FAST RF KICKER DESIGN

Transcription

1 FAST RF KICKER DESIGN David Alesini LNF-INFN, Frascati, Rome, Italy ICFA Mini-Workshop on Deflecting/Crabbing Cavity Applications in Accelerators, Shanghai, April 23-25, 2008

2 FAST STRIPLINE INJECTION KICKERS OUTLINE general considerations: injection with fast stripline kickers, pulse length and transverse field profile properties advantages of tapered striplines DAΦNE new injection kickers: design, HV pulsers R&D, beam coupling impedance, installation and first results on beam commissioning ILC kickers studies: effects of the non-uniformity of deflecting field, tapered strip advantages on beam coupling and transfer impedance INJECTION WITH RF DEFLECTORS: CTF3 case SW RF deflector for the Delay Loop TW RFDs for the Combiner Ring: beam vertical instability induced by RFD, design of damped RF deflectors

3 General considerations: injection with fast stripline kickers and pulse length beam E TEM odd mode +V B -V V IN Generator pulse shape V T ILC T f -2L/c=4σ B /c Injected bunch L=kicker length T r =rise time length Stored bunches T f =flat top length σ B =bunch length T B =bunch spacing T r T f T r t 2T B 2L/c+T r 2L/c+T r t assuming T r =300ps E [GeV] ILC DR 5 DAΦNE 0.51 V T DAΦNE Injection upgrade T B [ns] T B T f [ns] 2 5 t L [cm] 30 70

4 General considerations: transverse field profile properties and circular/elliptical cross sections φ a=25 mm Field uniformity B, E field lines Circular case φ Beam axis Vacuum chamber strip Horizontal component of the electric field (E T ) on the kicker axis as a function of the electrode coverage angle. Circular/elliptical cases Efficiency The profile of deflecting field depends on the coverage angle.

5 Advantages of tapered striplines Tapers are usually used to avoid abrupt steps in the vacuum chamber in order to reduce the intensity of wakefields and HOM (impedance of the machine). The uniformity of deflection depends on the coverage angle. Tapering the transition between the kicker structure and the adjacent beam pipe it is possible to minimize: the non uniformity of transverse deflection as a function of the transverse position; the contribution of the kicker to the machine impedance; the reflection coefficient at high frequency (short pulses) because of smoother transition between feedthrough coax line and strip line. Outer chamber Strip feedthrough Beam axis Kicker structure Constant impedance sections

6 DAΦNE new injection kickers: design (1/2) E [MeV] Bunch spacing [ns] L rings [m] Max numb. Of bunches DAΦNE MAIN PARAMETERS Input ports Strip ceramic supports Elliptical cross section BEAM Output ports (LOAD) HV feedthrough Tapered stripline

7 DAΦNE new injection kickers: design (2/2) The elliptical cross section was originally chosen to minimize the variation of the vertical dimension of the beam pipe between the injection region and the adjacent dipole region and to increase the deflection efficiency. By a tapered stripline kicker it is possible to minimize: a) the non uniformity of transverse deflection as a function of the transverse position; b) the contribution of the kicker to the machine impedance; c) the reflection coefficient at high frequency (short pulses) because of smoother transition between feedthrough coax line and stripline. EXPECTED BENEFITS higher maximum stored currents; Improved stability of colliding beams during injection; less background allowing data acquisition during injection; Field flatness by integration L k /2 L T ±3%

8 DAΦNE new injection kickers: HFSS model a) Ceramic stand-off effects Input ports b) HOM studies c) Real deflecting field calculation and frequency response Optimization of the whole structure Output ports d) Coaxial-strip transition optimization and beam transfer impedance Beam direction

9 DAΦNE new injection kickers: beam coupling and transfer impedance calculation Longitudinal impedance Transfer impedance

10 DAΦNE new injection kickers: parameters PARAMETERS Beam Energy E [MeV] Time spacing between bunches [ns] Deflection [mrad] Total deflecting voltage VT [MV] Total kicker length L [cm] Voltage per strip [kv] Input pulse length [ns] Pulse length seen by bunches [ns] Max rep rate [Hz]

