Download Fisher_10_08_02

Current Projects
Alan Fisher
APE Meeting
2010-08-03
Ongoing Projects
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Terahertz radiation from the LCLS electrons
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Measuring short SPEAR bunches by cross correlation of
synchrotron light with a Ti:sapphire laser
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With Jeff Corbett (SSRL) and Aaron Lindenberg
Imaging the LHC protons with synchrotron light
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With Aaron Lindenberg of PULSE
With several CERN collaborators
Beam-loss monitoring for machine protection at LCLS
Catching gas bursts near the LCLS gun
Assorted LCLS measurements
Terahertz Radiation
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Electrons pass through beryllium foil downstream of undulator
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2-µm foil at 45° to beam, with diamond window below
THz radiation extracted from wake of compressed beam
Femtosecond pulses with intense fields:
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Peak electric field expected to be ~3 V/Å
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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
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Requires a long THz transport line
Calculated Field and Spectrum at Focus
Calculation by Henrik Loos for 1 nC and 20 fs
Layout of THz Optical Table
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800-nm wavelength
20-fs pulses
68-MHz repetition rate
150 mW average power
T
Laser
BS
bolometer
Half wave plate
Bolometer and pyroelectric detector
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Pyroelectric camera moved through focus
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Electro-optic sampling
Michelson interferometer
First samples in focused THz beam
BS
THz
Autocorrelation
QWP
2A
2A
Pyro detector
T,2A
iris
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R
Balanced
Diodes/Andor
Pyro cam.
Later: Diagnostics and samples
R
ZnTe
EO sampling
flip mirror
Focal-spot size
R 3T
2A
flip mirror
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2A
Energy per pulse
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Sample stage
/pinhole
xyz stage
2A
First: THz characterization
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Delay stage
Laser specs
T
T
2A
Motorized
filter set
Alignment
laser
e−
Putting the First Optics on the Table
Restricted, Parasitic, and Full-Time Use
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Both electrons and x rays hit the foil
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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
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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?
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Thickness limited to 5 µm by Radiation Physics
LHC Synchrotron-Light Monitors
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Two applications:
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BSRT: Imaging telescope, for transverse beam profiles
BSRA: Abort-gap monitor, to verify that the gap is empty
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Two particle types:
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Particles passing through the abort kickers during their rise get a partial
kick and might quench a superconducting magnet.
Protons and lead ions
Three light sources:
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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
Cryostat
70 m
194 mm
To arc
To RF
cavities
and IP4
1.6 mrad
420 mm
D4
10 m
D3
Extracted light sent
to an optical table
below the beamline
U
560 mm
26 m
937 mm
Optical Table
Extraction
mirror
Beam
Table
Alignment
laser
Calibration light
and target
Shielding
PMT and 15% splitter for abort gap
F1 = 4 m
F2 = 0.75 m
Intermediate
image
Cameras
Slit
Focus
trombone
Table Coordinates [mm]
Telescope for Beam 1
Door to
RF cavities
Beam 1
Undulator
and dipole
Beam 2
Optical Table
Beam-1 Extraction mirror
(covered to hunt for light leak)
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.
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. Narrow slit.
Calibrating the Abort-Gap Monitor
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Inject a “pilot” bunch
Charge measured by bunchcharge 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
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Last bunch in fill
First bunch in fill
Pilot bunch
Abort gap
After coasting briefly,
bunch spreads out
Useful to test gap cleaning…
Time [100-ns bins]
Test of Abort-Gap Cleaning, December 2009
Abort gap (3 µs)
Beam dumped
Time (s)
Excitation had ringing
on the trailing edge
(improved in January)
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)
Gas Bursts in the LCLS Injector
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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.
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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)
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Pressure burst when Guardian is reset and L0A restarts
Gas Burst from L0A Restart
Pressure
RF Amplitude