1 Advanced Development of Particle Acceleration by Stimulated Emission of Radiation (PASER) W. D. Kimura, L. Schächter, S. Banna ATF Users Meeting April.

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Advanced Development of Particle Acceleration by Stimulated

Emission of Radiation (PASER)

W. D. Kimura, L. Schächter, S. Banna

ATF Users MeetingApril 4-6, 2007

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Brookhaven National Laboratory (Accelerator Test Facility)

- Marcus Babzien

- Karl Kusche

- Jangho Park

- Igor Pavlishin

- Igor Pogorelsky

- Daniil Stolyarov

- Vitaly Yakimenko

University of California, Los Angeles

- David Cline

- Xiaoping Ding

- Lei Shao

Collaborators

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Outline

Background

Goals of Proposed Experiment

Improvements to Experiment Design and Procedure

Description of Experimental Apparatus

Phase I – High-Gradient Demonstration

Phase II – Staged PASER Demonstration

Proposed Schedule and Runtime Needs

Conclusions

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Background

Particle Acceleration by Stimulated Emission of Radiation (PASER) successfully demonstrated for first time at ATF in Proof-of-Principle (POP) experiment

- PASER does not require multi-TW laser driver or subps e-beam bunch

- Only requires train of microbunches with spacing equal to activemedium transition wavelength

- Requires no phase-matching to stage PASER sections

POP experiment can be improved upon in many ways

- Hardware and operational improvements

- Better control and diagnosing of experimental parameters

- More extensive measurements and optimization of parameters

- More thorough investigation of new physics related to PASER

- More data to compare with model and theory

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Goals of Proposed Program Primary experimental goals of Advanced PASER Development are:

- Design and build improved PASER CO2 discharge system

- Demonstrate much higher energy gain and acceleration gradients(target is >50 MeV/m)

- Obtain more extensive data to characterize process, includinginvestigating new physics associated with PASER effect

- Demonstrate ease of staging process

- Compare with model and theory

Primary theoretical goals are:

- Investigate alternative active media, such as Ar+ plasma and solid-state media

Ar+ PASER operates at very low gas pressures

Solid-state PASER may be capable of ~1 GeV/m gradients andelectrons travel in a vacuum

- Develop theoretical basis for follow-on program

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Proposed Program Divided Into Two Phases

Phase I:

- Design, build, and test at STI improved PASER gas chamber

- Install improved PASER cell, plus diagnostics, on ATF beamline

- Using existing IFEL, produce microbunch train to drive PASER

- Perform extensive measurements to characterize and optimizesystem for maximum energy gain and gradient

Phase II:

- Install second PASER discharge system

- Measure characteristics of staged PASER system

- Perform any additional measurements as needed

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Summary of Experiment Improvements

Parameter POP Experiment

Proposed Program

Comments

E-beam energy 44.6 MeV 70 MeV Higher beam energy helps reduce space-charge and scattering effects.

Macrobunch duration

4 ps (effective)

2 – 5 ps Will vary in order to change number of microbunches M

Microbunch duration

3 fs 3 fs Dictated by CO2 laser wavelength. Cannot

change valve.

Macrobunch charge

0.1 nC Up to 5 nC Will increase until space-charge effects become an issue.

Microbunch charge

~0.2 pC Varies Charge in each microbunch depends on macrobunch charge and pulse length.

CO2 gas mixture

pressure

0.25 atm Up to 1 atm Pressure affects optimum energy density. Gas scattering and Cerenkov radiation loss are counter-effects. Will find best operating pressure that gives highest energy gain.

Applied voltage 30 kV Varies Voltage depends on electrode gap separation. Will utilize high voltage driver similar to commercial lasers, capable of higher voltages to permit adjusting energy density.

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Summary of Experiment Improvements (cont.)

Parameter POP Experiment

Proposed Program

Comments

Electrode length 40 cm Most likely 40 cm

The electrode length will be chosen during the design phase of the program. Issues such as space constraints on the ATF beamline may affect the length of the PASER cell.

Electrode gap spacing

2.5 cm <2.5 cm A smaller gap will make construction of the discharge system easier.

Pumping efficiency

~1% >10% (goal) Will take advantage of commercial CO2

laser technology and techniques.

Gas scattering compensation

None Use solenoid magnet

Will surround electrode to help control gas scattering.

E-beam windows 2 m thick diamond

2 m thick diamond

Will use same window design.

Measure gain of medium

Not done Incorporated in design

Will be used to determine stored energy density.

Main discharge trigger

Spark-gap Thyratron More reliable trigger with less jitter. Common in commercial lasers.

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Possible Design for Improved PASER System

New PASER gas chamber designed to hold two PASER discharge assemblies

Gas scattering traveling through last half of chamber will not affect PASER energy gain – only reduces beam charge slightly

Adjustable permanent magnet

triplet

Tospectrometer

Camera Camera

Foil Foil

PASER chamberDiamondwindow

Diamondwindow

FromIFEL

e-beam

Camera

Foil

H.V. H.V.

