Beam Source Requirements for High Beam Power...

A U.S. Department of EnergyOffice of Science LaboratoryOperated by The University of Chicago

Argonne National Laboratory

Office of ScienceU.S. Department of Energy

Beam Source Requirements forHigh Beam Power Accelerators

John W. LewellenAccelerator Systems DivisionAdvanced Photon Source

2

PioneeringScience andTechnology

Office of Science U.S. Department

of EnergyJohn Lewellen8 – 10 November 2004 Workshop on High-Power Beams

Acknowledgements

Charles Brau (Vanderbilt)Jym Clendennin (SLAC)Dave Dowell (SLAC/LCLS)Klaus Flöttmann (DESY)Fred Kiewit (Eindhoven)Yuelin Li (ANL)Kwang-Je Kim (ANL)John Noonan (ANL)

John Petillo (SAIC)John Power (ANL)John Schmerge (SLAC)Charles Sinclair (Cornell)Frank Stephan (DESY-PITZ)Todd Smith (Stanford)Alan Todd (AES)

and many others for interesting discussions ongun design & improvement

Katherine Harkay (ANL) – ongoing discussions & support

3




Overview of the Talk

• Acknowledgements & Introduction

- What this talk is not about

- Accelerator Taxonomy

- What drives the requirements?

• Source requirements

- Storage-ring replacements

- X-FELs

- MW-class IR-FEL facilities

• Conclusions

4




What am I not talking about?

• Injector & linac technology specifics

- DC vs. RF

- NC vs SRF

- recirculation

• Particular proposed future sources in any detail

- provide broad brush strokes

- generalization of performance figures & concepts

• “Operational” performance (i.e. uptime, MTBF, MTBM, etc.)

“User Perspective” (from machine builder’s standpoint)

5




Accelerator Taxonomy

• “Big Iron” accelerators

- Linear colliders

- Next-generation x-ray lightsources

• “Desktop” accelerators

- Electron microscopy

- Electron beam lithography

- Small laboratory experiments

• “Mini-Me” accelerators

- Radioisotope generation

- High-power free-electron lasers

- Slow positron production

- Pulse radiography

Courtesy S

LAC

/LCLS

Courtesy LLNL

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What drives the requirements?

• Linac-based light sources

- bifurcation between high-flux and high-brightness

- minimum performance enhancement to be worthwhile

- our ability to make magnetic structures

• “Mini-Me MW” FELs

- wavelength

- maximum desired electron beam power

- size requirements

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Sense of Scale: X-ray Facility

AdvancedPhotonSource

APS: 3rd-generation storage-ring light source

Circumference: ~ 1 km

Beamlines: ~ 35 bending magnet ~ 40 undulator/ID

Local user group offices

Users per year: thousands

Scheduled run hours per year: thousands

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What improvements can be made?

• Increase the x-ray beam flux (photons / sec unit bandwidth)

• Increase the x-ray beam brightness (peak or average)

- more “useful photons” per unit flux delivered

• Increase the temporal resolution (shorter x-ray pulses)

- time-resolved phenomena

- pump-probe experiments

• Coherent light production

- “light bulb-to-laser” transition

9




Storage Rings vs. Linac-Based Light Sources

• Electrons circulatemany times

• Beam power delivery =γ losses

• Electron beamproperties set by thering (injector irrelevant)

• Electrons make onepass

• Beam power delivery =Ebeam·Ibeam + γ losses

• Electron beamproperties set by thelinac (injector iseverything)

Storage Ring Light Sources Linac-Based Light Sources

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Storage-Ring Replacements & X-FELs

“1st-generation” Linac Based Light Sources

Storage-RingReplacement

(SRR):

Higher Peak Brightness

Improved TemporalResolution

X-ray Free ElectronLaser (X-FEL):

Coherence

Temporal Resolution

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SRR – Performance Requirements

• >102 peak brightness improvement over existing SR sources

• < ps temporal resolution (i.e. bunch length)

y,nx,n

22 INB

εε

γ∝ωωΔ

0.1

0.1

1.0

3 – 18

BunchCharge

[nC]

New Gun (II)

New Gun (I)

S-band Gun

APS

Source

13.5‡1351000† 0.1 0.1 x 0.1

135*1351000† 0.1 0.1 x 0.1

45*453000† 0.331 x 1

1*1300 20 - 4043 x 0.5

Avg.Brightness

Enhancement

PeakBrightness

Enhancement

PeakCurrent

[A]

BunchLength

[ps]

Norm.Transverse

Emittance [µm]

† With linac-based bunch compressor‡ Assuming 10 mA average beam current* Assuming 100 mA average beam current

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But here’s the kicker…Imean = Qb / Δt

where:

Imean = average beam current Δt = bunch-to-bunch interval

Qb = single-bunch charge

Δt: as large as possible fortiming exp.

