Experimental investigation of the longitudinal beam dynamics in a photoinjector using a two-macroparticle bunch

We have developed a two-macroparticle bunch to explore the longitudinal beam dynamics through various components of the Fermilab/NICADD photoinjector. Such a two-macroparticle bunch is generated by splitting the ultraviolet pulse from the photocathode drive laser. The presented method allows the exploration of radio-frequency-induced compression in the 1.625 cell radio frequency gun and the booster cavity. It also allows a direct measurement of the momentum compaction of the magnetic bunch compressor. The measurements are compared with analytical and numerical models.

Figure 1

Overview of the Fermilab/NICADD photoinjector. “X” refers to diagnostic stations (beam viewers and/or slit location), “L” to the solenoidal lenses, “Q” to quadrupoles, and “S” to skew quadrupoles. Distances are in meters. The magnetic bunch-compressor chicane bends the beam in the vertical plane while the spectrometer deflects the beam in the horizontal direction.

Figure 2

Streak camera image (left) and corresponding time-profile (right) of a two-pulse laser. In this example the separation between the two pulses is approximately 31 ps. The apparent intensity difference between the two pulses (noticeable on the profiles) is an artifact of the measurement (slight misalignment of the second pulse on the streak camera entrance slit).

Figure 3

Scan of the phase between the rf gun and the photocathode drive laser when only reference (delayed) or both uv pulses are incident on the photocathode. The data were obtained with the same uv-pulse configuration as in Fig. 2. The horizontal axis has an arbitrary offset.

Figure 4

Compression factor in the rf-gun cavity for various operating phases. Diamonds are experimental measurements, dashed red, solid green, and dashed green lines correspond, respectively, to numerical simulations with Astra, numerical integration of the equation of motion using the field ${\widehat{E}}_z$ obtained from Superfish, and numerical integration assuming ${\widehat{E}}_z=\cos (kz)$ (following Ref. 16).

Figure 5

Compression factor in the booster cavity for various operating phases. Diamonds are experimental measurements, solid and dashed lines correspond, respectively, to analytical calculation using Eq. (6), and numerical simulations performed with Astra . Note: The phase ${\psi }_0=90°$ corresponds to maximum acceleration.

Figure 6

Overview of the magnetic bunch-compressor chicane. Top: electron vertical trajectory superimposed to a false color map of the magnetic field $B_x(x=0,y,z)$; bottom: electron vertical deflection angle. The electron energy is 12.9 MeV, and the maximum B-field of the dipoles are 593.20 and $-592.35\mathrm{G}$ for, respectively, the outer and inner dipoles.

Figure 7

Example of energy distribution of the two-macroparticle bunch. The top figure is the beam transverse density recorded on a screen downstream of the spectrometer (horizontal axis is dispersive axis) and the bottom plot is the corresponding fractional momentum distribution. The bunch mean energy is $\mathcal{E}=12.9\mathrm{MeV}$ for the data presented.