Current Projects

1. Current Projects Alan Fisher APE Meeting 2010-08-03

2. Ongoing Projects • Terahertz radiation from the LCLS electrons • With Aaron Lindenberg of PULSE • Measuring short SPEAR bunches by cross correlation of synchrotron light with a Ti:sapphire laser • With Jeff Corbett (SSRL) and Aaron Lindenberg • Imaging the LHC protons with synchrotron light • With several CERN collaborators • Beam-loss monitoring for machine protection at LCLS • Catching gas bursts near the LCLS gun • Assorted LCLS measurements

3. Terahertz Radiation • Electrons pass through beryllium foil downstream of undulator • 2-µm foil at 45° to beam, with diamond window below • THz radiation extracted from wake of compressed beam • Femtosecond pulses with intense fields: • Peak electric field expected to be ~3 V/Å • Laboratory sources for 1-10 THz are much weaker: < 0.01 V/Å • Peak magnetic fields of order 100 T • Beam diagnostic: Single-shot characterization of fs bunches • User experiments: Switching materials at the level of atoms, optical manipulation and control of structural and electronic properties, measuring the speed of material transformations • Later: THz/X-ray pump/probe experiments in the NEH • Requires a long THz transport line

4. Calculated Field and Spectrum at Focus Calculation by Henrik Loos for 1 nC and 20 fs

5. Pyro cam. Layout of THz Optical Table • Laser specs • 800-nm wavelength • 20-fs pulses • 68-MHz repetition rate • 150 mW average power • First: THz characterization • Energy per pulse • Bolometer and pyroelectric detector • Focal-spot size • Pyroelectric camera moved through focus • Later: Diagnostics and samples • Electro-optic sampling • Michelson interferometer • First samples in focused THz beam Delay stage Laser e− T Sample stage /pinhole xyz stage BS 2A bolometer 3T R Half wave plate 2A 2A flip mirror R ZnTe EO sampling R Balanced Diodes/Andor QWP BS 2A flip mirror THz Auto- correlation T,2A iris 2A T T 2A Motorized filter set Pyro detector Alignment laser

6. Putting the First Optics on the Table

7. Restricted, Parasitic, and Full-Time Use • Both electrons and x rays hit the foil • Negligible loss for photons > 2 keV: parasitic use possible • Can’t insert foil when users want soft x rays • Perhaps…bump electrons away from x rays at foil • Electrons pass through foil, and then return to center of beampipe before bending down to the dump • Let x rays miss the foil by skirting its edge or by going through a hole • Foil could be used at all times • What bump amplitude can we make? • Can we cut a hole or support a free edge in a Be foil? • Thickness limited to 5 µm by Radiation Physics

8. LHC Synchrotron-Light Monitors • Two applications: • BSRT: Imaging telescope, for transverse beam profiles • BSRA: Abort-gap monitor, to verify that the gap is empty • Particles passing through the abort kickers during their rise get a partial kick and might quencha superconducting magnet. • Two particle types: • Protons and lead ions • Three light sources: • Undulator radiation at injection (0.45 to 1.2 TeV) • Dipole edge radiation at intermediate energy (1.2 to 3 TeV) • Central dipole radiation at collision energy (3 to 7 TeV) • Spectrum and focus change during ramp

9. Layout: Emission and Extraction Cryostat 70 m 194 mm To RF cavities and IP4 To arc 1.6 mrad 420 mm D4 10 m D3 U Extracted light sent to an optical table below the beamline 560 mm 26 m 937 mm

10. Optical Table Extraction mirror Beam Table Shielding Alignment laser Calibration light and target PMT and 15% splitter for abort gap F1 = 4 m F2 = 0.75 m Cameras Intermediate image Focustrombone Slit Table Coordinates [mm]

11. Telescope for Beam 1 Door toRF cavities Undulator and dipole Beam 1 Beam 2 Beam-1 Extraction mirror(covered to hunt for light leak) Optical Table

12. LHC Beams at Injection (450 GeV) Beam 1 Beam 2 Horizontal 1.3 mm 1.2 mm Vertical 0.9 mm 1.7 mm Light from undulator. No filters. Open slit.

13. LHC Beams at 3.5 TeV Beam 1 Beam 2 Horizontal 0.68 mm 0.70 mm Vertical 0.56 mm 1.05 mm Light from D3 dipole. Blue filter. Narrowslit.

14. Calibrating the Abort-Gap Monitor • Inject a “pilot” bunch • Charge measured by bunch-charge and DC-current electronics • Attenuate light by ratio 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 Pilot bunch Abort gap After coasting briefly, bunch spreads out Time [100-ns bins]

15. Test of Abort-Gap Cleaning, December 2009 Abort gap (3 µs) Beam dumped Excitation had ringing on the trailing edge (improved in January) Time (s) Cleaning excites beam at transverse tune. Applied to a 1-µs region: immediate effect. Charge drifting into gap RF off: coasting bunch in bucket just after gap Position in fill pattern (100-ns bins)

16. Gas Bursts in the LCLS Injector • Gas burst near gun can lower photocathode’s QE • Archiver records pressures at 1 Hz: too low to track the source • Beam-synchronous multichannel digitizer (120 Hz) records all pressures in a long circular buffer. • Buffer saved automatically after each burst • One culprit found: “Guardian” software that halted beam by shutting off low-level RF drive to first linac section (L0A) • Pressure burst when Guardian is reset and L0A restarts

17. Gas Burst from L0A Restart Pressure RF Amplitude