1. From LCLS to LHC: Light from Electrons, Protons, and even Lead Ions Alan Fisher ARD Status Meeting 2011 January 6 LHC

2. Terahertz Radiation from LCLS Electrons Electrons passing through a metal foil emit intense, coherent transition radiation (CTR) at wavelengths  bunch length. Highly compressed LCLS bunches:  10 to 100 µm Corresponding frequencies: 3 to 30 THz Intense pulses with the time structure of the electron beam Powerful diagnostic tool for the beam’s temporal profile When focused: Electric fields > 1 GV/m = 0.1 V/Å (depends on bunch charge, length) Magnetic fields > 3 T Well above other THz sources Duration of tens of fs Powerful enough to pump femtosecond chemistry and nonlinear behavior in materials

3. Terahertz Collaborators LCLS Henrik Loos Stefan Moeller Jim Turner Gene Kraft Dave Rich Rob McKinney Roenna del Rosario Frank Hoeflich John Wagner PULSE Aaron Lindenberg Dan Daranciang John Goodfellow Shambhu Ghimire FACET Ziran Wu University of Maryland Ralph Fiorito Anatoly Shkvarunets

4. Electric Field Calculations by Henrik Loos Calculated Field and Spectrum At the THz focus, for a 1-nC, 20-fs bunch Spectrum

5. Terahertz Layout Extract THz in the Undulator Hall Pneumatic actuator, 30 m past the end of the undulator, inserts a thin beryllium foil at 45° to the electron beam Electrons and hard x-rays pass through THz light goes downward through a diamond window to an optical table below Measurements Joulemeter: Energy Pyroelectric video camera: Size at focus Michelson interferometer: Spectrum Will soon install a 20-fs Ti:sapphire laser on the table Electro-optic measurements of electron bunch THz/laser pump/probe studies of materials Proof of principle for a future THz/x-ray pump/probe upgrade Requires a long THz transport line to the NEH

6. Beryllium Foil Thickness: 2 µm Diameter: 25 mm Electrons go through with little scattering or radiation Transparent to hard x rays Can be used parasitically Absorbs x rays below 2 keV For soft x-ray experiments, foil must be pulled out X-Ray Transmission Photon Energy [eV] 2-µm Be Foil at 45°

7. Beamline and Optical Table Pneumatic actuator Beryllium foil Diamond window Track for laser curtain Control rack Laser chiller e−e−

8. Laser Enclosure with Curtain

9. Foil in 6-way Cross Pneumatic actuator Beryllium foil Diamond window Bellows

10. (Enclosure 1) Pyrocam Joulemeter Fluorescent card Fluorescent card (flip up) Si HeNe Fluorescent card Filter wheel Optics for First Measurements Initial Characterization of the Terahertz Radiation: Energy and Profile at Focus 2010 October 12 Translation stage: Move through focus Elevation ViewPlan View Off-axis parabolic mirror

11. Complete Optics Layout Optics enclosure The optics will be enclosed for laser safety and for a dry-air purge. (Water has THz absorption lines.)

12. THz Energy versus Charge and Bunch Length Electric field E of a relativistic bunch varies with bunch charge q and duration  : E ~ q/  Energy in the pulse then follows: E 2  ~ q 2 /  Compare a q 2 /  fit (open circles) to the measured THz (solid) for two bunch-charge values

13. Transmission of GaAs versus THz Intensity Translate a GaAs wafer through the THz focus (at z = 0) Transmitted THz energy shows nonlinear absorption

14. Michelson Interferometer: THz Spectrum Scan of Michelson delay gives an autocorrelation Fourier transform of autocorrelation yields the power spectrum Still being commissioned, but we have preliminary data Pyroelectric detector Delay stage Si beamsplitter I(t) I(ω) Delay scan Fourier transform

15. Interferometer Scans

16. Insight from Simulations 1 nC, 20 fs 1 nC, 60 fs 350 pC, 50 fs 350 pC, 115 fs

17. Summary of Spectral Findings Short bunches show a broad spectrum peaking at 10 THz, consistent with a single-cycle pulse. Long bunches have nodes in the spectrum, corresponding to ripples in the time domain from imperfect compression. The width of the autocorrelation trace is consistently shorter than the value from the electron bunch-length monitor Better electronics are on order to greatly speed up the scans. A laser synchronized with the beam will be installed in February, allowing THz pump/optical probe experiments and direct measurements of time-dependent E-field.

