Zachary Conway

Variable CW RF power coupler for 345 MHz superconducting cavities

This paper reports the development of a CW variable coupler for 345 MHz spoke-loaded superconducting (SC) cavities. The coupler inserts an 80 K copper loop into a 5 cm (2 inch) interior diameter coupling port on several types of spoke-loaded cavity operating at 2 K or 4 K. The coupling loop can be moved during operation to vary the coupling over a range of 50 dB. The coupler is designed to facilitate high-pressure water rinsing and low-particulate clean assembly. Design details and operating characteristics are discussed.

Design and development of a new SRF cavity cryomodule for the ATLAS intensity upgrade

The ATLAS heavy ion linac at Argonne National Laboratory is undergoing an intensity upgrade that includes the development and implementation of a new cryomodule containing four superconducting solenoids and seven quarter-wave drift-tube-loaded superconducting rf cavities. The rf cavities extend the state of the art for this class of structure and feature ASME code stamped stainless steel liquid helium containment vessels. The cryomodule design is a further evolution of techniques recently implemented in a previous upgrade [1]. We provide a status report on the construction effort and describe the vacuum vessel, thermal shield, cold mass support and alignment, and other subsystems including couplers and tuners. Cavity mechanical design is also reviewed.

Beam Optics Simulations Study on the Pre-Stripper Linac for Rare Isotope Science Project

The rare isotope science project (RISP) under development in Korea aims to provide various heavy-ion beams for nuclear and applied science users. A prestripper linac is the first superconducting section to be constructed for the acceleration of both stable and radioisotope beams to the energy of 18.5 Mev/u with a DC equivalent voltage of 160 MV. The current baseline design consists of an ECR ion source, an RFQ, cryomodules with QWR and HWR cavities and quadruple focusing magnets in the warm sections between cryomodules. Recently we have developed an alternative design in collaboration with Argonne’s Linac Development Group to layout the linac based on stateof-the-art ANL’s QWR operating at 81.25 MHz and multi-cavity cryomodules of the type used for the ATLAS upgrade and Fermilab PIP-II projects. End-toend beam dynamics calculations have been performed to ensure an optimized design with no beam losses. The numbers of required cavities and cryomodules are significantly reduced in the alternative design. The results of beam optics simulations and error sensitivity studies are discussed. INTRODUCTION A next-generation rare isotope science facility using the in-flight fragment (IF) separation technique requires a high-current heavy-ion accelerator capable of delivering U beam with a few hundred kW power to a thin production target [1]. First, high currents of highly charged ions are needed to efficiently produce such highpower heavy ion beams. An ECR ion source operating at 28 GHz [2] has been developed, but for the heaviest ions, the beam current in a single charge state is still lower than required for a next generation IF facility. To fully utilize the available accelerating voltage of a heavy-ion linac, charge strippers are employed in the process of multi-step acceleration. To accelerate a uranium beam to 200 MeV/u, charge stripping at 18 MeV/u was determined to be optimal. A significant merit of a superconducting linac is that its longitudinal acceptance is large enough to simultaneously accelerate multiple charge states of uranium produced at the charge stripper. Therefore, a large fraction of the beam is accelerated after the stripper and beam losses in the charge state selection section are significantly reduced resulting in lower radiation levels in that region. The layout of the pre-stripper linac of the Rare Isotope Science Project (RISP) ongoing in Korea [3] is shown in Fig 1. The linac is designed to accelerate either radioisotope beams from the ISOL target or stable beams from the ECR. In fact, a plan is to accelerate both radioisotope and stable beams simultaneously when their charge-to-mass ratios are within 2%. For instance, Sn can be accelerated together with U. The isotope beam from ISOL is charge-bred before being injected into the pre-stripper linac. The charge breeding takes tens of ms in EBIS, which is under development at RISP [4], and stable ions can be accelerated during charge-breeding. Since the time duration for injection and extraction of isotope beams is much shorter than 1 ms and the breeding takes tens of ms, the fraction of stable beam can be over 90%. The simultaneous acceleration scheme of stable and radioisotope beams was devised for the proposed multi-user upgrade of the ATLAS linac at Argonne [5]. Figure 1: Layout of the pre-stripper linac of the RISP baseline design. The injector includes an RFQ and the beam energy to the first cavity is 500 keV/u. The pulsing of stable ions according to the time structure of the charge-bred radioisotope beam is formed by an electric chopper. At the end of pre-stripper linac the two beams are switched by a kicker magnet either to low energy experimental area or to the achromatic 180 bending section after charge stripping. CURRENT BEAM OPTICS DESIGN The current design of the pre-stripper linac is based on the use of two kinds of superconducting cavities: QWR (βopt=0.047) and HWR (βopt=0.12) operating at 81.25 and 162.5 MHz, respectively [6]. Transverse focusing components were decided to be quadrupole doublets based on the thought that superconducting solenoids located inside cryomodule can affect the cavity ___________________________________________ *Work supported by National Research Foundation Grant No. 20110018946 and the Rare Isotope Science Project. †jwkim@ibs.re.kr Proceedings of HB2016, Malmö, Sweden MOPM3P01 Beam Dynamics in Linacs ISBN 978-3-95450-178-6 31 C op yr ig ht

A new 2 Kelvin superconducting half-wave cavity Cryomodule for PIP-II

Argonne National Laboratory has developed and is implementing a novel 2 K superconducting cavity cryomodule operating at 162.5 MHz. This cryomodule is designed for the acceleration of 2 mA H-/proton beams from 2.1 to 10 MeV as part of the Fermilab Proton Improvement Project-II (PIP-II). This work is an evolution of techniques recently implemented in two previous heavy-ion accelerator cryomodules now operating at Argonne National Laboratory. The 2 K cryomodule is comprised of 8 half-wave cavities operated in the continuous wave mode with 8 superconducting magnets, one in front of each cavity. All of the solenoids and cavities operate off of a single gravity fed 2 K helium cryogenic system expected to provide up to 50 W of 2 K cooling. Here we review the mechanical design of the cavities and cryomodule which were developed using methods similar to those required in the ASME Boiler and Pressure Vessel Code. This will include an overview of the cryomodule layout, the alignment of the accelerator components via modifications of the cryomodule vacuum vessel and provide a status report on the cryomodule assembly.

Higher Order Mode Damping in a Higher Harmonic Cavity for the Advanced Photon Source Upgrade

A superconducting higher -harmonic cavity (HHC) is under development for the Advanced Photon Source Upgrade based on a Multi-Bend Achromat lattice. This cavity will be used to improve the Touschek lifetime and the single bunch current limit by lengthening the beam. A single-cell 1.4 GHz (the 4th harmonic of the main RF) cavity is designed based on the TESLA shape. Two adjustable fundamental mode power couplers are included. The harmonic cavity voltage of 0.84 MV will be driven by the 200 mA beam. The RMS bunch length with the harmonic cavity will be >50 ps. Higher-order modes (HOM) must be extracted and damped. This will be done with two silicon carbide beamline HOM absorbers to minimize heating of RF structures such as the superconducting cavity and/or couplers and suppress possible beam instabilities. The HHC system is designed such that 1) most monopole and dipole HOMs are extracted along the beam pipes and damped in the ‘beamline’ silicon carbide absorbers and 2) a few HOMs, resulting from introduction of the couplers, are extracted through the coupler and dissipated in a room temperature water-cooled load. We will present time and frequency domain simulation results and discuss damping of HOMs.