From LCLS to LHC: Light from Electrons, Protons, and even Lead Ions
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Transcript of From LCLS to LHC: Light from Electrons, Protons, and even Lead Ions
From LCLS to LHC:Light from Electrons, Protons,
and even Lead Ions
Alan Fisher
ARD Status Meeting2011 January 6
LHC
Terahertz Radiation from LCLS Electrons
Electrons passing through a metal foil emit intense, coherent transition radiation (CTR) at wavelengths l bunch length. Highly compressed LCLS bunches: l 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
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
Electric Field
Calculations by Henrik Loos
Calculated Field and Spectrum
At the THz focus, for a 1-nC, 20-fs bunch
Spectrum
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
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
Tra
nsm
issi
on
Photon Energy [eV]0 500 1000150020002500300035004000
0.0
0.1
0.2
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1.02-µm Be Foil at 45°
Beamline and Optical Table
Pneumaticactuator
Berylliumfoil
Diamond window
Track for laser curtain
Control rack
Laserchiller
e−
Laser Enclosure with Curtain
Foil in 6-way Cross
Pneumaticactuator
Berylliumfoil
Diamond window
Bellows
(Enclosure 1)
Pyrocam
Joulemeter
Fluorescent card
Fluorescent card (flip up)
SiHeNe
Fluorescent cardFilter 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 View Plan View
Off-axis parabolic
mirror
Complete Optics Layout
Optics enclosure
The optics will be enclosed for laser safety and for a dry-air purge.
(Water has THz absorption lines.)
THz Energy versus Charge and Bunch Length
Electric field E of a relativistic bunch varies with bunch charge q and duration t : E ~ q/t
Energy in the pulse then follows: E2t ~ q2/t Compare a q2/t fit (open circles) to the measured THz (solid)
for two bunch-charge values
Transmission of GaAs versus THz Intensity
Translate a GaAs wafer through the THz focus (at z = 0) Transmitted THz energy shows nonlinear absorption
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
e− bunch characteristics
I(t)200 nm steps, 10 shots/point, adjacent averaging
I(ω)Power spectrum
350 pC, 50 fs
350 pC, 115 fs
Interferometer Scans
Measured I(ω) Simulated electron form factor Simulated THz pulse
Insight from Simulations
1 nC, 20 fs
1 nC, 60 fs
350 pC, 50 fs
350 pC, 115 fs
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.
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!
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
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
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
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).
Power Radiated by a Charge in a Dipole
A relativistic charge q = Ze with mass m, velocity bc c, and energy E = eEeV = gmc2 travels through a dipole field Bd
Radius of curvature of the orbit:
Lead ions: Since r and Bd 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 g 4 makes the radiated power substantial—except in the LHC, where the protons have a g of 7500, but r = 6 km.
The factor of Z2g 4 means that ions radiate more than protons by a factor of 826/2084 = 162
2 22 4
20
23 4s
e cP Zcg
r
eV
d d
EmcqB ZcBgbr
Synchrotron Power in Various Rings
Radiusof
Curvature
Critical Wave-length
Powerin
Dipoles
Average Power
PerParticle
Per Turn
E I C r lc
[GeV] [A] [m] [m] [nm] [W/m] [W] [eV]
SPEAR-3 e − 3.0 5,870 0.5 234 7.86 0.163 9,230 460,000 912,000
PEP-2 HER e − 9.0 17,610 2.0 2,200 165 0.127 6,790 7,040,000 3,520,000
PEP-2 LER e + 3.15 6,160 2.5 2,200 13.75 0.246 18,300 1,580,000 633,000
LHC p 450 480 0.582 26,659 6,013 228,000 8.2E-07 0.0309 0.0531
LHC p 7,000 7,460 0.582 26,659 6,013 60.7 0.048 1,810 3,110
LHC 208Pb82+ 574,000 2,960 0.0061 26,659 6,013 968 0.084 3,170 520,000
Ring Circum-ference
Synchrotron RadiationDipoles
Ring ParticleType
Beam Energy
Beam Current
Normalized Energy
sI Pec2
Emc
g 2 sI Pec
r 2sP
cr
Spectrum and Critical Frequency
Spectrum near Critical Frequency
Normalized dipole emission, intergrated over vertical angle y, versus energy E/Ec
Long tail for E < Ec
Visible light << Ec for electron rings Rapid drop in emission for E > 10Ec
Peak is below Ec
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
Superconducting Dipole + Undulator
Add an undulator inside the dipole’s cryostat
Dipole: Field for 7 TeV
3.88 T Length
9.45 m Radius of bend
