Optical Stochastic Cooling Proof-of Principle Experiment

at MIT-Bates

Bill FranklinOSC Workshop

MIT-BatesFebruary 2, 2006

Feb. 2, 2006 2

Outline• Motivation

– Why do an OSC Demonstration Experiment– Why do it at Bates

• Experiment– Prerequisites – Design – Measurements

• Summary– Working plan– Schedule – Costs

Feb. 2, 2006 3

Motivation for OSC Demonstration

• Achievement of highest luminosity in collider experiments requires combination of complementary techniques

• Optical stochastic cooling based on interaction of particles with own undulator radiation within 2nd undulator provides cooling with right selection of parameters

• Promising technique for high energy protons, ions to lower cooling time under certain conditions

• Large investment for machines at high energy frontier• Technique has not been experimentally verified • Significant technical challenges in implementation• Can test much of physics with lower energy stored electron

beam

Feb. 2, 2006 4

Goals of OSC Experiment• Physics

– First demonstration of cooling based on undulator radiation– Test OSC formalism for cooling rates– Study cooling of beam as function of energy – Investigate designs for magnetic bypass– Realistic projections for ions based on electron results

• Technical– Development of high duty mid-IR amplifier – Diagnostics and Feedback for very sensitive phase stability of

beam and undulator radiation– Controls for dynamical cooling apparatus (e.g. amplifier gain

and bypass fields)– Significant components of apparatus transferable to high

energy ion rings

Feb. 2, 2006 5

OSC Proposal • New proposal to DOE Nuclear

Physics Office to do select beam physics experiments

• Physics uniquely explored in low-energy electron ring

• Transit-time OSC demonstration leading item

• Component on polarization, Complementary proposal on THz

• Previous successful models for small-scale accelerator experiments (e.g. electron cooling)

• Previous OSC proposals not funded (Duke, IUCF)

Feb. 2, 2006 6

Why OSC at Bates• Unique properties of accelerator complex

• Electron machine significantly eases amplifier requirements• Energy regime, weak bends permit long synchrotron damping

time • Flexible lattice• Long straight sections for OSC bypass• High RF frequency for South Hall Ring allows flexible design for

magnetic bypass delay line

• Practical considerations• Well developed controls, tunes, and diagnostics• Test accelerator, no user demands• University facilitates inclusion, education of new people in field• Bates seeking cohesive accelerator program for South Hall

Ring • Strong collaboration between Bates, BNL in several areas,

where OSC studied and considered for RHIC, eRHIC

Feb. 2, 2006 7

• Polarized source provides peak current > 4 mA, variable structure

• Linac accelerates electrons up to 500 MeV, rep < 600 Hz• Single pass recirculator permits doubling of energy• Stacking of electrons in South Hall Ring for long-lived CW beam

MIT-Bates Accelerator Center

• Nuclear physics experimental program completed in June 2005• Experimental areas and beamlines decommissioned, accelerator kept intact• MIT has assumed ownership of facility• Bates ops. staff reduced, physicist contribution required for accelerator expt.

Former Beam-lines

Feb. 2, 2006 8

Storage Mode for the South Hall Ring

• Highly automated filling cycle, reliable operation• Two-turn injection in SHR at 10 Hz, beam lifetime governed by target• Highly developed beam, albeit different needs from OSC demo

Stacking StorageTypical Storage Cycle (EPICS Control System)

• Precision experiments on nucleon in SHR using ABS and BLAST • Ran primarily at 850 MeV, beam intensity ~200 mA, longitudinal pol. ~.65

Feb. 2, 2006 9

Cooling Requirements

• Modest amplifier power, gain requirements• Amp gain depends on energy, power on number of bunches

1812bunches

• OSC time for electrons short • osc ~ 1 s achievable at SHR• Cooling rate dependent on SHR bunch charge, length, and amp bandwidth

Feb. 2, 2006 10

Energy for OSC Demo• OSC experiment with electrons requires long radiation damping

time• Energy regime, weak bends permit long synchrotron damping

time• SHR Energy: 0.3-1.0 GeV with injection at beam energy• SHR Circumference 190.2 m, 16 Bends, =9.144m • Radiation damping time of order few seconds

>1s

E<400MeV

Feb. 2, 2006 11

SHR Floor Space for OSC experiment and IR Beam lines

IR beam lines

OSC

• 190 m circumference

• Racetrack design with 2 long straight sections

• OSC experiment to reside in east straight section

• SHR tunnel > 4 m in width

• 16 dipoles w/sync light ports (THz studies and OSC diagnostics)

