The ESRF upgrade included an option for an increase of the storage ring current from 200 to 300 mA, which has already been tested successfully with the existing RF system. At this current level the higher-order mode tuning of the existing five-cell copper cavities becomes extremely delicate and in view of a future stable and reliable operation in user mode, strongly HOM-damped normal conducting cavities were developed at the ESRF. The design, manufacture and test of operational prototypes received funding from the European Union via the FP7/ESFRI/ESRFUP programme. The first of these cavities is already under test in the storage ring. Although the 300 mA option was not retained for the first phase of the ESRF upgrade, the aim is now to validate the new cavity design for a possible later increase in current.

Fig. 159: The first new HOM-damped cavity installed in the storage ring.

Figure 159 shows the first HOM damped cavity built by Research Instruments and installed for test in cell 25 at the location of the former five-cell cavity #5. The design is based on the 500 MHz cavity that was developed at BESSY and which is used at ALBA and the MLS ring in Berlin. However, the cavity has been fully redesigned in house for the ESRF frequency of 352.2 MHz, with several substantial improvements. The geometry was optimised numerically and checked with an aluminium model, yielding excellent accelerating mode characteristics and very efficient HOM damping by means of comparatively compact HOM dampers. In the BESSY/ALBA design, inevitable gaps remain between the ridges of the HOM dampers that protrude into the cavity and the sleeves of the cavity ports, which lead to unexpected thermal problems and a persistent HOM. The suppression of these gaps constitutes a major improvement of the ESRF design. It was achieved by electron beam welding the coupling sections of the HOM dampers to the cavity body and connecting the other elements of the HOM dampers face-to-face with RF spring contacts, at a sufficient distance from the body where the RF fields and surface currents are sufficiently decayed not to burn the RF springs [1].

Fig. 160: Residual HOM impedances are a factor two below the design threshold of 1 A for coupled bunch instabilities with 18 cavities on the storage ring.

Figure 160 shows the resulting extremely low residual HOM impedances measured on the first delivered cavity: with 18 such cavities installed in the ESRF storage ring, the threshold current for HOM driven beam instabilities would be about 2 A, well above the nominal beam current of the machine. The first beam tests with this cavity during the last run of 2011 were extremely promising. Vacuum conditioning for high RF power and high beam intensity was much easier than expected, even in 16 bunch filling mode, which is the most demanding in terms of power deposited by the beam into the HOM dampers: this cavity was transparent to normal user operation. Also the first tests with active operation of the cavity for beam acceleration were very successful.

A second cavity was delivered by SDMS and a third one is being manufactured by CINEL. They will also be tested one by one on cell 25 before all three will be installed on the first 7 m straight section in cell 23 during the summer in 2013, as shown in Figure 161. The cavities will then be powered by three 150 kW RF solid state amplifiers, which are currently being manufactured. The replacement of all existing five-cell cavities with 18 new cavities in total is envisaged for later upgrade phases.

figure_161_HL2011.jpg

Fig. 161: The first 7 m straight section in cell 23 with two undulators and three new RF cavities, planned for summer 2013.

 

References

[1] V. Serrière, J. Jacob, B. Ogier, L. Goirand, A. Triantafyllou, A.K. Bandyopadhyay and D. Boilot, 352.2 MHz HOM Damped Normal Conducting ESRF Cavity: Design and Fabrication, IPAC’2011, San Sebastian, Conference proceedings pp. 68-70 (2011).