Radiation Sources: Horses for Courses

Radiation Sources:Horses for Courses • Cockcroft Institute Laser Lectures • April 2008 Graeme HirstSTFC Central Laser Facility

Lecture 2 Plan • The big picture • The issues • Comparisons • Wrap-up

Lasers Lasers “Electronsin flight” “Electronsin flight” The Options Thermal Plasmas Electron impact Sunlight

Spectrum • Pulse length & timing • Transverse beam quality • Polarisation • Access • Flexibility • Reliability • Support & infrastructure • Future-proofing • Cost Brainstorm - What Matters to Users ? • TECHNICAL • NON-TECHNICAL

Health Warning ! Light sources occupy a parameter spacewith many dimensions Sources and user experiments are oftenquite “localised” within this parameter space(the degree of overlap is, perhaps, surprising !) Sources which can deliver unusual performancein more than one dimension simultaneously arevery much the exception, not the rule !

Photon Energy Range BENDING MAGNETS: Coherent enhancement ifbunch length < wavelength Output THz X-ray Photon Energy

Photon Energy Range UNDULATORS: Single ID tuningenvelope Fundamental tunedby B-field and/orelectron energy Output Harmonics THz X-ray Photon Energy

Photon Energy Range FELS: Tuningenvelope Fundamental tunedby B-field and/orelectron energy Output Harmonics THz X-ray Photon Energy

Photon Energy Range LASERS: Dyes Moleculargasmasers X-raylasers Lanthanideions Excimers (Energies inthe FIR andX-ray areindicativeonly) Output CO2 CO Nd F2 Ti 0.01 0.1 1 10 100 1000 Photon Energy (eV)

Photon Energy Range Extension LASERS: Nonlinear optics (frequency multiplying, sum- anddifference-frequency generation, optical parametricprocesses) can shift NIR laser photons to give completecoverage from <0.1eV to >6eV (limited by crystal opacity) with reasonable efficiency Harmonic generation gives complete coverage from10eV to several hundred eV with 10-5 - 10-8 efficiency Plasma conversion can give incoherent broadbandoutput from ~100eV to tens of keV and Ka emissionfrom ~2keV to many tens of keV

UNDULATORS AND FELS: Tuning is usually by changing the gap which is relatively easyfor undulators but gets increasingly difficult for FELs as thephoton energy rises. An order of magnitude range is possible. LASERS: Commercial Ti:S and dye lasers and optical parametricsystems are tuned by optic alignment, which can beautomated. An order of magnituderange is possible. Photon Energy Tuning/Scanning BENDING MAGNETS: Tuning can be by a dichroic mirror/monochromator or adispersive detector, which are easy but often inefficient,or (in the IR) by a Fourier Transform spectrometer.

Spectral Linewidth Linewidth Df can never be less than ~1/Dt (pulsewidth) This is the (Fourier) transform limit The exact constant of proportionality depends on pulseshape and on how “width” is defined LASERS: the beams are often “near transform limited” which means they are longitudinally (temporally) coherent. Spectral amplitude and phase can be stabilised. Longitudinal coherence can cause “speckle”. ACCELERATOR BASED SOURCES:the beams are usually not transform limited, the exceptions being cavity FELs, seeded FELs and spectrally filtered beams. Beams which are not transform limited are not temporally smooth.

Temporal Pulse Profiles Simulationsfrom DESY Temporal profile of a SASE FEL pulsegenerated by an electron bunchwith s = 250fs

Photon Energy Range - Summary ACCELERATOR BASED SOURCES:cover the widestspectral range from <0.01eV to >100keV. In principle tuning can be continuous. In practice the tuningrange depends on the source and the optical transport system. In general the spectral linewidth is broader than thetransform limit. This may be significant for some experiments(high resolution spectroscopy, coherent control etc). LASERS:cover the spectral range from <0.1eV to ~6eV withcontinuous tuning using nonlinear optics in crystals.Harmonic generation can extend this to several hundred eV. In general the spectral linewidth can be near to thetransform limit.

Important aside: Laser people would call this a 235fs pulse (FWHM). Accelerator people would call it a 100fs pulse (s).Laser people would call this one a50fs pulse on a pedestal 100 235 Pulse Duration Among others, this matters a) To users who need high peak power or intensity b) To users who want to do time-resolved studies c) To users whose sample explodes when irradiated (These seem to represent a large fraction of laser usersbut only a small fraction of synchrotron users)

BMs AND UNDULATORS: FELS: The pulse duration essentially reflects the electron bunchlength. In storage rings this can be as short as 10ps and inlinacs down to ~100fs. Exotic techniques (laser slicing, crabcavities, slits in chicanes …) can allow further shortening. Simulatedoutput fromsaturatedcavity FELdriven by250fs FWHMelectronbunch FEL gain is sensitiveto peak current. Slippage can lengthenpulses, superradiancecan shorten them, ascan seeding. 30fs FWHM ~10fs pulses are possible and exoticschemes promise lower. Pulse Duration

LASERS: “Grenouilles” fromSwamp Optics Incfor measuring ultrashort laser pulsesusing FROG Systems working from cw to <10fs FWHM are commerciallyavailable, as is the technology for CEP stabilisation. Harmonic techniques can deliver “attosecond” pulses. Frequency Resolved Optical Gatingis one of a number of “amphibian”measurement techniques (TOAD,TADPOLE, SPIDER …) Pulse Duration Short pulsesare measuredusing correlationtechniques. Below 10fs it hasbeen said thatmeasuring thepulses is moredifficult thangenerating them !

