EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH
CERN/PS 2000-072 (AE)
STATUS REPORT ON THE ANTIPROTON DECELERATOR (AD)
T. Erikson, S. Maury, D. Möhl
CERN's new Antiproton Decelerator (AD) has been delivering a 100 MeV/c
antiproton beam to three experiments (ASACUSA, ATHENA, and ATRAP) since
July 10th, 2000. In this status report, we summarise the initial performance of the AD,
draw provisional conclusions from the first month of operation and finally give some
prospects for the future.
LEAP 2000, 20th-28th August 2000, Venezia, Italy
20 November, 2000
Status Report on the Antiproton Decelerator (AD)
T. Erikson, S. Maury, D. Möhl
(for the AD machine team)
CERN, PS Division, Geneva, Switzerland
CERN's new Antiproton Decelerator (AD) has been delivering a 100 MeV/c
antiproton beam to three experiments (ASACUSA, ATHENA, and ATRAP)
since July 10th, 2000. In this status report, we summarise the initial performance
of the AD, draw provisional conclusions from the first month of operation and
finally give some prospects for the future.
In November 1999, the Antiproton Decelerator (Fig. 1) delivered its first
ejected antiprotons at 100 MeV/c. At that time, the emittances were still very
large and the intensity was low ( ~106 antiprotons) because several systems,
including the beam cooling at 100 MeV/c, were not yet working. After the shut-
down of the CERN machines, commissioning resumed in April 2000, and from
the 10th July onwards, a beam with the characteristics summarized in Table 1
was more or less routinely delivered to the three experiments ASACUSA,
ATHENA, and ATRAP.
The performances shown in Table 1, approach the design goal and were
consistently obtained when all systems worked "correctly". However there were
still many periods with reduced performance or complete break-down, due to
difficulties with various hardware and software components. In this status
report, we will draw first conclusions from the initial month of operation and
give prospects for future improvements.
Presented at : LEAP 2000, 20th-28th August 2000, Venezia, Italy
Figure 1 : Layout of the Antiproton Decelerator and the Experimental Area.
p for tests
26 GeV/c 3.5 GeV/c
Final Performance obtained at 100 MeV/c by August 2000
Extracted beam Obtained July 2000 Design aim
Momentum (MeV/c) 100 100
Intensity ( p per pulse) 2.0x107 2.0x107
Cycle time (sec) 140 60
Emittances (90% beam)
h (.mm.mrad) 4 1
v (.mm.mrad) 2 1
p/p (debunched) 1x10-4 1x10-4
p/p (bunched) 2x10-3 1x10-3
Shortest extracted bunch
Length (ns) 600 200
2 Critical systems
To reach the design pressure, baking of many parts of the machine is essential.
Some equipment, recuperated from the former AC (Antiproton Collector), can
only stand a very "gentle" bake-out with a very slow rate of temperature change.
Thus every bake-out takes of the order of two weeks.
During commissioning, a pressure close to the specified few 10-10 torr was
reached before a leak on the tank of an extraction kicker occurred. A proper
repair necessitates breaking the vacuum. Therefore the leak was provisionally
stopped "in situ" to avoid a new bake-out. As a result a local 'bump' in the
pressure remains and the composition of the residual gas is different. This
increases the scattering of the circulating antiprotons by at least a factor two.
Although this shortcoming will be solved during the next long shut down, it
highlights out vulnerability and the inconvenience of the long delay after each
breaking of the vacuum.
2.2 Power converters
The converters both for the bending magnets and for the quadrupoles form
complicated networks with interlaced main and trim supplies. The unusual
high stability required to handle the cooled beam is an additional challenge. In
the operation of the AD up to now, frequent interventions by the experts are
required, nevertheless long periods of stable operation have now become
During commissioning, several quadrupoles developed water leaks. This was
traced to small movements of the coils during the cycling of the machine,
resulting in fatigue of the copper conductors connecting the different sections of
the coil. The leaks were provisionally fixed "in situ" but a definite repair has to
be made in a long shut down. The coil movement on 28 large aperture
quadrupoles has been reduced by inserting spacers between the coil pancakes.
