In Situ Observation of Dark Current Emission in a High Gradient rf Photocathode Gun

Undesirable electron field emission (also known as dark current) in high gradient rf photocathode guns deteriorates the quality of the photoemission current and limits the operational gradient. To improve the understanding of dark current emission, a high-resolution ($\sim 100\mu \mathrm{m}$) dark current imaging experiment has been performed in an $L$-band photocathode gun operating at $\sim 100\mathrm{MV}/\mathrm{m}$ of surface gradient. Scattered strong emission areas with high current have been observed on the cathode. The field enhancement factor $\beta$ of selected regions on the cathode has been measured. The postexaminations with scanning electron microscopy and white light interferometry reveal the origins of $\sim 75\%$ strong emission areas overlap with the spots where rf breakdown has occurred.

Figure 1

The dark current imaging system at AWA. (a) The designed cathode with a reference pattern and the beam line layout: FC, Faraday cup; C, vacuum cross to house imaging components which were mounted on retractable actuators; FS, focusing solenoid; IS, imaging solenoid; and YAG, doped yttrium aluminum garnet phosphor screen. Inset: Top and side view of the novel shaped cathode with its sputtering pattern. (b) The equivalent optical imaging system. Insets: astra simulation results for the emission patterns on the cathode, at the gun exit, and in the imaging plane.

Figure 2

Dark current images on the last YAG screen. The white dashed circle indicates the boundary of the YAG screen. The white dashed square in (d) indicates the emission area for the resolution calculation. (a) Without collimator. (b)–(d) With collimator. The aperture diameters are 8, 1, and 0.2 mm, respectively. (a)–(b) Accumulation of 20 frames. (c)–(d) Accumulation of 100 frames.

Figure 3

Field enhancement factor measurement by the dark current imaging system. The white dashed circle indicates the boundary of the YAG screen. (a) Dark current image with the biggest aperture. White dashed squares ($610\mu \mathrm{m}\times 610\mu \mathrm{m}$ on the cathode) indicate selected regions for the measurement as shown in (b). $A$, strong emission area; $B$, weak emission area; and $C$, background. $A$ and $B$ both locate on the copper surface by the ex situ examination. (b) Fowler-Nordheim plot. Spots and lines are the measured data and linear fitting, respectively.

Figure 4

Comparison between strong and weak emission areas. (a) Model of the strong and weak emission areas. (b) $\alpha$ of the strong emitter (blue) and ${\beta }_{\text{strong}}$ (red) versus ${\beta }_1$.

Figure 5

Overlap of strong dark current emission areas and breakdown spots. (a) Overview of breakdown spots on the cathode. The red circles indicate the areas which contain breakdown spots. The white dashed circle indicates the maximum visible range by the dark current imaging system. (b)–(e) Zoom-in view of circles marked in (a). (b),(c) Breakdown spot on Cu which overlaps and does not overlap with a strong dark current emission area, respectively. (d) Breakdown spot on Au which overlaps with a strong dark current emission area. (e) Smooth undamaged surface on Mg. (f) Overlap of the strong dark current emission areas and the breakdown spots. The dark current imaging is in false color for better display. The overlapped strong emission areas and breakdown spots are marked by arrows. The strong emission areas with unknown origin are marked by the question mark.