R. Cappi

COLLECTIVE EFFECTS IN THE CERN-PS BEAM FOR LHC

This paper is an updated review of the collective effects observed and predicted in the CERN-PS machine for the LHC beam.

Collective Effects at Very High Intensity in the CERN-PS

The CERN-PS beam intensity is being steadily increased (1.55 10’ 3 protons per pulse achieved). In addition, a change of harmonic number by debunchingrebunching is performed to allow a clean bunch-intobucket injection into the SPS. Many collective phenomena had to be studied in this context lately, and selected results of interest to machine designers or operators are reported here. Topics covered are resistive-wall instabilities with peculiar characteristics due to space-charge detuning and fast-decaying wakes, microwave longitudinal instabilities, and problems associated with strong cavity beam-loading.

Adiabatic capture of charged particles in stable islands: a novel approach to multi-turn extraction

Recently a novel approach has been proposed aimed at performing multi-turn extraction from a circular machine. Such an approach consists of splitting the beam by means of stable islands created in transverse phase space by non-linear magnetic elements such as sextupoles and octupoles. Provided a slow time-variation of the linear tune is applied, adiabatic with respect to the betatron motion, the islands can be moved in phase space and eventually charged particles may be trapped inside the stable structures. This generates a certain number of well-separated beamlets. Originally, this principle was successfully tested using a fourth-order resonance. In this paper the approach is generalized to other type of resonances and some examples of adiabatic capture performed by using various low-order resonances are presented and described in detail.

Ultraslow Extraction with Good Duty Factor

In the framework of antiproton physics at CERN, a new Low Energy Antiproton Ring (LEAR) is being designed. In the basic mode, it will serve as a beam stretcher giving spill times in the region of an hour. The spill φ(t) must, of course, have a good duty factor 2/ 2. A method employing “stochastic extraction” has been studied theoretically and tried out at the CERN PS (∿ 9 s flat top) where an extremely good duty factor has been achieved, showing that much longer spill times will be practicable. The pulse length can be varied within wide limits given by the ripple, the momentum acceptance and the intermodulation distortion of the amplifier chain for the noise power. In addition, another method has been found effective which uses empty buckets. These methods need no servo system and both can easily be applied to other synchrotrons.

MULTITURN EXTRACTION BASED ON TRAPPING IN STABLE ISLANDS

Recently a novel approach to perform multi-turn extraction was proposed based on beam splitting in the transverse phase space by means of trapping inside stable islands. During the year 2002, preliminary measurements at the CERN Proton Synchrotron with a low-intensity, singlebunch, proton beam, confirmed the possibility of generating various beamlets starting from a single Gaussian beam. The experimental campaign continued also during the year 2003 to assess a number of key issues, such as the feasibility of trapping with high-intensity beam and capture efficiency. The experimental results are presented and discussed in detail in this paper.

Adiabatic beam trapping in stable islands of transverse phase space: measurement results at CERN Proton Synchrotron

Recently a novel approach to the problem of multi-turn extraction was proposed. It consists of splitting the beam by adiabatic capture inside stable islands created in the transverse phase space by sextupoles and octupoles. Numerical simulations indicate that such a technique should be feasible and potentially superior to the method presently used at the CERN Proton Synchrotron. During 2002, intense efforts were devoted to the experimental verification of this newly proposed extraction mode. Finally, beam capture into the islands was observed. In this paper, the extraction principle is briefly reviewed and the experimental results are presented and discussed in detail.

Measurement and reduction of transverse emittance blow-up induced by space charge effects

The CERN PS, as part of the LHC injector chain, will have to keep a high intensity, high brilliance beam for 1.2 s at the injection energy. The transverse particle density will exceed, by a factor of three, the highest currently attained. Careful experimental studies have recently been carried out in the PS to investigate transverse emittance blow-up in such a severe space charge regime. In addition, a new controlled longitudinal blow-up technique has been developed to produce bunches with flat-topped density profiles and, accordingly, reduced peak current. The results achieved so far are presented and discussed.<<ETX>>

The CERN PS complex as part of the LHC injector chain

The delivery of a beam with characteristics appropriate to the filling of the LHC (Large Hadron Collider) proton-proton collider requires that the CERN PS complex provide a beam whose transverse particle density exceeds, by a factor of three, the highest currently attained. The beam dynamics operations and the associated hardware modifications which would be required to achieve this goal are considered. The approach favored involves filling the Proton Synchrotron (PS) with two pulses from the Proton Synchrotron Booster (PSB) and requires and RF quadrupole (RFQ2) as a preinjector for the linac (LINAC2), an increase of the PSB extraction energy and additional RF systems, both in the PSB and PS.<<ETX>>

Acceleration and stacking of α particles in the CERN Linac, PS and ISR

The CERN Intersecting Storage Rings (ISR) have been successfully operated with sufficient α particles for experiments at total center of mass energies up to 120 GeV. Initially, the small beam currents obtainable from the (old) Linac hampered machine studies with the PS so that conclusive experiments similar to those done with deuterons were not possible. Recent attempts to increase the intensity by stripping a He+ beam at 520 keV succeeded and gave 10 mA of α particles from the Linac. Multiturn injection and acceleration in the PS produced 2×10¹¹ particles/pulse and stacking in the ISR resulted in a maximum stored beam intensity of 4.2 A at 52 GeV. After further acceleration by phase displacement in the ISR, luminosities of 4×10²⁸cm^-2s^-1 for α-α and 9×10²⁹cm^-2s^-1 for α-p experiments were obtained.

Upgrading the CERN PS booster to 1 GeV for improved antiproton production

For efficient antiproton production, a maximum number of protons must be concentrated within one quarter of the CERN Proton Synchrotron (PS) ring before sending the beam to the production target. With the Antiproton Collector (AC) added to the Antiproton Accumulator (AA), the bunch length has to be shorter (by about 20 ns) than before to allow bunch rotation in the AC. While a more ambitious scheme providing such a beam is being implemented, a funneling method, in which beams of two rings of the four-ring Proton Synchrotron Booster (PSB) are recombined in pairs by an RF dipole that permits longitudinal interleaving of successive bunches, has been in operation since the start-up of the AC. Preliminary experiments had shown that the PS space-charge limit had to be overcome in order to make the scheme feasible. After raising the PSB output energy from 815 MeV to 1 GeV, beams of >10/sup 13/ protons compressed into one quarter of the PS ring were achieved. Related to this development, a record proton beam for fixed-target physics was accelerated in the Super Proton Synchrotron, while beam losses in the Proton Synchrotron Booster-Proton Synchrotron Ring (PSB-PS) line were reduced.<<ETX>>