12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

REAL TIME MOMENTUM SPREAD MEASUREMENT OF THE
CERN ANTIPROTON DECELERATOR BEAM
P. Freyermuth*, B. Dupuy, D. Gamba, CERN, Geneva, Switzerland
Abstract∆𝑓 = 𝑛 𝑓 𝜂

∆

,

(1)

Constant optimisation and diagnostics of the coolingprocesses in the CERN antiproton decelerator (AD) relies
on a de-bunched beam momentum spread real time measurement. This article will describe the renovation of theacquisition chain of the longitudinal Schottky diagnostics
in the AD, using standard CERN hardware and software
to maximize reliability, ease maintenance, and meet therequirements for standard operational tools. The wholechain, from the pick-up to the operation software applications will be described with emphasis on the implementation of the data processing running on the front-end computer. Limitations will also be discussed and outlook forfurther development given.

where ∆𝑓 is the width of the Schottky band, 𝑛 is the harmonic number, 𝑓 is average revolution frequency, 𝜂 is theslip factor given by the synchrotron parameters and ∆p/p
is the momentum spread.
The band selection, therefore, of the harmonic, is a balance between signal-to-noise ratio, frequency resolution,and LPU and acquisition bandwidth.
To accommodate the different revolution frequencies of
the AD cooling plateaus, the signal from the LPU ismixed with a signal generated from a harmonic of therevolution frequency minus 50 kHz (Table 1).

THE AD LONGITUDINAL PICK-UP

Table 1: Down Mixing Frequencies

The AD longitudinal pick-up (LPU) consists of two
specially designed ultra-low noise ferrite-loaded beam
current transformers and amplifiers [1]. They are optimized respectively for high (0.25 – 30 MHz) and low(0.02 – 3 MHz) frequency ranges.
abdc

To downmixing

cFigure 1: LPU transformers (a & b), amplifiers (c) andsumming unit (d).
The output signals of the two transformers are filteredand added together by a summing unit to obtain a flat
frequency response over the 0.02-30 MHz bandwidth(Fig. 1). This system is used since the AD went operational in 2001.

SIGNAL DOWN-MIXINGThe longitudinal Schottky spectrum of a de-bunchedbeam consists of bands of frequencies (Schottky bands),
located around harmonics (n) of the average revolution
frequency. The momentum spread distribution can bederived from these Schottky bands. The power spectral
density of these bands decreases with n, but the width ofthese bands increases by a factor n (Eq. 1) [2].

Plateau
momentum[MeV/c]
3574
2000
300
100

Revolution
frequency[kHz]
1589.411
1487.722
500.465
174.155

Harmonic22
88Mixing
frequency[kHz]
3128.822
2925.445
3953.721
1343.240

The mixing unit includes filters for image frequency rejection. The result is a transposition of the signal to a
fixed centre frequency of 50 kHz, enabling fixed parameters for different cooling plateau energies and modest
analogue-to-digital converter (ADC) hardware specifications.
An additional 32 dB amplifier is also placed before themixer (Fig. 2), pulling the signal strength into the range of
the ADC sensitivity.
ba
From LPU
To acquisition
32 dB
From RF generator
Figure 2: Simplified schematic of the signal down-mixing
with a low-noise amplifier (a) and the mixing and filtering
unit (b).

___________________________________________* pierre.freyermuth@cern.chTUP045
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ISSN: 2 6 7 3 - 5 3 5 0
d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - T U P 0 4 512th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

IBIC2023, Saskatoon, Canada
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ISSN: 2 6 7 3 - 5 3 5 0d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - T U P 0 4 5

Front-end Computer
FromDownmixing

FMC ADC card
OASIS FESA class
Real-time
Control and acquisition
ServerData publication

Toprocessing

Figure 3: Overview of the acquisition system.
It consists of a 4 channel 100 million samples per second (Msps) 14-bit ADC card embedded on a FPGA Mezzanine Card format (FMC). The ADC card is operated ina Front-End Computer (FEC) by the OASIS system. OASIS is using the CERN Front-End Software Application(FESA) framework [4], enabling clients to subscribe
remotely to the digitalized signal data (Fig. 3).
A typical acquisition setting is a sampling rate of 3
Msps during a 100 ms segment, for a total of 3E5 samples
per segment. The acquisition segment is repeated andpublished continuously at 5 Hz. The voltage range is set
to the maximum sensitivity at +/- 50 mV.

