An Algorithm for Toroidal Field Harmonics Computation in Arbitrary Magnetic Configurations

Toroidal magnetic configurations are widely exploited in industry and scientific research, involving a vast spectrum of applications, such as thermonuclear fusion, particle detectors, SMES systems and medical devices. To properly design and analyse these sys-tems, it is crucial to determine the magnetic field generated by dif-ferent configurations. The multipole expansion theory can be ap-plied to the analysis of toroidal configurations, by solving the La-place equation for the magnetic scalar potential in toroidal coor-dinates. Contrarily to the case of accelerator magnets with straight axis, in this case the correlation between the current dis-tribution and the field harmonics cannot easily be identified. This paper proposes a methodology for the computation of field har-monics in toroidal coordinates, which is validated by comparison with the results obtained through the Biot-Savart law. This work was carried out in the frame of the GaToroid project ongoing at CERN.

Analysis of a Novel Toroidal Configuration for Hadron Therapy Gantries

Enrico Felcini

Hadron therapy refers to a medical treatment that uses hadron beams (i.e. protons and ions) to deliver localized energy that suppresses cancerous cells, sparing the neighbouring healthy tissues from unwanted radiation. The major technical components of a hadron therapy centre are the particle accelerator (cyclotron, synchrotron, or linac) and the beam delivery system that controls, shapes and orients the particles towards the area to be treated. The beam delivery can consist of fixed transfer lines, or it can include a gantry, a transfer line that rotates around the patient and allows radiation from multiple directions. The present work investigates a new toroidal gantry for hadron therapy, named GaToroid. This novel gantry configuration allows the dose delivery from a discrete number of angles avoiding magnets as well as patient rotation. Compared to traditional gantries that require rotating magnets, this improvement is made possible by a toroidal magnet operating in steady-state. This design constitutes the ideal conditions for the use of superconductors to generate a significantly higher magnetic field compared to normal-conducting solutions, as well as to reduce the weight and footprint of the magnets. The study of a GaToroid system requires the integration of several aspects of physics and engineering. In this framework, the focus of this research is on the design of the superconducting coils integrated with beam optics and particle tracking analyses. The first part of the thesis illustrates the optimization of the toroidal magnet. Coupling two-dimensional particle tracking and magnetic field calculations, an algorithm was developed to identify optimal gantry configurations that maximize the energy acceptance of the system. Two solutions composed of 16 coils, differing in high and low values of engineering current density, were investigated. In line with current clinical requirements, the beams converged at the isocenter within 1 mm over the whole treatment energy spectrum for both configurations. The second part of the thesis describes the algorithm implemented for the two- and three-dimensional particle tracking. Building upon the results of the magnetic optimization, a linear beam optics formalism was developed to determine the focusing properties of GaToroid. The third part of the thesis focuses on the engineering design of the low current density solution. Using two thermo-electric models, lumped and one-dimensional, the Nb-Ti and ReBCO cable geometries were validated, together with the quench protection system. Furthermore, analytical and numerical studies on mechanics made it possible to estimate the overall footprint and weight of the system. Results show that, compared with the state-of-the-art gantries, the proposed GaToroid solution has the potential to be more compact and lighter by at least a factor two. Finally, the last part of the thesis describes the design of a scaled-down demonstrator wound with ReBCO tapes. Studies on quench protection, mechanics and experimental implementation aimed at testing the use of ReBCO technology for GaToroid coils are discussed. In conclusion, this work presents the first overall description of a GaToroid system, ranging from the analytical definition, magnetic optimization, particle tracking and magnet engineering. The investigation of this new toroidal paradigm for gantries represents a quantum step toward more compact and less expensive solutions for hadron therapy centres.

EPFL2021

Investigating Scrape-Off Layer transport in alternative divertor geometries on the TCV tokamak

Roberto Maurizio

Thermonuclear fusion is a potentially clean and limitless energy source that can substantially change the current global electricity generation mix, which is highly dependent on limited fossil fuels. This thesis contributes to the development of fusion energy with experiments on the TCV tokamak, addressing power exhaust in the divertor, which remains a major issue for a fusion tokamak reactor. A large fraction of the heating power needed to keep the plasma at the parameters required for fusion reactions, is continuously transported by the plasma across closed magnetic surfaces into a thin layer with open field lines surrounding the plasma, the Scrape-Off Layer (SOL), where it follows magnetic field lines towards the divertor targets. Unmitigated target peak heat fluxes in a fusion reactor are projected to greatly exceed available material limits. While acceptable target conditions may be possible by operating the divertor in a detached regime, a degradation of core plasma performance may ensue. A unification of a performant core and a detached divertor may be possible using alternative magnetic configurations to the standard Single-Null (SN). This thesis assesses the power exhaust properties over an unprecedented range of alternative magnetic configurations with the goals of both reducing the power exhaust challenge through divertor geometry modifications and improving the current understanding of SOL transport physics. TCV's infrared thermography systems are extensively used to measure divertor target heat fluxes, fitted to extract the SOL heat power width

\lambda_{q,u}

and the spreading factor

S_u

. Power sharing between divertor targets significantly varies with the plasma and/or divertor geometry, mostly interpreted as the effect of parallel electron heat conduction. The

\lambda_{q,u}

is found sensitive to properties of the plasma core, shape of the plasma and the divertor, and is consistent with empirical cross-machine scalings. The dependence on plasma current can be mostly understood with a model based on the competition between parallel and perpendicular diffusive transport. Poloidal asymmetries and field direction effects indicate that more physics is needed. The

S_u

scales with the inverse of the target flux expansion. In the Low-Field-Side Snowflake Minus configuration, the secondary x-point enhances cross-field transport in the divertor, consistently with turbulence simulations. At the L- to H-mode transition, cross-field transport reduces in the main and divertor-SOL, suggesting that the edge transport barrier propagates into the SOL. The upstream

\lambda_{q,u}

between Type-I ELMs agrees with cross-machine scaling predictions only with the toroidal magnetic field as regression parameter, revealing that this field dependence is vital. A comparison of the ELM power deposition duration in TCV and JET supports the hypothesis of it increasing with the parallel connection length, and the ELM peak parallel energy fluence is consistent with a cross-machine scaling, supporting current extrapolations to ITER. Divertor geometry variations reveal that the ELM power width scales with the inverse of the target flux expansion. These results provide unique input for testing models required to reliably extrapolate to the divertor of a fusion reactor, and may have significant implications in the optimization of its magnetic configuration.

EPFL2020