• Can add shielding for proximity to apparatus

Feb. 2, 2006 12

SHR OSC Bypass

• Preliminary layout for OSC experiment in SHR east straight • Install 8 m long chicane, undulators bracket chicane • Utilize existing magnets, power supplies, and vacuum

equipment from decommissioned Bates X-Line• Optical amplifier replaces existing magnetics• Overall chicane delay of few ns, need very fast, stable amplifier• Proximity of access to diagnostics, amplifier highly desirable• Increase h from 1812 to 1817, path length modification of 50 cm

AmplifierUndulator Undulator

Undulator radiationMagnetic Delay Line

SHR beam

Feb. 2, 2006 13

The 2.856 GHz RF Cavity

• Unique 2856 MHz RF system, single cavity, 50 kW, CW klystron • Interbunch spacing of 10 cm provides significant flexibility in designing magnetic delay line• Tuning of RF frequency available within limited range• No modification of RF system envisioned for OSC experiment

Feb. 2, 2006 14

Prerequisites to OSC Demonstration

• Propose SHR Feasibility Study for this year– Demonstrate low energy SHR operation (E < 300 MeV)– Bunch control, single bunch SHR operation

(linac,recirculator, injection)– Investigate diagnostics for fast profile monitoring – Orbit control and response (fast fine control for bypass)– Test mode SHR operation, synergy with other proposed

accelerator research (THz, Stern-Gerlach)

• OSC Design Study can proceed in parallel– Magnetic Bypass Properties and Controls– Undulator system– Optical Amplifier– Diagnostics and Feedback

Feb. 2, 2006 15

SHR Low Energy Operation• Long radiation damping time essential for OSC, storage also difficult• SHR primarily operated at 850 MeV, past experiments at 569 MeV • Storage tests done at 330 MeV during early commissioning of SHR • Magnet calibration established down to 300 MeV• Very precise energy calibration (10-5) at 370 MeVthrough spin

precession of extracted beams (T. Zwart thesis)

• Small angle Coulomb scattering has stronger effect at low energy, cooling should improve SHR lifetime• No fundamental limit to low energy storage, should reach E < 300 MeV

Feb. 2, 2006 16

Present Bates Injector

• Quasi DC beam delivered by DC polarized photoinjector

• Linac structure set by RF chopper and RF prebuncher at 2856 MHz

• 2-3 ps pulse length at end of linac

• Injector fills every bunch (1 mA 350 fC accepted per bucket)

350

ps

634 ns

Existin

g

Electron ring

1812 Bunches

• SHR revolution = 1.576 MHz

• Ring cavity frequency = 2856 MHz

Feb. 2, 2006 17

Single Bunch Requirement

• Multibunch instabilities clearly observed during the 2005 runs for THz test

• OSC demo requires ability to control and vary bunch filling pattern

• Sync. SHR beam with OSC pulsed amplifier

• Presently building new mode-locked laser with electro-optical modulation to control the fill pattern and intensity of the SHR at the single bunch level.

• Solution will need phase-locked reference to SHR subharmonic with low jitter. Start with oscillator at low enough frequency to permit E-O modulation

Single

Bunch

63

4

ns

Pro

pos

ed

Electron ring

Feb. 2, 2006 18

60 cmYb-dopedfiber

30 cmSMR

133 cmSMR

435 cmSMF

Similariton Fiber Laser for Bates source (170fs, 10nJ, 28MHz)

J. R. Buckley, F.Wise, F. Ö . Ilday, T. Sosnowski, Opt. Lett., 30, 1888 2005

10-40 MHz fiber-based laser

(F. Kaertner, E. Tsentalovich)

Laser Specs

•T pulse < 100 ps

•Pulse Jitter < 10 ps (or shutter jitter < 50 ps)

•Rep rate ~ 10 Hz

• Wavelength 530 nm (doubled Yb-doped fiber) QE = ~1% on thin GaAs photocathode

Presently under construction, test soon

Feb. 2, 2006 19

SHR Beam Diagnostics

• Evaluate performance in single bunch mode for tuning, orbit control

• Need fast high resolution on profile

• Fast feedback for phase control of electron beam and undulator radiation in OSC experiment

• EPICS diagnostics for stored beams – 32 sets RF pickups as Beam Position Monitors during storage – Steering correctors distributed throughout SHR allow local orbit modifications– Framegrabber digitizes synchrotron light images (beam transverse profile)– Lifetime evaluated from DCCT measurements

– Generally update at 10 Hz

Feb. 2, 2006 20

• Investigate less expensive alternatives to streak camera for OSC

“BLAST” (original, z > 20ps)

