RF Synchronisation Issues

Xband Linacs for FELs

Lancaster University

October 2014

Red linac sections are X bandOff crest acceleration provides bunch compression Phase errors in linac RF cavities gives unwanted beam energy spread in undulatorAverage phase in linac RF cavities must be correct to within 0.02 degrees ~ 5 fs

RF Systems and Stability pmi@slac.stanford.edu

Linac Coherent Light Source Stanford Synchrotron Radiation LaboratoryStanford Linear Accelerator Center

LCLS Linac Review 12th December 2003 P. A. McIntosh, SLAC Klystron Dept.

X-Band Stability

NLCTA modulator voltage stability ~ 0.01%.Nominal operating voltage of 350 kV 35 V variation

XL-4 phase stability vs beam voltage = 0.0033o/VXL-4 phase stability then becomes ~ 0.12o without feedback!

L2 modulators are currently regulating to better than 0.01%!

NLCTA structure temperature tuning stability ~200 kHz/oCNeed to regulate structure temperature to 0.025oC

detuning 10 kHz

Phase variation = xs = 0.36o (for filling time s = 100 ns) without feedback!

Estimate for X-band system phase stability h = 0.48o

LCLS requirement is h = 0.50o

SLAC

SLAC achieving 0.08o stability translate to about 0.32o at XBand

LCLS Klystron

Timing Problems

• Stability Oscillators shift period with temperature, vibration etc. VCO shifts period with applied voltage Atomic clock f/f ~ 10-17 ~ 1 fs per minute

• Synchronisation Two clocks with different periods at same place (PLL) Identical delivery time to two places (Crab Cavity Problem) Same clock at two places

Resynchronisation requires constant propagation time of signal Detector with femtosecond accuracy

• Trigger an event at a later and a different location Needs two stable clocks which are synchronised Must be able to generate event from clock pulse with tiny jitter 10 fs looks achievable see work at DESY and MIT

Clock to cavity

Optical clock signal

Locked microwave oscillator

Solid state amplifier

IQ modulator

Solid state amplifier

TWT amplifier Klystron

Pulse compressor

Waveguide

Waveguide

Waveguide

Cavity

sensitive to temperature

Extremely sensitive to modulator

voltage

LLRF control - feedforward to next pulse based on last pulse and environment measurements

Every connector adds uncertainty

CLIC Cavity Synchronisation

Cavity to Cavity Phase synchronisation requirement

degrees1S

1

c

f7204rmsc

x

Target max. luminosity loss fraction S

f (GHz)

x

(nm)c

(rads)rms

(deg)t (fs) Pulse

Length (s)

0.98 12.0 45 0.020 0.0188 4.4 0.156

So need RF path lengths identical to better than c t = 1.3 microns

CLIC bunches ~ 45 nm horizontal by 0.9 nm vertical size at IP.

RF path length measurement

48MW 200ns pulsed 11.994 GHz Klystron

repetition 50Hz Vector

modulationControl

Phase Shifter

12 GHz Oscillator

Main beam outward pick up

Main beam outward pick up

From oscillator

Phase shifter trombone

(High power joint has been tested at SLAC)

Magic Tee

Waveguide path length phase and amplitude measurement and control

4kW 5s pulsed 11.8 GHz Klystron

repetition 5kHz

LLRF

Phase shifter trombone

LLRF

Cavity coupler 0dB or -40dB

Cavity coupler 0dB or -40dB

Expansion joint

Single moded copper plated Invar waveguide losses over 40m ~ 3dB -30 dB

coupler -30 dB coupler

Forward power

main pulse

12 MW

Reflected power main pulse ~ 600 W

Reflected power main pulse ~ 500 W

Waveguide from high power Klystron to magic tee can be

over moded

Expansion joint

RF path length is continuously measured and adjusted

Board Development

Front end electronics to enable phase to be measure during the short pulses to an accuracy of 2 milli-degrees has been prototyped

PLL controller MCU10.7 GHz VCODigital phase

detector

DBMs

Power MeterOutput

Wilkinson splitter

Input 1

Power MeterOutput

Input 2

Phase measurement accuracy

Pulse length Bandwidth Thermal calculation (milli-deg)

RMS resolution measured (milli-deg)

0.14 ms 7 kHz 0.56 1.0

5 μs 200 kHz 3.0 4.6

33 ns 30MHz 37 57

Reflection from cavity 1

Reflection from cavity 2

Voltage to oscilloscope /

ADC

High Speed op

amp

Double balanced mixer Variable LPF

Accuracy depends on measurement bandwidth due to noise limitations (bandwidth determines minimum measurement time).Table below shows data for a single mixer + amplifier with 14 dBm power input: can use 4 to double accuracy and use more power.

March 2012 10

Results (from slide 7)

7 kHz 200 kHz 30MHz

2.54 mV/mdeg 2.17 mV/mdeg 2.17 mV/mdeg

To oscilloscope

Mixer

12 GHz Source

Splitter

Coax lines

Coax line

Coax line stretchers

Waveguide choice

Waveguide type35 meters COPPER

Expansion = 17 ppm/K

Mode Transmission Timing error/0.3°CWidth

Timing error/0.3°C

length

No of modes

WR90(22.86x10.16mm) TE10 45.4% 210.5 fs 498.9 fs 1

Large Rectangular (25x14.5mm) TE10 57.9% 189.3 fs 507.8 fs 2

Cylindrical r =18mm TE01 66.9% 804.9 fs 315.9 fs 7

Cylindrical r =25mm TE01 90.4% 279.6 fs 471.4 fs 17

Copper coated extra pure INVAR 35 meters

Expansion = 0.65 ppm/K

Mode Transmission Timing error/0.3°CWidth

Timing error/0.3°C

length

No of modes

WR90(22.86x10.16mm) TE10 45.4% 8.13 fs 19.04 fs 1

Large Rectangular (25x14.5mm) TE10 57.9% 6.57 fs 19.69 fs 2

Cylindrical r =18mm TE01 66.9% 30.8 fs 12.1 fs 7

Cylindrical r =25mm TE01 90.4% 10.7 fs 18.02 fs 17

Rectangular invar is the best choice as it offers much better temperature stability-> Expands 2.3 microns for 35 m of waveguide per 0.1 °C.

LLRF Hardware Requirements

• Fast phase measurements during the pulse (~20 ns).

• Full scale linear phase measurements to centre mixers and for calibration.

• High accuracy differential phase measurements of RF path length difference (5 μs, 5 kHz).

• DSP control of phase shifters.

Prototype systems have been developed.

Linear Phase Detector

DSPADC

ADC

MagicTee

To Cavity To Cavity

Wilkinson splitters

-30 dB coupler

DBM

DBM DBM

10.7GHz Oscillator

-30 dB coupler

Manual phase shifter for initial setup

Fast piezoelectric phase shifter

DAC

Amp + LPF

Amp + LPF