Diagnostics & Lasers R&D at Daresbury

(1)

Diagnostics & Lasers

R&D at Daresbury

S.P. Jamison

Accelerator Science and Technology Centre,

STFC Daresbury Laboratory

(2)

Diagnostics & Lasers

• Fast beam position monitors

• Timing distribution & beam arrival monitors • Electro-optic longitudinal profile diagnostics • THz driven modulation of electron beam • Ultrafast Photon diagnostics

Daresbury accelerator projects

• ALICE - Accelerators and Lasers in Combined Experiments • EMMA - non-scaling FFAG demonstrator

• VELA -

(3)

ALICE

Accelerator and Lasers in Combined Experiments

30MeV Energy recovery linac

• Initial motivation as testbed for planned 100mA ERL • Principally for accelerator and light source R&D

• Some light-source user experiments

60pC, 81MHz, 100us train @10Hz

(8000bunches, 5mA peak)

CSR THz source 60pC, 16MHz, 100us train @10Hz IR FEL 10-40pC, single bunch @ 5Hz EMMA injector • Mid-IR FEL

• THz for cell irradiation

• “DICC” high current cryo-module currently being installed for with-beam testing • Synchronised lasers available – including 20 TW TiS (Compton scattering expt)

(4)

EMMA

Electron Model for Many Applications

First demonstration of non-scaling FFAG acceleration

Single bunch injection from ALICE

Acceleration from 10MeV – 20MeV in Serpentine acceleration mode

• 42 cells in 16m circumference

• One cell for each of injection, extraction

(5)

VELA

formally Electron Beam Test Facility

• High brightness RF Photoinjector

• Available for industry to develop new accelerator-based technologies

• Healthcare

• Security scanners • Water treatment • ….

• Two independent beam areas available • Funded August 2011

• Commissioning has started

Parameter Value Units

Frequency 2998.5 MHz

Bandwidth < 5 MHz

Accelerating Voltage < 6 MeV Accelerating Gradient <100 MV/m Peak RF Input Power up to 10 MW

(6)

(7)

(8)

Laser Transport Design

• The beam focusing done in stages

– Minimum 3 optics to achieve required demagnification – Mirror Box 2 compresses beam (two optics)

– Mirror Box 3 makes final demagnification (single optic)

• “

off-the-shelf” spherical mirrors

– not a true image at the cathode

– Build-up of astigmatism from successive spheres even

– Main aim is for an illuminated spot of about the correct size

• custom toroids required for true image relay

FM1 = 750 mm FL FM2 = 100 mm FL FM3 = 2500 mm FL

Wavefront propagation

using FOCUS code

(9)

To develop a normal conducting test accelerator able to generate longitudinally

and transversely bright electron bunches and to use these bunches in the

experimental production of

stable

,

synchronised

,

ultra short

photon pulses of

coherent light from a single pass FEL with techniques directly applicable to the

future generation of light source facilities.

• Stable in terms of transverse position, angle, and intensity from shot to shot.

• A target synchronisation level for the photon pulse ‘arrival time’ of better than 10 fs

rms is proposed.

• In this context “ultra short” means less than the FEL cooperation length, which is

typically ~100 wavelengths long (i.e. this equates to a pulse length of 400 as at 1keV,

or 40 as at 10 keV). A SASE FEL normally generates pulses that are dictated by the

electron bunch length, which can be orders of magnitude larger than the cooperation

length.

