SLAC i PUB - 4545
February 1988
WA)
Colliding a Linear Electron Beam with a Storage Ring Beam*
P. GROSSE- WIESMANN
Stanford Linear
Accelerator Center
Stanford University, Stanford, California 94305
ABSTRACT
We investigate the possibility of colliding a linear accelerator electron beam
with a particle beam stored in a circular storage ring. Such a scheme allows e+e-
colliders with a center-of-mass energy of a few hundred GeV and eP colliders with
a center-of-mass energy of several TeV. High luminosities are possible for both
colliders.
Submitted to Nuclear Instruments & Methods
* W&k supported by the Department of Energy, contract DE - A C 0 3 - 76 SF 00 5 15.
1. Introduction
In order to get a better understanding of the standard model of particle
physics, higher energies and higher luminosities in particle collisions are desir-
able. It is generally recognized that particle collisions involving leptons have
a considerable advantage for experimental studies over purely hadronic interac-
tions. The initial state is better defined and cleaner event samples are achieved.
Unfortunately, the center-of-mass energy and the luminosity achievable in an
electron storage ring are limited. The energy losses from synchrotron radiation
increase rapidly with the beam energy. Even with the largest storage rings un-
der construction or under discussion the beam energy cannot be extended far
beyond
100
GeV [l].
I
n addition, the luminosity in a storage ring is strongly
limited by the beam-beam interaction. Event rates desirable to investigate the
known particles are not even available at existing storage rings.
One way out of this problem is the linear collider scheme [2,3]. Here, syn-
chrotron radiation losses are avoided and the beam-beam interaction limits are
much weaker. In order to get the desired high luminosity, very tiny beam sizes at
the collision point have to be achieved. So far, experience with linear colliders is
very limited [4]. Although high energy, high luminosity linear colliders are under
discussion [4,5],
a technically detailed proposal does not yet exist.
Here we discuss a scheme, in which the electron beam from a linear accelerator
(linac) is collided with a beam stored in a ring 161. Such a concept allows the
economy of a storage ring to be used, but at the same time can potentially avoid
its energy and luminosity limitations.
A high intensity proton or positron beam can be stored at an energy inde-
pendent of the electron beam.
The proton beam can be stored at a very high
energy using superconducting bending magnets. For positrons, in contrast to the
linear collider concept, a high intensity positron source and the adjunct cooling
rings are unnecessary.
2
The particle density of the stored beam at the collision point is not limited
by the usual tune shift induced by the opposite beam, since the electron beam is
not recycled. The electron beam has to be of relatively low current, so that the
storage ring beam is not disrupted. Linac and storage ring beams are relatively
decoupled. This gives more freedom to choose parameters and, in addition, the
linac beam can be instantaneously adjusted to the parameter of the storage ring
beam.
A very important advantage of such a scheme is that a low emittance electron
beam can be directly produced by a suitable source [7], so that cooling rings are
not necessary, and polarized electron beams can be achieved by using a special
cathode and polarized laser light [8].
To see if such a scheme is practical we have to study the achievable luminosity.
The luminosity in a collider, assuming a Gaussian particle density distribution,
is
L=f*4:oNJ -Ho ,
= Y
(1)
where f is the collision frequency, Ne is the number of electrons, Np is the num-
ber of stored particles, oZ and cry are the horizontal and vertical beam size at
the collision point and
HD
is the luminosity enhancement due to beam-beam
focusing.
In principle, the luminosity can be increased by increasing fNeNp or by de-
creasing the beam spot size. Unfortunately, technical and financial limits tightly
constrain these options.
In this paper we first discuss the various constraints of such a collider scheme.
We then apply these constraints to four different configurations: a high luminosity
b-factory, a high luminosity Z”-factory, a few hundred GeV e+e- collider and a
few TeV eP collider.
2. Constraints Affecting the Machine Parameter Choices
2.1 POWER CONSUMPTION FOR THE BEAMS
One of the major limitations for any high energy electron machine is the
overall power consumption. The power consumption for a linac is
Ne f E
p,rl.6 .e.-.-.
1-(MW) ,
lOlo KHz TeV qla
(2)
where
E
is the beam energy and qlo is the acceleration efficiency for the linear
accelerator. r]la for conventional disk loaded linac structures is at most a few
percent [9,10]. Superconducting cavities can potentially achieve efficiencies of
- 50%, but are rather expensive [ 111.
The power needed to restore energy lost by the stored positrons due to syn-
chrotron radiation is
P8 = 2.6 -
$S(25~eV)4'(~)2'$(MWI 9
(3)
where nb is the number of stored bunches, p is the bending radius and qea is the
efficiency to restore the synchrotron radiation losses. nb is related to f and p by
nb =
e . Here, a superconducting accelerator structure with high efficiency is
the appropriate choice.
2.2
CURRENT LIMITATIONS FOR THE LINAC BEAM
The number of electrons per bunch in the linac beam is limited by its effect
on the storage ring beam.
This is traditionally expressed in terms of the tune
shift limit AQ [12]:
EP
Ne = 4.36 - - -
~~(0~ + ay> cm
GeV (pm)2
- p - AQY.
lo8
.
Y
PY
is the vertical ,8 function at the collision point. Based on experimental
experience, A& 5 0.06 for electron storage rings and A& 5 0.003 for proton
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storage rings are considered to be possible [13]. This limit will demand low
intensity beams in the linac.
2.3 CURRENT LIMITATIONS
FOR THE STORAGE RING
Assuming fN, fixed by power considerations, the ratio & determines the
achievable luminosity. Therefore, one would like to get as many particles per
storage ring bunch as possible and tiny spot sizes at the collision point.
Besides power considerations, single beam instabilities, beamstrahlung by the
electron beam and intrabeam scattering limit the maximum number of particles
in the storage ring.
The peak currents which can be kept stable in a storage ring are determined
by the transverse impedance of the accelerator. The transverse impedance de-
pends on the detailed structure .of the accelerator. In existing and planned storage
rings, - 1Or2 particles are achieved or planned.
The intense electric and magnetic field of the storage ring beam causes the
electrons of the linac beam to radiate photons (‘beamstrahlung’). Specifying a
certain tolerable beamstrahlung loss constrains the number of particles and the
bunch size of the storage ring beam. The storage ring beam acts like a lens on
the electron beam and causes it to oscillate around the beam axis. In contrast to
a pure linear collider, where the focusing effect happens in both beams and can
cause instabilities due to plasma oscillations, here only the linac beam is affected.
The focusing effect for the storage ring beam is small and is quantified in the
tune shift limit discussed above. An oscillation of the electron beam around the
center of the storage ring beam increases the luminosity by - 1.5. This beam
focus effect also removes the constraints that p be greater than a, at the collision
point. The focus of the electron beam has only to ensure that the particles are
caught by the storage ring bunch. The fractional energy loss of the electrons can
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