The advantages of contemporary particle injectors are high bunch charges and good beam quality in the case of normal conducting RF guns and increased repetition rates in the one of DC injectors. The technological edge of the concept of superconducting radio frequency injectors is to combine the strengths of both these sides. As many future accelerator concepts, such as energy recovery linacs, high power free electron lasers and certain collider designs, demand particle sources with high bunch charges and high repetition rates combined, applying the superconductivity of the accelerator modules to the injector itself is the next logical step. However, emittance compensation — the cornerstone for high beam quality — in case of a superconducting injector is much more challenging than in the normal conducting one. The use of simple electromagnets generating a solenoid field around the gun’s resonator interferes with its superconducting state. Hence, it requires novel and sophisticated techniques to maintain the high energy gain inside the gun cavity, while at the same time alleviating the detrimental fast transverse emittance growth of the bunch. In the case of the ELBE accelerator at the Helmholtz-Zentrum Dresden-Rossendorf, a superconducting electron accelerator provides beam for several independent beamlines in continuous wave mode. The applications include IR to THz free electron lasers, neutron and positron generation, to Thompson backscattering with an inhouse TW laser, and hence, call for a flexible CW injector. Therefore, the development of a 3.5 cell superconducting electron gun was initiated in 1997. The focus of this thesis lies on three approaches of transverse emittance compensation for this photoinjector: RF focusing, the installation of a superconducting solenoid close to the cavity’s exit, and the introduction of a transverse electrical mode of the RF field in the resonator. All three methods are described in theory, examined by numerical simulation, and experimentally reviewed in the particular case of the ELBE SRF Gun II at HZDR and a copy of its niobium resonator at Thomas Jefferson National Laboratory, Newport News, VA, USA.

The advantages of contemporary particle injectors are high bunch charges and good beam quality in the case of normal conducting RF guns and increased repetition rates in the one of DC injectors. The technological edge of the concept of superconducting radio frequency injectors is to combine the strengths of both these sides. As many future accelerator concepts, such as energy recovery linacs, high power free electron lasers and certain collider designs, demand particle sources with high bunch charges and high repetition rates combined, applying the superconductivity of the accelerator modules to the injector itself is the next logical step. However, emittance compensation — the cornerstone for high beam quality — in case of a superconducting injector is much more challenging than in the normal conducting one. The use of simple electromagnets generating a solenoid field around the gun’s resonator interferes with its superconducting state. Hence, it requires novel and sophisticated techniques to maintain the high energy gain inside the gun cavity, while at the same time alleviating the detrimental fast transverse emittance growth of the bunch. In the case of the ELBE accelerator at the Helmholtz-Zentrum Dresden-Rossendorf, a superconducting electron accelerator provides beam for several independent beamlines in continuous wave mode. The applications include IR to THz free electron lasers, neutron and positron generation, to Thompson backscattering with an inhouse TW laser, and hence, call for a flexible CW injector. Therefore, the development of a 3.5 cell superconducting electron gun was initiated in 1997. The focus of this thesis lies on three approaches of transverse emittance compensation for this photoinjector: RF focusing, the installation of a superconducting solenoid close to the cavity’s exit, and the introduction of a transverse electrical mode of the RF field in the resonator. All three methods are described in theory, examined by numerical simulation, and experimentally reviewed in the particular case of the ELBE SRF Gun II at HZDR and a copy of its niobium resonator at Thomas Jefferson National Laboratory, Newport News, VA, USA.