Electrode

Electrode

Solenoid

Solenoid

Adjustable permanent magnet

triplet

(Location of second set of discharge

electrodes)

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Possible PASER Chamber Design

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Can Use Permanent-Magnet Quadrupoles for Triplets Before and After PASER Cell

STI manufactures PM quads

Compact design

- Magnetic field tunable usingmotors to move magnets

- Permits obtaining tight focusof beam into cell

Can also use hybrid focusing configuration

- Use existing upstreamelectromagnet quads

- Use single PM quad justbefore entrance to PASERcell

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Solenoid Around Electrode Can be Made With Permanent Magnets

Permanent magnet (PM) solenoid has advantages over electromagnet solenoid

- More compact; does not require water-cooling or high-current powersupply; is inherently stable; and can have stronger fields

- STI has already performed preliminary magnetic analysis of PM solenoidfor photocathode electron gun

Example of component layout

Magnetic field plot

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Electrode

Solenoid

Solenoid

CO2 laser probe beam

e-beam

Measuring Gain of CO2 Discharge Gives Excited-State Energy Density

Use CO2 laser probe beam to measure gain versus discharge parameters

Will also use to optimize pumping efficiency

PASER theory predicts optimum energy density is a function of other parameters

- Gain measurementsimportant for verifying thisdependence

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Optimum Energy Density Dependence Reveals Interesting New Physics

Optimum energy density of active medium (wact) shows oscillating dependence on number of microbunches M, but not beam size (Rb)

- Implies collective effects of entire ensemble of electrons affects abilityto extract energy from medium

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Summary of Major Phase I Tasks Measure gain as function of CO2 gas mixture pressure, composition, and

high-voltage settings at STI

Once installed at ATF, determine optimum tune for e-beam through cell

- Adjust quads and solenoid

- Maximize delivered charge

Use double-period STELLA undulator for operation of IFEL at 70 MeV

- Adjust CO2 laser power to undulator to achieve desired modulation

- May possibly utilize STELLA chicane to reduce drift space

Use CTR to monitor bunching efficiency

Perform PASER experiments

- Vary number of microbunches by varying e-beam pulse length

- Systematically scan over other parameters

- Measure energy spectrum with and without discharge present

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Schematic of Staged PASER System

Adjustable permanent magnet

triplet

Tospectrometer

Camera Camera

Foil Foil

PASER chamberDiamondwindow

Diamondwindow

FromIFEL

e-beam

H.V. H.V.

Electrode

Electrode

Solenoid

Solenoid

Camera

Foil

H.V. H.V.

Electrode

Electrode

Solenoid

Solenoid

2nd PASER stage 1st PASER stage

Adjustable permanent magnet

triplet

Second discharge system would be identical to first one

Note, staging requires no special positioning of second discharge with respect to first one

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Summary of Major Phase II Tasks

Determine optimum e-beam tune through both stages

- May primarily affect downstream e-beam optics, e.g., exit triplet

- Again, aim for maximum charge throughput

Measure energy spectrum with and without second discharge on

- Should see doubling of energy gain

- Vary parameters to determine dependence

- Compare with model predictions

Use Phase I results to find optimum operating condition for second stage

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Advanced Concepts: Ar+ PASER Advanced PASER concepts will be investigated in parallel with experimental

effort and will focus on studying alternative active media

Challenge is making microbunch train with 476.5 nm bunch separation

- Possibly use seeded FEL driven by Nd:YAG pumped dye laser

Creates microbunch train as by-product of FEL process

Approach being pursued at UCLA Neptune Lab in collaboration with STI

Argon ion laser active medium

- Breakdown of medium less of issue because medium is already a plasma

- Argon ion photons are 50 times more energetic than CO2 photons

- Highest gain in pulsed argon lasers is at 476.5 nm at 20 – 30 mTorr

- Pinch effect has been observed that might help enhance local excited-state density

- Low pressure means may be able to use discharges similar to gas-filled capillaries (eliminates windows)

- Use wire-mesh technique to generate custom microbunch train

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Advanced Concepts: Solid-State PASER

Challenge is making microbunch train with 1.06 m bunch separation AND with 1 GeV energy

- Possibly make train at low energy and accelerate in damping ring

Issues such as effects of coherent synchrotron radiation must be studied

Use Nd:YAG rod as active medium with e-beam traveling through hole in center of rod

- All scattering/breakdown effects eliminated because electrons travelthrough vacuum

- Nd:YAG photons are 10 times more energetic than CO2 photons

- Excited-state energy density is ~10 times higher

- Hence, energy density may be 100 times larger → ~1 GeV/m possible

- BUT, field from electrons does not appreciably penetrate into rod unlesselectrons are highly relativistic, i.e., 1 GeV

- In principle, can use wire-mesh technique, but 1 GeV beam must have low

emittance

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Proposed Program Schedule and Runtime Needs

Estimate for runtime requirements

- Phase I: 6 weeks

- Phase II: 4 weeks

Year 1 Year 2 Year 3

Design improved

cell

Fabricate improved

cell

Test cellat STI

Install cellat ATF

Perform Phase IExperiments

Activate 2nddischarge

Perform Phase II:Staging Experiments

Analysis and modeling for experiments and advanced concepts

Prepare papersfor publication

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Role of Collaborators

ATF staff responsible for

- Generating microbunch train using STELLA undulator

- Optimizing e-beam tune through system

- Operation of CTR diagnostics

UCLA (Prof. Dave Cline) responsible for

- Graduate and postdoc support

- Similar role as during STELLA

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Conclusions Advanced PASER Development program offers unique opportunity to

investigate a new paradigm in advanced acceleration schemes

- PASER is potentially a simpler scheme capable of comparableacceleration gradients as other advanced methods

- If wire-mesh technique is used to make microbunch train, then this eliminates need for IFEL

PASER effect also has interesting physics

- Collective e-beam effects on excited molecules affect energyexchange process

- Opportunities to test better active media

There is a synergism between PASER, wire-mesh technique, STELLA, and inverse Cerenkov acceleration that is unique to the ATF