Gilbert Guignard

Simulation package based on PLACET

The program PLACET is used to simulate transverse and longitudinal beam effects in the main linac, the drive-beam accelerator and the drive-beam decelerators of CLIC, as well as in the linac of CTF3. It provides different models of accelerating and decelerating structures, linear optics and thin multipoles. Several methods of beam-based alignment, including emittance tuning bumps and feedbacks, and different failure modes can be simulated. An interface to the beam-beam simulation code GUINEA-PIG exists. Currently, interfaces to MAD and TRANSPORT are under development and an extension to transfer lines and bunch compressors is also being made. In the future, the simulations will need to be performed by many users, which requires a simplified user interface. The paper describes the status of PLACET and plans for the future.

Scaling laws for e+/e− linear colliders

Abstract Design studies of a future TeV e+e− Linear Collider (TLC) are presently being made by five major laboratories within the framework of a world-wide collaboration. A figure of merit is defined which enables an objective comparison of these different designs. This figure of merit is shown to depend only on a small number of parameters. General scaling laws for the main beam parameters and linac parameters are derived and prove to be very effective when used as guidelines to optimize the linear collider design. By adopting appropriate parameters for beam stability, the figure of merit becomes nearly independent of accelerating gradient and RF frequency of the accelerating structures. In spite of the strong dependence of the wake fields with frequency, the single-bunch emittance blow-up during acceleration along the linac is also shown to be independent of the RF frequency when using equivalent trajectory correction schemes. In this situation, beam acceleration using high-frequency structures becomes very advantageous because it enables high accelerating fields to be obtained, which reduces the overall length and consequently the total cost of the linac.

A new method for RF power generation for two-beam linear colliders

In this paper we discuss a new approach to two-beam acceleration. The energy for RF production is initially stored in a long-pulse electron beam which is efficiently accelerated to about 1.2 GeV by a fully loaded, conventional, low frequency (∼1 GHz) linac. The beam pulse length is twice the length of the high-gradient linac. Segments of this long pulse beam are compressed using combiner rings to create a sequence of higher peak power drive beams with gaps in between. This train of drive beams is distributed from the end of the linac against the main beam direction down a common transport line so that each drive beam can power a section of the main linac. After a 180-degree turn, each high-current, low-energy drive beam is decelerated in low-impedance decelerator structures, and the resulting power is used to accelerate the low-current, high-energy beam in the main linac. The method discussed here seems relatively inexpensive, is very flexible and can be used to accelerate beams for linear colliders over ...

CLIC-a compact and efficient high energy linear collider

We discuss the RF system, the drive linac, drive beam generation, the isochronous ring drive beam scheme, the main linac injector system, machine parameters, beam dynamics and final focus studies and the alignment test facility and beam monitor test results.

Operational Experience with the Superconducting High-Luminosity Insertion in the CERN Intersecting Storage Rings (ISR)

The eight superconducting quadrupoles and their cryogenic equipment for this insertion were installed in the ISR at the end of 1980. The insertion has been used to assess the problems of running a superconducting insertion in a storage ring as well as to provide high luminosity for physics. The luminosity is increased at intersection 8 by a factor of 7. By means of dedicated collimators and orbit corrections, safe working conditions could be established for the superconducting magnets during injection, accumulation, stable beam periods and when dumping the beams. Quenches were mainly caused by large accidental beam losses. Operating parameters for all standard beam energies, including acceleration to 31.4 GeV/c, have been established. At 26.6 GeV/c, with currents of 30.6 A in ring 1 and 30.3 A in ring 2, a luminosity of 1.4 1032 cm-2s-1 was obtained in the insertion. This is the highest luminosity reached so far in storage rings and it was obtained during a physics run. Satisfactory beam conditions could also be provided for antiproton physics at 26.6 GeV/c in ISR with both low-ß insertions on, in I1 and I8, respectively.