11 DAΦNE new injection kickers: HV R&D Input ports Elliptical cross section HV feedthrough Strip ceramic supports BEAM Output ports (LOAD) Tapered stripline

12 DAΦNE new injection kickers: R&D on HV feedthrough (1/2) When HV is applied the possibility of discharges is higher in the end-section of the kicker electrodes, where the electrode itself is closer to the vacuum tube. HV 50 Ohm (wide band) commercial feedthroughs do not exist and an R&D activity has been necessary. The wide band of the feedthroughs is important to keep low the beam impedance of the kicker even well beyond the frequencies content of the input pulse. A stripline with the same dimension and the same distance from the chamber of the kicker stripline in the end section has been built. Coax ceramic feedthrough have been mounted on this test device and HV tests have been done.

13 DAΦNE new injection kickers: R&D on HV feedthrough (2/2) Several FID GmbH HV pulser have been tested up to the final version under specification: 45 kv, flat top 5 ns A commercial feedthrough (not 50 Ohm) has been initially tested without success. An HV feedthrough at 50 Ohm has been designed, realized and tested at LNF with complete success up to 50 kv with the FID pulser.

14 DAΦNE new injection kickers: HV tests on the new kickers (1/3)

15 DAΦNE new injection kickers: HV tests on the new kickers (2/3) Old pulser (LNF) New pulser (FID) 45 kv 25 kv 5 ns 250 ns

16 DAΦNE new injection kickers: HV tests on the new kickers (3/3) New pulser (FID) Old pulser (LNF)

17 DAΦNE new injection kickers: RF characterization Connectors for RF test with NA

18 DAΦNE new injection kickers: Installation in the DAΦNE rings (Nov. 07) e+ e+ IP The new kickers can be feed by the old pulsers (200 ns) or by the new pulsers (6 ns). First test on e+ ring with fast pulsers have been successfully done. Unfortunately we had problems with the new fast FID pulsers after few hours of operation.

19 ILC kickers studies: effects of the non-uniformity of deflecting field (1/4) STARTING POINT PARAMETERS β x_kick =65 m; β y_kick =20 m A x_max =A y_max =0.09 m rad (injected) Bunches distance = 3.08 ns Considered stripline geometries Geometry 1 y φ/2=45 deg x Geometry 2 25 mm

20 ILC kickers studies: effects of the non-uniformity of deflecting field (2/4) Geometry 1 Geometry 2

21 ILC kickers studies: effects of the non-uniformity of deflecting field (3/4) Horizontal plane Geometry 1 Vertical plane A x_max =0.15 m rad A y_max =0.13 m rad A x_98% =0.11 m rad A y_98% =0.09 m rad

22 ILC kickers studies: effects of the non-uniformity of deflecting field (4/4) Geometry 1 V strip =10kV

23 ILC kickers studies: Tapered strip advantages L tap =50mm L tot =300mm We will built a dedicated tapered stripline kicker to be installed in the KEK

24 INJECTION WITH RF DEFLECTORS: CLIC Test Facility (CTF) CERN Aim: build a small-scale version of the CLIC RF power source, in order to: -demonstrate full beam-loading acceleration -demonstrate electron beam pulse compression and frequency multiplication using RF deflectors: -provide the RF power to test the CLIC accelerating structures and components 3GHz The RF deflectors for the DL and CR have been The DL RFD is SW while the CR RFDs are TW structures.

