S. Belomestnykh

Suppression of multipacting in rectangular coupler waveguides.

Although a rectangular waveguide coupler has the conceptual advantages of simplicity and capability of withstanding higher power, builders of modern superconducting accelerators are routinely choosing instead a coaxial coupler for its proven performance. Multipacting induced discharge has been found to be an operating mechanism that prevents a rectangular waveguide coupler from reaching its full potential. Earlier calculations predicted the existence of two-sided multipacting in a rectangular waveguide geometry. In the present study, special waveguide sections of CESR type were built and tested. Multipacting characteristics of the waveguide were identified. Two multipacting suppression methods, the slotted waveguide method and the DC magnetic bias method, were experimentally evaluated. The multipacting current is suppressed by a factor of more than 2 by opening a slot on the broad wall. Complete multipacting suppression can be realized by using the DC magnetic bias method.

The Cornell ERL prototype project

Synchrotron light sources based on Energy Recovery Linacs (ERLs) show promise to deliver X-ray beams with both brilliance and X-ray pulse duration far superior to the values that can be achieved with storage ring technology. Cornell University, in collaboration with Jefferson Laboratory, has proposed the construction of a prototype ERL. This 100MeV, 100mA CW superconducting electron accelerator will be used to study and resolve the many accelerator physics and technology issues of this type of machine. These studies are essential before ERLs can be confidently proposed for large-scale applications such as synchrotron light sources. Key issues include the generation of high average current, high brightness electron beams; acceleration and transport of these beams while preserving their brightness; adequate damping of higher order modes (HOMs) to assure beam stability; removal of large amounts of HOM power from the cryogenic environment; stable RF control of cavities operating at very high external Q; reduction of beam losses to very low levels; and the development of precision non-intercepting diagnostics to allow beam setup, control and characterization. Our prototype design allows us to address these and other issues over a broad range of parameter space. This design, along with recent progress on understanding these issues, will be presented.

Dipole-mode-free and kick-free 2-cell cavity for the SC ERL injector

For the ERL injector, superconducting cavities are needed to deliver to the beam a 100 kWCW RF power. With a beam current of 100...33 mA, gap voltage of 1...3 MV, the coupler must have an external g-factor in the range of 4.6/spl times/10/sup 4/...4.1/spl times/10/sup 5/. The cavity shape and coupler design presented provide the possibility of working in the range of parameters without substantial transverse kick to the beam and HOM-losses in the system. In order to preserve field flatness while the dipole mode is driven out, the 2-cell cavity has a protruding iris between the cell and the larger beam pipe. A twin-coaxial coupler has high coupling but low kick because of its symmetry. Calculation and optimization of the coupler-cavity system are performed with a 2D SLANS and 3D Microwave Studio/sup /spl reg// codes.

Commissioning of the superconducting RF cavities for the CESR Luminosity Upgrade

The new superconducting RF system consisting of four single-cell cavity modules is an important part of the CESR Luminosity Upgrade. We describe the commissioning of the first three accelerating modules. This includes in situ testing and conditioning, pulsed power and beam processing of RF windows, commissioning of various cryogenic feedback loops, measuring cavity spacing and phasing with beam, and high-current operation.

Design, prototyping, and testing of a compact superconducting double quarter wave crab cavity

A novel design of superconducting Crab Cavity was proposed and designed at Brookhaven National Laboratory. The new cavity shape is a Double Quarter Wave or DQWCC. After fabrication and surface treatments, the niobium proof-of-principle cavity was cryogenically tested in a vertical cryostat. The cavity is extremely compact yet has a low frequency of 400 MHz, an essential property for service for the Large Hadron Collider luminosity upgrade. The electromagnetic properties of the cavity are also well matched for this demanding task. The demonstrated deflecting voltage of 4.6 MV is well above the requirement for a crab cavity in the future High Luminosity LHC of 3.34 MV. In this paper we present the design, prototyping and test results of the DQWCC.

Design of the CW Cornell ERL Injector Cryomodule

The Cornell ERL Prototype injector will accelerate bunches from an electron source to an energy of several MeV, while preserving the ultra-low emittance of the beam. The injector linac will be based on superconducting RF technology with five 2-cell RF cavities operated in cw mode. The beam tubes on one side of the cavities have been enlarged to propagate Higher-Order-Mode power from the cavities to broadband RF ring-absorbers located at 80 K between the cavities. The axial symmetry of these ferrite based absorbers, together with two symmetrically placed input couplers per cavity, avoids transverse on-axis fields, which would cause emittance growth. Each cavity is surrounded by a LHe vessel and equipped with a frequency tuner. The cryomodule provides the support and alignment for the cavity string, the 80 K cooling of the ferrite loads, and the 2K LHe cryogenic system for the high cw heat load of the cavities. In this paper we give an overview of the ERL injector cryomodule design.

Buncher cavity for ERL

Design of the buncher cavity for Cornell/JLab ERL project is presented. This is a reentrant spherical copper cavity at a frequency of 1300 MHz. It will be installed between a 500 keV electron gun and superconducting accelerating sections in the injector part of ERL. The cavity has Q of 20,000 and a shunt impedance of 2.1 MOhm. For a design cavity voltage of 200 kV, power dissipated in cavity is as much as 9.6 kW. The cavity has a coaxial loop coupler and will be driven by a 17.5 kW klystron. The estimates of cavity influence on beam dynamics are also discussed.

Comparison of the predicted and measured loss factor of the superconducting cavity assembly for the CESR upgrade

Superconducting cavities have been chosen to replace the existing copper cavities for the future upgrade of CESR. The use of superconducting cavity modules, specially designed for a high current collider, allows us to lower the cavity impedance and the loss factor of the accelerating system and thereby increase the threshold for multi- and single-bunch instabilities. The loss factor has been measured in a beam test using calorimetric method; we measure the water temperature rise and the flow rate of the cooling water for a higher order mode load. To measure the loss factor vs. bunch length (10 to 25 mm), we used two different sets of CESR optics and different RF voltages. The experimental data points are in a good agreement with predicted values.

Photocathode behavior during high current running in the Cornell energy recovery linac photoinjector

The Cornell University energy recovery linac (ERL) photoinjector has recently demonstrated operation at 20 mA for approximately 8 hours, utilizing a multialkali photocathode deposited on a Si substrate. We describe the recipe for photocathode deposition, and will detail the parameters of the run. Post-run analysis of the photocathode indicates the presence of significant damage to the substrate, perhaps due to ion back-bombardment from the residual beam line gas. While the exact cause of the substrate damage remains unknown, we describe multiple surface characterization techniques (x-ray fluorescence spectroscopy, x-ray diffraction, atomic force, and scanning electron microscopy) used to study the interesting morphological and crystallographic features of the photocathode surface after its use for high current beam production. Finally, we present a simple model of crystal damage due to ion back-bombardment, which agrees qualitatively with the distribution of damage on the substrate surface.

Superconducting RF system upgrade for short bunch operation of CESR

The CESR luminosity upgrade plan calls for shortening bunch length to 1 cm. Such bunch length can be achieved by installing two more superconducting cavities to increase total RF voltage. The RF system upgrade necessary to accommodate this change is discussed.