18. Transport to the NEH A 100-m transport line is needed to relay the THz light from the source to an NEH hutch. Sequence of off-axis parabolic (OAP) mirrors, to alternate between collimation and focusing to a waist Diffraction demands large mirrors and frequent refocusing For example, 400-mm-diameter mirrors with a focal length of 5 m, spaced every 10 m 90° bend at each OAP lengthens the path THz arrives tens of nanoseconds after the x rays We want THz/x-ray pump/probe, but this gives probe/pump!

19. Two Electron Bunches To get the THz radiation to a user before the x rays arrive, we’ll need two electron bunches, separated by tens of ns First bunch has a high charge, optimized for THz That would lase poorly in the FEL, but we can also spoil the gain: Turn up the laser heater: Excessive energy spread Or add a fast kicker: Orbit oscillation along the undulator Choose an RF bucket for first pulse so that it arrives ps before x rays THz “optical trombone” delay line then tweaks the arrival time Second bunch makes x rays A second THz pulse arrives tens of ns after pump/probe No problem for users because it is so late If necessary, a fast kicker could force electrons to miss the foil Test in July: 2 equal bunches, 8.4 ns apart Both bunches were lasing in the FEL

20. 2010-07-28: Two-Bunch Test in LCLS 2 bunches, 8.4 ns apart Photodiode viewing 2 x-ray pulses on YAG screen 10 ns/div Both pulses, adjusted to have slightly different energies, on the SXR spectrometer Upgrade of instrumentation (BPMs, toroids) required to measure individual bunches

21. Synchrotron-Light Monitors for the LHC Five applications: BSRT: Imaging telescope, for transverse beam profiles BSRA: Abort-gap monitor, to verify that the gap is empty When the kicker fires, particles in the gap get a partial kick and might cause a quench. Abort-gap cleaning Longitudinal density monitor (in development) Halo monitor (future upgrade) Two particle types: Protons Lead ions (1 month a year): First ion run was in November/December Three light sources: Undulator radiation at injection (0.45 to 1.2 TeV protons) Dipole edge radiation at intermediate energy (1.2 to 3 TeV) Central dipole radiation at collision energy (3 to 7 TeV) Consequently, the spectrum and focus change during the energy ramp

22. CERN Collaborators Stéphane Bart-Pedersen Andrea Boccardi Enrico Bravin Stéphane Burger Gérard Burtin Ana Guerrero Wolfgang Hofle Adam Jeff Thibaut Lefevre Malika Meddahi Aurélie Rabiller Federico Roncarolo My work at CERN has been supported by SLAC through the DOE’s LHC Accelerator Research Program (LARP).

23. Power Radiated by a Charge in a Dipole A relativistic charge q = Ze with mass m, velocity  c  c, and energy E = eE eV =  mc 2 travels through a dipole field B d Radius of curvature of the orbit: Lead ions: Since  and B d don’t change, the maximum energy must be Z = 82 times higher—574 TeV, or 100  J per ion! Equal to the kinetic energy of 82 mosquitos flying at 1 m/s Or, a 1-mm grain of sand thrown at 40 km/h Power emitted in synchrotron radiation: The factor of  4 makes the radiated power substantial—except in the LHC, where the protons have a  of 7500, but  = 6 km. The factor of Z 2  4 means that ions radiate more than protons by a factor of 82 6 /208 4 = 162

24. Synchrotron Power in Various Rings

25. Spectrum and Critical Frequency Spectrum near Critical Frequency Normalized dipole emission, intergrated over vertical angle , versus energy E  E c Long tail for E < E c Visible light << E c for electron rings Rapid drop in emission for E > 10E c Peak is below E c Cameras respond from near IR to near UV Proton emission wavelengths are too long to see below ~1.2 TeV Ion emission is too long below ~3 TeV Ion energy given as “equivalent proton energy”: Dipole set to the same field as for 3-TeV protons Critical Wavelength vs. LHC Beam Energy Lead ions Protons Camera response

