Accelerators are affected by the cavities detuning variation caused by external mechanical disturbances (microphonics). The paper presents microphonics estimation and prediction methods applicable for superconducting accelerators operating in pulsed mode. A mathematical model is built using the estimates of detuning during previous RF pulses. The model can be used for predictions of disturbances for the future time step and setup of the fast tuners accordingly. The proposed method was successfully verified with measurements conducted at the FLASH linac. INTRODUCTION Located at DESY, FLASH and currently constructed XFEL [1] are examples of the Linear Particle Accelerators (linacs) built using superconducting technology. Those linacs operate in pulsed mode regime. Because of the high operating gradients reaching up to 40 MV/m, during the pulse, RF cavities are detuned by the magnetic field pressure phenomena. In a RF cavity, electromagnetic field induces surface current and surface charges on the wall of the cavity. Interaction of the surface currents with a standing wave inside cavity generates a pressure, which mechanically deforms the cavity and as a consequence detunes it from the nominal frequency. In the literature it is known as Lorentz force detuning (LFD). Another source of the detuning is caused by external mechanical forces acting on the RF cavities called microphonics. In opposite to the Lorentz force detuning, microphonics are generally not synchronized to the RF operation. Some sources of the microphonics are the helium plant, ground motions and man made machinery [2]. Pulsed mode operation at FLASH consists of the approx. 1.3 ms RF pulses with a repetition rate of 10 Hz. Time during the RF pulse is insufficient to measure the microphonic disturbance and compensate for it during the same pulse. Due to a very high quality factor of superconducting cavities, detuning decreases the power transferred from the RF system. This reduces the RF system efficiency and also makes controlling stability of the accelerating gradient more difficult. In a situationwhere a high power amplifier is shared between many cavities, accelerating field control becomes more complicated, because detuning affects each cavity separately. Conceptually (Fig. 1) it is possible to compensate for microphonics by predicting its level based on the information ∗ radoslaw.rybaniec@desy.de Cavity Microphonics model PIEZO tuner C avity pk-up Forw rd, reected R F snals LLRF Controller LFD compensation Klystron Microphonics compensation PIEZO sensor Figure 1: Block diagram of the proposed microphonics compensation method for a linac operating in pulsed mode. from previous pulses. To enable this, a microphonics model has to be build during an operation of a facility. Possible information sources are RF signals during the pulse and (optionally) piezo sensors output in the time between the pulses. Piezo actuator can be used for fast tuning of the cavity based on the knowledge from the model, in the similar way as it is used for Lorentz force detuning compensation. This could decrease the amount of microphonic disturbances affecting the cavity. In the paper a first implementation of this concept is evaluated. In the first attempt a simple auto-regressive (AR) model for the microphonics is used. Disturbances are estimated using the detuning computed from the RF signals. The method was validated with the open-loop measurements at the FLASH linac. First results show potential reduction of the detuning caused by microphonic disturbances in the accelerators operating in a pulsed mode. MICROPHONIC DISTURBANCES ESTIMATION AND PREDICTION During an operation of the facility, detuning of SRF cavities can be computed [3] using RF signals. This is accomplished with the first order cavity model [4]: dVc dt + (ω1/2 − jΔω)Vc = Kg · Vg, where Vc cavity pick-up signal, ω1/2 half-bandwidth of the cavity, Δω detuning and Kg · Vg is the calibrated generator voltage, calculated from the forward and reflected power signals. After separation of the real (I) and imaginary (Q) parts of the complex numbers, detuning can be Proceedings of IPAC2015, Richmond, VA, USA WEPMN032 7: Accelerator Technology T31 Subsystems, Technology, and Components, Other ISBN 978-3-95450-168-7 2997 Co py rig ht © 20 15 CC -B Y3. 0 an d by th er es pe ct iv ea ut ho rs 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 200 0 200 400 Time (ms) D et un in g (H z)