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Gai, W

Publications and source records attributed to Gai, W.

17 records
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DOE OSTI2012

Photocathode emission studies: Dark current and Schottky-enabled photo-electrons in a high-field RF gun

We report on dark current and Schottky-enabled photoemission from a copper photocathode surface. Field-emitted dark current is a major gradient-limiting factor in RF cavities. Field emission is generally attributed to geometrical projections on the bulk surface whose field enhancement factor ({beta}) and the emitting area (A{sub e}) can be extracted from the Fowler-Nordheim (FN) plot. Measurements were made at Tsinghua S-band RF gun facility in two separate experiments. Using the traditional FN formula for RF fields we discovered that field enhancement factor ({beta}) alone cannot explain the full data set. Instead, we found that a low work function at some sites is required. In addition, surface analysis of the cathode after the experiment shows that geometric {beta} indicated would be < 10. Thus we conclude that low work function sites with a small {beta} are responsible for dark current emission and subsequent breakdown in high-gradient structures. The origin of these sites is unclear but could be due to defects or impurities.

43 PARTICLE ACCELERATORS↗
DOE OSTI2012

Start-to-end beam dynamics simulation of double triangular current profile generation in Argonne Wakefield Accelerator

Double triangular current profile (DT) gives a high transformer ratio which is the determining factor of the performance of collinear wakefield accelerator. This current profile can be generated using the emittance exchange (EEX) beam line. Argonne Wakefield Accelerator (AWA) facility plans to generate DT using the EEX beam line. We conducted start-to-end simulation for the AWA beam line using PARMELA code. Also, we discuss requirements of beam parameters for the generation of DT.

43 PARTICLE ACCELERATORS↗
DOE OSTI2012

Generation of Picosecond Electron-Bunch Trains with Variable Spacing Using a Multi-Pulse Photocathode Laser

We demonstrate the generation of a train of electron bunches with variable spacing at the Argonne Wakefield Accelerator. The photocathode ultraviolet laser pulse consists of a train of four pulses produced via polarization splitting using two alpha-BBO crystals. The photoemitted electron bunches are then manipulated in a horizontally-bending dogleg with variable longitudinal dispersion. A downstream vertically-deflecting cavity is then used to diagnose the temporal profile of the electron beam. The generation of a train composed of four bunches with tunable spacing is demonstrated. Such a train of bunch could have application to, e.g., the resonant excitation of wakefield in dielectric-lined structures. We have presented preliminary measurements on a simple technique to generate a train of electron bunches with variable separation. In the initial experiment appreciable density modulation down to wavelengths of {approx}1.8 mm (corresponding to a temporal separation of {approx}6 ps) were achieved for a total charge of 0.5 nC. Finding ways to reach smaller separations is being explored with the help of numerical simulations and will be presented elsewhere.

43 PARTICLE ACCELERATORS↗
DOE OSTI2011

The Design of the Positron Source for the International Linear Collider

The high luminosity requirements and the option of a polarized positron beam present a great challenge for the positron source of a future linear collider. This paper provides a comprehensive overview of the latest proposed design for the baseline positron source of the International Linear Collider (ILC). We report on recent progress and results concerning the main components of the source: including the undulator, capture optics, and target.

43 PARTICLE ACCELERATORS↗
DOE OSTI2011

Experimental Demonstration of Wakefield Acceleration in a Tunable Dielectric Loaded Accelerating Structure

We report on a collinear wakefield experiment using the first tunable dielectric loaded accelerating structure. By introducing an extra layer of nonlinear ferroelectric, which has a dielectric constant sensitive to temperature and dc bias, the frequency of a dielectric loaded accelerating structure can be tuned. During the experiment, the energy of a witness bunch at a fixed delay with respect to the drive beam was measured while the temperature of the structure was scanned over a 50 deg. C range. The energy change corresponded to a change of more than half of the nominal structure wavelength.

