Gap Transient Suppression using Increased Bunch Density Andrew Hutton

HL-LHC Collaboration Meeting Oct 2018 Gap Transient Problem In a high-current multi-bunch storage ring, an abort gap is required to protect the hardware from errant beams A gap is also usually required to mitigate fast ion or electron cloud instabilities The gap produces a large transient in the RF system The transient is hard to correct because the beam loading from the beam is large compared to the available RF drive power The RF transient produces an energy slope along the bunch train This can shift the collision point and/or the time of collision at the IP From my presentation 2018-11-29 HL-LHC Collaboration Meeting Oct 2018

HL-LHC Collaboration Meeting Oct 2018 Proposed solutions Brute force – add more RF generator power Prohibitively expensive Pulse the RF systems to reduce the RF power requirements Reduces the klystron power by locking RF phase to actual bunch phase Does not solve the problem of bunch energy variation along the bunch train Solution proposed by Daniel Boussard for LHC, implemented by Themis Mastorides Match the energy slope for the two beams All bunches collide at the same position where the beta functions are optimized Solution adopted by PEP II and LHC Hard to match electrons and ions Increase bunch charge before and after the gap so average current is constant Solution proposed by John Bird for ALS Tested successfully by Bob Rimmer and Dmitry Teytelman at BEPC Increasing the bunch charge may cause instabilities Doesn’t work with gear-changing Can insert additional bunches in empty buckets in ion ring – how? This solution removes all energy and position transients From my presentation 2018-11-29 HL-LHC Collaboration Meeting Oct 2018

Bunch and Charge Distribution There are two ways of increasing the bunch current either side of the gap: Increase the bunch charge with the same bunch distribution Increase the bunch density with the same bunch charge Increasing the bunch charge brings problems of instability, halo, etc. which might be acceptable Increasing the bunch density does not have these problems but creating the correct bunch pattern stalled this approach This option is available because the RF frequency will be 952 MHz while the bunch repetition frequency is 476 MHz From an email to Bob Rimmer 2018-7-12: Your idea to increase the bunch charge on either side of the gap fixes the RF gap transient   A conceptually simple solution would be to fill the intervening buckets in these areas; this doubles the current but preserves the beam-beam interactions   However, I have no idea how to create this filling pattern HL-LHC Collaboration Meeting Oct 2018

HL-LHC Collaboration Meeting Oct 2018 Large Booster The change to the JLEIC ion injection chain to include a full-size booster changed the situation This full-size booster has several advantages, the biggest impact is on the average luminosity if the bunches are prepared in the booster during collisions so they are ready when the colliding bunches need to be replaced  This moves all of the bunch splitting into the full-size booster and out of the collider ring   There is an additional advantage If all the bunch splitting is done in the full-sized booster, additional bunches could be created in a second fill and transferred (bucket to bucket) into the empty buckets There are two options: Interleave the bunches in the second fill between the circulating bunches Create the double-frequency bunch pattern in the full-size booster and inject the bunch train into an empty gap HL-LHC Collaboration Meeting Oct 2018

HL-LHC Collaboration Meeting Oct 2018 Options Interleaved transfer Circulating Bunches Abort Gap Circulating Bunches Second Injection Second Injection Bunch train transfer Kicker Gaps HL-LHC Collaboration Meeting Oct 2018

Comments on Bunch Train Transfer Bunch train transfer is the easiest Requires additional gaps for kicker rise and fall times which also need to be compensated From Jiquan’ presentation 2019-2-7 Abort gap = 267 ns Kicker gaps = 2 x 20 ns Double current regions 2 x 134 ns Values only approximate – need to be an integral number of wavelengths Additional hardware required: Small 976 MHz RF system in full-size booster for additional bunch splitting Fast kicker: Either two kickers or one kicker capable of two pulses with short inter-pulse charging time HL-LHC Collaboration Meeting Oct 2018

Comments on Interleaved Transfer Interleaved transfer is much harder Requires an injection system of increased complexity There is a theorem that there exists no system of linear or nonlinear optics which can simultaneously close multiple local orbit bumps and dispersion through a single beam transport region  https://www.sciencedirect.com/science/article/pii/S0168900216304673#bib1 This means that either an unclosed injection kick is acceptable or a time- varying kick (e.g. an RF deflector) would be needed  HL-LHC Collaboration Meeting Oct 2018

Unclosed Injection Kick The following slides are from a talk by Chiara Bracco, CERN https://indico.cern.ch/event/451905/contributions/2159032/attachments/1425 216/2188137/CAS_ERICE_Hadrons_Injection.pdf HL-LHC Collaboration Meeting Oct 2018

Unclosed Injected Kick = Filamentation HL-LHC Collaboration Meeting Oct 2018

Injection with Dispersion If the incoming particles have a different energy from the circulating bunches and there is dispersion at the septum: It is possible to inject such that the bunch separation equals the energy difference x dispersion There is then no transverse filamentation BUT, since the energies are different, the incoming bunches will oscillate with the synchrotron frequency Result – filamentation in the longitudinal direction leading to long bunches Energy-related halo is liable to be a problem HL-LHC Collaboration Meeting Oct 2018

Matched Interleaved Injection Involves an RF transverse kicker 90 degrees downstream from the septum operating at 476 MHz with a dipole associated such that the circulating beam sees zero kick The incoming bunches are offset by one half wavelength at 476 MHz and therefore see a kick from the RF kicker and the dipole This can be used to put the injected bunches on the circulating orbit so that they can be captured by the 952MHz RF system in the ion ring.     The rise and fall time of the RF kicker/dipole pair should be of the order of 50% of the revolution time in the ion ring (a few microseconds) The peak power required is huge (Jiquan estimated 12.5 MW for 10 meters of RF structure), but the duration of the power requirement is really short A possible solution is based on the KEKB ARES cavities, which have a large storage cavity, an intermediate coupling cavity and the accelerating cavity itself (http://epaper.kek.jp/a98/APAC98/6D039.PDF) and switching the coupling cavity with ferroelectrics   RF curvature is also a problem HL-LHC Collaboration Meeting Oct 2018

HL-LHC Collaboration Meeting Oct 2018 Overall comments Unmatched injection is unacceptable Unmatched injection leads to longitudinal halo Interleaved injection with an RF kicker is technically possible, but looks expensive and would be a long R&D project to demonstrate Best for the moment is the batch train transfer Would be nice to find a way to avoid the additional kicker gaps HL-LHC Collaboration Meeting Oct 2018