Chamber Surface Roughness and Electron Cloud for the Advanced Photon Source Superconducting Undulator

The electron cloud is a possible heat source in the superconducting undulator (SCU) designed for the Advanced Photon Source (APS), a 7-GeV electron synchrotron radiation source at Argonne National Laboratory. In electron cloud generation extensive research has been done, and is continuing, to understand the secondary electron component. However, little work has been done to understand the parameters of photoemission in the accelerator environment. To better understand the primary electron generation in the APS; a beamline at the Australian Light Source synchrotron was used to characterize two samples of the Al APS vacuum chamber. The total photoelectron yield and the photoemission spectra were measured. Four parameters were varied: surface roughness, sample temperature, incident photon energy, and incident photon angle, with their results presented here.Comment: presented at ECLOUD'12: Joint INFN-CERN-EuCARD-AccNet Workshop on Electron-Cloud Effects, La Biodola, Isola d'Elba, Italy, 5-9 June 201

Electron Sources for Future Lightsources, Summary and Conclusions for the Activities during FLS 2012

This paper summarizes the discussions, presentations, and activity of the Future Light Sources Workshop 2012 (FLS 2012) working group dedicated to Electron Sources. The focus of the working group was to discuss concepts and technologies that might enable much higher peak and average brightness from electron beam sources. Furthermore the working group was asked to consider methods to greatly improve the robustness of operation and lower the costs of providing electrons.Comment: 11 pages, 7 figures, summary paper from working group Future Light Sources 2012 Workshop at Newport News, Virginia, USA (http://www.jlab.org/conferences/FLS2012/

The optimization of a DC injector for the Energy Recovery Linac upgrade to APS

An energy recovery linac based light source is a potential revolutionary upgrade to the Advanced Photon Source (APS) at Argonne National Laboratory. The concept relies on several key research areas, one of which is the generation of ultra-low emittance, high-average-current electron beams. In this paper, we present our investigation of a dc-gun-based system for ultra-low emittance bunches in the 20 pC range. A parallel multi-objective numerical optimization is performed in multi-parameter space. Parameters varied include experimentally feasible drive-laser shapes, the DC gun voltage, the thermal energy of the emitted photo-electrons and other electric or magnetic field strengths, RF cavity phase etc. Our goal is to deliver a {approx}10 MeV, 20 pC bunch at the entrance of the linac with an emittance of 0.1 {micro}m or lower, rms bunch length of 2 {approx} 3 ps, and energy spread no larger than 140 keV. We present the machine parameters needed to generate such an injector beam, albeit without a merger

Kelvin Probe Studies of Cesium Telluride Photocathode for AWA Photoinjector

Cesium telluride is an important photocathode as an electron source for particle accelerators. It has a relatively high quantum efficiency (>1%), is sufficiently robust in a photoinjector, and has a long lifetime. This photocathode is grown in-house for a new Argonne Wakefield Accelerator (AWA) beamline to produce high charge per bunch (~50 nC) in a long bunch train. Here, we present a study of the work function of cesium telluride photocathode using the Kelvin Probe technique. The study includes an investigation of the correlation between the quantum efficiency and the work function, the effect of photocathode aging, the effect of UV exposure on the work function, and the evolution of the work function during and after photocathode rejuvenation via heating.Comment: 5 pages, 6 figure

Characterizing Transverse Beam Dynamics at the APS Storage Ring Using a Dual-Sweep Streak Camera

Abstract. We present a novel techniquefor characterizingtransverse beam dynamics using a dual-sweep streak camera. The camera is used to record the front view of successive beam bunches and/or successive turns of the bunches. This extension of the dual-sweep technique makesit possible to display non-repeatablebeam transverse motion in two fast and slow time scales of choice, and in a single shot. We present a study of a transverse multi-bunch instability in the AM storage ring. The positions, sizes, and shapes of 20 bunches (2.84 ns apart) in the train, in 3 to 14 successive turns (3.68 w apart) are recorded in a single image, providing rich information about the unstable beam. These include the amplitude of the oscillation(-0.0 at the head of the train and -2 mm towards the end of the train), the bunch-tobunch phase difference, and the significant transverse size growth withh the train. In the second example, the technique is used to characterize the injection-kicker induced beam motion, in support of the planned storagering top-up operation. By adjustingthe time scale of the dual sweep, it clearly shows the amplitude (d.8mm) and direction of tie kick, and the subsequent decoherence (-500 turns) and damping (-20 ms) of the stored beam. Since the storagering has an insertion device chamber with full vertical aperture of 5 mm, it is of special interestto track the vertical motion of the beam. An intensified gated camera was used for this purpose. The turn-by-turn x-y motion of a single-bunch beam was recorded and used as a diagnosticfor coupling correction. Images taken with uncorrectedcoupling will be presented. .