“LMC-4” (z= 3.6 ps)

• LB9 SR Visible light transport to accessible area• Bunch Longitudinal profile from Streak Camera (C6860) B. Podobedov• Synchroscan f: 81.6 MHz (2856/35), integration time ~ 100ms

SHR Low momentum compaction Lattice operation (Dec. 2004)

350ps

Feb. 2, 2006 21

• Low Lattices- Based on quadrupole regroup and polarity switches • Ability to manipulate bunch length with SHR lattice, RF parameters, • Prefer long bunch for OSC, correlates well with sync

• Studies demonstrate MAD calculations of SHR lattice reasonably accurate in modeling momentum compaction at 10-4 level

SHR Bunch Length Studies

Feb. 2, 2006 22

SHR OSC Magnetic Bypass Design

• Need precise control of bypass path integrals for phase stability for golden orbit electron and own amplified undulator radiation

• Simulation to define relation which will permit optimal cooling• Dipoles designed for 22 degree bends =4.57 m) quads from

X-Line• Sextupoles need to be purchased• Multiple BPM’s in bypass insertion• Define power supply stability requirements• Consider air-core magnets for fast modulation and dynamical

cooling

Feb. 2, 2006 23

• Two identical precisely tuned undulators to generate coherent radiation

• Tunable field to permit OSC at different energies with fixed wavelength

• λ=λu(2+K2)/(4γ2) with a bandwidth of ΔωFWHM=ω/Nu, where Nu is the number of undulator periods, K=qBuλu/(2πmc), Bu is the undulator field strength, and λu is the undulator wavelength

• 10 cm period typical, > 1m in length for light sources

Undulator properties

Feb. 2, 2006 24

Optical Amplifier (F. Kaertner)

• Wide bandwidth nonlinear optical system– Pump laser locked to SHR RF subharmonic– Scalability in central wavelength (2 m for Bates), avg. power– Precise phase delay control– Pulsed operation

• Build and test at RLE, coordinate design with BNL

1.047 mYb:fiberlaser20ps,1nJ ,28MHz

Nd:YLFpoweramplifier

20ps,1J ,28MHz

Yb:fiberpre-amplifier20ps,100nJ ,28MHz

10-100J

Signalbeamat2m

MgO:PPLNcrystal

ChirpedMirrors

Filter

Feb. 2, 2006 25

OSC Equipment Budget

Item Year 1 (k$)

Year 2 (k$)

Year 3 (k$)

Beam instrumentation 10 10 10 Electron Bypass magnets, power supplies

0 50 0

Vacuum hardware 0 50 0 Undulator system 0 500 0 Optical amplifier system

20 ps fiber laser 20 0 0 5 W Fiber amplifier 30 0 0 IR Spectrometer 25 0 0 Enhancement cavity, MgO:PPLN crystals and oven

25 0 0

Solid state amplifier to 30 W level

0 70 0

Optics and electronics 15 15 15 Total 125 695 25

• Budget from OSC proposal• Investigating undulator alternatives• Improve spec on beam instrumentation• Synergies between OSC, THz proposal

Feb. 2, 2006 26

OSC Proposal Schedule Overview

• Year 1 – Carry out feasibility study in SHR– Develop coherent plan for OSC, THz, Stern-Gerlach expts.– Cooling simulations for South Hall Ring– Amplifier development underway

• Year 2– Magnetic bypass installation and commissioning run– Shielding area construction and survey– Undulator fabrication– Amplifier completion and bench tests

• Year 3– Undulator, Amplifier installation in SHR and commissioning– Initial cooling measurements

Feb. 2, 2006 27

Measurements

• Well documented beam profile (x,y,s), lifetime pre-cooling• Optical transmission, noise, and gain control in ring

environment• Synchronization of amp with electron beam and dynamic

phase stability• Variation of bypass properties and dynamic cooling• Control of cooling rate through regulation of amplifier power • Measurement of cooling rates as function of bunch charge,

beam energy, bunch charge, length• Multibunch effects• Synchrotron-betatron coupling for transverse cooling

Feb. 2, 2006 28

Summary• The Bates South Hall Ring provides a number of unique

features which could permit for the first time a detailed and economical laboratory for the study of optical stochastic cooling

• Bates accelerator physicists and laboratory leadership have interest in pursuing this area of research as part of accelerator research program

• Proposal submitted to DOE, review pending• Prospects for success in both obtaining funding and

successfully carrying out experimental program depend crucially on strong collaboration with accelerator community on experiment design and program

• Seek to define program with maximum possible impact in basic research and applicability to larger scale facility (RHIC)