PROCESSING
The computation of the Schottky spectrum is performedby compiled C++ code running on the same FEC as theacquisition, using the FESA Framework (Fig. 4).
Front-end Computer

Schottky spectrum
FESA class

a3

1.5

2.5

a2

1

1.5
10.5
Momen…
Rev. Freq.

0.5
0050
Cycle time (s)bb0100

Such real time publication of the longitudinal Schottkyspectrum enables live animation in multiple applications
used daily in operation. Live data availability is particularly relevant for a slow cycling machine, giving the operators the opportunity to apply corrections before thestart of the next cycle, reducing setting-up and diagnostictime.

Real-time
OASIS subscription
FFTSpectrum
ServerData publication

3.5

Figure 5: The two stochastic (a) and the two electron (b)cooling plateaus of the AD deceleration cycle, and thekey moments indicated by black arrows.

FFT library

FromOASIS

AD OPERATION
The main goal of the renovation of the acquisition andprocessing chain was to be able to get a visual representation of the momentum envelop along all the AD coolingplateaus. (Fig. 5).
Revolution frequency (MHz)The signal acquisition was previously performed by a
now obsolete National Instrument ADC module. It has
been replaced by a CERN Open Analogue Signal Information System (OASIS) [3].

The real time layer of the FESA class is subscribed tothe OASIS device and receives the raw data. The optimised library FFTW [5] is used for the fast Fourier transform (FFT) of each acquired segments.
The power spectrum is computed, and only a frequency
range centred around 50 kHz is stored. The typical frequency range corresponding to the AD momentum spread
capabilities is 48 – 52 kHz. It allows a data reduction
down to 500 points per spectrum, easing the data transferand storage.
The server layer of the FESA class takes care of thepublishing of computed data to the subscribed clients.
The FEC is fitted with an Intel Core i5 CPU and canrun two instances of the FESA class, producing and publishing Schottky spectra at 5 Hz per device.
Additionally, another FESA class generates snapshotsof a Schottky spectrum, only updated at key moments
along the AD deceleration cycle (Fig. 5).

Momentum (GeV/c)

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SIGNAL ACQUISITIONThe Operation Application
Toclients

Figure 4: Overview of the signal data processing.

A java application has been developed for the operators, available on control rooms computers (Fig. 6).
Schottky spectra are displayed in a three-dimensions
chart. The time axis is kept horizontal to be synchronisedwith other two dimensions time charts. The power spectrum value is colour coded, and the vertical axis represents the frequency.

TUP045294

09 Data Acquisition and Processing Platforms12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

IBIC2023, Saskatoon, Canada
JACoW Publishing
ISSN: 2 6 7 3 - 5 3 5 0d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - T U P 0 4 5The full cycle length Schottky spectrum recording ofeach AD cycles are also saved as an image. By averaging
images of several cycles during a stable operation period,
one can enhance the details of the envelop of the longitudinal cooling processes performance (Fig. 8).
5251504948

3.5 GeV/c 2 GeV/c 3.5 GeV/c 2 GeV/c691 cycles
Single cycle

Figure 8: A single Schottky recording of the stochastic
cooling plateaus (left) and the average of 691 recordings(right).

The Anti-Proton Production Status Screen
Displayed on large screens in control rooms and published worldwide via a CERN webpage [7], the status
screen (also called “fix-display”) of the AD now integrates the real time animated Schottky spectrum. Thisaddition enables better, and faster diagnostics of the AD
cooling processes for a prompt action from the operationteam.
Figure 6: Real time longitudinal Schottky spectrum showing a de-tuned stochastic cooling system during AD startup (top) and a nominal AD operation cycle (bottom).

