# Tevatron Beam position monitor upgrade\*

S. Wolbers, B. Banerjee, B. Barker, S. Bledsoe, T. Boes, M. Bowden, G.
Cancelo, B. Forster, G. Duerling, B. Haynes, B. Hendricks, T. Kasza, R.
Kutschke, R. Mahlum, M. Martens, M. Mengel, M. Olson, V. Pavlicek, T.
Pham, L. Piccoli, J. Steimel, K. Treptow, M. Votava, B. West, R. Webber,
D. Zhang Fermilab, Batavia, IL

Abstract

The Tevatron Beam Position Monitor (BPM) readout electronics and
software have been upgraded to improve measurement precision,
functionality and reliability. The original system, designed and built
in the early 1980's, became inadequate for current and future operations
of the Tevatron. The upgraded system consists of 960 channels of new
electronics to process analog signals from 240 BPMs, new front-end
software, new online and controls software, and modified applications to
take advantage of the improved measurements and support the new
functionality. The new system reads signals from both ends of the
existing directional stripline pickups to provide simultaneous proton
and antiproton position measurements. Measurements using the new system
are presented that demonstrate its improved resolution and overall
performance.

## upgrade requirements and goals 

The Tevatron BPM upgrade provides significantly better accuracy,
precision, and reliability as well as new measurement capabilities. The
requirements for measurements during various beam conditions reflect the
needs of Tevatron operations in the Run 2 era. The key specifications
taken from the upgrade requirements\[1\] can be found in Table 1. The
system is required to provide turn-by-turn and closed orbit
measurements. A significant new capability is the measurement of
antiproton positions.

An important goal for the upgraded system was that it be available at
the earliest possible date for use during Tevatron Run 2. This
influenced some of the decisions on hardware and signal processing
choices.

## Upgrade details

### BPM Pickups

The pickups in the Tevatron ring are part of the superconducting
quadrupole assemblies and were not modified as part of the BPM upgrade.
Each BPM is a pair of 50 ohm striplines 18 cm long, each subtending 110
degrees of arc, with a circular aperture of 7.0 cm diameter \[2\]. Each
BPM measures either the vertical or the horizontal coordinate, and there
are approximately 240 BPMs situated around the Tevatron ring. The
pickups are directional (26dB), and are read out on both ends. Special
half-length BPMs are installed near the B0 and D0 interaction regions.

Table 1 : Tevatron BPM Specifications

+-----------------------------------------------------------------------+
| #### Key Specifications (Protons)\*:                                  |
|                                                                       |
| Measurement Range: 15mm                                               |
|                                                                       |
| Absolute Position Accuracy: \< 1.0 mm                                 |
|                                                                       |
| Long Term Position Stability: \< 0.05 mm                              |
|                                                                       |
| Best Orbit Position Resolution: \< 0.02mm                             |
|                                                                       |
| Position Linearity: \< 1.5%                                           |
|                                                                       |
| Relative Position Accuracy: \< 5%                                     |
|                                                                       |
| Intensity Stability: \< 2%                                            |
+-----------------------------------------------------------------------+
| #### Key Specifications (Pbars)\*:                                    |
|                                                                       |
| Measurement Range: 15mm                                               |
|                                                                       |
| Absolute Position Accuracy: \< 1.0 mm                                 |
|                                                                       |
| Long Term Position Stability: \< 0.05 mm                              |
|                                                                       |
| Best Orbit Position Resolution: \< 0.05mm                             |
|                                                                       |
| Position Linearity: \< 1.5%                                           |
|                                                                       |
| Relative Position Accuracy: \< 5%                                     |
|                                                                       |
| Intensity Stability: \< 2%                                            |
|                                                                       |
| ^\*^*All values are 3*                                                |
+-----------------------------------------------------------------------+

### Electronics and Signal Processing

The 53 MHz component of the BPM pickup response is used to measure beam
positions. A schematic of the signal processing path is shown in
Figure 1. Signals from the BPM pickups in the Tevatron tunnel are
carried over foam RG-8 coaxial cables to the electronics system in one
of 27 service buildings situated on the surface above the ring. Each VME
subrack contains a Motorola MVME2400-0361 processor module, a timing
board providing clock and interrupt signals, front-end analog filter
boards providing 53 MHz bandpass filtering and signal attenuation, and
8-channel 80 MHz digital signal receiver boards from the Echotek
corporation (model ECDR-GC814-FV-A). All interconnections are made using
double-shielded coax cables. A photograph of a completed and installed
VME subrack is shown in Figure 2.

The choice of the Echotek board was made after extensive evaluation of
different signal processing strategies. It was chosen because it is a
common technology for other BPM upgrades, it could be acquired quickly,
and compared to in-house boards required less engineering and testing
effort. The board is very similar to those used for the Recycler BPMs at
Fermilab and identical to boards acquired for the Main Injector and
transfer line BPM upgrades. The Echotek digital signal receiver board
consists of a 14-bit A/D converter, Graychip digital down-converter,
FPGA, RAM, and VME interface. Signals are synchronously digitized at 74
MHz. Narrow-band (about 1 kHz) and wide-band (47 kHz) digital filters
provide closed-orbit (CO) and turn-by-turn (TBT) measurements.