Again this is only a temporary measure. All magnetic elements were designed
for the former dc-operated AC, and are now cycled at a rate up to 1/min. A
"consolidation programme " to make these elements "cycle proof" is therefore
2.4 Cooling systems
The performance of the cooling systems is summarised in Table 2. Whereas
the cooling at 3.5 and 2 GeV/c works very satisfactorily, the cooling at the lower
momenta needs further work to reach the design performance. The final
emittances reached at 300 and 100 MeV/c will be improved once the "multiple
scattering heating" by the residual gas is reduced by improving the vacuum of
the ring. To reach shorter cooling times, several measures are envisaged : the
overlap of the electrons with the antiproton beam will be improved; drifts in the
energy will be avoided by stabilisation; including a feedback on the electron
energy; the neutralisation of the electron beam will be controlled to avoid
fluctuations of the space charge forces at higher electron currents.
A very high stability in the orbit and the energy of the antiproton beam is as
important as the quality of the cooling beam. For the main power supplies
tolerances as low as a few 10-4 at 100 MeV/c, which is a challenge, given the
range of 35 between injection and ejection momentum on the one hand and the
complexity of the AD supplies on the other. The cooling performance is not only
intimately linked to the vacuum and the power supply systems, it also depends
on the optics of the ring. Special settings, different from the "high energy optics"
used so far during the whole cycle, will be tried on the electron cooling plateaus.
Performances of the Cooling System with h , v (horizontal and vertical
emittances in .mm.mrad) and p/p (dispersion of the debunched beam in %)
Momentum Cooling Final emittances Total cooling time
(GeV/c) System (95% beam) t(sec)
Obtained July 2000 Design aim Jul.2000 Aim
3.5 Stochastic 3 4 0.100 5 5 0.100 20 20
2.0 Stochastic 4 4 0.015 5 5 0.030 15 15
0.3 Electron 1 0.5 0.010 2 2 0.100 28 6
0.1 Electron 5 2.5 0.015 1 1 0.010 16 1
2.5 Beam Diagnostics
New diagnostic devices have been developed to monitor the beam (intensity,
size and position) with as little as 107 p . At 100 MeV/c this corresponds to a
circulating current of only 260 nA, which cannot be resolved by a "normal" beam
current transformer nor by "straightforward" Schottky noise analysis. In the
beginning, before these new monitors were operational, commissioning was
therefore done with beams of about 109 test protons injected through the former
AA (Antiproton Accumulator) ejection line. This "TST" beam is very useful for
debugging. But as it circulates in the opposite direction to the antiprotons,
neither the cooling systems nor the directional Schottky pick-ups are usable.
In addition to the Schottky noise system foreseen to monitor the debunched
beam on the different plateaus, the AD is equipped with an advanced
'electrostatic' orbit observation system that works with the bunched beam. After
some effort, the noise level on the electrostatic pickups and their very sensitive
head amplifiers could be sufficiently reduced to monitor the position of the
antiproton bunch. This makes it possible to survey and correct the closed orbit of
the antiprotons, even at 100 MeV/c. An intensity measurement is also obtained
at 100 MeV/c using the bunched beam signal from the pickup normally used for
The new, extremely-low-noise Schottky system works for the longitudinal
signal from the coasting beam. It serves as monitor for the momentum width
p/p and provides a good signal, especially when the beam is well cooled. The
special Schottky monitors for the transverse plane, foreseen to survey betatron
tune and beam size, are not yet operational. Also the profile detectors,
observing beam size via the ions (and/or electrons) created by ionisation of the
residual gas, need further development. Thus for the moment, antiproton beam
emittances at low momentum can only be measured destructively by moving
scrapers and observing the beam loss with scintillators. For the Q-measurement,
the beam transfer function technique is used, based on the observation, via a
transverse Schottky pickup, of the beam response to a swept sine wave applied
to a transverse kicker.
