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Linac4 is the new negative hydrogen ion (H$^-$) linear accelerator of the European Organization for Nuclear Research (CERN). Its ion source operates on the principle of Radio-Frequency Inductively Coupled Plasma (RF-ICP) and it is required to provide 50~mA of H$^-$ beam in pulses of 600~$\mu$s with a repetition rate up to 2 Hz and within an RMS emittance of 0.25~$\pi$~mm~mrad in order to fullfil the requirements of the accelerator. This thesis is dedicated to the characterization of the hydrogen plasma in the Linac4 H$^-$ ion source. We have developed a Particle-In-Cell Monte Carlo Collision (PIC-MCC) code to simulate the RF-ICP heating mechanism and performed measurements to benchmark the fraction of the simulation outputs that can be experimentally accessed. The code solves self-consistently the interaction between the electromagnetic field generated by the RF coil and the resulting plasma response, including a kinetic description of charged and neutral species. A fully-implicit implementation allowed to simulate the high density regime of the Linac4 H$^-$ ion source, ensuring the energy conservation while maintaining the computational resources tractable. We studied the capacitive to inductive transition characteristic of the initial phase of the pulsed discharge. The simulation results were confirmed by time-resolved photometry measurements and allowed quantifying the effect of the hydrogen pressure and of the external magnetic cusp field on the transition dynamics. This provided insights into possible modifications to the magnetic cusp field configuration to maximize the power deposited to the plasma. The optimal ion source configuration maximizes the density of volume produced H$^-$, the flux of H$^0$ atoms onto the cesiated molybdenum plasma electrode surface at the origin of H$^-$ emission, and minimizes the electron density and energy in the beam formation region. We simulated the high-density regime (10$^{19}$ m$^{-3}$) representative of the nominal operation of the Linac4 ion source during beam extraction. We performed a parametric study of the RF current, hydrogen pressure and magnetic configuration (cusp and filter) to assess their impact on the plasma parameters. The simulation results allowed assessment of these parameters and provided guidelines for the optimization of the ion source operational and design parameters. The simulation results of electron density, electron energy and hydrogen dissociation degree showed excellent agreement with optical emission spectroscopy measurements both as a function of RF coil current and magnetic configuration. The outputs of these simulations provide crucial inputs to beam formation and extraction physics models. Dedicated PIC software packages are being developed that will eventually shed insight into essential beam parameters such as the intensity and emittance of the H$^-$ beam and the intensity of the co-extracted electrons.

The ring cyclotron of the Paul Scherrer Institute (PSI) accelerates an intense proton beam from 72MeV up to 590MeV. This happens in four cavities of very high quality factor, oscillating in the fundamental mode. The beam can excite parasitic oscillation modes (HOMs), because of its time structure. Measurements showed that their field can leak out into the vacuum chamber. Until now, there is no tool available to predict the potentially harmful effect of these HOMs onto the beam operation of the cyclotron. It is foreseeable that these effects might play a role if even higher beam currents have to be accelerated. This dissertation therefore deals with the numerical analysis and measurement of beam-cavity interactions. First calculations for a single cavity, interacting with a proton bunch were performed with MAFIA's eigenmode- (E3), time domain- (T3) and particle-in-cell (TS3) solvers. However, the structured grid and the limited computing performance of MAFIA make realistic simulations impossible. A simplified computation method is developed in this dissertation since a self-consistent simulation is impossible on today's computers: The parallel eigensolver Omega3P of the Stanford Linear Accelerator Center (SLAC) allowed us to calculate eigenmodes of the entire ring cyclotron for the first time ever. The rf fields are expanded onto a superposition of these modes and the excitation is calculated in frequency domain. Trajectories of the particles in the static magnetic field, superposed with the space charge fields and the beam excited HOMs, are then simulated. However, the quantitative accuracy of this model is still limited. On the one hand, because of the simplification in the geometry of the simulated rf structure, which otherwise would lead to a problem size going beyond the available computing resources. On the other hand, because it is not yet possible to simulate strongly absorbing boundaries more accurately. The simulation results confirm that up to proton beam currents of 2mA, corresponding to the routinely accelerated beam intensities, only a small deformation of the charge distribution appears. This thesis leads to a new simulation tool for further studies of intensity increases in high power cyclotrons.

As a result of extremely high upper critical fields B_c2, high temperature superconductors (HTS) have the potential to be used as high field insert coils in magnet systems where the background field is provided by low temperature superconductors (LTS) with the aim of application of such systems for high energy physics, nuclear magnetic resonance and energy storage. Among the superconductors discovered in late 1980s, one that is widely studied is YBCO coated conductor (CC). Despite the advanced 2G conductor technology in recent years allowing for manufacturing of long lengths of YBCO CC, up to 1 km, the crucial issue in practical application of the hybrid systems with YBCO insert coil remain the quench detection and protection of the insert. Unlike in LTS magnets, the quench propagation in HTS is significantly slower, which makes quench detection and consecutively protection, very challenging. Following a new approach for quench protection a non-insulated HTS insert coil was manufactured at the CRPP. In case of quench, the current is expected to by-pass the normal zone and prevent the coil from a permanent damage. The idea of non-insulated free-standing coils has been already studied and brought promising results in terms of self-protection of such coils. However, in a hybrid system of LTS-HTS, the quench behaviour needs careful evaluation that includes magneto-thermo-electrical study. The objective of this work is to study and discuss the applicability of HTS insert coils (insulated and non-insulated) by addressing the issue of their quench detection and protection schemes. The starting point for the analysis is studying the existing design according to CRPP specifications taking into account various operating modes together with detection and protection schemes. Finally, general guidelines for the design of a successful LTS-HTS magnet system will be discussed.

