Beam-cavity interactions in high power cyclotrons

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.

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.

EPFL2011

H- ion source for CERN's Linac4 accelerator

Stefano Mattei

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 us 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 H0 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.

EPFL2017

Electron Spin Resonance Magnetometers for Particle Accelerators

Anthony Jean Beaumont

CERN is the European Organization for Nuclear Research located in Geneva, Switzerland. Its main goal is to explore fundamental physics and it exists primarily to provide physicists the necessary tools for doing this, namely accelerators. These tools bring particles to almost the speed of light and then use them in different experiments. The CERN Accelerator Complex includes a chain of interconnected accelerators allowing the acceleration of charged particles up to 6.5 TeV per beam (for protons) in 2018 by the Large Hadron Collider (LHC). Different types of accelerator exist, but at CERN the most common are the synchrotrons. For those, the magnetic field is synchronous with the beam momentum. Magnetic models cannot be always used to predict the field within the required reproducibility, which may be as low as 10^(-5), due to non-linear effects (eddy currents, saturation and hysteresis). Therefore, most of the CERN's synchrotrons are equipped with a real time magnetic field measurement system, the so called "B-Train". The magnetic field measurement devices are composed of absolute field sensors (called "markers"), field tracking sensors (called "induction coils"), an acquisition system, a control system that calculates the magnetic field from the sensors, and a distribution system that delivers the field value to the users. Due to the configuration of the CERN accelerator chain, if one of the injectors fails, the LHC cannot have beam. Consequently, a huge consolidation program of the real time magnetic measurement systems was launched to guarantee operation in the long term. Thus the markers are critical devices for operations at CERN and must be carefully developed to fulfil the high performance and reliability requirements of the B-Train systems. This thesis propose new designs for electron spin resonance (ESR) field markers. We discuss in details the design, the operation, and performance of the ESR sensors based on resonator and oscillator structures including comparison between paramagnetic and ferrimagnetic samples. We propose four field marker levels at 36 mT, 106 mT, 360 mT and 710 mT that correspond to resonance frequencies of 1GHz, 3GHz, 10GHz and 20 GHz. Those measurement values cover most of the marker level requirements of the CERN accelerators and achieving resolution up to 0.1 nT/Hz^(1/2) for field ramps as fast as 5 T/s and field gradients as strong as 12 T/m. We conclude with measurements validation performed on the Proton Synchrotron (PS) and on the Low Energy Ion Ring (LEIR) accelerators.

EPFL2020