Radiation protection studies for CERN Linac4/SPL accelerator complex

CERN is presently designing a new chain of accelerators to replace the present Proton Synchrotron (PS) complex: a 160 MeV room-temperature H- linac (Linac4) to replace the present 50 MeV proton linac injector, a 3.5 GeV Superconducting Proton Linac (SPL) to replace the 1.4 GeV PS booster (PSB) and a 50 GeV synchrotron (named PS2) to replace the 26 GeV PS. Linac4 has been funded and the civil engineering work started in October 2008, whilst the SPL is in an advanced stage of design. Beyond injecting into the future 50 GeV PS, the ultimate goal of the SPL is to generate a 4 MW beam for the production of intense neutrino beams. The radiation protection design is driven by the latter requirement. This thesis summarizes the radiation protection studies conducted for Linac4. FLUKA Monte Carlo simulations, complemented by analytical estimates, were performed 1) to evaluate the propagation of neutrons through the waveguide, ventilation and cable ducts placed along the accelerator, 2) to estimate the radiological impact of the accelerator in its low energy section, where the access area is located, and 3) to calculate the induced radioactivity in the air and in the components of the accelerator. The latter study is particularly important for maintenance interventions and final disposal of radioactive waste. Two possible layouts for the CCDTL section of the machine were considered in order to evaluate the feasibility, from the radiological standpoint, of replacing electromagnetic quadrupoles with permanent magnet quadrupoles with high content of cobalt. The present work provides complete information on beam loss assumptions, accelerator structure and duct and maze design, in order to make the present results of sufficiently general interest and provide guidelines for similar studies for intermediate energy proton accelerators. In addition to the Linac4-specific studies, this thesis also discusses FLUKA simulations performed to test the capability of the code, in a proton therapy application, in predicting induced radioactivity from intermediate energy protons. The thesis also reviews the analytical models mostly used for the calculation of neutron streaming through penetration traversing shielding barriers, discusses the FLUKA simulations performed to test the reliability of these models and, on the basis of the simulations, derives a universal expression that can be used to estimate the neutron transmission through a straight duct in direct view of the source, model missing so far in the literature.

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

Space Charge Effects and Advanced Modelling for CERN Low Energy Machines

Adrian Oeftiger

The strong space charge regime of future operation of CERN's circular particle accelerators is investigated andmitigation strategies are developed in the framework of the present thesis. The intensity upgrade of the injector chain of Large Hadron Collider (LHC) prepares the particle accelerators to meet the requirements of the High-Luminosity LHC project. Producing the specified characteristics of the future LHC beams imperatively relies on injecting brighter bunches into the Proton Synchrotron Booster (PSB), the downstream Proton Synchrotron (PS) and eventually the Super Proton Synchrotron (SPS). The increased brightness, i.e. bunch intensity per transverse emittance, entails stronger beam self-fields which can lead to harmful interaction with betatron resonances. Possible beamemittance growth and losses as a consequence thereof threaten to degrade the beam brightness. These space charge effects are partly mitigated by the upgrade of the PSB and PS injection energies. Nevertheless, the space charge tune spreads of the future injector beams are found to exceed the values reached by present LHC or other intense fixed target physics beams. This thesis project comprises three key tasks: detailed modelling of space charge effects, measurement at the CERN machines and mitigation of space charge impact. Throughout the course of this thesis, the simulation tool PyHEADTAIL has been developed and extended to model 3D space charge effects in circular accelerators across the wide energy range from PSB to SPS. The implementation for hardwareaccelerating GPU architectures enables extensive studies, especially when employing the self-consistent particle-in-cell algorithm. The implemented models have been benchmarked with analytical results for space charge beam dynamics. In particular, the spectra of quadrupolar pick-ups - which provide a direct measurement method for the space charge tune shift - have been simulated and compared with the derived theory. The space charge situation at the SPS injection plateau has been extensively investigated in the course of comprehensive measurement studies, resulting in the identification of an optimal working point region for the SPS. The interplay of space charge and the horizontal quarter-integer resonance has been scrutinised in measurement, theory and simulation. Last but not least, a new LHC beam type with a hollow longitudinal phase space distribution has been developed for the PSB and proved to substantially mitigate space charge impact on the PS injection plateau.

EPFL2016

Activation studies for a beta-beam Decay Ring (DR): residual dose rates during maintenance and airborne activity

Stefania Trovati

In a future beta-beam facility radioactive ions (6He and 18Ne) are produced, accelerated and then stored in a large Decay Ring (DR), where they eventually produce anti-neutrino and neutrino beams through β± decay. CERN is one of the candidate sites for the beta-beam facility, as existing machines like the Proton Synchrotron (PS) and the Super Proton Synchrotron (SPS) could be used for the acceleration. The DR is composed of two long straight sections and two arcs. The straight sections host the collimators and two bumps. The beam is injected into the ring at nearly 92 GeV per nucleon. Beam losses occur in different sections of the machine: relevant losses include collimation losses in the collimation section and the bumps, and decay losses in the magnets in the arcs. This work focuses on two radiation protection aspects related to the operation of the DR, namely the induced radioactivity and the air activation generated by collimation and decay losses. All the calculations are performed with the Monte Carlo transport code FLUKA and are based on a continuous three-month operation. The induced radioactivity in the machine components and the expected residual dose rates inside the tunnel during maintenance are calculated for three different waiting times. Airborne activity is evaluated through the convolution of predicted particle spectra in the tunnel with isotope-production cross-sections. Using activity-to-dose coefficients, previously calculated for the ISOLDE facility at CERN, and a laminar flow model for the air diffusion, the airborne activity is converted into effective dose to the reference population group. The results show that residual dose rates during maintenance decrease significantly in a week after the shutdown of the machine, reaching values that correspond, according to CERN area classification, to a limited stay area. The effective dose given to the reference population in one year of operation is below the reference value for CERN emissions into the environment.

2011