The quality factor and oscillating mode of a gyrotron cavity are essential parameters to consider when trying to obtain a high power (>500 kW), high efficiency (∼50%) microwave source, which oscillates in a stable manner in the principal mode of the cavity. The study and development of an 8 GHz gyrotron whose resonant cavity is formed by a cylindrical waveguide of slowly varying radius, is undertaken. The study is principally concerned with the phenomena associated with the low quality factor of the TE°011 mode of the cavity. The power at optimal efficiency of a given resonant mode (TE°011 ) is increased when the diffractive quality factor (Qdiff) of the mode is decreased. Experiments performed with the gyrotron delivering power into a matched load show that during the process of decreasing Qdiff, the chosen mode of oscillation is perturbed by the appearance of oscillations in parasitic modes of the cavity; composed of higher order longitudinal modes (TE°O1q'q=2.3), and a propagating mode TE°21' gyro BWO), corresponding to an absolute instability of the system. These modes limit the power and efficiency of the principal mode by either oscillating simultaneously with, or in place of, the principal mode. When the gyro BWO mode oscillates, even at low power (several kilowatts), the efficiency of the principal mode of a cavity with Qdiff=160 is limited to less than 25%. This parasitic oscillation can be eliminated by the addition of a gradient in the magnetic field over the length of the interaction region. The maximum efficiency of the principal mode is then greater thon 40%; however, the power delivered remains lower than that of the cavity with Qdiff=225. The study of the influence of power reflections on the oscillation characteristics of the gyrotron reveals that the lowering of the quality factor of the cavity increases the sensitivity of the gyrotron to power reflections due to mismatched loads. A gyrotron with a quality factor which is too low can only deliver its maximum power to a matched load. In the presence of power reflections, the mode of oscillation changes, or the pulse is interrupted by an arc in the interior of the gyrotron. Even though a maximum power of 310 kW at an efficiency of 35% is obtained by the cavity with Qdiff=225, the sensitivity of this gyrotron to power reflections could limit the possibility of its use as a source for current generation or heating in a plasma, for example. These results show that there is a lower limit to the quality factor of a gyrotron cavity. This limit is situated between 410 and 225 for the TE°011 mode at 8 GHz. Below this limit the power in the mode decreases, the oscillation is perturbed or eliminated by the simultaneous oscillation of parasitic modes, and the sensitivity to power reflections could render the tube unusable for practical applications. A determination of the average velocity ratio α of the electrons in the electron beam (α=/) is made by measuring the oscillation frequency of the gyro BWO mode. The results obtained using a non-linear simulation of the interaction between the gyro BWO and the electron beam show that the value of α at the entrance of the cavity corresponds to the α-value which the electron gun is supposed to produce. It seems, however, to indicate the presence of a larger velocity dispersion than expected. This original method of measuring the α of an electron beam can be applied to any source in which a gyro BWO mode is observed, or directly as a method of characterising the beam generated by a given electron gun.

One of the biggest goals for the international thermonuclear experimental reactor (ITER) is the steady state operation. For this reason all its coils will be superconducting and Nb3Sn material is used for both the toroidal field (TF) and the central solenoid (CS) coils. The primary current carrying unit in a coil is the cable, which contains hundreds of superconducting strands. The ITER coils will be made using a kind of cable called cable in conduit conductor (CICC). Both CS and TF magnets have to withstand several electromagnetic (EM) cycles in their lifetime. This has been discovered to cause a severe reduction of the magnet performances. Nb3Sn is a brittle material, therefore it is very strain sensitive. The knowledge of the strain state in a CICC allows the explanation of the performances of the cable itself, providing also information regarding the permanent damaging of the Nb3Sn structure. The measurement of the Nb3Sn strain state in a CICC is a very challenging task because it is not possible to make it directly. An indirect technique that allows the measurement of the thermal strain through the measurement of the magnetic susceptibility vs. temperature (χ(T )) for a CICC has been developed. A CICC is a complex system and as such it behaves in a way which is not directly predictable studying the single components. The elastic deformation of the superconducting layers produces reversible phenomena which interact with irreversible phenomena due to Nb3Sn fractures and to plastic deformations of the matrix surrounding the superconducting layers. Reversible and irreversible phenomena occurs together and are deeply shuffled such that it is not straightforward the explanation of the cable performance. The magnetic susceptibility measurements, via an appropriate data analysis, allow to quantify and to separate the thermal strain from the jumble of parameters necessary to describe the cable performance. Several TF and CS samples have been measured with this technique in the SULTAN test facility. Each cable behaves differently from the others but at least it is possible to identify common features. For some samples, the evolution of the performance with the EM cycles is dominated by reversible rearrangements of the strand position in the cable and consequently by the change of the strain distribution during the EM load, for others it is dominated by breakages and irreversible phenomena.