Reinventing the accelerator for the highenergy frontier

1
Reinventing the accelerator for the high-energy
frontier

• J. B. Rosenzweig
• UCLA Department of Physics and Astronomy
• Fermilab Colloquium - January 4, 2006

The history of discovery in high-energy physics
has been intimately connected with progress in
methods of accelerating particles for the past 75
years. This remains true today, as the post-LHC
era in particle physics will require significant
innovation and investment in a superconducting
linear collider (LC). The choice of the LC as
the next-generation discovery machine, and the
selection of superconducting technology has
thrown promising competing techniques -- such as
very large hadron colliders, muon colliders, and
high-field, high frequency LCs -- into the
background. We discuss the state of such
conventional options, and the likelihood of their
eventual success. We then follow with a much
longer view a survey of a new, burgeoning
frontier in high energy accelerators, where
intense lasers, charged particle beams, and
plasmas are all combined in a cross-disciplinary
effort to reinvent the accelerator from its
fundamental principles on up.
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Outline

• Historical overview of accelerators in particle
  physics
• Limitations of present accelerators
• Options for future accelerators
• Connections to other scientific fields
• Near-term future accelerators
• Some effects of linear collider technology
  decision
• Exotic acceleration techniques
• Incredible progress
• Will stewardship continue?

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Particle physics and particle accelerators have a
shared history (and destiny?)

• Groundbreaking discoveries always associated with
  innovations in accelerator and beam
  capabilities, e.g.
• Lawrence (cyclotron, radioactive elements)
• Rubbia and van der Meer (antiproton cooling, W/Z)
• Tevatron
• Astroparticle experiments complement measurements
  at the energy frontier in accelerators
• Consensus in the field emphasize the centrality
  of accelerator-based HEP
• Large Hadron Collider (LHC)
• International Linear Collider (ILC)

4
Schematic view of accelerators for particle
physics related fields
Betatron
Superconducting Circular Collider
FFAG, etc.
Circular Collider
Synchrotron
VLHC?
Cyclotron
Muon Collider?
2030
1930
Ion Linear Accelerators
Ultra-High Energy LC?
Electrostatic Accelerators
Electron Linear Accelerators
Electron Linear Colliders
Laser/Plasma Accelerators?
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Now mature ideas have driven HEP accelerators
forward

• Induction acceleration (betatron)
• Resonant electromagnetic acceleration (cyclotron)
• Normal and superconducting RF cavities (linac,
  synchrotron)
• Alternating gradient magnetic focusing
  (synchrotron)
• Fixed targetry, exotic particle sources
  (synchrotron, linac)
• Colliding beams in synchrotrons (circular
  collider)
• Colliding beams in linear accelerators (linear
  collider)
• Cooling of particle beam phase space (colliders)
• Particle polarization (fixed targets/colliders)

Are these enough for the future? Do we need to
re-invent the accelerator?
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The process of discovery collisions

• Scattering elastic and inelastic processes
• Tradition since Rutherford well known beam
  initial state, defines ?c impact parameter not
  known scanned over
• In collider, beam is probe and target
• Need dense beams for collisions

Scattering center
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Why colliders?

• Fixed target energy for particle creation
• Colliding beams (e.g. ee-) makes lab frame into
  center of mass
• Exponential growth in COM energy over time
• Livingston plot Moores Law for accelerators
• Challenge in energy, but not onlyluminosity as
  well

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Present limitations of collider energy

• Synchrotron radiation power loss
• Forces future e-e- colliders to be linear
• LEP (180 GeV COM) is last of breed
• Large(!) circular machines for heavier particles
• Consider muons for lepton colliders?
• Scaling in size/cost
• Approaching unitary limits
• Few 104 m in dimension
• Few 109

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The energy challenge

• Avoid giantism
• Cost above all
• Higher fields give physics challenges
• Magnets in circular machines
• Accelerating fields in linear machines
• Enter new world of high energy density physics
• Impacts luminosity challenge

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The Luminosity Challenge

• Circular colliders provide high repetition rate
• Linear colliders have much lower repetition rate
• Use many particles/bunch solution brings
  problems
• Inherent scaling for higher energy not enough

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Luminosity through particle number (N)

• Particle sources
• Polarized electronssolved at SLAC/KEK (80)
• Polarized positrons difficult, very high
• Exotics require many primary protons (p-bar, ?)
• Accelerating beam power enormous in LC
• 10s of MW in ILC
• Superconducting machine adds efficiency
• Large charge gt collective effects
• Space-charge, intra-beam scattering
• Coherent instabilities
• Beam-beam interaction

Schematic of polarized e source
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Luminosity through lower emittance (?n)

• Emittance area in phase space plane
• Collider design ? always smaller than sources
  deliver
• Damping employed
• Synchrotron radiation in damping ring (e e-)
  naturally gives ?n,x gtgt?n,y
• Stochastic cooling (p-bars)
• Ionization cooling (new) for muons
• Needs time (collision rate limit)
• Needs infrastructure (expertise and )

Schematic of ionization cooling (note similarity
to synch. radiation)
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Luminosity through better focusing (??)

