Rostock University, Faculty of Computer Science and Electrical Engineering, Rostock, Germany
Eigenmode simulations of accelerating structures often involve a large number of computed modes that need to be catalogued and compared. In order to effectively process all the information gathered from eigenmode simulations a new algorithm was developed to automatically recognize modes’ field patterns. In this paper we present the principles of the algorithm and investigate its applicability by means of different single and multi cell cavities. The highest achievable order of correctly recognized modes is of particular interest.
Rostock University, Faculty of Computer Science and Electrical Engineering, Rostock, Germany
BESSY-VSR is a planned scheme to upgrade the existing BESSY II storage ring to support variable electron pulse lengths. In addition to the present 0.5 GHz energy replenishment cavity, two additional SRF bunch compressing cavities operating at 1.5 GHz (3rd harmonic) and 1.75 GHz (sub-harmonic), will be installed. These cavities are essential to produce short 1.5 ps bunches with current of up to 0.8 mA per bunch. In order to achieve such high beam currents, higher order modes must be damped in the superconducting cavities. In this work we present analysis of higher order modes in cavities with different mid-cell shapes.
Rostock University, Faculty of Computer Science and Electrical Engineering, Rostock, Germany
The electromagnetic properties of SRF cavities are mostly determined by their shape. Due to fabrication tolerances, tuning and limited resolution of measurement systems, the exact shape remains uncertain. In order to make assessments for the real life behaviour it is important to quantify how these geometrical uncertainties propagate through the mathematical system and influence certain electromagnetic properties, like the resonant frequencies of the structure's eigenmodes. This can be done by using non-intrusive straightforward methods like Monte-Carlo (MC) simulations. However, such simulations require a large number of deterministic problem solutions to obtain a sufficient accuracy. In order to avoid this scaling behaviour, the so-called polynomial chaos (PC) expansion is used. This technique allows for the relatively fast computation of uncertainty propagation for few uncertain parameters in the case of computationally expensive deterministic models. In this paper we use the PC expansion to quantify the propagation of uncertain geometry on the example of single cell cavities used for BESSY VSR as well as to compare the obtained results with the MC simulation.
Rostock University, Faculty of Computer Science and Electrical Engineering, Rostock, Germany
* J. Qiang, S. Lidia, R. D. Ryne, and C. Limborg-Deprey, “A Three-Dimensional Quasi-Static Model for High Brightness Beam Dynamics simulation,” Phys. Rev. ST Accel. Beams, vol 9, 044204 (2006).
Rostock University, Faculty of Computer Science and Electrical Engineering, Rostock, Germany
HZDR, Dresden, Germany
Superconducting radio frequency (SRF) structures may be subjected to electron multipacting (MP). The electrons emitted from one of the structure’s wall under certain conditions are accelerated by the RF field, thereby they may impact the wall again based on the field pattern in the structure. Accordingly the number of electrons increases exponentially caused by secondary electron emission*. The latter depends on the secondary emission coefficient of the surface material and the electron trajectory in the device under study**. This phenomenon limits the accelerating gradient in the cavity, moreover, it might cause an impair of RF components and distortion of the RF signal. Therefore, there should be an efficient countermeasure to suppress MP in order to boost the performance of the SRF gun. In this paper, three techniques of suppression of MP from the vicinity of the cathode, such as DC-bias, geometric modification and the microstructure of the cathode's surface, in the Rossendorf SRF gun are presented. The simulation has been done using CST Microwave Studio® and CST Particle Studio®***. Eventually, the efficient suppression method would be chosen for this particular case.
* H.Padamsee, J. Knobloch and T. Hays, 1998, Ch. 10.
** E. T. Tulu, A. Arnold and U. van Rienen, 16th International Conference on SRF, Paris, France, 2013.
*** CST AG, http://www.cst.com.
Rostock University, Faculty of Computer Science and Electrical Engineering, Rostock, Germany
The computation of eigenmodes for complex accelerating structures is a challenging and important task for the design and operation of particle accelerators. Discretizing long and complex structures to determine its eigenmodes leads to demanding computations typically performed on super computers. This contribution presents an application example of a method to compute eigenmodes and other parameters derived from these eigenmodes for long and complex structures using standard workstation computers. This is accomplished by the decomposition of the complex structure into several single segments. In a next step, the electromagnetic properties of the segments are described in terms of a compact state space model. Subsequently, the state space models of the single structures are concatenated to the full structure. The results of direct calculations are compared with results obtained by the concatenation scheme in terms of computational time and accuracy.