43 PARTICLE ACCELERATORS↗
DOE OSTI2011

Emittance-Exchange-Based High Harmonic Generation Scheme for a Short-Wavelength Free Electron Laser

Generation of short-wavelength radiation by a free-electron laser using up-frequency conversion of an electron bunch density modulation is currently an area of active research. We propose a new scheme for producing the longitudinal electron bunch density modulation similar to the recently proposed echo-enabled harmonic generation but based on an emittance exchange beam line and a multislit mask. Beam line analysis and start-to-end simulation are presented.

43 PARTICLE ACCELERATORS↗
DOE OSTI2010

Development of X-Band Dielectric-Loaded Accelerating Structures

This paper presents a progress report on the development and testing of X-band dielectric-loaded accelerating structures. Recent tests on several quartz DLA structures with different inner diameters are reported. Designs for gap-free DLA structures are presented. Also, planned new experiments are discussed, including higher gradient traveling-wave and standing-wave structures and special grooved structures for multipactor suppression.

43 PARTICLE ACCELERATORS↗
DOE OSTI2010

Recent Experiment on Wakefield Transformer Ratio Enhancement at AWA

One technique to enhance the transformer ratio beyond the ordinary limit of 2 in a collinear wakefield acceleration scheme is to use a ramped bunched train (RBT). The first experimental demonstration has been reported in [1]. However, due to the mismatch between the beam bunch length and frequency of the accelerating structure, the observed transformer ratio was only marginally above 2 in the earlier experiment. We recently revisited this experiment with an optimized bunch length using the laser stacking technique at Argonne Wakefield Accelerator (AWA) facility. A transformer ratio of 3.4 has been measured using two drive bunches. Attempting to use four drive bunches met with major challenges. In this article, measurement results and data analysis from these experiments are presented in detail.

43 PARTICLE ACCELERATORS↗
DOE OSTI2010

Dielectric-Based Wakefield Power Extractor

In the Two Beam Accelerator (TBA), wakefield power extractors which extract high power RF from a high current beam are used to power high gradient accelerating structures. A dielectric-based Wakefield Power Extractor (DWPE) is one option in addition to the metallic structures considered previously, like the CLIC PETS (Power Extraction and Transfer Structure). 7.8 GHz and 26 GHz DWPE prototypes have been successfully built and tested at the Argonne Wakefield Accelerator (AWA) facility. We are currently designing an X-band version for a potential application with the CLIC beam. In this article, we report on test results of the 26 GHz DWPE and the preliminary design of the X-band structure. Future plan and possible difficulties in the development of DWPEs are also discussed.

43 PARTICLE ACCELERATORS↗
DOE OSTI2010

Design and Analysis of a Single-Shot Longitudinal Phase Space Measurement Diagnostics at the Argonne Wakefield Accelerator

A single-shot longitudinal phase space diagnostics is currently being assembled at the Argonne Wakefield Accelerator (AWA). The diagnostics consists of mapping the longitudinal phase space coordinates to the transverse position space plane using a beamline that incorporates two magnetic dipoles arranged as a dogleg followed by an rf deflecting cavity. The deflecting cavity streaks the beam vertically while the dipole magnets bend horizontally. In this paper, we analyze analytically and via numerical simulations the performances of such diagnostics and their limitations. Simulations done in IMPACT-T shows good reconstruction of the longitudinal phase space.

43 PARTICLE ACCELERATORS↗
DOE OSTI2010

Using an Emittance Exchange Scheme for High Harmonic Generation by Free-Electron Lasers

We propose combining a transverse-to-longitudinal emittance exchange beam line and a conventional modulator to generate high harmonics with short-wavelength free-electron lasers. The key point of this scheme is the use of a multi-slit mask to create a transverse-shaped beam. In an emittance exchange beam line, the transverse-shaped bunch will produce a longitudinal-shaped one, resulting in separated energy bands. The modulator that follows this step will generate micro-bunches containing high harmonics. A coherent synchrotron radiation effect is also considered in this paper; compared to the two-modulator scheme, the method investigated here is more flexible and can be used to relax the coherent synchrotron radiation effect.

43 PARTICLE ACCELERATORS↗