Polarized photocathodes make the grade
Future linear colliders will require high levels of performance from their electron sources. A group at SLAC has recently tested a structure that substantially exceeds current collider polarized electron source pulse-profile requirements.
Surface charge limit in NEA superlattice photocathodes of polarized electron source
The “surface charge limit (SCL)” phenomenon in negative electron affinity (NEA) photocathodes with GaAs–AlGaAs superlattice and InGaAs–AlGaAs strained-layer superlattice structures has been investigated systematically using a 70 keV polarized electron gun and a nanosecond multi-bunch laser. The space-charge-limited beam with multi-bunch structure (1.6 A peak current, 12 ns bunch width and 15 or 25 ns bunch separation) could be produced from the superlattice photocathodes without suffering the SCL phenomenon. From the experimental results, it has been confirmed that the SCL phenomenon is governed by two physical mechanisms at the NEA surface region, the tunneling of conduction electrons against the surface potential barrier (escaping process) and that of valence holes against the surface band bending barrier (recombination process); these effects can be enhanced using the superlattice structure and heavy p-doping at the surface, respectively. We conclude that a superlattice with heavily p-doped surface is the best photocathode for producing the multi-bunch electron beam required for future linear colliders.
Substation automation based on IEC 61850 with new process-close technologies
New technologies in primary equipment like nonconventional instrument transformers require new interfaces to the substation automation system. The future standard IEC 61850 perfectly supports this requirement. With the introduction of the new technologies and the use of IEC 61850, a more decentralized architecture of the substation automation system will be possible. This provides several benefits but a careful system design is required to maintain the overall reliability of the system.