K. Harkay

A rudimentary electron energy analyzer for accelerator diagnostics.

Abstract We have constructed a compact, planar retarding field analyzer for the diagnostics of low-energy, background electrons in a high-energy particle accelerator. Bench measurements of the analyzer have been made to characterize it, and the results are reasonable in light of models of this type of analyzer. Comparisons to results obtained using a beam-position monitor (BPM) show the advantages of this analyzer for electron diagnostics. Sample results from analyzers installed at the Advanced Photon Source storage ring at Argonne National Laboratory and the Proton Storage Ring at Los Alamos National Laboratory show how the analyzers can be used for studying the intensity, energy, and time structure of electrons in an accelerator environment.

Planned use of pulsed crab cavities for short X-ray pulsed generation at the Advanced Photon Source

Recently, we have explored application to the Advanced Photon Source (APS) of Zholents[1] crab cavity scheme for production of short x-ray pulses. We assumed use of superconducting (SC) cavities in order to have a continuous stream of crabbed bunches and flexibility of operating modes. The challenges of the SC approach are related to the size, cost, and development time of the cavities and associated systems. A good case can be made [2] for a pulsed system using room-temperature cavities. APS has elected to pursue such a system in the near term, with the SC-based system planned for a later date. This paper describes the motivation for the pulsed system and gives an overview of the planned implementation and issues. Among these are overall configuration options and constraints, cavity design options, frequency choice, cavity design challenges, tolerances, instabilities, and diagnostics plans.

Anomalous work function anisotropy in ternary acetylides

and C2 refers to the acetylide ion C 2 , with the rods embedded into an alkali cation matrix. It is shown that the conversion of the seasoned Cs2Te photo-emissive material to ternary acetylide Cs2TeC2 results in substantial reduction of its 3 eV workfunction down to 1.71-2.44 eV on the Cs2TeC2(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, eld emission displays, detectors and scanners.

Horizontal coupling impedance of the APS storage ring

The three-dimensional wake potentials of the APS storage ring have been reconstructed according to the impedance database concept. Every wakefield-generating component in the ring was considered including small-gap insertion device (ID) chambers, rf cavities, shielded bellows, beam position monitors, synchrotron radiation absorbers, scrapers, flags, various chamber transitions, septum chambers, and pulsed kickers. In this paper the result for the horizontal wake potentials and its impedance are presented. The numerically obtained impedance has been used to investigate the experimental results. Tune shift was calculated and compared with the measurement. We also observed a horizontal focusing in the calculated wake potential of the shallow transition without rotational symmetry.

Longitudinal coupling impedance of the APS storage ring

The three-dimensional wake potentials of the APS storage ring have been reconstructed according to the impedance database concept. Every wakefield-generating component in the ring was considered including small-gap insertion device (ID) chambers, rf cavities, shielded bellows, beam position monitors, synchrotron radiation absorbers, scrapers, flags, various chamber transitions, septum chambers, and pulsed kickers. In this paper the result for the longitudinal wake potentials and its impedance are presented. The total impedance is not model-based but the direct sum of numerical functions. The fit parameters for the broadband resonator model are also included as a convenient representation. We used this impedance model to investigate the observed microwave instability, namely bunch lengthening, anomalous energy spread, and coherent excitation at high synchrotron sideband.

Generation of Short X-Ray Pulses Using Crab Cavities at the Advanced Photon Source

There is growing interest within the user community to utilize the pulsed nature of synchrotron radiation from storage ring sources. Conventional third-generation light sources can provide pulses on the order of 100 ps but typically cannot provide pulses of about 1 ps that some users now require to advance their research programs. However, it was recently proposed by A. Zholents et al. to use rf orbit deflection to generate sub-ps x-ray pulses [1]. In this scheme, two crab cavities are used to deliver a longitudinally dependent vertical kick to the beam, thus exciting longitudinally correlated vertical motion of the electrons. This makes it possible to spatially separate the radiation coming from different longitudinal parts of the beam. An optical slit can then be used to slice out a short part of the radiation pulse, or an asymmetrically cut crystal can be used to compress the radiation in time. In this paper we present a feasibility study of this method applied to the Advanced Photon Source (APS). We find that the pulse length can be decreased down to a 1 ps range using superconducting crab cavities.

Vertical coupling impedance of the APS storage ring

The three-dimensional wake potentials of the APS storage ring have been reconstructed according to the impedance database concept. Every wakefield-generating component in the ring was considered including small-gap insertion device (ID) chambers, rf cavities, shielded bellows, beam position monitors, synchrotron radiation absorbers, scrapers, flags, various chamber transitions, septum chambers, and pulsed kickers. In this paper the result for the vertical wake potentials and its impedance are presented. Dominant contributors are the ID chambers whose heights are 5 mm and 8 mm. Since more small-gap chambers are envisioned for installation in the APS storage ring, prediction of their effect on the beam is very important not only for the APS but also for all third-generation light sources. We used the vertical impedance reported here to investigate the measured tune slope and single-bunch current limit in the APS storage ring. The program elegant was used for particle tracking, and its results are presented. We also report that we observed a vertical focusing in the calculated wake potential of the shallow transition without rotational symmetry.

Characterizing transverse beam dynamics at the APS storage ring using a dual-sweep streak camera.

We present a novel technique for characterizing transverse 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 makes it possible to display non-repeatable beam 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 APS 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 μs apart) are recorded in a single image, providing rich information about the unstable beam. These include the amplitude of the oscillation (∼0 mm at the head of the train and ∼2 mm towards the end of the train), the bunch-to-bunch phase difference, and the significant transverse size growth within the train. In the second example, the technique is used to characterize the injection kicker-induced beam motion, in support of th...

A potpourri of impedance measurements at the Advanced Photon Source storage ring

Machine coupling impedances were determined in the APS storage ring from measurements of the bunch length, synchronous phase, and synchrotron and betatron tunes vs. single-bunch current. The transverse measurements were performed for various numbers of small gap insertion device (ID) chambers installed in the ring. The transverse impedance is determined from measurements of the transverse tunes and bunch length as a function of single-bunch current. The shift in the synchrotron tune was measured as a function of bunch current from which the total cavity impedance was extracted. The loss factor was determined by measuring the relative synchronous phase as a function of bunch current. The longitudinal resistive impedance is calculated using the loss factor dependence on the bunch length. From these results, we can estimate what the impedance would be for a full set of ID chambers.

Longitudinal instability analysis for the IPNS upgrade

The proposed 1-MW spallation neutron source upgrade calls for a 2-GeV rapidly-cycling synchrotron (RCS) with an intensity of 1.04/spl times/10/sup 14/ protons per pulse. Due to the high intensity, the potential exists for collective instabilities. Emphasis is placed on controlling these by (a) minimizing the machine impedance by using a contour-following RF shield and (b) maximizing the momentum spread to make use of Landau damping. The coupling impedance is estimated and is dominated by space charge effects. It is found that the longitudinal microwave stability limit can be exceeded unless the momentum spread is sufficient. A longitudinal tracking code was developed to simulate injection and acceleration, including the effects of space charge and other sources of impedance. With the aid of the simulation, and under the assumptions of the instability theory, we arrive at an RF voltage profile and beam injection parameters which avoid both the instability and beam loss through the entire cycle. The limitations of the analysis are explored.