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

0.3

0.4

0.5

0.6

0.7

0.8

0.9

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