THE DA  NE 3 RD HARMONIC CAVITY A. Gallo, with D. Alesini, R. Boni, S. Guiducci, F.Marcellini, M. Migliorati, L. Palumbo, M. Zobov

SUMMARY: A)Introduction B)DA  NE 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 DA  NE 3 rd Harmonic Cavity

INTRODUCTION: Motivations for Installing a Harmonic Cavity at DA  NE 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 DA  NE 3 rd Harmonic Cavity

Working point of the main and harmonic RF systems 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 A. Gallo: The DA  NE 3 rd 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 DA  NE 3 rd 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 DA  NE 3 rd Harmonic Cavity

Shift of the coherent synchrotron frequencies of the coupled bunch modes The interaction between the beam and the impedance of the two cavities perturbs the coupled bunch coherent motion (mainly for mode “0”, “1” and “N b -1”) shifting the synchrotron frequency accordingly to: 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 DA  NE 3 rd Harmonic Cavity (R/Q) H =25 

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 DA  NE 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 self- generated long range wakes. A spread of the bunch parasitic losses results. 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. 47 out of 60 bunches A. Gallo: The DA  NE 3 rd 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 DA  NE 3 rd 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. 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 simu- lated 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. LFB ONLFB OFF A. Gallo: The DA  NE 3 rd Harmonic Cavity

Touschek lifetime expected improvements The blue plots are normalized to the KLOE 2002 run case (V RF =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 V RF =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 DA  NE dynamic aperture has been considerably increased. 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. A. Gallo: The DA  NE 3 rd 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.   n=2.5 A. Gallo: The DA  NE 3 rd Harmonic Cavity

DA  NE 3 rd Harmonic Cavity Main Features Round Shape Aluminium Cell; Low R/Q (  25  ); Wide Tunability (-1.5 f rev  3.5 f rev ); KEK-B SBP Ferrite Damper; Cavity to Damper Tapered Connection; No direct Ferrite exposure to the Beam A. Gallo: The DA  NE 3 rd Harmonic Cavity

DA  NE 3 rd Harmonic Cavity: HFSS Simulations Fundamental M 1 mode confined in the cell High Order M 4 monopole damped in the ferrite load |s 21 | monopole port to port transmission by HFSS simulations |s 21 | dipole port to port transmission by HFSS simulations A. Gallo: The DA  NE 3 rd Harmonic Cavity

DA  NE 3 rd Harmonic Cavity: bench measurements DA  NE 3 rd harmonic cavity (with no ferrite damper and tuner) on bench DA  NE 3 rd harmonic cavity port-to-port measurements and mode identification A. Gallo: The DA  NE 3 rd Harmonic Cavity

DA  NE 3 rd Harmonic Cavity: longitudinal impedance measurements M4 monopole no wire wire meas. The impedance of the cavity longitudinal modes have been evaluated by measuring the Q-factors from port- to-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 M 1, because the resonant impedances were not distinguishable. In the case of the M 4 monopole, the wire changed so much the field configuration that the mode resulted undamped and its impedance value unrealistic. A. Gallo: The DA  NE 3 rd Harmonic Cavity

DA  NE 3 rd Harmonic Cavity: transverse impedance measurements In the wire measurements of the transverse impedance of the cavity dipoles the D 1, D 3 and D 6 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. A. Gallo: The DA  NE 3 rd Harmonic Cavity

DA  NE 3 rd Harmonic Cavity: Tuning and Parking DA  NE 3 rd harmonic cavity measured tuning range Port-to-port measurements of the parked cavity (  n=+2.5) MAFIA model of the cavity tuner A. Gallo: The DA  NE 3 rd Harmonic Cavity

CONCLUSIONS: One harmonic cavity per ring passively powered by the beam will be installed in DA  NE 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 DA  NE 3 rd 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 3 rd harmonic) with a tuning plunger with a long stroke.

A. Gallo: The DA  NE 3 rd 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.