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GSI Scientific Report 2014 - GSI Report 2015-1

This paper continues the presentation of pumps begun in ‘Vacuum I’. The main topic here is gauges and partial-pressure measurements. Starting from the kinetics of gases, the various strategies for measuring vacuum pressures are presented at an introductory level, with some reference to hardware devices. Partial-pressure measurement techniques are introduced, showing that the principles of ion selection have a direct similarity to particle dynamics in accelerators.

A study of the electron-cloud pinch dynamics during a bunch passage under the effect of a single arbitrary-order multipole was presented earlier [1]. However, in a realistic situation, the proton beam will not be located in the center of the vacuum chamber. If the beam is offset a new pinch regime is encountered, where feed-down effects and asymmetry of pinch density render the dynamics more challenging. More recently we initiated an approach to investigate the dynamics of the field created by the pinched electrons, including transverse displacements the beam, the vacuum chamber of the surrounding magnet [2]. In the present paper we continue this study and develop a field map approach to the modeling of the proton beam dynamics under the effect of the pinched electrons. We compute the detuning with amplitude under various conditions through tracking simulations and compare the results with a static prediction based on the gradient of the electric field experienced on the beam axis.

This report summarizes the disucssions and conclusions from the "Space Charge 2013" (SC-13) workshop at CERN, 16-19 April 2013. SC-13 was jointly organized by EUCARD, ACCNET, ICFA, HIC4FAIR and LIU. Strong activities on space-charge related topics are ongoing at CERN (LIU), GSI (FAIR), and RAL (ISIS upgrade). Several studies include experimental work. The issue of code benchmarking is important with regard to long-term tracking. In particular, the noise created by PIC codes evoked intense discussions. Interesting from a theoretical stand point has been a discussion about equating PIC noise with intrabeam-scattering. A decision has been taken to use the GSI test suite for benchmarking of frozen space charge models also for the benchmarking of PIC codes. Firm plans have been made to benchmark Synergia and Orbit, plus perhaps also IMPACT and WARP. Other discussion focused on the role of GPU for high intensity beam dynamics. Corresponding efforts were reported from GSI, RAL, and FNAL. The final consensus is that many of the topics discussed still require further studies.

A study on the pinch dynamics of electron cloud during a bunch passage under the effect of a single arbitrary order multipole was presented at IPAC2011. The complexity of the pinch pattern is directly related to the order of the multipolar field. However, in a realistic situation, the proton beam will not be located in the center of the vacuum chamber. If the beam is offset a new pinch regime is encountered, where feed-down effects and asymmetry of pinch density render the dynamics more challenging. In this paper we discuss the pinch dynamics with orbit offset, including the resulting orbit variation along a bunch, and address their relevance for the incoherent effect of the electron cloud.

Tracking of high intensity effects for few turns of a circular accelerator is at reach of present computational capabilities. The situation is very different when the prediction of beam behavior is extended to hundred of thousands of turns, where special approaches for the control of computer artifact are necessary sometimes to the expense of a complete physical modeling. The identification of the key physical ingredients helps to the development of computer algorithms capable of treating the long term tracking. Here we present the latest status of long-term tracking simulations of high intensity bunches for SIS100, and also discuss a more realistic modeling of the incoherent effect of an electron cloud for the LHC.

The pinch of the electron cloud due to a passing proton bunch was extensively studied in a field free region and in a dipolar magnetic field. For the latter study, a strong field approximation helped to formulate the equations of motion and to understand the complex electron pinch dynamics, which exhibited some similarities with the field-free situation. Here we extend the analysis to the case of electron pinch in quadrupoles and in sextupoles. We discuss the limits of validity for the strong field approximation and we evaluate the relative magnitude of the peak tune shift along the bunch expected for the different fields.

The knowledge of the distribution of the nonlinear components in a ring is important for the resonance compensation. A method to measure the lattice nonlinear components based on the nonlinear tune response to a controlled closed orbit deformation (NTRM) was suggested in [1]. First benchmarking of this method was carried out with controlled sextupolar errors in the SIS18 synchrotron at GSI. We present here a further benchmarking of NTRM by using controlled octupolar errors at CERN SPS.

During the passage of a proton bunch through an electron cloud, a complicated electron density modulation arises, with characteristic ring or stripe patterns that move radially outward along the bunch. We present simulation results for field-free and dipole regions, which reveal the morphology and main features of this phenomenon, explain the physical origin of the stripes in either case, and discuss the dependence on key parameters.

Besides beam cooling, an electron cooler also acts as a non-linear optical element. This may lead to the excitation of resonances possibly resulting in an increase of the beam emittance. The aim of this work is the calculation of resonances driven by the electron space charge field in the cooler installed in the SIS heavy ion synchrotron at GSI Darmstadt. For our calculations, we used a numerical model consisting of a rotation matrix representing the ideal lattice together with a non-linear transverse kick element representing the electron cooler. Within this model, we studied the dominant resonance lines resulting from the interaction with the cooler.

When a bunch passes through an electron cloud, the transverse electron density distribution is enhanced and modulated in time as a consequence of the motion of individual electrons under the action of the nonlinear beam field. The effect of this “structured” electron pinch together with the synchrotron motion of beam particles leads to an incoherent emittance growth via the excitation and repeated crossing of resonances, that can give rise to either stochastic “scattering” (“diffusion”) or trapping. We study these effects via a toy model of an idealized pinch, and present applications to the CERN SPS and the GSI SIS100.

We present here the relevant space charge issues for long-term beam storage. The impact on the choice of the working point along with the prediction of the beam loss is discussed for the example of the SIS100. We present a first estimate on the effect of self consistency and discuss the equivalence of space charge, and electron clouds induced ”quasi” incoherent effect.