6013 m Bend angle
1.57 mrad Undulator:
Peak field 5 T
Periods 2 Period length
280 mm Pole gap 60
mm Wavelength for
609 nm450-GeVprotons (injection)
Undulator layout and field map
Undulator inside cryostat
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
Fisher — Imaging with Synchrotron Light
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
Photoelectrons per Particle at the Camera
Protons Lead Ions
Dipole center
UndulatorCombined
Dipole edgeUndulator
CombinedDipole 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 w >> wc
Particles per Bunch and per Fill
Protons Lead Ions
Particles in a “pilot” bunch 5109 7107
Particles in a nominal bunch 1.151011 7107
Bunches in early fills 1 to 43 1 to 62
Bunches in a full ring 2808 592
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 m 937 mm560 mm
420 mm
D3 U
10 mD4
194 mm
RF Cavities
BSRT for Beam 1
B1 Extraction mirror(covered to hunt for a light
leak)
Door toRF cavities
Undulator and dipole
Beam 1 Beam 2
Optical Table
Table Enclosure under Extraction Mirror
Beam 1 Beam 2
B1 Extraction mirror
Extraction Mirror
Protons—with their small heat load—can use a simple mirror without cooling.
Optical Table
Layout of the Optics
Alignment laser
Focustrombone
F1 = 4 m
PMT and 15% splitter for abort gap monitor
Intermediate image
Table Coordinates [mm]
CamerasSlit
Calibration light and
target
F2 = 0.75 m
Beam
Optical Table
Extraction mirror
Shielding
Beam 1 Beam 2
Light from undulator.No filters.
LHC Beams at Injection (450 GeV)
Horizontal1.3 mm
1.2 mm
Vertical0.9 mm
1.7 mm
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
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.
First Lead-Ion Images, 2.3 TeV
First ramp of one bunch of lead ions2011 November 5
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 bx,y
Only possible with a small ring current Beam bump
Compare bump of image centroid with shift seen by BPMs
5 mm
Emittance Comparisons at 450 GeV
Beam 2 Vertical
LHC Synchrotron Light
Nominal
Time [h] Time [h]
Beam 1 Horizontal
Beam 2 Horizontal
Beam 1 Vertical
LHC Wire Scanner
From SPS
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
b 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
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
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?
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
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
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 dumpedGap cleaningRF on(poor lifetime)
0 25 minutes
Charge in Abort Gap
Abort gap (3 µs)
Position in fill pattern (100-ns bins)
Tim
e (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
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
Tim
e (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
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: 103 photons/bunch at 1 TeV, and 3106 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
5105 protons (410-6 of a nominal full bunch) Only 1 photon every 200 turns
May require 2 APDs: APD for slow mode gated off during full bunches
Testing the Longitudinal-Density Monitor
Adam Jeff
1 turn
1 train
28 ns
One bunch, but protons are also in
neighboring buckets
Observing the Solar Corona
Lyot invented a coronagraph in the 1930s to image the corona Huge dynamic range: Sun is 106 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
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
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
Haloimage
Source Steering mirrors
Masking mirror
Halo Monitor with Masking Mirror
Alignment laser
Focustrombone
F1 = 4 m
PMT and 15% splitter for abort gap monitor
Intermediate image
Table Coordinates [mm]
Cameras
Slit
Calibration lightand target
F2 = 0.75 mMasking mirror
Diffraction stop
Zoom lens
During halo measurements:Insert zoom lens, masking mirror, and return
mirror.
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.
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
Halo Monitor with Digital Mirror Array
Alignment laser
Focustrombone
F1 = 4 m
PMT and 15% splitter for abort gap monitor
Intermediate image
Table Coordinates [mm]
CamerasSlit
Calibration lightand target
F2 = 0.75 mDMA
Diffraction stop
During halo measurements:Insert DMA and return mirror; rotate camera.
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.
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