N Mariani

LIMITS FOR BEAM INDUCED DAMAGE: RECKLESS OR TOO CAUTIOUS?

Accidental events implying direct beam impacts on collimators are of the utmost importance as they may lead to serious limitations of the overall LHC Performance. In order to assess damage threshold of components impacted by high energy density beams, entailing changes of phase and extreme pressures, state-of-the-art numerical simulation methods are required. In this paper, a review of the different dynamic response regimes induced by particle beams is given along with an indication of the most suited tools to treat each regime. Particular attention is paid to the most critical case, that of shock waves, for which standard Finite Element codes are totally unfit. A novel category of numerical tools, named Hydrocodes, has been adapted and used to analyse the consequences of an asynchronous beam abort on Phase 1 Tertiary Collimators (TCT). A number of simulations has been carried out with varying beam energy, number of bunches and bunch sizes allowing to identify different damage levels for the TCT up to catastrophic failure.

Behaviour of advanced materials impacted by high energy particle beams

Beam Intercepting Devices (BID) are designed to operate in a harsh radioactive environment and are highly loaded from a thermo-structural point of view. Moreover, modern particle accelerators, storing unprecedented energy, may be exposed to severe accidental events triggered by direct beam impacts. In this context, impulse has been given to the development of novel materials for advanced thermal management with high thermal shock resistance like metal-diamond and metal-graphite composites on top of refractory metals such as molybdenum, tungsten and copper alloys. This paper presents the results of a first-of-its-kind experiment which exploited 440 GeV proton beams at different intensities to impact samples of the aforementioned materials. Effects of thermally induced shockwaves were acquired via high speed acquisition system including strain gauges, laser Doppler vibrometer and high speed camera. Preliminary information of beam induced damages on materials were also collected. State-of-the-art hydrodynamic codes (like Autodyn®), relying on complex material models including equation of state (EOS), strength and failure models, have been used for the simulation of the experiment. Preliminary results confirm the effectiveness and reliability of these numerical methods when material constitutive models are completely available (W and Cu alloys). For novel composite materials a reverse engineering approach will be used to build appropriate constitutive models, thus allowing a realistic representation of these complex phenomena. These results are of paramount importance for understanding and predicting the response of novel advanced composites to beam impacts in modern particle accelerators.

submitter : Proton Irradiation Effects on the Physio-Mechanical Properties and Microstructure of Cold-Worked Molybdenum

High temperature refractory materials and alloys including Mo and TZM have been considered and studied to assess their applicability in fusion reactor applications in addition to spallation targets in particle accelerators. The impacts of neutron, proton and ion irradiation on the properties and microstructure of pure Mo and its combination TZM have been evaluated through illumination damage studies. Cold- worked molybdenum (CW half), described by a microstructure comprising of non-consistently extended grains, has been considered for use in the Large Hadron Collider 7 TeV shaft halo cleaning framework has incited the present investigation. To assess the degradation of key physio-mechanical properties of the cold-worked structure following protracted exposure to proton irradiation as well as the impact of the irradiation temperature on the degradation irradiations with 200 MeV protons at 960°C to fluencies ~2 × 1021 p/cm2 and with 28 MeV at below 600°C to fluency of ~6 × 1020 p/cm2 were performed at Brookhaven National Laboratory. High energy X-rays at the NSLS and NSLS II synchrotrons were utilized in the post-irradiation evaluation (PIE) to assess the evolution of the microstructure. It was revealed that the cold-worked Mo and in agreement with neutron irradiation studies at high temperatures, suffers serious reduction in tensile strength due to the evolution of defects into dislocation networks. Further, irradiation at temperatures near the full re-crystallization temperature of the cold-worked structure removes the texture of the microstructure induced by cold working.

Investigation of Dynamic Fracture Behavior of Graphite

The strain-rate sensitivity of brittle materials, such as glass, ceramics or concrete-like materials, is usually easier to be performed in compression. However, also the tensile behavior, which affects phenomena such as spalling, scabbing and fragmentation, has to be investigated to achieve an exhaustive characterization. In last decades, a lot of researchers suggested spalling test as one of the best ways to characterize dynamically brittle materials. This type of test is based on propagation and reflection of elastic waves: the fracture for spalling occurs when, in the material, the tensile stress state, obtained by the reflection on a free surface of a compressive pulse, exceeds the strength limit. These conditions are usually reached using a SHPB setup: a striker bar is launched against the input bar, which is in contact with a long bar specimen free at the opposite surface. In this work, the spalling test has been performed to investigate the dynamic tensile behavior of graphite. The apparatus is actuated by a pneumatic gas-gun (1.5 m long). Striker and input bars are made of high-strength steel 10 mm of diameter. Different striker lengths are used (100 and 80 mm) to obtain different pulse lengths and amplitudes. The input bar is 3.4 m long and is instrumented in the middle. The specimens are 200 mm long and 10 mm of diameter, instrumented at 80 from the free surface with strain-gages.