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

2. Previous episodes Alain gave the foundations in a talk at TLEP6: “Beam Polarization and Energy Calibration” 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 ration between B+ and B- of 6.25

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

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 ≥2x10 12 particles/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 mins we 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