Qb: as small as possible for εn

define SRR in terms ofbrightness, not flux

1 10 1000.1

1

10

100

1?103

1 nC100 pC10 pC

Bunch spacing [ns]

Average current [mA]

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The Pierce Parameter and X-FEL Performance

3

x,n2p

x2

2b

22u

A

332

x

bu

A

p I

64

fK

I

1

2

1

22

Kf

I

I

εγ⋅

βπ

λ⋅=

γ

πσ

λ=ρ

The larger the Pierce parameter…• the smaller the gain length• the shorter the X-FEL undulator• the less expensive the facility is

0

25

50

75

100

125

150

175

200

225

0.1 0.6 1.1 1.6 2.1

Norm. emittance [µm]

Satu

rati

on

len

gth

[m

]

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

η/(η

d+η

+η

)

LCLS OperatingPoint

Emittance too big? Pump up K andλu … but there’s a price.

γ-2 λu for fixed λ, so slow progress

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X-FEL – Performance Requirements

• Maintain LCLS wavelength performance

• Minimize overall systems cost

+=2

12

2

2

Ku

γλ

λ

Fixed Parameterλ = x-ray wavelength = 1.5 Å

Parameters we can adjustλu = undulator period > 1_ cmK = undulator strength ~ 1

Parameter to Minimizeγ = electron beam Lorentz factor

With existing undulatortechnology, need abouta 4 GeV beam to make1.5 Å x-rays … assumingwe can get the gain.

15




SCALE: 1 in = 100 m

100 m

APS Linac

PAR

Storage Ring

Booster Synchrotron

APS VUV-FEL

N

10 2.400

1.000

Primary Linac SecondaryLinac

UndulatorFarm

Storage RingInjection Branchline

Interleaving Dipole Energy Separator

X-FEL Design Optimization…

Parameter LCLS High -Energy Low -? design #1 Low -? design #2 Beam Energy 13.35 GeV 7 GeV

(APS beam energy) 4 GeV

(alternate params) ?n 1.2 ?m 0.1 ?m 0.1 ?m (0.15 ?m) Ipeak 3400 A 1000 A 500 A (1000 A) ? E/E 0.01 % 0.025 % 0.025 %

Undulator period 3.0 cm 3.0 cm 1.25 cm K 3.7 1.3 1.0 Optimized beta fcn. 7.5 m 3 m 2 m Saturation length 91 m 31 m 24 m

Assume:• Existing undulator technology• Existing linac technology• “Classic” SASE design

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Summary: SRR and X-FEL sources

• Single-bunch parameters for X-FEL and SRRsources are remarkably similar:~ 0.1 µm normalized emittance (or better)

~ 1 kA peak beam current (post-compressor)

• SRR will need 10 – 100 mA beam current to maintainreasonable average brightness

• X-FELs might want long inter-pulse spacing tochange out samples

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A Word on Linear Collider Guns…

• Why are these desired?

- Damping rings are expensive as well as challenging

- Makes most sense when LC combined with X-FEL

• Basic parameters not dissimilar from X-FEL guns

• Additional requirements:

- flat-beam production (100:1 emittance ratios)

- polarized electron beam production

• A positron gun would be nice…

18




IR-FEL – Driving Parameters• “1-ish” µm operating wavelength

• ~ 100 MeV max. beam energy

• 1 MW optical power output

• Notional FEL extraction efficiency: ~ 1%

⇒1-A average beam current

⇒100 MW circulating e-beam power

• Large outcoupling (50-ish %)

⇒ reduce circulating optical power (save the mirrors)

⇒high gain per pass

⇒ fairly high beam quality

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High-power IR and UV FELs

Injector

7 MeV 50 MeV 100 - 150 MeV

Beam ParametersIpeak ~ 500 – 1000 Aεn ~ 5 – 10 µm

δE/E ~ 0.1 – 0.25%

Electron beam power: 100 MWOptical beam power: 1 - 2 MW

RF power used: 7 MW (to dump)Average beam current: 1A

Wallplug efficiency: ~ 10 – 20%: 2 MW (FEL)

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Source Parameter Estimation

• For the moment, assume no “exotic” designs

- no tapered undulator

- no linac/wiggler combinations

- etc.

