Longitudinal Schottky Spectrum of the Peak Bunch Amplitude Signal

Diagnostic techniques based on the Schottky spectrum of the peak detected signal have been used at CERN for a long time to study the behaviour of bunched beams. In this paper it is shown how the measured spectrum is related to the particle distribution in synchrotron frequency. The experimental set-up used and its limitations are also presented together with examples of beam measurements in the SPS and LHC

An RF System for Landau Damping in the LHC

A Landau system in the LHC could significantly increase the longitudinal stability of the LHC beams in the absence of wide-band longitudinal feedback and provide more freedom to define the bunch parameters even during the initial stages of LHC operation. This technique for stabilizing beams, used already in many accelerators, has proven to be very useful in the SPS, raising the instability thresholds by a factor five. One of the luminosity upgrade paths for LHC requires an RF system at 1.2 GHz with ~ 60 MV per beam for bunch shortening. A much smaller RF system at this frequency with ~3 MV per beam would be sufficient to provide Landau damping. This Note analyses the possible benefits and recommends that an R & D programme, leading to one prototype cryostat per ring to be installed in the LHC machine, be launched as soon as possible

RF scenarios and longitudinal dynamics in SPS and LHC for 12.5 and 75 NS bunch spacings

Beams with proton bunches spaced at 12.5 ns and twice shorter than nominal, or spaced at 75 ns and twice longer than nominal, are the key ingredients for two main scenarios considered for an LHC upgrade. These two options are analysed from the point of view of the choice of both RF systems and beam parameters in the SPS and LHC and the corresponding longitudinal beam stability

Longitudinal impedance monitoring in the SPS with single short bunches at 26 GeV/c (RF on)

In the course of monitoring the low frequency impedance of the SPS over the years usually the quadrupole frequency shift as a function of intensity is measured with single bunches at 26 GeV/c with RF on using the peak detected signal. The additional acquisition of longitudinal bunch profiles allows the evaluation of various parameters of the injected beam, details of its quadrupole oscillation and the evolution of the bunch length as a function of time. Data acquired between 1999 and 2007 will be analysed in this respect. It will be shown that the bunch length data at 600 ms indicates clearly the effect of the SPS impedance reduction programme realised in 2000/2001 and that from then on the absolute value of the quadrupole frequency shift shows a tendency to increase over the years, indicating an impedance increase. However, it does not allow to monitor unambiguously the changes from one year to the next. The reason that the quadrupole frequency shift is not very well determined is attributed to the lack of reproducibility of bunch length and emittance at injection. In the future more attention will have to be paid to make these parameters reproducible

Measuring the Resonance Structure of Accelerator Impedance with Single Bunches

We describe a new method used in the CERN SPS accelerator to measure the longitudinal impedance in the frequency range 100 MHz to 4 GHz. Single high intensity proton bunches were injected and their spectrum observed during slow debunching. The presence of different resonant impedances leads to line density modulation at the resonant frequencies. This instability reaches some maximum modulation amplitude which was recorded as a function of frequency for many bunches. Using sufficiently long bunches the SPS impedance structure was observed and previously unknown sources were identified

Longitudinal beam parameters and quality checks of the LHC beam in the SPS: further results and comparisons

Controlled longitudinal emittance blow up is used, along with other measures, to stabilize the nominal LHC beam in the SPS. Two Machine Development studies (MDs) were carried out in 2007 to evaluate the effectiveness of different noise settings for the longitudinal blow up of the beam. The noise settings are affected by both the presence of the 800 MHz RF system and intensity effects which modify the synchrotron frequency distribution inside the bunch. The results for the first MD are reported in Note [1]. This Note reports on the results of the second MD, carried out on 2007-10-17, as well as the comparison between the two in order to analyse the differences between the two occasions. Figures of merit are used that allow rapid evaluation of the quality of the beam as for example stability and bunch length uniformity across batches

The Fine Structure of the Longitudinal Machine Impedance Observed with Single Bunches

Using a beam spectrum measurement the detailed structure of the CERN SPS longitudinal machine impedance has been observed over a wide frequency range. Single high intensity proton bunches were injected into the machine with RF off. The spectrum of the unstable bunch modes, produced by the different resonant impedances in the ring, was measured during slow debunching. The maximum mode amplitude as a function of frequency was recorded for many injected bunches. The central frequency of each bunch mode is close to the resonant frequency of the impedance and the width depends upon the bunch length. Using sufficiently long bunches allowed the machine impedance to be seen in detail in the frequency range 100MHz to 4GHz. The low frequency of this range is defined by the initial stable bunch spectrum, and the high frequency by the vacuum chamber cut-off frequency. The dominant peaks in the spectrum have been identified with different impedance sources in the machine - some previously unknown

Longitudinal emittance blow-up and hollow bunches with arbitrarily-shaped noise in the SPS as LHC test-bed

In the SPS the LHC type high intensity beam can be kept stable longitudinally during acceleration to 450 GeV/c by using the 800 MHz higher harmonic system and, in addition, by making a longitudinal controlled emittance increase by a factor of about 1.5. This latter is obtained by applying band-limited RF phase noise via the main RF system. In LHC itself, which does not have a higher harmonic system, an emittance increase by a factor 2.5 is required. This was attempted in the SPS, as a test-bed for LHC, with shaped RF phase noise and with the 800 MHz system switched off - limiting the maximum stable beam intensity. The emittance of a single (LHC-) pilot bunch has been blown up to a factor 2.5 in coast at 270 GeV/c with a final âgoodâ bunch shape. It remains to be demonstrated that this technique can also be applied successfully for many high intensity bunches with differing synchrotron frequency profiles along the batches. Also a first very quick test to create hollow bunches was done

Study of Controlled Longitudinal Emittance Blow-Up for High Intensity LHC Beams in the CERN SPS

Preventive longitudinal emittance blow-up, in addition to a fourth harmonic Landau damping RF system, is required to keep the LHC beam in the SPS stable up to extraction. The beam is blown-up in a controlled way during the acceleration ramp by using band-limited phase noise targeted to act inside the synchrotron frequency spread, which is itself modified both by the second RF system and by intensity effects (beam loading and others). For a high intensity beam these latter effects can lead to a non-uniform emittance blow-up and even loss of stability for certain bunches in the batch. In this paper we present studies of the emittance blow-up achieved with high intensity beams under different conditions of both RF and noise parameters

Barrier Buckets in the CERN SPS

The use of barrier buckets to modify the azimuthal distribution of a beam in a circular accelerator has been described and demonstrated in several machines. Barrier buckets could become interesting for future modes of operation in the CERN SPS. Up to now the specially designed RF cavities used for this purpose have been very wide-band, capable of producing a single sine-wave. The existing 200 MHz Travelling Wave cavities in the SPS are not as wide-band but nonetheless have rise-times that are small compared with the revolution period. The possibility of using these cavities to provide "thick" barriers has been studied theoretically and experimentally in the SPS