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THE DAFNE 3RD HARMONIC CAVITY A. Gallo , with D. Alesini, R. Boni, S. Guiducci, F.Marcellini, M. Migliorati, L. Palumbo, M. Zobov A. Gallo: The DAFNE 3rd Harmonic Cavity SUMMARY: A) Introduction B) DAFNE Beam Dynamics with the harmonic cavity • • • • Shift of the coupled bunch mode coherent synchrotron frequencies Spread of the bunch synchronous phases generated by a gap in the bunch filling pattern Bunch lengthening process in the double RF voltage regime Touschek lifetime expected improvements C) Harmonic Cavity Design, Construction and Measurements • • • Cavity profile design Integration of the KEK-B SC cavity HOM damper in the design Cavity construction and bench measurements A. Gallo: The DAFNE 3rd Harmonic Cavity INTRODUCTION: Motivations for Installing a Harmonic Cavity at DAFNE • Improving the Touschek lifetime by lengthening the bunch (*) • Improving the Landau damping coming from the non-linearity of the voltage along the bunch • Adding one degree of freedom to the ring longitudinal focusing, that allows setting more independently the bunch length and the RF acceptance. In this way a whole class of beam experiments becomes available. (*) not only: also by increasing the ring dynamic aperture and the RF acceptance A. Gallo: The DAFNE 3rd Harmonic Cavity If the harmonic voltage is phased to reduce the total RF slope, the bunch natural length increases. The further lengthening produced by the ring wakes can be estimated by a multiparticle tracking simulation Working point of the main and harmonic RF systems Main RF voltage Main cavity shunt impedance Main cavity Q-factor Main cavity input coupling factor RF harmonic frequency RF harmonic voltage Harmonic cavity shunt impedance Harmonic cavity Q-factor Natural bunch length Bunch length (lengthening regime) RF acceptance Typical lifetime (colliding beams) VRF [kV] KLOE Run 2002 110 2004 Run 200 Rs=V2/2PRF [M] 1.9 1.9 Q0 31500 31500 4.6 4.6 fH [MHz] VH [kV] ----- 1104.87=3 fRF 56 RH=V2/2PH [M] --- 0.48 Q 0H z0 [cm] --1.6 (@Ib 0 mA) 18500 2.5 (@Ib 0 mA) z [cm] 2.5 (@Ib 20 mA) 3.1 (@Ib 35 mA) RF/E 0.5 % 0.7 % 3600 (@Ib 17 mA) 2500 (@Ib 34 mA) T [s] 1300 (@Ib 17 mA) A. Gallo: The DAFNE 3rd Harmonic Cavity The required harmonic voltage is very moderate (56 kV), while the stored multibunch current is quite large (typically 1A). The harmonic voltage can be easily generated from the passive beam excitation of one cavity per ring. The passive option is far less complicated and expensive compared to the active one. In this case the cavity efficiency is not the first priority and the cavity design can be mainly addressed to the HOM suppression In the passive mode the cavity has to be progressively detuned from the 3rd harmonic line as the current increases to keep the harmonic voltage constant. The harmonic voltage can be switched off by tuning the cavity far from the 3rd harmonic line of the beam spectrum and in between two revolution harmonics (parking option). A. Gallo: The DAFNE 3rd Harmonic Cavity Beam Dynamics Issues: A) Shift of the coherent synchrotron frequencies of the coupled bunch modes B) Spread of the bunch synchronous phases generated by a gap in the bunch filling pattern C) Bunch lengthening process in the double RF voltage regime D) Touschek lifetime expected improvements E) Beam dynamics in the cavity parking option A. Gallo: The DAFNE 3rd Harmonic Cavity Shift of the coherent synchrotron frequencies of the coupled bunch modes The interaction between the beam and impedance of the two cavities perturbs coupled bunch coherent motion (mainly mode “0”, “1” and “Nb-1”) shifting synchrotron frequency accordingly to: 2 c 1 2 I c i n i2 2h E e 2 i2 RF c the the for the pZ i ( p r )e p r t ; i any integer 2 p iN b n c VRF sin s 3VH sin H 2h E e (R/Q)H=25 The reduction of the mode #0 coherent frequency is not dangerous since the motion is heavily Robinson damped. A weak excitation of mode #1 is expected to be damped by the Longitudinal Feedback System. A. Gallo: The DAFNE 3rd Harmonic Cavity Spread of the bunch synchronous phases generated by a gap in the bunch filling pattern A 15÷25 % gap in the bunch filling pattern is required in DAFNE operation to avoid ion trapping in the e- ring. Macro-particle tracking simulations predict the behaviour of non-uniform beams in the ring. Different bunches along the train experience different kicks from the selfgenerated long range wakes. A spread of the bunch parasitic losses results. 