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ILC Crab Cavity Collaboration. Cockcroft Institute : Richard Carter (Lancaster University) Amos Dexter (Lancaster University) Graeme Burt (Lancaster University) Imran Tahir (Lancaster University) Philippe Goudket (ASTeC) Peter McIntosh (ASTeC) Alex Kalinin (ASTeC) Carl Beard (ASTeC)

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Ilc crab cavity collaboration
ILC Crab Cavity Collaboration

  • Cockcroft Institute :

    • Richard Carter (Lancaster University)

    • Amos Dexter (Lancaster University)

    • Graeme Burt (Lancaster University)

    • Imran Tahir (Lancaster University)

    • Philippe Goudket (ASTeC)

    • Peter McIntosh (ASTeC)

    • Alex Kalinin (ASTeC)

    • Carl Beard (ASTeC)

    • Lili Ma (ASTeC)

    • Roger Jones (Manchester University)

  • FNAL

    • Leo Bellantoni

    • Mike Church

    • Tim Koeth

    • Timergali Khabiboulline

    • Nikolay Solyak

  • SLAC

    • Chris Adolphson

    • Kwok Ko

    • Zenghai Li

    • Cho Ng

LC-ABD plenary April 2007


Crab cavity function
Crab Cavity Function

The crab cavity is a deflection cavity operated with a 90o phase shift.

A particle at the centre of the bunch gets no transverse momentum kick and hence no deflection at the IP.

A particle at the front gets a transverse momentum that is equal and opposite to a particle at the back.

The quadrupoles change the rate of rotation of the bunch.

LC-ABD plenary April 2007


Rdr crab cavity parameters
RDR Crab Cavity Parameters

For 500 GeV CM we might use 1 nine cell cavity or two 5 cell cavities

LC-ABD plenary April 2007


Anticipated rf system
Anticipated RF system

spent beam

BPM

Cryostat

  • Minimum requirement for 14 mrad crossing is 1  9 cell or 2  5 - cell cavities per linac

  • 2  9 – cells would provide full redundancy in case of failure

  • Need space for cryostat, input/output couplers, tuning mechanisms…

IP

~ 14 m

RF Amplifier

RF Amplifier

kick reference

from spent beam

Feedbackloop

Feedback loop

luminosity reference from IP

DSP Phase Control

DSP Phase Control

Linac timing

Reference Phase 2

Reference Phase 1

References to and from symmetrically placed crab cavities on other beam

LC-ABD plenary April 2007


Tm110 dipole mode cavity

Beam

TM110 Dipole mode cavity

Electric Field

in red

View from top

Magnetic field

in green

  • The electric and magnetic fields are 90o out of phase.

  • For a crab cavity the bunch centre is at the cell centre when E is max and B is zero.

  • A particle at the centre of the bunch sees no electric or magnetic field throughout .

LC-ABD plenary April 2007


Beamloading
Beamloading

  • Longitudinal electric field on axis is zero for dipole mode

  • Beamloading loading is zero for on axis bunches

  • Bunches pass cavity centre when B transverse = 0 hence of axis E = maximum

  • Crab cavities are loaded by off axis bunches

  • Dipole deflection cavities are not loaded by off axis bunches

  • Power requirement for 9 cells (500 GeV CoM) ~ a few kW

LC-ABD plenary April 2007


Using amplifier to extract cavity energy
Using amplifier to extract cavity energy

For the crab cavity the bunches can supply or remove energy.

Whilst in principle the amplifier can be used to reduce cavity energy after shifting its phase by 180o this is undesirable when one is trying to control cavity phase.

It is desirable to chose a low external Q so this never needs to happen.

Bunch goes through

cavity at time t =0

LC-ABD plenary April 2007


Modelling of cavity amplitude with microphonics
Modelling of cavity amplitude with microphonics

Oscillatory bunch offset of 1 mm and random arrival phase of 1 degree

Drive frequency in GHz = 3.9 GHz

Centre cavity frequency in GHz = 3.9 GHz

Cavity Q factor = 1.0E+09

External Q factor = 3.0E+06

Cavity R over Q (2xFNAL=53 per cell) = 53 ohms

Energy point ILC crab~0.0284J per cell) = 0.0284 J

Amplitude set point = 301670 V

Maximum Amplifier Power per cell = 300 W

Maximum voltage set point (no beam) = 617740 V

Maximum beam offset = 1.0 mm

Maximum bunch phase error = 1.0 deg

Beam offset frequency = 2kHz

Bunch charge (ILC=3.2e-9) = 3.2E-09 C

RF cycles between bunches = 1200

Delay for control system in cycles = 3900

Bunch length = 1 ms

Cavity frequency shift from microphonics = 600 Hz

Cavity vibration frequency = 230 Hz

Initial vibration phase (degrees) = 20 deg

PI controller with 1 ms delay

cpr = 2.5e-6  Qe

cir = 1.0e-10  Qe

cpi = 2.5e-6  Qe

cii = 1.0e-10  Qe

LC-ABD plenary April 2007


Modelling of cavity drive with microphonics
Modelling of cavity drive with microphonics

follows microphonics

follows beam offset

LC-ABD plenary April 2007


Modelling of cavity drive power with microphonics
Modelling of cavity drive power with microphonics

  • A single klystron can’t easily do this but solid state amplifiers can.

