Conceptual Design of a Transverse Deflecting Structure for Longitudinal Diagnostics at DALI

Abstract

A conceptual design study of a Transverse Deflecting Structure (TDS) for longitudinal beam diagnostics at the DALI accelerator facility is presented. The TDS provides a time-dependent transverse kick to the electron bunch, enabling direct measurement of the longitudinal bunch profile and reconstruction of the longitudinal phase space when combined with a dispersive spectrometer section. The report reviews the physical principles of RF deflecting cavities, including the transverse kick mechanism, temporal-to-spatial mapping, and the relationship between beam optics and achievable temporal resolution. Engineering considerations such as RF frequency choice, cavity design, wakefield effects, timing synchronization, and mechanical stability are also discussed.

Figures (7)

• Figure 1: TM$_{11}$-type RF deflecting structure (LOLA) developed at SLAC, reproduced from Fig. 1 of Loew1965RFDeflecting. The disk-loaded traveling-wave geometry supports a dipole mode used to generate transverse momentum for charged particles.
• Figure 2: Principle of TDS streaking. At RF zero-crossing, the cavity imparts a transverse kick proportional to the arrival time within the bunch. After transport with matrix element $R_{12}$, time is mapped to transverse position on the screen ($x=S\,z$).
• Figure 3: Longitudinal phase-space measurement with a TDS and a spectrometer dipole. The TDS maps arrival time to horizontal position ($x\!\propto\!t$), while the dipole introduces vertical dispersion so that $y\!\propto\!\delta$. The image on the screen encodes $(t,\delta)$ for each slice, enabling reconstruction of the longitudinal phase space.
• Figure 4: Indicative comparison of cavity outer diameter and beam aperture for S-, C-, and X-band TDS cavities. Higher frequency (X-band) yields smaller structures and apertures, which improves streaking efficiency but tightens alignment and increases wakefield sensitivity.
• Figure 5: RF feeding and integration of a TDS: the RF source (klystron or solid-state) delivers power through waveguides and phase control to a matched input coupler exciting the TM$_{110}$ deflecting mode. Higher-order mode (HOM) damping and active cooling maintain stability, while the beamline axis sets constraints on aperture, alignment, and vacuum.