Accessing Accelerator Parameters with Mathematica via the Passerelle

To access the control system of the accelerators in the PS Complex at CERN from an office PC running Mathematica in a Windows environment, the package PasserelleUtils has been implemented. This package is based on the functions provided by the Passerelle, a bridge between office PCs and the accelerator control system at CERN running Linux. It allows read and write access by executing synchronous function calls to accelerator parameters directly from Mathematica. The seamless integration of data from the accelerators into the powerful mathematical programming and graphics environment of Mathematica facilitates their analysis and post-processing. Parameter settings can be derived and sent to the accelerator equipment directly in Mathematica. Dedicated functions allow for easy read access to vector tables and sampler data as used to control and operate equipment in the PS Complex. Furthermore, it removes limitations inherent to the conventional way of using EXCEL to exploit the functions of the Passerelle. This note can be referred to as a manual of the PasserelleUtils package. After a brief introduction to its functionality, a detailed description of all functions provided is given. Several simple examples illustrate the usefulness

Bunch Length along the Batch of the LHC Beam at Extraction from the PS

During machine development experiments with multi-bunch LHC beams in the PS, it has been observed that the bunch length at extraction increases slightly towards the end of the batch. This effect does not only get stronger with increasing intensity, but it is also strongly amplified when instead of the usual two all three 80 MHz cavities are operated to produce bunches significantly shorter than nominal. Systematic measurements show that the bunch length gradient along the batch can be attributed to the effect of the impedance of the 80 MHz cavities

Excitation of longitudinal coupled-bunch oscillations with the wide-band cavity in the CERN PS

Longitudinal coupled-bunch oscillations in the CERN Proton Synchrotron have been studied in the past years and they have been recognized as one of the major challenges to reach the high brightness beam required by the High Luminosity LHC project. In the frame of the LHC Injectors Upgrade project in 2014 a new wide-band Finemet cavity has been installed in the Proton Synchrotron as a part of the coupled-bunch feedback system. To explore the functionality of the Finemet cavity during 2015 a dedicated measurement campaign has been performed. Coupled-bunch oscillations have been excited with the cavity around each harmonic of the revolution frequency with both a uniform and nominal filling pattern. In the following the measurements procedure and results are presented

Study of the beam-cavity interaction in the PS 10 MHz RF system

The eleven main accelerating cavities of the Proton Synchrotron (PS) at CERN consist of two ferrite-loaded coaxial lambda/4 resonators each. Both resonators oscillate in phase, as their gaps are electrically connected by short bars. They are in addition magnetically coupled via the bias loop used for cavity tuning. The cavities are equipped with a wide-band feedback system, limiting the beam loading, and a further reduction of the beam induced voltage is achieved by relays which short-circuit each half-resonator gap when the cavity is not in use. Asymmetries of the beam induced voltage observed in the two half-cavities indicate that the coupling between the two resonators is not as tight as expected. The total cavity impedance coupling to the beam may be affected differently by the contributions of both resonators. A dedicated measurement campaign with high-intensity proton beam and numerical simulation have been performed to investigate the beam-cavity interaction. This paper reports the result of the study and the work aiming at the development of a model of the system, including the wide-band feedback, which reproduces this interaction

New Control Structure of the 200 MHz RF System in the CERN PS

The 200 MHz RF system is an essential tool for the preparation of high-intensity beams in the CERN PS. Presently, six RF cavities are operated to control the longitudinal bunch emittance and rebunching of the beam before the transfer to the SPS. Cavities are selected for the various processes with a dedicated hardware matrix, switching the individual timing pulses and voltage programs per cavity. However, the electronics used for the matrix hardware is obsolete and its reliability cannot be guaranteed due to a lack of spare modules and components. Instead of replacing the old hardware matrix by modern hardware, this note describes a new control structure for the 200MHz RF system so that no dedicated hardware will be required anymore. The implementation of the new control structure is based on two main concepts. Firstly, linked timing trees per blow-up or rebunching are used to handle all related timing and to store one row of the matrix. Secondly, as a reflection of the RF signal generation for the 200 MHz system, where all units always receive the same RF signal, the new implementation features a single global voltage program function. Internal stops in the voltage program function are restarted by timings in the linked trees and assure inherent coherence between timings and voltage programs for the cavities. After a description of the requirements of the present hardware in terms of timing signals and functions, functional s pecification of the new application program is given

