Bosotti, A

Michelato, P

Pagani, C

Panzeri, N

Paparella, R

Pierini, P

CERN Document Server

 SRF     THE FAST PIEZO-BLADE TUNER FOR SCRF RESONATORS  C. Pagani, A. Bosotti, P. Michelato, N. Panzeri, R. Paparella, P. Pierini INFN Milano LASA    Abstract   The blade tuner concept has been extended to provide the fast tuning capabilities needed for Lorentz Force Detuning (LFD) compensation and microphonics stabilization of superconducting RF cavities. This functionality is achieved in addition to the slow tuning capabilities, extensively proven with the successful experimental program on the superstructures at TTF. Two complete fast/slow tuning systems are being developed in the context of CARE subprograms, one for the high-end ILC case, currently under fabrication, and one for reduced beta elliptical cavities for high intensity pulsed proton injectors.    Contribution to the SRF 2005, Ithaca, New York, USA Work supported by the European Community-Research Infrastructure Activity under the FP6 “Structuring the European Research Area” programme (CARE, contract number RII3-CT-2003-506395).  EU contract number RII3CT-2003-506395 CARE Conf-05-051-SRF1THE FAST PIEZO-BLADE TUNER FOR SCRF RESONATORS C. Pagani, A. Bosotti, P. Michelato, N. Panzeri, R. Paparella, P. Pierini INFN Milano LASAAbstract The blade tuner concept has been extended to provide the fast tuning capabilities needed for Lorentz Force Detuning (LFD) compensation and microphonics stabilization of superconducting RF cavities. This functionality is achieved in addition to the slow tuning capabilities, extensively proven with the successful experimental program on the superstructures at TTF. Two complete fast/slow tuning systems are being developed in the context of CARE subprograms, one for the high-end ILC case, currently under fabrication, and one for reduced beta elliptical cavities for high intensity pulsed proton injectors. 1 INTRODUCTION In the ILC project an accelerating field of 35 MV/m and a filling factor higher than that of the TTF are expected. Unfortunately a such high accelerating field require a suitable solution in order to reduce the Lorentz forces detuning. It has been proven that a piezoelectric tuner system will permit efficient cavity operation at the gradient of 35MV/m [1] but the design requirements are very tight due to the particular working conditions (cryogenic temperature) and to the need to increase the filling factor. For this reason we developed a coaxial tuner solution (blade tuner) inspired by the idea described in [2] and two prototypes were built and tested in CHECHIA horizontal cryostat and on the so called superstructure in 2002 [3]. During both tests the blade tuner performed as expected in terms of stiffness, frequency sensitivity and tuning capabilities. Later on in order to compensate the Lorentz forces detuning foreseen at 35 MV/m, fast elements have been integrated in the blade tuner and the designed configuration is presented in this paper. The compatibility of the proposed solution with the TESLA/ILC cavities has been proved by means of FE analysis while two complete systems are under fabrication for experimental tests. Its main characteristics are no additional longitudinal space required, usability with minor modification with every SC cavity and possible use of different piezoelectric or magnetostrictive actuators. 2 DESIGN REQUIREMENTS In the designing of the tuning device it has to be considered that the static Lorentz force detuning strongly depends from the global stiffness of the cavity. As shown in [4], increasing the boundary stiffness will decrease the static Lorentz force detuning and increase the global stability and strength of the cavity. Moreover in [5] it is shown that an eventual interaction between the cavity natural frequencies and the RF repetition rate could lead to an increase of the Lorentz forces detuning effect. For these reasons the end dishes, helium tank and tuner system have to be as stiffer as possible, but such as to avoid the RF repetition rate. In the case of the TESLA cavities the following parameters have been considered: • bandwidth: 434 Hz • cavity sensitivity to axial stretch: 404 kHz/mm In order to fulfill the tunability requirements of the TESLA project and on the bases of currents warm-cold frequency reproducibility test at the TTF, the values reported in table 1 have been taken into consideration as minimum frequency shift that the tuner must be capable to impose to the cavity.  