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Now 30 MeV stage of the ARIEL (The Advanced Rare Isotope Laboratory) e-Linac is under commissioning which includes an injector cryomodule (ICM) and the first accelerator cryomodule (ACM1). The two ACM1 cavities are driven by a single klystron with vector-sum control and running in CW mode. During the commissioning, the ACM1 cavities gradient and stability were limited by a ponderomotive effect [1-4]. Acoustic noise from the environment including vibration sources from waterand aircooling systems, cryogenic system and vacuum system have been identified. In this paper, the progress of the microphonics suppression of ACM1 is presented. INTRODUCTION ARIEL e-Linac is a continuous-wave (CW) superconducting electron linear accelerator. The accelerator is divided into three cryomodules [5, 6]: an ICM with one cavity and two ACM with two 1.3 GHz nine-cell cavities each. The ‘Demonstrator’ phase of ARIEL was installed for initial technical and beam tests with successful beam acceleration to 22 MeV [7]. The ACM1 cryomodule, initially installed with one cavity, was then updated to 2 cavities [8] but still driven by a single klystron in vector sum. During commissioning, acoustic noise from the environment vibration generated by the cooling water system, cooling air, cryogenic system and vacuum system seeded a coupled cavity instability driven by the Lorentz force that impacted the ACM1 rf performance and final beam energy stability. The final energy gain of ACM1 was limited to around 17~18 MeV with significant energy instability [ ]. Extensive damping has been implemented during a recent shutdown. The new RF test results show 20 MeV acceleration gain can be reached. PONDERMOTIVE EFFECT Both ICM and ACM1 work in phase-locked loop (PLL) self-excited loop (SEL) in CW mode. Unlike the ICM with one cavity and one SEL loop, the two ACM1 cavities are driven by a single klystron with vector-sum control of the two cavity SEL PLL. There is no gradient regulation for each individual cavity in the ACM1 [9]. Individual tuner loops are established to maintain the cavity frequency with respect to the established reference phases. Microphonics can excite mechanical resonances in either cavity, perturbing the RF resonant frequency, which can couple to the other cavity via vector-sum regulation. The resonance modulations lead to further amplitude variations in the two cavities which in turn drive the mechanical resonances, leading to a ponderomotive instability [10-14]. While under vector-sum regulation, the ACM1 cavities pickup signals indicated a slowly developing (over seconds), large amplitude oscillation at 160Hz in counterphase (Fig. 1) impacting the energy stability and ultimately the loop stability. Figure 1: The two-cavity oscillation of ACM1 pickup signals. (a) The yellow & cyan waveforms are the 1st and 2nd cavity pickup signals, respectively. (b) Without oscillation, the 160 Hz peak is not notable(red). When the oscillation happens, the 160 Hz peak becomes much stronger(blue). The field threshold for excitation of this instability is impacted both by ambient microphonic noise that can seed the instability and the precise settings of the two cavity phase loops. The coupled oscillation can deteriorate the eLinac final energy stability outside specification and needs to be controlled. MICROPHONICS SOURCE SEARCHING AND DAMPING The external vibrations couple to the mechanical system constituted by the cavity and auxiliary components and excite mechanical modes at resonance. RF Waveguide System Damping The e-Linac high power RF system consists of two klystrons, six high power dummy loads, two circulators and waveguide components. The water system cooling these components comprises the main vibration source for the -100 -80 -60 -40 -20 0 -400 -300 -200 -100 0 100 200 300 400 A m pl itu de (d Bm ) Frequency(Hz) with oscillation without oscillation  * TRIUMF receives funding via a contribution agreement with the National Research Council of Canada † mayanyun@triumf.ca Th is is a pr ep ri nt — th e fin al ve rs io n is pu bl ish ed w ith IO P 10th Int. Partile Accelerator Conf. IPAC2019, Melbourne, Australia JACoW Publishing ISBN: 978-3-95450-208-0 doi:10.18429/JACoW-IPAC2019-MOPGW004 MC2: Photon Sources and Electron Accelerators A08 Linear Accelerators MOPGW004 65 Co nt en tf ro m th is w or k m ay be us ed un de rt he te rm so ft he CC BY 3. 0 lic en ce (© 20 19 ). A ny di str ib ut io n of th is w or k m us tm ai nt ai n at tri bu tio n to th e au th or (s ), tit le of th e w or k, pu bl ish er ,a nd D O I

Microphonics Suppression In Ariel Acm1 Cry- Omodule