Minutes by Paolo
Participants: C. Milardi, K. Oide, A. De Santis, O. Etiksen, S. Ozdmenir, P. Craievich, F. Zimmermann, T. Raubenheimer

• The first topic raised by Frank concerns the asymmetric (not uniform) filling of the damping ring. A symmetrical (uniform) filling or a periodic system has its advantages and can be ensured by increasing the circumference. With non-uniform filling there can be problems with phase and amplitude transients. 
  • Note by Tor in a previous email:“The nine trains were spaced non-uniformly with the gap between two of them much larger than others.  The design was sized for 8 uniformly distributed trains.  I suggest we return to that.  I also think that having a 50 ns flat top allows for flexibility in the future - where it is accelerating 3 bunches separated by 25 ns (or 15 ns) or having two bunches separated by 40 ns.  50 ns rise/fall should be straightforward to achieve. “
  • it must be kept in mind that non-uniform filling was introduced to guarantee the 2.5 ms spacing in the common linac (see the Antonio's email below)
     
• Damping time.
  Actually the damping time is 10 ms but is has to be reduced. (Why?)
  • Tor proposed to increase the DR energy but the damping time depends on the number of bunches. To ensure the possibility of polarised positrons in the future, the energy of the DR must avoid resonance energies. 
  • Oide: Number of buckets in BR divide number of buckets in DR mustn’t be an integer otherwise we can not choose the right bucket in the BR. However, changing the repetition rate of the linac can ensure correct injection into the booster. 
  • Ozgur proposed a different layout for the damping ring (with higher magnetic field and Robinson wiggler) and he got a damping time of 6 ms but the emittance is high with the actual design, around 15 mm mead (normalised). Catia will organise a dedicated meeting on this new DR layout. 
• Another possibility would be to go with an even higher energy DR, more than 3.0 GeV. In this case one could inject positrons and electrons into the DR at 3.0 GeV and after the DR accelerate them into the HE linac at 20 GeV. This solution would greatly simplify the injector by avoiding return lines and guaranteeing the beam quality of both species.

Email from Antonio on 6.10.22:

Dear Tor,

here my answer/comment to your mail. I have added also Catia Milardi and Paolo Craievich to share these considerations about timing also with the DR and Injector complex project leaders.

1. The harmonic number is the one that minimize the difference between the effective RF frequency (399.75MHz) w.r.t. the nominal (400 MHz) as result of the DR length. If needed it would be possible to change RF frequency and harmonic number to have a more regular fill structure with some intervention on the lattice, still under development.
2. When the new injector baseline (e.g. separate 1.54 GeV LINACs at  for positron and electron with 200 Hz rep. rate and one common 6 GeV LINAC at 400 Hz rep. rate) was proposed, we studied the possibility to accommodate the needed 2.5 ms time delay without impacting on DR performance. The number of pulses stored in the DR is needed to not reduce the damping time with respect to the nominal baseline of the CDR (40 ms). The damping could be shortened if the incoming positron absolute emittance will be the one foreseen in CDR (1.2 mm mrad), but this parameter is still under investigation together with the DR acceptance. Following the scheme proposed and assuming 10 RF periods time delay between bunches in the same pulse, the spacing in the DR will be 24e,1f,10e,1f,24e bunches (e)mpty/(f)illed respectively.  Kickers rise time and flat top duration have to satisfy these conditions: rise_time < 22 T_RF, flat_top_duration > 14 RF, 2xrise_time+flat_top_duration < 60 RF.  Where at least 2 RF tolerance have been considered. If these requirements are too strong for the kickers, a shorter damping time have to be considered.  
3. In the new baseline for the injector, the extraction have to include the 2.5 ms shift to fill the positron pulses in the common LINAC alternate with respect to the electron pulses. For this reason a transient time with a "hole" in the DR fill is unavoidable. As reported in the slides, synchronicity conditions has been considered for the electron gun in order to fill the DR in the right place during the injection. Correspondingly similar condition should be applied for the extraction to maintain the nominal rate of the injector complex (200 Hz). The time resolution at the level of the extraction will be the revolution period of the DR (~0.8 us) and presumably could be adjusted within some tolerance depending on the equipment used in the LINAC and/or Transfer Lines.
4. Concerning your question about the injection in the booster (BR), i assume that this is connected with the problem of modulating the charge of the single bunch in order to compensate charge asymmetry in the collider (MR). If this is the case, the timing scheme is strictly connected to the second stage of acceleration: pre-booster option (PBR), or High Energy LINAC (HEL). In HEL case the extraction time could be adjusted at the level of single DR turn (0.8 us) within the limit of the common LINAC operation while in the PBR scenario, the intermediate buffer of the second accelerating stage imply a double mapping between the different rings: DR_2_PBR and PBR_2_BR. In both case i would  "reverse" the problem: once is known the mapping between main rings bucket and LINAC pulse timing, the charge of the pulse have to be adapted to the requested one in MR's for the corresponding position (e.g. firing the LINAC at a given time will fill up a well defined pair of buckets in the BR. The charge of this two bunches have to be modulated according to the needs in the main ring).

I hope that my answers could be useful to further understand the timing of the whole complex,

Cheers.

Antonio