The CLIC study of magnet stability and time-dependent luminosity performance

The present parameters of the CLIC study require the collision of small emittance beams with a vertical spot size of 1 nm. The tolerances on vertical quadrupole vibration (above a few Hz) are as small as a few nm in the linac and most of the Final Focus. The final focusing quadrupole has a stability requirement of 4 nm in the horizontal and 0.2 nm in the vertical direction. Those tolerances can only be achieved with the use of damped support structures for CLIC. A study has been set-up at CERN to explore the application of stabilization devices from specialized industry and to predict the time-dependent luminosity performance for CLIC. The results will guide the specification of required technological improvements and will help to verify the feasibility of the present CLIC parameters.

Magnetic performance of the LEP bending magnets

The 3304 steel-concrete cores of the LEP (Large Electron Positron colliding beam accelerator) bending magnets have been individually measured to determine the excitation characteristics (field integral on central orbit versus current), and one out of ten has been submitted to a complete measurement of the field pattern in the aperture. The end and junction effects, as well as the field distortion due to the vacuum chamber both in static conditions (due to residual magnetism of the chamber materials) and during field ramping (influence of eddy currents), have been determined on some average cores. The authors briefly describe the measuring benches and give the average values and RMS (root-mean-square) dispersions of the dipole, quadrupole, sextupole, octupole, and decapole components seen by the beam at different field levels and during ramping. The effects on the machine parameters are analyzed for each component and compared with results obtained in the first injection tests.<<ETX>>

Measurement of the Excitation of the Coupling Resonance Qh - Qv = 0

To take advantage of the large resonance-free regions close to the diagonal Qh = Qv in the tune diagram, the Intersecting Storage Rings (ISR) operate with nearly equal tunes. Thus, the excitation of the coupling resonance Qh - Qv = 0 is of importance and this has stimulated the study of its effects, the measurement of its excitation by axial and skew quadrupole fields and its compensation. A complex coupling coefficient C can be defined in terms of axial and skew quadrupole fields, and the unperturbed machine parameters. An electronic device has been built to measure |C| by kicking a small beam and analysing the coherent oscillation. By combining different coupling vectors, phase measurements are also possible. Examples are given of coupling vectors measured in the ISR for magnet tilts, skew quadrupoles and solenoids. The outlines of two methods for directly measuring both amplitude and phase are also given. Some ideas are extended to higher order resonances.

Colliding nanobeams in CLIC with magnets stabilized to the sub-nm level

The Compact Linear Collider (CLIC) aims at colliding e/sup +/e/sup -/ beams at 1.5 TeV with effective transverse spot sizes of 60 nm (horizontal) times 0.7 nm (vertical). Strict stability tolerances must be respected in order to achieve a sufficient overlap of the two colliding beams. A stability test stand has been set up at CERN, bringing latest stabilization technology to the accelerator field. Using this technology, a CLIC prototype magnet was stabilized in a normal CERN working environment to less than 1-nm vertical RMS motion above 4 Hz. Detailed simulations of the time-dependent luminosity performance of CLIC are discussed. They include the beam-beam interaction, the beam-based feedbacks and the measured data on magnet stability.

Theory of single bunch stability and dynamics in linacs with strong wakefields and misalignments

Abstract The basic method we propose in order to solve analytically the equation of motion of a relativistic single-bunch travelling in a linac, in the presence of wakefields, has been summarized in a preceding report (Guignard and Hagel, CERN-SL-98-015 (AP) and CLIC Note 362, 1998). The extended treatment presented here includes the quadrupole transverse displacements, the chromatic variation of the magnetic focusing, the energy spread along the bunch and possible microwave quadrupoles. It deals with a Gaussian distribution of charge, linear variation of the wakefields within the bunch and smooth focusing. The energy is assumed to be constant in linac sectors, but increases from one sector to the next to simulate acceleration. The longitudinal and transverse equations of motion are solved, the second by using the perturbation method with partial expansions developed for this theory. The localized nature of the misalignment kicks and their superposition property are preserved by using thin lenses. The causality of the downstream oscillations due to these kicks is introduced via Heaviside functions. These ideas make it possible to build an analytical model for quadrupole misalignments and correlated displacements due to trajectory corrections. The resulting theory provides algebraic expressions for BNS damping, tune shifts, transverse off-sets and emittance dilution. It represents a significant break-through complementing the simulations and reproducing the oscillations observed numerically.