Experimental Demonstration of Emittance Compensation with Velocity Bunching

In this Letter we report the first experiments aimed at the simultaneous demonstration of the emittance compensation process and velocity bunching in a high brightness electron source, the SPARC photoinjector in INFN-LNF. While a maximum compression ratio up to a factor 14 has been observed, in a particular case of interest a compression factor of 3, yielding a slice current of 120 A with less than $2\mu \mathrm{m}$ slice emittance, has been measured. This technique may be crucial in achieving high brightness beams in photoinjectors aiming at optimized performance of short wavelength single-pass free electron lasers or other advanced applications in laser-plasma accelerators.

Figure 1

Horizontal slice emittance and corresponding current profile (continuous lines), for the noncompressed beam. Gun solenoid set to 2.46 kG, long solenoids off.

Figure 2

Measured rms bunch length (black) and corresponding compression factor (red) of a 280 pC beam versus the phase of the first traveling wave structure. PARMELA simulations are also shown with dashed red line.

Figure 3

Measured rms normalized emittance at $C=3$ as a function of the gun solenoid field, long solenoid set to 450 G. For comparison black and green dots represent best results without compression, long solenoid off.

Figure 4

Measured envelopes and PARMELA simulations (left plot). Emittance evolution along the linac, PARMELA simulations (right plot). No compression (curves $a$), compression with long solenoids off (curves $b$), same compression with long solenoids set to 450 G (curves $c$).

Figure 5

Horizontal slice emittance profiles for three levels of the gun solenoid field together with beam current profile ($C=3$). Long solenoids set to 450 G.