LASERS: ACCELERATORS: Laser oscillator pulses can be synchronised to one anotherand to RF references with ~20fs rms jitter in experimentallaboratories and sub-fs jitter in standards labs. Accelerators are large and many beam transport elementsare dispersive. This makes bunch timing control difficult.Stabilisation and feedback control are veryactive research areas. Timing and Synchronisation Multi-beam experiments usually require the timing of thedifferent pulses to be managed. In 10fs light travels just 3mm. In some cases “management” can just be measurementand the experimental data can be time-stamped.

LASERS: Oscillator pulse rates vary fromtens of MHz to GHz and are setby optical cavity length. The timestructure is inherently uniform. Amplifiers can be pulsed from single-shot to hundreds of kHzor cw. But matching to few hundred MHzaccelerator rates is rare. Pulse Rates and Time Structures These matter if they a) affect average power b) interrupt relaxation/recovery of sample and/or diagnostic c) are seriously mismatched in multi-beam set-ups EO and AO choppers work for NIR/vis/UV photon energiesand mechanical ones work into the X-ray (on MHz beams !)

Pulse Rates and Time Structures ACCELERATORS: Maximum pulse rates are set by the machine RF,typically hundreds of MHz to few GHz (30GHz for CLIC !) Not every RF bucket needs to have electrons in,but gaps reduce current and may affect stability Secondary time structures are imposed byorbit length (storage rings) and macrobunchingto limit average power (linacs) On multi-user machines there can be conflict overtime-structures (SRS is currently on single-bunch) FEL pulse rates can be limited by opticalpower handling

Average Power Average power = Pulse energy × Pulse rate It is important for: Improving signal-to-noise Acquiring data before a sample degrades Acquiring data before beamtime runs out Supplying multiple experiments It may be limited by: Drive capacity and conversion efficiency to light Nonlinear effects in the light source (due to peak power ?) Heat removal from the light source Damage/distortion to the transport optics Damage/disruption to the sample and/or detectors Controlling beam power without affectingother parameters can besurprisingly difficult.

ACCELERATORS: High current machines (storage rings, ERLs etc) should becapable of watts or tens of watts from undulators. A 14kW IR FEL has been built and a 250W VUV FEL designed. Linac average powers are orders of magnitude lowerbecause of inherent inefficiency. LASERS: Commercial “research” lasers tend to be limited to a fewtens of watts if tuneable short pulses are required. Over the next few years lasers in the 100W-1kW class areexpected to be developed and may be commercialised. HHG conversion at 10-6 efficiency promises mW powers. Average Power

4GLS IR FEL 4GLS VUV FEL Commercial 1kHz OPA (ph/s) 4GLS Bending Magnet 4GLS XUV FEL Diamond undulators HU64 4GLS undulators U30, 10m U23 U60, 5m Max III U62.5 4GLS 3.5T wiggler Average Power Data from 4GLS CDR Commercial lasers compare well withhigh current accelerators.

Beam focusing is important for intensityand for microscopy/small samples Without tricks, focal spot diameterf cannot be less than ~fl/d This “diffraction limit” requires full transverse coherence f d f Transverse Beam Quality LASERS: the beams are often “near diffraction limited”. ACCELERATOR BASED SOURCES:can be effectively “point like”. Long, thin undulators can act as spatial filters, imposing transverse coherence. Positional stability may be an issue. Transport optics can distort under high heat load. Achieving adequate surface figure is hard at short wavelength.

BENDING MAGNETS: UNDULATORS AND FELS: Output is inherently linearly polarised. Polarisation can be controlledby using an APPLE helicalundulator section to generatelinear, elliptical or circularpolarisation. LASERS: Oscillators and nonlinear optical converters are generallylinearly polarised. Over the photon energy range typical oflasers polarisation can easily be adjusted by passive optics. Harmonic generation is inherently linearly polarised. Polarisation

Source Combinations • These can be considered in (at least) two ways: • A composite light source could be produced using lasers to cover, say, the photon energy range 0.1eV-6eV and accelerator-based sources for higher and lower energies • Users should be unaware of the source of their photons • (concentration on similarities) • Alternatively: • A laser could be used to do something for which it is uniquely suited – e.g. create a warm-dense-matter plasma or a Bose-Einstein condensate • An accelerator-based source could then probe the result • (concentration on differences)

LASERS: ACCELERATORS: • Can deliver high energy pulses • Can deliver sub-femtosecond pulses • Can be very tightly controlled (spectral, spatial and temporal phases and amplitudes, time structures and synchronisation) • Are relatively cheap and quick to produce, so the field develops rapidly and small groups can build custom systems • Can reach the spectral extremes with high average powers • Can operate at high photon energy with variable polarisation • Can support multiple users • Are large enough to justify the provision of substantial supporting infrastructure Summary - Source Strengths

“ ... ... ” Report from the Review of UK Light Sources Jan 2008 Wrap-Up • Comparisons are complicated by the very significant difference ininvestment to date between a typical laser source (few £M) and atypical accelerator-based source (few hundred £M) • The choice between a laser and an accelerator-based light source will depend on the application and on the user’s preferred “way of working” as much as on the sources’ technical performance

Thank you !

Radiation Sources: Horses for Courses