Analysis of a Novel Toroidal Configuration for Hadron Therapy Gantries

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.

Stability modeling of the LHC Nb-Ti Rutherford cables subjected to beam losses

Luca Bottura, Enrico Felcini

The Large Hadron Collider (LHC) at CERN is being prepared for its full energy exploitation during Run III, i.e. an increase of the beam energy beyond the present 6.5 TeV, targeting the maximum discovery potential attainable. This requires an increase of the operating field of the superconducting dipole and quadrupole magnets, which in turn will result in more demanding working conditions due to a reduction of the operating margin while the energy deposited by particle loss will increase. Beam-induced magnet quenches, i.e. the transition to normal conducting state, will become an increasing concern, because they could affect the availability of the LHC. It is hence very important to understand and be able to predict the quench levels of the main LHC magnets for the required values of current and generated magnetic fields. This information will be used to set accurate operating limits of beam loss, with sufficient but not excessive margin, so to achieve maximal beam delivery to the experiments. In this study we used a one dimensional, multi-strand thermal-electric model to analyze the maximum beam-losses that can be sustained by the LHC magnets, still remaining superconducting. The heat deposition distribution due to the beam losses is given as an input for the stability analysis. Critical elements of the model are the ability to capture heat and current distribution among strands, and heat transfer to the superfluid helium bath. The computational model has been benchmarked against energy densities reconstructed from beam-induced MB (Main Bending) dipole quenches during LHC operation at 6.5 TeV. The model was then used to evaluate the stability margin of both MB and MQ (Main Quadrupole) magnets at different beam energies, up to the expected ultimate operating energy of the LHC, 7.5 TeV. The comparison between the quench levels underlines how the increase of beam energy implies a substantial reduction of magnets stability and will require much stricter setting on the allowable beam losses to avoid resistive transitions during operation.

2019

Magnetic Design of a Superconducting Toroidal Gantry for Hadron Therapy

Luca Bottura, Bertrand Dutoit, Enrico Felcini

Hadron and proton therapy are cutting edge techniques for cancer treatment and a great development of specialized medical centers and research facilities is foreseen in the next decades. One of the main obstacles to the penetration of the use of charged particles for therapy is the construction of complex and expensive accelerating structures and rotating transfer lines, i.e. gantries, able to bend and focus the beam down to the patient. GaToroid is a novel concept of a fixed toroidal gantry, able to deliver the dose at discrete angles in the whole range of treatment energies in steady-state configuration. The steady-state current and magnet-ic field are appealing features, implying simplified demands on stability, powering, mechanics and cooling, as well as for the clinical perspective, allowing rapid variations of beam energy and treatment angle. In this work, we present the magnetic design of the toroidal coils composing the first instance of GaToroid, focusing the analysis on an option for a proton machine with an energy range of 70 MeV to 250 MeV. To create a proper magnetic field distribution, the coils have been designed with peculiar asymmetric shape and the windings have been graded. An initial winding geometry was obtained with an optimization aiming at maximum energy acceptance of the gantry. We are now progressing to the detailed engineering design. We describe here the over-all magnet design, coil and conductor layout (LTS and HTS options), and mechanical studies involving the general torus structure. Quench protection is evaluated for LTS (Nb-Ti) configuration, as well as more innovative HTS (REBCO) options. Finally, we present the design and the construction of a scaled-down demonstrator, intended as the proof of principle of winding procedure and mechanical coil structure.

2020

Cyclotron Designs for Ion Beam Therapy with Cyclinacs

Adriano Garonna

This thesis presents new superconducting compact (as opposed to separated-sector ) cyclotron designs for injection in CABOTO, a linac developed by the TERA Foundation delivering C6+/H2+ beams up to 400 MeV/u for ion beam therapy. This association of a variable energy linac injected by a fixed energy cyclotron is called cyclinac. Two superconducting cyclotron designs are compared under the same design constraints and methods: a synchrocyclotron and an isochronous cyclotron, both at the highest possible magnetic field and with an output energy of 230 MeV/u. This energy allows to use the cyclotron as a stand-alone accelerator for protontherapy. Once the optimal cyclotron is determined, lower energy cyclotrons can easily be designed. The short pulse length (1.5 µs), fast repetition rate (100-300 Hz) and small beam transmission of the cyclinac (0.2%) require intense pulsed ion sources. To deliver the desired clinical dose rate, the average pulse current of 60 eµA of C6+ at 300 Hz can be produced by three commercial EBIS (EBIS-SC by Dreebit Gmbh) operating at 100 Hz and connected to the beamline in alternating mode. A multicusp ion source is sufficient to produce compatible H2+ beams. The synchrocyclotron design features a central magnetic field of 5 T, an axisymmetric pole and a constant field index of 0.02. The beam is injected axially with a spiral inflector (K = 1.4). A static magnetic perturbation of 0.1 T and 5° width boosts the beam radial gain per turn (with no emittance degradation) by exciting the first radial integer resonance and thus allows beam ejection with moderate beam losses (30%). The RF system operates in first harmonic (Q = 2500). The 180° Dee provides 28 kV peak voltage and the RF is modulated (30-38 MHz) by a rotating capacitor (90-900 pF). The synchrocyclotron's best features are the simple and compact magnet (300 tons) and the low RF power requirements (30 kW power supply). The isochronous cyclotron design features a 3.2 T central magnetic field, four sectors and a pole characterized by elliptical gaps in the hills (3-30 mm) and in the valleys (11-50 cm). Spiraling is minimized (80° total hill axis rotation) and beam ejection is achieved with a single electrostatic deflector placed inside an empty valley. The two RF cavities operate in fourth harmonic at 98 MHz (Q = 7100). The RF system provides peak voltages of 70-120 kV and is powered by a single 100 kW unit. The synchrocyclotron reliability is brought into question by the need of a rotating capacitor and by the complexity of the injection and ejection systems. However, the isochronous cyclotron requires a much more complex magnet. Overall, the isochronous cyclotron is a better solution compared to the synchrocyclotron, because it is as compact but more reliable. To quantitatively determine the industrial and clinical optimum for the CABOTO injection energy, three complementary isochronous cyclotrons of 70, 120 and 170 MeV/u are studied, based on the 230 MeV/u design. The optimal cyclotron energy strongly depends on the clinical aim of the facility. For a dual proton and carbon ion centre, the best compromise between clinical flexibility, accelerator size and power consumption is to accelerate particles up to 150 MeV/u in the cyclotron. In this configuration, the 150 MeV/u isochronous cyclotron has similar weight and spiraling as the most widely used cyclotron for protontherapy (C235 by IBA S.A.), CABOTO is 24 m long and the overall power consumption of the cyclinac is 650 kW. Adding to these characteristics, the property of fast energy variation of the linac makes the cyclinac presented in this thesis a strongly competitive accelerator for dual proton and carbon ion therapy.