Development of a Superconducting RF Harmonic Cavity for eRHIC
In the proposed collider eRHIC, hadrons from one of the RHIC storage rings will interact with electrons accelerated in a six-pass energy recovery linac (ERL). The maximum energy of protons will be 250 GeV and the maximum energy of electrons will be 30 GeV. The machine will heavily rely on SRF technology. One accelerator physics issue being considered in the design is that the nonlinearities of the accelerating linac in the ERL produce a spread in energy which will degrade the polarization of the electron beam. One way to compensate these nonlinearities is to use a harmonic SRF linac at a multiple of the main linac frequency of 704 MHz. There are a number of challenges in developing these SRF cavities. First, related to the very high current in the ERL, is minimizing the Higher Order Mode (HOM) power generation and damping this power efficiently. Second is maximizing the real-estate gradient in the limited space available. Third is to design a cryogenic-efficient cavity. In the Phase I project, an innovative design for these harmonic cavities was identified a multi- cell accelerating cavity with a special coupling cell based on a photonic band gap (PBG) structure. This structure allows very strong damping of higher-order modes which leak through the photonic-band gap array, it increases the real-estate gradient by eliminating the end groups which are usually used for fundamental and higher-order mode coupling in SRF multicell cavities, and has a potential many-cell upgrade path with excellent cryogenic efficiency. The Phase II project will build the first-ever multi-cell superconducting accelerating cavity with a PBG coupling cell, addressing the mechanical design and manufacturing issues along with the cavity tuning and power coupling challenges. In Phase III, Niowave will collaborate with Brookhaven National Laboratory (host for eRHIC) and Los Alamos National Laboratory (developers of the photonic band gap SRF cavity designs) to offer complete harmonic cavity systems for eRHIC and to develop and market highly damped SRF cavities using PBG cells for high-current accelerator projects worldwide.
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