CLARA

(10)

Other Aims and Prerequisites

To deliver the ultimate objectives of CLARA will encompass

development across many areas:

NC RF photoinjectors

and

seed laser systems

Generation and control

of bright electron

bunches

– manipulation by externally injected

radiation fields

– mitigation against unwanted short

electron bunch effects

High temporal coherence

and wavelength stability

through seeding or other

methods

Generation of coherent

_{higher harmonics of a}

seed source

Photon pulse diagnostics

for single shot

characterisation and

arrival time monitoring

Low charge single

bunch

diagnostics

Synchronisation

systems

Advanced low level

RF systems

Novel short period

(11)

CLARA Flexible FEL Layout

Chicane (1m long)

Diagnostic/Matching Section

Modulator Undulator (1.5m long)

Radiator Undulator (2.5m long)

e-beam

Laser seed

0m

3m

6m

9m

12m

15m

18m

21m

• By implementing a flexible FEL layout,

especially in the modulator region, it will be

possible to test several of the most promising

schemes.

• We are carefully comparing the various

schemes and their detailed requirements –

we do not anticipate testing them all!

• We aim to design in this flexibility from the

start.

Current principle seed/modulation wavelengths

(12)

EXAMPLES OF FEL SCHEMES ON CLARA

SINGLE SPIKE SASE

100pC tracked bunch compressed via velocity bunching

SLICING + CHIRP/TAPER

Short pulse generation using an energy chirped electron bunch

and a tapered undulator

E. L. Saldin et al, Phys. Rev. STAB 9, 050702, 2006

MODE-LOCKING

Mode-locked amplifier FEL using the standard CLARA lattice

with electron beam delays between undulators

N. R. Thompson and B. W. J. McNeil, Phys. Rev. Lett. 100, 203901, 2008

MATCHED MODE-LOCKING

Electron beam delays matched to the rms electron bunch

length to distinguish a single spike from the pulse train

(13)

Parameters

The parameters have now been broken down to cover

5 different operating

modes

. This helps us understand which parameters we need simultaneously.

FEL output wavelengths from 400nm to 100nm

• Can make use of 800nm laser for harmonic generation experiments

• Can use well established laser diagnostics for single shot pulse length measurements

• No need for long photon beamlines, can deflect by 90 degrees

Operating Mode Seeded (long pulse) Unseeded (SASE) Ultra Short Pulse Multibunch High Rep Rate

Motivation Flat top for seeded FEL experiments

SASE FEL Single Spike SASE FEL Oscillator FEL (RAFEL)

Technology demonstration & commercial applications Max Beam Energy

(MeV)

250 250 250 250 100

Max Macropule Rep Rate (Hz)

100 100 100 100 400

Bunch Charge (pC) 250 250 20 to 100 25 250

Number of Bunches per macropulse

1 + pilot 1 + pilot 1 + pilot 20 1

Peak Current at FEL Entrance (A)

400/125 400 1500 25 N/A

Nominal Bunch Length (fs)

(14)

(15)

CLARA - Next Steps

CDR now being drafted

TDR will follow

CLARA funding still to be secured

If procurement starts April 2014 then could install in first

half of 2016

(16)

Diagnostics & Lasers

• Fast beam position monitors

• Timing distribution & beam arrival monitors • Electro-optic longitudinal profile diagnostics • THz driven modulation of electron beam • Ultrafast Photon diagnostics

Daresbury accelerator projects

• ALICE - Accelerators and Lasers in Combined Experiments • EMMA - non-scaling FFAG demonstrator

• VELA -

(17)

rapid serpentine acceleration with large tune variation.

EMMA was constructed for study of non-scaling FFAG acceleration

During accelerating the bunch executes up to ten turns

• Expanding trajectory sweeps about a half of the pickup aperture. • For machine tuning, the bunch can be kept circulating >1000turns. • Revolution period is T=55.2ns,

• bunch charge is up to 30pC, the bunch length is about 10ps.

The rapid dynamics needs advanced diagnostics.

EMMA Diagnostics

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EMMA Beam Position Monitor System

world-fastest-rate BPM system, ASTeC designed, built and commissioned The system is applicable to ERL machines for bunch-by-bunch-in-train measurements, in particular, to ALICE.

faced a problem of a BPM noise caused by a RF power leakage from the EMMA RF system waveguide.

Noise suppression required improvement to Front-End module shielding.

Meeting requirements of EMMA beam diagnostics.