Feb. 2, 2006 29

An OSC Demo Collaboration

• Experiment would complement ongoing OSC work • Interested institutions

– BNL - RHIC requirements, amp and diagnostic development

– LBL - Modeling and bypass design– Indiana - students, analysis– MIT RLE - amplifier and feedback systems– MIT-Bates, LNS - SHR optics, beam hardware, operations– Bogazici - Simulations

• Others encouraged to join

Feb. 2, 2006 30

Discussion Topics• Applicability of OSC demo pieces to RHIC

– Implications of electron results for ion beams – Bypass design and control– Amplifier– Beam instrumentation

• Clarify requirements for OSC demo and design study– Electron beam optics

• SHR (lattice properties)• Magnetic Bypass (stability, path integrals for OSC,

magnetics)

– Undulator (periodicity, tolerance)– Amplifier (gain, timing)– Instrumentation (resolution, feedback)

END OF PRESENTATION

Feb. 2, 2006 32

Transverse effects

As the particle gains or loses energy by its interaction with the electric field of itself and its sampling partners, the corresponding momentum closed orbit is also modified. Thus the betatron phase space coordinates are changed as well. This may generate heating and cooling effect to the beam. The change of transverse betatron coordinates are (for ID>0)

xi2c=xi2+D2Gsin(ΔΦi+ψij), x'i2c=x'i2+D'2Gsin(ΔΦi+ψij).

where (xi2,x'i2) and (xi2c,x'i2c) are the betatron phase space coordinates of the i-th particle before and after correction at the second undulator location, and D2,D'2 are the value of the dispersion function at the second undulator location.

Feb. 2, 2006 33

• In the first undulator, a test particle radiates an EM wave propagating in the s-direction: Ei=E0 sin (ks-ωt +Φi) with electric field amplitude E0 and phase Φi. The wave number and frequency are k=2π/λ and ω= kc. This radiation propagates to the optical amplifier, while the particle follows the bypass and traverses it in a time Δti=ℓi/βc, where βc is the speed of the particle.

• The time Δt0 required for radiation to pass all the way between undulators, including the amplifier delay, must be constrained and maintained by a feedback system to yield the condition ℓ0-cΔt0=(n±¼)λ, where n=0, 1, 2, ..., and the ± sign depends on the beam transport property in the bypass. The test particle arrives at the second undulator with a time delay δ(Δt) = Δti- Δt0 and with a phase shift ΔΦi=k(ℓi-ℓ0)=k[xiI1+x'iI2+δiID] relative to the phase of the electric field at zero crossing. For simplicity, hereafter, we use (xi, x’i), and δi as the betatron phase-space coordinates and fractional off-momentum variable of the ith particle at the first undulator location.

• In the second undulator, the particle interacts with the electric field of its own radiation. The fractional change of its momentum is: δPi/P=-G sinΔΦi, where ID >0 is assumed, G=gqE0NuλuK[JJ]/(2cγP) is the amplitude of the fractional momentum gain-factor, q is the magnitude of the particle charge, Nu is the number of undulator periods, g is the amplification factor of the optical amplifier, and δPi is the amount of the momentum change related to the coherent longitudinal kick Δδi=δPi/P.

• Let D2 and D'2 be the dispersion function and its derivative at the second undulator. The changes of the particle betatron coordinates at the exit of the second undulator are Δxi2=-D2(δPi/P) and Δx'i2=-D'2(δPi/P), where xi2 and x’i2 are the phase space coordinates of the i-th particle at the second undulator location.

OSC Mechanism (SY Lee, IUCF)

Feb. 2, 2006 34

• Each particle also interacts with the EM waves emitted by other particles in a sample within a distance less than Nuλ. Assume that a test particle interacts with Ns electromagnetic waves (including its own wave) in a sample. The change of the particle's momentum at the exit of the cooling insertion becomes

δic=δi-Gsin(ΔΦi+ψij) where ψij=ΔΦj-ΔΦi,

• Longitudinal effects We assume ID>0. A test particle interacts with the electromagnetic waves

radiated from the sample of Ns particles. We have to evaluate the ensemble average of the quadratic change:

Δ(δi2)=δic

2-δi2= -2δiGsin(ΔΦi+ψij)+G2[sin(ΔΦi+ψij)]2.

obtain the longitudinal damping decrement

αδ=-(δic2-δi

2)/σδ2=2GkID exp(-u) -G2Ns/(2σδ

2) where u=½ k2[(β1I1

2-2α1I1I2+γ1I22)εx+ID

2σδ2]

is a measure of the total thermal energy of the beam. The optimal momentum gain-factor and the maximum damping decrement are

Gδ=2kIDσδ2 exp(-u)/Ns with αδmax=(2k2ID

2σδ2/Ns)exp(-2u).

