Abstract
One of the important issues of the in-vacuum undulator design is the coupling impedance of the vacuum chamber, which includes tapered transitions with variable gap size. To get complete and reliable information on the impedance, analytical estimate, numerical simulations and beam-based measurements have been performed at Diamond Light Source, a forthcoming upgrade of which includes introducing additional insertion device (ID) straights. The impedance of an already existing ID vessel geometrically similar to the new one has been measured using the orbit bump method. The measurement results in comparison with analytical estimations and numerical simulations are discussed in this paper.
Highlights
In-vacuum undulators with a small vertical gap are the major contributors to the total coupling impedance of modern synchrotron light sources
It is proposed to convert some of the double-bend achromat (DBA) lattice cells into a double-DBA, with a new insertion device (ID) straight between the two achromats [1]
The vertical kick factor of ID16 has been measured for seven values of the ID gap height
Summary
In-vacuum undulators with a small vertical gap are the major contributors to the total coupling impedance of modern synchrotron light sources. A series of analytical estimations, numerical simulations and beam-based measurements have been performed to get complete and reliable information on the ID coupling. For a variable-gap in-vacuum ID, this is the first-time comparison of measured impedance with analytical estimations and numerical simulations. The vacuum chamber of the ID section has a complex geometry including tapers, foils, transition between elliptic and flat cross sections, etc., see Fig. 1. It is impossible to derive accurate analytical formulas for the impedance of the whole chamber. Analytical formulas for the geometric and resistive-wall impedances of a simplified flat taper model are available, so we can estimate the impedance as a function of gap to compare with the measurement results. For the same flat rectangular tapered structure, wakefield simulations have been carried out using finite-difference simulation codes GDFIDL [2] and CST PARTICLE STUDIO [3]
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