ELSA, Bonn, Germany
Rostock University, Faculty of Computer Science and Electrical Engineering, Rostock, Germany
HZB, Berlin, Germany
In the ELSA stretcher ring electrons are accelerated by a fast energy ramp of 6 GeV/s to a beam energy of 3.2 GeV. The high energetic electrons ionize the residual gas molecules in the beam pipe by collisions or synchrotron radiation. The generated ions in turn accumulate inside the beam potential, causing several undesired effects such as tune shifts and beam instabilities. These effects are studied experimentally at ELSA using its full diagnostic capabilities. Both tune shifts due to beam neutralization and transversal beam-ion instabilities can be determined from the beam spectrum. Additionally the beam's transfer function can be measured using a broadband transversal kicker. In the stretcher ring at a beam energy of 1.2 GeV, a periodic beam blow-up was detected in the horizontal plane. Additional measurements of the transversal beam spectrum and ns-time resolution observations with a streak camera identified this blow-up as a coherent dipole oscillation of the beam. This horizontal instability is presumably caused by trapped ions, as there is a strong correlation with the high voltage-bias of the clearing electrodes.
Rostock University, Faculty of Computer Science and Electrical Engineering, Rostock, Germany
ELSA, Bonn, Germany
HZB, Berlin, Germany
Ions generated by synchrotron radiation and collisions of the beam with the rest gas in the vacuum chamber could be a limiting factor for the operation of electron storage rings and Energy Recovery Linacs (ERL). In order to develop beam instability mitigation strategies, a deeper understanding of the ion-cloud behaviour is needed. Numerical simulations of the interaction between electron beams and parasitic ions verified with dedicated measurements can help to acquire that knowledge. This paper presents results of detailed simulations of the interaction in quadrupole magnets and drift sections of the Electron Stretcher Accelerator ELSA in Bonn. The focus is on the evaluation of the dynamics of different ion species and their characteristic distribution in quadrupole magnets.
CERN, Geneva, Switzerland
Rostock University, Faculty of Computer Science and Electrical Engineering, Rostock, Germany
In this paper we present the progress of the Higher-Order-Mode (HOM) coupler design for the high beta CERN SPL (Superconducting Proton Linac) cavities. This includes the RF transmission behavior as well as mechanical and thermal requirements and their optimizations. Warm RF measurements are presented for the first four high beta SPL Cavities made of bulk niobium. Moreover the first prototype of a HOMcoupler will be introduced and we discuss its characteristics and its tuning possibilities.
Rostock University, Faculty of Computer Science and Electrical Engineering, Rostock, Germany
HZDR, Dresden, Germany
The efficient reduction of the pulse length and the energy width of electron beams plays a crucial role in the generation of short pulses in the range of sub-picoseconds at future light sources. At the radiation source ELBE in Dresden Rossendorf short pulses are required for coherent THz generation and laser-electron beam interaction experiments such as X-ray Thomson scattering. Energy dechirping can be carried out passively by wakefields generated when the electron beam passes through suitable structures, namely corrugated and dielectrically lined cylindrical pipes or dielectrically lined rectangular waveguides (*,**,***). All structures offer the possibility to tune the resulting wakefield and therefore the resulting energy chirp through a variation of purely geometrical or material parameters. In this paper we present a semi-analytical approach to determine the wakefield in dielectrically lined rectangular waveguide, starting with the expression of the electric field in terms of the structure's eigenmodes.
* Bane, Stupakov, SLAC-PUB-14925 (2012)
** Mosnier, Novokhatski, in: Proceedings of PAC97, Vancouver, Canada, 1997
*** Antipov et al., in: Proceedings of IPAC2012, New Orleans, USA, 2012