Ivan Gonin

Fabrication and test of the first normal-conducting crossbar H-type accelerating cavity at Fermilab for HINS

The proposed high intensity neutrino source (HINS) at Fermilab is based on an 8 GeV linear proton accelerator that consists of a normal-conducting (warm) and a superconducting section. The warm section is composed of an ion source, a radio frequency quadrupole, a medium energy beam transport (MEBT) and 16 warm Crossbar H-type (CH) cavities that accelerate the beam from 2.5 MeV to 10 MeV (from beta=0.0744 to beta=0.1422). These warm cavities are separated by superconducting solenoids enclosed in individual cryostats. Beyond 10 MeV, the design uses superconducting spoke resonators to accelerate the beam up to 8 GeV. In this paper, we illustrate the completion of the first warm CH cavity (beta=0.0744) explaining in detail the mechanical engineering aspects related to the machining and brazing processes. The radio-frequency (RF) measurements and tuning performed at Fermilab on the resonator and comparisons with simulations are also discussed.

Production of 325 MHZ single spoke resonators at FNAL

The High Intensity Neutrino Source (HINS) project represents the current effort at Fermi National Accelerator Laboratory to produce an 8-GeV proton linac based on about 400 independently phased superconducting resonators. Eighteen beta= 0.21 single spoke resonators, operating at 325 MHz, comprise the first stage of the linac cold section. We present the production status of the first two of these resonators along with progress on the slow tuning prototyping. In particular, we report on the construction phases, the pre-weld tuning process and the comparison of low power RF measurements with calculations.

Development of 325 MHz Single Spoke Resonators at Fermilab

The High Intensity Neutrino Source (HINS) project represents the current effort at Fermilab to produce an 8-GeV proton linac based on 400 independently phased superconducting cavities. Eighteen beta = 0.21 single spoke resonators, operating at 325 MHz, comprise the first stage of the linac cold section. In this paper we present the current status of the production and testing of the first two prototype cavities. This includes descriptions of the fabrication, frequency tuning, chemical polishing, high pressure rinse, and high-gradient cold tests.

Design of 325 MHz Single and Triple Spoke Resonators at FNAL

The proposed 8-GeV driver at FNAL is based on approximately 400 independently phased superconducting resonators. In this paper the design of 325 MHz spoke resonators, two single spoke resonators (beta = 0.21 and beta = 0.4) and a triple spoke resonator (beta = 0.62), for the high intensity neutrino source (HINS) front end is presented. We describe the optimization of the spoke resonator geometry, the goal being to minimize the EPEAK/EACC and BPEAK/EACC ratios. We report on the coupled ANSYS-MWS analysis on the resonator mechanical properties and power coupler RF design. The current status of mechanical design and slow tuning mechanism are also presented.

Recent Innovations in Muon Beam Cooling

Eight new ideas are being developed under SBIR/STTR grants to cool muon beams for colliders, neutrino factories, and muon experiments. Analytical and simulation studies have confirmed that a six-dimensional (6D) cooling channel based on helical magnets surrounding RF cavities filled with dense hydrogen gas can provide effective beam cooling. This helical cooling channel (HCC) has solenoidal, helical dipole, helical quadrupole, and helical sextupole magnetic fields to generate emittance exchange and achieve 6D emittance reduction of over 3 orders of magnitude in a 100 m segment. Four such sequential HCC segments, where the RF frequencies are increased and transverse physical dimensions reduced as the beams become cooler, implies a 6D emittance reduction of almost five orders of magnitude. Two new cooling ideas, Parametric-resonance Ionization Cooling and Reverse Emittance Exchange, then can be employed to reduce transverse emittances to a few mm-mr, which allows high luminosity with fewer muons than previously imagined. We describe these new ideas as well as a new precooling idea based on a HCC with z dependent fields that can be used as MANX, an exceptional 6D cooling demonstration experiment.

First results of testing 3.9 GHz TM/sub 010/ superconducting cavity

Fermilab is developing third a harmonic 3.9 GHz superconducting cavity to improve performances of A0 and TTF photoinjectors. In the frame work of this project we have built and tested two nine-cell copper models and one 3-cell niobium cavity. Properties of the high order modes were carefully studied in a chain of two copper cavities at room temperature. In this paper we discuss results of cold tests of the 3-cell cavity before and after surface treatment.

Multiphysics analysis of frequency detuning in superconducting RF cavities for proton particle accelerators

Multiphysics analyses for superconducting cavities are essential in the course of cavity design to meet stringent requirements on cavity frequency detuning. Superconducting RF cavities are the core accelerating elements in modern particle accelerators whether it is proton or electron machine, as they offer extremely high quality factors thus reducing the RF losses per cavity. However, the superior quality factor comes with the challenge of controlling the resonance frequency of the cavity within few tens of hertz bandwidth. In this paper, we investigate how the multiphysics analysis plays a major role in proactively minimizing sources of frequency detuning, specifically; microphonics and Lorentz Force Detuning (LFD) in the stage of RF design of the cavity and mechanical design of the niobium shell and the helium vessel.

