Title: Design of a Multicell Superconducting Cavity for the Cornell ERL Linac
Mentor: Matthias Liepe

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. The 390 RF cavities in this linac have to be 
operated at a very low temperature of 2 K, and will have to accelerate 
a 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 excitation of so-called Higher-Order-Modes in the 
RF cavities by the electron beam. These electromagnetic modes can have a 
detrimental influence on the beam quality. In addition the cavities 
have to be operated at high electric accelerating fields (15 to 20 MV/m), 
and even though the cavities are superconducting, there are still some 
losses from the RF fields at the cavity walls (some watts per cavity), 
which need to be intercepted at a temperature of 2 K. To intercept one 
watt at 2 K, about 1 kW has to be spend in the refrigerator, so with 390 
cavities the total power requirements are very significant. Accordingly 
we would like to optimize the cavity shape with respect to two objectives: 
First, to reduce the cryogenic losses at a given accelerating field and 
second, to minimize the excitation of detrimental Higher-Order-Modes. 
Unfortunately, these two design goals are partly mutually exclusive, 
which obviously complicates the design optimization significantly. 
Within this project we plan to study the effect of cavity shape changes 
on the two objectives, with to goal of arriving at an optimized cavity 
shape. A 3-dimensional computer model will be used for this work