BEAM QUALITY MONITORING IN THE CERN ANTIPROTONDECELERATOR (AD)T.Eriksson
CERN, CH-1211 Geneva 23, SwitzerlandAbstract
The CERN Antiproton Decelerator (AD) has now reacheda stable mode of operation, regularly delivering batchesof 100 MeV/c antiprotons to the ATRAP, ATHENA and
ASACUSA collaborations.
Experience during the commissioning and initial physics
runs revealed difficulties in achieving design goals andmaintaining stable performance levels.
This paper will deal with the specific difficulties inmonitoring and improving beam quality in a slow cyclingmachine operating with low beam intensities of a few 107
circulating particles.

1 INTRODUCTION
AD, the low energy antiproton facility at CERN (Fig.1)
routinely produces and delivers 100 MeV/c antiprotons to
the three experiments ASACUSA, ATHENA andATRAP. Commissioning of the AD started in fall 1998
and after a lengthy period of running-in, physics started inJuly 2000.
Since the start of AD physics, regular machinedevelopment sessions have been organized with the aimof improving machine performance, improving stability,
recovering from performance degradations that occur
during the run and installing and testing new beamdiagnostic equipment.

stochastically cooled, bunched again and decelerated
down to 2 GeV/c. There it is stochastically cooled once
more, primarily to reduce the momentum spread to fit therequirements of the deceleration RF system. After thestochastic cooling, the AD working point is moved from
Qx=5.385, Qy=5.37 to Qx=5.45, Qy=5.42 taking advantageof the small emittances to cross the 5th order resonance
5Qx=27. The first working point provides maximum
machine acceptance at injection, while the second places
the beam in a region of the tune diagram where moreresonance free space is available. This is particularly
important at low momenta.
The antiproton beam is then decelerated down to 300
MeV/c and cooled by the beam from the electron cooler.
After cooling, the beam is rebunched on harmonic
number 3 (the deceleration RF cavity only operates in therange 0.5 – 1.6 MHz) and decelerated to the ejection
momentum of 100 MeV/c. The antiprotons are again
cooled by the electron beam, rebunched on harmonic
number 1 (this is necessary to extract all the particles in
one bunch, for which the RF cavity resonant frequency islowered to 174 kHz by means of a relay-switchedcapacitor), rotated by 90 degrees in longitudinal phasespace (if experiments demand shorter beam, which istypically the case) and finally ejected.

Figure 2: AD cycle.
Figure 1: AD layout.
To produce the low energy antiproton beam, protons of26 GeV/c are ejected from the PS and transferred to atarget. There, antiprotons are produced and transferred to
the AD ring via a curved transfer line. After injection at
3.57 GeV/c (Fig.2) the antiproton bunches are rotated by90 degrees in longitudinal phase space, taking advantageof the short bunch length of about 25 ns to reduce theinitial momentum spread. Then the beam is debunched,

2 PERFORMACE REVIEW 2000-2002
During the commissioning and initial startup, it was
found that setting-up the AD and improving/maintaining
its performance is very time-consuming. Making accurate
measurements of intensity, tunes, orbits and machine
acceptance with beam intensities in the order of 1-4*107
pbars requires sophisticated equipment and patience. Thelong (1-2 minutes) cycle with 4 different flat parts forbeam cooling also makes adjustments very slow.

Nevertheless, machine performance was gradually
improved over the first years of operation and is now up
to or exceeding the design specifications in most respects(Table 1). Efforts will continue in order to further reducethe cycle duration by attempting to improve beam-cooling
speed and reduce the duration of the deceleration ramps.
Note the relatively large number of hours spent on
startups and machine development. This is in part
explained by the need to regularly re-optimize themachine, which is often suffering from drifts in beam
trajectories and beam cooling performance.
Table 1: AD performance and operation 2000-2002.
2000
Intensity at 3.5
7GeV/c (*10 )Intensity at 100
7MeV/c (*10 )Deceleration
efficiency (%)
Cycle repetition
rate (s)Flux (pbars/s)
Emittances at
100 MeV/c(85% beam)

End
2002
3.3

Design

3.0

End
2001
3.2

2.0

3.0

3.0

1.2

65

95

90

25

140-110

96

86

60

1.8*1053*10

5

3.5*10552*10εh(π.mm.mrad)

4

1

1

1εv(π.mm.mrad)

2

1

11

∆p/p (aftercooling)

1.5*10∆p/p (bunched,after rotation)
Minimum
extracted bunch
length (ns)

Total hours forphysics
Total hours forstartup/md/recovery
Downtime (%)