47 out of 60 bunches The parasitic loss spread is converted in a synchronous phase spread by the RF voltage curve. Being the local RF slope reduced by the harmonic voltage contribution, the synchronous phase spread is much enlarged compared to the no harmonic cavity case. A. Gallo: The DAFNE 3rd Harmonic Cavity Lengthening process of the bunches along the train in the double RF voltage regime The bunch centroids occupy different positions along the total RF voltage (which is largely non-linear). Then each bunch seats at a different RF slope and have its own synchrotron frequency and charge distribution. So, each bunch has its own "natural" length, its equilibrium profile (in the lengthening regime) and, in the end, its own Touschek lifetime. A. Gallo: The DAFNE 3rd Harmonic Cavity Spread of the bunch synchronous phases: effects on the Longitudinal Feedback System performances A large spread of the synchronous phases can change the position of the interaction point (IP) from bunch to bunch affecting the luminosity if some bunches collide out of the vertical -function waist. If the synchronous phase spread is equal in the two beams, the IP positions remain fixed and only the collision times vary with respect to the RF clock. LFB ON LFB OFF A large synchronous phase spread can also affect the LFB performances if the front-end and/or the back-end hardware lose its synchronization over some bunches. The tracked oscillations of bunches #1, 24 and 47 simulated for a 1.6 A, 47 bunches beam are shown. Under these conditions the LFB seems still effective, while also a Landau contribution to the beam stability is visible. A. Gallo: The DAFNE 3rd Harmonic Cavity Touschek lifetime expected improvements Lifetime evaluations have been made by a dedicated code that takes into account the limited physical aperture of the vacuum chamber but not the limitations in the momentum acceptance coming from the ring dynamic aperture. Bunch currents of 17 and 34 mA have been considered. The blue plots are normalized to the KLOE 2002 run case (VRF=110 kV) and the average lifetime improvement over the bunch train is expected to be 80%. The red plots are normalized to the the case of a single voltage VRF=200 kV and the average lifetime improvement over the train, coming only by the bunch lengthening in this case, is 35%. These computed factors are now more realistic since recently the DAFNE dynamic aperture has been considerably increased. A. Gallo: The DAFNE 3rd Harmonic Cavity The cavity parking option The harmonic voltage can be almost completely switched off by tuning the cavity away from the 3rd harmonic of the RF and in-between two beam revolution harmonics. The coupled bunch modes having their unstable sidebands near the adjacent harmonics are only weakly excited, while the synchronous phase spread much less emphasized. Dn=2.5 A. Gallo: The DAFNE 3rd Harmonic Cavity DAFNE 3rd Harmonic Cavity Main Features • Round Shape Aluminium Cell; • Low R/Q (25 ); • Wide Tunability (-1.5 frev 3.5 frev); • KEK-B SBP Ferrite Damper; • Cavity to Damper Tapered Connection; • No direct Ferrite exposure to the Beam A. Gallo: The DAFNE 3rd Harmonic Cavity DAFNE 3rd Harmonic Cavity: HFSS Simulations |s21| monopole port to port transmission by HFSS simulations Fundamental M1 mode confined in the cell High Order M4 monopole damped in the ferrite load |s21| dipole port to port transmission by HFSS simulations A. Gallo: The DAFNE 3rd Harmonic Cavity DAFNE 3rd Harmonic Cavity: bench measurements DAFNE 3rd