(19)

hor. and vert. time-multiplexed converter outputs (4ns/div)

• developed concept of BPM self-synchronisation with beam, when the BPM detector reference signals and the ADC clock are being manufactured from the BPM input signal, which makes them automatically synchronous with the beam signal.

• We devised a way of use of a fast and precise pipe-line-type ADC chip for single

bunch/train measurements which opened a possibility to use such ADCs in fast-rate BPMs.

(20)

• The system comprises total 53 of BPMs that is about 400 boards & cards.

• Functional architecture, solutions and design of electronics was done by

ASTeC.

• In-house EPICS implementation

• In collaboration, a VME interface and its firmware was designed by

WareWorks Ltd (UK).

Poincare map.

(21)

* Leakage in vertical plane due to pick-up geometry and spurious vertical dispersion

Combined BPM/BAM/FEL

Diagnostics at ALICE

"20121213"" ""22:30:57.522037" 0 500 1000 1500 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Horizontal BPM 0 500 1000 1500 0.4 0.3 0.2 0.1 0.0 Vertical BPM 0 0 0 0 0 Horizontal BPM Charge

Bunch Number Bunch Number Bunch Number Po sit io n ( mm) 0 500 1000 1500 45 50 55 60 65 Charge

Vertical BPM* FEL Output

Bunch Number

* Leakage in vertical plane due to pick-up geometry and spurious vertical dispersion

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Optical Clock Distribution &

beam arrival monitors

Single Mode Distribution Fibre (100m) Dispersion Comp. Fibre Faraday Rotating Mirror (50:50) RF pickup Beamline

BEAM ARRIVAL MONITOR

MZM

Scope PLL

Fibre Stretcher

STABILIZED FIBRE LINK

Mode-locked fibre ring laser (81.25MHz)

~

RF crystal oscillator (81.25MHz) Pol. Contr. λ/2

A pulsed timing system, similar to DESY, is being used to deliver short

pulses for accelerator diagnostics.

Currently linked to ALICE, will migrate to VELA/CLARA

The delivered clock stability is aimed at the few femtosecond level.

Delay detector

 The laser master oscillator is a mode-locked fibre ring laser at 81.25 MHz

(81.08MHz for CLARA)

 Mode-locked Eribum laser from Toptica Photonics  65fs output pulses

 An actively stabilized fibre delivers the short pulses to diagnostic locations

 Beam arrival monitors have been implemented using the delivered short pulses

Fast BPM electronics will be modified for use in BAM

(23)

Dt

RF pickup Beamline Mod. Scope From stabilized link

The Optical Link

 Link stabilization is currently achieved using a balanced optical cross-correlator to measure the group delay between the reference and reflected beams

 10.2fs jitter in link (some issues with the RF master oscillator at time of experiment)

 The links on ALICE are ~100m long and run though the basement (no environmental control)

Beam Arrival Monitors

 Beam arrival monitors convert the timing jitter information into amplitude jitter

 Some issues with sensitivity at low bunch charges since we are using existing stripline BPM.

Optical Clock Distribution System

phase noise.

103 ₁₀4 ₁₀5 ₁₀6 102

(24)

Carrier-phase studies

– towards 1fs links



Monitoring effect of fibre stretching

on changes in carrier phase offset



Deliberate stretching of fibre enable

studies of fibre response at different

frequencies



Feasibility study on locking both

group and phase velocity in

distribution link.

(25)

ALICE

Study of beam dynamics with combined

diagnostics

As well as clock delivery, the distributed femtosecond pulses are used to implement beam arrival

monitoring.

 A combined experiment using multiple diagnostics was performed to study instabilities in the FEL and ALICE as a whole.