• Beam size ? at focus is (?????)1/2
• For linear coliders ?yfew nm(!), ?x100 nm.
• Need chromatic corrections for ?y
• ? is depth of focus minimized at bunch length
• Smaller beams with stronger focusing
• Exotic solutions
• Ultra-strong magnets
• Permanent magnet quadrupoles
• Superconducting quadrupoles
• Plasma lenses?
• Asymmetric beams in LC to mitigate
• effects of extremely high charge density

Worlds strongest quadrupole (570 T/m field
gradient), built at UCLA PBPL
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High energy density in future colliders I the
accelerator

• High fields in violent accelerating systems
• Relativistic oscillations
• Diseases
• Breakdown, dark current
• Peak/stored energy
• Power dissipation
• Approaches
• High frequency, normal cond.
• Superconducting
• Lasers and/or plasma waves!

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High energy density in future colliders II
collective effects

• Many particles/bunch, extremely high charge
  densities
• Huge collective (focusing) fields luminosity
  limit
• Circular machines tune shift
• Linear colliders
• Disruption (two-stream instability)
• Beamstrahlung energy loss/spread, nuisance
  particles
• Classical electrodynamics and quantum processes
• LC initial state not as clean as naively thought

0
2Ub
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Approaches to new collider paradigms

• Advancement of existing techniques
• Higher field (SC) magnets (VLHC)
• Use of more exotic colliding particles (muons)
• Higher gradient RF cavities (X-band LC)
• Superconducting RF cavities (TESLA LC)
• Revolutionary new approaches (high
  gradient frontier)
• New sources i.e., lasers
• New accelerating structures and/or media i.e.,
  plasmas
• Truly immersed in high energy density physics

Another Talk
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The road to the next accelerator

• First fork in the road Snowmass 2001
• Consensus that LC is next machine post-LHC
• VLHC and muons demoted
• Second fork ITRP Selection of Linear Collider
  Technology
• Barish committee evaluates warm v. cold
  accelerator technology
• Somewhat surprisingly (to many in field),
  superconducting option favored

The crystal ball clears due to ITRP decision,
8/19/2004
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The LC technology selection
X-band, high gradient, normal conducting
traveling wave linac
Superconducting, L-band standing wave cavity

• ITRP committee determined that both technologies
  were viable
• Decision forced by need to concentrate global LC
  RD resources
• What drove the decision to endorse the cold
  option?
• What are the implications of this choice on
  accelerator RD, in
• and outside of the LC?

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The short answer

• Warm technology allows greater energy reach
• Now double accelerating gradient perhaps more
  soon
• Logical path towards even higher gradient
  approaches
• A future consideration?
• SC technology enables relaxed bunch format, low
  wakefields
• SC cavity has lower risk
• industrialization well advanced
• Reduced power consumption
• Synergistic development of technology for 4th
  generation light sources X-ray FELs
• X-band spin-off to medical linacs, not as
  compelling

For more information, see ITRP report
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The path not taken the warm linear collider

• X-band chosen to mitigate power demands
• X-band traveling wave cavities developed,
• gives gt65 MV/m unloaded gradient
• Serious breakdown issues recently resolved
• Important work on the road to higher gradient
• Klystron power an issue, fixed with RF pulse
  compression (SLED, etc.)
• Conclusion still many complications

X-band klystron
X-band linac section
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NLC testing has been aggressive, diverse
N. Phinney (Victoria, 2004)
Many efforts directly applicable to cold machine
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The linear collider technologyTESLA
Superconducting RF cavities

• Very high intrinsic Q (gt109), 6 orders of
  magnitude higher than NC
• Extremely beam-loaded operation
• Many pulses, ms apart, in ms fill
• Power goes into beam, not wall
• Even with tax from Carnot efficiency, SC gt2x
  efficient
• Large apertures (L-band), wakes and beam-breakup
  much less an issue
• Excellent high power technology
• Other applications (FEL, spallation, muons)

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A few TESLA/ILC challenges

• Particle sources are demanding
• Damping rings very large
• Positron sources (even unpolarized) difficult
• Maximize gradient
• Large effort at TTF (working FEL facility)
• Intrinsic limit on surface field not more than
  50 MV/m
• Intra-bunch-train feedback
• Message from ITRP adopt lessons from other
  designs
• Beam delivery, final focus, etc.

