A superconducting, 3.9 GHz, third harmonic accelerating cavity, developed at Fermilab, required a completely new main power coupler design to meet performance requirements, cost, and manufacturability. The RF design and optimization, multipactor problem analysis, and solid modeling were completed for non-adjustable version of the coupler. We have also begun a new power coupler design for the 325 MHz single, double and triple spoke cavities. The analysis of the couplers included magnetic and electrical coupling versions. In this paper, we discuss the status of the coupler development for 3.9 GHz and 325 MHz cavities . INTRODUCTION Fermilab is working on a few SC projects. One of them is 3rd harmonic 3.9 GHz accelerating cavity, developing for TTF/FEL facility to increase beam peak current [1-3]. Another project is Proton Driver (PD), designing to generate powerful 1GeV proton beam for neutrino physics [2]. The medium energy part of the PD linac will explore 325 MHz single, double and triple spoke SC cavities. Needed power level to feed SC cavities in both projects is from tens to hundreds of kW peak power. Since the chosen cavity frequencies are quite different from what is used in other SC projects (SNS, TESLA, etc.) none of existing power coupler can be easily adopted for projects.. COUPLER DESIGN FOR THIRD HARMONIC CAVITY Fig. 1. Coupler layout in cryostat The module with the four 3rd harmonic cavities is planning to install at DESY TTF/FEL facility. Since the available space is limited, the cryostat with cavities will be build as an extension of existing TTF cryostat. So, geometrical and assembly constrains for the coupler, as well as requirements are similar to the 1.3 GHz TTF coupler [5]. The required power level ~50 kW is defined by accelerating gradient 14 MV/m and beam loading. Layout of the final coupler in cryostat is shown on Fig.1. Coaxial part has 50Ω with a 30mm outer diameter to prevent excitation of the asymmetrical modes. All components of the coupler: cold and warm windows, bellow section, coax-to-waveguide transition, vacuum and diagnostic ports, were optimized by HFSS for low reflection (S11<0.05) at the operating frequency.

Fermilab has refined the mechanical design of the Superconducting, 3.9 GHz, 3rd Harmonic System over the past two years. This 2 meter long, 4 cavity cryomodule will be installed at DESY’s TTF in 2007. Our design is summarized in this paper to include: the development of the 9 cell niobium cavity; the design of the helium vessel, bladetuner, and cryostat; and the HOM and Main Input coupler designs. This paper reflects the current status of our design. CRYOMODULE DESIGN Working closely with DESY personnel, the cryomodule has evolved into the final design. The 3-D design is complete (see Fig. 1) and the 2-D detailing is near completion. The cryomodule design is a combination of DESY’s Cryo-1, -2, and -3 designs. This 3 Harmonic module is installed between the TTF Turn-a-round Endcan and a type 2 cryomodule. Due to its location and mounting at TTF, the design required an interface to an earlier TTF cryomodule. However, many of the newer design features found in the Cryo-3 design were incorporated wherever possible. Figure 1: Cryomodule section view. CAVITY STRING Four, 3.9 GHz cavities, four cold main coupler assemblies, two gate valves, helium piping, and interconnect bellows will be assembled, in 2006, into one cavity string inside a class 10 cleanroom. Cavities and helium vessels are being fabricated. Two bladetuners have been fabricated and tested. HOM couplers are in procurement and will arrive at Fermilab in spring 2006. Figure 2: Cavity string top. Figure 3: Cavity string isometric. Niobium Cavity This year we suffered a setback when our final equatorial weld on our first 3 Harmonic cavity failed. Attempts to repair the hole created during the final ebeam weld failed as well. We are in the process of cutting out a cavity cell and re-welding a replacement cell. This cavity will be repaired and ready for testing in November 2006. Our second 3 Harmonic cavity is also near completion. JLAB and Fermilab have contracted to produce an additional six cavities. Figure 4: CAD rendered niobium cavity. Proceedings of the 12th International Workshop on RF Superconductivity, Cornell University, Ithaca, New York, USA

A 4-cavity 3.9 GHz cryomodule has been constructed at Fermilab and delivered to DESY. Its intended use is to linearize the non-linear beam energy-time profile produced by the 1.3 GHz accelerating gradient and thus improve the operating characteristics of FLASH for its users. First cold testing of the module is expected in the near future prior to its installation. We will report on the performance of the cavities, assembly and transport of the module as well as anticipated testing, installation, and commissioning plans.