26. Superconducting Dipole + Undulator Add an undulator inside the dipole’s cryostat Dipole: Field for 7 TeV3.88 T Length9.45 m Radius of bend6013 m Bend angle1.57 mrad Undulator: Peak field5 T Periods2 Period length280 mm Pole gap60 mm Wavelength for609 nm 450-GeVprotons (injection) Undulator layout and field map Undulator inside cryostat

27. Undulator Emission versus Beam Energy Undulator peak is red for injected protons, but moves into the ultraviolet at 1 TeV. Dipole light is still in the infrared. Injected ions can be seen only with the weak high-energy tail. Radiation from a 2-period undulator has a broad bandwidth. Camera response Lead ions Protons

28. Short Dipoles and Edge Radiation Radiation from the dipole’s edge fills the gap during the energy ramp between the undulator and the central dipole light. If the dipole is short, the observer sees a faster “blip” of radiation, which pushes the spectrum to higher frequencies. Rapidly rising edge field of a (long) dipole has the same effect. 2010-01-18 Fisher — Imaging with Synchrotron Light28

29. Photoelectrons per Particle at the Camera ProtonsLead Ions Dipole center Undulator Combined Dipole edge Undulator Combined Dipole center Dipole edge In the crossover region between undulator and dipole radiation: Weak signal Two comparable sources: poor focus over a narrow energy range Focus changes with energy: from undulator, to dipole edge, to dipole center Dipole edge radiation is distinct from central radiation only for  >>  c

30. Particles per Bunch and per Fill ProtonsLead Ions Particles in a “pilot” bunch 5  10 9 7  10 7 Particles in a nominal bunch 1.15  10 11 7  10 7 Bunches in early fills1 to 431 to 62 Bunches in a full ring2808592

31. Layout: Dipole and Undulator To RF cavities and IP4 To arc Cryostat Extracted light sent to an optical table below the beamline 1.6 mrad 70 m 26 m937 mm 560 mm 420 mm D3U 10 m D4 194 mm

32. RF Cavities

33. BSRT for Beam 1 B1 Extraction mirror (covered to hunt for a light leak) Door to RF cavities Undulator and dipole Beam 1Beam 2 Optical Table

34. Table Enclosure under Extraction Mirror Beam 1Beam 2 B1 Extraction mirror

35. Extraction Mirror Protons—with their small heat load—can use a simple mirror without cooling.

36. Optical Table

37. Layout of the Optics Alignment laser Focus trombone F1 = 4 m PMT and 15% splitter for abort gap monitor Intermediate image Table Coordinates [mm] Cameras Slit Calibration light and target F2 = 0.75 m Beam Optical Table Extraction mirror Shielding

38. Beam 1Beam 2 Light from undulator. No filters. LHC Beams at Injection (450 GeV) Horizontal 1.3 mm1.2 mm Vertical 0.9 mm1.7 mm

39. Beam 1 at 1.18 TeV 1.18 TeV has the weakest emission in the camera’s band. Undulator’s peak has moved from red to the ultraviolet Dipole’s critical energy is still in the infrared Nevertheless, there is enough light for an adequate image. Some blurring from two comparable sources at different distances Vertical emittance growth before and after ramp Comparing synchrotron light to wire scanner Synchrotron Light Wire Scanner Vertical Emittance Proton Energy

40. Beam 1Beam 2 Horizontal 0.68 mm0.70 mm Vertical 0.56 mm1.05 mm LHC Beams at 3.5 TeV Light from D3 dipole. Blue filter.

41. First Lead-Ion Images, 2.3 TeV First ramp of one bunch of lead ions 2011 November 5

42. Calibration Techniques Target Incoherently illuminated target (and alignment laser) on the optical table Folded calibration path on table matches optical path of entering light Wire scanners Compare with size from synchrotron light, after adjusting for different  x,y Only possible with a small ring current Beam bump Compare bump of image centroid with shift seen by BPMs 5 mm

43. Emittance Comparisons at 450 GeV Beam 2 Vertical LHC Synchrotron Light Nominal  Time [h] Beam 1 Horizontal Beam 2 Horizontal Beam 1 Vertical LHC Wire Scanner From SPS

