Andrei Lunin

Analysis and measurement of the transfer matrix of a 9-cell, 1.3-GHz superconducting cavity

Superconducting linacs are capable of producing intense, stable, high-quality electron beams that have found widespread applications in science and industry. The 9-cell, 1.3-GHz superconducting standing-wave accelerating rf cavity originally developed for

The Variable Input Coupler for the Fermilab Vertical Cavity Test Facility

{e}^{+}/{e}^{\ensuremath{-}}

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

linear-collider applications [B. Aunes, et al. Phys. Rev. ST Accel. Beams 3, 092001 (2000)] has been broadly employed in various superconducting-linac designs. In this paper we discuss the transfer matrix of such a cavity and present its measurement performed at the Fermilab Accelerator Science and Technology (FAST) facility. The experimental results are found to be in agreement with analytical calculations and numerical simulations.

Monopole HOMs Dumping in the LCLS-II 1.3 GHz Structure

A variable input coupler has been designed for the Fermilab vertical cavity test facility (VCTF), a facility for testing bare 1.3 GHz 9-cell superconducting radiofrequency (SRF) cavities at 2 K, to provide some flexibility in the test stand measurements. The variable coupler allows the cavity to be critically coupled for all tests, including all TM010 passband modes, which will simplify or make possible the measurement of those modes with very low end-cell fields, e.g., pi/9 mode. The variable coupler assembly mounts to the standard input coupler port on the cavity, and uses a cryogenic motor submerged in superfluid helium to control the antenna position. The RF and mechanical design and RF test results are described.

Mechanical Optimization of High Beta 650 MHz Cavity for Pulse and CW Operation of PIP-II Project

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

Simulations of 3.9 GHz CW Coupler for LCLS-II Project

Developing an upgrade of Linac Coherent Light Source (LCLS-II) is currently underway. The central part of LCLS-II is a continuous wave superconducting RF (CW SRF) electron linac. High order modes (HOMs) excited in SRF structures by passing beam may deteriorate beam quality and affect beam stability. In this paper, we report the simulation results of monopole High Order Modes (HOM) spectrum in the 1.3 GHz accelerating structure. We suggest optimum parameters of the HOM feedthrough for minimizing RF losses on the HOM antenna tip and for preserving an efficiency of monopole HOMs damping simultaneously. INTRODUCTION A continuous operation regime of the 1.3 GHz LCSL-II accelerating structure at the nominal gradient of 16 MV/m sets an extra caution on possible overheating of HOM couplers feedthroughs [1]. 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 and the niobium surface is in a superconducting state. Nevertheless, a radiation of HOMs and an operating mode leaking through the notch filter can cause RF heating of the feedthrough internal parts and then a heating of the antenna itself by a thermal conductivity. This effect may initiate a thermal runaway process when increasing the antenna temperature leads to larger RF losses and generate an additional antenna heating by itself. Eventually it will produce a sharp temperature rise and end up by a cavity quench. In order to avoid such a scenario, one has to minimize the antenna RF heating by using smaller antenna tip and increasing the gap between the antenna and the f-part of HOM coupler. At the same time, we should not compromise the coupler capability to damp the cavity HOM spectrum. Bellow we compare ILC and XFEL design of the HOM feedthrough and analyse various antenna positions for finding optimum parameters. 3 MONOPOLE HOMS SPECTRUM The detailed study of resonant HOM excitation in the LCLS-II accelerating structure is performed in [2]. For the nominal parameters of LCLS-II linac, the most dangerous are monopole HOMs with high shunt impedances, which may result additional radiation of RF power to the HOM coupler port. Based on it we limited our investigation by the monopole HOMs only. Originally the spectrum of monopole modes in the 2D model of the TESLA 9-cell cavity is presented in [3] for first three TM-monopole passbands. Recently the detailed study of a thermal quench initiated by the overheating of the HOM antenna is published in [4] for various designs of the HOM feedthrough and trapped monopole HOMs below the beam pipe cut-off frequency of 2.942 GHz for the TM01 mode. Table 1: Parameters of Monopole HOMs Mode # Frequency, [GHz] R/Q [Ω] Qext

