Cavity beam position monitor system for the Accelerator Test Facility 2

The Accelerator Test Facility 2 (ATF2) is a scaled demonstrator system for final focus beam lines of linear high energy colliders. This paper describes the high resolution cavity beam position monitor (BPM) system, which is a part of the ATF2 diagnostics. Two types of cavity BPMs are used, $C$-band operating at 6.423 GHz, and $S$-band at 2.888 GHz with an increased beam aperture. The cavities, electronics, and digital processing are described. The resolution of the $C$-band system with attenuators was determined to be approximately 250 nm and $1\mu \mathrm{m}$ for the $S$-band system. Without attenuation the best recorded $C$-band cavity resolution was 27 nm.

Figure 1

ATF2 beam line, highlighting the quadrupoles containing CBPMs.

Figure 2

ATF2 beta $\beta$ and dispersion $\eta$ as a function of distance along the beam line $S$ from the first extraction kicker. Three beam line regions are marked which are instrumented with $C$-band nonmover, $C$-band mover, and $S$-band mover CBPM systems. The first magnet in Fig. 1, BH3X is located at $s=19.1\mathrm{m}$.

Figure 3

$C$-band cavity BPM located inside quadrupole magnet (QD6FF).

Figure 4

$S$-band cavity BPM and associated quadrupole magnet (QF1FF).

Figure 5

Schematic of rf signal processing electronics for the $C$-band system. The $S$-band system lacks the initial combiner hybrid and filter.

Figure 6

The linearity of $C$- and $S$-band electronics.

Figure 7

Example of raw and digitally down-converted signals for a dipole $C$-band cavity: raw signal including the calibration tone burst (top left), raw pedestal subtracted beam generated signal only (top right), DDC amplitude (bottom left), and DDC phase (bottom right).

Figure 8

Quadrupole magnet (QD6FF) and $C$-band CBPM assembly on a ATF2 mover. The rf processing electronics is mounted at the bottom of the concrete girder.

Figure 9

Schematic vertical section of an ATF2 mover at its midrange position.

Figure 10

Plot of the down-converted signal frequency $\delta {\omega }_{\mathrm{DDC}}$ as a function of digital oscillator frequency ${\omega }_{\mathrm{DDC}}$.

Figure 11

Example of the DDC signal decay constant (${\tau }_{\mathrm{DDC}}$) determination.

Figure 12

A typical mover calibration: $I$ and $Q$ values vs pulse number taken during the calibration (top left), $Q$ vs $I$ averaged for each step and fitted with a straight line to extract the rotation angle (top right), rotated $I$ vs mover position averaged for each step and fitted to a straight line to extract the position scale (bottom left), and rotated $Q$ vs mover position serving as an error estimate (bottom right).

Figure 13

BPM software and controls schematic.

Figure 14

A screen shot of the main EDM CBPM panel. The window shows all the CBPMs in beam-line order, with status indicators indicating the calibration status (notune, uncalibrated, normal), digitizer signal status (normal or staturated), and controls to set the digital processing parameters (Tune), perform calibration (BCal or MCal), and save CBPM (Log) data.

Figure 15

A screen shot of a single BPM control panel. The top of the window shows the input (boxed and editable numbers) and output parameters (unboxed numbers) of the digital processing algorithm. The lower portion shows the raw data for horizontal $X$, vertical $Y$, and monopole $Q$ cavities and different steps of the digital signal processing.

Figure 16

Example of resolution determination using an SVD based MIA: $x$ (top left) and $y$ (top right) positions measured by the BPM in question, and their distributions (bottom left), $x$ and $y$ residuals (bottom right).

Figure 17

Resolution of all of the BPMs along the beam line using data from April 2010.

Figure 18

Resolution of all of the BPMs along the beam line using data from February 2011.

Figure 19

Resolution of attenuated $C$-band BPMs in $x$ (top) and $y$ (bottom).

Figure 20

Resolution of all of the BPMs along the beam line using data from April 2010. The orbit was tuned to unsaturate the 4 $C$-band CBPMs without attenuators.

Figure 21

Resolution of MQM15FF in the $y$ direction as a function of bunch charge. The dashed line is a fit to the function $\sqrt{A/q^2+B^2}$.

Figure 22

Two measurements of the processing electronics relative amplitude and phase separated by 4 days.