Beam-beam studies for the Tevatron Page: 1 of 12
This article is part of the collection entitled: Office of Scientific & Technical Information Technical Reports and was provided to Digital Library by the UNT Libraries Government Documents Department.
The following text was automatically extracted from the image on this page using optical character recognition software:
FERMILAB-Conf-00/124-T June 2000
-beam studies for the Tevatron
Tanaji Sen, FNAL, Batavia, IL 60510
1 MOTIVATION FOR RUNII
In the first stage of Run II, the Tevatron will be operated
with 36 bunches in each beam with bunch separations of
396 nanoseconds. The expected peak luminosity is L
8.6 x 1031 cm 2sec with an average number of 2.3 in-
teractions per bunch crossing. In the second stage of Run
II, the goal is to increase the luminosity to about 1.5 x 1032
cm-2sec 1. If the bunch spacing were kept constant, the
average number of interactions per bunch crossing would
increase to about 4. This is thought to be unacceptably large
and might saturate the efficiency of the detectors. This is
the main reason for decreasing the bunch spacing at higher
One possibility is to reduce the bunch spacing to 132
nanoseconds which lowers the average number of interac-
tions to an acceptable value of 1.4. This shorter bunch spac-
ing though has several consequences on beam dynamics.
Collisions between bunches will now occur every 19.78m.
This is shorter than the distance of the nearest separators
from the main IPs at BO and DO. Consequently the beams
will not be separated at the parasitic collisions nearest to the
IPs if the geometry of the orbit is left unchanged. A sketch
of this orbit is seen in the top part of Figure 1. This will lead
to unacceptably large beam losses and background. Mov-
ing the separators closer to the detectors does not separate
the beams sufficiently at the locations PC1L and PC1R. The
phase advance from the first available position for the sepa-
rators to these points is too small for the separator strengths
that are available .
One way to increase the transverse separation between
the beams is to make the beams cross at an angle at the
IPs. The optimum crossing angle depends upon a num-
ber of issues and requires a detailed investigation. The is-
sues include a reduction in the luminosity, change in the
beam-beam tune spreads, excitation of synchro-betatron
resonances, orbit offset in IR quadrupoles which increases
the nonlinear fields seen by the beams, required separation
between the beams at the nearest parasitic collisions, the
dispersion wave generated by the orbit offset, increase in
the strength of the coupling etc. A crossing angle of ~
200prad in the 45 degree plane separates the beams by
~ 4Q at the first parasitic collision. A sketch of the orbits
with a crossing angle is shown in the bottom part of Figure
The crossing angles that are thought to be necessary have
a major impact on the luminosity. If Lo is the nominal lu-
minosity without a crossing angle, then the luminosity with
PC2L PC1L PC1R PC2R
e - - --1 -pr- e - oton
---- --- - e --- --- --- ---- -- -e ------------- -----------
* ..]L L - atpoo
separators Q2 Q3 Q4 Q4 Q3 Q2 separators
132nsec bunch spacing without a crossing angle
- - IP
132nsec bunch spacing with a crossing angle
Figure 1: Sketch of the locations of the main beam-beam
collisions and the next two parasitic collisions, e.g. PC1R,
PR2R on the right, with respect to the triplet quadrupoles
and the separators. The top figure shows the geometry with-
out a crossing angle, the bottom figure shows the geometry
with a crossing angle.
a total crossing angle of 20 is
1 + (0s / l)2 o
where aL is the transverse beam size at the IP. Figure 2
shows the relative loss in luminosity as the crossing an-
gle is increased. For example at a half crossing angle
of 200pradians, the luminosity is only 38% of its value
without a crossing angle. The smaller overlap between
the beams which lowers the luminosity also decreases tbe
beam-beam tune shift. If one assumes that we can replace
the beam size a at the IP by QL 1 + (asp / al)2 then the
head-on tune shift parameter is reduced from its value o at
zero crossing angle to = R2 0. Figure 2 shows that with
this assumption, the relative tune shift at a half crossing an-
gle of 200pradians is about 28% of its value at zero crossing
angle. This hand-waving estimate of the beam-beam tune
shift with a crossing angle is useful only as a rough guide.
The beam-beam tune shift with a crossing angle depends on
the synchrotron oscillation amplitude so it is not enough to
specify only the transverse amplitudes when computing the
tune shift. However it is true that at any betatron amplitude,
the tune shift at all synchrotron amplitudes except zero is
smaller than the tune shift without a crossing angle.
Here’s what’s next.
This article can be searched. Note: Results may vary based on the legibility of text within the document.
Tools / Downloads
Get a copy of this page or view the extracted text.
Citing and Sharing
Basic information for referencing this web page. We also provide extended guidance on usage rights, references, copying or embedding.
Reference the current page of this Article.
Sen, Tanaji. Beam-beam studies for the Tevatron, article, June 13, 2000; Batavia, Illinois. (https://digital.library.unt.edu/ark:/67531/metadc710905/m1/1/: accessed April 18, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.