1. Mehr Licht! The last words of Johann Wolfgang von Goethe (1749 - 1832)
Goethe had a deep fascination for the physical and metaphorical effects of light on humans. Whilst being best remembered now for his literary works, he himself believed the scientific treatise The Theory of Colours (1810), to be his most important work.
“As to what I have done as a poet,... I take no pride in it... But that in my century I am the only person who knows the truth in the difficult science of colours – of that, I say, I am not a little proud, and here I have a consciousness of a superiority to many.”
— Johann Eckermann, Conversations with Goethe
This might lead us to believe that his celebrated deathbed cry was a plea for increased enlightenment before dying - “more illumination, more knowledge, more reality.” What he actually said was:
"Do open the shutter of the bedroom so that more light may enter".

2. HSP / Axion Limits from Accelerator Based Light Sources
Peter Williams, Cockcroft Institute & Daresbury Laboratory,
Axion-Like Particles Workshop, IPPP, 13th April 2016

3. Light Shining Through the Wall
Limits / discovery of axions / hidden photons can be set by LSW experiments
Purely laboratory based – confirmation of astrophysical limits should be seen as worthwhile as well as absolute limits
Sensitivity proportional to e-m field strength = photon flux = stored energy in a cavity, so we need more light
Either raw average power, enhancement through resonance or both!
Really we want to create “astrophysical” conditions in the lab

4. Synchrotron Light Sources
High Flux: high beam current in e storage ring (typically 500mA) determines flux = photons / (s x BW)
High Brightness: small beam divergence = photons / (s x mrad2 x BW)
High Brilliance: small beam emittance ( “=“ invariant beam size + divergence) determines brilliance (“=“ luminosity in a collider) = photons / (s x mm2 x mrad2 x BW)
Achieved by filling storage ring with undulators – exploit constructive interference = narrow bandwidth at intermediate wavelength, additional bending enhances broadband intensity at short wavelength

5. Synchrotron Light Sources
High Flux: high beam current in e storage ring (typically 500mA) determines flux = photons / (s x BW)
High Brightness: small beam divergence = photons / (s x mrad2 x BW)
High Brilliance: small beam emittance ( “=“ invariant beam size + divergence) determines brilliance (“=“ luminosity in a collider) = photons / (s x mm2 x mrad2 x BW)
Achieved by filling storage ring with undulators – exploit constructive interference = narrow bandwidth at intermediate wavelength, additional bending enhances broadband intensity at short wavelength
Full polarisation: either linear or circular
Pulsed time structure: typically 10ps pulses CW
High Stability: submicron variations over many days
High availability: mature facilities, uptimes of >98% for months, top up injection = no beam fills a la LHC
Many simultaneous user experiments – typically 20 – 40 beamlines per synchrotron
Broadband from VUV to hard X-ray (10’s eV to 10’s keV)

6. Axion Search at ESRF: 2010
ESRF ID06 delivered 1.2 x1012 photons / s at 50.2 keV and 3.1 x 1010 at 90.7 keV – selected by monochometer
Two magnets, each 3 T. 5mm think Ge detector, LN2 cooled, QE
Integration time of 2 hours for each wavelength (diminishing returns as limit  1/t)

7. Axion Search at ESRF: 2010
ESRF ID06 delivered 1.2 x1012 photons / s at 50.2 keV and 3.1 x 1010 at 90.7 keV
Two magnets, each 3 T. 5mm think Ge detector, LN2 cooled, QE
Integration time of 2 hours for each wavelength (diminishing returns as limit  1/t)
Not good enough!
Increase the flux by orders of magnitude to improve (1 needed to exceed NOMAD limit from CERN-SPS neutrino beam, 3 to compete with laser LSW)
Possible to use CAST at a synchrotron X-ray beamline?

8. “Ultimate” Axion Search at a Synchrotron using CAST magnet

9. HSP Search at Spring-8: 2013
SPring-8 BL19LXU delivered between 0.4 and 7 x 1013 photons / s at 9 photon energies between 7 and 26 keV
Very similar Ge detector to that at ESRF
Integration time of ~8 hours at each photon energy

10. “Ultimate” HSP Search at a Synchrotron

11. “Ultimate, ultimate” HSP Search: ILC Positron Source Spontaneous Undulator
Baseline ILC design uses spontaneous undulator radiation generated by polarized electron beam to generate polarized positrons via pair production from MeV gammas
Courtesy Ian Bailey

12. Free Electron Lasers – Next Generation SR Light Source
Long undulator: narrowband radiation intensity growing exponentially until… radiation field modulates energy of electron bunch… then relativistic slippage … microbunching at radiation wavelength…. Coherent radiation of the microbunches
Low gain -> high gain -> saturation
transitions on ALICE IR-FEL
at Daresbury

13. Free Electron Lasers – The Good
Peak radiation powers increase by 106
Full coherence: free propagating radiation is therefore “cavity like”
Full Polarisation: either linear or circular to 100%
Short Pulses: typically 100fs, possibly 1 as!

