F. Pedersen

Beam Loading Effects in the CERN PS Booster

At intensities > 1.5 × 1012 protons per ring, and at reduced RF voltage, fast growing beam loading instabilities occur in the Booster (PSB) with loss of beam bunching. The observed thresholds are reported; they are different from those given by Robinsons1 criterion and occur with a different frequency. To explain this and other observations, a model of the beam loading interaction including the feedback loops controlling RF amplitude, phase and cavity tuning, is presented. In the general case, beam loading causes cross-coupling between all three loops, which for high beam loading makes the system unstable. For low beam loading, which was assumed for the basic systems design, the loops are practically independent. The Robinson criterion is a limiting case of this model (no feedback loops). A method for designing better loops taking into account the cross-coupling is described. It has the advantage over other cures, such as lowering of the cavity Q or feeding the cavity with a current equal to the beam current but of opposite phase, that it does not require extra RF power.

Theory and Performance of the Longitudinal Active Damping System for the CERN PS Booster

Longitudinal instabilities have long been trouble-some in the Booster1). They are described by the coupledbunch mode number n = 0 to 4 for five bunches, and by the within-bunch mode number m = 1 for dipole, m = 2 for quadrupole, m = 3 for sextupole, and so on (Fig. 1). The normal beam control system damps the rigid-bunch oscillation (m = 1) when all five bunches move togethier (n = 0)2). The new feedback system3) damps the other coupled-bunch modes n = 1 to 4 for the three lowest orders, m = l to 3. With the damping system off, one can display the evolution of any mode along the cycle, which helps in locating coupling impedances that cause instability. One can also excite the various modes and measure the amplitude-phase response (RF knockout applied to a bunched beam). This gives the frequency spread within the bunch, the coherent frequent shifts, plus the usual stability diagram in the U-V plane.

Shaping of Proton Distribution for Raising the Space-Charge of the CERN PS Booster

The intensity of the PS Booster is limited by space-charge defocusing forces which create a spread in the betatron tunes of up to ΔQ ≃ 0.5. Shaping of the transverse and longitudinal distributions was used for accommodating more particles in a given working area and enabled the Booster to accelerate 2 × 1013 protons per pulse, twice the design intensity. Modifying the RF potential well by an experimental second-harmonic cavity yields beam intensities and densities well beyond the present performance. The corresponding PSB experiments and improvements are described and an outlook on future developments is given.

Electronics for the Longitudinal Active Damping System for the CERN PS Booster

Precisely tracking band-pass filters centred at the sixth and seventh harmonic of the revolution frequency are required1). During the accelerating cycle of 0.6 sec the frequency changes by a factor 2.7. The resulting tracking problem is solved by active two-path filters, where the centre frequency is governed by the frequency of a pair of sinusoidal signals in quadrature, which are generated from the accelerating RF frequency (fifth harmonic) by means of a phase-locked loop and a loop-controlled phase shifter. The phase change caused by the large frequency sweep (6 or 7 MHz) in conjunction with the delay in the feedback loop (cables, etc.) is compensated by a digital system, which computes the required phase advance from the value of the RF frequency and controls digitally the phase shift of the two-path filters. A low-frequency quadrature VCO is made to track the synchrotron frequency or harmonics hereof from analogue information about bending magnet field (momentum) and RF voltage. This quadrature pair ensures tracking of single sideband filters which permit each individual mode sideband to be examined throughout the cycle. A drive system can, by means of a similar VCO, generate any desired mode sideband, and thus excite any given mode.

A Second Harmonic (6-16 MHz) RF System with Feedback-Reduced Gap Impedance for Accelerating Flat-Topped Bunches in the CERN PS Booster

Since all experimental and theoretical evidence indicates that the beam intensity in the PSB is at present limited by the Laslett Q shift after trapping, a second harmonic accelerating system is being added with the aim of producing flat-topped bunches. Calculations indicate that the resulting improvement in bunching factor should permit an intensity increase of 25 to 40 %. Additionally, the bucket area is increased by 25 to 45 % depending on intensity.

The new digital-receiver-based system for antiproton beam diagnostics

An innovative system to measure antiproton beam intensity, momentum spread and mean momentum in CERNs Antiproton Decelerator (AD) is described. This system is based on a state-of-the-art digital receiver (DRX) board, consisting of 8 digital down-converter (DDC) chips and one digital signal processor (DSP). An ultra-low-noise, wide-band AC beam transformer (0.2 MHz -30 MHz) is used to measure AC beam current modulation. For bunched beams, the intensity is obtained by measuring the amplitude of the fundamental and second RF Fourier components. On the magnetic plateaus the beam is debunched for stochastic or electron cooling and longitudinal beam properties (intensity, momentum spread and mean momentum) are measured by FFT-based spectral analysis of Schottky signals. The system thus provides real time information characterising the machine performance; it has been used for troubleshooting and to fine-tune the AD, thus achieving further improved performances. This system has been operating since May 2000 and typical results are presented.

Beam Dynamics Experiments in the PS Booster

The main problems encountered on the way to 1013 ppp have been emittance blow-up and coherent instabilities. The observations and counter measures are described in the text.

Evolution of the RF Systfem of the CPS Booster Since the Beginning of Its Operation

Since the first beams were accelerated in 1972, the RF system has performed very satisfactorily. In particular, the simple air-cooled design of the cavity and final amplifier has proved to be very trouble free in operation. As the bean intensities gradually reached and then exceeded the design value of 2.5 × 1012 protons per ring, a number of improvements were introduced to cope with the increased beam loading and to permit the special beam gymnastics that have been found useful at the highest intensities. Modifications include: i) provision for reducing the Q of the cavity during trapping; ii) an improved system for synchronizing the four rings prior to ejection; iii) measures taken to control longitudinal instabilities; iv) safety precautions to protect the cavity and amplifier against transients.

Upgrades to the digital receiver-based low-intensity beam diagnostics for CERN AD

The CERN AD low-intensity beam multidiagnostics (LIMD) has been upgraded as planned since 2001 by adding tune measurements during ramps and plateaus, based on the beam transfer function (BTF) method. This relies on transversally exciting the beam by a deflector and deriving the BTF and coherence function from FFTs of excitation and beam response recorded by digital receivers (DRX). These, continuously tuned to a betatron sideband, pass data to a digital signal processor (DSP) on the DRX board for data processing. The upgrades discussed also include increased longitudinal frequency range, noise reduction measures and digital flags for setup of data acquisition (DAQ) and processing parameters.

A Low-Noise Charge Amplifier for the ELENA Trajectory, Orbit, and Intensity Measurement System

A low-noise head amplifier has been developed for the extra low energy antiproton ring beam trajectory, orbit, and intensity measurement system at CERN. This system is based on 24 double-electrode electrostatic beam position monitors installed around the ring. A head amplifier is placed close to each beam position monitor to amplify the electrode signals and generate a difference and a sum signal. These signals are sent to the digital acquisition system, about 50 m away from the ring, where they are digitized and further processed. The beam position can be measured by dividing the difference signal by the sum signal while the sum signal gives information relative to the beam intensity. The head amplifier consists of two discrete charge preamplifiers with junction field effect transistor (JFET) inputs, a sum and a difference stage, and two cable drivers. Special attention has been paid to the amplifier printed circuit board design to minimize the parasitic capacitances and inductances at the charge amplifier stages to meet the gain and noise requirements. The measurements carried out on the head amplifier showed a gain of 40.5 and 46.5 dB for the sum and difference outputs with a bandwidth from 200 Hz to 75 MHz and an input voltage noise density lower than