Y.W. Kang

FEL development at the Advanced Photon Source

Construction of a single-pass free-electron laser (FEL) based on the self-amplified spontaneous emission (SASE) mode of operation is nearing completion at the Advanced Photon Source (APS) with initial experiments imminent. The APS SASE FEL is a proof-of-principle fourth-generation light source. As of January 1999 the undulator hall, end-station building, necessary transfer lines, electron and optical diagnostics, injectors, and initial undulators have been constructed and, with the exception of the undulators, installed. All preliminary code development and simulations have also been completed. The undulator hall is now ready to accept first beam for characterization of the output radiation. It is the project goal to push towards full FEL saturation, initially in the visible, but ultimately to UV and VUV, wavelengths.

Damping spurious harmonic resonances in the APS storage ring beam chamber

The APS storage ring beam chamber has been storing the beam up to 100 mA successfully. However, in some beam chambers, spurious signals corrupted the BPM outputs. The cause of the unwanted signals was investigated, and it was found that transverse electric (TE) longitudinal harmonic resonances of the beam chamber were responsible. The beam chambers have small height in the area between the oval beam chamber and the antechamber. The structure behaves like a ridge waveguide so that the cut-off frequency of the waveguide mode becomes lower. The pass-band then includes the frequency around 350 MHz that is important to the beam position monitors (BPMs). The spurious harmonic resonances are damped with two types of dampers to restore the useful signals of the BPMs; coaxial loop dampers and lossy ceramic slab loading are used.

Twisted waveguide accelerating structure.

Regular straight hollow waveguides have phase velocities of propagating electromagnetic waves greater than the free-space speed of light. However, it has been found that, if the waveguide is twisted, the phase velocities of the waveguide modes become slower. By choosing optimum shape of the cross section and pitch angle of a twisted waveguide, a desired phase velocity of the accelerating mode can be obtained for accelerating charged particles. This type of accelerating structure may have advantages over the conventional irisloaded structures. The twisted waveguide structure has been modeled and computer simulated in 3-D electromagnetic solvers to show the slow-wave properties of the accelerating mode.

LIGA fabrication of mm-wave accelerating cavity structures at the Advanced Photon Source (APS)

Recent microfabrication technologies based on the LIGA (German acronym for Lithographe, Galvanoformung, und Abformung) process have been applied to build high-aspect-ratio, metallic or dielectric planar structures suitable for high-frequency rf cavity structures. The cavity structures would be used as parts of linear accelerators, microwave undulators, and mm-wave amplifiers. The microfabrication process includes manufacture of precision X-ray masks, exposure of positive resist by X-rays through the mask, resist development, and electroforming of the final microstructure. Prototypes of a 32-cell, 108-GHz constant-impedance cavity and a 66-cell, 94-GHz constant-gradient cavity were fabricated with the synchrotron radiation sources at APS and NSLS. This paper presents an overview of the new technology and details of the mm-wave cavity fabrication.

A 50-MeV mm-wave electron linear accelerator system for production of tunable short wavelength synchrotron radiation

The Advanced Photon Source (APS) at Argonne in collaboration with the University of Illinois at Chicago and the University of Wisconsin at Madison is developing a new millimeter wavelength, 50-MeV electron linear accelerator system for production of coherent tunable wavelength synchroton radiation. Modern micromachining techniques based on deep etch x-ray lithography, LIGA (Lithografie, Galvanoforming, Abformung), capable of producing high-aspect ratio structures are being considered for the fabrication of the accelerating components.<<ETX>>

Coaxial higher-order mode damper employing a high-pass filter

Two different types of coaxial higher-order mode (HOM) dampers have been investigated for the Advanced Photon Source (APS) storage ring cavities: e-probe dampers and h-loop dampers. Realization of the h-loop dampers has been difficult because the loop antenna couples not only to the HOMs but also to the accelerating mode and results in loss of Q at the fundamental frequency. Previously, a first-order fundamental rejection filter was tested with unsatisfactory rejection characteristics. This problem can be overcome by using a higher-order high-pass filter between the loop and the matched load. Prototype dampers have been fabricated and tested in a storage ring single-cell cavity and the damping characteristic was analyzed.

Design and testing of a high power, ultra-high vacuum, dual-directional coupler for the Advanced Photon Source (APS) linear accelerator

Leaks and cracks have developed in the vacuum windows of the linac WR 284 waveguide directional couplers. In the existing coupler design the vacuum window is brazed to the waveguide. Replacement of a cracked window requires the removal of the component from the waveguide system resulting in a loss of vacuum in the waveguide. A new design has been developed and a prototype tested that utilizes bolted-in vacuum windows and allows for easier replacement of the windows in the system, while still providing suitable radio-frequency specifications.

Storage ring cavity higher-order mode dampers for the Advanced Photon Source

Coaxial, mode selective higher-order mode dampers for the Advanced Photon Source storage ring cavities have been fabricated and tested. Two types of dampers will be employed. Electric field probe dampers are positioned in the equatorial plane of the cavity so as to not couple to the fundamental TM/sub 01/ accelerating mode. Additionally, two magnetic field probe dampers with quarter-wavelength stub rejection filters are positioned in the cavity equatorial plane 90 degrees apart to facilitate dipole mode damping. Both damper types use a vacuum compatible, aluminum nitride (AlN) ceramic RF absorber as the matched load. Measurements were made to optimize the frequency response of the tapered absorber. The design eliminates the need for a ceramic vacuum window.

A mm-wave planar microcavity structure for electron linear accelerator system

The muffin-tin cavity structure is planar and well suited for a mm-wave accelerator with silicon etching techniques. A constant impedance travelling-wave structure is considered for design simplicity. The RF parameters are calculated and the shunt impedance is compared with the shunt impedance of a disk loaded cylindrical structure.<<ETX>>

Beam chopper for the low-energy undulator test line (LEUTL) in the APS

The low-energy undulator test line (LEUTL) is being built and will be tested with a short beam pulse from an rf gun in the Advanced Photon Source (APS) at the Argonne National Laboratory. In the LEUTL a beam chopper is used after the rf gun to deflect the unwanted beam to a beam dump. The beam chopper consists of a permanent magnet and an electric deflector that can compensate for the magnetic deflection. A 30-kV pulsed power supply is used for the electric deflector. The chopper subsystem was assembled and tested for beamline installation. The electrical and beam properties of the chopper assembly are presented.