Measurements of the CERN PS longitudinal resistive coupling impedance

The longitudinal coupling impedance of the CERN PS has been studied in the past years in order to better understand collective effects which could produce beam intensity limitations for the LHC Injectors Upgrade project. By measuring the incoherent quadrupole synchrotron frequency vs beam intensity, the inductive impedance was evaluated and compared with the impedance model obtained by taking into account the contribution of the most important machine devices. In this paper, we present the results of the measurements performed during a dedicated campaign, of the real part of the longitudinal coupling impedance by means of the synchronous phase shift vs beam intensity. The phase shift has been measured by using two different techniques: in one case, we injected in the machine two bunches, one used as a reference with constant intensity, and the second one changing its intensity; in the second case, more conventional, we measured the bunch position with respect to the RF signal of the 40 MHz cavities. The obtained dependence of the synchrotron phase with intensity is then related to the loss factor and the resistive coupling impedance, which is compared to the real part of the PS impedance model

Longitudinal Tomographic Reconstruction of LHC-type Bunches in the SPS

Longitudinal tomographic reconstruction on the basis of measured profiles is an important technique to measure the particle density distribution of a bunch in longitudinal phase space. This measurement technique, well established in all circular machines of the PS complex, has been applied to the SPS for the first time. Due to recent improvements of the data acquisition of the signals from the longitudinal pick-ups in the SPS and a new LHC type wall current monitor, the quality of the bunch profiles is now more appropriate for tomography. Longitudinal beam signals from the wall current pick-ups APWL-10 and WC-2 are used as input for the reconstruction algorithm. It is shown that, due to short bunches and long cables in the SPS, the correction of the signal with the transfer function of the transmission system is indispensable. The analysis of the longitudinal distribution of a batch of 48 bunches of an LHC type beam at injection into the SPS, averaged over more than ten cycles, showed that any systematic variation of the bunch parameters along the batch is shadowed by statistical errors due to the quality of the measured bunch profiles. Avoiding the long coaxial cables from the SPS tunnel to the surface is a crucial issue for improving the quality of the bunch profiles suitable for tomographic reconstruction

The PS 10 MHz High Level RF System Upgrade

In view of the upgrade of the injectors for the High Luminosity LHC, significantly higher bunch intensity is required for LHC-type beams. In this context an upgrade of the main accelerating RF system of the Proton Synchrotron (PS) is necessary, aiming at reducing the cavity impedance which is the source of longitudinal coupled-bunch oscillations. These instabilities pose as a major limitation for the increase of the beam intensity as planned after LS2. The 10 MHz RF system consists in 11 ferrite loaded cavities, driven by tube-based power amplifiers for reasons of radiation hardness. The cavity-amplifier system is equipped with a wide-band feedback that reduces the beam induced voltage. A further reduction of the beam loading is foreseen by upgrading the feedback system, which can be reasonably achieved by increasing the loop gain of the existing amplification chain. This paper describes the progress of the design of the upgraded feedback system and shows the results of the tests on the new amplifier prototype, installed in the PS during the 2015-16 technical stop. It also reports the first results of its performance with beam, observed in the beginning of the 2016 run

Electron Cloud Mitigation by Fast Bunch Compression in the CERN PS

A fast transverse instability has been observed with nominal LHC beams in the CERN Proton Synchrotron (PS) in 2006. The instability develops within less than 1 ms, starting when the bunch length decreases below a threshold of 11.5 ns during the RF procedure to shorten the bunches immediately prior to extraction. An alternative longitudinal beam manipulation, double bunch rotation, has been proposed to compress the bunches from 14 ns to the 4 ns required at extraction within 0.9 ms, saving some 4.5 ms with respect to the present compression scheme. The resultant bunch length is found to be equivalent for both schemes. In addition, electron cloud and vacuum measurements confirm that the development of an electron cloud and the onset of an associated fast pressure rise are delayed with the new compression scheme. Beam dynamics simulations and measurements of the double bunch rotation are presented as well as evidence for its beneficial effect from the electron cloud standpoint

Implementation of synchronised PS-SPS transfer with barrier buckets

For the future intensity increase of the fixed-target beams in the CERN accelerator complex, a barrier-bucket scheme has been developed to reduce the beam loss during the 5-turn extraction from the PS towards the SPS, the so-called Multi-Turn Extraction. The low-level RF system must synchronise the barrier phase with the PS extraction and SPS injection kickers to minimise the number of particles lost during the rise times of their fields. As the RF voltage of the wide-band cavity generating the barrier bucket would be too low for a conventional synchronisation, a combination of a feedforward cogging manipulation and the real-time control of the barrier phase has been developed and tested. A deterministic frequency bump has been added to compensate for the imperfect circumference ratio between PS and SPS. This contribution presents the concept and implementation of the synchronised barrier-bucket transfer. Measurements with high-intensity beam demonstrate the feasibility of the proposed transfer scheme.Comment: Talk presented at LLRF Workshop 2022 (LLRF2022, arXiv:2208.13680