Table 1: Guidelines for a TESLA cavity tuner.  Frequency range [kHz] Axial movement [µm] Slow tuning ~ 500 ~ 1500 Fast tuning ~ 1 ~ 3  3 TUNER DESIGN The tuning system is made of the blade tuner, driven by a stepping motor which provides the slow tuning capabilities, and a fast part that provides the Lorentz forces compensation. In the present design the fast part can accommodates two piezos of different length (from 36 to 72 mm) but if necessary they can be substituted with magneto-strictive elements. 3.1 The blade tuner for the slow tuning The main tuner mechanism consists of a three-ring bending system connected by welded blades at an angle which transforms the azimuthal rotation (in opposite directions) of the two halves of the central ring into a variation of the distance between the end rings. The azimuthal rotation is provided by a leverage system connected to a stepping motor (see figure 1). The leverage system amplifies the torque of the stepping motor, contemporarily increasing the tuning sensitivity. The maximum axial displacement  foreseen for the blade tuner is w = ±1.5 mm.  In the standard design (reported in [2]) the two end rings are rigidly connected to the He tank, which has been split in two halves joined by a bellow in order to allow the variation of the cavity length.  EU contract number RII3CT-2003-506395 CARE Conf-05-051-SRF2  Figure 1: cinematic description of the tuning system The ring-blade system has been designed with a parametrical model in order to define the structure kinematics and to provide the axial stiffness and tuning force compatibility. The total stiffness matrix obtained by finite element analyses is here reported, while the symbol used are described in figure 2.  +++++−+−+=−MzFzFxwuLCLRLC2791.06056.41675.0422.1827.3126144.33349.08072.1473.4810 6ψ  LCRFZFZFXLCRFX2FXCRMZL2MZMZz xy Figure 2: symbol used in the tuner stiffness matrix 3.2 Integration of fast elements The detailed information obtained by the finite element analysis of the blade tuner has then been used for the integration of a fast tuning action based on piezoelectric actuators, which are subject to several requirements limiting the choice of possible design configurations. First of all, the piezoelectric actuators (piezos) cannot be subject to tension forces, and bending and shear actions need to be carefully avoided. Moreover, in order to maximize their lifetime, a correct preload range needs to be guaranteed under all operating condition. In addition to that, the characteristic behavior of piezo operation at cryogenic temperatures, in terms of stroke and preload effects, is still a subject undergoing a systematic investigation, especially within the CARE program [6]. From all the considerations expressed above, and using as guidelines the typical values of the TESLA type structures in terms of cavity elasticity and tuning requirements (see table 1), we have integrated the piezoelectric elements in the tuner design, by inserting two elements between one of the end rings and the corresponding flange on the He vessel, as shown in figure 3. The position of the piezo actuators has been carefully chosen in order to avoid possible stresses due to the deflection and vibration of the helium tank. Moreover four bars have been introduced in the design: they can be used both for transportation and safety means in case of piezo breakdown. The active elements are kept in position by means of  supports shown in figure 4: they can accommodate elements of length up to 72 mm and a spherical head is used in order to minimize shear and bending forces.  Figure 3: the piezo assisted tuner  Figure 4: supports for the fixing of fast elements 3.3 Characterization of the full system The axial mechanical behavior of the full system (cavity + He tank + tuner) has been investigated parametrically by means of the spring model shown in figure 5. The stiffness of every part, for the TESLA cavity case, is reported in table 2, together with the material types, the elastic properties (Young module) and the legend of symbols used in figure 5. The stiffness values were either derived from FEM analysis or measured experimentally [3]. EU contract number RII3CT-2003-506395 CARE Conf-05-051-SRF3 Figure 5: Equivalent spring models The forces and displacements of every part in the system corresponding to a tuner displacement (δT) of 1 mm (slow tuning action) or a piezo displacement (δp) of 1 µm (fast tuning action) have been evaluated and are reported in table 3.  