Experimental studies of dark current in a superconducting rf photoinjector

A superconducting rf photoinjector (SRF gun) has been put in operation successfully at the radiation source ELBE. It produces a 13 MHz, low emittance, cw beam with beam current up to $400\mu \mathrm{A}$ and energy of 3 MeV. During the gun operation, the field emission from the niobium cavity and the ${\mathrm{Cs}}_2\mathrm{Te}$ cathodes produces dark current. In this paper, we study the dark current of the SRF gun by using the existing diagnostics beam line. The dependency of the intensity and the energy spectrum on the cavity gradient and the cathode voltage are examined, and the emission sources are analyzed. Furthermore, the new project of installing a strip line kicker to reduce the dark current effect is presented.

Figure 1

Top: The cavity shape of the ELBE SRF gun. Bottom: The distribution of the electric surface field $|E|$ and the axis field $E_z$, simulated using Superfish [21] with $E_{z\mathrm{.}\max }=16.2\mathrm{MV}/\mathrm{m}$. The solid red line and the blue dashed line are respectively the case with cathode plug or without cathode plug, while the black dots show the acceleration field $E_z$ along the $z$ axis (without cathode). Inset: The distribution of the electric surface field in the zone close to the cathode ($z=2.6\mathrm{cm}$ to 3.2 cm) [20].

Figure 2

The schematic view of the gun, the diagnostic beam line, and the dogleg beam line connecting the setup to the ELBE linac. The $-{45}^{°}$ dipole, 2.39 m downstream from the cathode, can guide the beam through the achromatic dogleg into the ELBE Linac. Alternatively, the electron beam reaches the 180°-bending dipole for energy and energy spread measurements. Beam position monitors and steerers are not shown here.

Figure 3

The image of the dark current focused with the solenoid on the YAG screen 2.

Figure 4

Display on oscilloscope, $E_{z.\max }=16.2\mathrm{MV}/\mathrm{m}$. The blue line (top) gives out the Faraday cup signal of the sum of the dark current and the photocurrent; the green line (bottom) shows the macropulse signal of the solid state amplifier; and the magenta line (middle) represents the trigger signal of the cathode drive laser.

Figure 5

Dark current from the gun without any cathode and with different photocathodes vs axis peak field. The red dots are from a measurement with a Pb-coated Nb cathode. Pink triangles represent the case with uncoated Mo cathode. The hollow squares and the green balls are with ${\mathrm{Cs}}_2\mathrm{Te}$ emission layer on Mo substratum. The blue squares give the result of the case without cathode plug.

Figure 6

Energy spectra of the dark current from the SRF gun with $E_{z.\max }=16.2\mathrm{MV}/\mathrm{m}$. The intensity is normalized to the total dark current. The curve in (a) represents the case only from the Nb cavity. Part (b) shows the result of the cavity with Mo plug, and part (c) is from the case of the cavity with ${\mathrm{Cs}}_2\mathrm{Te}$ photocathode. The orange arrow points out the measured photoelectron energy.

Figure 7

Dark current vs peak field $E_{z.\max }$ for the cavity with Mo cathode. Measurement was done with pulsed mode.

Figure 8

Dark current vs cathode voltage for the cavity with Mo cathode.

Figure 9

Energy spectra of the dark current from the SRF gun with Mo cathode plug. The measurement was done with various rf fields $E_{z.\max }$ and cathode voltages.

Figure 10

Energy spectra of the dark current vs gradient for the SRF gun with the same ${\mathrm{Cs}}_2\mathrm{Te}$ photocathode. The cathode voltage was set to 3 kV. The orange arrows indicate the energy of the photoelectron beam (laser phase 30°).

Figure 11

F-N plot of the dark current from the niobium cavity. In the horizontal axis $E(\mathrm{V}/\mathrm{m})$ is the $E$-field amplitude. The star is the data extended from the linear fit with respect to $E_{z.\max }=30\mathrm{MV}/\mathrm{m}$, $I=1\times {10}^{-3}\mathrm{A}$.

Figure 12

The basic concept of the kicker designed for the SRF gun [19].

Figure 13

The model of the strip line kicker for the SRF gun, which is 250 mm long with a 150 mm strip and 50 mm taper at each side. The structure consists of two strip lines with four $50\Omega$ ports and two ground fenders for homogenizing the field inside the chamber.