rf traveling-wave electron gun for photoinjectors

The design of a photoinjector, in particular that of the electron source, is of central importance for free electron laser (FEL) machines where a high beam brightness is required. In comparison to standard designs, an rf traveling-wave photocathode gun can provide a more rigid beam with a higher brightness and a shorter pulse. This is illustrated by applying a specific optimization procedure to the SwissFEL photoinjector, for which a brightness improvement up to a factor 3 could be achieved together with a double gun output energy compared to the reference setup foreseeing a state-of-the-art S-band rf standing-wave gun. The higher brightness is mainly given by a (at least) double peak current at the exit of the gun which brings benefits for both the beam dynamics in the linac and the efficiency of the FEL process. The gun design foresees an innovative coaxial rf coupling at both ends of the structure which allows a solenoid with integrated bucking coil to be placed around the cathode in order to provide the necessary focusing right after emission.

Figure 1

Contour plot of the penalty function $f_{\mathrm{p}}({\overline{B}}_{\mathrm{n},\mathrm{inj}},\overline{\zeta })$ in the typical optimization range of brightness ${\overline{B}}_{\mathrm{n},\mathrm{inj}}$ and mean mismatch parameter $\overline{\zeta }$ for the SwissFEL injector.

Figure 2

Flow diagram of the injector optimization.

Figure 3

Detailed inspection of the stretching method by comparing modulus (top) and phase (bottom) of the initial and final part (note the abscissa discontinuity) of the gun field map. The second part of the phase plot was offset for displaying reasons. The input coupling cell of the reference field map computed with hfss (solid red) is stretched by 1 mm (dash-dotted blue) and is compared to that of a design computed again in hfss (solid blue) also by providing their difference (green, right scales). The position of the first iris is indicated for both cases (dashed gray).

Figure 4

Kinetic energy $E_{\mathrm{kin}}$ and rms energy spread $\mathrm{\Delta }{E}_{\mathrm{rms}}$ (top), horizontal bunch size ${\sigma }_x$ and longitudinal bunch length ${\sigma }_z$ (middle), and normalized projected emittance ${ϵ}_{x,\mathrm{n}}$ (bottom) along the trajectory position $s$. Above the plots, the beam line elements (accelerating structures and solenoids in copper and green color, respectively) used in the simulations are sketched in the correct position.

Figure 5

Longitudinal phase space distribution $(z,p_z)$, with respect to the reference particle having $p_{z,0}=140\mathrm{MeV}/\mathrm{c}$, and corresponding charge density $\rho (Q)$ (top), slice mismatch parameter ${\zeta }_s$ (middle) and normalized slice emittance ${ϵ}_{x,\mathrm{n},s}$ (bottom) along the bunch position $z$ at $s=12.6\mathrm{m}$, after two booster structures. The last two quantities refer to a longitudinal subdivision in $N_{\mathrm{s}}=20$ slices with constant charge and the dashed lines indicate the average values $\overline{\zeta }$ and ${\overline{ϵ}}_{x,\mathrm{n}}$ (neglecting $k_{\mathrm{s}}=2$ slices), respectively.

Figure 6

Top: Upper half of the longitudinal cut of the cavity geometry for the 60 deg (top) and 120 deg (bottom) phase advance designs displaying the modulus of the complex electric field $|\tilde{\mathit{E}}(x,s)|$ in the beam region for operating conditions. Middle: Corresponding on-axis field map for both traveling-wave designs (solid) and for a standing-wave (dashed) together with the solenoid field map (green, right scale). Bottom: Examples of accelerating electric field $E_z(s)$ experienced by the bunch centroid.

Figure 7

Relativistic $\beta$ factor of the electron bunch (solid) along the initial part of the C-band guns. The first three iris positions of the TW guns are indicated, as well as the first two of the SW gun (dashed).

Figure 8

One quarter of the full cavity geometry with input and output waveguide-to-coaxial transition. The modulus of the complex electric $|\tilde{\mathit{E}}|$ and magnetic $|\tilde{\mathit{H}}|$ field is displayed for operating conditions, in logarithmic scale in order to emphasize the field distribution in the region of the transitions and to demonstrate the effectiveness of the filtering cells and choke.

Figure 9

S11 parameter of the two traveling-wave gun designs as a function of the rf frequency $f$. The operating frequency $f_0=5.712\mathrm{GHz}$ is indicated (dashed gray).

Figure 10

Two possible feeding schemes for the traveling-wave gun with 60 deg phase advance: pulse compression with a BOC (top) and recirculation with a 180 deg hybrid (bottom). The power (blue) and the voltage phase (green) are given at the klystron output (dashed) and at the gun input (solid) as a function of time $t$. Bottom: The situation with a long klystron pulse is also indicated (gray). Note the different time scale on the abscissa.

Figure 11

Top: Upper half of the longitudinal cut of the combined solenoid incorporating main and bucking coil (blue), which are driven with opposed current polarity, around the gun cavity volume. The magnetic field lines are displayed (magenta). Bottom: Corresponding on-axis field map of the longitudinal magnetic field $B_z$ with the nominal (solid) and maximum (dashed) allowed bucking coil current.

Figure 12

The mechanical concept is currently under study: the cathode (green) and the output coaxial section (yellow) are not brazed to the main gun body (blue) in order to, respectively, be compatible with the load-lock concept and to allow the mounting of the solenoid.