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6/R-012 - conference room (CERN)
Minutes from the FCC-ee Accelerator meeting of 13 July 2015
Participants: Sandra Aumon, Alain Blondel, Anton Bogomyagkov, Andreas Doblhammer, Bernhard Holzer, Luis Medina, Yukiyoshi Ohnishi, Katsunobu Oide, Pavel Piminov, Dmitry Shatilov, Sergey Sinyatkin, Marco Alan Valdivia, Frank Zimmermann
Remote participants: Sergey Glukhov, Ivan Koop, Mauro Migliorati, Leonid Rivkin
0) Minutes and actions from the past two meetings
There were no comments to the minutes of the meetings from 4 and 8 June 2015.
Bernhard Holzer requested the FCC-ee design meetings to be maintained on Mondays at 16:00. He recommended a doodle poll to determine the optimum date of the next meeting in August.
1) Energy calibration issues for FCC-ee
Ivan Koop presented the energy calibration concepts, Compton polarimeter, free precession frequency approach, spin precession de-phasing rates, spin tracking results, accuracy and some conclusions.
The proposed energy calibration concept is based on the production of a polarized positron beam in a damping ring at 1-2 GeV, acceleration of polarized beams via linac, SPS and, finally, a 100-km booster ring with Siberian snakes. Injection from the booster into the collider ring will be with horizontal spin orientation. Then the turn-by-turn precession frequency will be measured using a longitudinal Compton polarimeter. The number of polarimeters should be large, in order to be able to measure the beam energy at several points around the ring using the magnetic spectrometer.
The free-precession approach avoid problems with spin resonances in the collider, i.e. this method works at all energies. The spectrometer would be calibrated with laser Compton backscattering. An accuracy of 1-10 micron is needed for the position measurements in order to arrive at 1e-5 energy accuracy. Compton scattering has a high asymmetry at the edge of the spectrum. For this reason it is better to detect the scattered electrons rather than photons.
Example tracking results showed the spin modulation. The loss of polarization degree due to dephasing is small for a high enough synchrotron tune. Spin recoherence at the synchrotron frequency is clearly visible. Results for different values of the synchrotron tune illustrated that with smaller synchrotron tune, the polarization approaches some equilibrium value, unequal zero. For a value of the spin modulation index lager than 1.7 the central peak disappears in the frequency spectrum. Other simulation examples were presented for beam energies of 80 and 120 GeV.
An accuracy analysis was also presented. The error of the frequency decreases with the inverse third power of the number of turns. The error of the phase measurement is of order 0.04 rad. On one shot one can measure the spin phase advance with a relative accuracy of 4e-4 (total spin tune ~100).
In conclusion, the free precession approach provides an extremely fast method of spin frequency measurement. In general high synchrotron tunes are preferred. FCC-ee crab-waist parameters do not satisfy this requirement, so that special modes of operation might be needed for the calibration.
Plans for future work include the development of the accelerator chain for polarized beams, considering the resonant depolarization technique as an alternative, incorporation of 90-degree spin rotator insertions, simulation tool for suppression of spin resonances, etc. Ivan Koop highlighted that there are more experts at BINP available to contribute to these tasks.
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Alain Blondel commented that the Compton polarimeter could be used for various purposes. A primary challenge is the energy sawtooth, which makes the energy at the IP different from the average energy. Ivan Koop remarked that the Compton magnetic spectrometer was a key tool to understand the energy sawtooth effects, and to explore other systematic error sources, by running at lower energy.
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Alain Blondel highlighted the need for developing a cost estimate for the polarization effort. Ivan Koop estimated a few dozens of millions of CHF. The rotators and Siberian snakes would not be terribly expensive. Katsunobu Oide remarked that the polarized positron damping ring will be expensive though.
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Ivan Koop announced that he has prepared a report which summarizes the results.
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Bernhard Holzer commented that lower beam energy will require extremely low dipole fields.
2) IR with elliptical compensated solenoid in FCC-ee
Sergey Sinyatkin estimated the solenoid fringe field, presented a simple optical model of elliptical solenoids, magnet field simulations, and effect on ring optics.
The layout of the IR solenoids was sketched.
The elliptical solenoids can be represented by ordinary round solenoid and skew quadrupoles.
To reduce the vertical emittance the field of compensated solenoid was decreased to -2 T and its length increased to 1 m.
Longitudinal and radial fields, and their integrals, were calculated for various configurations. The integral of the radial field created by the elliptical solenoid is non zero. In case of using elliptical solenoids both for compensation and screening the integral of the radial field is reduced.
The betatron coupling is not suppressed by elliptical compensated solenoid.
Splitting the elements into small slices allows taking into account the solenoid fringe field more precisely.
For reaching the design luminosity, the compensation solenoid should not be shorter than 1 m.
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Replying to a question by Frank Zimmermann, Anton Bogomyagkov elaborated that the conclusion of Sergey Sinyatkin’s work is that a round solenoid with reduced field and extended length is preferred over the use of elliptical solenoids, which would require additional skew quadrupoles.
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Alain Blondel commented that 15-20 mrad angles are needed for luminosity measurements in addition to larger angles.
3) Tunnel geometry
Katsunobu Oide compared the tunnel geometry of FCC-hh and FCC-ee (asymmetric case). The two machines lie up to a few hundred metres apart.
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Frank Zimmermann asked whether the hadron collider layout could perhaps be adapted, e.g. by extending the length of the experimental insertions.
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