Introduction to FACET and the Plasma Wakefield Accelerator Experiment

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Introduction to FACET and thePlasma Wakefield
Accelerator Experiment

• Tor Raubenheimer
• SLAC PPA Assistant Director for Accelerator
  Research
• FACET/BELLA Joint DOE Review
• July 21 - 23, 2008

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FACET Presentation Agenda
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Talk Outline

• The Promise of Plasma Acceleration
• A Plasma Wakefield-Based Linear Collider concept
  (PWFA-LC)
• The PWFA-LC RD Program
• The Facility for Advanced Accelerator
  Experimental Tests (FACET)

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Plasma Wakefield Linear Collider

• Plasma acceleration offers ultra-high gradients
  that may provide a cost-effective path to TeV or
  multi-TeV collider
• 50 GV/m demonstrated at SLAC FFTB facility over
  85 cm
• Developed a Plasma Wakefield Linear Collider
  (PWFA-LC) concept to understand PWFA RD
  requirements
• Issues in plasma acceleration are common for all
  acc techniques
• Issues for electron acceleration are relatively
  well understood
• Still extensive exploration of physics required
  for positrons
• FACET designed to address major issues of a PWFA
  stage
• Successful completion of FACET ? define all
  parameters of PWFA-LC
• Next would be a pre-construction demo of the
  final configuration

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Promise of Plasma Acceleration

• Dramatic gains overlast few years in electron
  acceleration(3000x SLAC gradient)
• Material breakdownavoided ? potentialfor
  compact linearaccelerators
• Experiments andbenchmarked simulations
  providebasis for furtherdevelopment

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Highlights of the PWFA RD at FFTB
Details to be discussed by Tom Katsouleas in
following talk
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A Concept for a PWLA-LC

• Developed a concept for a 1 TeV PWFA-LC
• Makes use of 30 years of conventional linear
  collider RD
• Focus RD on plasma acceleration not other LC
  subsystems
• Used to understand primary issues and define
  FACET RD plan

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Linear Collider Luminosity

• Luminosity is critical in a linear collider
• Physics studies have been based on 1x1034
  cm-2sec-1
• Need large beam powers, large charges per bunch,
  and small spot sizes
• Conventional LC studies of damping rings for gex
  few mm-mrad and gey 1 of gex with bunches of
  few 1010
• ATF prototype damping ring and CESR-TA test
  facility
• Conventional LC studies of final foci can deliver
  flat beams with beta functions 10 mm x 0.1 mm
  looking at round beams also
• Final Focus Test Beam and ATF2 at KEK
• CLIC physics studies have been made for dB 30

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PWFA and the Drive Beam Concept

• Lots of possible PWFA-LC configurations
• Afterburner multi-stage afterburner two-beam
  PWFA
• Multiple PWFA stages in a CLIC-like two-beam
  scheme allows re-use of drive beam accelerator ?
  cost savings
• Drive beam accelerator could be normal-conducting
  or superconducting
• Require highly efficient transfer of energy to
  drive beam
• CLIC Test Facility 3 hasdemonstrated 95 xferof
  rf energy to beam
• Parameters similar tothose needed for a PWFA-LC

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Why Plasma-Wakefield Acceleration?

• High gradient acceleration demonstrated
• 45 GeV acceleration in a single stage seen in
  FFTB
• Experiments at FFTB indicate feasibility of beam
  transport and emittance preservation
• Matching to plasma beta-functions
• High beam-loading with small energy spreads
  illustrated in simulations
• Possible to generate drive power extremely
  efficiently
• Two-beam accelerator using decades of accelerator
  experience
• No show-stoppers
• Of course, many problems and questions need
  resolution? FACET

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Why SLAC?