Anomalous Workfunction Anisotropy in Ternary Acetylides

Anomalous anisotropy of workfunction values in ternary alkali metal transition metal acetylides is reported. Workfunction values of some characteristic surfaces in these emerging semiconducting materials may differ by more than $\approx$ 2 eV as predicted by Density Functional Theory calculations. This large anisotropy is a consequence of the relative orientation of rod-like [MC$_{2}$]$_{\infty}$ negatively charged polymeric subunits and the surfaces, with M being a transition metal or metalloid element and C$_{2}$ refers to the acetylide ion C$_{2}^{2-}$, with the rods embedded into an alkali cation matrix. It is shown that the conversion of the seasoned Cs$_{2}$Te photo-emissive material to ternary acetylide Cs$_{2}$TeC$_{2}$ results in substantial reduction of its $\approx$ 3 eV workfunction down to 1.71-2.44 eV on the Cs$_{2}$TeC$_{2}$(010) surface while its high quantum yield is preserved. Similar low workfunction values are predicted for other ternary acetylides as well, allowing for a broad range of applications from improved electron- and light-sources to solar cells, field emission displays, detectors and scanners.Comment: Accepted for publication in Phys. Rev.

A study of longitudinal instabilities and emittance growth in the Fermilab Booster synchrotron

Attempts to measure and describe beam instabilities have been made ever since they were first observed in particle accelerators thirty years ago. Such collective, coherent effects arise due to the electromagnetic interaction of the beam with its environment, namely, the elements in the beamline. With sufficient intensity, the motion can become unstable, possibly leading to phase space dilution and beam loss. A coupled-bunch instability has long been observed in the Booster, an 8-GeV proton synchrotron at Fermi National Accelerator Laboratory. The accompanying longitudinal emittance growth is a major limit to beam brightness, limiting also the performance of subsequent accelerator stages. Previous studies have indicated that the coupled-bunch mode fluctuations are likely due to the influence of higher-order modes (HOM) in the radio-frequency (RF) accelerating cavities. However, the physics, especially that of the emittance growth, was only partially characterized. It is my goal in this thesis to expand what we understand about coherent longitudinal phenomena and integrate it with a real machine which does not readily give up her secrets. Building upon prior observations and coupled with the advent of more sophisticated diagnostic and computational tools, this research seeks to characterize the unstable beam behavior in a rapidly cycling synchrotron. Experimental studies are designed to systematically vary parameters in order to establish functional dependencies. Bench measurements are made of the impedance due to RF cavity HOMs. These data are compared with analytic results derived from the standard linear perturbation treatment as well as with simulation. The major finding of this research is that the theoretical predictions of linear growth rates of the longitudinal coupled-bunch instability based on the measured impedance show quantitative agreement with the data, but only when the beam momentum spread and nonlinearity of the RF potential are incorporated self-consistently. Development and installation in the cavities of passive HOM dampers proved to reduce the emittance by a factor of three and allowed, for the first time, an experimental test of instability thresholds. The linear theory is inadequate in describing the observed emittance growth, for which simulation results are invoked instead to provide a scaling rule

Electron Proton Two-Stream Instability at the Proton Storage Ring (PSR)

A strong, fast, transverse instability has long been observed at the Los Alamos Proton Storage Ring (PSR) where it is a limiting factor on peak intensity. Most of the available evidence, based on measurements of the unstable proton beam motion, is consistent with an electron-proton two-stream instability. The need for higher beam intensity at PSR and for future high-intensity, proton drivers has motivated a multi-lab collaboration (LANL, ANL, FNAL, LBNL, BNL, ORNL, and PPPL) to coordinate research on the causes, dynamics and cures for this instability. Important characteristics of the electron cloud were recently measured with retarding field electron analyzers and various collection electrodes. Suppression of the electron cloud formation by TiN coatings has confirmed the importance of secondary emission processes in its generation. New tests of potential controls included dual harmonic rf, damping by higher order multipoles, damping by X,Y coupling and the use of inductive inserts to compensate longitudinal space charge forces. With these controls and higher rf voltage the PSR has accumulated stable beam intensity up to 9.7 {micro}C/pulse (6x1013 protons), which is a 60% increase over the previous maximum