Feb. 2, 2006 35

SHR Orbit Control

• Orbit control software to apply series of local corrections to trajectory

• Based on BPM information

Feb. 2, 2006 36

Second test: CSR Power, spectrum measurement & lattice study June 5-7, 2005

CSR detector system L. Carr

FTIR spectrometer Nicolet Magna 860

LHe cooled Si detector

M1

M2

B16 Line

Quartz viewport3.8” opening

Side View

Top ViewM3

Source to M1 ~3.6m

Movable horn GHz detectors

Quartz view port (6 mm) transmission

Feb. 2, 2006 37

Instrumentation for Spectroscopic Analysis of Coherent SR L. Carr

electronicselectronicsdetectordetector

InterferometerInterferometer

sourcesource

parabolicreflectorsparabolicreflectors

Feb. 2, 2006 38

Final interferometer and bolometer assembly

Microwave detector

for CSR in time domainB. Podobedov

Setup for THz run: second test

•Remote RF frequency control: Electron path length adjustment for thermal closed orbit changes.•B16 line & instrumentation for CSR measurement.

Feb. 2, 2006 39

Bunch length (Streak Camera) vs. bunch current &

Spectrum comparison to Gaussian beam ( 3.5ps rms)

Significant lengthening when Ib > 1.1A (I=2 mA)

Notice:

The bunch length is measured over 100 ms integration time. (over ~1.57x105 turns and 2.85x108 bunches)

Distinguish of bunch “lengthening” caused by instabilities and other mechanisms is not possible.

Low spectrum frequency, similar to Gaussian beam.

Low bunch intensity & insignificant bunch distortion

Feb. 2, 2006 40

Bolometer

75-110 GHz detector

50-75 GHzdetector

Sub-THz signal in time domain (Microwave detector)

At low current, only transverse beam instabilityI=2 mA, transverse damping x=200 ms

Longitudinal beam instability at higher currentI= 10 mA, longitudinal damping s= 100 ms

Feb. 2, 2006 41

Beam Profiles

• Streak camera integrates over defined period• Fewer pictures but better statistics for large t• Histogram other 2-D correlations (centroids, 2, bgd)• N-tuples possible to generate cuts• Analyze large data sets together

t = 1001 ms t = 111 ms

Feb. 2, 2006 42

• Plateau visible in pulse ht, vs. beam intensity• Achievement of high average intensity difficult for low alpha• Automating analysis for intensity scans

Feb. 2, 2006 43

2856 MHz

1.576 MHz

~

/2048 (1.4 MHz)

mode locked Laser

Prescaler

1-10 Hz

~

635ns

Timing schematic for the bunch-bunch control

Fiber

(10 fs)

GaA

s

Photo

cath

ode

/512 (5.6 MHz)

/1024 (2.8 MHz)

……

/128 (22.4 MHz)

/64 (44.8 MHz)

EOM1 EOM2

530 nm

Feb. 2, 2006 44

The particle emits EM wave:

∫∫∫ ===

′+′++=

2

1

2

1

2

1)(

)(,

)(

),(,

)(

),(

location at particleth of scoordiante space phase are ),(

1122

1111

111

21110

s

s

D

s

s

s

s

ii

Diiii

dss

sDIds

s

ssMIds

s

ssMI

sixx

IIxIx

ρρρ

δll

)sin(0 ii tks φω +−Ε=Ε

The path length of a particle from location s1 to s2 is

The EM wave propagates to the optical amplifiers, and the charged Particle travels through the beam bypass. They would arrive at theSecond (corrector) undulator with a time difference of Δti = ℓI /βc.

The time difference between the test and the reference particlesat the corrector undulator Is Δti – Δt0 . The phase difference becomes

ΔΦi = ω(Δti – Δt0 ) , or

)()( 21110 Diiiii IIxIxkk δφ +′+=−= ll

Feb. 2, 2006 45

Feedback

• Phase stability requires control to fraction of undulator wavelength

Feb. 2, 2006 46

SHR OSC Magnetic Bypass Design

• Cooling relies on precise difference in phase for electron and own undulator radiation

• Precise control of path integrals depending on (x,x’,δ)• Accurate representation of SHR distribution at entrance• Chicane should permit versatile set of conditions for

path integrals, incorporating into SHR MAD calculations• Develop method for chicane setting control

Particle travel length in bypass line