Off-axis beam dynamics in rf-gun-based electron photoinjectors

The need to operate an rf-gun-based electron photoinjector with a beam emitted away from the cathode center can occur under various circumstances. First, in some cases the cathode can be affected by ion back-bombardment that progressively reduces the quantum efficiency (QE) in its center, making off-axis operation mandatory; second, in some cases the drive laser intensity can be sufficiently high to generate QE depletion in the cathode area illuminated by the laser, forcing off-axis operation; last, in cathodes with nonuniform QE distribution it could be convenient to operate off axis to exploit a better QE. However, operation in this mode may lead to growth of the projected transverse beam emittances due to correlations between the transverse and longitudinal degrees of freedom that are introduced within the gun and downstream rf cavities. A strategy is described to mitigate this emittance growth by allowing the beam to propagate along a carefully tuned off-axis trajectory in downstream rf cavities to remove the time-dependent rf kicks introduced in the gun. Along this trajectory, short range wakefields do not degrade the emittance, and long range wakefields degrade the emittance for very high repetition rate only.

Figure 1

Schematic of the photoinjector based on the APEX VHF-Gun used for these studies. The injector exit is located 15 m downstream from the cathode.

Figure 2

$z-p_x$ correlation introduced by a rf cavity for different initial offsets. An initially uncorrelated beam enters each cavity with an initial horizontal offset $x_0$. The final $z-p_x$ correlation is calculated at the exit of each cavity, using astra simulation. The upper figure shows the correlation introduced by the rf gun and the lower shows the correlation introduced by the rf buncher cavity. The lines denote linear fits to the simulation data, with the slopes defining the correlation coefficients (${\partial }^2{p}_r/\partial r\partial z$).

Figure 3

Particle distribution at the cathode. Red dots are particles emitted on-axis near the center of the cathode. Blue dots are particles emitted off-axis with 2.0 mm horizontal misalignment. (Upper) X-Y projection. (Lower) Z-X projection.

Figure 4

Evolution of the transverse centroid (upper) and transverse emittance (lower) along the length of the injector system of Fig. 1. The red and blue lines are, respectively, the horizontal and vertical results for a beam emitted 2.0 mm horizontally off-axis. Results for the on-axis beam are shown in black.

Figure 5

A close-up view of the behavior in Fig. 4 from the cathode to the exit of the rf buncher, illustrating the effects of rf defocusing in the gun. (Red) Evolution of the centroid divergence angle $r^{\prime }=p_r/p_z$ for a beam emitted 2 mm horizontally off-axis at the cathode. (Blue) Geometric mean of the horizontal and vertical normalized transverse emittances for a beam emitted 2 mm horizontally off-axis at the cathode. (Black) The corresponding transverse emittance for a beam emitted on-axis at the cathode.

Figure 6

Particle distribution of a beam emitted 2.0 mm off-axis at the cathode, after tracking to the exit of the injector shown in Fig. 1. Transverse-longitudinal correlations are visible in the final beam phase space. Here color denotes particle density, with red-black denoting regions of highest particle density and blue denoting regions of lowest particle density. (Upper) Z-Y projection. (Lower) Z-Py projection.

Figure 7

A Pareto-optimal front [22] showing the set of solutions that minimize the horizontal and vertical normalized emittances at the injector exit obtained by varying the strengths of the correctors upstream of the buncher. The result is shown for a 100 pC beam emitted 2.0 mm off-axis. Space charge is not included.

Figure 8

The beam offset required in each cavity to compensate for the emittance growth due to off-axis beam emission is shown for different values of the initial laser offset (crosses). The values of the optimal beam offset at the injector exit are shown as black crosses. The colored lines denote linear fits to the data.

Figure 9

Simulation results showing the residual emittance growth (after the compensation technique is applied) as a function of the initial laser offset at the cathode. Wakefield effects not included.

Figure 10

Simulation of a beam emitted 2 mm off-axis at the cathode, with space charge included. Top: Beam centroid position along the beam line for the optimally corrected (green line) and noncorrected (black line) cases. Bottom: Normalized transverse emittance evolution along the beam line for the optimally corrected (dashed lines) and noncorrected (solid lines) cases. Red and blue curves are used to identify the horizontal and vertical planes, respectively.