1 / 17

Au Beam Lifetime at Low Energy

This presentation discusses the effects of working points, intensity dependence, and octupoles on the lifetime of Au beam at low energy. It also explores the impact of cooling and compares different store configurations. Practical impediments and possible solutions are discussed.

puglisi
Download Presentation

Au Beam Lifetime at Low Energy

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Au Beam Lifetime at Low Energy V. Schoefer (for everyone) 7/31/2019 RHIC Retreat Run19

  2. Outline • Working point effects • Intensity dependence • Fixed target 3.85 GeV lifetime • Octupoles and detuning • Lifetime during cooling comparison • Flat bunches • Au beam only (not about the effect of the electrons) • Almost exclusively about 3.85 GeV/N

  3. Statement of the Problem LEReC is designed only to counteract growth from IBS. Mechanisms for loss or growth faster than IBS diminish the baseline luminosity (without cooling) and the expectation of gain from cooling BETACOOL simulations (Chuyu) showed growth rates from IBS very similar to measured. Focus for this talk is particle loss.

  4. Uncontroversial Background The decay rate (DCCT) for Au beam: At 3.85 GeV/N: 100%/hr ‘easily’ achieved (tunes/chroms) 20-30%/hr achievable with octupoles At 4.59 GeV/N: 25%/hr easily achievable, no octupoles Beam-beam has a small enough impact on the lifetime that comparing colliding vs non-colliding lifetime is ok. Blue lifetime is systematically worse than yellow (I don’t know why), lots of data for yellow only (because of LEReC layout).

  5. Working point summary • Three working points attempted: • Under 0.1 -> Not obviously better and often difficult to reproduce optimum • Lifetime degradation from e-beam was worse • ‘near’ 0.5 -> A few hours of setup attempted, very difficult, abandoned • 0.23-0.24 -> ‘classic’ RHIC tunes with some benefit from pushing further up toward 0.25 (6/16/19): Vadim scanned along diagonal in reasonably small steps from 0.18 to 0.44, no obvious minimum better than 0.23 neighborhood

  6. Typical 3.85 GeV/N Stores Representative physics stores (no octupoles) DCCT Decay Rates

  7. Optimized Lifetime (with octupoles) DCCT Decay Rates

  8. Octupoles and Detuning Octupoles significantly decrease the measured tune spread and improve the lifetime (at 3.85 GeV/N) [Not typical behavior at 9.8 GeV/N) Where does the amplitude dependent tune spread come from? (quad edges?) Lattice sextupole detuning not enough (MADX calculation) Does improvement in lifetime come from tune spread, driving terms or both? Do we need to look into custom octupole families (software)? Tune Distributions

  9. Intensity Dependence (6/16/19) • Chroms set to zero (highly linear) • Octupole optimized for lifetime • Measure bunched beam decay with exponential fit to WCM after first 25 seconds • Two different tunes • Some sign of faster decay at 0.23 over 0.5e9/b • BETACOOL simulation of bunch beam lifetime if only losses are out of the bucket (no transverse aperture) • Simulation only at 0.5e9 (for now), scaled linearly to other intensities Bunched Beam Decay Constants Projecting measurements to zero intensity, decay is zero (Thanks to Alexei for BETACOOL calculation)

  10. A Clue from Fixed Target at 3.85 GeV (6/9/19) The bunched beam lifetime in the fixed target lattice (beta* = 10 m) is good without octupole Bunched Beam Decay Constants • High end intensity: 7e8/bunch • In line with debunching only expectation, without octupole correction • Plausible differences • IP6 physical aperture restrictions • Octupole from triplet nonlinearities due to change in beta* (NO BTFs from the fixed target run!) Fixed target, no oct

  11. Can we run with low tune spread? An illustrative attempt (6/26/19) A lower intensity store, lifetime optimized with octupoles E-beam cur E-beam on High loss rate BMMPS trip SRF trip, good lifetime DCCT Loss % DCCT,WCM Instability (low tune spread) Low avg bunch intensity

  12. Can we run this way? An illustrative attempt (6/26/19) More than double intensity, low tune spread Fast coherence losses Good lifetime w/oct Low tune spread Revert all lifetime improving changes Comparison stores Cooling on/off

  13. Bunched beam lifetime for cooled store Bunched Beam Decay Constants Adding the cooled and uncooled comparison stores from late on 6/26 The baseline lifetime is factor 2-4 times worse than optimal case Intensity dependence much steeper and very steep in the cooled case. The best comparison case we have (40% gain in integrated lumi) is fundamentally different from our most optimized state.

  14. Practical impediments • Stability problems • Instability both during injections and after 15 minutes of circulating • Is there a chrom/octupole setup that is good for both lifetime and stability? • If lifetime is driving term driven, then orthogonal detuning/driving term knobs would help • Do we need the BBB transverse damper? • Keeping this on continuously for stores establishes a low noise tolerance • Requires development (i.e Kevin Mernick’s already overbooked time) • Collide at an addition IP? • Maybe stabilizing with minimum lifetime penalty • Collimation • All lifetime studies/tuning done with collimators out. • Low beta* at low energy in collisions often requires aggressive collimation (“aggressive” means noticeable on the lifetime). • Background considerations + LEReC requirements for beam lifetime may limit the beta*. • Should we consider commissioning LEReC at 10m and walking down? • Pushing for peak lumi may not be the best way to optimize under the existing constraints.

  15. Flattened bunches Apply 28 MHz voltage out of phase with 9 MHz bucket to defocus longitudinally • Two main advantages: • Reduced peak current (space charge) for ion lifetime • Ion focusing of electrons is even during the ‘plateau’ WCM profiles • Two main disadvantages: • Peak lumi down by ~30% • For all bunches, requires 9 MHz @ h=123. Challenging trigger mode for BPMs at low peak current

  16. Flattened bunches Apply 28 MHz voltage out of phase with 9 MHz bucket to defocus longitudinally • Small improvement in early lifetime • Space charge? • Bucket area? • Long term lifetime identical to ordinary bunch • Longitudinal cooling rate (not shown) identical • No dedicated attempt to optimize electron beam for flat bunches Bunch Intensity Small benefit to initial lifetime For discussion: Should we set up at h=123 at startup to keep this option open?

  17. Conclusions • Bunched beam decay rates consistent with IBS demonstrated (we did it!) • This is good news! Rules out difficult to diagnose lifetime issues. • Achievable only at very low tune spread. “Tuning” may or may not be enough to find a compromise • Cooling comparison stores were never performed in the most optimized configurations • Should have IBS simulation for Run20 • configurations before the run. • Outstanding questions concerning • Octupole families • BBB damper • ’flat’ bunches (harmonic number) • Beta* strategy Electron heating C-AD Ion lifetime

More Related