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An international team is currently investigating the best way to use HigherOrder Modes (HOM) for beam diagnostics in 3.9 GHz cavities. HOMs are excited bycharged particles when passing through an accelerating structure. Thirdharmonic cavities working at 3.9 GHz have been installed in FLASH to linearizethe bunch energy profile. A proof-of-principle of using HOMs for beammonitoring has been made at FLASH in the TESLA 1.3 GHz cavities. Since thewakefields generated in the 3.9 GHz cavities are significantly larger, theirimpact on the beam should be carefully minimized. Therefore our target is tomonitor HOMs and minimize them by aligning the beam on the cavity axis. Thedifficulty is that, in comparison to the 1.3 GHz cavities, the HOM-spectrum isdense, making it difficult to identify individual modes. Also, most modespropagate through the whole cryo-module containing several cavities, making itdifficult to measure local beam properties. In this paper the options for theHOM-based beam position monitors are discussed.

Higher Order Modes for Beam Diagnostics in Third Harmonic 3.9 GHz Accelerating Module