A high resolution cavity BPM for the CLIC Test Facility

The proposed design of the 0.8 GeV PIP-II SC Linac employs two families of 650 MHz 5-cell elliptical cavities with 2 different beta. The β=0.61 will cover the 185-500 MeV range and the β=0.92 will cover the 500-800 MeV range. In this paper we will present update of RF and mechanical design of dressed high beta cavity (β=0.92) HB650 optimized for pulse regime of operation at 2 mA beam current. In previous CW version of PIP-II project the mechanical design was concentrated on minimization of frequency shift due to helium pressure fluctuation. In current case of pulse regime operation the main goal is Lorentz force detuning minimization. We present the scope of coupled RF-Mechanical issues and their resolution. Also detailed stress analysis of dresses cavity will be presented. INTRODUCTION HB650 cavity originally was developed for CW operation. Current cryogenic power deficiency forced to consider switching of operation regime from CW to pulse mode in order to reduce cryogenic losses. During pulsed operation electromagnetic field energy stored in the cavity change with time and RF filed pressure to cavity walls also change causing resonance frequency modulation. Low beam current of 2 mA require relatively low RF power, therefore operating loaded Q of the cavity is high and operating frequency bandwidth 60 Hz is very narrow. Requirements for accelerating field amplitude 0.1% and phase 0.1° are very tight [1]. Lorenz force detuning (LFD) factor became a critical factor for cavity design optimization [2]. We introduce some modifications to dressed cavity design in order to reduce LFD coefficient. Also we need to keep very low sensitivity of operating mode resonance frequency to Helium pressure variations. Some modifications were added to simplify cavity tuner installation. Cavity Mechanical Design Optimization As mentioned above, the original design of the cavity and Helium vessel has been done for CW version of PIPII project. For original design “as is” LFD~-1.33 Hz/(MV/m) [3] i.e. 530 Hz of detuning for Eacc=20 MV/m. Fig. 1 shows the cavity wall deformation corresponded to Eacc=20 MV/m. Figure 1: Wall deformation in original design. On Fig. 1 are shown also 2 μm deformation of “coupler end” (left) and 0.42 μm of “tuner end” (right). Cavity sensitivity is ~ 160 Hz/μm. It means that Helium vessel walls deformations have 70% impact on the LFD value. Having the goal to minimize the Helium vessel modifications, the obvious way to significantly reduce the LFD value is to strengthen its walls. Also to reduce LFD coefficient, reducing process included optimization of position and number for stiffening rings of the cavity. One and two rings were considered with radius of rings as a parameter for optimization. Low sensitivity to helium pressure variations and external vibrations is still necessary parameter during optimization. One of the possible solution to reduce the LFD is adding the second stiffening ring in the cavity. Figure 2 shows the 3D view of COMSOL solid model. Two rings with radiuses R1 and R2 used in optimization process. Radius of stiffening ring in the end groups are the same as in original design. In these simulations we keep beam pipe flanges fixed. Figure 2: 3D view of COMSOL model with 2 rings. In 2 rings option the cavity stiffness only depend on the position of the ring R2. Figure 3 shows the dependence of the stiffness vs. R2 radius. We decided to keep radius R2 not higher than 120mm to avoid the cavity over stiffening. Figure 3: Cavity stiffness vs. position if the ring R2. ________________________________________ * Operated by Fermi Research Alliance, LLC under Contract No. DEAC02-07CH11359 with the United States Department of Energy. khabibul@fnal.gov Proceedings of SRF2015, Whistler, BC, Canada THPB014 SRF Technology Cavity E01-Elliptical design ISBN 978-3-95450-178-6 1093 C op yr ig ht

SINGLE SPOKE CAVITIES FOR LOW-ENERGY PART OF CW LINAC OF PROJECT X

LCLS-II linac is based on XFEL/ILC superconducting technology. Third harmonic cavity of 3.9 GHz is used to compensate nonlinear distortion of the beam longitudinal phase space. The TTF-III fundamental power coupler for the 3.9 GHz 9-cell cavities has been modified to satisfy to LCLS-II requirements and operation in the CW regime. In this paper we discuss the results of thermal analysis for proposed modifications of the power coupler design suitable for various operating regimes of the LCLS-II linac. The results of mechanical study are also presented INTRODUCTION The LCLS-II SCRF linac consists of 35 1.3 GHz, 8cavity Cryomodules (CM), and two 3.9 GHz, 8-cavity CMs. 3.9 GHz third harmonic superconducting cavities are used to increase the peak bunch current and to compensate non-linear distortions in the beam longitudinal phase space due to sinusoidal 1.3 GHz accelerating cavity voltage [1]. The fundamental power coupler (FPC) is an important and complicate component of the third harmonic system developed for the LCLS-II project. Table 1 shows main parameters of the 3.9 GHz cavity and cryomodule. Table 1: Main 3.9 GHz CM and Cavity Parameters