25 Apr 2008 ... FAST RF KICKER DESIGN. David Alesini. LNF-INFN, Frascati, Rome, Italy. ICFA
Mini-Workshop on Deflecting/Crabbing Cavity Applications in ...
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
OUTLINE FAST STRIPLINE INJECTION KICKERS ⇒ 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
General considerations: injection with fast stripline kickers and pulse length beam
E
TEM odd mode B
+V
-V
VIN L=kicker length Tr=rise time length Tf=flat top length σB=bunch length TB=bunch spacing assuming Tr=300ps
VT
Generator pulse shape
Tf-2L/c=4σB/c
ILC
Injected bunch Stored bunches
Tf
2TB
Tr
Tr ILC DR
DAΦNE
E [GeV]
5
0.51
TB [ns]
3.08
2.7
Tf [ns]
∼2
∼5
L [cm]
∼30
∼70
t
2L/c+Tr
t 2L/c+Tr
VT 2TB
DAΦNE Injection upgrade t
General considerations: transverse field profile properties and circular/elliptical cross sections B, E field lines Vacuum chamber strip
φ
φ
a=25 mm
Efficiency
Horizontal component of the electric field (ET) on the kicker axis as a function of the electrode coverage angle.
Beam axis
Field uniformity Circular case
Circular/elliptical cases
The profile of deflecting field depends on the coverage angle.
Advantages of tapered striplines Tapering the transition between the kicker structure and the adjacent beam pipe it is possible to minimize:
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 non uniformity of transverse deflection as a function of the transverse position;
The uniformity of deflection depends on the coverage angle.
• 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
DAΦNE new injection kickers: design (1/2) DAΦNE MAIN PARAMETERS E [MeV]
510
Bunch spacing [ns]
2.7
L rings [m]
∼97
Max numb. Of bunches
120
Input ports Strip ceramic supports
Elliptical cross section HV feedthrough
BEAM
Output ports (LOAD)
Tapered stripline
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.
EXPECTED BENEFITS
By a tapered stripline kicker it is possible to minimize:
•higher maximum currents;
stored
a) the non uniformity of transverse deflection as a function of the transverse position;
•Improved stability of colliding beams during injection;
b) the contribution of the kicker to the machine impedance;
•less background allowing data acquisition during injection;
c) the reflection coefficient at high frequency (short pulses) because of smoother transition between feedthrough coax line and stripline. Field flatness by integration
LT
Lk/2
±3%
DAΦNE new injection kickers: HFSS model a) Ceramic stand-off effects
Input ports
b) HOM studies c) Real deflecting field calculation and frequency response
Output ports
Beam direction
d) Coaxial-strip transition optimization and beam transfer impedance
Optimization of the whole structure
DAΦNE new injection kickers: beam coupling and transfer impedance calculation
Longitudinal impedance
Transfer impedance
DAΦNE new injection kickers: parameters PARAMETERS Beam Energy E [MeV]
510
Time spacing between bunches [ns]
2.7
Deflection [mrad]
5
Total deflecting voltage VT [MV]
2.5
Total kicker length L [cm]
∼90
Voltage per strip [kV]
45
Input pulse length [ns]
∼5
Pulse length “seen” by bunches [ns]
∼10
Max rep rate [Hz]
10
DAΦNE new injection kickers: HV R&D
Input ports Elliptical cross section
HV feedthrough
Tapered stripline
BEAM
Output ports (LOAD)
Strip ceramic supports
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.
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.
DAΦNE new injection kickers: HV tests on the new kickers (1/3)
DAΦNE new injection kickers: HV tests on the new kickers (2/3)
Old pulser (LNF)
New pulser (FID)
25 kV
45 kV
5 ns
250 ns
DAΦNE new injection kickers: HV tests on the new kickers (3/3) New pulser (FID)
Old pulser (LNF)
DAΦNE new injection kickers: RF characterization Connectors for RF test with NA
DAΦNE new injection kickers: Installation in the DAΦNE rings (Nov. 07)
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.
e+
ILC kickers studies: effects of the non-uniformity of deflecting field (1/4) STARTING POINT PARAMETERS βx_KICK=65 m; βy_KICK=20 m Ax_max=Ay_max=0.09 m⋅rad (injected) Bunches distance = 3.08 ns
Considered stripline geometries Geometry 1
y
φ/2=45 deg
25 mm Geometry 2
x
ILC kickers studies: effects of the non-uniformity of deflecting field (2/4) Geometry 1
Geometry 2
ILC kickers studies: effects of the non-uniformity of deflecting field (3/4) Horizontal plane
Geometry 1
Ax_max=0.15 m⋅rad
Vertical plane
Ay_max=0.13 m⋅rad
Ax_98%=0.11 m⋅rad Ay_98%=0.09 m⋅rad
ILC kickers studies: effects of the non-uniformity of deflecting field (4/4) Geometry 1
Vstrip=10kV
ILC kickers studies: Tapered strip advantages
Ltap=50mm Ltot=300mm
We will built a dedicated tapered stripline kicker to be installed in the ATF @ KEK
INJECTION WITH RF DEFLECTORS: CLIC Test Facility (CTF) 3 @ 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 designed @LNF-INFN. The DL RFD is SW while the CR RFDs are TW structures.
INJECTION WITH RF DEFLECTORS: SW RFD for the DL
DESIGN PARAMETER
SW
TW
efficency (deflection vs. rf power) per unit length
high
Low
filling time
generally slow (proportional to the quality factor)
generally fast (proportional to the group velocity and the structure length)
necessary
NOT necessary
circulator
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 3500.
Frequency [GHz]
1.5
angle of deflection [mrad]
15
Max. Beam energy [MeV]
300
Klystron output Power [MW]
20
Defl. voltage [MV]
4.5
Instead of circulator a 90 deg hybrid juction has been used to protect the source from reflections K
K W
See F. Marcellini, LHC-CC08
L
W
L
H
W
W
W
C
C
C
Obtained recombination
INJECTION WITH RF DEFLECTORS: TW RFD for the CR
Frequency Number of cells TW mode
3 [GHz] 10 2π/3
Length
33 [cm]
Group velocity
-0.024c
Filling time
47 [ns]
Input power
2 MW
Deflection angle
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 2.8855GHz and RF generator.
∆f≅50 MHz
INJECTION WITH RF DEFLECTORS: recombination and vert. instability (1/3) BUNCH RECOMBINATION AT LOW CHARGE
x
Streak camera images
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.
333 ps
BUNCH RECOMBINATION AT HIGH CHARGE 83 ps In the last commissioning the recombination has been tried at high charge but a strong vertical instability occurred.
Stored current
Vertical position
Horizontal position
t
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. One particular vertical mode corresponding to the 2π/3 cell phase advance has the strongest shunt impedance
Q
11500
fRF
∼ 3.0443 GHz
RT
∼1.6 MΩ
Spectrum of a 200 bunches in the combiner ring in 4 turns (fREV≅3.56 MHz)
VT (τ ) ≅ q
2 ω RF RT
c Q
−
ye
ω RF 2Q
τ
sin(ω RFτ )
q=2.33 nC y=5 mm
Vertical mode resonance
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).
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;
Vertical beam offset y=2 mm Beam emittance 0.4 mm mrad
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.
CONCLUSIONS FAST STRIPLINE INJECTION KICKERS ⇒ advantages of tapered striplines with respect to conventional design ⇒ DAΦNE new injection kickers successfully installed in the collider ⇒ 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