Table 2: Characteristics used in the spring model. Part Material Stiffness symbol E (MPa) K (N/µm) He tank Ti KH 105000 302.4 Tuner Ti KT 105000 25 Cavity Nb KC 102700 3.023 End dishes and lips NbTi KW 62055 14.0 Piezo PIC 255 KP 45000 2 x 105 Tank bellow Ti KB 105000 0.19  We note here that due to the low stiffness of the end dishes and lips, only 80 % of the displacement applied by the tuner is transferred to the cavity. For the fast tuning action a further contribution comes from the tuner stiffness, and only 74 % of the displacement applied by the piezos is transferred to the cavity. In particular, to provide the values reported in table 1, the piezos have to assure a maximum stroke of ~ 4 µm at 2 K. The use of stiffer lips for the He tank connection would slightly reduce the requirements on tuner/piezo excursion. It is also important to point out that the piezo preload is varying with the slow tuner action, therefore a suitable pre-tuning strategy needs to be assessed to ensure that during the slow tuning phase the preload remains within valid operation limits. Furthermore, we have to be sure that the piezo dynamic load is always lower than its blocking force (the force above which the piezo do not provide stroke). For the prototype case of two piezos, with a cross section of 10 x 10 mm, a blocking force of 4 kN (each) is expected therefore if we assume a load limit of 8 kN (obtained subtracting the 4 kN blocking force from the 12 kN nominal load limit) a maximum slow tuning action of ~ 1600 kHz can be safely applied, which is at least a factor of four higher than the current warm-cold frequency reproducibility at the TTF. For sake of safety a lower preload value is preferable.. The experimental tests planned in the context of the CARE program to assess the thermal expansion of different piezo-ceramic materials together with the cold tests of the tuner prototypes (see below for details), will further reduce the overall uncertainty of the preload at the tuner working point. Moreover, the needed cold tuning action is expected to further reduce for the case of a big industrial cavity production, like the one needed by the XFEL or ILC cases.   Table 3: Axial forces and displacements for a tuner elongation of 1 mm (slow tuning action) and for a piezo elongation of 1 µm (fast tuning action). Load case: δT = 1 mm (slow tuning) Part Axial force (N) Axial displ. (mm) He + dishes -2435.1 -0.182 Tuner -2622.7 1.000 Cavity 2435.1 0.805 Piezo -2622.7 / 2 -0.013 He bellow 187.6 0.987 Load case δP = 1 µm (fast tuning) Part Axial force (N) Axial displ. (µm) He + dishes -2229.1 -0.167 Tuner -2400.9 -0.096 Cavity 2229.1 0.737 Piezo -2400.9 1.000 He bellow 171.7 0.904  4 COMPATIBILITY Compatibility of the presented solution with the TTF design has been checked in terms of transverse displacements (due to the gravity load) and mechanical eigenmodes. 4.1 Bending analysis A bending analysis has been performed in order to check the vertical displacements of the cavity subject to its weight and to the weight of the tuner. This analysis is necessary because of the modification introduced in the helium tank design. We point out that, in order to reduce the displacements, the pads at the coupler side (on the right in figure 6) have been shifted by 150 mm towards to the tuner position. An axisymmetric model with non axisymmetric load has been implemented: it consists of  1713 2-nodes SHELL elements, 539 4-nodes PLANE elements and a total of 2369 nodes (see figure 6), it accounts for the gravity load and neglects any vibration or external perturbation. In this configuration the tuner is considered only as a load and the stiffener effects due to the four safety bars are not considered. The total weight (cavity plus helium tank and piezo blade tuner) is of 598 N and the maximum vertical displacement of the cavity is of 0.144 mm in the up EU contract number RII3CT-2003-506395 CARE Conf-05-051-SRF4direction at the right end. Because the central lowering is of 0.115 mm (see figure 7) the maximum sagging of the cavity respect to its ends is of 0.115 + 0.144 = 0.259 mm, smaller than the specifications required for the fabrication tolerances of the cells (0.6 mm). The most important displacements are reported in Table 4.   