Direct Measurement of Sub-10 fs Relativistic Electron Beams with Ultralow Emittance

Ultralow emittance ($\le 20\mathrm{nm}$, normalized) electron beams with $10^5$ electrons per bunch are obtained by tightly focusing an ultrafast ($\sim 100\mathrm{fs}$) laser pulse on the cathode of a 1.6 cell radio frequency photoinjector. Taking advantage of the small initial longitudinal emittance, a downstream velocity bunching cavity is used to compress the beam to $<10\mathrm{fs}$ rms bunch length. The measurement is performed using a thick high-voltage deflecting cavity which is shown to be well suited to measure ultrashort durations of bunching beams, provided that the beam reaches a ballistic longitudinal focus at the cavity center.

Figure 1

Experimental layout with element positions (not to scale). (a) The UV drive laser reaches a final focus at the copper photocathode of a 1.6 cell $S$-band photogun. The inset shows an image of the laser intensity distribution at the photocathode plane. (b) An 11 cell linac is used as a buncher by applying a negative $z\text{−}\gamma$ correlation to the bunch. The red and blue arrows signify the applied force on the tail and head of the electron bunch, respectively. (c) An in-vacuum microscope objective images a $20\mu \mathrm{m}$ thick YAG:Ce screen onto a CCD in air, providing a high spatial resolution ($1.4\mu \mathrm{m}/\mathrm{pixel}$) beam profile monitor for emittance measurements. (d) The electron bunch reaches a nonlaminar longitudinal focus at the center of a 9 cell $X$-band deflection cavity. (e) The streaked electron profile is detected on a $50\mu \mathrm{m}$ thick YAG:Ce with an intensified camera. Two focusing solenoids (not shown) are located at $z=0.24$ and 3.3 m.

Figure 2

Normalized transverse emittances in $y$ (a) and $x$ (b) for a ${\sigma }_{t,uv}=1.1\mathrm{ps}$ laser pulse on the photocathode, accelerated on crest in the linac. (c) and (d) show normalized emittances for ${\sigma }_{t,uv}=100\mathrm{fs}$, in the velocity bunching configuration.

Figure 3

(a) Bunch length vs position for ${\sigma }_{t,uv}=100\mathrm{fs}$ (solid curve) and 1 ps (dashed curve). Inset: LPS and current profile of the 100 fs case at the longitudinal waist, with the LPS colored by the particle transverse radius. (b) Evolution of longitudinal and transverse emittances along the beam line for both laser pulse lengths.

Figure 4

(a) Bunch length vs relative buncher phase for deflector voltages of 400 and 200 kV. (b) Example deflector-on shot (red curve), shown with the mean deflector-off distribution (blue curve), and the result of convolution with specified ${\sigma }_t$ (black curve).