44. Disagreement with Wire Scanners The horizontal size—but not the vertical—measured with synchrotron light is larger than the size from the wire scanners. Beam 1: Factor of 2 in x emittance (  2 in beam size) Beam 2: Factor of 1.3 in x emittance  beat isn’t large enough to explain this. I tested the full system in the lab in 2009 Found distortions on the first focusing mirrors (old, perhaps left-overs from LEP?) Replacements arrived just before tunnel was locked: No time for tests

45. Nov 2010: Bench Tests with Duplicate Optics I set up a new 4.8-m  0.8-m table in the lab with a copy of the tunnel optics. Alignment of tunnel optics: Images shift while focusing: mirrors not properly filled More diffractive blurring? New alignment procedure Entering light needs one more motorized mirror Camera and digitizer: Fixed hexagonal pattern from intensified camera Increased magnification reduces the effect Digitizer grabs every other line 500-µm line width 400-µm line width

46. BSRA: Abort-Gap Monitor Gated photomultiplier receives ~15% of collected light PMT is gated off except during the 3-  s abort gap: High gain needed during gap Avoid saturation when full buckets pass by Beamsplitter is before all slits or filters, to get maximum light Gap signal is digitized in 30 100-ns bins Summed over 100 ms and 1 s Requirement: Every 100 ms, detect whether any bin has a population over 10% of the quench threshold Integration over 1 s is needed where PMT signal is weak Protons near 1.2 TeV Worst case signal-to-noise is 10 for 1-s integration with a population of 10% of quench threshold No PMT signal observed for an ion bunch at injection energy Calculations said there should be a signal Does PMT not extend into the near IR as far as the datasheet claims?

47. Protons/100ns at the Quench Threshold Original thresholds, specified only for 0.45 and 7 TeV, were too generous Must detect levels well below a pilot bunch BLM group provided improved models: using Sapinski’s calculation Ion threshold is scaled from proton threshold: Ion fragments on beam screen Deposits same energy as Z protons at same point in ramp Original specification Model for BSRA (Q4 quench) (M. Sapinski) General quench model (B. Dehning) Protons in a pilot bunch

48. Calibration of BSRA Inject a pilot bunch Charge measured by bunch- charge and DC-current electronics Attenuate light by a factor of  bunch charge / quench threshold Move BSRA gate to include the pilot bunch Find PMT counts per proton (adjusted for attenuation) as a function of PMT voltage and beam energy Turn RF off (coast) for 5 minutes to observe a small, nearly uniform fill of the gap Useful to test gap cleaning… Last bunch in fill First bunch in fill Abort gap Time [100-ns bins] After coasting briefly, bunch spreads out Pilot bunch

49. 2009 Dec 16: Test of Abort-Gap Cleaning Injected 4 bunches into Beam 2 Poor lifetime, but not important for this experiment Turned off RF, and coasted for 5 minutes Abort-gap monitor detected charge drifting into the abort gap Excited 1 µs of the 3-µs gap at a transverse tune for 5 minutes How well did this work? Look inside the gap... Beam charge (injection) Total PMT signal (negative going) in all 30 bins RF off Beam dumped Gap cleaning RF on (poor lifetime) 025 minutes

50. Charge in Abort Gap Abort gap (3 µs) Position in fill pattern (100-ns bins) Time (s) Charge drifting from first bunch after gap Cleaning started in 1-µs region: Immediate effect Excitation had ringing on the trailing edge (improved in January) Beam dumped RF off: coasting beam

51. 2010 Sep 30: Test with Improved RF Pulse Injected 2 bunches (1 and 1201), 3 µs apart; no ramp Another test at 3.5 TeV took place on 2010 Oct 22 Cleaning pulse applied between the bunches at the x or y tune RF voltage reduced in steps; debunched protons drift into gap Time (s) 3-µs Gap (in 30 100-ns bins) Darker = More protons Cleaning resumed. RF voltage lowered. Low/high momentum protons drift into gap from bunches on left/right. Encounter cleaning region. Cleaning turned off. Protons repopulate cleaning region. Voltage lowered by another step. Cleaning off

