RU}=\text{right}\text{−}\text{upstream}$, $\mathrm{RD}=\text{right}\text{−}\text{downstream}$). The BPM signals are flipped several times in order to obtain repeated measurements to counter for changes in the beam orbit and intensity which affect the electrode signals.

Figure 4

Example of a BPM-based alignment of the TCSP.A4R6.B1, showing the left-up (LU), left-down (LD), right-up (RU) and right-down (RD) jaw corner positions and electrode signals, as well as the upstream (UP) and downstream (DW) beam positions measured relative to the collimator.

Figure 5

Time required to align the 8 TCTs in IR1 and IR5 in 2012 with the BLM-based technique (top) and all 16 TCTPs in 2015 with the BPM-based technique (bottom). Each pair of jaws is marked in a separate color.

Figure 6

The total time required for TCT alignment, the number of TCT alignments and the average time required to align a TCT collimator over the years.

Figure 7

A comparison of the absolute beam centers measured using the BLM-based and BPM-based techniques, and the differences between the average of the upstream and downstream BPM-based positions and the BLM-based positions, at injection and flat top.

Figure 8

Measurements of tilts in three collimators from BPM-based and BLM-based alignment.

Figure 9

Comparison of upstream and downstream beam positions in three collimators expected from MADX and those found in the BPM-based alignment.

Figure 10

Nonlinear relationship between $X_{\mathrm{beam}}$ and $X_{\mathrm{bpm}}$ for TCTPV.4R2.B2, depending on the beam offset and jaw gap (see legend).

Figure 11

Linearity factor as a function of the beam offset normalized to the BPM aperture for the TCTPV.4R2.B2 for different jaw gaps (see legend).

Figure 12

The Poly53 2D polynomial fit used to correct the nonlinear BPM readings of the TCTPV.4R2.B2 for the operational jaw motion range.

Figure 13

The 2D fit coefficients $c_{pq}$ obtained for all 36 collimator BPMs during the collimator scans at flat top.

Figure 14

The resulting corrected plot of $X_{\mathrm{corr}}$ as a function of $X_{\mathrm{beam}}$.

Figure 15

Estimated change in $X_{\mathrm{abs}}$ for various orbit bumps in the orthogonal plane of three BPM collimators.

Figure 16

Beam positions measured relative to the centers of selected collimators during the ramp (top), squeeze (center), and stable beams (bottom) in 2015, before they were used in the collimator settings generation. Each number in the legend corresponds to a separate fill for physics.

Figure 17

The distribution of the measured orbit at the TCTPH.4L1.B1 during all proton-proton stable beams periods in 2016. 99% of the orbit drifts with respect to the collimator center are within $235\mu \mathrm{m}$.

Figure 18

Distribution of the 99% of the all orbit drifts at each of the BPM collimators in stable beams.

Figure 19

The average beam positions measured during the stable beams period each proton physics fill in 2016, together with the standard deviation as the error bar, for B1 (top) and B2 (bottom) collimators.

Figure 20

Average of upstream and downstream beam positions measured at each pair of neighboring horizontal and vertical TCTPs in B1 (top) and B2 (bottom), averaged over the entire stable beams duration of each proton physics fill in 2016.

Figure 21

The absolute measured beam orbit at the collimator together with the setting for the TCTPH.4L5.B1 during the combined ramp and squeeze.

Figure 22

Number of predicted dumps during stable beams for different BPM interlock thresholds for TCTP and TCSP collimators separated by IR configuration in nominal beam $\sigma$. The red line indicates the proposed threshold for each configuration.