title: Carbon-Based Materials for High-Power Accelerator Components Exposed to Extreme Radiation Conditions
Creators: name: Simon, Pascal
Publisher: name: UNSPECIFIED
Year: 2023

description: The increasing beam power of next-generation particle accelerator facilities demands specific functional materials that can withstand extreme operation conditions, especially when interacting directly with the beam. These conditions include large thermal loads and mechanical stresses that are induced by high-intensity/high-power particle beams, which also induce radiation damage. In comparison to electron and proton accelerators, radiation damage is a particularly severe issue at heavy ion accelerators like the Facility for Antiproton and Ion Research (FAIR) under construction in Darmstadt. Carbon-based materials are commonly used in accelerator components. Graphite, for example, is employed in beam-intercepting devices like beam dumps and secondary particle production targets, while diamond is the active material in high-performance particle detectors. Both graphite and diamond are characterized by their large robustness towards radiation damage due to lower stopping power and lower activation in high-dose environments in comparison to metals. While structural radiation damage has been extensively studied in graphite and diamond, this thesis thus focuses (i) on the potential application of diamond-based composites for high-intensity heavy ion luminescence screens and (ii) on the effects of high-power single beam pulses on graphitic materials. 

Diamond-based metal matrix composites, containing type Ib diamond powder, bulk monocrystalline type Ib and type IIa diamonds were irradiated with different swift heavy ions at the UNILAC accelerator of the GSI Helmholtz Centre for Heavy Ion Research, Darmstadt. On-line ionoluminescence spectroscopy was combined with in-situ UV/vis and infrared absorption spectroscopy to characterize the radiation-induced evolution of intrinsic and extrinsic defects within the diamond lattice. Radiation damage effects along the ion range were investigated with depth-resolved Raman and photoluminescence spectroscopy.

The ionoluminescence signal of diamond-based composite and type Ib diamond degraded rapidly under irradiation with swift heavy ions. UV/vis absorption spectroscopy showed no clear difference in the evolution of radiation-induced defects between type Ib and type IIa diamonds with increasing radiation fluence. While optical microscopy indicated severe loss of transmission in irradiated diamonds, significant increase of absorption occurred only at the highest ion fluences and is thus not contributing to the degradation of the ionoluminescence signal. Photoluminescence measurements along the ion trajectory reveal that color centers are produced predominantly in regions of high electronic energy loss. All color centers exhibit a non-linear trend with increasing radiation fluence that is attributed to a radiation-induced vacancy density threshold. In summary, the irradiation experiments performed on various diamond samples within this thesis indicate that diamond-based metal matrix composites have major drawbacks as a luminescence screen for high-intensity heavy ion beams.

To investigate the effects of high-power beam pulses, various graphitic materials were exposed to 440 GeV/c proton beams in a dedicated experiment at the High-Radiation to Materials (HiRadMat) facility at CERN. Such beams produce high local energy densities of a few kJ/g, which induce a dynamic response that is akin to a mechanical shock, which was monitored on-line via laser Doppler vibrometry. 

Different polycrystalline graphite grades displayed a correlation between the mesostructure and damping of the beam-induced dynamic response. Smaller particle sizes and the absence of a binder phase decrease the damping of elastic pressure waves. Thermo-mechanical finite element simulations of the dynamic response of SGL R6650 polycrystalline graphite indicated a fully elastic material response up to the maximum beam intensity. Hence, these results demonstrate that this graphite grade can be safely used in the Superconducting Fragment Separator (Super-FRS) target at FAIR. A set of composite samples, that comprised tantalum cores embedded in different graphite shells, were used to create beam-induced stresses to potentially invoke failure in the graphite. The maximum surface velocity of the samples exhibited a non-linear trend with increasing beam-induced energy density within the tantalum cores, indicative of the onset of (local) failure within the graphite shells. The dynamic response of high-strength carbon-fiber reinforced graphite grades degraded more rapidly in comparison to lower strength material grades such as polycrystalline graphite or even low-density graphite foam. The broad overview of the beam-induced dynamic behavior presented within this thesis provides experimental validation of graphite grades for potential use in the FAIR accelerators and is a basis for the development of advanced material models for thermo-mechanical simulations of (anisotropic) graphite materials.