RF DEFLECTORS AT CTF3

BNL/US-LARP/CARE-HHH MINI-WORKSHOP ON CRAB CAVITIES FOR THE LHCBrookhaven National Laboratory, Upton NY Feb 25-26, 2008

Fabio Marcellini

Presentation OUTLINE

• CTF3 and bunch combination with RF deflectors.

• RF deflector design for the Delay Loop.• Results of operation with DL RFD.

• RFDs design for the Combiner Ring.• Results of operation with CR RFDs.

• Beam vertical instability induced by RFD.• New deflector design (in progress).

Motivation and goals of CTF3 collaboration

Build a small-scale version of the CLIC RF power source, in order to demonstrate:

full beam-loading accelerator operation electron beam pulse compression and frequency multiplication using RF deflectors

Provide the RF power to test the CLIC accelerating structures and components

CTF3 – Layout

DRIVE BEAM LINAC

fully loaded acceleration

DELAY LOOPrecombination x 2

phase-coding

bunch length control

COMBINER RING

recombination x 4

PETS on-off

two-beamacceleration

structures 12 GHz

structures 30 GHz

4 A – 1.2 s150 Mev

30 A – 140 ns150 Mev

10 m

CLEXCLIC Experimental Area

R. Corsini: CTF3 collaboration meeting, Jan 08

Streak camera images of the beam, showing the bunch combination process in the CTF3 preliminary phase

Bunch combination at low charge

RF deflectors are special injection kickers essential in the bunch interleaving process to obtain compressed bunch trains with bunch rep rate beyond 10 GHz.

Bunch combination has been obtained for the first time during the CTF3 preliminary phase using the TW deflectors designed for the CR.

333 ps

t

x

83 ps

INFN-Frascati has designed and built RF deflectors for injection and extraction in:

DELAY LOOP

• STANDING WAVE STRUCTURE•high efficency (deflection vs. rf power) per unit length.•filling time proportional to the quality factor: generally slow.•deflecting fields scales as the square root of the number of cells.•in pulsed regime a circulator is generally needed but...

• 2 CAVITIES and HYBRID JUNCTION → NO CIRCULATOR

COMBINER RING

• TRAVELING WAVE STRUCTURE•low efficency (deflection vs. rf power) per unit length.•filling time proportional to the group velocity and the structure length: generally fast.•deflecting fields scales as number of cells.•circulator not necessary.

Even and odd trains are deflected by kicks of the same amplitude but opposite sign. Only even trains are injected into the ring.

DESIGN PARAMETER

Frequency [GHz] 1.499275

angle of deflection [mrad] 15

Max. Beam energy [MeV] 300

Klystron output Power [MW]

20

Pulse length [s] 5

The required deflection is too large to get with a traveling wave structure of reasonable dimensions. It is necessary to resort to a standing wave resonator.

The CTF3 Delay Loop

The major drawback of this choice is the slow filling time of a resonant SW cavity. To keep acceptable the difference (less than 1%) of deflection angle between the head and the tail of the train the cavity Q must be reduced, but not beyond a certain threshold, when the shunt impedance becomes too low.A good compromise is obtained with a loaded Q value between 3000 and 3500.

Coupling hole70X38 rectangular beam pipe

WR650 wg

Pill box shaped cavity

CAVITY VOLTAGE vs. TIME

0 1 2 3 4 5 60

0.2

0.4

0.6

0.8

1

t [ns]

V/V

ma

x

t' t''

[V(t')-V(t'')] < 0.01

Train length

[V(t’)-V(t”)]<0.01

The cavity deflector has been designed starting from a simple pill box shape.The cavity is externally coupled to a rectangular waveguide (WR650, the same standard of the klystron output) through a hole. The hole dimensions set the input coupling coefficient and they have been chosen to obtain the wanted cavity loaded Q.

Delay Loop Deflector

b

-0.001

-0.0005

0

0.0005

0.001

-50

0

50

100

150

200

250

300

40 60 80 100 120 140

f -f0 [GHz] delta Q

L [%]

b [mm]

-0.001

-0.0005

0

0.0005

0.001

-10

-5

0

5

10

15

24.6 24.8 25 25.2 25.4

f -f0 [GHz]

delta QL [%]

coupling hole radius [mm]-0.001

-0.0005

0

0.0005

0.001

-10

0

10

20

30

40

3.5 4.2 4.9 5.6 6.3 7

f -f0 [GHz]

delta QL [%]

coupling hole thickness [mm]

