Nikolay Solyak

A CONCEPT OF A WIDE APERTURE KLYSTRON WITH RF ABSORBING DRIFT TUBES FOR A LINEAR COLLIDER

Abstract This paper is devoted to a problem of the optimal design of the electrodynamic structure of the X-band klystron for a linear collider. It is shown that the optimal design should provide a large aperture and a high power gain, about 80 dB. The most severe problem arising here is that of parasitic self-excitation of the klystron, which becomes more complicated at increasing aperture and power gain. Our investigations have shown that traditional methods for suppressing the self-excitation become ineffective at the desired technical parameters of the klystron. In this paper we present a novel concept of a wide aperture klystron with distributed suppression of parasitic oscillations. Results of an experimental study of the wide-aperture relativistic klystron for VLEPP are presented. Investigations have been performed using the driving beam of the JINR LIA-3000 induction accelerator ( E = 1 MeV, I = 250 A, τ = 250 ns). To suppress self-excitation parasitic modes we have used the technique of RF absorbing drift tubes. As a result, we have obtained design output parameters of the klystron and achieved a level of 100 MW output power.

Study of 14 GHz VLEPP klystrons with 11 and 15 mm aperture

Results of experimental study of two variants of relativistic klystron for VLEPP are presented. Investigations have been performed using the driving beam of the JINR LIA‐3000 induction accelerator (E=1 MeV, I=300 A, τ=250 ns). The main emphasis is put on the study of the self‐excitation parasitic modes and their temporal evolution. A concept of relativistic klystron with RF absorbing drift tubes is proposed to solve the problem of the parasitic oscillations suppression. The results of preliminary experiments with such a klystron are presented, too.

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

Monopole HOMs Dumping in the LCLS-II 1.3 GHz Structure

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

Integrated High-Power Tests of Dressed N-doped 1.3 GHz SRF Cavities for LCLS-II

New auxiliary components have been designed and fabricated for the 1.3 GHz SRF cavities comprising the LCLS-II linac. In particular, the LCLS-II cavity’s helium vessel, high-power input coupler, higher-order mode (HOM) feedthroughs, magnetic shielding, and cavity tuning system were all designed to meet LCLS-II specifications. Integrated tests of the cavity and these components were done at Fermilab’s Horizontal Test Stand (HTS) using several kilowatts of continuous-wave (CW) RF power. The results of the tests are summarized here. INTRODUCTION The LCLS-II 4 GeV superconducting linac [1] is based on XFEL/ILC technology intensively developed over the last couple of decades. A major difference however is that LCLS-II operates in the CW regime, whereas the XFEL/ILC will operate in pulsed mode. This required modifications to or complete re-design of some of the basic components: cavity Helium vessel, tuner, power coupler, and other cryomodule parts in order to accommodate the much higher cryogenic loads expected in the CW regime. To accelerate the production of two pre-production cryomodules, it was decided to use existing ILC bare cavities and fundamental power couplers, which led to some constraints. The major LCLS-II modifications of the dressed cavity and auxiliaries are as follows: uf0b7 Nitrogen doped cavity to reduce losses in CW regime. LCLS-II requirements: Q0 > 2.7 x 10 at the nominal gradient of 16 MV/m. uf0b7 Helium vessel with a larger diameter two-phase connection to accommodate higher heat flux, and two helium supply inlets to provide more uniform thermal gradients during cooldown, which are crucial to effective magnetic flux expulsion, and hence low surface resistance. uf0b7 Two layers of magnetic shielding to reduce residual magnetic field at the cavity below 5mG. uf0b7 New end-lever tuner design which had to remain compatible with the “short-short” version of the ILC cavity adopted for the pre-production cryomodule. This design must also fit the “short-long” XFEL version of the cavity, which was adopted for production cryomodules. uf0b7 Design of the fundamental power coupler (FPC) was modified to fulfil LCLS-II requirements: loaded Q=4 x 10 and average power up to 6.4kW (includes 1.6kW of reflected power). Major modifications include reduction of the antenna length by 8.5mm and increase in the thickness of copper plating on the inner conductor of warm section to reduce coupler temperature. To minimize the risks to the project all technical solutions and new designs have to be prototyped and tested in a cryomodule. Testing was focused on the most critical components and technical solutions, and performed in the Horizontal Test Stand cryostat (HTS) under conditions approximating the final cryomodule configuration. An integrated cavity test was the last stage of the design verification program. In this test a nitrogen doped cavity (AES021), previously qualified in a vertical cryostat, was dressed and fully assembled with all components (fundamental power coupler, two-layer magnetic shielding, XFEL-type feedthroughs, end-lever tuner). All components were previously individually tested in the HTS with cavities, but not as a complete integrated system. One major goal of this integrated test was to demonstrate that high Q0 values demonstrated in vertical test can be preserved even when additional sources of heating from the power coupler and tuner and potential additional external magnetic fields from auxiliary components are present. Other important studies related to design verification included thermal performance and power handling of the power coupler, heating of HOM couplers and tuner components, tuner performance, sensitivity to microphonics, and frequency control. Data from this test program allows component design to be verified and certain other aspects of cryomodule design (e.g., component thermal anchoring) to be finalized. TEST PREPARATION AND CAVITY CONFIGURATION Dressed cavity AES021 was tested previously in a vertical test stand (VTS) without HOM feedthroughs. HOM feedthroughs were later installed in a clean room and after a brief high pressure water rinse, a pumping manifold was installed, the cavity evacuated, and successfully leak checked. The cavity field probe was not removed or replaced. The cavity was transported to a different clean room for installation of the coupler cold section. No additional cleaning of the cavity surfaces took place either as part of or subsequent to coupler installation. HOM feedthroughs were later installed in a clean room and after brief high pressure water rinsing, a pumping manifold was installed and cavity was leak tight. Cavity was transported to assembly clean room for ___________________________________________ # solyak@fnal.gov N. Solyak , T. Arkan, B. Chase, A. Crawford, E. Cullerton, I. Gonin, A. Grassellino, C. Grimm, A. Hocker, J. Holzbauer, T. Khabiboulline, O. Melnychuk, J. Ozelis, T. Peterson, Y. Pischalnikov, K. Premo, A. Romanenko, A. Rowe, W. Schappert, D. Sergatskov, R. Stanek, G. Wu, FNAL, Batavia, IL 60510, USA MOPB087 Proceedings of SRF2015, Whistler, BC, Canada ISBN 978-3-95450-178-6 342 C op yr ig ht

Simulations of 3.9 GHz CW Coupler for LCLS-II Project

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