Low energy dark current collimation system in single-pass linacs

The dark current emitted from a surface of a radio frequency cavity may be a severe issue for the activation and the protection of the components of linear accelerators, if this current is lost in an uncontrolled manner. For a single-pass linac based on a photo-injector, we studied the possibility of using a collimator installed at low energy (below 10 MeV) to dump the maximum fraction of the dark current before it is transported along the linac. We developed and experimentally verified an emission and tracking model that we used to study and optimize the dark current mitigation at SwissFEL test facility. We optimized a collimator, which is expected to reduce by two orders of magnitude the transport of the dark current to the first compressor. We have also verified the effects of wakefield excited by the beam itself passing through the collimator at such a low energy, comparing the results of beam-based measurements with an analytical model.

Figure 1

SITF schematic layout (not in scale). The position of the faraday cup and the integrated current transformer used to measure the dark current are also indicated. The total length of the facility is about 60 m.

Figure 2

Geometry of the CTF2 gun and amplitude of the electric field in the horizontal and vertical section.

Figure 3

Normalized on-axis longitudinal electric field profiles for the CTF2 and SwissFEL rf guns.

Figure 4

Cutaway 3D views of the SwissFEL gun. Details of the backplane and the cathode plug are also visible in the upper right figure.

Figure 5

Standard polycristalline copper cathode plug (left) and cathodes (right) where a disc of 10 mm diameter of ${\mathrm{Cs}}_2\mathrm{Te}$ has been deposited on the front surface. Both types were used at SITF.

Figure 6

Interferometric roughness measurement of cathode surface. Evaluated $R_a=5.2\mathrm{nm}$.

Figure 7

Electric field pattern of the CTF2 and the SwissFEL gun obtained from numerical simulation. The axes are in mm.

Figure 8

Trajectories of particles emitted at distance $r=4$, 8, 10, 12, 16, 20 mm from the center of the cathode. Each line corresponds to a different distance. The emission is at the operating phase of the gun corresponding to the center of the emission time of the nominal beam. The dashed line corresponds to a starting position of $r=10\mathrm{mm}$, where the gap of the cathode plug lies. The top plot refers to the CTF2 and the bottom one to the PSI gun, respectively.

Figure 9

Trajectories of particles emitted at distance $r=4$, 8, 10, 12, 16, 20 mm from the center of the cathode. Each line corresponds to a different distance. The emission is at the time corresponding to the maximum field of the gun. The dashed line corresponds to a starting position of $r=10\mathrm{mm}$, where the gap of the cathode plug lies. The top plot refers to the CTF2 and the bottom one to the PSI gun, respectively.

Figure 10

Dark current versus the maximum electric field at the cathode measured for the CTF2 (top) and SwissFEL (bottom) guns. The repetition rate during the measurement, $f$, and the rf full pulse width, $T_{\text{pulse}}$, are also indicated.

Figure 11

Rf field amplitude of the SwissFEL and CTF2 gun (top plot), comparison of the FC measured trace and those obtained from the Fowler-Nordheim fit (dashed line) corresponding to the CTF2 (middle plot) and the SwissFEL (bottom plot) gun.

Figure 12

$\beta$ as a function of work function $\varphi$ using the values of $E_c$ from table 1. Each point in the blue region on the plot represents a possible combination of field enhancement factor and local work function which match the data with a 95% confidence bound.

Figure 13

Dark current beam imaged on a YAG:Ce screen near the gun exit for the CTF2 (left) and SwissFEL (right) gun. In both cases the electrons are focused with the gun solenoid.

Figure 14

Dark current measured by the ICT at $2\mu \mathrm{s}$ and $1.5\mu \mathrm{s}$ gun rf pulse lengths.

Figure 15

Top plot: transmission to the end of the second accelerating cavity of the dark current as a function of the gun peak field. Middle plots: rf pulse assumed in the simulations assuming different pulse lengths. The buckets considered are also indicated. Bottom plot: expected transmission of the dark current to the end of the second accelerating cavity as a function of the rf pulse length normalized to the case corresponding to $2\mu \mathrm{s}$ rf pulse length.

Figure 16

Simulated energy spectrum of the dark current at (a) the location of the Faraday cup, (b) the entrance of the first accelerating cavity, (c) the exit of the second structure and (d) the end of the fourth structure. The percentage of the number macroparticles transmitted normalized to those at the FC is also shown. The rf pulse length assumed is $2\mu \mathrm{s}$.

Figure 17

Collimator array installed at SITF.

Figure 18

ICT signals at the end of SITF using several aperture diameters, $\mathrm{\Phi }$, of the collimator.

Figure 19

Measured field emitted charge at the end of SITF as a function of the collimator aperture compared with the simulations. Each integrated current is normalized to the transmitted charge with the largest collimator aperture (16 mm).

Figure 20

Measured slice energy spread along the bunch for several collimator apertures compared to the case with the collimator removed from the beam path (“Plate out”). The full bunch length is divided in 15 parts of constant length.

Figure 21

Measured projected emittance increase versus the hole diameter of the collimator normalized to the case with the collimator removed from the beam path. The slice emittance of the central slice along the longitudinal coordinate of the bunch is reported for comparison to give an idea of the jitter of the machine.

Figure 22

Measured slice emittance along the bunch for the several diameter holes of the low energy collimator. The full bunch length is divided in 15 parts of constant size (not a constant charge per slice).

Figure 23

Measured vertical position of the centroid of the electron beam passing the 3 mm diameter aperture of the collimator versus the displacement of the collimator axis with respect to the bunch on a downstream YAG screen. The horizontal centroid is shown to give an idea of the machine jitter.

Figure 24

Measured projected emittances with the 9 mm diameter hole as a function of the collimator offset making the beam passing through the 9 mm diameter hole of the collimator and comparison with Eq. (11) (top plot). Expectations from the model assuming several hole diameters (bottom plot).

Figure 25

Simulated transmission of the dark current at the exit of the second S-band structure at SwissFEL. The number of particles is normalized to those at the FC. The first accelerating structure is installed at 3.3 m from the cathode.