Photoelectron shield for the first mirror of a soft X‐ray beamline

At a soft X‐ray beamline with an undulator source, significant heat generation at the first‐mirror chamber and light emission at the viewport were found, which can be explained by photoelectrons from the mirror. The chamber temperature increases up to approximately 50°C over a period of several hours. A photoelectron shield consisting of thin copper plates not only prevents the heat generation and light emission but also improves the pressure of the vacuum chamber, if a voltage of a few tens of V is applied to the shield. The total electron yield of the shield reached as much as 58 mA under high heat‐load conditions, indicating the emission of numerous photoelectrons from the first mirror. Heat‐balance analyses suggest that approximately 30% or more of the heat load on the first mirror is transferred to the surroundings. Significant heat generation at a first‐mirror chamber and light emission at a viewport were found at a soft X‐ray beamline with an undulator source, which can be explained by photoelectrons from the mirror. It was confirmed that a photoelectron shield consisting of thin copper plates prevents both phenomena and improves the pressure of the vacuum chamber.