Lasers as Accelerator Subsystems or What’s the problem Bob ? I don’t know Alice, it must be the laser

1. Lasers as Accelerator SubsystemsorWhat’s the problem Bob ?I don’t know Alice, it must be the laser • Cockcroft Institute Laser Lectures • April 2008 Graeme HirstSTFC Central Laser Facility

2. Lecture 5 Plan • Photoinjectors • Beam heating • Compton scattering • Beam diagnosis - spatial • Clocking • Beam diagnosis - temporal • Time slicing • FEL seeding

3. However the excellent performanceof the 1ns thermionic gun on SCSSshows that photoinjectors arenot the only option Photoinjectors Photoinjector electron bunches are made using a laser and aphotocathode They promise precise controlof the bunches’ spatial andtemporal profiles and henceof emittance and synchronisation The beam’s overall time structurecan also be modulated easily

4. Cs2Te co-evaporated films can be ~10% efficient butneed ~10-9 mbar and hn>4.5eV. Lifetime depends onextracted charge. Film response can be nonuniform Cs:GaAs can be >10% efficient with hn>1.5eV.There is a slow response. Cs lifetime requires <10-11 mbar operation which is impractical in RFguns. Uniformity is again an issue Brookhaven have proposed current amplificationby secondary emission in diamond. Gains >100may be possible, allowing useof exotic cathodes Photocathodes Metal photocathodes (Cu, Mg) are simple, durable,uniform and gas-tolerant but have a low QE (<0.1%)and high work function (so need a UV laser)

5. 20 psstacking HG 0.01-100 msslicing 1-20 Hzchopping Oscillator Amp 1 Coding Noisecontrol Preamp Faraday HG HG 1.54 msslicing 1 pulseslicing Oscillator Thermallensing correction Amp 2 Photoinjector Lasers ALICE: Cs:GaAs, tpulse ~20ps,hn = 2.33eV (2w Nd:YVO4),Epulse= 20nJ (80pC, 1% QE)fpulse = 81.25MHz100ms macropulses at 20Hz CTF3: Cs2Te, tpulse ~7ps,hn = 4.73eV (4w Nd:YLF),Epulse= 370nJ (2.33nC, 3% QE)fpulse = 1.5GHz,1.54ms macropulses at 5Hz Other requirements: average power, halo, rise andfall times, reliability over long periods,amplitude and timing stability

6. Photoinjector Laser Developments BEAM CONTROL: Improvements in temporal shaping(“water bag”) will require broader laser bandwidth. Transverseprofiles are currently modified by aperturing (inefficient)and homogenising/flattening (small depth of field). Customoptics and improved source profile should help. AVERAGE POWER: The laser power (~100W) for a 100mAbeam is within reach if an efficient, green-sensitive cathodeis used. But UV and/or inefficient cathodes need powersbeyond what has been demonstrated. Yb3+ fibre lasers and, perhaps, thin disk lasers promise highpowers in good quality beams with sub-picosecondcapacity

7. It is planned to dothis on the LCLSby propagating alaser pulse with theelectrons throughan undulator Beam Heating The high brightness electron beams needed for FELs arevulnerable to microbunching instabilities. These can besuppressed by adding energy spread (“heat”) to the beam At the 120Hz LCLS bunch rate the 40mJ laser is modest. Butscaling to MHz rates or more would quickly become challenging This is just one of a number of schemes formodifying electron bunch propertiesusing a laser

8. Transverse laser Head on laser Electrons X-rays 30 X-ray energy (keV) Diff cross-section (barn/rad) 800nm photons 35MeVelectrons 20 10 0 0 10 20 30 40 50 mrad Compton Scattering - Basics Laser photons can be inverse Compton scattered from electronswith a very large upshift of photonenergy (~4g2 head-on, ~2g2 transverse) Polarisation is preserved inCompton scattering so circularlypolarised X-rays can be made The pulse duration is that ofthe electrons (head-on) orthe laser convolved withthe transit time (transverse) The X-rays are relatively narrowband and tuneable - importantfor e.g. contrast enhancement anddose reduction in radiography

9. Photons Timing jitter moves the IP away fromthe common focus and with short optical“pancakes” can significantly reduce yield Electrons Compton Scattering - Practicalities The cross-section is small, favouring tight focusing, cavityenhancement and large lasers Minimising the larger of the focal spots maximises the X-ray yield Minimising the smaller of the spots reduces source size, andincreases yield if beams are bright in the centre (NB divergence) Laser bandwidth converts to X-ray bandwidth. In the head-ongeometry short pulse lasers may “just” add excess bandwidth Applications include radiography, nuclear spectroscopy,polarised positron production and ultrahigh energy photon science

10. April 4th, 2008 Compton Scattering - Application A “joule class”10ps laser(3w, Nd3+) isscatteredby 120MeVelectron bunchesto produce 776keV photons to drivenuclear fluorescence

11. Truck Radiography (Not using a Compton source)

12. Laser Wave Undulators With a sufficiently intense laser and bright electron beam thelaser EM field may act as a short period FEL undulator The FEL wavelength is lFEL* for aa laser wavelength lW, undulatorstrength kW andlaser intensity IW A 10J, 100ps CO2 laser focused to 100mm has IW = 1013 MW/m2.With 30MeV electrons (g=60) lFEL is 0.75nm i.e. 1.7keV ! Other requirements are extreme. Undulator field uniformity andFEL saturation need a long laser Rayleigh range. High currentand good beam overlap need exceptional electron emittance.Significant developments beyond the presentstate of the art are required. *Phys Letts A 257 26 (1999)

