TEMF, TU Darmstadt, Darmstadt
GSI, Darmstadt
Beam coupling impedances have been identified as an appropriate quantity to describe collective instabilities caused through beam-induced fields in heavy ion synchrotron accelerators such as the SIS-18 and the SIS-100 at the GSI facility. The impedance contributions caused by the multiple types of beamline components need to be determined to serve as input condition for later stability studies. This paper will discuss different approaches to calculate the Coupling Impedance contribution of ferrite devices, exploiting the abilities of both commercial codes such as CST STUDIO SUITE® and specific extensions of this code to address kicker related problems in particular. Before addressing actual beamline devices, benchmark problems with cylindrical and rectangular geometry will be simulated and the results will be compared with the corresponding analytical formulations.
TEMF, TU Darmstadt, Darmstadt
GSI, Darmstadt
For the future operation as an injector for the FAIR project the SIS18 synchrotron has to deliver intense and high quality ion bunches with high repetition rate. One requirement is that the initial momentum spread of the injected coasting beam should not exceed the limit set by the SIS18 rf bucket area. Also the Schottky spectrum should be used to routinely measure the momentum spread and revolution frequency directly after injection. During the transverse multi-turn injection the SIS18 is filled withμbunches from the UNILAC linac at 36 MHz. For low beam intensities theμbunches debunch within a few turns and form a coasting beam with a Gaussian-like momentum spread distribution. With increasing intensity we observe persistent current fluctuations and an accompanying pseudo-Schottky spectrum. We will explain that the multi-stream instability of theμbunch filaments is responsible for the turbulent current spectrum that can be observed a few 100 turns after injection. The current spectrum observed in the SIS18 and the results from a longitudinal simulation code will compared to an analytical model of the multi-stream instability induced by the space charge impedance.
TEMF, TU Darmstadt, Darmstadt
GSI, Darmstadt
Electron clouds generated by residual gas ionization pose a potential threat to the stability of the circulating heavy ion beams in the existing SIS-18 synchrotron and in the projected SIS-100. The electrons can potentially accumulate in the space charge potential of the long bunches. As an extreme case we study the accumulation of electrons in a coasting beam under conditions relevant in the SIS-18. Previous studies of electron clouds in coasting beams used Particle-In-Cell (PIC) codes to describe the generation of the cloud and the interaction with the ion beam. PIC beams exhibit much larger fluctuation amplitudes than real beams. The fluctuations heat the electrons. Therefore the obtained neutralization degree is strongly reduced, relative to a real beam. In our simulation model we add a Langevin term to the electron equation of motion in order to account for the heating process. The effect of natural beam fluctuations on the neutralization degree is studied. The modification of the beam response function as well as the stability limits in the presence of the electrons is discussed. Finally we will also address the electron accumulation in long bunches.
IAP, Frankfurt am Main
GSI, Darmstadt
For high current synchrotrons and for the SIS18 operation as booster of the projected SIS100 it is important to improve the multi-turn injection efficiency. This can be achieved by coupling the transverse planes with skew quadrupoles, which can move the particles away from the septum. Linear betatron coupling by skew quadrupole components in SIS18 including space charge effect was studied in an experiment using different diagnostic methods during the crossing of the difference coupling resonance. The beam loss was measured using a fast current transformer, the transverse emittance exchange was observed using a residual gas monitor and the coupled tunes were obtained from the Schottky noise spectrum. We compared the experimental results with simulation using PARMTRA which is a code developed at GSI.
GSI, Darmstadt
ITEP, Moscow
Octupole magnets can be used as a passive cure against transverse collective instabilities. The octupole field creates a betatron frequency spread due to amplitude-dependent tune shift and thus enhances Landau damping. The drawback is the reduction of the dynamic aperture (DA). Ultimately, a balance between collective damping and DA must be found. Here we analyse the transverse coherent instability thresholds in SIS100 with octupoles and nonlinear space-charge taken into account. As the major impedance sources at low frequencies, the resistive wall and the kickers are considered. A coasting beam is assumed, which results in a conservative stability estimation. On the other hand, we simulate the DA of the SIS100 lattice using the MADX code, with systematic multipole errors, random multipole errors, and closed-orbit errors taken into account.
GSI, Darmstadt
The FAIR accelerator project at GSI should increase the intensity of primary proton and heavy ion beams by up to two orders of magnitude, relative to the existing GSI facility. In addition to the design of the new synchrotron SIS-100 and the storage rings, the intensity upgrade of the SIS-18 synchrotron plays a key role for the FAIR project. Recently a new record beam intensity for intermediate charge state uranium ions has been achieved in the SIS-18. Still several challenges related to beam intensity effects and phase space conservation have to be mastered in order to reach the beam parameters required for the injection into SIS-100. In SIS-100 beam loss control and machine protection are of major concern. Lost energetic heavy ions can cause a more severe damage of accelerator components than the corresponding amount of protons. Gradual beam loss of energetic ions is expected to occur in SIS-100 mainly during slow extraction of intense beams. Coherent transverse instabilities induced by the beam pipe impedance are a potential cause of fast beam loss and emittance increase. Cures and protection measures together with the result of simulation studies will be summarized.
GSI, Darmstadt
ITEP, Moscow
The heavy ion synchrotron SIS-100 will play a key role within the future FAIR project underway at GSI. Although this synchrotron is optimized for fast extraction, also slow extraction will be used. Slow extraction is based on beam excitation due to a third order resonance. The spread in the particle momenta generating a tune spread causes particle loss leading to an irradiation of the machine especially in a high-current operation. A major part of the losses is assumed to occur at the electro-static separator. In the present study we apply a tracking method to model the extraction process to predict the losses, where, in a first step, high current effects are not taken into account.