Compensation of Detector Solenoid with Large Crossing Angle

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Compensation of Detector Solenoid with Large
Crossing Angle

• Y. Nosochkov, A. Seryi

ILC-Americas Workshop, SLAC, October 14-16, 2004
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• Detector solenoid model
• Large horizontal crossing angle (35 mrad) in
  the IR-2 for g-g collisions.
• Beams travel at an half-crossing angle to the
  solenoid field.
• Solenoid field overlaps one or both the Final
  Focus quadrupoles.

Example of solenoid field model for Silicon
Detector (SiD) and Large Detector (LD) in the NLC
FFS (IP is at z 0). The plotted field is on a
beam path at 10 mrad angle with respect to the
solenoid axis (for 20 mrad crossing angle).
SiD 16.7 Tm
LD 14.4 Tm
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• Solenoid perturbation of beam optics
• Coupling of x-y betatron motion.
• Vertical orbit due to beam passing at horizontal
  angle and offset.
• Vertical dispersion due to the horizontal angle
  and FF horizontal dispersion.
• Focusing in x-y planes.
• If FF quadrupoles are outside of detector
  solenoid
• Solenoid coupling is just an x g y beam rotation
  causing a modest vertical beam size growth at IP
  (less than a factor of 2).
• Vertical orbit and dispersion from solenoid
  cancel at IP due to opposite effects of the
  longitudinal and radial solenoid fields (but the
  IP angle is not zero).
• If FF quadrupoles are inside the solenoid
• Solenoid and quad overlap generates the new
  dominant coupling term ltyxgt (x g y) causing a
  large growth of vertical beam size at IP (a
  factor of 100).
• Overlap creates residual vertical orbit and
  dispersion at IP.
• Solenoid analysis below is based on the NLC study
  for 20 mrad crossing angle and the design IP
  parameters L3.51 m, bx8 mm, by0.11 mm,
  hx0.0094, sx243 nm, sy3 nm, sxp30.4 mrad,
  syp27.3 mrad, sE0.25, and beam energy E250
  GeV (details in SLAC-PUB-10592).

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Strong coupling due to solenoid and quad
overlap Example of the test Tiny solenoid model
without quad overlap (red) and with additional
small field D10.1 Tm (blue) and D20.5 Tm
(black) added on top of the first FF quad. IP
phase space is normalized to the ideal beam sigma
(green no solenoid, red with Tiny
solenoid). Even a small part of the solenoid
field inside the FF quad significantly increases
the IP vertical beam size.
Tiny 18 Tm
0 Tm
0.1 Tm
0.5 Tm
The left figure presents a test, where the
detector solenoid is replaced by a short and weak
test solenoid (0.5 Tm), and its effect on the IP
beam size, coupling (ltyxgt), vertical dispersion
(ltyEgt), normalized, and IP orbit is shown versus
the solenoid position z. Clearly, the largest
effect is created when the solenoid overlaps the
FF quads, at z3.5-10 m.
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Comparison of solenoid effects at IP for 250 GeV
beam with 20 mrad crossing angle for different
detector models without solenoid compensation.
Here, BL is the total solenoid field on one side
of IP, and BLFD is its fraction over the FF
quads. The normalized coupling terms are in units
of beam size growth. Note the strong dependence
of vertical beam size growth on the amount of
BLFD field.
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Phase space at IP (normalized to ideal sigma) for
different detector models without solenoid
compensation. Green no solenoid, red with
solenoid. Note that the beam size growth is of
similar magnitude for zero and 20 mrad crossing
angles.
LD model, q 20 mrad
Tiny model, q 20 mrad
SiD model, q 20 mrad
LD model, q 0
A compensation system for detector solenoid is
required to achieve the design beam size and
position at IP.
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• Since the dominant solenoid aberrations are
  generated in the overlapped FF quadrupoles, it is
  important that the compensation system is
  effective against this part of the solenoid
  field.
• In earlier studies, the main approach for
  solenoid correction was to use a skew quadrupole
  placed near the FF quad (or a rotation of the FF
  quad). This compensates the major part of IP
  coupling, but further correction, including orbit
  and dispersion, requires the use of additional
  correctors such as tuning knobs. Note that for
  the lower energy beam options (to 50 GeV), all
  these correctors have to be optically stronger.
• It is desirable to achieve the most local
  compensation of the aberrations created by the
  solenoid overlap with the FF quads. Therefore
  why not to physically reduce this part of the
  solenoid field by an opposite compensating
  solenoid field? This leads to the idea of using
  short and weak antisolenoids placed at the FF
  quad locations as part of the detector. This way,
  the detector solenoid field in the FF quads is
  directly reduced by the antisolenoids for the
  most local correction. And since the overlapped
  field is typically small, rather weak
  antisolenoids are needed. The extra advantage is
  that by removing the most of the overlapped
  field, the antisolenoids restore the properties
  of a bare solenoid with self-compensation of the
  vertical orbit and dispersion. It has been shown
  that the antisolenoid method provides a better
  correction than the skew quads, and it is more
  robust at lower beam energies. However, the
  technical implications and impact on detector
  design have not been studied.