In dieser Arbeit adressieren wir die Schwierigkeit, Präzisionsvorhersagen mit dem kompletten Satz theoretischer Unsicherheiten in der perturbativen Quantenchromodynamik im Rahmen von Monte-Carlo-Simulationen zu treffen, angesichts der zunehmenden Komplexität der dazu nötigen Berechnungen. Die Anforderungen an die Rechenleistung können so groß sein, dass nicht in jeder Anwendung die bestmögliche Präzision erzielt wird. Wir präsentieren eine Reweighting-Methode für den Monte-Carlo-Ereignisgenerator SHERPA. Diese erstellt Variationen der nominellen Vorhersage mit vergleichsweise geringem zusätzlichen Zeitaufwand. Die Methode ist kompatibel mit aktuellen Multijet-Berechnungen nächsthöherer Ordnung, die mit Korrekturen von allen Ordnungen durch einen Partonschauer versehen sind. Zusätzlich diskutieren wir neue Entwicklungen für einen weiteren Reweighting-Ansatz, der auf QCD-Interpolationsgittern beruht. Diese ermöglichen noch schnellere Variationen für Berechnungen fester Ordnung. Solche Gitter können für Monte-Carlo-Simulationen automatisiert erstellt werden mithilfe von Interfaces wie MCgrid. Unsere Verbesserungen für MCgrid ermöglichen die Erstellung vielseitigerer Gitter, die eine größere Klasse von Berechnungen, Gitter-Implementierungen und Skalenvariationen unterstützen. Darüber hinaus diskutieren wir, auf welche Weise solche Gitter für die Unterstützung von Resummationseffekten erweitert werden müssten. Neben dem Reweighting studieren wir noch die Verwendung von Extrapolationsmethoden für die Vorhersage von Jet-Raten hoher Multiplizitäten, welche an zukünftigen Hochenergiebeschleunigern allgegenwärtig sein werden. Diese Methoden basieren auf dem Skalierungsverhalten der Jet-Raten. Eingebettet ist diese Studie in eine allgemeinere Diskussion der zu erwartenden Jet-Aktivität an einem Proton-Proton-Beschleuniger mit einer Schwerpunktsenergie von 100 TeV.

The reaction 12C(α,γ)16O is of paramount importance for the nucleosynthesis of heavier elements in stars. It takes place during helium burning and determines the abundance of 12C and 16O at the end of this burning stage and therefore influences subsequent nuclear reactions. Currently the cross section at astrophysically relevant energies is not known with satisfactory precision. Due to the low cross section of the reaction, low background, high beam intensities and target thicknesses are necessary for experiments. Therefore a new laboratory hosting a 5 MV ion accelerator, was built in the shallow-underground tunnels of Felsenkeller. The main background component in such laboratories was investigated with a muon telescope in this thesis. It was found, that the rock overburden of about 45 m vertical depth reduces the muons by a factor of about 40 compared to the surface. Furthermore the results of the measurements were compared to a simulation based on the geometry of the facility and showed good agreement. In the next step the accelerator was put into operation. Since the experiment on 12C(α,γ)16O will be done in inverse kinematics, an intense carbon beam is necessary to reach sufficient statistics. For this, the creation and extraction of carbon ions in an external ion source was improved. The external source now provides steady currents of 12C− of above 100 μA. In the following the transmission through the accelerator and the high-energy beamline was tested with a beam restricted in width. The pressure of the gas stripper in the centre of the accelerator and the parameters of different focusing elements after the accelerator were varied. It was found, that for a desired carbon beam energy of below 9 MeV, the 2+ charge state is suited best, where up to 35% of the inserted beam could be transmitted. To ease the planning of future experiments and aid the analysis of the data, the target chamber and two different kinds of cluster detectors were modelled in Geant4. The low-energy region was verified by comparing the simulations to measurements with radioactive calibration sources. Deviations for the detectors were below 10% without target chamber, and up to 30% for individual germanium crystals of the Cluster Detectors with the target chamber. A first test measurement was undertaken to investigate the capabilities of the new laboratory. Solid tantalum targets implanted with 4 He were prepared. An ERDA analysis of the used solid targets showed contaminations with carbon and oxygen. These led to beam-induced background in the region of interest during the irradiation. Then the targets were irradiated with a carbon beam at two different energies. While no clear signal of 12C(α,γ)16O could be observed, the beam could be steered on the target for the whole duration of the beam time spanning five days. Problems during this test, like low beam current, were identified. These could be partly remedied in the scope of this thesis. Suggestions for improvements for a second test run were developed as well.

