1Jefferson Lab

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A Medium Energy Collider at JLab
Alex Bogacz1, Tanja Horn1, Franz Klein2, Pawel
Nadel-Turonski2

• 1Jefferson Lab
• 2Catholic University of America

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The Next JLab Upgrade
A strategic choice has to be made

• Fixed target experiments
• Increase CEBAF energy beyond 12 GeV
• Beam emittance will deteriorate
• Collider experiments
• Use CEBAF as part of injector
• Further energy upgrades are detrimental for
  luminosity
• Design criteria for high and low energy colliders
  are very different

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High vs. Medium Energy Colliders
Concepts are mutually exclusive

• High energy colliders can only operate close to
  maximum energy
• Widening the energy range reduces the luminosity
  significantly
• Only weak focusing (ß) possible to accommodate
  quadrupole strengths at high energy and aperture
  sizes (smax) at low energy.

Detector must fit in the interaction region (f,
in the thin lens limit)

• Multi-purpose design cannot compete with an
  optimized design
• Physics Options
• Inclusive at high energies
• Exclusive at medium energies

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Physics at a medium energy collider
Physics opportunities and requirements

• Exclusive processes at high Q2 or low x
  resolution
• Compton Scattering (DVCS, double DVCS)
• Meson production
• Hadron spectroscopy
• Charmed baryons (Lattice QCD)
• Hybrid baryons?
• (Semi-) Inclusive measurements kinematic reach
  and flavor tagging
• Down to x10-3 at fixed target energy similar to
  COMPASS
• Charge symmetry violation

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A Medium Energy Collider at JLab
Emphasizes the strengths of JLab
Physics of exclusive processes drives the design

• Kinematic reach
• For exclusive reactions limited by detector
  resolution

• Figure of merit
• Luminosity x Acceptance x Polarization2

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Symmetric Collider
Max fixed target equivalent energy of 170 GeV

• In general, colliders have the advantage of
• Small backgrounds (no Mollers)
• Good figure of merit, in particular for
  transverse polarization
• Symmetric colliders also offer
• Lowest lab momenta for a given s
• Optimal momentum resolution
• Good particle identification
• Improved acceptance

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Our Collider Design

• Design Goal
• Luminosity gt 1033 cm-2 s-1 in 4 GeV/c on 4 GeV/c
  electron-proton collisions
• Components
• Figure-8 intersecting storage rings
• CEBAF used as part of electron injector
• Ion injector based on Fermilab Project X
• 2 GeV superconducting Linac (prototype for
  Project X)
• Proton Linac technology important for energy
  applications
• Interaction Regions
• Up to four collision points
• CEBAF experimental end stations A-D as for 12 GeV
• New fixed target area for protons
• Long pulse neutron spallation source?

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Collider Performance Summary

• Luminosity for 4 on 4 GeV/c
• 2x1033 cm-2 s-1 (conservative)
• 2x1034 cm-2 s-1 (using COSY estimate)

• Energies

• Ion Species p, d, He-3, He-4, Li-7

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Contributions to Luminosity
Luminosity per IR 2 1033 cm-2 s-1 in 4 on 4
GeV/c kinematics.
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Collider Performance Summary cont.

• Electromagnetic beam-beam forces between
  colliding bunches, may
• cause emittance growth
• induce coherent instabilities
• decrease luminosity and its lifetime

• Severity of the effect is measured by the
  beam-beam interaction parameter, ?

• Acceptable values for ?

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ProjectX style ion injector
Accumulator/electron cooling ring
SRF linac
ProjectX-like linac
polarized ion source

• IUCF style polarized ion source (p, d, He-3,
  He-4, Li-7)
• polarized ion gun (RFQs) 1 Amp
• SRF linac HINS-like (300 MeV/u pulsed)
• Accumulator/Electron cooling ring cooling with
  200 keV DC electrons (0.01 sec) - 80 mA
• SRF linac based on CW Project X (2 GeV)
• 0.5 MW average power (for polarized beam)

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ProjectX style ion injector

• Stacking/Accumulation/Bunching Booster Ring
• Multi-turn (10 15) injection from 4 GeV SRF
  linac to the Stacking Ring
• Electron stripping and phase space painting
• Accumulation of 0.8 A coasted beam at space
  charge limited emittance
• Stochastic cooling down to the equilibrium
  emittance eN 0. 210-6 m rad
• RF bunching (1.5 GHz) with tunable RF (500-1500
  MHz)
• Further acceleration and ramping

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Collider Rings
Pair of Figure-8 Rings (900 m circumference)
ProjectX style ion injector
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Interaction Region design
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Interaction Region design - ions

• Triplet based low beta Optics
• larger distance IP - first quad
• larger crossing angle 50-100 mrad
• 5 Tesla/m FF gradients - no need for SC quads
• Reasonable longitudinal acceptance Dp/p 10-4
• effective chromatic compensation with sextupoles

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Interaction Region design - electrons

• Relaxed Triplet based IR Optics (electrons) with
  large b 18 cm
• first FF quad far from the IP (8-10 m)
• no need for chromatic compensation

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Staged Construction Plan

• Proton Linac and Fixed Target Area
• Ion Linac providing 80mA of 2 GeV/c protons (50
  M)
• Unpolarized high-current ion source (5 M)
• Beamline and fixed target areas for both basic
  and applied research
• Collider with Unpolarized Ions
• Intersecting storage rings (20 M)
• Accumulator ring for lepton injector (10 M)
• Simple detector for storage ring
• High-Luminosity Collider with Polarized Ions
• Polarized ion source (10 M)
• Small ring for electron cooling (2 M)
• Booster ring (40 M)
• Detector(s) for storage ring

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Add COSY to CEBAF?
May not be as simple as one may think

• Limitations of a COSY collider
• Lacks the polarization advantage of a Figure-8
  design
• Luminosity at least an order of magnitude lower
• Proton energy limited to 3.7 GeV
• Upgrade potential limited
• Cost benefits are limited extensive
  modifications required
• Individual COSY components can be a valuable
  addition
• Experiments (e.g. WASA, ANKE)
• Ring could be used as ion accumulator (no RF)
  NOT needed here
• Beam pipes, diagnostics, ion sources, and parts
  of the cooling system

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Fixed Target Physics
WASA detector

• Physics opportunities without additional
  investment
• Good way of expanding the user community
• High quality low-energy nuclear physics
• Will attract a new hadronic user community
• Fills an important void

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Accelerator Applications
Transmutation of Nuclear Waste

• Challenge Energy Independence and global warming
• Nuclear energy will be part of the solution
• Fast reactors may be perceived as unsafe
• No passive safeguards (void, Doppler)
• A sub-critical system with accelerator driven
  spallation source could be easier to accept
• Requires reliable high-current proton linac
  technology (Ep gt 1 GeV)

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Neutron Spallation Source
Research into atomic structure and dynamics of
materials

• Oak Ridge 1.4 MW short-pulsed SNS
• European Spallation Source (ESS) planned 5 MW
  long-pulsed
• Large user community
• Unlikely to materialize due to high cost and lack
  of political support.
• With all infrastructure in place, JLab could
  provide ESS performance at a fraction of the cost