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Example of SiD solenoid compensation using one
weak antisolenoid (1.74 Tm) placed at the first
FF quad.
Total field with and w/o antisolenoid
IP phase space with an antisolenoid, dsy 29
With antisolenoid and linear knobs, dsy 0.3
Corrected IP orbit. Although y¹0, ee- collide
head-on in y-plane due to ee- orbit antisymmetry.
Orbit and coupling at IP versus half-crossing
angle without (top) and with antisolenoid
(bottom). Correction is optimized at 10 mrad. It
can be reoptimized for other angles.
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Example of LD solenoid compensation using two
weak antisolenoids (2.26 and 0.07 Tm) placed at
two FF quads.
Total field with and w/o antisolenoids
IP phase space with antisolenoid correction , dsy
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With antisolenoids and linear knobs, dsy 0.9
Corrected vertical IP orbit
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• Energy dependence of antisolenoid compensation
  for SiD
• Solenoid aberrations are significantly increased
  at lower beam energies, and the vertical focusing
  ltyygt becomes the 2nd largest linear term after
  ltyxgt.
• For the same antisolenoid field, a good
  correction of the dominant coupling term ltyxgt,
  vertical dispersion and orbit is maintained for a
  full range of energies (50-300 GeV).
• Additional tuning of IP beam size using the
  linear and second order optics knobs is needed at
  the lowest energies.

Relative beam size growth at IP versus beam
energy (50-300 GeV) for SiD compensation using
one antisolenoid and several linear optics knobs.
The second order knobs may be used at low beam
energies to reduce the enhanced high order terms
such as ltyxxgt.
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• Technical issues (not studied)
• Antisolenoid at location of the first FF quad
  (closest to IP) should be an integral part of the
  detector solenoid and aligned on the detector
  axis.
• It should have two coils for adjustment of its
  field and longitudinal position.
• It should be compact to minimize interference
  and space taken from the detector.
• It should be able to withstand the longitudinal
  forces from the detector solenoid field. These
  forces prohibit aligning this antisolenoid on
  other than the detector axis.
• The 2nd antisolenoid (if needed) may be actually
  wound and aligned on the 2nd FF quad since the
  forces from the detector field should be already
  small.

Position of antisolenoid in SiD
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• Conclusions
• It is found that the method of weak
  antisolenoids provides a good compensation of the
  detector solenoid aberrations at IP in a linear
  collider.
• The antisolenoid method is found to be more
  efficient than a skew quad correction.
• The optimum parameters of the antisolenoid
  depend on the detector solenoid model and the
  crossing angle, but not on IP b-functions. With a
  proper optimization, this method should provide
  an adequate compensation at the larger crossing
  angle of 35 mrad for g-g.
• The antisolenoid compensation is found excellent
  at the nominal beam energy of 250 GeV and should
  be sufficient at much lower energies when using
  additional linear and 2nd order tuning knobs.
• Antisolenoid has to be placed at the FF quad
  location and designed as part of the detector. It
  is desirable to have means for fine tuning of
  antisolenoid strength and effective position. The
  technical issues of this design and impact on the
  detector need to be studied.
• As a 2nd choice, the skew quad correction with
  the aid of tuning knobs should still be able to
  provide the correction.