1. Jim Clarke ASTeC and Cockcroft Institute ALPHA-X Workshop, May 2013
Overview of CLARA
Jim Clarke
ASTeC and Cockcroft Institute
ALPHA-X Workshop, May 2013

2. Update:Versatile Electron Linear Accelerator
VELA is the new name for EBTF
High brightness RF Photoinjector
Essential technology for advanced electron facilities
Light sources
Colliders
First RF Photoinjector in the UK
New tool for industry to develop new accelerator-based technologies
Healthcare
Security scanners
Water treatment ….
Two independent beam areas available
Funded August 2011

3. VELA Beam Transport Layout
Beam Injector
Beam
Enclosure 1
Beam
Enclosure 2

4. VELA Injector

5. VELA

6. VELA Status VELA Approval – August 2011
VELA hardware commissioning started – Oct 2012
RF Conditioning started – 30th Nov
5.3 MW peak power achieved – 20th Dec
RF window vacuum leak identified – 11th Jan 2013
RF Conditioning restarted – 21st Feb
Multipactoring observed as solenoid powered
4.8 MW, RF window cracked – 6th March
Ceramic window fitted (Strathclyde)
RF Conditioning restarted – 19th Mar
5.7 MW peak power achieved – 2nd Apr
First electrons – 5th Apr 2013
Shutdown for installation of all systems

7. CLARA Major upgrade of VELA
Compact Linear Accelerator for Research and Applications
Major upgrade of VELA
A world class FEL test facility that can try out new ideas so they can be implemented directly into a future light source facility – we know there is a strong demand for these improvements from the NLS Science Case and direct interactions with users
In parallel we will also be able test advanced accelerator technologies
The investment in CLARA will pay for itself by reducing future risk and timescales
More importantly, it will also make any UK future light source a world beater !

8. 4th Generation Light Sources
Free Electron Lasers
Ultra high peak intensity
Very short pulses of light
Tuneable
Basic FEL unstable in intensity and wavelength
Immature as a technology, plenty of scope for improvement
Fortunately lots of ideas exist for improving FEL stability (wavelength and intensity) and to make even shorter pulses of light but very few have been tested
Can’t propose a major new facility based on an untested idea! Need test facility

9. Reviewing the field – Facilities
Currently there are only five (soon to be three?) dedicated single pass FEL test facilities worldwide, two in the US (NLCTA and SDU), one in Asia (SDUV-FEL) and two in Europe (SPARC and MAX).
Highest current priority for FELs is improving temporal coherence.
Reducing size and cost is another common theme.
An opportunity exists for a new FEL test facility looking at next frontiers.
Extract from:
A Review of Worldwide Test Facilities for Free Electron Lasers
David Dunning, ASTeC

10. Ultimate aims of CLARA
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.

11. Other Aims and Prerequisites
To deliver the ultimate objectives of CLARA will encompass development across many areas:
Generation and control of bright electron bunches
manipulation by externally injected radiation fields
mitigation against unwanted short electron bunch effects
Low charge single bunch diagnostics
NC RF photoinjectors and
seed laser systems
High temporal coherence and wavelength stability through seeding or other methods
Synchronisation systems
Generation of coherent higher harmonics of a seed source
Advanced digital low level RF systems
Photon pulse diagnostics for single shot characterisation and arrival time monitoring
Novel short period undulators

12. Goals, Opportunities and Benefits
The proof of principle demonstrations of ultra-short photon pulse generation using schemes which are applicable to X-ray FELs and with extreme levels of synchronisation.
The ability to test other novel schemes for increasing the intrinsic FEL output intensity stability, wavelength stability, or the longitudinal coherence using external seeding, self-seeding or other methods.
The ability to generate higher harmonic radiation of a seed source using EEHG, HGHG, etc.
The generation and characterisation of very bright (in 6D) electron bunches and the manipulation of the bunch properties with externally injected lasers.
The development of advanced accelerator technologies, such as a high repetition rate NCRF photoinjector, single bunch low charge diagnostics, and novel photocathode materials and preparation techniques.
The enhancement of VELA, in terms of energy, beam power, and repetition rate.
The development of vital skills within the UK accelerator community, including providing excellent opportunities for PhD students and post docs to work on a world class accelerator test facility.
To use the electron beam for other applications: ultrafast electron diffraction, plasma wakefield accelerator research, Compton scattering source of X-rays or gamma photons, and other novel acceleration schemes such as dielectric wakefield accelerators.