The 3rd harmonic 3.9GHz accelerating cavity was proposed to improve the beam performance of the electron/positron linear accelerators. In the frame of a collaborative agreement, Fermilab will provide DESY with a cryomodule containing a string of four cavities. Seven 9-cell Nb cavities were tested and six of them did reach accelerating gradient up to 24 MV/m almost twice more than design value of 14 MV/m. Two of these cavities are with new HOM couplers with improved design. In this paper we present all results of the vertical and horizontal tests.

The superconducting 3.9GHz deflecting mode cavity design that has been under development [1] as a beam slice diagnostic is planned for use as the ILC crab cavity. We describe the applications and review the status of the R & D, giving both prototype test results and computational studies of the beam-cavity interaction. RECENT PROTOTYPE TEST RESULTS In late 2005 and early 2006, a 3-cell prototype built in conjunction with Advanced Energy Systems of Medford NY (U.S.A.) was cold-tested. The cavity was processed only with buffered chemical polishing (BCP) using a 1:1:2 mix of nitric, fluoric, and phosphoric acids, and baked at 600° C for 10 to expel hydrogen. A highpressure rinse of 18M ultra-pure water for 3.5 hours removed particulate contamination. No couplers other than the main power coupler and a small pickup probe were installed. At 3.9GHz and at the 1.8K test temperature, the intrinsic resistance from the BCS mechanism is, by extrapolation of DESY data, about 41n. The magnetic field inside of our test Dewar has been measured at about 25mG, and using the rough rule R SURF 0.3n f GHz () H EXT mG

The XFEL facilities are planning to use a section with several third harmonic cavities (3.9 GHz) to improve beam performance [1]. Fermilab is developing the superconducting third harmonic section for the FLASH (TTF/DESY) upgrade. This section will include four cavities equiped with couplers and blade tuners installed in a cryostat. Currently, two cavities are completed and one of them is under vertical test. The gradient of this cavity was limited by multipacting in the HOM coupler. Visual inspection of the HOM couplers after cold tests showed that both couplers were damaged. In this paper we discuss the results of the vertical tests, an analysis of multipactoring in the HOM coupler, and a new design for the HOM coupler.

Peak current and emittance of the high brightness photoinjector are limited by non-linear energy distribution in the bunch. Adding a weighted amount of third harmonic accelerating voltage allows compensate this non-linearity. For FNAL-NICADD photoinjector it can result in significant improvement of its performance reducing emittance and increasing peak current. These benefits have triggered designing and prototyping of a superconducting 3.9 GHz (3 harmonic to 1.3GHz used as a main accelerating frequency) cavity at FNAL. At first stage it was built two models: 9-cell copper cavity and 3-cell niobium cavity. In this paper the status of the cavity design and results of RF measurements of the two models are presented.

The third harmonic 3.9 GHz superconducting cavity was recently proposed by DESY for a new generation of high brightness photo-injector (TTF photoinjector-2) to compensate nonlinear distortion of the longitudinal phase space due to RF curvature of the 1.3 GHz TESLA cavities. Installation of the 3rd harmonic cavity will allow us to generate ultra-short (<50 /spl mu/m rms) highly charged electron bunches with an extremely small transverse normalized emittance (<1 /spl mu/m). This is required to support a new generation of linear colliders, free electron lasers and synchrotron radiation sources. In this paper we present the current status of the 3rd harmonic cavity being developed at Fermilab. We discuss the design procedure, the building and testing of the copper and niobium half-cells and components, the design of input and HOM couplers.