13. Transverse extent can bemeasured by scanning a physicalwire across the beam. A non-invasive,durable alternative for electronsis to scan a focused laser andmeasure Compton yield - “laser wire”. TEM01 Interference TEM00 Spatial resolution is set by the 3Dextent of the laser beam’s smallestfeatures and by pointing stability. Feature size is minimised by small f#optics (but beware depth of focus),high order modes/interference andshort wavelength. Beam Diagnosis - Spatial

14. Results from KEK ATF showa) shoriz=110mm and b) svert=10mm The SLC result showsss=1.14mm Beam Diagnosis - Spatial Sensitivity is set by S/N where noise arises from bremsstrahlung,SR etc. Correlation between emission angle and photon energyis one of several possible discriminators for Compton photons. Signal depends onlaser power whichis maximised bycavity enhancementand/or pulsing

15. RF ReferenceFree running fibre laserLocked fibre laser However PLL performance isalways improved by minimisingreference noise A fibre laser oscillator lockedat low offset frequencies to amicrowave synthesiser ischeaper than a comparableall-electronic source and has the advantage of optical output Clock Generation Accelerator system elements are synchronised by locking to amaster clock, often using phase-locked loops Clock stability is only needed over a finite range of timescales,with limits set by the slowest and quickest responding elements

16. Two fibre-based systems have met this specification: Distribution fibre lengths cancan be measured by detectingthe timing offset of pulsesover a round-trip of the fibre This technique stabilises thegroup transit time Clock Distribution Clock signals may need to be distributed over hundreds ofmetres or kilometres with as low as 10fs jitter This corresponds to a path length stability of a part in 108(~10 times better than can be delivered by coaxial cable)

17. Two fibre-based systems have met this specification: Alternatively a distributionfibre can be included within anoptical interferometer. A longcoherence length cw laser canmeasure the fibre’s phasetransit time to much less thanan optical period Clock Distribution Clock signals may need to be distributed over hundreds ofmetres or kilometres with as low as 10fs jitter This corresponds to a path length stability of a part in 108(~10 times better than can be delivered by coaxial cable)

18. Beam Diagnosis - Temporal Measurements can be split into “bunch profile” and “arrival time”. Bunch profile is important for machine setup, tuning and faultdiagnosis. It should, in general, be inherently stable. Arrival time matters for synchronisation. Next generationmachines will probably need active stabilisation. Existing techniques include: Transverse deflection (LOLA)Very high resolution but inherently destructive Measurement of SRStreak cameras have limited resolution, cross-correlationneeds intensity, photodiodes cannot measure bunch by bunch Spectroscopy of emitted radiation (e.g. OTR)Indirect - needs deconvolutionand interpretation

19. Electro-Optic Sensing Electrons Electrons’ linear charge density produces Coulomb field whichchanges birefringence in EO crystal Laser intensity after analyser reflects bunch profile, andchirping maps this into spectrum Resolution limited to ~200fs by the appearance ofmodulation-induced spectral features

20. Electrons Result fromFLASH/DESY Electro-Optic Sensing Single-shot cross-correlator maps temporal profile directly Result is insensitive to spectral artefacts but needs~1mJ/pulse from the laser Resolution is limited to ~80fs with GaP crystal and 50fs laser Schemes to improve resolution and reducerequired energy are in hand

21. Timing reference laser pulses Darker transmitted pulses are synchronous with electron bunches EO modulator Coaxial RF connection Pickup electrode Electron bunches ADCs Beam Diagnosis - Temporal Developed atFLASH/DESY Arrival time sensors need high resolution and fast sampling rateand would ideally use low power fibre-distributed clock pulses Commercial EO modulators outside the beampipe, overdrivenby RF from pickup, modulate clock pulse amplitudes. Bunch bybunch data are corrected for lateral position and charge 30fs resolution has been achievedand 10fs seems possible

22. Time Slicing A short pulse lasermodulates theenergy of someelectrons. Magnetdispersion spatiallydisplaces them.Their subsequentradiation can beoptically selected. The output of a mJ laser is sufficient for this process. But thephoton yield is small as only a fraction of each bunch is used. Laser average power limits the number of sliced bunches.With dispersion management this may beextended to the few femtosecondregime.

23. FEL Seeding SASE FELspectra arefar fromtransformlimited, makingtemporal profiles spiky. Seeding with a goodquality beam shouldovercome this problem. Simulationsfrom DESY Simulations disagree on the seed power needed but from 6eV to100eV the tens of nJ available from HHG sources should suffice Absolute phase appears to fluctuate in FEL amplification,so phase coherent pump-probe and controlexperiments may not be viable

24. FEL Seeding SCSS experimentshave shown strongseeding at 7.75eVwith just 0.53nJ Output at H3 andH5 was observed Matching the HHG source and FEL gainwavelengths was an issue

25. FEL Seeding Issues foroperationalseed lasersinclude: Can the photon energy be tunedcontinuously ? Does tuning alter anything else ? Can the pulse rate beincreased to matchthe accelerator ? Can the laser be reliably synchronised ? What fraction of the science actuallyneeds the FEL amplifier ... ?

26. Conclusion • What’s the problem Bob ? • I don’t know Alice but don’t worry,whatever it is we can fix it with a laser !

27. Thank you !