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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH
CERN-PS DIVISION
CERN/PS 2000-072 (AE)
STATUS REPORT ON THE ANTIPROTON DECELERATOR (AD)
T. Erikson, S. Maury, D. Möhl
Abstract
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
Abstract
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.
1 Introduction
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
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Figure 1 : Layout of the Antiproton Decelerator and the Experimental Area.
Table 1:
Final Performance obtained at 100 MeV/c by August 2000
Extracted beamCharacteristics / Obtained July 2000 / Design aim
Momentum (MeV/c) / 100 / 100
Intensity ( per pulse) / 2.0x107 / 2.0x107
Cycle time (sec) / 140 / 60
Emittances (90% beam)
h (.mm.mrad)
v (.mm.mrad)
p/p (debunched)
p/p (bunched) / 4
2
1x10-4
2x10-3 / 1
1
1x10-4
1x10-3
Shortest extracted bunch
Length (ns) / 600 / 200
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2Critical systems
2.1Vacuum
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.
2.2Power 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 possible.
2.3Magnets
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 necessary.
2.4Cooling 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.
Table 2
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(GeV/c) / Cooling System / Final emittances
(95% beam)
Obtained July 2000Design aim / Total cooling time
t(sec)
Obtained Design
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
2.5Beam Diagnostics
New diagnostic devices have been developed to monitor the beam (intensity, size and position) with as little as 107. 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 Schottky measurements.
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.
2.6Controls 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 experience gained.
2.7Future 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 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.
- Conclusion
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.