Long Term Logging

Frequency (kHz)

51
50.5
50

LIMITATIONS
Due to hardware limitations, the generated signal derived from the revolution frequency, used for the down
mixing process, works only for stable energy plateaus. Inaddition, the dynamic of the signal for bunched beam is
too high for the acquisition chain. This prevents the system to give any valid measurements during ramps.
The LPU rather weak signal during the 300 MeV/c and100 MeV/c low energy plateaus is also a known limitationof the system. Consequently, less details of the momentum envelop during the electron cooling processes arevisible.

CONCLUSION AND OUTLOOK

49.5
4902
Time (Day)

4Figure 7: Example of historical data of the longitudinal
Schottky spectrum at the end of the 300 MeV/C coolingplateau, over 5 days.
Schottky spectra at the start and the end of each of thefour cooling plateaus (Fig. 5) are recorded for every AD
cycle using NXCALS, the CERN logging infrastructure[6]. It allows machine stability analysis over several days
and observation of any drifts of the cooling systems(Fig. 7).

The AD longitudinal Schottky measurement for operation has been renovated and successfully deployed withstandard CERN hardware and software. It is used for dayto-day operation, and it is a key asset for setup and diagnostics.
Further improvements are under studies, like the computation of the actual momentum spread in percent instead of the signal frequency.
A feasibility study to deploy a similar system for theELENA machine, where the beam energy is even lower, isongoing.

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Averaging

Frequency (kHz)The AD magnetic field and circulating intensity chart issynchronized with the time horizontal axis and displayed
at the bottom. The actual spectrum is displayed on theright and synchronized with the vertical frequency axis.
All the charts are animated with the real time publishingof the data at a rate of 5 Hz.
This application is daily used by the operators for diagnostic and fine tuning. The resulting picture is easily
memorised by operators, and any drift or shape changes
can be spotted efficiently in one look. It is also a useful
asset for time consuming setup and adjustment, such as
the tuning of the stochastic cooling, where the representation of the evolution of the beam momentum envelop
along the cooling process is instrumental.

12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3IBIC2023, Saskatoon, Canada
JACoW Publishing
ISSN: 2 6 7 3 - 5 3 5 0d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - T U P 0 4 5

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ACKNOWLEDGEMENTS
We would like to thank the CERN OASIS, RF and OPAD team for their installation work, support and advises.
In particular B. Ninet, D. Lampridis, M.E. Angoletta, A.
Findlay, G. Tranquille, and L. Ponce.

REFERENCES[1] C. Gonzalez and F. Pedersen, “An ultra-low noise ac beamtransformer for deceleration and diagnostics of low intensity
beams.”, Particle Accelerator Conference, New York, USA,
29 March - 2 April, 1999, pp. 474-476.
doi: 10.1109/PAC.1999.795736[2] D. Boussard, “Schottky noise and beam transfer function
diagnostics 1995 ed.”, in CAS - CERN Accelerator School:
5th Advanced Accelerator Physics Course, pp.749-782.

[3] What is OASIS?,
https://be-dep-css.web.cern.ch/oasis[4] What is FESA?,

https://be-dep-css.web.cern.ch/fesa[5] FFTW,
https://www.fftw.org[6] J.P. Wozniak and C. Roderick, “NXCALS - Architecture andChallenges of the Next CERN Accelerator Logging Service”, in Proc. ICALEPCS'19, New York, NY, USA, Oct.
2019, pp. 1465-1469.
doi:10.18429/JACoW-ICALEPCS2019-WEPHA163[7] AD-ELENA,

https://opwebtools.web.cern.ch/Vistar/vistars.php?usr=ADE

doi: 10.5170/CERN-1995-006.749

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