Der Einfluss von Nichtlinearitäten höherer Ordnung der Magnetfelder auf die Leistung des Large Hadron Collider (LHC) und dessen geplante High-Luminosity-Aufrüstung, dem HL-LHC, wurde umfangreich untersucht. Insbesondere hat sich gezeigt, dass das Vorhandensein solcher Fehler in den Insertion Regions (IR) erhebliche Auswirkungen hat, bedingt durch hohe Beta-Funktionen und Feed-Down auf niedrigere Ordnungen aufgrund der Kreuzungsschemata. Augenmerk dieser Arbeit ist auf die Erforschung diverser Methoden zur effektiven Behandlung dieser Nichtlinearitäten höherer Ordnung gerichtet, mit dem Ziel, sie zu identifizieren und korrigieren, um die Strahloptik zu optimieren und die Maschinenleistung zu verbessern. Simulationsstudien werden eingesetzt, in denen mit verschiedenen Fehlerquellen assoziierte Resonanzantreibende Terme (RDTs) gezielt angegangen werden. Besondere Aufmerksamkeit gilt Dekapol- und Dodekapolfehlern, die in früheren Messungen im LHC schädliche Auswirkungen durch Feed-Down auf Amplituden-Detuning gezeigt haben. Die erwartete Erhöhung der Sensitivität der Optik gegenüber Fehlern in den IRs des HL-LHC unterstreicht weiter die Bedeutung der Behandlung dieser Fehler. Des Weitern werden Korrekturoptionen mit Hilfe der nichtlinearen Korrektorpaketen entwickelt. Experimentelle Studien werden durchgeführt, um die Ergebnisse zu validieren. Erhebliche Anstrengungen wurden unternommen, um die Feed-Down Effekte von Dekapol- und Dodekapol-Feldfehlern zu mindern. Um diese Herausforderung anzugehen, wurden neuartige Korrekturalgorithmen eingeführt, die erstmals die Dodekapol-Korrektoren in den IRs im operationellen Betrieb ansteuern. Die Ergebnisse dieser Experimente liefern wertvolle Erkenntnisse zur Minderung von Fehlern höherer Ordnung und tragen zum besseren Verständnis der Strahldynamik in modernen und zukünftigen Teilchenbeschleunigern bei. / The impact of high-order nonlinear magnetic field errors on the performance of the Large Hadron Collider (LHC) and its planned High-Luminosity upgrade, the HL-LHC, has been extensively studied. Particularly, the presence of such errors in the Insertion Regions (IR) has shown significant repercussions due to the high beta-functions and feed-down to lower orders caused by crossing schemes. This thesis aims to explore different methods for effectively addressing these high-order errors, with the ultimate goal of identifying and correcting them to optimize beam optics and enhance machine performance. Simulation studies are employed, using a novel and flexible correction algorithm developed during the course of this PhD research. Various strategies are investigated to improve corrections by targeting Resonance Driving Terms (RDTs) associated with diverse error sources. Special attention is devoted to decapole and dodecapole errors, which have demonstrated detrimental effects on amplitude detuning due to feed-down based on previous measurements in the LHC. The anticipated increase in optics sensitivity to errors in the IRs of the HL-LHC further underscores the importance of addressing these errors. Correction options are evaluated, focusing on the utilization of the nonlinear corrector packages to address errors in the new separation and recombination dipoles in the HL-LHC, where increased decapole errors had been expected. Experimental studies are conducted to validate the findings. Significant efforts are dedicated to mitigating the feed-down effects arising from decapole and dodecapole field errors. To address this challenge, novel corrections involving the operational implementation of dodecapole correctors in the IRs have been introduced for the first time. The results of these experiments provide valuable insights into the mitigation of high-order errors and contribute to the overall understanding of beam dynamics in advanced particle accelerators.