Abstract
The electromagnetic characterisation of different materials for the inner wall coating of beam pipes is a long-standing problem in accelerator physics, regardless the purpose they are used for, since their presence may affect in an unpredictable way the beam coupling impedance and therefore the machine performance. Moreover, in particle accelerators and storage rings of new generation very short bunches might be required, extending far in frequency the exploration of the beam spectrum and rendering therefore more and more important to assess the coating material response up to hundreds of GHz. This paper describes a time domain method based on THz waveguide spectroscopy to infer the coating properties at very high frequencies. The technique has been tested on Non Evaporable Getter thick films deposited by DC magnetron sputtering on copper plates.
Highlights
An important step towards the development of a new generation of accelerators and light sources is the special treatment of the vacuum chamber surface, in order to avoid electron cloud (e-cloud) effects that may degrade the machine performance and limit its maximum luminosity
If the value of secondary electron yield (SEY) of the surface material is larger than unity, the number of electrons starts growing exponentially and may lead to beam instabilities and other detrimental side effects [2,3]
Sub-THz measurements are carried out using a time domain spectrometer (TDS) operating in transmission mode
Summary
An important step towards the development of a new generation of accelerators and light sources is the special treatment of the vacuum chamber surface, in order to avoid electron cloud (e-cloud) effects that may degrade the machine performance and limit its maximum luminosity. The impedance of NEG films has been measured in frequency domain in the sub-THz range, directly depositing 1–2 μm of the material on the lateral walls of a calibrated waveguide [10,11] This method can be extended to the characterisation of other coating in thin film form, has its own drawbacks, local in-homogeneity with blistering and peel-off, constraints in sample dimensions, and impossibility to re-use the test system (the waveguide) for further measurements. The design allows us to measure in a simple way large area coating deposited on metallic plates as in the case of accelerators, where averaged quantities are needed This technique has been successfully used to characterize NEG samples deposited on both sides of thin copper slabs inserted in a circular waveguide [16]. In order to validate the technique, we measured the sub-THz response of two coating NEG layers about 4 μm thick deposited using DC magnetron sputtering on both sides of copper plates, and evaluate their conductivity
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