Electron beam characterization of technical surfaces at cryogenic temperatures

This dissertation presents applied research on the electron irradiation-induced emission of electrons and molecules and thermally controlled gas adsorption and desorption at cryogenic temperatures. Various technical-grade metal surfaces and functional surface coatings and treatments are studied under conditions relevant to many technical applications. A particular focus is on understanding the electron cloud and dynamic vacuum phenomena in CERN’s Large Hadron Collider (LHC), which operates at cryogenic temperatures below 20 K. Its electron cloud is characterised by low energies in 0–1 keV range but high doses up to 10 mC.mm−2. Such conditions are controllably reproduced in a newly developed cryogenic laboratory setup designed for collector-based measurements of Secondary electron emission (SEY), electron stimulated desorption (ESD), and temperature programmed desorption (TPD) at high sensitivity, precision, and accuracy. The experimental results are acquired, analysed and systematically discussed in detail. Finally, semiempirical parametric models of the SEY and ESD yields are developed to capture the energy, dose, angle, temperature and composition dependencies, allowing further use in the field. While emphasising the LHC’s electron cloud-induced dynamic vacuum effect and related phenomena, the research findings are interpreted in a generalist manner, making them relevant to other accelerators and technical applications

Electron conditioning of technical surfaces at cryogenic and room temperature in the 0–1 keV energy range

In the superconducting magnets of the Large Hadron Collider (LHC) at CERN, most of the beam-induced heat load is intercepted by a beam-screen (BS) cryogenically cooled to 5–20 K. When circulating the bunched proton beam, an electron cloud (EC) can form and bombard the BS copper surface with high doses of predominantly low-energy electrons, which desorb gas and consequently increase the pressure. The beam-induced pressure rise decreases during operation as the electron irradiation diminishes the secondary electron yield (SEY) and the electron-stimulated desorption (ESD) yield, a phenomenon referred to as ‘beam conditioning’. Low ESD and SEY values achieved rapidly are requisite to mitigate EC and maintain UHV in storage rings. We report data on ESD and SEY electron conditioning completed at cryogenic temperature with 0–1 keV electrons up to an electron dose of 5.10−3 C mm−2. Our results show that SEY conditioning depends on the primary electron energy and also that ESD yield significantly decreases with temperature. At 15 K, the amorphous-carbon coating and laser-treated copper present SEY below 1.1 and have initial ESD yields 3–6 times lower than OFE copper. Our results conform to the SEY and ESD's general understanding and extend it towards cryogenic temperatures. •Technical copper has 4–80x lower electrodesorption yields at 15 K than at 260 K.•Electrodesorption energy threshold and conditioning rate remain unchanged at 15 K.•Low-energy e− have a limited conditioning effect on the secondary electron yield.•Carbon-coated and laser-treated copper retain low SEY and ESD at 15 K

Collector-based measurement of gas desorption and secondary electron emission induced by 0–1.4 keV electrons from LHC-grade copper at 15 K

CERN’s Large Hadron Collider cryomagnets embed a 1.9 K UHV chamber lined with a 5–20 K beam-screen (BS) that intercepts the synchrotron radiation and electron cloud (EC). The low–energy EC irradiates the BS and desorbs gas, creating a dynamic vacuum effect. A novel setup controllably reproduces this by irradiating an unbaked as-received BS copper sample at 15 K with 0–1.4 keV electrons, representing a slice of the EC spectrum. This collector-based setup is qualified using a HOPG reference for secondary electron yield (SEY) and $^{15}$N$_2$ as a tracer in low-energy electron stimulated desorption (ESD) measurements. Measurement at 15 K revealed sub-10 eV ESD thresholds and a maximum around 300 eV of 0.18 H2/e$^-$ and 0.13 CO/e$^-$. Irradiation with 300 eV and 1 keV electrons at ∼ 8.10$^{-4}$ C.mm$^{-2}$ conditioned ESD and SEY alike. Similar dose at 17 eV only caused minor SEY reduction and no ESD decline. The as-received H2 and CO2 yields at 300 eV decreased 5-150x between 15 and 265 K, respectively

Electron Beam Characterization of REBCO-Coated Conductors at Cryogenic Conditions

Particle accelerators with superconducting magnets operating at cryogenic temperatures use a beam screen (BS) liner that extracts heat generated by the circulating bunched charge particle beam before it can reach the magnets. The BS surface, commonly made of high–conductivity copper, provides a low impedance for beam stability reasons, low secondary electron yield (SEY) to mitigate the electron–cloud (EC) effect, and low electron–stimulated desorption yield (ESD) to limit the dynamic pressure rise due to EC. Rare–earth barium copper oxide (REBCO) high–temperature superconductors (HTSs) recently reached technical maturity, are produced as coated conductor tapes (REBCO–CCs), and will be considered for application in future colliders to decrease the BS impedance and enable operation at around 50 K, consequently relaxing the cryogenic requirements. Aside from HTS properties, industry–grade REBCO–CCs also need qualification for EC and dynamic vacuum compatibility under accelerator–like conditions. Hence, we report the SEY and ESD measured at cryogenic temperatures of 12 K under low–energy electron irradiation of 0–1.4 keV. We also verify the sample compositions and morphologies using the XPS, SEM, and EDS methods. The energy and dose dependencies of ESD are comparable to those of technical–grade metals and one sample reached SEY (Formula presented.) = 1.2 after electron conditioning.The authors acknowledge the support from the HL–LHC project and CERN funding FCCGOV–CC-0208 (KE4947/ATS). The authors acknowledge the support and samples provided by Superox and SuNAM. M. Haubner acknowledges partial support from the Czech Technical University in Prague, grant number: SGS21/149/OHK2/3T/12. A. Romanov acknowledges MSCA-COFUND-2016-754397 for the PhD grant. A. Romanov, T. Puig, J. Gutierrez acknowledge funding from PID2021-127297OB-C21 and CEX2019-000917-S.With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe