Title: Design Optimization of a HOM Broadband Absorber for the Cornell ERL
Mentors: Matthias Liepe and Valery Shemelin

Description:
One of the main projects in the Superconducting Radio-Frequency (SRF) Group 
is the design of a superconducting linac for the proposed Cornell 
Energy-Recovery-Linac Light Source. The superconducting RF cavities in 
this linac will have to accelerate an electron beam with a high current 
of up to 100 mA and very short bunches. This beam parameter set is well 
beyond present state of the art. One of the critical issues is the damping 
of so-called Higher-Order-Modes (HOMs) in the RF cavities. These 
electromagnetic modes are excited by the beam and can have a detrimental 
influence on the beam quality. Strong mode damping is required to avoid this. 
For damping we plan to use broadband HOM dampers based on exotic materials, 
which absorb RF waves in the GHz regime. These dampers will be placed in 
the beam tubes between the RF cavities. 
A new computer code has become 
available recently, allowing calculating Higher-Order-Modes and their 
damping in 3-dimentional structures. Within this project we plan to make 
a 3-D computer model of an entire ERL cavity with HOM absorbers.  This 
model will then be used to optimize the design of the absorber. The 
calculated HOM damping values will be compared to damping measurements 
on real cavities to benchmark the code. This project involves both 
computer work as well as experimental work.