Dither Luminosity feedback versus Fast IP feedback A.Drago Dec/17/2008

Introduction SuperB wants to make stable collisions between beams with 5pm emittance Problems to a perfect stable match at the IP can come from A) Mechanical vibrations from Seismic sources Vehicular traffic around the collider buildings B) Ripples in the electromagnet power supplies Both cases can make “slow” shift/drift of the beams decreasing luminosity

Two approaches We are going to compare two very different feedback approaches to be used in the SuperB interaction point: A) Dither Luminosity feedback Evolution from the dither coil based luminosity feedback designed for PEP-II by SLAC team B) Fast IP feedback inspired by the IntraTrain IP feedback designed for ILC by Phil Burrows team (Oxford Un.) see PAC07, “THE FONT4 ILC INTRA-TRAIN BEAM-BASED DIGITAL FEEDBACK”

Common points Both feedbacks have the goal to compensate for residual vibrations or ripples induced jitter in the IP by steering the electron and positron beams into collision Both systems are faster than the supposed noise frequency bandwidth A third competitor that in principle should be considered, is an orbit feedback. Like for example the LIBERA-based feedback (working at ~1kHz) tha has been implemented in circular light sources with very low emittance

Dither Luminosity Feedback (from Mike Sullivan’s doc) The idea is to use air coils that can be driven at frequencies of around 100 Hz. We simultaneously excite the coils at 3 different frequencies – one for each dimension and detect the 3 signals with separate lock-in amplifiers. The lock-ins deliver an amplitude and sign for each dimension. The sign indicates which direction to move. The feedback algorithm then uses this information to decide how much to step in each dimension. The 3 steps are then added together and the magnets are moved in all 3 dimensions at once.

Fast IP feedback The key components of each such system are beam position monitors (BPMs) to pickup the beam orbit; fast signal processors to translate the raw BPM signals into a position output; feedback circuits, including delay loops, for applying gain and taking account of system latency; amplifiers to provide the required output drive signals; kickers for applying position (or angle) correction to the beam. A schematic of the IP intratrain feedback is shown in Figure 1, (next slide) for the case in which the electron and positron beams cross with a small angle; the current ILC design incorporates a crossing angle of 14 mrad. The ILC prototype system incorporates a digital feedback processor based on a FPGA (Field Programmable Gate Array) device. The use of a digital processor will allow for the implementation of more sophisticated algorithms which can be optimised for possible beam jitter scenarios at the IP.

Fast IP feedback Two possible implementation

Dither-LUM-FB (PEPII) FAST-IP-FB (ILC) Pickup Dither coils BPM or striplines Actuators Magnet power supplies kicker processing technol. Mixed FPGA Excitation to beam yes No Working planes X, Y, Yp position or angle Main algorithm Newton zone finder FIR (?) sensitivity ? Response time ~1ms <200ns Low emittance impact ??? very low Feedforward or b-b-b no tested @ PEP-II ATF2

Question & Comments The excitation given by the Dither FB can produce problems to a very low emittance beam? Where can be measured performance and impact of the dither feedback? (KEK???) Apart from goals, the two systems seem very different for implementation & technology The Dither_fb could be defined a complete luminosity fb while the Fast_IP is more a very fast local orbit fb The FAST_IP_FB can be easily implemented by the fast bunch-by-bunch feedback hardware The dither fb should be implemented as in PEP-II In principle the two approaches can cohabit together without mixing each others (apart for the needed space!)

Fast dither for SuperB (from Mike Sullivan’s doc) We will need to make special beam pipes that pass 1kHz magnetic fields but block RF and HOMs for the dither coils. We would also need fast beam position correctors. In PEP-II we used our standard DC correctors which limited our correction speed to 8 Hz. We need a luminosity signal similar to what we had in PEP-II however it must have a faster response time. We will need to develop a dedicated feedback software package that also has a scanning option for finding collisions. We need a good simulation package to test the feedback algorithm. At lower luminosity the signal fluctuations can be larger, but we still need to make it work for finding initial collisions, etc.

Conclusions It seem interesting try to implement both approaches and maintaining them separate The two systems should cohabit without problems The “Dither Coil Feedback” should be developed considering fast data exchange with the luminosity monitor The “Fast IP Feedback” should be implemented in close connection with the fast bunch by bunch feedback design