Systematic studies of the beam dynamics with a superconducting damping wiggler at KARA
Creators: name: Gethmann, Julian
Publisher: name: Karlsruher Institut für Technologie (KIT)
Year: 2021
Subjects: wiggler
undulator
insertion-device
beam-dynamics
accelerator physics
optics
superconducting
nb-ti
kara
anka
clic
cern
x-ray
physics

description: Particle accelerators exist in different variants serving different purposes in scientific, industrial and medical applications. In the field of high energy physics, hadron colliders are often seen as “exploration machines” and lepton colliders as facilities for precise measurements of the particles. The Large-Hadron-Collider (LHC) is such a hadron collider at which the Higgs boson was found, that couldn’t be characterized completely [et.20], yet. It is a storage ring collider where the colliding particles are accelerated in forward direction by the radio frequency cavities and perpendicularly, to form a closed ring, by bending magnets. This results in strong radiation, called synchrotron radiation named after the synchrotron circular accelerator in which it was discovered [et.20]. For colliders of light particles this is a big disadvantage and was one of the limiting factors of LHC’s predecessor, the Large Electron-Positron collider (LEP), because the radiation power losses scale with 1/m4 [Hue, Wie07]. Hence, there are concepts for linear colliders as the next large high energy physics experiment machine, namely the International Linear Collider (ILC) and the Compact Linear Collider (CLIC) [Eur20, The18]. Nonetheless, the synchrotron radiation can also be used for scientific purposes. Nowadays, dedicated synchrotrons exist that utilise this broadband and intense light. The Karlsruhe Institute of Technology (KIT) has got a storage ring called KArlsruhe Research Accelerator (KARA) providing its radiation for a variety of use cases ranging from imaging with hard x-rays to lithography, and THz and infrared spectroscopy [ANK14]. Modern synchrotron light sources make use of insertion devices (ID) that provide much higher intensities than bending magnets do. One kind of such an ID is a so-called wiggler. Its spectrum is similar broadband as that of a bending magnet, but much stronger enabling hard x-ray imaging at KARA’s IMAGE beamline. Though in general synchrotron radiation is not wanted at high energy physics accelerators, the next generation of colliders will make use of it, too. They produce it on purpose before accelerating the beam to its final energy to actively reduce the transverse momentum. Thereby the luminosity and thus the statistics of the collision processes can be increased. For this purpose very strong wigglers, then called damping wiggler, are used. For the CLIC project it is planned to have two damping rings each with two straight sections with 26 2-m-long superconducting wiggler with a magnetic field of 3 T and a period length of 0.05 m [The18] each. The layout of the CLIC facility and one damping ring is sketched in Fig. 1.1. After the injection and pre-acceleration the electron beams’ oscillations are damped in the pre damping rings (labelled PDR in the figure) and damping rings (DR) with the damping wigglers, before the beams are longitudinally compressed (in BC1, BC2), guided (in TA, BDS), and accelerated (in booster linac, main linac) untill they collide in the interaction point (IP). A novel cooling technique for superconducting wigglers was developed by the Budker Institute of Nuclear Physics (BINP) to make individual wigglers more accessible to maintenance by using conduction cooling instead of bath cooling. This and the expected high heat load from the upstream wigglers are challenging for devices with these high fields and relatively short period length. Also, damping rings with superconducting wigglers have not been built yet, so the demand to test a prototype of such a wiggler emerged. The Conseil Européen pour la Recherche Nucléaire (CERN), the KIT and the BINP have established a collaboration to investigate the maturity of the technology and to investigate beam dynamic effects caused by a prototype of such a wiggler build by BINP and installed in KARA. The wiggler serves as a prototype for CERN and as a light source for KIT’s IMAGE beamline at KARA. In this thesis, the question if damping wigglers can be used in such large scale, as planned for damping rings, will be tackeled. Can the beam dynamics of the wigglers be simulated properly and can the effects be experimentally confirmed? Or does it turn out that beam dynamics of damping wigglers are not understood sufficiently well to rely the next generation collider physics accelerators thereon when ca. 50 % of the damping rings’ circumference are damping wigglers? Can one find effects appearing in real devices that are not covered by simulations so far? These are questions, this thesis tries to answer by simulating the damping wiggler prototype and doing measurements with it in KARA. For the CLIC project emerging from a conceptual design study to a technical design study it is of interest if this prototype can fulfill the expectations and work reliably in a real accelerator. This is the topic of this thesis. As collective effects showed to be of importance for the CLIC damping rings [The18] in the past, it is also of interest to do first experiments with this wiggler with regard to collective effects. At KARA there is also a strong research on one collective effect, the so-called “micro-bunching instability” which is under investigation in theory, simulation and experiments. Therefore it makes sense to experimentally investigate the wiggler’s influence on this particular collective effect within this work. After the necessary theoretical background on accelerator physics, beam dynamics, synchrotron radiation and insertion devices—in Chapter 2—, the experimental setup of the accelerator and its insertion devices and then the simulation and measurement techniques are presented in Chapter 3. Because often IDs are simulated as many alternating dipoles or quadrupoles only acting in the vertical plane no common approach exists that satisfies our needs for the simulation, in particular higher order multipoles represented in actual field data. So different approaches for including the wiggler into the storage ring models are evaluated and compared against each other. This encompasses, firstly, the transformation of magnetic field data to Fourier components as input for different wiggler implementation of the particle tracking code elegant. And, secondly, the selection of the actual implementation of the wiggler model. The available options and choices made will be discussed in Chapter 4. The basic functionality, features and influence of the wiggler on beam dynamics are tested experimentally in the 2.50 GeV operation mode. In this mode also heat load studies were conducted. Some further beam dynamics investigations and experiments were carried out in the short-bunch, low-α mode at 1.30 GeV beam-energy to better understand the mechanisms of coherent synchrotron radiation and the so-called micro-bunching instability. Accordingly, the development of two different models of the storage ring is presented in Chapter 5. Chapter 6 describes the experimental characterisation of the wiggler. This includes the findings of the heat load studies as well as the beam dynamics investigations that yielded additional octupole components of the integrated magnetic field. The wiggler was not only used to investigate its transverse beam dynamics, but it was also used to investigate the influence of the damping time on the THz radiation emitted by the electron beam. This happened in the special short-bunch, low-α mode. The optics manipulation needed to operate the wiggler in this special operation mode along with the experimental results of these efforts are presented in Chapter 7.