RF thermal and new cold part design studies on a TTF-III input coupler for Project-X

An RF power coupler is one of the key components in a superconducting (SC) linac. It provides RF power to the SC cavity and interconnects different temperature layers (1.8 K, 4.2 K, 70 K and 300 K). The TTF-III coupler is one of the most promising candidates for the High Energy (HE) linac of Project X, but it cannot meet the average power requirements because of the relatively high temperature rise on the warm inner conductor, so some design modifications will be required. In this paper, we describe our simulation studies on the copper coating thickness on the warm inner conductor with RRR values of 10 and 100. Our purpose is to rebalance the dynamic and static loads, and finally lower the temperature rise along the warm inner conductor. In addition, to get stronger coupling, better power handling and less multipacting probability, one new cold part design was proposed using a 60 mm coaxial line; the corresponding multipacting simulation studies have also been investigated.

Production and Test Results of Superconducting 3.9-GHz Accelerating Cavity at Fermilab

The 3rd harmonic 3.9 GHz accelerating cavity was proposed to improve beam performances for TTF-FEL facility. In the frame of collaboration Fermilab will provide DESY with a cryomodule containing a string of four cavities. In addition, a second cryomodule with one cavity will be fabricated for installation in the Fermilab photo-injector, which will be upgraded for the International Linear Collider (ILC) accelerator test facility. In this paper we will discuss the status of the cavity and coupler production and the first result of cavity tests. It is hoped that this project will be completed during the first half of 2007 and the cryomodule delivered to DESY in this time span.

Redesign of the End Group in the 3.9 GHz LCLS-II Cavity

Development and production of Linac Coherent Light Source II (LCLS-II) is underway. The central part of LCLS-II is a continuous wave superconducting RF (CW SCRF) electron linac. The 3.9 GHz third harmonic cavity similar to the XFEL design will be used in the linac for linearizing the longitudinal beam profile [1]. The initial design of the 3.9 GHz cavity developed for the XFEL project has a large, 40 mm, beam pipe aperture for ensuring a low (< 106) cavity loaded quality factor. It is resulted in dipole HOMs with frequencies nearby the operating mode, which causes difficulties with HOM coupler notch filter tuning. The CW linac operation requires an extra caution in the design of the HOM coupler in order to prevent its possible overheating. In this paper, we present the modified 3.9 GHz cavity End Group for meeting to the LCLS-II requirements. INTRODUCTION A continuous operation regime of the 3.9 GHz LCSL-II accelerating structure at the maximum gradient of 14.9 MV/m sets an extra caution on possible overheating of HOM couplers feedthroughs [2, 3]. The HOM feedthrough coupling antenna is made of a solid Niobium, which does not produce significant amount of RF losses until its temperature is keeping below critical, but it may initiate a thermal runaway process and end up by a cavity quench due to a leak of an operating mode or a resonant excitation of the cavity HOM spectrum [4]. In order to avoid such a scenario, one has to minimize the antenna RF heating by using smaller antenna tip and increasing the size of the f-part snag. The proposed HOM coupler modification in the 3.9 GHz cavity is illustrated in Fig. 1. The height of antenna tip is decreased from 5 mm to 1 mm and the height of the f-part snag is increased to 7.8 mm in order to make a shallow antenna penetration and, thus, to lower a surface magnetic field. The nominal gap between the antenna and the f-part is chosen equal to 0.5 mm. Figure 1: Modifications of the HOM coupler for the 3.9 GHz cavity: a) XFEL design and b) LCLS-II design. Another drawback of the original XFEL End Group design is an oversized 40 mm aperture of the beam pipe, which has a cut off frequency of the lowest dipole mode very close to an operating mode. Eventually it makes quite difficult tuning the HOM coupler notch filter in a close proximity of dipole HOMs in the cavity End Group [5]. As a remedy, we decided to decrease slightly both apertures of the beam pipe and interconnecting bellows to 38 mm aiming to shift up frequencies of nearby dipole HOMs by at least of 100 MHz Modified design of the 3.9 GHz cavity End Group is illustrated in Fig. 2. The geometry of the cavity end cell remains untouched, while the tapering to a smaller aperture is made within the Nb transition ring. Figure 2: New design of the 3.9 GHz cavity End Group. OPERATING MODE RF LOSSES Parameters of operating mode for both designs of 3.9 GHz cavities, XFEL and LCLS-II, are compared in the Table 1. Since only the End Group was modified, there are little changes in the cavity performance Table 1: Parameters of 3.9 GHz Cavities Operating Mode Parameters XFEL LCLS-II Frequency, [GHz] 3.9 3.9 Stored Energy, [J] 1 1