1. 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, Jefferson Ave., Newport News, VA and Peter Ostroumov, ANL, Illinois
Abstract We report on progress in conceptual development of the proposed high luminosity (up to 10^35/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 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
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?
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.
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.
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
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
100
Bend Radius in Dipoles. 75
Arc Angle
grad
240x2
# of Oscillations in Ion Arcs 20
Arc Length
470x2
Revolution Freq.
KHz
200
Crossing Straights Length
346x2
Ion Freq. Change at Acceleration %
0.1
Straights Crossing Angle 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.
Multiple IBS
Touschek scattering
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 5
1/25
4.2
2.9
*Work supported by the U.S. Department of Energy, contract DE-AC05-84ER401050