This rudimentary set of diagnostics proved just sufficient to set up and
operate the machine down to the lowest energy, but the advent of the new
transverse Schottky pickups and the non-destructive beam-ionisation profile
monitors is eagerly awaited. In addition, it is planned to get the "stacking mode"
operational to accumulate a factor of perhaps five in intensity at injection. This
will ease the diagnostics and perhaps also serve some users, although the
stacking is time-consuming because, although the intensity per pulse increases,
the flux to the experiments does not increase as much.
2.6 Controls and operation
The controls system was of great help during the commissioning phase. But it
is still at an early stage of its evolution towards the high degree of automation
Report CERN/PS 96-43 (AR) required for routine operation, as defined in the
design report. Although the philosophy is well established, a lot of application
programs remain to be developed. One example is a powerful fault diagnostics
program, acquiring, storing and displaying the key parameters on all the
plateaus of the cycle.
For the moment, AD is operated from the local control room (ACR). A
"machine supervisor" is responsible for the operation on a daily basis. He is
assisted by an "operation technician" from 08h00 to 15h00 (first shift) and 15h00
to 23h00 (second shift). During the night, (23h00 to 08h00) the machine runs in
"automatic mode" without an operator on shift and there is no recovery if it
breaks down. The morning shift is at present taken by the machine team for
setting up and development, but frequently beam is given to the experiments
during this period in parallel. The machine stops from Friday 23h00 to Monday
08h00. It is foreseen to transfer later the routine operation from the local to the
main PS control room. The situation will be reviewed in the light of the
2.7 Future Developments
To reach and to consolidate the design performance, further work is
necessary. A special effort is needed to obtain the high stability and the small
emittances of the extracted beam required for 'post-deceleration' by the radio
frequency quadrupole (RFQ) to be installed in October 2000.
Further developments will concern the intensity per pulse and the time
structure of the ejected beam. Stacking at injection will be implemented if a
higher intensity per pulse is required for some experiments. To make this work,
a curious phenomenon, recently observed, has to be cured : once the intensity of
p exceeds about 2x107 particles, the extracted bunch tends to develop a double-
humped time structure, whereas at 1x107 particles it is still nicely bell-shaped. It
is not yet clear whether this is caused by a coherent instability. If it is, the active
feedback system (installed to counteract coherent instability, but not yet
operational) should help.
As another option "multiple ejection" could be envisaged, e.g. by capturing
the 100 MeV/c beam into several buckets which are kicked out individually.
Three bunches can be provided relatively easily with the rf-system that works on
harmonic 3 from 300 to 100 MeV/c. The time between ejections is constrained by
the repetition time of the kicker and by the beam lifetime.
The options mentioned so far are more or less straightforward extrapolations
from the design performance. Developments beyond this are constrained mainly
by the space available in the Hall and by scheduling difficulties. This applies to
the request for more experiments and/or more exotic extracted beam patterns.
One may think of using the "beam measurement zone" for an additional
experiment. The magnetic elements and power supplies limit its momentum to
about 300 MeV/c. This DEM line has also been designed for checking the
ejected beam characteristics. A small controlled area is attached to this line.
Once the measurements on the ejected beam are no longer essential, or if they
can co-exist with experimental apparatus, then an additional small experiment
could be attached to the DEM line.
A preliminary study of slow extraction from the AD was done in 1995. No
satisfactory solution was found and the conclusion was that slow extraction is
not feasible without major modifications of the AD.
The AD has started operation with characteristics not far from design
expectations. Work is still needed to improve and consolidate the performance
and to ease the operation. Simple developments concerning e.g. the time
structure of the ejected bunches or the intensity per pulse can be envisaged.
More radical improvements like the installation of a further experiment, or using
a higher extracted energy require further study.