 Developing bunch-by-bunch understanding of how beam affects FEL and how FEL affects beam

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26

Study of FEL with combined

diagnostics



Combine with fast FEL detector and BAM

measurements, similar instabilities

observed



Correlations of diagnostics give

information about Arc 2



Tracing of trends though pre-lasing and

lasing parts of pulse train.

position charge

Frequency (MHz) FEL pulse energy

Frequency (MHz)



Several instabilities observed in beam by

fast BPM system



100 kHz bunch position oscillation



300 kHz charge oscillation. Confirmed

in faraday cup and PI laser power



On-going investigation into laser position

stability

courtesy F. Jackson

(27)

Electro-Optic temporal profile monitors

Spectral Decoding

Spatial Encoding

Temporal Decoding

Spectral upconversion**

o Chirped optical input o Spectral readout

o Use time-wavelength relationship o >1ps limited

o Ultrashort optical input o Spatial readout (EO crystal) o Use time-space relationship

o Long pulse + ultrashort pulse gate

o Spatial readout (cross-correlator crystal) o Use time-space relationship

o monochomatic optical input (long pulse)

o Spectral readout

o **Implicit time domain

(28)

χ

(2)

(ω;ω

thz

,ω

opt

)

ω

_opt

+ ω

_thz

convolution over all

combinations of optical

and Coulomb frequencies

Electro-optic detection bandwidth

ω

_thz

ω

_opt

ω

_opt

- ω

_thz

ω

_opt

description of EO detection as sum- and difference-frequency mixing

THz spectrum

(complex)

propagation

& nonlinear

efficiency

geometry

dependent

(repeat for each principle axis)

optical probe

spectrum

(complex)

E

O

cr

yst

a

l

(29)

DC “THz” field....

_{phase shift}

(pockels cell)

temporal

sampling

of THz field

Monochromatic

THz & optical

Chirped optical

Parameter dependent results

optical

sidebands

(30)

Spectral decoding as optical Fourier transform

Why does it work, when does it fail?

Consider (positive) optical frequencies from mixing

Positive and negative

Coulomb (THz) frequencies; sum and diff mixing

Linear chirped pulse:

Assume broad input probe spectrum

Fourier transform form

Convolution function limits time resolution…

(31)

ALICE Electro-optic experiments

o

Energy recovery test-accelerator

intratrain diagnostics must be non-invasive

o

low charge, high repetition rate operation

typically 40pC, 81MHz trains for 100us

Spectral decoding results for 40pC bunch

o confirming compression for FEL commissioning o examine compression and arrival timing along train

(32)

Deconvolution possible.

“Balanced detection”

χ

(2)

_{optical pulse interferes with input probe}

(phase information retained)

“Crossed polariser detection”

input probe extinguished...phase information lost

Deconvolution not possible [ Kramers-Kronig(?)]

Oscillations from interference with probe bandwidth

⇒ resolution limited to probe duration

(33)

Spectral decoding

Kramer-Kronig deconvolution

Phase inferred from

measured amplitude spectrum

Widely used in CSR/CTR temporal diagnostic…

…can it work for spectral decoding?

“measured” spectral amplitude KKphase actual phase (chirp removed)

Surprisingly close

retrieval of phase information

- Does not recover chirp - Some turning points missing

“known” &

retrieved

temporal profiles

Linear chirp phase reintroduced - independently

(34)

Spectral upconversion diagnostic

measure the bunch Fourier spectrum...

... accepting loss of phase information & explicit temporal information

... gaining potential for determining

information on even shorter structure ... gaining measurement simplicity

Long pulse, narrow bandwidth, probe laser

→ δ-function

NOTE: the long probe is still converted to optical replica

same physics as “standard” EO

(35)

Spectral upconversion

difference

frequency mixing

sum

frequency mixing

Spectral sidebands contain the

temporal (phase) information

ALICE single shot

CTR expt

• Femtosecond diagnostic without femtosecond laser

• Capability for <20fs resolution

F E LI X F E L ex p t A pp Ph ys L et t (201 0) Sidebands generated by 2.0THz FEL output

• Measure octave spanning THz spectrum in single optical spectrometer

• Add temporal readout as extension. (FROG, SPIDER)

(36)

Wavelength [um]

Observe non-propagating spectral components which are

not accessible to radiative techniques (CSR/CTR/SP)

Expected

Upconversion

spectra for short

bunches

& narrow

bandwidth probe

(37)

Δν <50GHz (Δ t >9ps)

Laser based test-bed at Daresbury

Femtosecond laser pulse spectrally filtered to produce narrow bandwidth probe

• Photoconductive antenna THz source mimics

Coulomb field.