Figure 6: FE model for the analysis of displacements   Figure 7: vertical displacements (dead loads)  Table 4: vertical displacements  Position Helium tank displ. (mm) Cavity displ. (mm) Left end +0.027 +0.063 Center -0.074 -0.115 Right end +0.100 +0.144 Sagging -0.127 -0.259  Neglecting the stress concentrations near the welding point, that are not accurately meshed, the maximum stress in the bellow is equal to 6.3 MPa, lower than the admissible stress (see figure 8).   Figure 8: stresses in the bellow due to the dead loads and tuner weight 4.2 Eigenfrequency analysis As seen in the design requirements it is important to check that the structure eigenfrequencies does not coincide with the frequency of RF repetition rate (1÷10 Hz). This is verified as shown in table 5. Table 5: eigenfrequencies of assembly HT + cavity n. f [Hz] Mode Shape 1 27.77  2 51.66  3 116.1  4 305.9  5 ADAPTATION TO LOW BETA ELLIPTICAL STRUCTURES The simple geometry of the tuning device allows to adapt it to other cavities with minor modification. In particular, in the context of the HIPPI subproject of the CARE program [7], our group is going to equip the two low beta (0.47) 700 MHz elliptical cavities, developed for the TRASCO project, for high power pulsed measurements in CRYHOLAB at CEA/Saclay, to verify the feasibility of LFD compensation of these structures. The cavities have been tested in CW conditions up to 17 MV/m (61 MV/m peak electric and 100 mT peak magnetic fields) [8]. The test of these structures under pulsed operation is particularly significant for the feasibility studies of superconducting high power proton drivers.   Figure 9: The preliminary layout of the scaled coaxial tuner for the 700 MHz 0.5 beta TRASCO cavity.  On the basis of the design procedures described in the preceding sections, a scaling of the coaxial tuner, and its adaptation to the TRASCO cavity case, has been performed, and the layout is shown in figure 9. The EU contract number RII3CT-2003-506395 CARE Conf-05-051-SRF5construction of the tuner prototypes for the HIPPI program is foreseen in late 2006, thus the final detailed engineering of the tuner will be finalized after the end of the tests that will be performed at DESY on the TESLA type tuner. 6 CONCLUSIONS The functionalities of the coaxial tuner equipped with fast element have been described pointing out the requirements that drove its design. Several analyses by means of simple spring models or by means of finite element method allowed us to verify the compatibility of the device with the specification of the TTF/Tesla cavities. A major topic is represented by the performances of piezo actuators (or any other fast element used) at cryogenic temperature, but their behaviour is under study in many laboratories. We plan to perform soon a test of a complete blade tuner prototype integrating an fast capability for LFD and microphonics compensation. We expect the tests will provide the final qualification of this solution, which is attractive in terms of compactness and longitudinal clearance needs.  7 REFERENCES [1] M. Liepe, W.D.-Moeller, S.N. Simrock, “Dynamic Lorentz Force Compensation with a Fast Piezoelectric Tuner”, Proceedings of the 2001 Particle Accelerator Conference, Chicago, p. 1074-1076 [2]  H. Kaiser, “New Approaches to Tuning of TESLA Resonators”, in Proceedings of SRF Workshop 1999, Santa Fe, USA. [3] D. Barni, A. Bosotti, C. Pagani, R. Lange, H.B. Peters, “A new tuner for Tesla”, Proceedings of EPAC 2002, Paris, France, 2205-2207 [4] TESLA Technical Design Report, DESY 2001-011, 2001 [5]  R. Mitchell, K. Matsumoto, G. Ciovati, K. Davis, K. Macha, R.Sundelin, “Lorentz force detuning analysis of the spallation neutron source (SNS) accelerating cavities”, 10th Workshop On RF Superconductivity (SRF 2001), Tsukuba (JP), 09/06/2001--09/11/2001. [6] P. Sekalski et al., “Static Absolute Force Measurement for Preloaded Piezoelements used for Fast Lorentz Force Detuning System”, in Proceedings of Linac 2004, Lubeck, Germany, p. 486. [7] See http://mgt-hippi.web.cern.ch/mgt-hippi/     and http://esgard.lal.in2p3.fr/  for an outline of the CARE program and the HIPPI activities. [8] A. Bosotti et al. “RF Tests of the Beta=0.5 Five Cell TRASCO Cavities”, in Proceedings of EPAC2004, Lucerne, Switzerland, p. 1024.   EU contract number RII3CT-2003-506395 CARE Conf-05-051-SRF6

The Fast Piezo-blade Tuner For Scrf Resonators

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