  • To work with a Klystron we must lower the external Q

LC-ABD plenary April 2007


Modelling of cavity phase with microphonics
Modelling of cavity phase with microphonics

Oscillatory bunch offset of 1 mm and random arrival phase of 1 degree

Modelling suggests that we might be able to control the phase of the cavity to within 6 milli-degrees of the measured phase error with respect to a local reference.

LC-ABD plenary April 2007


Wire measurement technique
Wire Measurement Technique

Technique employed extensively on X-band structures at SLAC.

Bench measurement provides characterization of:

- mode frequencies

- kick factors

- loss factors

LC-ABD plenary April 2007


Wire theory
Wire Theory

A wire through a uniform reference tube can be regarded as a transmission line characterised by Ro , Lo and Co

A wire through the cavity under investigation is modelled with an additional series impedance Zll / l

The impedance Zll is large close to each

cavity mode . One measures the phase lag q of a wave passing along the wire for

the cavity (DUT) with respect to the plain tube (Ref) then determines Zlland hence kloss from theequations opposite.

As q is measured as a function of frequency one obtains a loss factor at each frequency where Zll is large i.e. for each mode.

LC-ABD plenary April 2007


Impedance measurements
Impedance Measurements

Measurements are underway to measure the loss factors of the dominant modes in the crab cavity.

The coupling impedance has been calculated for a 3 cell cavity in order to investigate systematic errors in the measurements.

LC-ABD plenary April 2007


Long range wakefields
Long Range Wakefields

The long range wakes are found by summing over all modes and all bunches

LC-ABD plenary April 2007


Prototype coupler
Prototype Coupler

A prototype Coupler has been constructed with replaceable tips and variable cavity separation to measure the external Q for a variety of coupler configurations.

A probe coupler will be matched to the ohmic Q of the cavity and calibrated using S11,with the loaded Q being measured using the bandwidth. Then the external Q can be measured from

LC-ABD plenary April 2007


Placet
Placet

The crab cavity has now been inserted into PLACET and bunches have been tracked through the final focus, with and without crab cavities.

The results give good agreement with the previous analytical results, and show little emittance growth.

LC-ABD plenary April 2007


Phase control development
Phase Control Development

Digital Phase Detector Calibration Chart

The phase control work at is focusing on the use of a 1.3GHz digital phase detector with fast 16 bit ADC and DAC conversion.

The digital phase detector offers an absolute measurement without calibration. They may eventually be used alongside phase quadrature measurements with double balanced mixers at an intermediate frequency for enhanced resolution. Used in quadrature double balanced mixers give both phase and amplitude.

A diode detector being used for amplitude measurements at the moment.

Vector modulation available to 4 GHz

Degrees per Volt of a digital phase detector can be amplified to overcome ADC noise.

Ultimate resolution is 5 milli-degree rms for 100kHz bandwidth

LC-ABD plenary April 2007


Phase control implementation for single cavity
Phase Control Implementation for single Cavity

A randomly vibrating tuning stub simulates microphonics in a warm cavity

LC-ABD plenary April 2007


Development of 16 bit DAC & ADC Boards

16 bit ADC Sample Rate = 105MSPS, Latency = 130ns

16 bit DAC Sample Rate = 40MSPS, Settling time = 10ns

Have potential to react to a phase errorof 5 mdeg in 2-3 ms.

LC-ABD plenary April 2007


Status of measurement precision
Status of measurement precision

Precision of the measurement is determined by looking at the jitter on a DAC output using digital amplification inside DSP, when a controller in the DSP is used to phase lock a low Q cavity.

Programmed basic control software in DSP and have demonstrated phase locking of warm cavity to better than 0.01 degrees rms within bandwidth of 50kHz.

Have yet to implement FPGAs hence slow read & write.

Read time by DSP CPU~300 ns

Write time to DAC ~200 ns.

Still using in built functions to get sine and cosine for vector modulator hence processing is a little slow.

Steady State pk-pk phase jitter = +/- 15 mdeg

LC-ABD plenary April 2007


Phase synchronisation i
Phase synchronisation I

  • If cavities are stabilized with respect to local references to ± 0.01 degrees we then seek synchronisation between local references to 0.06 degrees at 3.9 GHz = 43 fs

  • Optical systems have been developed elsewhere that achieve this.

  • Initial plans are to see what can be achieved with coax and s.o.a. RF components.

simple synchronisation scheme

LC-ABD plenary April 2007


Phase synchronisation ii

divide or mix

divide or mix

to 1.3 GHz

to 1.3 GHz

1.3 GHz

master

phase shifter

oscillator

3.9 GHz

master

oscillator

Phase Synchronisation II

Cavity Control

Cavity Control

vector

vector

Note that the phase detectors for each system should be as close as possible to their cavity output couplers.

mod.

mod.

digital phase

digital phase

Ü

Ü

Ü

Ü

D/A

DSP

A/D

D/A

DSP

A/D

detector

detector

synchronous

synchronous

digital phase

output

output

digital pha

se

detector

detector

loop

loop

Load

filter

filter

~ 50 metre

coax link

phase

phase

shifter

shifter

3 dB directional

3dB directional

coupler

coupler

i

nterferometer line

p

recision

length adjustment

reflector

  • managing reflection on the interferometer is the biggest challenge

LC-ABD plenary April 2007


Vertical cryostat phase control tests
Vertical Cryostat Phase Control Tests

Local reference and controller

Local reference and controller

Phase measurement

Line to synchronise the local references must have its length continuously measured to with an accuracy of a few microns

LC-ABD plenary April 2007


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