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Transverse polarization: do we need wigglers?

Transverse polarization: do we need wigglers?. M. Koratzinos With valuable input from Alain Blondel TLEP ACC meeting no. 4, 24/3/2014. FCC- ee accelerator meeting no. 6, 26/5/2014. Previous episodes.

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Transverse polarization: do we need wigglers?

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  1. Transverse polarization: do we need wigglers? M. Koratzinos With valuable input from Alain Blondel TLEP ACC meeting no. 4, 24/3/2014 FCC-eeaccelerator meeting no. 6, 26/5/2014

  2. Previous episodes • Alain gave the foundations in a talk at TLEP6: “BeamPolarization and EnergyCalibration” • I will here only concentrate on transverse polarization at the Z peak, needed for energy calibration • The problem is this: • We need continuous energy calibration from the beginning of physics till the end of the fill • Polarization time is ~200 hours at TLEP (scales as circumference to the third power)

  3. wigglers • Alain proposed to solve the problem of long polarization time by introducing wigglers • However, wigglers not only decrease the polarization time, they also increase the energy spread that leads to depolarization • Alain considered that we can afford to increase the energy spread up to 52MeV • Alain used the design and number of wigglers built for LEP: 12 wigglers with a ratio between B+ and B- of 6.25

  4. LEP wigglers Quick and dirty design, troublesome operationally • For TLEP some optimization would be needed • Alain says that changing the wiggler parameters “makes little difference”

  5. A.Blondel TLEP6 polrization 17/10/2013

  6. The problem • The problem with the wigglers is that (at least in the current design) when operating, they eat up 21% of the power budget, meaning that our luminosity will be decreased by 21% • Is this inevitable?

  7. Degree of polarization • Judging from the LEP experience, to achieve a depolarization measurement, 10% polarization is sufficient • I believe that with the design of a good polarimeter, we can bring this figure down: a realistic value we can aim at is 5% • With the use of wigglers, we can achieve a 5% polarization in 45 minutes and a 10% polarization in 90 minutes

  8. Some more facts about TLEP-Z • Strangely, the bunch population at LEP and TLEPZ is the same: 1.8E11 •  I will assume that for depolarization measurements we need bunches with the same number of electrons as normal, colliding, bunches. • Beam lifetime is 200mins • What is the fillup time? • How often do we need to perform depolarization measurements and how many bunches do we need?

  9. Fillup time • According to our recent parameter document (EDMS 1346082): • “The required flux of e+ and of e- is currently estimated to ≥2x1012particles/s for each species.” • I have used 2E12 particles/s and a bhabha lifetime of 200 mins. This results in a fill up time at the Z of 28 minutes:

  10. Number of depol. measurements • I believe that having 5 depolarization measurements per hour is sufficient (at LEP we had less than 1 depol. measurement per fill) • A year’s running would give us a sample of more than 10,000 measurements – it might even be an overkill • We only need to destroy 1 bunch per depolarization measurement • We need to measure electrons and positrons separately • The bunches to be depolarized should not collide

  11. Strategy • Therefore, a strategy emerges: • We start with a fresh machine and 100 bunches per species only. These will be our depolarizing bunches, they will not be colliding. • for 15 minswe operate the wigglers at full blast • After 15 minutes we start injecting the normal beam and the machine is full in 30 minites. During this period we operate the wigglers at full blast. • Now, 45 minutes has passed since the injection of the depolarizing bunches, so they have a polarization of 5% • We switch the wigglers off • We start physics and start depolarizing measurements every 12 minutes, meaning that we use up 5 bunches per hour. • Every time we depolarize a bunch, we replace it with a fresh bunch (non colliding) which has no polarization. • By the time we have used up bunch 100, 20 hours have passed, and our oldest “fresh” bunch has attained a polarization level of 10%

  12. What about TLEP-W? • Calculations show that polarization will be available at the W pair running (this was not the case at LEP) • Polarization time is 9 hours, so 5% is achieved in 30 minutes and 10% in 60 minutes • No wigglers are needed for TLEP-W running, as natular polarization time is lower than the polarization time using wigglers at TLEP-Z

  13. Conclusions • Wigglers are necessary for the Z running • Wigglers need only be used at the initial period of every fill and can be switched off later • Therefore, wigglers will not be eating up our power budget which can be all used to deliver luminosity

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