3*10-4-3

1*10
2*10-4-3

1*10
8*10-4-4

600

220

90

1500

2250

2100

2050

800

700

14

11

10

1*10

5

some extent, beam steering can be done blindly byscanning steering dipole currents, but this is rarely
successful due to the low cycle repetition rate (1-2minutes). Complete trajectory correction through thedogleg would take several hours, during which time othermachine and beam parameters often change. The correct
method is to setup the AD with protons. In this mode, 3.5
GeV/c protons are injected and made to circulate in theAD in the reverse direction via a special transfer line.
Beam intensities of a few 109 protons are sufficient forcorrect operation of transformers and screens. Setting up
the proton scheme is however not a simple task and also
very time-consuming. It is only done when no othersolutions can be found.
For routine beam intensity monitoring during the physics
runs, a special Digital Signal Processing-based system(Fig.3) has been implemented to measure circulating
beam intensities and momentum spread throughout thedeceleration cycle. The system performs FFT spectral
analysis on the signal from two combined low-noise
longitudinal Schottky pickups covering 0.02-30 MHz.
Averaging of many scans is necessary for analysis of thelow intensity beam. This is perhaps the most important
tool used in operation since it gives non-destructive,continuous and reliable information in real-time (Fig.4).

-4

1.7*10

-3

200

3000

3 MONITORING AND IMPROVINGBEAM QUALITY
3.1 Beam intensity
Measuring proton beam intensity and trajectory throughthe injection line up to the production target is done using
beam current transformers and screens equipped with TVcameras. Production beam intensities in the order of
1.5*1013 protons are common. After the target andthrough the “dog-leg” only 3-4*107 antiprotons arecollected making the use of such devices impossible. To

Figure 3: DSP Schottky analysis system.

Figure 5: Measured intensity over AD cycle.

Figure 6: Vertical beam profiles before and after coolingat 3.5 GeV/c.

3.2 Beam emittances
To keep beam emittances low and losses duringdeceleration to a minimum, beam cooling must work well.
At low beam energies, there are problems maintaining
good beam cooling performance, which results in higher
than normal beam emittances and losses. Regular tuningof the electron cooling is necessary almost every week.
To optimize cooling, the position and angle of the pbar
and/or electron beams are scanned and transverse
emittances measured for each step. Emittancemeasurements are done by stepwise entry of a scraper
blade into the beam, and simultaneously detecting createdsecondary particles with a scintillator/photomultipliersystem. (Fig.5) Beam profiles can be measured with goodprecision at all energies. (Fig.6) This method is however
destructive and very slow due to the long cycle and theneed to stop the cycle for measurements at certain
energies. It is not unusual to spend several hours on suchtuning.

To improve the situation, a new Beam Ionization Profile
Monitor system is now under development. This device
allows non-destructive monitoring of beam position andtransverse emittances throughout the cycle. Identificationof sources for cooling degradation would also be greatly
helped with this device. Promising results have beenobtained, but more work is necessary before routine
measurements are possible. (Fig7).

Fig.7: Horizontal beam width and position during thecycle as seen with the beam ionisation profile monitor.

Figure 5: Beam profile measurement layout.

An additional way of analyzing longitudinal Schottky
noise is used routinely in the AD. The signal is downmixed and analyzed by a commercial FFT signal analyzer
in real-time. No numerical results are produced, but avisual interpretation of the frequency domain of thecirculating beam is possible. (Fig.8) This is an extremely
useful device for general beam quality monitoring overthe machine cycle, giving the operator a good “feeling”
and a very quick initial diagnosis of:• Distribution in frequency domain.
• Intensity.
• Cooling speed and centering.
• Initial and final momentum spread.
• Losses during the cycle.
• Instabilities.

∆∆

∆

∆

∆

Σ
Σ

Σ

Σ

Σ

Σ

∆
∆
Σ

∆

∆/Σ
Σ

Σ

∆

Σ

Figure 9: AD H/V ring pickup system.

4 CONCLUSION

Fig.8: Longitudinal Schottky analysis at 3.5, 2, 0.3 and0.1 GeV/c.

3.3 Stability of orbit and ejection trajectory
In 2000 and 2001, the AD experiments suffered frominstabilities and drifts in the trajectory of the ejected pbars.
The RFQ decelerator in the ASACUSA experimental area
requires very high transverse and longitudinal stability inorder to maintain good efficiency. Much time was spentby the operations team to retune the 100 MeV/c transferlines. Analysis showed that the machine orbit before
extraction was the cause. However, both slow drifts andfast jumps occurred pointing either to a single or toseveral locations around the ring. Fortunately, closed
orbit measurements and corrections can be made at low
beam energies thanks to the improved low-noise beam
position pickups and closed orbit measurement system(Fig. 9), which allow accurate measurements with as little
as 1*107 circulating particles at 100 MeV/c. During
normal operation, orbits are now continuously measured
and logged at 300 and 100 MeV/c in order to detect any
abnormalities. The situation improved during 2002 whenseveral possible hardware and software causes wereeliminated.

Initial setup problems were encountered in the AD,
mainly due to the low beam intensities and a slow cycle.
Beam monitoring and improvements turned out to be
more difficult and slower than foreseen.
The long startup period and subsequent improvements
and tune-up sessions have made it possible for theoperation team to get to know the AD quite well. Not
always using the latest or most high-tech diagnostic
equipment makes things slower, but it can be moreinteresting for the operator, giving opportunities to learn
more about accelerator physics.