IR Optimization, DID and anti-DID Page: 4 of 5
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Os = y 01p (g/2-1) = 01p E (GeV) / 0.44065. The maximum
practical angle from Table.1 is 01p =122grad which for the
spin angle gives Os =69mrad (and does not depend on
energy, since for fixed detector field Op -l/E). The
polarization behaves as cosine of the angles cos(Op+Os)
and thus it does not allow full factorization of the effects
of the angles at the IP and at the upstream polarimeter. In
the assumption that Op<Os and that the beam orbit angle at
the IP can be measured with precision better than about
10rad, the precision of polarization knowledge at the IP
with respect to measurement at the upstream polarimeter
is dominated by the spin misalignment at the polarimeter
and could be expressed as rms(Op)*Os. This gives about
0.17% rms for the polarization precision which is better
than the target goal of 0.25%.
Moreover, when the vertical IP angle is not corrected,
one can still tilt the beam orbit in the downstream
polarimeter (where the SR beam emittance increase does
not matter, in contrary to the upstream polarimeter) to
match the IP angle.
These considerations suggest that while it is possible to
zero or decrease the vertical angle at the IP, this may not
be necessary, since polarization precision goals can be
Anti-DID and TPC Operation
Let us discuss compatibility of DID (or anti-DID)
transverse field with Time Projection Chamber operation.
Traditionally, TPC specify requirements for field
uniformity with certain high precision. However, precise
3D field maps are used in tracking reconstructions
anyway. Therefore, providing 3D map of solenoid field
with DID (for several settings) would solve this particular
TPC-DID concern. However, there is another issue related
to TPC track-based calibration. It was suggested by Dan
Peterson  that uniform magnetic field is required in
some region about half-a-meter around the IP in order to
perform a track-based calibration the magnetic field. Such
uniform field region would allow isolating the effects of
the field distortions on track trajectories from the effects
of field distortions on the drift path. It was suggested 
that the uniformity requirement is dB/B < 4*10-4 for Izi <
50 cm, while the uniformity is less important at larger z -
the current DID design field of 0.07T at Izl= 2.2m (in
LDC at 20mrad) would be acceptable. Details of TPC
operations and specifics of the field-map requirements
due to the anti-DID will be discussed in details in
upcoming notes .
To address the above challenge, we suggested to
modify the design of DID coils and construct the field
using two coils, a shorter and a longer one. The 3D
models of the coils were created, with the same radius of
3.5m and with pattern length of 1.5m and 3m, as
illustrated in Fig.11. The resulting field was used in the
In the field calculation the effect of detector iron was
neglected (we checked earlier that this is a reasonable
approximation) but eventually the iron should be included
in detailed simulations. The "short" and "long" DID coils
were combined and the currents were adjusted to flatten
the field in the center. It was found that in order to flatten
the field, the current ratio for the short/long coils should
be equal to -1.245, and both currents need to be increased
2.5 times to have the same max field for the combined
DID as for single coil.
With combined DID coil, reduction of the field in the
central region (Izl < 0.5m) was found to be about 65 times
with respect to the single long DID, as illustrated in
Fig.12. Such modification of the DID field shape should
ease TPC calibration. The DID field shape used in this
paper for GLD and LDC was similar to the one shown in
Fig. 12, but had sharper decay after IzI>5m due to effect of
the iron of the detector yoke (shown in Fig.8).
Figure 11: 3D model of a longer DID coil. Coil radius is
R=3.5m, length 3m, effective magnetic length 3.97m. The
shorter DID coil had the same radius and length of 1.5m.
Bx of single DID coils and of combined DID
- Single DID (ong coil)
- Single DID (Short coil)
Figure 12: Field of the shorter (1.5m pattern length) and
longer (3m) DID coils and the field of the combined DID
coil optimized for detectors with TPC.
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Seryi, Andrei; Maruyama, Takashi; /SLAC; Parker, Brett & /Brookhaven. IR Optimization, DID and anti-DID, article, February 3, 2006; [Menlo Park, California]. (digital.library.unt.edu/ark:/67531/metadc878368/m1/4/: accessed November 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.