14. Free Electron Lasers – The Bad
Only possible with “fresh” beam
A beam in equilibrium with SR in a storage ring is too diffuse
Linacs only = 1 pass = expensive and low average power
Not mature “facilities” (FLASH, LCLS, FERMI, SACLA)

15. HSP/Axion Plans at SACLA: Proposed
Would be first use of an X-ray FEL for HSP/Axion search
Axions: “Improvement by factor 40 from NOMAD is expected”
No mention of utilising the full coherence of the source – is it relevant?

16. But, we don’t want a low average power X-ray laser if we’re looking at meV, we want a high average power THz / IR laser – A “Blowtorch”
So far the “killer app” for FELs is “molecular movies = time resolved X-ray diffraction so the demand for hard X-ray: 10’s – 100’s keV
ALSO the machine repetition rate kills the average brilliance compared to the 1010 passes in synchrotron!
SOLUTION: High flux recoverable using SC linac - abolishing electrical resistance makes getting the energy into the beam much more efficient (60Hz LCLS vs macropulse EU-XFEL 4.5MHz for 600us at 10Hz).
IDEAL SOLUTION: Full CW beam at MHz repetition rates = another 104 flux = a SC energy recovery linac
Daresbury Operational for ~10 years - Unfortunately not CW, only 100us trains pulsed at 10 Hz. ERL-FEL designs are underway for ~100kW average power FELs at photon energies ~100eV…….. But see later…….

17. HSP/Axion Searches at Jlab ERL IR-FEL
So not surprising that the only searches to date using an FEL have been at Jlab IR-FEL. It’s an ERL and therefore capable of high average powers, up to 10 kW demonstrated (originally funded by US Navy for Star Wars) = a blowtorch

18. HSP/Axion Searches at Jlab ERL IR-FEL
LIPSS (2009): 935nm but no cavity enhancement and 200W average power – re-doing this would be competitive with ALPS-II table-top laser

19. HSP / Axion Search at a CW X-ray FEL
Naively – just extend the blue and yellow lines down by 6 orders of magnitude!
This would require the equivalent of LCLS-2 using energy recovery operating with ~200mA average current – a la storage ring
Much more likely that a EUV machine would be built – 100eV instead of 100keV

20. If someone builds a IR / UV / X-ray “Blowtorch” – we will get high power coherent THz “for free”!
Provided a SC, CW linac in ERL configuration drives the FEL, it doesn’t matter what the final resonant wavelength is because…..
Coherent THz will be generated parasitically from the bunch compression process – at about the same average power as the FEL!
E.g
Joerg: “But, what about single photon THz detectors?”

21. Single Photon THz Detection?
There is activity developing single photon THz detectors – “charge sensitive infrared phototransistors”
“In terms of the noise equivalent power (NEP), the detectors show experimental values of 7x10-20 W/Hz1/2“
“Additionally, intrinsic dynamic range expected to be 109, covering attowatt to nanowatt”
Wavelength range of um, operate at 4.2K

22. Various ERLs Running / In Construction / Proposed
ERL’s are proposed for other applications, not just FELs e.g. high current electron source for EIC, electron cooler for ion beams, etc
Running: Jlab FEL, ALICE, Budker Inst THz Recuperator
Commisioning: cERL at KEK, Brookhaven ERL
Prototyping: bERLinPro at HZ-Berlin
Proposed: Cornell test ERL, LHeC, eRHIC
More info …

23. Conclusions
Synchrotron, storage ring, third generation light sources struggle to break new ground and spectra peak at 10’s keV
Free Electron Lasers should be capable of setting new lab bounds on HSPs / Axions
In particular, FELs driven by SC energy recovery linacs would give a ~6 order of magnitude increase in average power
High average power (100s kW) coherent THz for searching meV range – parasitic generation at FEL’s of any fundamental wavelength
Material science / chemistry / biochemistry demands that these facilities will continue to expand – there are ~50 synchrotron light sources, developed in over 30 years. FELs will develop over the next 30 = many billions £££
If we can use the light, we should…….