Projects
Use the links below to learn more about some of our projects.
Metal Photocathodes R&D for the LCLS - LCLS
Quantum efficiency (QE), defined as photo-emitted electrons per incident photon, is one measure of a surface's ability to produce useable beams for injection into electron accelerators. Metal photocathodes are commonly used in high-field radio frequency guns for this purpose because they are robust, produce high-quality beams and tolerate relatively poor vacuum compared to semiconductor cathodes. However, metal cathodes have very low QE even at UV wavelengths, and still require some form of cleaning after installation in the gun. Ideally, a process which produces an atomically clean, but unaltered surface is needed.
SMS, along with LCLS colleagues, are using a hydrogen ion (H-ion) beam to clean copper cathodes, with the purposes of 1) Ensuring that copper photocathodes are capable of attaining maximum QE prior to insertion into an rf gun and, 2) Be able to clean the cathode in-situ without an injector vault entry. Experiments in SMS have produced surfaces that exceed the required LCLS, Generation I, QE minimum.
The longer term goal is to develop cathodes with yet higher QE and lower emittance than copper, for the LCLS machine GEN II upgrade in 2009. This would likely involve the deposition of a semiconductor cathode in a laboratory system, qualify it for QE and emittance, and transfer it, without breaking vacuum, to the LCLS injector. An integrated rf gun load lock would be used in the cathode insertion step.
In-Situ Cleaning of Metal Cathodes Using a Hydrogen Ion Beam SLAC-PUB-11455
Quantum Efficiency and Topography of Heated and Plasma-Cleaned Copper SLAC-PUB-11355
Collective Instabilities - ILC
“Electron cloud” is a special type of electron resonant multiplication effect, called multipactoring. A “cloud” of electrons forms around intense positively-charged circulating bunches. The cloud is spatially-coupled to the transverse motion of the bunches, leading to a potential disruption of trailing bunches. The source of the original electrons, that subsequently form the cloud, is synchrotron radiation striking the chamber walls. These photoelectrons generate secondary electrons that further multiply at the walls and can even be accelerated to the opposite wall by the passing positive bunches. The electron cloud density depends on characteristics of the positively-charged circulating beam (bunch length, charge and spacing) and the electron secondary yield (SEY) of the wall surface from which the starting electrons arise. SMS measures the SEY of chamber material and its SE-suppressing coatings, first for SPEAR and PEP, and now for the proposed ILC positron damping rings.
Both grooving the beam chamber surface and low-energy ion bombardment of the surface lower the SEY.
Secondary electron emission yields from PEP-II accelerator materials Nuclear Instruments and Methods A551 (2005) 187
High-Polarization Electron Sources - ILC
This program is focused on producing high ( > 90%) polarization, high ( > 1012 electrons) charge bunchs/bunch trains for the International Linear Collider (ILC). The Polarized Photocathode Research Collaboration includes SMS, ILC, ARDA and Univ. of Wisconsin.
High polarization development has progressed from bulk GaAs (22% P at RT) photocathodes to high-gradient-doped strained-layer superlattice structures based on GaAs1-xPx compounds. Exciting laser wavelength is 780 nmeters. Both charge and polarization requirements for the ILC, Generation I, have been demonstrated. A DC electron gun for the ILC injector will be based on the successful Stanfor Linear Collider design.
The longer term goal is to produce a spin-polarized electron radio frequency L-band gun for the ILC upgrade. This project is a multi-company, multi-lab and multi-country effort. One of the difficult R&D problems involves ion feedback damage of the sensitive superlattice cathode. SMS has an ion gun that can be used to make damage-dose measurements, as well as electron and atomic force microscopes for topography imaging. In addition, an existing load-lock transfer system exists for moving bombarded cathodes to the ILC Gun Test Facility for polarization and QE measurements.
A very high charge, high polarization gradient-doped strained GaAs photocathode Nuclear Instruments and Methods A492 (2002) 199-211
Electric Breakdown on Surfaces-Advanced Accelerator R&D
Quality Assurance and Failure Analysis
SMS provides QA and other analytical services to SLAC, including failure analyses (optical and electron microscopy, XPS, Auger spectroscopy, metallographic bulk analysis, energy-dispersive x-ray analysis), measurement techniques (microbalances, surface profilometry, atomic force microscopy, spectrophotometry, low-energy x-ray fluorescence for film thickness measurement), materials qualification (metallography, quantitative energy-dispersive x-ray spectrometry), and vacuum deposition of metal films. Recent examples of this service support are autopsy examination of SPEAR3 electron injector dispenser cathodes, contamination analyses of EXO chamber materials, post-mortems on LCLS test facility copper photocathodes, and QC on stainless flanges for LCLS construction. Our metallographer/Materials Scientist QCs critical klystron-manufacturing materials, particularly OFE copper.