harmonic cavity (with no ferrite damper and tuner) on bench DAFNE 3rd harmonic cavity port-to-port measurements and mode identification A. Gallo: The DAFNE 3rd Harmonic Cavity DAFNE 3rd Harmonic Cavity: longitudinal impedance measurements MAFIA Simulations Measurements M4 monopole M4 monopole no wire wire meas. M1 M2 M3 M4 M5 M6 M7 M8 M9 f [GHz] 1.105 1.335 1.600 1.650 1.899 2.094 2.270 2.495 2.524 Q R/Q [] 23000 26 10 16 30 6 50 2 50 4 110 7 120 9 170 3 230 10 f [GHz] 1.105 Q R/Q [] 16500 21.4 not measurable (n.m.) n.m. n.m. n.m. n.m. n.m. 1.65 168 16.8 n.m. n.m. n.m. 2.100 2289 2.466 2.507 224 60 140 278 n.m. n.m. n.m. n.m. The impedance of the cavity longitudinal modes have been evaluated by measuring the Q-factors from portto-port measurements and the R/Q factors with the wire excitation method. The wire measurements have given no clear results, with the exception of the fundamental mode M1, because the resonant impedances were not distinguishable. In the case of the M4 monopole, the wire changed so much the field configuration that the mode resulted undamped and its impedance value unrealistic. A. Gallo: The DAFNE 3rd Harmonic Cavity DAFNE 3rd Harmonic Cavity: transverse impedance measurements In the wire measurements of the transverse impedance of the cavity dipoles the D1, D3 and D6 modes were recognizable. Their estimated impedance is reported in the table below, and their contribution to the machine transverse impedance is substantially smaller than that given by the less damped dipole modes of the main RF cavity. D1 D2 D3 D4 D5 D6 f [GHz] 1.089 1.244 1.445 1.618 1.797 1.886 Simulations Q R/Q [/m] 438 66 35 26 158 22 158 29 266 37 283 24 Measurements f [GHz] Q R/Q [/m] 1.070 450 146 not measurable (n.m.) n.m. n.m. 1.400 1.560 1.725 1.865 139 175 163 190 29 n.m. n.m. 74 A. Gallo: The DAFNE 3rd Harmonic Cavity DAFNE 3rd Harmonic Cavity: Tuning and Parking MAFIA model of the cavity tuner Port-to-port measurements of the parked cavity (Dn=+2.5) DAFNE 3rd harmonic cavity measured tuning range A. Gallo: The DAFNE 3rd Harmonic Cavity CONCLUSIONS: • One harmonic cavity per ring passively powered by the beam will be installed in DAFNE in the near future (middle 2004?) to lengthen the bunches. The expected Touschek lifetime increase, due also to improvements of the dynamic aperture and RF acceptance, is 80%; • Positive effects on the beam dynamics due to larger Landau damping and larger natural bunch length are also expected; • The shift of the coupled bunch coherent synchrotron frequencies are under control, since the R/Q of the passive cavity is not too high; • The presence of a gap in the bunch filling pattern will produce a large spread of the synchronous phases. Different bunches will collide at slightly different IPs and the synchronization of the bunch-by-bunch feedback systems may be affected; • The actual tolerability of such effects can not be exactly predicted since it depends on the operating conditions (such as the gap width); A. Gallo: The DAFNE 3rd Harmonic Cavity CONCLUSIONS (cnt’d): • The bunch charge distribution changes for different bunches along the train and the Touschek lifetime gain is not uniform over the train; • The “parking option”, which virtually switch-off the harmonic voltage, can be considered as a reliable back-up procedure in case the effects of the gap in the filling pattern will result unmanageable; • Two cavities have been designed, built and tested on bench. A very good suppression of the HOMs has been obtained by incorporating in the design the SBP ferrite damper of the KEK-B SC cavities; • The bench measurements are in substantial agreement with computer simulations based on the MAFIA and HFSS codes; • The cavity can be tuned over a wide range (5 revolution harmonics around the RF 3rd harmonic) with a tuning plunger with a long stroke. A. Gallo: The DAFNE 3rd Harmonic Cavity Thanks to Dr. Furuja and to the KEK-B staff for providing us 3 SBP ferrite dampers, together with their expertise to make them work.