-0.01

-0.005

0

0.005

0.01

-1

0

1

2

3

4

5

119 119.3 119.7 120 120.4

f -f0 [GHz] delta Q

L [%]

cavity radius [mm]

Sensitivity of the resonant frequency and of the loaded Q to small variations of the main dimensional parameters has been evaluated.

coupling hole axis

Delay Loop Deflector

=5.42

9.2 19max

4.152 15

6.257 20

9.562 30

diametertunerDepthInsertionID

MHzfmmID

mmkHzID

fmm

mmkHzID

fmm

mmkHzID

fmm

tuner

Tuning sensitivity and range of tuning

Delay Loop Deflector

1.45 1.46 1.47 1.48 1.49 1.5 1.51 1.52-110

-100

-90

-80

-70

-60

-50

f [GHz]

S2

1 [

dB

]

TM110horizontal polarization

TM110vertical polarization

Resonant frequencies of the most dangerous parasitic modes for the beam dynamics (monopoles and dipoles) are far enough from the lines of the beam power spectrum.

Example of HOM (octupole–2.98GHz). Electric field (magnitude) representation.

CAVITY PARASITIC MODES

The vertical polarization of the TM110 results more than 40 MHz apart from the horizontal one.

Distribution of losses on the cavity walls.

Average power dissipated in each cavity is

2.5 kW at full power operation (i.e. 20 MW

klystron output peak power).

Each cavity has been provided with 5 coils

for cooling.

THERMAL ANALYSIS

Delay Loop Deflector

TIME DOMAIN CAVITY RESPONSE

The reflected power depends on:• the cavity input coupling coefficient• the pulse rise time

Reflected power for 2 arbitrary slopes of the input pulseBlue – RF input pulse. Red – cavity reflected power.

Power coming from klystron is split in equal parts by the hybrid and feeds two cavities, excited in the TM110 deflecting mode. Power reflected at the cavity inputs add in phase at the fourth port of the hybrid, where it is connected a load. In principle no power reaches the klystron.With 2 cavities the total shunt impedance is doubled. Deflecting voltage results increased by a factor √2.

Klystron needs to be isolated from the reflected power.This can be done by a circulator......or by a 90 deg hybrid junction similarly to the sled scheme used in the linac technology.

K

W

L

W

C

K

W

L

W

C

H

K

W

WW

L

CC

H

K

W

WW

L

CC

Delay Loop Deflector

WHOLE DEFLECTOR STRUCTURE DESIGN

Basic components:

1)Two identical cavities (the same described above).

2) One 3dB hybrid coupler.

The hybrid is longitudinally aligned at the center of the two cavities.

The fields in the two cavities resonates in quadrature.

Then the cavities have to be placed an odd multiple integer of /4 of the RF wavelength apart along the beam line to kick the beam with the same amplitude and phase.

For reasons of space the distance between the gaps has been chosen 250mm, i.e. 5/4 RF

Delay Loop Deflector

Septa magnets

port for vacuum pumping

directional couplers

RF windows SF6/vacuum

3 dB coupler

deflector cavities

Delay Loop Deflector

1.45 1.46 1.47 1.48 1.49 1.5 1.51 1.52-110

-100

-90

-80

-70

-60

-50

f [GHz]

S21

[dB

]

TM110horizontal polarization

TM110vertical polarization

1.44 1.46 1.48 1.5 1.52 1.54 1.56

x 109

-100

-90

-80

-70

-60

-50

-40

1.4986 1.4988 1.499 1.4992 1.4994 1.4996 1.4998

x 109

-39

-38

-37

-36

-35

-34

-33

-32

-31

-30mags11

1.4986 1.4988 1.499 1.4992 1.4994 1.4996 1.4998

x 109

0.7

0.75

0.8

0.85

0.9

0.95mags21

1.4986 1.4988 1.499 1.4992 1.4994 1.4996 1.4998

x 109

-50

-48

-46

-44

-42

-40

-38

-36

-34

-32

-30mags22

1.4986 1.4988 1.499 1.4992 1.4994 1.4996 1.4998

x 109

1

1.005

1.01

1.015

1.02

1.025

1.03

1.035

1.04

1.045

1.05vswr

The frequency responses of each component of the system have been measured to verify their correspondence with the design specification and with the results obtained from HFSS simulation code.To optimize the isolation of the klystron from the reflected power the impedance sees from the cavity side ports of the HJ must be as equal as possible. Accurate tuning of the resonant frequency of the cavity operating mode is therefore necessary.