17 km (!) dogbone damping ring
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Spin-offf X-ray SASE FEL based on SC RF linear
accelerator

• Synchrotron radiation is (again) converted from
  vice to virtue
• SASE FEL microbunching instability
• Coherent X-rays from multi-GeV e- beam
• Unprecedented brightness
• Spin-off of TESLA program split from TESLA
  project in late 2001
• Approval from German govt, pending EU
• High average beam power than warm technologies
  (e.g. LCLS at Stanford)
• Many SASE FEL projects worldwide

10 orders of magnitude beyond 3rd gen X-ray light
source!
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Are we on the road to a 3 TeV LC?

• Surprise? SC LC option does not scale well
• Intrinsic low gradient
• 24 MV/m TESLA 500 GeV
• 35 MV/m TESLA 800 GeV
• 42 MV/m theoretical limit
• X-band still difficult
• Power sources, efficiencies
• High gradient means high frequency
• Where is power source?
• Look to wakefields (vice-to-virtue, again)
• Source of energy is coherent radiation from
  bunched, relativistic e- beam
• Also powers more exotic schemes

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High gradients, high frequency, EM power from
wakefields CLIC
CLIC wakefield-powered scheme
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Linacs where have we been, and why?

• NC linac development was driven by post-WWII
  availability of high power ?wave sources
• Basic acceleration scheme hasnt changed much,
  nor have ?wave sources
• Newer high freq. EM sources have ultra-high peak
  power
• Wakefield sources (CLIC and beyond)
• Optical source ultra-high power (gtTW) lasers
• Can we use these new sources for linear
  accelerators?
• Fields from 100 MV/m -gt 100 GV/m

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The optical accelerator

• Scale the linac from 1-10 cm to 1-10 mm laser!
• Resonant structure (like linac)
• Slab symmetry
• Take advantage of copious power
• Allow high beam charge
• Suppress wakefields
• Limit on gradient?
• 1-2 GV/m, avalanche ionization
• Experiments
• ongoing at SLAC (1 mm)
• planned at UCLA (340 mm)

Resonant dielectric structure schematic
e-beam
Simulated field profile (OOPIC) half structure
Laser power input
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Evading material breakdownThe inverse FEL
accelerator

• Run FEL resonance backwards with ultra-high power
  laser
• No nearby material laser very intense
• Magnetic field ltgt synchrotron rad.
• Accleration dynamics similar to ion linac
• Experiment at UCLA Neptune Lab 15 MeV beam
  accelerated to over 35 MeV
• Higher harmonic interaction observed
• Capture at 5 improve to near 100 with
  configuration improvements
• IFEL is now workhorse microbuncher

IFEL undulator (50 cm length)
Neptune IFEL single shot energy spectrum
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Inverse Cerenkov Acceleration

• Coherent Cerenkov wakes can be extremely strong
• Short beam, small aperture
• SLAC FFTB, Nb3E10, sz 20 mm, a50 mm, gt 11
  GV/m!
• New experiment in Aug. 2005 UCLA/SLAC/LLNL
  collab.

Simulated GV/m Cerenkov wakes for typical FFTB
parameters (OOPIC -)
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FFTB Cerenkov Wake Results

• Quartz fibers 350 ?m OD, 100-200 ?m ID, 1 cm
  length
• Energy differences not observable w/o longer
  tubes
• Observed breakdown threshold
• 4 GV/m surface field
• 2 GV/m acceleration field!
• Vaporization of 1 ?m Al cladding

View end of dielectric tube frames sorted by
increasing peak current
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Past the breakdown limitPlasma Accelerators

• Very high energy density laser or electron beam
  excites plasma waves as it propagates
• Excitation by ponderomotive forces (laser) or
  space-charge (beam)
• Extremely high fields possible

Schematic of laser wakefield Accelerator (LWFA)
Ex tenous gas density
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Plasma Wakefield Acceleration (PWFA)

• Electron beam shock-excites plasma
• Same scaling as Cerenkov wakes, maximum field
  scales in strength as

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The PWFA Blowout Regime

• Beam much denser than plasma
• Very nonlinear plasma waves
• Plasma electrons exit beam channel
• Very linear wakefield response
• Ez (accel) constant in r (EM wave)
• Focusing linear in r (ES ion field)
• Like linac quadrupoles!
• Good fields because no free-electron charges or
  currents in beam channel
• or are they?