### 

Figure 1: Tevatron BPM signal processing path

## Installation and commissioning the new system

A working Tevatron BPM system is required for the operation of the
accelerator. The installation plan allowed for replacement of the old
electronics with upgraded electronics service-building by
service-building between colliding beams periods, or "stores". In this
way only one service building (typically 8 BPMs) was unavailable for
accelerator operations at any given time. The installation of the new
system began in December, 2004, as the accelerator returned to operation
after the fall 2004 shutdown.

Each VME subrack was assembled and tested before it was installed in a
service building. Once installed and connected in the service building
the system was checked for proper cable connections using diagnostic
signals and with beam. Timing was adjusted for TBT measurements. All
measurement modes were implemented and tested for proper functionality.
Subtle problems with timing and triggering for TBT measurements were
found during early phases of the commissioning. The problems were
diagnosed and resolved on the test stand and were corrected during the
installation and commissioning of the 27 systems. The final system was
installed and connected in May 2005.

During the entire commissioning period the Tevatron was operated using a
mix of old and new BPM measurements for orbit closure at injection,
orbit smoothing, and other activities that required BPM information.
Each service building was configured in the controls system to use
either the new BPM system in that building or the old one and the switch
was made when the proton signal cables were moved from the old to the
new system.

![](media/image2.jpeg){width="3.196527777777778in"
height="4.074305555555555in"}Figure 2: Completed VME subrack in the E3
service building.

## System performance

Measurements have been made using the new system to establish its
functionality and performance. The system has performed as expected and
has produced improved measurements compared to the previous system.

### Closed Orbit

Transverse positions are computed using the following formula:

*P =* 26.\* (*\|A\| - \|B)\|/(\|A\| + \|B\|)*

where *A* and *B* are the BPM response from the two plates and 26. is
the scaling factor to convert from BPM response to position (in mm) for
this pickup geometry. A final calibration of the system and higher order
corrections are not included in this calculation.

An example of the performance of the new system can be seen in Figure 3.
In this figure the difference in proton positions at each BPM at 150 GeV
for two stores are shown with essentially all of the BPMs upgraded to
the new system. Orbit oscillations of about 100 m are clearly seen.

![](media/image3.png){width="3.2444444444444445in"
height="1.8604166666666666in"}Figure 3 : Horizontal (top) and vertical
(bottom) orbit differences between two stores, measured at 150 GeV.
Vertical scale is +- 400 m.

### Antiproton Position Measurements 

The antiproton beam positions are determined by a "deconvolution"
technique that subtracts the proton signal contamination on the
antiproton pickup. This subtraction is required because of the imperfect
directionality of the pickups. This technique is described in more
detail in a companion paper \[3\]. The subtraction is implemented using
the following formulae:

*A'~Pbar~ = A~Pbar~ -- aA~P~ - bB~P~*

*B'~Pbar~ = B~Pbar~ -- cB~P~ - dA~P~*

The coefficients *a, b, c, d* are determined empirically. The
coefficients depend on the beam position in the pickup so it is
important to determine these at the beginning of every store. After the
deconvolution the proton and antiproton positions at the beginning of a
proton-pbar store are shown in Figure 4. The technique is being
commissioned ring-wide.

### Turn by Turn Measurements

The upgraded BPM system provides TBT measurements at beam injection and
on request. The system provides 8192 (configurable) measurements. An
example of TBT measurements on injection can be seen in Figure 5,
clearly showing synchrotron oscillations.

## Conclusion

The Tevatron BPM upgrade has been installed and commissioned. The
upgrade was accomplished in an orderly fashion, replacing old with new
electronics during short accelerator downtimes, always maintaining a
combination of old and new BPM measurements for Tevatron operations. The
new system provides much improved resolution and new functionality and
early indications show it to be stable and reliable.

![](media/image4.jpeg){width="3.102777777777778in"
height="2.9472222222222224in"}

Figure 4 : Top plot: proton and antiproton sum signals. Bottom plot:
Antiproton position (bottom curve) during injection and beginning of
collisions.

![](media/image5.wmf){width="3.2083333333333335in"
height="3.1680555555555556in"}

Figure 5 : Proton position at 4 locations during the first 8192 turns
after injection into the Tevatron.

## References

\[1\] M. Martens, et. al., "Tevatron Beam Position Monitor Upgrade
Requirements", Beams-doc-554,
<http://beamdocs.fnal.gov/cgi-bin/DocDB/>ShowDocument?docid=554

\[2\] R.E. Shafer, R.C. Webber, and T.H. Nicol, "Fermilab Energy Doubler
Beam Position Detector", IEEE NS-28, **3**, (3390) June, 1981.

\[3\] R. Kutschke, et. al., "Simultaneous position measurements of
protons and antiprotons in the Tevatron", submitted to PAC'05, May,
2005.