ELECTRON-ION COLLIDER AT CEBAF: NEW INSIGHTS AND CONCEPTUAL PROGRESS

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ELECTRON-ION COLLIDER AT CEBAF NEW INSIGHTS AND
CONCEPTUAL PROGRESS Ya. Derbenev, A. Afanasev,
K. Beard, A. Bogacz, P. Degtiarenko, J. Delayen,
A. Hutton, G.A. Krafft, R. Li, L. Merminga, M.
Poelker, B. Yunn, Y. Zhang, Jefferson Lab, 12000
Jefferson Ave., Newport News, VA 23606 and Peter
Ostroumov, ANL, Illinois
Abstract We report on progress in conceptual
development of the proposed high luminosity (up
to 1035/cm2s) and efficient spin manipulation
(using figure 8 boosters and collider rings)
Electron-Ion Collider at CEBAF based on use of
polarized 5-7 GeV electrons in superconduction
energy recovering linac (ERL with circulator
ring, kicker-operated) and 30-150 GeV ion storage
ring (polarized p, d. He3, Li and unpolarized
nuclei up to Ar, all totally stripped).
Ultra-high luminosity is envisioned to be
achievable with short ion bunches and
crab-crossing at 1.5 GHz bunch collision rate
interaction points. Our recent studies
concentrated on simulation of beam-beam
interaction, preventing the electron cloud
instability, calculating luminosity lifetime due
to Touschek effect in ion beam and background
scattering of ions, experiments on energy
recovery at CEBAF, and other. These studies have
been incorporated in the development of the
luminosity calculator and in formulating minimum
requirements to the polarized electron and ion
sources
Beam - Beam Simulations
Nuclear Physics Motivation
ERL-ring synchronization issue
Synchronization between electron and ion bunches
is a common constraint of EIC design. It is
expressed by the relationship f qefe qifi
between RF frequency f and revolution frequencies
fe ve/Ce, fi vi/Ci, where qe and qi are
integers, and Ce, Ci are the beam orbits
circumference. The constraint is due to the fact
that ion velocity, vi, changes by a factor of
about 10-3 in energy range of an EIC the related
change of revolution frequency would be very
difficult to compensate by change of ion orbit
length with energy. In ELIC design where the
beams are driven by RF of very high qi (about
7500 at f 1.5 GHz), possible solution consists
of varying the integer qi yet admitting
residual change of ion path length in arcs up
to one bunch spacing (about 20 cm, corresponding
to 12 mm orbit displacement in arcs ). Ion
acceleration in collider ring can be performed
using warm resonators of changeable frequency,
after that one can switch (via beam re-bunching)
to high voltage superconducting resonators.

• A high luminosity polarized electron light ion
  collider has been proposed as a powerful new
  microscope to probe the partonic (quarks and
  gluons) 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 the proton and neutron
  in terms of their quark and gluon constituents?
• How do quarks and gluons evolve into hadrons?
• What is the quark-gluon origin of nuclear binding?

Beam Energy GeV 150/7 Beta Mm 5
Collision rate MHz 1500 Horiz. Norm. emit. µm 1/90
particle/bunch 1010 1 / 0.2 Ver. Norm. Emit. µm 0.01/9
Beam Current A 2.4/0.5 of Interaction pts. 1
Energy Spread RMS 10-4 4 Vert. Tune Shift/IP 10-2 2.2
Bunch Length RMS Mm 5/5 Luminosity/IP 1034 6
Nuclear Physics Requirements

• The features of the facility necessary to address
  these issues
• Center-of-mass energy between 20 GeV and 65 GeV
  with energy asymmetry of 10, which yields
• Ee 3 GeV on Ei 30 GeV up to Ee 7 GeV on Ei
  150 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 ? 50 80 required for the
  study of generalized parton distributions and
  transversity
• Transverse polarization of ions extremely
  desirable
• Spin-flip of both beams extremely desirable

