Wolfgang Schnell

The Large Hadron Collider (LHC) in the LEP tunnel

After the remarkable start-up of LEP, the installation of a Large Hadron Collider, LHC, in the LEP tunnel will open a new era for the High Energy Physics. This report summarizes the main LHC parameters and subsytems and describes the more recent studies and developments.

Stochastic cooling of a stored proton beam

Abstract A setup to damp the incoherent betatron oscillations of the particles in a coasting beam is described. First experimental results obtained in the CERN Intersecting Storage Ring (ISR) are presented.

Design Concept for a 100 GeV E+-E- Storage Ring

A conceptual design of a 100 GeV e+-e- storage ring (LEP) being studied at CERN and some of the problems encountered are presented. The 20 GeV fastcycling injector synchrotron is studied at the Rutherford Laboratory.1 To obtain a luminosity L = 1 × 1032 cm-2 s-1 at 100 GeV, the product of bending radius ¿ and the RF power PB delivered to both beams must be PB ¿ = 136 GWm, assuming optimum coupling, a maximum permissible beam-beam tune shift ¿Q = 0.06, and a vertical amplitude function ßy* = 0.01 m at the crossings. The bending radius ¿ = 6.1 km was obtained by cost optimisation.2

Compensation of Beam Loading in the ISR RF Cavities

The RF cavities employed for stacking in the ISR are equipped with a powerful feedback system, which compensates for most of the beam-induced voltage. This system is, however, limited - mainly by the current in the final amplifier tube - to injected beam intensities of about 5 × 1012 protons at most. Higher intensities are now available from the PS. Additional beam-load compensation has, therefore, been added to the existing cavities and feedback amplifiers. This beam-load compensation consists of a pick-up, a one-turn delay cable, a wide-band amplifier chain and a final amplifier, directly connected to the accelerating gap. This system injects into the cavity a current approximately equal and opposite to the beam current.

Design Study of a Large Electron-Positron Colliding Beam Machine - LEP

Early in 1976 a study group at CERN began to examine design concepts for a Large Electron-Positron storage ring, LEP. At that time, 50 km circumference and 100 GeV per beam - to be obtained with a conventional radio frequency system - were chosen. This first study was terminated with several problems still unsolved, including extreme sensitivity of orbit stability to closed-orbit tolerances, operation in collision mode with electrostatically separated beams and technical difficulties due to the low magnetic field at injection. In addition, the estimated cost was considered high. A fresh start was made in the second half of 1977. In order to explore the variation of difficulties and cost with machine size and in an attempt to arrive at a solid base for an entirely feasible machine, it was decided to reduce the nominal energy to 70 GeV while retaining the target for maximum luminosity at 1032cm-2s-1. The optimum radius for this design - later confirmed by the outcome of the study - is 3.5 km. This phase of the study ended in August 1978 with the issuing of a detailed Design Report including a cost estimate. The conclusions are that such a machine is not only feasible but that it can be developed to reach 100 GeV per beam when suitable superconducting cavities become available. In fact, the design is made such that this extension of energy requires no major change other than the substitution of cavities.

Experiments with Stochastic Cooling in the ISR

Recent results with stochastic cooling of vertical betatron oscillations in the frequency ranges 1-2 GHz and 80-360 MHz are presented. The new experimental set-up for cooling of momentum spread in the 50-180 MHz range for low intensities is also described.

Report on the ISR

During 1974, the period covered by this report, the ISR were in operation for 3500 h, of which 80% were for physics runs and their preparation. 20 physics experiments took data at six of the beam intersections. Ten of these experiments were completed. Several new ones will start this year, after the annual shut-down ending March 1st. Again six intersections will be used for physics, including one (I 7) that has now been equipped with a high luminosity insertion. Operational performance throughout the year has profited from the regular application of the methods developed at the end of 1973, viz. gradual compensation of the space-charge tune shift during stacking followed by fine tune corrections with the help of Schottky diagnostics Circulating currents above 15 A and luminosities around 5 × 1030 cm30 s-1 became routine performance, while the beams remained confined to the space of the tune diagram marked 8C in Fig. 1. This space is free of non-linear resonances below the order 8. In addition, a further improvement of the pressure to values well below 10-11 torr average around the two rings, has led to improved clearing of ionization electrons. As a result, decay rates as low as 3 × 10-5 fractional loss of current per hour have often been observed. More than half of this decay is due to beam-beam collisions, the rest to nuclear collisions with the residual gas. These conditions permitted running at the highest luminosity while all experiments could take data, undisturbed by excessive background.

Longitudinal Bunch Dilution Due to RF Noise

The effect of phase noise on a tightly bunched proton beam is investigated taking into account the frequency spread in the beam, the wall impedance and the phase feedback loop. Under normal conditions the measured dilution rates in the ISR correspond typically to a doubling of the bunch length in one to a few hours, and are consistent with the measured noise spectra. Amplitude noise is not investigated.

A radio-frequency transfer structure for the CERN Linear Collider

The results from experimental and three-dimensional computational studies of a radio-frequency structure for use in the CERN Linear Collider are presented. The structure would perform the function of converting beam energy from a UHF accelerated drive beam to 30-GHz radiation for powering the high gradient cavities of a second parallel beam, which would be accelerated to an energy of 1 TeV. Design considerations of this unusual component are discussed, along with the results of low-power radio-frequency tests on a scaled model which reveal the electrical characteristics of the structure. In addition, a comparison of the experimental findings with 3-D computations, performed with the MAFIA codes, is given, and good agreement is found. The results of tests with the structure excited by beams from the LEP (Large Electron-Positron Collider) preinjector linac are also presented. The transfer structure is found to satisfy many of the requirements needed for the role it would play in a two-beam accelerator.<<ETX>>

Structure studies for the CERN Linear Collider CLIC

Each linac proposed for the CERN Linear Collider (CLIC) is composed of 50000 25-cm-long-accelerator sections operating at 29 GHz with gradients of 80 MV/m to produce beam energies of 1 TeV. Basic structure parameters have been established, and fabrication of these sections by the electroforming and machine-and-braze techniques is being investigated. Results obtained from various prototype test pieces are given and discussed.<<ETX>>