52. Longitudinal-Density Monitor Monitor is being developed by Adam Jeff Photon counting using an avalanche photodiode (APD) Fiber collects 1% of the BSRT’s synchrotron light Sufficient signal: 10 3 photons/bunch at 1 TeV, and 3  10 6 for E ≥ 3.5 TeV Measure time from ring-turn clock to photodiode pulse Accumulate counts in 50-ps bins One unit has been installed on Beam 2 for testing Modes: Fast mode: 1-ms accumulation, for bunch length, shape, and density Requires corrections for APD deadtime and for photon pile-up Slow mode: 10-s accumulation, for tails and ghost bunches down to 5  10 5 protons (4  10 -6 of a nominal full bunch) Only 1 photon every 200 turns May require 2 APDs: APD for slow mode gated off during full bunches

53. Testing the Longitudinal-Density Monitor Adam Jeff 1 turn 1 train 28 ns One bunch, but protons are also in neighboring buckets

54. Observing the Solar Corona Lyot invented a coronagraph in the 1930s to image the corona Huge dynamic range: Sun is 10 6 times brighter than its corona Block light from solar disc with a circular mask B on image plane Diffraction from edge of first lens (A, limiting aperture) exceeds corona Circumferential stop D around of image of lens A formed by lens C Can we apply this to measuring the halo of a particle beam? Bernard Lyot, Monthly Notices of the Royal Astronomical Society, 99 (1939) 580

55. Beam-Halo Monitor Halo monitoring is part of the original specification for the synchrotron-light monitor SLAC’s involvement, through LARP, in both light monitors and collimation makes this a natural extension to the SLM project But the coronagraph needs some changes: The Sun has a constant diameter and a sharp edge The beam has a varying diameter and a Gaussian profile An adjustable mask is needed

56. Fixed Mask with Adjustable Optics SLM images a broad bandwidth: Near IR to near UV Reflective zoom is difficult compared to a zoom lens Bandwidth is a problem for refractive optics Limited by need for radiation-hard materials But a blue filter is used for higher currents: Fused silica lenses could work Zoom lens Halo image Source Steering mirrors Masking mirror

57. Halo Monitor with Masking Mirror Alignment laser Focus trombone F1 = 4 m PMT and 15% splitter for abort gap monitor Intermediate image Table Coordinates [mm] Cameras Slit Calibration light and target F2 = 0.75 m Masking mirror Diffraction stop Zoom lens During halo measurements: Insert zoom lens, masking mirror, and return mirror.

58. Digital Micro-Mirror Array 1024  768 grid of 13.68-µm square pixels Pixel tilt toggles about diagonal by ±12° Mirror array mounted on a control board, which is tilted by 45° so that the reflections are horizontal.

59. Digital Micro-Mirror Array Advantages: Flexible masking due to individually addressable pixels Adapts well to flat beams in electron rings But the LHC beams are nearly circular Disadvantages: The pixels are somewhat large for the LHC F1 is far from source: Intermediate image is demagnified by 7 RMS size: 14 pixels at 450 GeV, but only 3.4 pixels at 7 TeV Reflected wavefront is tilted DMA has features of a mirror and a grating Camera face must tilt by 24° to compensate for tilt Known as Scheimflug compensation Will test this first at SPEAR

60. Halo Monitor with Digital Mirror Array Alignment laser Focus trombone F1 = 4 m PMT and 15% splitter for abort gap monitor Intermediate image Table Coordinates [mm] Cameras Slit Calibration light and target F2 = 0.75 m DMA Diffraction stop During halo measurements: Insert DMA and return mirror; rotate camera.

61. Summary: LHC Synchrotron Light Synchrotron light is in routine use for observing the beam size and for monitoring the abort gap. Improvements proposed for better beam images Two tests of abort-gap cleaning have been successful, with the abort-gap monitor showing changes in the gap population. A longitudinal-density monitor is being developed. Now considering how to add a halo monitor. First measure the halo at SPEAR, where access to the light is easy.

62. Summary: LCLS Terahertz Source First THz light measured in October Measurements so far: Scans of THz energy versus charge and bunch length Spectrum using interferometer Nonlinear absorption in GaAs Titanium-sapphire laser to be installed this winter: Mapping temporal profile of the THz electric field using polarization rotation in an electro-optic crystal THz/laser pump/probe Later, design a transport line to the NEH