Tunable Subpicosecond Electron-Bunch-Train Generation Using a Transverse-To-Longitudinal Phase-Space Exchange Technique

We report on the experimental generation of a train of subpicosecond electron bunches. The bunch train generation is accomplished using a beam line capable of exchanging the coordinates between the horizontal and longitudinal degrees of freedom. An initial beam consisting of a set of horizontally separated beamlets is converted into a train of bunches temporally separated with tunable bunch duration and separation. The experiment reported in this Letter unambiguously demonstrates the conversion process and its versatility.

Figure 1

Top view of the experimental setup displaying elements pertinent to the present experiment. The X refers to diagnostic stations (beam viewers and/or multislit masks location), Q the quadrupole magnets and D the dipole magnets. Distances are in millimeters and referenced to the photocathode surface. The spectrometer dipole magnet downstream of the EEX beam line bends the beam in the vertical direction.

Figure 2

Transverse initial beam density at X5 (a), XS3 (b) and corresponding final beam density at X23 with deflecting cavity off (c) and on (e), and at XS4 with deflecting cavity off (d) and on (f). The corresponding relevant intensity-normalized horizontal profile at X23 (g) and fractional energy spread (h) profiles obtained from XS3 (red) and XS4 for the cases when the cavity is on (green) and off (dashed blue) are also displayed.

Figure 3

Total normalized CTR energy detected at X24 as a function of quadrupole magnets currents $I_{Q_1}$ and $I_{Q_2}$ with X3 slits out (a) and in (b) the beam line. The bolometer signal is representative of the inverse of the bunch duration ${\sigma }_t$. The white dots in (b) indicate loci where more detailed measurements were performed; see Fig. 5.

Figure 4

(a) Normalized autocorrelation function $\Gamma (\tau )/\Gamma (0)$ of the CTR signal (a) recorded with (solid) and without (dashed) the X3 slits inserted as a function of the optical path difference $\tau$. The corresponding beam transverse densities at XS4 appear in (b) and (c). The vertical axis on the bottom image is proportional to the beam’s fractional momentum spread ($\delta$). The nominal bunch charge is $550\pm 30\mathrm{pC}$ and reduces to $\sim 15\pm 3\mathrm{pC}$ when the slits are inserted.

Figure 5

Fractional momentum spread separation $\Delta \delta$ versus longitudinal position separation $\Delta z$ between the bunches within the train for different initial beam conditions. The different data points are obtained from the autocorrelation functions recorded for settings $I_{Q1}=1.0$, 1.2, 1.4, 1.6, and 1.8 A (from left to right) shown as white dots in Fig. 3. The current $I_{Q2}$ is kept constant at $-0.6\mathrm{A}$.