ACCELERATOR PHYSICS ISSUES FOR FUTURE ELECTRON ION COLLIDERS. Page: 1 of 5
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2001 Particle Accelerator Conference
June 18 - 22, 2001, Chicago, IL BNL-68754
ACCELERATOR PHYSICS ISSUES FOR FUTURE ELECTRON-ION
S. Peggs, I. Ben-Zvi, J. Kewisch, J. Murphy, BNL.
Interest continues to grow in the physics of collisions
between electrons and heavy ions, and between polarized
electrons and polarized protons [1, 2, 3]. Table 1 compares
the parameters of some machines under discussion. DESY
has begun to explore the possibility of upgrading the ex-
isting HERA-p ring to store heavy ions, in order to collide
them with electrons (or positrons) in the HERA-e ring, or
from TESLA . An upgrade to store polarized protons in
the HERA-p ring is also under discussion . BNL is con-
sidering adding polarized electrons to the RHIC repertoire,
which already includes heavy and light ions, and polarized
protons. The authors of this paper have made a first pass
analysis of this "eRHIC" possibility . MIT-BATES is
also considering electron ion collider designs .
Ring-Ring and Linac-Ring scenarios. Figure 1 com-
pares the ring-ring and linac-ring scenarios, using eR-
HIC as a convenient example. In the "ring-ring" scenario
(TOP), pre-polarized electrons are injected into an electron
storage ring from a full energy linac (or from a booster).
Collisions are possible with the clockwise rotating ions
at up to 5 interaction points. The average electron beam
power passing a single point - a few GW - is contained as
a stored beam energy which is conserved except for syn-
chrotron radiation losses of about 1 MW. In the linac-ring
scenario (BOTTOM) the beam circulates only once, before
the average electron beam power - about I GW - is re-
covered by passing the beam back through the linac. The
recirculation ring may, or may not, share a tunnel with the
ion ring. The Energy Recovery Linac (ERL) must be su-
perconducting in a linac-ring design, constructed of nio-
bium superconducting cavities (for example using 1.3 GHz
TESLA cavities). In the ring-ring scenario the linac could
alternately be constructed with copper cavities (for exam-
ple at the SLAC linac frequency of 2.856 GHz, where cav-
ities and RF sources are readily available). Such a copper
linac has no particularly new issues or difficulties, except
in the need for an electron gun which can provide polar-
ized electrons at up to 80% polarization [7, 8].
2 ULTIMATE PERFORMANCE
The luminosity is given by
where F, = 11/bun,. is the collision frequency, N, and
; are the single bunch populations for electrons and ions,
and r-* is the round beam collision size (assumed to be the
K Abort lnstnunentation
ion rings / Linac
Figure 1: In the ring-ring scenario (TOP) electrons are
stored for hours in their own ring. In the linac-ring sce-
nario (BOTTOM) electrons circulate the ring once, before
re-entering the superconducting Energy Recovery Linac.
same for both beams). The RMS electron and ion beam
sizes are written as
where F; is the normalized RMS ion emittance (no 4r or
6r), and where the electron emittance ,, is unnormalized.
The electron and ion beam-beam parameters (, and a;
depend only on the bunch population of the other beam,
*Work performed under the auspices of the U.S. Department of Energy
(* = f F<
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PEGGS,S.; BEN-ZVI,I.; KEWISCH,J. & MURPHY,J. ACCELERATOR PHYSICS ISSUES FOR FUTURE ELECTRON ION COLLIDERS., article, June 18, 2001; Upton, New York. (https://digital.library.unt.edu/ark:/67531/metadc723383/m1/1/: accessed April 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.