Experimental studies of 7-cell dual axis asymmetric cavity for energy recovery linac

Abstract

High average current, transportable energy recovery linacs (ERLs) can be very attractive tools for a number of applications including next generation high-luminosity, compact light sources. Conventional ERLs are based on an electron beam circulating through the same set of rf cavity cells. This leads to an accumulation of high-order modes inside the cavity cells, resulting in the development of a beam breakup (BBU) instability, unless the beam current is kept below the BBU start current. This limits the maximum current which can be transported through the ERL and hence the intensity of the photon beam generated. It has recently been proposed that splitting the accelerating and decelerating stages, tuning them separately and coupling them via a resonance coupler can increase the BBU start current. The paper presents the first experimental rf studies of a dual axis 7-cell asymmetric cavity and confirms the properties predicted by the theoretical model. The field structures of the symmetric and asymmetric modes are measured and good agreement with the numerical predictions is demonstrated. The operating mode field flatness was also measured and discussed. A novel approach based on the coupled mode (Fano-like) model has been developed for the description of the cavity eigenmode spectrum and good agreement between analytical theory, numerical predictions and experimental data is shown. Numerical and experimental results observed are analyzed, discussed and a good agreement between theory and experiment is demonstrated.

Studies of high order modes in asymmetric dual-axis cavity

“…This is desirable for next generation high power light sources capable of generating high brilliance light in the THz and X-ray frequency range [16][17][18][19][20][21][22] as well as for beam cooling in electron-ion colliders. In previous works, 13,14 theoretical and preliminary experimental studies of modes in the pass band have been carried out and some confirmation of the validity of the numerical model has been demonstrated. In this paper, the measurements and rigorous experimental studies of HOMs have been conducted.…”

“…To overcome the limitations imposed by the instabilities, the use of a dual axis [8][9][10] asymmetric cavity has been suggested. [11][12][13][14] The asymmetric configuration exploits the splitting of the accelerating and decelerating processes onto two slightly different sets of cavity cells centred on two parallel axes and resonantly coupled by a bridge cell [ Fig. 1(a)].…”

“…The predicted advantage of the dual-axis asymmetric cavity is the possibility of confining HOMs to either accelerating or decelerating sections implying a decaying electromagnetic field on one of the axes. This would prevent the feedback loop between sets of cells positioned on different axes becoming closed, thereby achieving the overall goal of increasing BBU start current and allowing the average current driven through such an ERL to be dramatically increased 13,14 whilst meeting efficiency requirements via the use of an energy recovery stage. This is desirable for next generation high power light sources capable of generating high brilliance light in the THz and X-ray frequency range [16][17][18][19][20][21][22] as well as for beam cooling in electron-ion colliders.…”

“…The 7-cell test cavity (Fig. 1) 13,14 was set-up upon an optical table, which was vibrationally isolated so environmental impact on the measurements could be minimised. Additionally, the measurements were carried out overnight to avoid other interferences.…”

“…A pair of electric dipole probes (couplers) connected to the two-port VNA were inserted through specific cavity ports, and both couplers were used to excite and probe the EM field inside the cavity. As opposed to the previous setups 14 for these experiments, the couplers were inserted vertically through ports which were vertically aligned with the cavity axes, located along the bridge cell and at the opposite side of the cavity, as shown by the circles in Fig. 1.…”