• Field strengths up to 1 MV/cm.

1.5mm 150μm

Asymmetry in sum and difference spectra

- not explainable by (co-linear) phase matching

Due angular separation of sum & difference waves

- general implications for THz-TDS and EO diagnostics

ZnTe Probe

Sum Freq.

THz _{Diff Freq.}Detection

(38)

fs time domain diagnostic without fs laser

Problem: Up-conversion is relatively weak – our calculations suggest energies of a few nJ.

Signal needs amplifying without loss of information.

Solution: Non-collinear Chirped Pulse Amplification (NCPA)

~800nm femtosecond signal

Stretcher

Compressor

Stretching factor 103_{or more to prevent}

saturation, damage, NL effects

Amplified pulse then recompressed

BBO

Routinely used to produce “single-cycle” optical pulses Amplification with robust nanosecond pulse lasers

High gains of 107_{or more}

Gain bandwidths >100nm (50THz)

Preservation of phase of pulse is possible

Beams ~1.5mm diameter

Gain >1000x (~300MW/cm2₎

(39)

Laser-lab development system

(2) Amplification

Stretcher Compressor Single Shot FROG

NL crystal

(3) Measure: 𝐸� 𝜔 = 𝑆 𝜔 𝑒−𝑖𝜑 𝜔

(4) Calculate properties at NL crystal (to remove remaining spectral amplitude and any residual phase distortion) 50ps 60mJ 1064nm Nd:YAG (doubled) Spectrally filtered Ti:Sapphire THz Source

(40)

(2) Amplification

Stretcher Compressor Single Shot FROG

NL crystal

(3) Measure: 𝐸� 𝜔 = 𝑆 𝜔 𝑒−𝑖𝜑 𝜔

(4) Calculate properties at NL crystal (to remove remaining spectral amplitude and any residual phase distortion) (1) up-convert

Coulomb field

(Spectral intensity and phase distortions can be both modelled and measured)

In beam pipe

Commercial nanosecond Nd Laser Integrated frequency conversion

(OPO)

(41)

Picosecond periods match time scale of compressed bunches

lengths in conventional accelerators.

• No oscillatory smearing as in optical bunch slicing • Controllable field profile on sub-ps time scale. • Octave spanning spectrum possible

Terahertz carrier-phase is synchronised to laser pulse envelope

• Potential for the whole bunch to be “resynchronised” or compressed (in contrast to the selection/tagging from within the bunch)

Laser driven synchronisation ?

(42)

Energy gain for 20 MeV beam

AEMITR

ALICE Energy Modulation by Interaction with THz Radiation

Vacuum acceleration of bunch with TEM

₁₀

-like single-cycle THz pulses



100 MV/m fields achievable

(43)

E

_y

Radial bias

(120kV pulse)

Longitudinal polarised THz pulses from Photoconductive antenna

E

_x

Simple & efficient

but Lacks temporal shaping capability

Transverse field from current surge generates

charge separation

origin of longitudinal

field

Longitudinal field implicit from

now working on nonlinear generation of longitudinal beams

(44)

AEMITR layout

Energy spread diagnostic

• Two-bunch train, separation • 790ns (reference & modulated) • YAG:Ce screen (t~100ns)

• Double shutter gated camera, measuring both reference & modulated bunches

• 20MeV, 20pC

• Minimising projected energy spread “on-crest” acceleration. <50keV spread