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

Area of Science:

• Physics
• Accelerator Science
• Materials Science

Background:

• Undesirable electron field emission, or dark current, degrades photoemission quality in high-gradient RF photocathode guns.
• This dark current limits the operational gradient achievable in these systems.

Purpose of the Study:

• To enhance the understanding of dark current emission mechanisms in RF photocathode guns.
• To investigate the spatial correlation between dark current emission and cathode surface properties.

Main Methods:

• A high-resolution (∼100 μm) dark current imaging experiment was conducted in an L-band photocathode gun.
• The experiment operated at a surface gradient of ∼100 MV/m.
• Field enhancement factor (β) was measured for specific emission regions, followed by post-examination using scanning electron microscopy and white light interferometry.

Main Results:

• Scattered areas of strong dark current emission with high current density were observed on the cathode.
• The field enhancement factor (β) was quantified for these emission regions.
• Approximately 75% of the strong emission areas were found to coincide with locations where radiofrequency (RF) breakdown had previously occurred.

Conclusions:

• The study identifies a strong correlation between RF breakdown events and significant dark current emission sites on the cathode.
• These findings suggest that RF breakdown plays a crucial role in initiating or exacerbating dark current in high-gradient photocathode guns.
• Understanding this link is vital for mitigating dark current and improving the performance of RF photocathode guns.