• The SLAC linac provides unique infrastructure to
  develop the PWFA technology
• 20 GeV low emittance high current electron and
  positron beams
• Only place in the world to develop this promising
  approach
• Bring 45 years of high energy accelerator design
  and construction experience
• Have excellent team for construction and
  operation of accelerator experiments
• Bring 30 years of experience designing linear
  colliders
• SLAC group is critical to development of a linear
  collider design
• Group is very motivated and has been committed to
  advancing TeV-scale lepton physics for decades
• FFTB studies built a strong plasma physics
  collaboration

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Plasma Simulation for PWFA-LC

• Missing figure from Warren / Mark

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High-Level Parameters for PWFA-LC
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Primary Issues for any Plasma-based LC

• Need to understand acceleration of electrons
  positrons
• Luminosity drives many issues
• High beam power (20 MW) ? efficient ac-to-beam
  conversion
• Well defined cms energy ? small energy spread
• Small IP spot sizes ? small energy spread and
  small De
• These translate into requirements on the plasma
  acceleration
• High beam loading of e and e- (for efficiency)
• Acceleration with small energy spread
• Preservation of small transverse emittances
  maybe flat beams
• Bunch repetition rates of 10s of kHz
• Multiple stages allow better beam control and use
  of drive-beam ? possible to demonstrate single
  stage before full system test

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Plasma Cell Operating Regime

• Density of 1x1017 cm-3 and a gradient of 25 GV/m
  over 1m
• Operate using field ionization regime
• Drive beam creates plasma channel and allows
  dense plasmas
• Drive beam with gex,y lt 100 mm-mrad with many nC
  per bunch
• Easily satisfied with rf gun
• Need to regenerate plasma between pulses
• Operate in nonlinear plasma regime
• With 1010 particle per bunch and small
  emittances? nb gtgt np and have to be in the
  nonlinear regime
• Two advantages of nonlinear regime
• Linear focusing forces (at least for electrons)
• Large voltage gradient which allows for heavy
  beam-loading while compensating energy spread
  critical for linear collider
• Discussed by Tom Katsouleas

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Simulation Codes

• Extensive suite of simulation codes
• Linear collider
• GuineaPig IP beam-beam simulation
• ???
• Lucreta linac tracking and tolerances
• DIMAD particle tracking
• LiTrack longitudinal phase space tracking
• Plasma cell (covered by Warren Mori)
• OSIRIS
• QuickPIC
• Extensive benchmarking against other codes and
  against experiments

OSIRIS
VORPAL
QuickPIC
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Present PWFA Understanding

• Electron physics is relatively well understood
• Simulations benchmarked against FFTB results
• Scaling understood to perform parameter
  optimization
• Need further understanding of tolerances and
  sensitivities
• Some issues remain but expect FACET will confirm
  basic parameter choices
• Positron case is different
• Two different options e/e and e-/e
  acceleration
• Both have difficulties with both acceleration and
  emittance preservation
• Different PWFA-LC configurations use e/e or
  e-/e
• Simulations are more difficult and less developed
• Need more extensive studies
• Need experimental exploration of parameter space

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FACET Facility for Advanced Accelerator
Experimental Tests

• Use the SLAC injector complex and 2/3 of the SLAC
  linac to deliver electrons and positrons
• Compressed 20 GeV beams ? 20 kA peak current
• Small spots necessary for plasma acceleration
  studies
• Two separate installations
• Final bunch compression and focusing system in
  Sector 20
• Expanded Sector 10 bunch compressor for positrons

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FACET Sector 20 Accelerator Science Facility
(ASF)
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FACET Experimental Parameters
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FACET Parameter Space

• Designed FACET to cover the important parameter
  space for studying a PWFA-LC
• gt2e10 electrons or positrons per pulse
• Multi-stage bunch compressors to achieve lt25 mm
  bunch length
• 20 kA peak current
• Low emittances from damping rings for small spots
  lt10 mm
• Can deliver flat beams or round beams
• Final focus system to match beta functions to
  plasma cells
• Choose asymmetric or symmetric IP beta-functions
• Can vary beta functions, emittances, bunch length
• Two bunch operation (drive/witness) with e-/e-,
  e/e, and e-/e (high charge e-/e after an
  upgrade)
• Demonstrate real witness beam acceleration

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Two-Bunch Operation

• Critical to demonstrate drive/witness
  acceleration
• Use notch collimator in Sector-10 bunch
  compressor to generate two separate bunches
• Technique worksfor either e-/e-or e/e
• Varying collimatorshape and positionallows
  flexibilityin two-bunch format

