Optimal choice of cell geometry for a multicell superconducting cavity

Abstract

An algorithm for optimization of the multicell cavity cells is proposed. Inner cells are optimized for minimal losses or minimal magnetic field, when the aperture diameter, E pk =E acc-the ratio of peak electric field to the accelerating field, and the wall slope angle are given. Optimization of the end cells is done for minimal losses or maximal acceleration in them. Two shapes of the end cells-with and without the end irises-are analyzed. This approach facilitates further optimization for higher order modes extraction because it permits keeping the achieved optimal values nearly the same while changing some dimensions of the cells. Comparison of the proposed cavity geometry with the TESLA cavity geometry illustrates the traits of the presented approach. It is also shown that lower values of the wall slope angle, which lead to the reentrant shape for the inner cells, are also beneficial for the end cells. For the Cornell Energy Recovery Linac most dangerous are dipole modes causing the beam breakup (BBU). Minimization of power of higher order modes (HOMs) in a multicell cavity was done using derivatives of the BBU parameter with respect to geometric parameters of the cavity cells. As a starting point of optimization, the shape with minimal losses at the fundamental mode was taken. Further changing the shape for better propagation of HOMs was done with degradation of the fundamental mode loss parameter G Á R sh =Q within 1% while decrease of the BBU parameter was nearly 3 orders of magnitude. The BBU threshold current tends to be inversely proportional to this parameter.

Electromagnetic Design of a $\beta_{g} = 0.9$ 650-MHz Superconducting-Radio-Frequency Cavity

“…This is in agreement with the observations reported in Ref. [9] that the re-entrant geometry is better for achieving higher acceleration gradient as well as minimum cavity loss.…”

“…We are thus left with four independent parameters to consider, while optimizing the geometry. In the existing literature [5,9], there are two types of cavity performance optimizations that have been discussed -(i) optimization for the maximum acceleration gradient E acc , such that the accelerator can be made as compact as possible. This optimizes the construction cost of the accelerator, (ii) optimization for the minimum power loss on cavity wall such that there is less cryogenic load to be handled.…”

“…Either of the two approaches mentioned above have been used in the literature [5,9] for the optimization of mid cell geometry. In this paper, we have optimized our design for maximum acceleration gradient, but we finally show that it is moderately optimized even for minimum power loss.…”

“…In contrast, in the procedure that we have developed, we perform step by step, one dimensional optimization, which is relatively simple. After optimizing the mid cell geometry, the end-cell geometry is optimized, keeping the requirement on field flatness in the cavity [9,10]. A two-dimensional electromagnetic design code SUPERFISH [11] has been used for performing these calculations.…”

“…The dimensions of mid-cell geometry have been optimized for achieving maximum acceleration gradient, for which a simple and general procedure is evolved in this paper. Belomestnykh [5] and Shemelin [9] have discussed a procedure for the optimization of the cavity geometry, which is essentially a multi dimensional optimization. In contrast, in the procedure that we have developed, we perform step by step, one dimensional optimization, which is relatively simple.…”