Precise polarimetry with 0.25% accuracy is needed.[1] Compton polarimeters are being designed to achieve this and have been included in the baseline beam delivery design.[2,3] Preliminary designs for polarimeter diagnostic chicanes are included upstream and downstream of the IP for both the 2mrad and 20mrad IR designs. Detailed studies are underway to refine these designs and evaluate their performance capabilities. To achieve the best accuracy for polarimetry and to aid in the alignment of the spin vector, it is desirable to implement polarimeters both upstream and downstream of the IR. The beam optics need to be designed such that the polarization vector can be fully longitudinal simultaneously at the collider IP and the two polarimeter IPs.
The upstream polarimeter measures the undisturbed beam during collisions. The relatively clean environment allows a laser system that measures every single bunch in the train and a large lever arm in analyzing power for a multi-channel polarimeter, which facilitates internal systematic checks.
The downstream polarimeter measures a priory the polarization of the outgoing beam after collision. The average depolarization for colliding beams is 0.3%, and for the outgoing beam 1%. Due to a clever choice of the extraction line optics the beam can, however, be focused such that its polarization is very similar to the luminosity-weighted polarization. The polarization of the undisturbed beam can be measured as well with non-colliding beams. The much higher background requires a high power laser that can only probe one or a few bunches per train and the lever arm in analyzing power is smaller.
The upstream polarimeters are located ~1400 meters before the e+e- IP. The design has evolved from an earlier study for the TESLA machine.[4] Most major aspects of this work, except for the spectrometer configuration, remain valid for the ILC. In particular it is foreseen to retain the TTF-style laser developed by Max-Born Institute,[5] as it is well adapted to the particular bunch and pulse structure of the cold machine.
Dedicated 4-magnet chicane spectrometers will be employed, similar to those at the extraction line polarimeters. This will eliminate some of the operational shortcomings inherent in the old TESLA design that relied on beamline magnets in the existing BDS lattice. A schematic layout of the chicane spectrometer is shown in Figure 1.The horizontal width of the good field region of the individual dipoles is chosen to accommodate a maximum dispersion of 11 cm for the lowest expected beam energy of 45.6 GeV for the Giga-Z option. The laser beam enters and exits between the inner two dipoles, which must be separated by some 8 meters for a vertical beam crossing of 10 mrad. A possible optical arrangement was given at LCWS-05.[3]
Compton electrons generated at the laser IP at mid-chicane will propagate essentially along the electron beam direction . The third dipole D3 will fan out the Compton electron spectrum, while the fourth dipole can be used to restore the angular direction, if it has sufficient width. The Compton electrons are detected behind the last dipole in a gas Cerenkov hodoscope with 20 identical channels.[3]
The layout for the 20 mrad crossing angle interaction region shown in Figure 2 has the Compton interaction point approximately 142 meters downstream from the e+e- Interaction Point. All bends are in the vertical plane. The extraction line apertures are designed to accommodate the ±0.75mrad cone of beamsstrahlung photons produced in the e+e- interaction and the low energy disrupted electrons. The 2 mrad crossing angle extraction line first moves the extracted beam away from the incoming beam line and then bends the beam back to the direction it had at the e+e- interaction point . This is done in the horizontal plane as shown in Figure 3. The Compton polarimeter is located 226 meters downstream from the 2 mrad crossing angle e+e- interaction point and the polarimeter chicane bends in the vertical plane.
The Compton interaction point is located at a secondary focus in the middle of a chicane with 20 mm dispersion, but with no net bend angle with respect to the primary IP.[6] At the middle of the chicane the Compton scattering occurs and the scattered electron is confined to a cone having a half-angle of 2µrad and is effectively collinear with the initial electron direction. The beam-beam depolarization effects are measured in the extraction line polarimeter directly by comparing beams in and out of collision. Also, spin precession effects due to the final focus optics and beam-beam deflections can be studied by correlating the polarization and Interaction Point beam position monitor measurements.
A 532-nm (2.33eV) circularly polarized laser beam collides with the electron beam in the middle of the Polarimeter Chicane . Compton-scattered electrons near the kinematic edge at 25.1 GeV are detected in segmented detectors near the last chicane magnet.
References
1. The role of polarized positrons and electrons in revealing
fundamental interactions at the Linear Collider,
G. Moortgat-Pick et al. e-Print Archive:
hep-ph/0507011
(submitted to Physics Reports).
2. Polarization Setup and Polarimetry for 2 IRs, and Status of
Downstream Polarimeter Designs, K.C Moffeit, M. Woods,
Y. Nosochkov , K.P. Schuler, K. Moenig, and W. Oliver,.
SLAC-PUB-11322, 2005.
3. Upstream
Polarimetry with 4-magnet Chicane, N. Meyners,
V. Gharibyan, K.P. Schuler,
presentation at LCWS05.
4. The TESLA Compton
Polarimeter, V. Gharibyan, N.
Meyners, K.P. Schüler, LC-DET-2001-047,
http://www.desy.de/~lcnotes
5.
Running Experience with the Laser System for the rf Gun Based Injector
at the TESLA Test Facility Linac,
S. Schreiber et al., Nucl.
Instr. Meth. A 445 (2000) 427.
6. Studies for a Downstream Compton Polarimeter at the ILC, K.C.
Moffeit and M. Woods,
SLAC-PUB-10669, 2004.
7. A Fabry-Perot Cavity for FLC Polarimetry, A. Variola,
presentation at
LCWS05.