Charles Sinclair

Energy recovery linacs as synchrotron radiation sources (invited)

Practically all synchrotron x-ray sources to data are based on the use of storage rings to produce the high current electron (or positron) beams needed for synchrotron radiation (SR). The ultimate limitations on the quality of the electron beam, which are directly reflected in many of the most important characteristics of the SR beams, arise from the physics of equilibrium processes fundamental to the operation of storage rings. It is possible to produce electron beams with superior characteristics for SR via photoinjected electron sources and high-energy linacs; however, the energy consumption of such machines is prohibitive. This limitation can be overcome by the use of an energy recovery linac (ERL), which involves configuring the electron-beam path to use the same superconducting linac as a decelerator of the electron beam after SR production, thereby recovering the beam energy for acceleration of new electrons. ERLs have the potential to produce SR beams with brilliance, coherence, time structure, and source size and shape which are superior to even the best third-generation storage ring sources, while maintaining flexible machine operation and competitive costs. Here, we describe a project to produce a hard x-ray ERL SR source at Cornell University, with emphasis on the characteristics, promise, and challenges of such an ERL machine.

New energy recovery linac source of synchrotron X-rays

Introduction Cornell University and Jefferson Laboratory physicists have been studying the properties of a new type of synchrotron radiation machine, called an Energy Recovery Linac (ERL), based on a superconducting linac configured for energy recovery with a return ring. A high energy, high current ERL could produce electron beams of order 10 microns in diameter. These could be used as an ultra-high brilliance x-ray source with many desirable characteristics, including: transversely coherent, diffraction-limited hard x-ray beams, very short (~100 fs) frequent (1 – 2 GHz) pulses, no limits on beam lifetime, and very flexible modes of operation. This combination of characteristics opens up new possibilities and could significantly advance the state of the art in x-ray research.

Recent advances in polarized electron sources

Current experimental physics programs at a number of electron accelerator laboratories worldwide require the delivery of high average current highly polarized electron beams for long periods of time. The polarized electrons are produced by near bandgap photoemission from certain semiconductor photocathodes. We observe the quantum efficiency of these cathodes to be inversely related to the total charge they have delivered. Recent developments in ultrahigh vacuum technology, electron trajectory control, photocathode preparation, and lasers have led to operationally reliable delivery of many hundreds of coulombs of polarized electrons, at rates as high as 8 coulombs/day. Currently, our photocathode operational lifetime is almost completely dominated by ion backbombardment. Further gains in the high average current lifetime of these cathodes may be expected, which will allow photoemission electron guns to be used for accelerator applications other than polarized sources.

Polarized source performance and developments at Jefferson Lab

The polarized photoinjector at Jefferson Lab continues to provide high average current, high polarization, high quality beam to nuclear physics Users in as many as three endstations simultaneously. Long lifetime operation has been obtained from two identical polarized guns. A new high power modelocked ti-sapphire laser has been constructed to enhance the effective operating lifetime of the photoinjector. Efforts to enhance beam polarization and reduce helicity correlated beam systematic effects are underway.

FEL design using the CEBAF linac

Conceptual studies of two free-electron lasers (FELs) located at the output of the front end and north linac of the CEBAF (Continuous Electron Beam Accelerator Facility) accelerator are conducted. The high average beam power and the superior electron beam quality produced by the linac yield projections of tunable output power that substantially exceed existing and most proposed sources. The tolerances for most FEL components are not severe but the high optical power requires careful consideration and, perhaps, special optical cavity arrangements and mirror designs. >

Atomic hydrogen cleaning of semiconductor photocathodes

Negative electron affinity (NEA) semiconductor photocathodes are widely used for the production of polarized electron beams, and are also useful for the production of high brightness electron beams which can be modulated at very high frequencies. Preparation of an atomically clean semiconductor surface is an essential step in the fabrication of a NEA photocathode. This cleaning step is difficult for certain semiconductors, such as the very thin materials which produce the highest beam polarization, and those which have tightly bound oxides and carbides. Using a small RF dissociation atomic hydrogen source, we have reproducibly cleaned GaAs wafers which have been only degreased prior to installation in vacuum. We have consistently prepared very high quantum efficiency photocathodes following atomic hydrogen cleaning. Details of our apparatus and most recent results are presented.

Advances in DC photocathode electron guns

At Jefferson Lab, a DC photoemission gun using GaAs and GaAs-like cathodes provides a source of polarized electrons for the main accelerator. The gun is required to produce high average current with long operational lifetimes and high system throughput. Recent work has shown that careful control of the parameters affecting cathode lifetime lead to dramatic improvements in source operation. These conditions include vacuum and the related effect of ion backbombardment, and precise control of all of the electrons emitted from the cathode. In this paper, we will review recent results and discuss implications for future photocathode guns.

5 MeV Mott polarimeter development at Jefferson Lab

Low energy (Ek=100u2009keV) Mott scattering polarimeters are ill-suited to support operations foreseen for the polarized electron injector at Jefferson Lab. One solution is to measure the polarization at 5 MeV where multiple and plural scattering are unimportant and precision beam monitoring is straightforward. The higher injector beam current offsets the lower cross-sections. Recent improvements in the CEBAF injector polarimeter scattering chamber have improved signal to noise.

Precision intercomparison of beam current monitors at CEBAF

The CEBAF accelerator delivers a CW electron beam at a fundamental frequency of 1497 MHz, with an average beam current up to 200 /spl mu/A. Accurate and stable non-intercepting beam current monitors are required for a number of applications. These include setup and control of the accelerator, monitoring of both beam current and beam losses for machine protection and personnel safety purposes, and providing beam current information to the experimental users. Fundamental frequency stainless steel RF cavities have been chosen for these beam current monitors. This paper reports on a precision intercomparison between two such RF cavities, an Unser monitor, and two Faraday cups, all located in the injector area. At the low beam energy in the injector, it is straightforward to verify the high efficiency of the Faraday cups, and the Unser monitor included a wire through it to permit an absolute calibration. The cavity intensity monitors have proven to be capable of stable, high precision monitoring of the beam current.

Effect of Atomic Hydrogen Exposure on Electron Beam Polarization from Strained GaAs photocathodes

Strained‐layer GaAs photocathodes are used at Jefferson Lab to obtain highly polarized electrons. Exposure to atomic hydrogen (or deuterium) is used to clean the wafer surface before the activation with cesium and nitrogen trifluoride to consistently produce high quantum yield photocathodes. The hydrogen‐cleaning method is easy, reliable and inexpensive. However, recent tests indicate that exposure to atomic hydrogen may affect the polarization of the electron beam. This paper presents preliminary results of a series of tests conducted to study the effect of atomic H exposure on the polarized electron beam from a strained‐layer GaAs sample. The experimental setup is described and the first measurements of the beam polarization as a function of exposure dose to atomic hydrogen are presented.