Light Source Photocathode Performance and Development
EMSL Project ID
40992
Abstract
Accelerator photocathode technology must be advanced to meet the needs of fourth generation light sources. The ultimate output characteristics and cost of next generation UV or X-ray sources, whether based on free electron laser (FEL) or storage ring designs, is heavily dependent upon photocathode brightness and emittance characteristics. Novel photocathode designs could dramatically reduce light source construction costs, by a factor of two or more, by significantly simplifying downstream accelerator or FEL design. Traditionally, the international light source community has successfully applied an engineering approach to photocathode design but has not engaged in systematic photocathode development efforts. This community clearly recognizes the need for a more scientific approach to new photocathode development and is in the initial stages of addressing this issue by organizing symposia and workshops. The need to include researchers from fields outside the accelerator community, in this effort, is also appreciated but remains largely unaddressed. It is apparent that the fields of materials science, solid-state photochemistry, surface chemistry and analysis can make an immediate and timely contribution to this essential activity. Pacific Northwest National Laboratory (PNNL) has ideal personnel and capabilities to impact next generation photocathode design, has developed a working relationship with photocathode specialists at Jefferson Lab (JLab), and is poised to bridge to the broader accelerator community. Photocathode materials and designs must be highly robust, under pulsed or continuous laser irradiation, and tolerant to a variety of high field accelerator operating conditions. Existing photocathodes degrade in use leading to lower electron bunch intensity, higher emittance, the need for frequent replacement, and consequently, expensive machine downtime. Several photocathode degradation processes are suspected including ion back bombardment, photochemistry of surface adsorbed species, and irradiation induced surface and bulk defect formation. At present, no consensus exists within the user community as to the mechanisms of photocathode damage. Better understanding of degradation mechanisms of existing photocathode materials could lead to improved emission properties and greater durability (longer operating lifetime). Existing photocathode materials range from metallic (e.g. copper) to semiconducting (e.g. GaAs) with various structures, dopants, and surface preparations. Photocathode emission requirements include high electron yield (intensity) and low thermal emittance (spatial dispersion) at high repetition rate. The exact details regarding electron bunch dynamics change significantly with accelerator type and a variety of robust new photocathode materials and designs need to be developed. The ultimate goal is to develop new photocathode designs required by next generation light sources. To achieve this goal, we have combined PNNL expertise in materials science, surface chemistry, photophysics and surface analysis with FEL photocathode expertise from scientific staff at Jefferson Lab (JLab).
Project Details
Start Date
2010-08-23
End Date
2011-08-28
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
Related Publications
Peppernick SJ, AG Joly, KM Beck, and WP Hess. 2012. "Near-Field Focused Photoemission from Polystyrene Microspheres Studied with Photoemission Electron Microscopy." Journal of Chemical Physics 137(1):124202-1/9. doi:10.1063/1.4730598
Shutthanandan V, Z Zhu, ML Stutzman, F Hannon, C Hernandez-Garcia, MI Nandasiri, SVNT Kuchibhatla, S Thevuthasan, and WP Hess. 2012. "Surface Science Analysis of GaAs Photocathodes Following Sustained Electron Beam Delivery." Physical Review Special Topics - Accelerators and Beams 15(6):063501. doi:10.1103/PhysRevSTAB.15.063501