Thomas Jefferson National Accelerator Facility

1
ELIC A HIGH LUMINOSITY AND EFFICIENT SPIN
MANIPULATION ELECTRON-LIGHT ION
COLLIDER BASED AT CEBAF L. Merminga, Ya.
Derbenev, A. Hutton, M. Poelker, and Y. Zhang,
Jefferson Lab, 12000 Jefferson Ave., Newport
News, VA 23606
Thomas Jefferson National Accelerator
Facility Newport News, Virginia, USA
Abstract Electron-light ion colliders with center
of mass energy between 20 and 100 GeV, luminosity
between 1033 and 1035 cm-2 sec-1, and
polarization of both beams at or above 80 has
been proposed for the study of hadronic
structure. The proposed scheme would accelerate
the electron beam using the CEBAF recirculating
linac with energy recovery. If 20-40 MV/m
accelerating structures are installed in the
CEBAF tunnel, then a single recirculation can
result in electron beam energy of about 5-10 GeV.
After colliding with protons/light ions
circulating in a storage ring under electron
cooling at an energy of 50-100 GeV or above, the
electrons are re-injected into the CEBAF
accelerator for deceleration and energy recovery.
In this report several innovative features of
electron and ion beam designs and their
advantages in delivering the luminosity and spin
are described. These features include electron
circulator ring to reduce electron polarized
source and energy recovering linac requirements,
twisted spin booster and collider ring low
energy electron cooling accumulator as option for
stacking beams from positive polarized ion
sources interaction points with low beta-star
and crab-crossing using the short, cooled ion
bunches. Accelerator physics and technology
issues for both protons/ions and electrons are
discussed. The feasibility of an integrated fixed
target program at 25 GeV and collider program
with center of mass energy between 20 and 45 GeV
is also explored.
INTEGRATION WITH 25 GeV FIXED TARGET PROGRAM 1
NUCLEAR PHYSICS MOTIVATION

• A high luminosity polarized electron light ion
  collider has been proposed as a powerful new
  microscope to probe the partonic structure of
  matter
• Over the past two decades we have learned a great
  amount about the hadronic structure
• Some crucial questions remain open
• What is the structure of hadrons in terms of
  their quark and gluon constituents?
• How do quarks and gluons evolve into hadrons?

The same electron accelerator can also provide 25
GeV electrons for fixed target experiments for
physics.
THE ELIC PROPOSAL 1
INTRABEAM SCATTERING AND FLAT BEAMS 2
ELECTRON COOLING IN ELIC 4
NUCLEAR PHYSICS REQUIREMENTS
Electron cooling time grows with beam energy in
the first or second power and with normalized
beam emittances - in the third power. Therefore,
it seems critically important to organize the
cooling process in two stages cool the ion beam
initially at injection energy after stacking it
in collider ring (in parallel or after
re-bunching), and continue the cooling during
and/or after acceleration to a high energy.
It is important to distinguish between multiple
IBS and single scattering or Touschek effect.
Multiple scattering contributs to ion
Focker-Plank equilibrium i.e. beam core, while
single scattering kicks the particles out of the
core. To overwhelm multiple IBS, current of
cooling beam must well exceed a critical value,
proportional to ion current. After the cooling
starts, the ion beam will shrink to the
Focker-Plank equilibrium. Following this stage,
an interplay between Touschek scattering and
particle damping due to electron cooling beyond
the core will determine the core i.e. luminosity
lifetime. At ion energies far above transition
value, the area of the cooling beam should
frequently exceed that of the ion beam, in order
to extend the ion core lifetime. Using this
phenomenology one can estimate an optimum set of
parameters for maximum average luminosity of a
collider. See Table below. At energies above
the transition value, energy exchange at
intra-beam collisions leads to horizontal
emittance blow up due to energy-orbit coupling,
and vertical emittance due to x-y coupling.
Since luminosity is determined by the product of
two emittances, reduction of transverse coupling
to a minimum - while conserving the beam area -
would result in decrease of energy scattering,
hence, decrease of the impact of IBS on
luminosity. Electron cooling then leads to a flat
equilibrium with a large aspect ratio 5. A flat
ion beam should collide with a correspondently
flat electron beam.

• Implement 5-pass recirculator, at 5 GeV/pass, as
  in present CEBAF
• (One accelerating one decelerating pass through
  CEBAF ? 20-45 GeV CM Collider Program)

• Center-of-mass energy between 20 GeV and 100 GeV
  with energy asymmetry of 10, which yields
• Ee 3 GeV on Ei 30 GeV up to Ee 5 GeV on Ei
  100 GeV
• CW Luminosity from 1033 to 1035 cm-2 sec-1
• Ion species of interest protons, deuterons, 3He
• Longitudinal polarization of both beams in the
  interaction region ? 80 required for the study
  of generalized parton distributions and
  transversity
• Transverse polarization of ions extremely
  desirable
• Spin-flip of both beams extremely desirable

RD STRATEGY
Electron Cooler for ELIC

• Conceptual development
• Circulator Ring concept promises to ease high
  current polarized photoinjector and ERL
  requirements significantly
• Additional concepts for luminosity improvements
  are being explored
• Analysis/Simulations
• Electron cooling and short bunches
• Beam-beam physics
• Circulator ring dynamics
• ERL physics
• Experiments
• JLab FEL (10mA), Cornell/JLab ERL Prototype
  (100mA), BNL Cooling Prototype (100mA) to address
  high current ERL issues
• CEBAF-ER The Energy Recovery experiment at CEBAF
  to address ERL issues in large scale systems 6

• ELIC DESIGN INNOVATIONS 2,3
• Electron circulator ring
• Twisted spin booster and collider ring
• Short ion bunches resulting from electron
  cooling
• Low beta-star ( 5 mm)
• Crab-crossing
• Traveling ion focus

BASIS OF ELIC PROPOSAL
CEBAF with upgraded cryomodules and energy
recovery
Figure 8 storage ring and boosters
CONCLUSIONS

• The hadron physics community is asking for a
  high luminosity, polarized electron-light ion
  collider
• Our design studies have led to an approach that
  promises luminosities up to 1035 cm-2 sec-1
• This design can be realized cost-effectively
  using energy recovery on the JLab site and can be
  integrated with a 25 GeV fixed target program for
  physics
• Planned RD will address open issues

Circulator Ring

• CEBAF with energy recovery is used for rf power
  savings and beam dump requirements
• Figure-8 storage ring is used for the ions for
  flexible spin manipulations of all light-ion
  species of interest
• Circulator ring for the electrons may be used to
  ease high current polarized photoinjector
  requirements

1 L. Merminga, et al., ELIC An
Electron-Light Ion Collider Based at CEBAF,
Proceedings of EPAC 2002 2 Ya. Derbenev,
Luminosity Potentials in Colliders with Electron
Cooling, These proceedings TPPB081 3 Ya.
Derbenev, Advanced Concepts for Electron-Ion
Collider, Proceedings of EPAC 2002 4 Ya.
Derbenev, Proceedings of ECOOL03 Workshop 5
Ya. Derbenev, Proceedings of EPAC 2000 6 A.
Bogacz et al., and C. Tennant et al., These
proceedings TOAC006 and RPPG032
REFERENCES
Work supported by the U.S. Department of Energy,
contract DE-AC05-84ER401050