Plasma wake (Ez) response, blowout regime,
OOPIC. Below radial dependence of fields in beam
region
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PWFA Experiments Large fractional energy gain
and loss at FNAL

• 15 MeV Beam nearly stopped in 7 cm of plasma in
  UCLA/FNAL A0 experiment
• Accelerating wake is also stable good efficiency

Acceleration to gt 24.3 MeV (130 MeV/m), 60 gain.
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Ultra-high gradient PWFA E164 experiment at SLAC
FFTB

• Uses ultra-short beam (20 ?m)
• Beam causes field ionization to create dense
  plasma
• Over 4 GeV(!) energy gain over 10 cm 40 GV/m
  fields
• Self-trapping of plasma e- s
• X-rays from betatron oscillations

ne2.5x10 17 cm-3 plasma
M. Hogan, et al.
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Plasma wave excitation with laser creation of
very high quality beam

• Trapped plasma electrons in LWFA give ?n1
  mm-mrad at Nbgt1010
• Narrow energy spreads can be produced
• accelerating in plasma channels
• Not every shot (yet)
• Looks like a beam!
• Less expensive than photo-injector/linac/compresor
  
• Very popular
• LBL, Imperial, Ecole Polytech.

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Energy doubling of LC beams the PWFA
Afterburner Concept
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Final Focus Plasma Lenses
Magnetic Quadrupoles
Underdense Plasma Lens
Uses electrostatic forces to focus electron beam
in both dimensions.
Uses magnetic forces to focus electron beam in
one dimension at a time.
Ex superconducting quad
Even low density plasma lens are impressvely
strong 150 T/m at 5x1012 cm-3 (FNAL
experiment)
For LC, density is 6 orders of magnitude higher
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FNAL Underdense Plasma Focusing Results
Beam Spot Before x FWHM
1200 µm y FWHM 1100 µm nb 5 x 1012 cm-3
Beam Spot After (Ave.)
x FWHM 200 µm y FWHM 300 µm nb
1 x 1014 cm-3
Unfocused 5 electron pulses
Plasma focused 1 pulse
The beam area is reduced by a factor of 22.
Equivalent to luminosity enhancement
Plasma on
Plasma off
Also demonstrated - time dependent focusing -
focusing with asymmetric beam for LC
Time-resolved intensity profile (w/streak camera)
13.5 Beam Aspect Ratio
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Storm clouds on the horizon

• Linear collider-like beams are extremely dense
  when subject to focusing in PWFA
• Field ionization
• Ultra-relativistic plasma e- response
• Instabilities (hosing, etc.)
• Ion collapse
• Ions move to axis under enormous beam fields
• Complete collapse for afterburner parameters
• Implications for plasma lenses
• Vice-to-virtue fusion scenario!

OOPIC simulation of ion density inside of
afterburner beam (from J.B. Rosenzweig, A.M.
Cook, A. Scott, M.C. Thompson, R. Yoder, 95,
195002 (2005))
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Challenges and prospects for advanced accelerator
application to future LCs

• Optical and plasma accelerators a challenge in
  experiment
• Very large fields
• Very small dimensions and time scales
• Multidisciplinary in the extreme
• Many collective effects to worry about (HED)
• We have orders of magnitude in learning curve
• Breath-taking recent progress
• More people needed students eager/welcome

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Overall Status of Advanced Accelerators for HEP

• People have worked on future accelerator concepts
  with some urgency for gt20 years
• Despite lack of resources, we have many
  accomplishments to show for this effort options
  that look promising
• How do we take advantage?

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Observations on proceeding

• With the LC technology decision, significant
  resources efforts will be thrown into LC
  development
• Re-organize and reprioritize
• High frequency RF acceleration initiative (DoE)
• HEPAP subpanel on advanced accelerators
• DoE/NSF very positive HEP accelerator
  stewardship at stake
• Programs healthy now
• Synergy with nearby fields (FEL, etc.)
• Overseas competition is heating up US leadership
  in doubt
• Future support needs to increase
• Need vocal support from HEP field also man/brain
  power!