13. Flexible FEL Layout
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 are designing in this flexibility from the start.

14. 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, , 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, , 2008
MATCHED MODE-LOCKING Electron beam delays matched to the rms electron bunch length to distinguish a single spike from the pulse train
Plots courtesy of Ian Martin and Neil Thompson

15. Slicing Scheme Example
Few-cycle seed laser is used to modulate the electron beam energy to an amplitude greater than the natural bandwidth of the FEL.
By tapering the gap of the undulator, only the sections of the electron bunch where the energy chirp is correctly matched to the undulator taper will experience high FEL gain.
CLARA Example:
50 mm, 10 mJ, 500 fs seed laser

16. Parameters
The parameters have now been broken down to cover 5 different operating modes. This helps us understand which parameters we need simultaneously. 25
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

17. RF Frequency & Rep Rate
The current VELA gun (ALPHA-X) and the DLS 1 kHz gun are both SEU
There is significant effort within Europe on advanced RF guns using SEU
CLARA will have an SEU gun and an SEU linac with an XEU linearizing cavity
Higher rep rates (>100 Hz) will be useful for
Industrial applications
Feedback systems for stabilization
Sharing pulses amongst multiple facilities
Technology demonstration and leadership
A future national facility
CLARA has a maximum energy specified to be 250 MeV but it is not necessary (or even important) to attain this energy at the highest rep rates
The vast majority of industrial users will be satisfied with 100 MeV
We have proposed that CLARA should have a repetition rate of 400 Hz but at a reduced linac gradient (and so beam energy). The infrastructure (eg RF modulators, lasers) should therefore be capable of up to 400 Hz operation. Full beam energy should be possible at 100 Hz.

18. High Repetition Rate NCRF
Initial EBTF gun cavity (ALPHA-X) will operate at up to 10 Hz repetition rate
High Rep Rate Gun Development
Scaled to S-band FLASH/XFEL gun cavity fabricated by DLS could be tested at up to 400 Hz with EBTF/CLARA (no RF pick up)
New design is being generated and compared against DLS solution
CLARA Linac
Assuming use of 3 x SwissFEL Injector linac structures ( to 25 MeV/m, 10MeV/m, 4.3 m long)
A study on practical realisation of high gradient, 20 to 25 MeV/m, 400 Hz linac structures has carried out by AES – solution proposed looks feasible and realistic
X-Band RF Source Collaboration initiated with CERN
Development of low cost RF source for linearising cavity

19. CLARA Layout 4th Harmonic Cavity Further exploitation line
Photon Diagnostics
Gun
4m linac
FEL afterburner
Laser Heater
2m linac
4m linacs
70MeV
250MeV
Bunch Compressor
TDC2
FEL radiators
TDC1
FEL modulator
Dump
EBTF exploitation area
Total Length ~ 90m

20. Impact on VELA

21. Impact on VELA CLARA axis is offset by 1m from VELA
CLARA Front End
CLARA axis is offset by 1m from VELA
VELA photoinjector laser/RF system used by CLARA gun (VELA gun cavity initially as well)
Existing VELA beamline will stay in place (after first 5m)
CLARA will then feed VELA at up to 25MeV (cf 5MeV)
Option for new themionic gun for VELA available if required

22. Other Opportunities Time Resolved Electron Diffraction
Grant proposal submitted for ultrafast electron diffraction
Will be incorporated into existing VELA for demo experiments, aiming to incorporate within CLARA later
Plasma accelerator research
see next talk
THz source for science
Compton fs gamma or X-ray source
High energy beamline for industrial exploitation
Dielectric Wakefield Acceleration Experiments
Exotic storage rings (eg optical stochastic cooling, non-equilibrium light source, integrable optics lattice)

23. Next Steps CDR now being drafted TDR will follow
CLARA funding still to be secured
SwissFEL linacs released Jan 2015
If procurement starts April 2014 then could install in first half of 2016
CLARA first commissioning – mid 2016

25. Acknowledgement
Many thanks to colleagues from ASTeC, Technology Department, Strathclyde University, SwissFEL, LAL, the Cockcroft Institute, the John Adams Institute, and Diamond Light Source for their contributions to this talk and the CLARA project