25 INJECTION WITH RF DEFLECTORS: SW RFD for the DL See F. Marcellini, LHC-CC08 efficency (deflection vs. rf power) per unit length filling time circulator SW high To keep acceptable the difference (less than 1%) of deflection angle between the head and the tail of the train the loaded cavity Q has been reduced. A good compromise between filling time and shunt impedance reduction has been a loaded Q value between 3000 and generally slow (proportional to the quality factor) necessary L K W TW Low generally fast (proportional to the group velocity and the structure length) NOT necessary Instead of circulator a 90 deg hybrid juction has been used to protect the source from reflections L H K W Frequency [GHz] Defl. voltage [MV] DESIGN PARAMETER angle of deflection [mrad] Max. Beam energy [MeV] Klystron output Power [MW] Obtained recombination W W W C C C

26 INJECTION WITH RF DEFLECTORS: TW RFD for the CR Frequency Number of cells TW mode Length Group velocity Filling time Input power Deflection angle 3 [GHz] 10 2π/3 33 [cm] c 47 [ns] 2 MW 5 [mrad] Metallic rods have been inserted to shift the frequency of the deflecting mode with vertical polarity. The dimensions and position of the rods have been choosed in order to avoid the excitation of the vertical modes from the beam power spectrum line at GHz and RF generator. f 50 MHz

27 INJECTION WITH RF DEFLECTORS: recombination and vert. instability (1/3) BUNCH RECOMBINATION AT LOW CHARGE Bunch combination has been obtained for the first time during the CTF3 preliminary phase using the TW deflectors designed for the CR at low charge. x Streak camera images 333 ps BUNCH RECOMBINATION AT HIGH CHARGE In the last commissioning the recombination has been tried at high charge but a strong vertical instability occurred. 83 ps t Stored current Vertical position Horizontal position

28 INJECTION WITH RF DEFLECTORS: recombination and vert. instability (2/3) The instability phenomenology has been studied and, finally, it has been associated to the vertical SW deflecting modes. In fact even if they are shifted in frequency by the rods, beam spectrum lines can excite it and generate the instability it the beam passes vertically off axis. 2 ωrf ω τ One particular vertical mode RF R T 2Q VT () τ q ye sin( ωrfτ ) corresponding to the 2π/3 cell c Q phase advance has the strongest shunt impedance Q f RF GHz R T 1.6 MΩ q=2.33 nc y=5 mm Spectrum of a 200 bunches in the combiner ring in 4 turns (f REV 3.56 MHz) Vertical mode resonance

29 INJECTION WITH RF DEFLECTORS: recombination and vert. instability (3/3) A dedicated tracking code has been written to study the multi-bunch multi-turn effects (see D. Alesini. CTF3 Coll. Meeting, Jan 2008). Vertical beam offset y=2 mm Beam emittance 0.4 mm mrad 1) Good agreement between analytical model/tracking and phenomenology; 2) Strong instability driven by few mm off-axis beam; 3) Tuning dependence study seems suggest that a vertical tune near half integer can reduce the effects on beam dynamics; 4) Particular bunch patterns can also reduce the effects on beam dynamics; 5) The reduction of the vertical β-function at the deflector can also help in the control of the instability; 6) Localized vertical bumps at the deflectors to minimize the vertical residual orbit can reduce the effects of the trapped modes; 7) The reduction of the Q-factors of the modes can reduce the driven force of such vertical modes;

30 INJECTION WITH RF DEFLECTORS: new damped RFD The most efficient way to figth the instability is to absorb the vertical deflecting modes excited by the beam.. New RF deflectors are now in costruction. In the deflectors the rods of each cell have been moved toward the center in order to increase the frequency shift. Moreover each rod has been modified in order to be, in the same time, an absorbing antenna.

31 CONCLUSIONS FAST STRIPLINE INJECTION KICKERS advantages of tapered striplines with respect to conventional design DAΦNE new injection kickers collider successfully installed in the ILC kickers studies: effects of the non-uniformity of deflecting field and advantages of tapered stripline on beam impedance INJECTION WITH RF DEFLECTORS: CTF3 case SW RF deflector for the Delay Loop: design including a 90 deg hybrid junction to avoid circulators TW RFDs for the Combiner Ring: good operation at low charge but vertical beam instability at high currents induced by the vertical modes. Changed of vertical tune will give possibilities to mitigate the instability and new damped RF deflectors are now in conrruction