• Assume some “reasonable” undulator parameters- λu ~ 3 cm

- K ~ 2− β ~ 10 m

• Assume some “bulk” beam properties

- Ebeam ~ 100 MeV- ΔE/E ~ 0.25% (rms)

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What do we get…?

geometr ic beam emit tance

?x0 . 7 1 5m m? beam size in the undulator (average)

aw1 . 4 1 4? wiggler parameter the rest of the known universe uses

?r1 . 1 7 5?m? l a s e r w a v e l e n g t h

fB0 . 8 0 8? helical undulator Pierce parameter reduction factor

? 3 . 8 4 81 03?

??Pierce parameter

L1D0 . 3 5 8m?one-d imens iona l ga in length

Lg0 . 6 1 6m? three-dimensional gain length

Pbeam1 0 0G W? peak particle beam power

Pn0 . 0 6 1W? peak noise power at startup

Psat2 0 7 . 9 4MW? peak optical power at saturation

Lsat1 4 . 8 8 7m? saturation length of the device

ps 1 012?sec???Particle beam properties Undulator propert ies

particle beam energy:Eb1 0 0MeV??? wiggler parameter

(NOT rms):K 2??

normalized electron beam emit tance: ?n1 0?? m m? mrad???

undulator per iod: ?w3cm???rms relative energy spread on the beam:??0 . 2 5%?? average design undulator

beta function:?f1 0m??

peak current: Ip1 n C?1 ps?

??

Ip1 1 03? A?

Calcu la ted proper t ies

? 1 9 5 . 6 9 5? Lorentz factor for the beam

? 5 1 . 1n mrad??

σx = 0.715 mmSpot size in undulator isreasonable; no exoticfocusing schemes needed

λr = 1.175 µmLaser wavelength is in thedesired range

Lg = 0.616 mGain length (1 e-foldinglength) is a reasonable matchfor a few-m undulator in ahigh-loss cavity.

εnx = 10 µm, Ip = 1 kAAdjust to get reasonable Lg

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Summary – MW-class ERL-FEL Sources

• 1-A average beam current; Qb = 1 amp / frf

• Transverse emittance: ~ 10 µm (rms norm) @ ~ 1.5nC / bunch

• Longitudinal emittance: ~ 100 keV ps (rms)

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LCLS gun

TESLA X-FEL Gun

IR/UV ERL

MicroscopeGun

X-FEL, SRR Gun

0.0001

0.001

0.01

0.1

1

10

0.0001 0.001 0.01 0.1 1 10 100

Bunch charge [nC]

Tra

ns.

em

itta

nce

[u

m]

Specified Guns

New Gun Designs

TESLA X-FEL GunLCLS gun

new X-FEL gun

MicroscopeGun

SRR Gun

IR/UV ERL

0.0001

0.001

0.01

0.1

1

10

0.0001 0.001 0.01 0.1 1 10 100 1000

Average beam current [mA]

Tra

ns

. e

mit

tan

ce

[u

m]

Specified Guns

New Gun Designs

General Source Requirements

TESLA X-FEL Gun

LCLS gun

New X-FEL gun

MicroscopeGun

SRR Gun

IR/UV ERL

0.0001

0.001

0.01

0.1

1

10

100

1000

0.0001 0.001 0.01 0.1 1 10 100

Bunch charge [nC]

Ave

rag

e b

eam

cu

rren

t [m

A]

Specified Guns

New Gun Designs

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A Word on Recirculation Losses…

• Megawatt-class IR FEL

- 1 A x 100 MeV = 100 MW

- 1 kW losses = Pbeam x 10-5 = 0.001%

- 1 W loss = 10-8 ≈ 90 electrons / bunch (from 1.4 nC)

• Storage-Ring Replacement X-ray Source

- 100 mA x 5 GeV = 500 MW

- 1 kW losses = 0.0002% beam loss

- 1 W loss = 2·10-9 ≈ 1_ electrons / bunch (from 100 pC)

• Controlling halo no longer means winning an X-Box game.

• Source parameters must be consistent with zero-lossoperation.

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Summary

• X-FEL wants:

- order-of-magnitude improvement in emittance

• MW IR-FEL wants:

- true CW operation

- high beam currents (ca. 1 A) at modest energy

- ~ 10-8 recirculation losses

• SRR wants:

- true CW operation

- high beam currents (ca. 10 – 100 mA) at 7-ish GeV

- order-of-magnitude improvement in emittance

- True zero-loss recirculation

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