Current Projects

Alan Fisher

APE Meeting2010-08-03

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

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

Calculated Field and Spectrum at Focus

Calculation by Henrik Loos for 1 nC and 20 fs

bolometer

Pyro detector

Pyro cam.

THzAuto-correlation

Sample stage/pinhole xyz stage

flip mirror

iris

Motorized filter set

Laser

QWP

ZnTe

BS

EO samplingBalanced

Diodes/Andor

T2A

T,2A

T

2A

3T2A

RR

T

Alignmentlaser

2A

Delay stage

2A

BS2A

Half wave plate R

flip mirror

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

e−

Putting the First Optics on the Table

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

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 quench a 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

Layout: Emission and Extraction

To RF cavities and IP4

To arc

Cryostat

Extracted light sent to an optical table below the

beamline

1.6 mrad

70 m

26 m 937 mm560 mm

420 mm

D3 U

10 mD4

194 mm

Optical Table

Alignment laser

Focustrombone

F1 = 4 m

PMT and 15% splitter for abort gap

Intermediate image

Table Coordinates [mm]

CamerasSlit

Calibration light and

target

F2 = 0.75 m

Beam

Table

Extraction mirror

Shielding

Telescope for Beam 1

Beam-1 Extraction mirror(covered to hunt for light

leak)

Door toRF cavities

Undulator and dipole

Beam 1 Beam 2

Optical Table

Beam 1 Beam 2

Light from undulator.No filters. Open slit.

LHC Beams at Injection (450 GeV)

Horizontal1.3 mm

1.2 mm

Vertical0.9 mm

1.7 mm

Beam 1 Beam 2

Horizontal0.68 mm

0.70 mm

Vertical0.56 mm

1.05 mm

LHC Beams at 3.5 TeV

Light from D3 dipole.Blue filter. Narrow slit.

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

Abort gap

Time [100-ns bins]

After coasting briefly, bunch spreads out

Pilot bunch

Test of Abort-Gap Cleaning, December 2009Abort gap (3 µs)

Position in fill pattern (100-ns bins)

Tim

e (s

)

Charge drifting into gap

Cleaning excites beam at transverse tune. Applied to a 1-µs region: immediate effect.

Excitation had ringing on the trailing edge (improved in January)

Beam dumped

RF off: coasting bunch in bucket just after gap

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

Gas Burst from L0A Restart

Pressure RF Amplitude