1.497 1.498 1.499 1.5 1.501 1.5020

0.2

0.4

0.6

0.8

1

f [GHz]

mag

nitu

de S11

S21

_____ S21_____ S11

1.497 1.498 1.499 1.5 1.501 1.5020

0.2

0.4

0.6

0.8

1

f [GHz]

mag

nitu

de S11

S21

_____ S21_____ S11

The two polarizations of TM110 deflecting mode.HFSS simulation (above)Measured (below)Measured transmission

between klystron and load ports.

Reflection at the klystron port (red). Transmission between klystron and load ports (green). HFSS result.

1.498775 1.499275 1.499775

-36

-34

-32

-30

-28

-26

-24

-22

frequency [GHz]m

ag [d

B]

S11 measured at the HJ input port

1.489275 1.494275 1.499275 1.504275 1.509275

89.5

90

90.5

frequency [GHz]

phas

e [d

eg]

phase difference between the signals taken from the two cavities fed through the HJ

Delay Loop Deflector

Beam current: in transfer line before the DL (black), in the DL (red)and in transfer line after recombination (blue)

Beam recombination in the Delay Loop (factor 2)

2nd deflector

RF deflector fieldo

injection lineseptum

localinner orbits

1st deflector

o/4

• The orbit must have the lengthC = (nt ± ¼) (it is „-” in our case)so the beam arrives exactly:

The CTF3 Combiner Ring

– 1st turn: at zero crossing phase

-- 2nd turn: at the crest

– 3rd turn: at zero crossing again

But opposite amplitude than at the injection

• Adjust the length with the wiggler

MECHANICAL DRAWING

FABRICATION STEPS

PROTOTYPE

Combiner Ring Deflector

SCALING LENGELER’S RESULTS FOR 2.855GHz T.W. DEFLECTING STRUCTURES

2.6 A

8.5 A

10.4 A

Achieved recombination:

• Linac current lower than nominal

• DL bypassed (no holes, missing factor 2)

• Losses during recombination (instability…)

R. Corsini: CTF3 collaboration meeting, Jan 08

7.5 A

30 A current !

Vertical Instability: what has been observed.

Sum & difference signals, show the instability develops in the vertical plane from turn to turn

Stored current

Horizontal position

Vertical position

Analog signals in a CR BPM

Very repetitive pattern

several consecutive shots with the scope in persistence mode

Losses depend strongly on the pulse length

230ns

130ns

60ns

• Increasing the charge in the bunches the instability becomes stronger.

• From FFT, the frequency content of the vertical difference signal has a peak at 47 MHz from the fundamental frequency (2.99855 GHz).

• The instability occurs in case of both a single train doing different turns and different recombined trains.

Moreover:

VERTICAL MODES IN THE RF DEFLECTORS

Metallic rods have been inserted to shift the frequencies of the vertical polarity deflecting modes.

Dimensions and position of the rods have been chosen to move the vertical modes out of the klystron bandwidth.

Dispersion curve of the vertical modes have been measured before the final assembling.

VERTICAL MODES IN THE RF DEFLECTORS (HFSS)

frequency shunt impedance

Q

Frequency of the highest impedance mode is 3.0443 GHz 46 MHz above the RF frequency!

SPECTRUM OF A 200 BUNCHES IN THE COMBINER RING IN 4 TURNS (FREV3.56 MHZ)

1) Good agreement between analytical model/tracking and observed phenomena;

2) Strong instability driven by only few mm off-axis beam;3) Reduction of vertical -function at the deflector, minimization of the vertical

residual orbit, proper choice of tune and bunch patterns, can help in the control of the instability;

4) Lowering the Q-factors of the vertical modes by damping can reduce the driving force of such modes.

TRACKING CODERF deflector

MCR

A dedicated tracking code has been written to study the multi-bunch multi- turn effects.

Only 1 RF deflector and 1 mode (dominant) considered.

RESULTS (see D. Alesini: CTF3 collaboration meeting, Jan 08)

MODIFY THE PRESENT RF DEFLECTORS

>30 mm

ADDING TWO WAVEGUIDE WHICH COUPLE THE VERTICAL POLARITY

MAKE NEW RF DEFLECTORS

Titanium surface coating

Aluminum instead of copperWelded, not brazed

Pout_2/Pin=6%

Damping could be not sufficient in any case!

Vertical wgs should be moved on more

central cells.

MAKE NEW RF DEFLECTORS

Stronger damping could be realized if the vertical polarity is coupled out of every single cell (except the two terminals) through holes or antennas.