J.J. Song

Enhanced adhesion buffer layer for deep x-ray lithography using hard x rays

The first step in the fabrication of microstructure using deep x-ray lithography (DXRL) is the irradiation of a x-ray sensitive resist like polymethylmethacrylate (PMMA) by hard x-rays. At the Advanced Photon Source, a dedicated beamline allows the proper exposure of very thick (several mm) resists. To fabricate electroformed metal microstructure with heights of several mm, a PMMA sheet is glued onto a metallic plating base. An important requirement is that the PMMA layer must adhere well to the plating base. The adhesion is greatly reduced by the penetration of even a small fraction of hard x-rays through the mask absorber into the substrate. In this work we will show a novel technique to improve the adhesion of PMMA onto high-Z substrates for DXRL. Results of the improved adhesion are shown for different exposure/substrate conditions.

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.

Bunch length measurements at the Advanced Photon Source (APS) linear accelerator

Measurements of the APS linac micro-bunch length are performed by backphasing a single 2856-MHz, S-band linac waveguide and using a downstream spectrometer to observe the beam. By measuring the beam width in the dispersive plane as a function of RF power into the linac waveguide, the bunch length can be determined absolutely provided the beam energy and dispersion at the spectrometer are known. The bunch length determined in this fashion is used to calibrate a fifth-harmonic bunch length cavity which is used for real-time bunch length monitoring.

An overview of the APS 352-MHz RF systems

The Advanced Photon Source (APS) is a 7-GeV full energy positron storage ring for generating synchrotron radiation with an injector. The booster synchrotron RF system consists of a single 1-MW klystron which drives four five-cell cavities at 352 MHz. The storage ring cavities consist of four groups of four single cells powered by two 1-MW klystrons for 100-mA operation. An overview of the operation of the APS 352-MHz RF systems is presented.

Compensation of longitudinal coupled-bunch instability in the Advanced Photon Source storage ring

A longitudinal coupled-bunch (CB) instability was encountered in the 7-GeV storage ring. This instability was found to depend on the bunch fill pattern as well as on the beam intensity. The beam spectrum exhibited a coupled-bunch signature, which could be reproduced by an analytical model. The oscillations were also observed on a horizontal photon monitor. The beam fluctuations exhibited two periodicities, which were found to be correlated with the rf cavity temperatures. This correlation is consistent with the measured temperature dependence of the higher-order mode (HOM) frequencies. The HOM impedance drives the beam when brought into resonance with the CB mode by the temperature variation. Increasing the inlet cavity water temperature suppressed the instability. The experimental results are compared to an analytical model which characterizes the fill-pattern dependence. Studies to identify the offending HOMs are also presented.

Fabrication of mm-wave undulator/linear accelerator cavities, using deep x-ray lithography.

The possibility of fabricating mm-wave radio frequency cavities using deep x-ray lithography (DXRL) is being investigated. The frequency of operation can be from 30 GHz to 300 GHz, operating mode in either TM or TE-mode, depending on the application. For most applications, a complete structure consists of two mirror-image planar half structures assembled face-to-face. The fabrication 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. The precision hard x-ray mask was made by means of an surface mask, using soft x-ray lithography for pattern transfer into poly-methylmethacrylate (PMMA) on a 200-micrometers thick Si wafer, followed by electroplating of 35-micrometers Au at CXrL (Center of X-ray Lithography) in Wisconsin. For the DXRL process, PMMA was used as the positive resist, either as an 1-mm sheet glued or 200-micrometers film cast onto a Cu substrate. The NSLS (National Synchrotron Light Source) X- 26C beamline in Brookhaven was used to expose the resist. 99.9% OFC (oxygen free copper) was electroplated onto the developed PMMA structure, and then polished by the diamond- lapping. The cavity will be aligned with the optical fibers on the grooves and then initial test will be performed with HP 8510 network analyzer.

Feasibility studies of a compact mm-wave linac FEL

Abstract Microfabrication technology offers an alternative method for fabricating precision, miniature-size components suitable for use in accelerator physics and commercial applications. The orginal R&D work at Argonne, in collaboration with the University of Illinois at Chicago and University of Wisconsin-Madison, has produced encouraging results in the area of rf accelerating structure design, optical and X-ray masks production, deep X-ray lithography (LIGA exposures), and precision structural alignments. In this paper we present feasibility studies for a compact single pass mm-linac FEL to produce short wavelength radiation. This system will consist of a photocathode rf gun operated at 30 GHz, a 50-MeV superconducting constant gradient structure operated at 60 GHz, and a microundulator with 1-mm period.

RF cavities for the Positron Accumulator Ring (PAR) of the Advanced Photon Source (APS)

The cavities for the dual frequency system of the APS PAR are described. The system uses two frequencies: a 9.78 MHz fundamental system for the particle accumulation and a 117.3 MHz twelfth harmonic system for the bunch compression. The cavities have been built, installed, tested, and used for storing the beam in the PAR for about a year. The fundamental cavity is a reentrant coaxial type with a capacitive loading plunger and has 1.6 m length. The harmonic cavity is a symmetrical reentrant coaxial type and is 0.8 m long. Ferrite tuners are used for frequency tuning. During the accumulation period, the ferrite tuner of the harmonic cavity works as a damper to disable the cavity. During an injection cycle the 9.78 MHz system accumulates 24 positron bunches in a bucket and the 117.3 MHz system compresses the bunch into a shorter bunch. Measurements were made on the RF properties of the cavities.

RF radiation measurement for the Advanced Photon Source (APS) personnel safety system

The Advanced Photon Source (APS) booster and storage ring RF system consists of five 1-MW klystrons, four 5-cell cavities, and sixteen single-cell cavities. The RF power is distributed through many hundreds of feet of WR2300 waveguide with H-hybrids and circulators. In order to protect personnel from the danger of RF radiation due to loose flanges or other openings in the waveguide system, three detector systems were implemented: an RF radiation detector, a waveguide pressure switch, and a Radiax aperture detector (RAD). This paper describes RF radiation measurements on the WR2300 waveguide system.

HOM (higher‐order mode) damper tests of the storage ring single‐cell cavity with a 20‐MeV e− beam for advanced photon source

A beamline has been assembled with the ANL Chemistry Division linac (20‐MeV e− beam with FWHM of 20 ps) to test the effectiveness of damping techniques of the APS storage ring single‐cell cavity. The beamline consists of two sections—the beam collimating section and the cavity measurement section—separated by two single Al foil windows, allowing the beam to propagate on‐ and off‐axis of the cavity. The beam diagnostics include a beam position monitor, integrating current transformers, fluorescent screens, and a Faraday cup. The EPICS (Experimental Physics and Industrial Control System) is used for beamline control, monitoring, and data acquisition. The diagnostic system used for beam image capture and analysis is PV WAVE. The cavity was excited by the electron beam to investigate the HOMs. The HOMs were measured when the cavity was unloaded as well as loaded into the waveguide system. An rf cavity was also tested with and without various types of dampers. The HOM measurements were made with H‐loops and E‐...