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Possible FACET Upgrades

• Three possible upgrades being developed at this
  time
• Sector 20 Sailboat chicane (discussed further
  next slide)
• Allow acceleration of e bunch followed by e-
  bunch 5cm behind
• Different path lengths through final compressor
  chicane so that e follows e- drive beam by 100
  mm?Allows for high charge drive/witness
  demonstration for e-/e
• Injector upgrade
• Replacement of the electron gun with an rf gun to
  improve e- emittances (piggy-back on linac
  upgrade plan)? Improves transverse matching ?
  Allows for multi-bunch studies to understand
  plasma stability and heating and cell operation
• Lower damping ring energy
• Decreasing the energy would allow for lower
  transverse and longitudinal emittances from the
  ring? Improved two-bunch performance

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FACET Sector-20 Sailboat Chicane Upgrade

• After DOE review in February, we looked at
  improvements ? Sailboat Chicane
• Provides an option for demonstrating
  drive/witness acceleration of e-/e at high
  charge (4e10 total charge)
• Full design was not ready for inclusion in
  proposal
• Short side has same cost as old dogleg and
  facilitates upgrade
• Ready for inclusion or as an upgrade on the
  2013-timescale

Focal point
Positron chicane (upgrade)
Dogleg compressor (February)
Electron or positron compressor chicane
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PWFA-LC Experimental Program
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PWFA-LC Simulation/Design Program

• Extensive simulation and theoretical development
  program in parallel with FACET experimental
  program
• Also directed design and engineering program for
  PWFA-LC

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PWFA-LC FACET Program Summary

• FACET is designed to answer critical PWFA
  questions in support of a linear collider design
• Demonstrate electron acceleration with high
  efficiency and small energy spreads
• Demonstrate electron emittance preservation
• Understand positron acceleration, choose optimal
  positron acceleration configuration and
  demonstrate along with positron emittance
  preservation
• Complete program with high charge e-/e
  drive/witness configuration requires upgrade of
  Sector 20 sailboat chicane
• Understand plasma stability and heating issues
• Complete program requires injector upgrade
• After successful conclusion, all parameters for a
  PWFA-LC could be specified

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PWFA-LC Beyond FACET

• After FACET, the technology should be ready for a
  demonstration of the resulting optimized
  configuration
• Will require multi-stage acceleration with
  electrons and positrons
• Will need multi-bunch drive beam and multi-bunch
  primary beam
• Expect that this second test facility will be a
  large scale linear collider test facility similar
  in size to present conventional rf linear
  collider test facilities 100 M scale
• Strong synergies with CLIC Two-Beam Acceleration
  test requirements
• Possible on SLAC site or an upgrade of a CLIC
  test facility
• Possible to consider FACET-II demonstration on
  2020 timescale

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PWFA-LC Collaboration

• The PWFA-LC collaboration will be formed from the
  core FFTB experimental group (SLAC, USC, UCLA,
  and Duke) and the SLAC Linear Collider Department
• Excellent plasma theory and simulation
  contributions
• Strong experimental group with FFTB experience
• Strong linear collider design group
• Collaboration must grow to support full PWFA-LC
  program both experimental and design efforts
• Open structure to engage additional
  groups/laboratories
• Working on MOUs and detailed collaboration
  structure see final talk
• Interest from CERN in drive beam design
• Expect interest from other national laboratories
  and universities

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Summary

• Plasma acceleration offers ultra-high gradients
  that may provide a cost-effective path to TeV or
  multi-TeV collider
• 50 GV/m demonstrated at SLAC FFTB facility over
  85 cm
• Developed a Plasma Wakefield Linear Collider
  (PWFA-LC) concept to understand PWFA RD
  requirements
• Issues in plasma acceleration are common for all
  acc techniques
• Issues for electron acceleration are relatively
  well understood
• Still extensive exploration of physics required
  for positrons
• FACET designed to address major issues of a PWFA
  stage and lead to the next step
• Successful completion of FACET ? define all
  parameters of PWFA-LC
• Next would be a pre-construction demo of the
  final configuration

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Agenda