ELIC Layout
ELIC Rings Basic Paramenters
Circumference M 1532 Bend Field in Ion Arcs, Max. T 7
Arc Radius M 100 Bend Radius in Dipoles. M 75
Arc Angle grad 240x2 of Oscillations in Ion Arcs 20
Arc Length M 470x2 Revolution Freq. KHz 200
Crossing Straights Length M 346x2 Ion Freq. Change at Acceleration 0.1
Straights Crossing Angle grad 60
ELIC Parameters at different CM energies
ELIC Interaction Region
Electron Cooling for ELIC
Research group of Jefferson laboratory is
proceeding with conceptual development of 75 MeV
EC for ELIC at CEBAF. High luminosity of ELIC
requires a high electron cooling current (up to
2-3 A) that forces one to consider an optional EC
design implicating a circulator-cooler ring
incorporated with the injector and ERL. A
particular important advantage of ERL based
cooler is the possibility to use staged cooling
for reduction of initial cooling time. As an
alternative to electron beam magnetization by a
long superconducting solenoid, we also have
started to explore EC version using a strong
quadrupole or axial focusing along the entire
electron track while using a non-magnetized
electron gun. Besides easier focusing lattice and
acceleration, an important advantage of such
transport concept is effective beam diagnostics
and alignment control while the magnetization
feature of EC remains important. These transport
concepts are addressed to future EC for
luminosity upgrade in hadron colliders, as well.
Intrabeam Scattering
To implement very tight focusing for the
interaction region, while maintaining its
compactness, it is beneficial to employ a triplet
focusing (DFD) providing a net focal length of
about 3 meters at the collision energy of 150
GeV. Here, for the lattice design one uses two
varieties of quads (D defocusing quad 1.12
meter long and F focusing quad 1.96 meter long)
with transverse aperture radius of 3 cm and the
peak field of 7.5 Tesla, which defines a maximum
gradient of 250 Tesla/m. Our first cut of the
final focus lattice design assumes as target
parameters of and as illustrated in the
Figure. One can configure the final focus lattice
into a mirror symmetric configuration (DFDODFD)
or an anti-symmetric one (FDFODFD). The
advantages of the last one is a more stable
solution - less sensitive to ground motion,
magnet power supply fluctuation etc. Assuming
the horizontal emittance after cooling , yields
the beam width in final triplet of about 5
mm. Further, more aggressive lattice
optimization assumes going to stronger final
triplet quads a peak field of 9 Tesla with the
same aperture radius of 3 cm would allow us to
reduce to about 5 mm. However, much shorter
focal length of the triplet (less than 5 m) would
significantly reduce free space around the
interaction point available for the detector (to
about 4 m). The interaction region will consist
of two final focus points separated by about 60
meters of the beam extension FODO section. The IR
region will then be matched through another beam
extension FODO lattice insert to much tighter
FODO lattice of the arc with of about 12 m or
less.
IBS heating mechanism energy exchange at
intra-beam collisions increase the energyy spread
and excites the transverse oscillators via orbit
dispersion At low x-y coupling, IBS can be
reduced in flat beam

• Luminosity is determined by the beam area
• IBS effect is reduced by a factor of the aspect
  ratio
• Cooling effect at equilibrium can be enhanced by
  flattening the electron beam in cooling section
  solenoid
• Luminosity lifetime is determined by Touschek
  scattering beyond the cooling beam area    

The numerical results on multiple IBS for current
ELIC design parameters are summarized in the
following table
E (GeV) Ni ?s (mm) ?x?y (m) R (m) ?x,norm (µm) K ?\\ Long. Life time (min.) Horiz. Life time (min.)
20 2x109 80 10 100 4 1 3x10-4 120 1.72x105
150 2x109 5 10 100 1 1/25 3x10-4 4.2 2.9
Work supported by the U.S. Department of Energy,
contract DE-AC05-84ER401050