Transverse-Longitudinal Coupling Effect in Laser Bunch Slicing

We report turn-by-turn observation of coherent synchrotron radiation (CSR) produced by the laser bunch slicing technique at an electron storage ring operated with a small momentum compaction factor. CSR emission was intermittent, and its interval depended strongly on the betatron tune. This peculiar behavior of the CSR could be interpreted as a result of coupling between the transverse and longitudinal motion of the electrons. This is the first observation of such an effect, which would be important not only for controlling the CSR emission but also for generating and transporting ultrashort electron bunches or electron bunches with microdensity structures in advanced accelerators.

Figure 1

Schematic drawing of laser bunch slicing system at UVSOR-II.

Figure 2

CSR signals of three diode detectors at the low-alpha optics. Revolution time of the ring is 177 ns. Panels (a)– (c) are with the low-alpha ($1/2$) optics and a laser pulse FWHM of 460 fs. Sensitivity ranges are (a) $11.0–16.6{\mathrm{cm}}^{-1}$, (b) $7.3–11.0{\mathrm{cm}}^{-1}$, and (c) $3.7–5.7{\mathrm{cm}}^{-1}$. Panel (d) is with low-alpha ($1/3$) optics and a laser pulse FWHM of 1 ps. Sensitivity range is $7.3—11.0{\mathrm{cm}}^{-1}$.

Figure 3

Development of $R_{51}$, $R_{52}$, and $R_{56}$ elements of transport matrices at each turn at normal and two low-alpha optics.

Figure 4

Numerical estimates of CSR signals at the low-alpha optics. Panels (a)–(c) are with betatron tune 3.53 and a laser pulse FWHM of 460 fs. Sensitivity ranges are (a) $11.0–16.6{\mathrm{cm}}^{-1}$, (b) $7.3–11.0{\mathrm{cm}}^{-1}$, (c) $3.7–5.7{\mathrm{cm}}^{-1}$, and (d) is with betatron tune 3.68 and a laser pulse FWHM of 1 ps. Sensitivity range is $7.3–11.0{\mathrm{cm}}^{-1}$.

Figure 5

An example of evolution of fragments and a dip structure in longitudinal phase space (upper) and its corresponding longitudinal density distribution (lower). The horizontal axis is an arrival order after the revolutions.