1. Simulations of collimation losses at RHIC
G. Robert-Demolaize, A. Drees
Work supported, in parts, by the European Commission under the Capacities 7th Framework Programme, Grant Agreement (EuCARD-2 Project).
Collimation - WP5 Tracking Workshop, CERN, Oct. 30, 2015

2. Collimation - WP5 Tracking Workshop, CERN, Oct. 30, 2015
I – Introduction II – Simulations of pp operations III – Loss maps from tracking IV – Machine protection studies V - Conclusion
Primary purpose of RHIC collimation system: background control for the STAR and PHENIX experiments.
System layout:
Since Run15, additional masks have been installed to help protect the detectors from abort system pre- fires.
Typical running conditions for 200 GeV collisions:
Species
No. of bunches
Intensity/bunch [*1e9]
β* [m]
Emitt. (rms) [mm.mrad]
Lstore avg. [cm-2s-1] pp
111
2.25
0.85
2.8 → 4.0
63x1030 Au
1.6
0.7 → 0.5
2.5 → 0.65
50x1026
Collimation - WP5 Tracking Workshop, CERN, Oct. 30, 2015

3. Collimation - WP5 Tracking Workshop, CERN, Oct. 30, 2015
I – Introduction II – Simulations of pp operations III – Loss maps from tracking IV – Machine protection studies V - Conclusion
RHIC: 1primary (H+V) + 3 secondary collimators (H / V / H) for each beamline.
Pin diodes are installed downstream of each collimator to get a direct loss signal when setting their position.
One movable O-shaped silicon crystal was installed in the yellow RHIC ring prior to Run1, forming a 2- stage collimation system together with the existing L-shaped copper scrapers. However, the crystal collimator was removed in 2003 after no configuration could be found in which the crystal actually  reduced the experimental backgrounds due to unfavorable angular divergence of the RHIC beam in the available warm sections
RHIC primary scraper
LHC horizontal collimator
Collimation - WP5 Tracking Workshop, CERN, Oct. 30, 2015

4. Collimation - WP5 Tracking Workshop, CERN, Oct. 30, 2015
I – Introduction II – Simulations of pp operations III – Loss maps from tracking IV – Machine protection studies V - Conclusion
The following will present work that was performed in part as benchmarking studies for SixTrack’s collimation module after its first official release.
Main goal: reproduce beam conditions for RHIC operations with polarized protons, i.e. generate a lattice with MAD-X (offline model) and convert it to SixTrack for tracking with collimation system.
Particles’ tracks are then processed through an aperture model: the resulting loss maps are then compared with measurements from BLM’s.
Simulated machine settings (Run5 pp):
Parameter
Achieved value
Injection energy [GeV]
24.3
Store energy [GeV]
100
Transverse norm. emittance at store [mm.mrad]
20.0
Working point at store [Qx / Qy]
0.690 / 0.685
Protons per bunch
2.0 x 1011
Bunches per ring
111
Average store Luminosity [cm2.s-1]
7.0 x 1030
β* in STAR and PHENIX [m]
1.0
β* at other IPs [m]
10.0
Collimation - WP5 Tracking Workshop, CERN, Oct. 30, 2015

5. Collimation - WP5 Tracking Workshop, CERN, Oct. 30, 2015
I – Introduction II – Simulations of pp operations III – Loss maps from tracking IV – Machine protection studies V - Conclusion
The following will present work that was performed in part as benchmarking studies for SixTrack’s collimation module after its first official release.
Main goal: reproduce beam conditions for RHIC operations with polarized protons, i.e. generate a lattice with MAD-X (offline model) and convert it to SixTrack for tracking with collimation system.
Particles’ tracks are then processed through an aperture model: the resulting loss maps are then compared with measurements from BLM’s.
What the aperture model looks like:
Collimation - WP5 Tracking Workshop, CERN, Oct. 30, 2015

6. Collimation - WP5 Tracking Workshop, CERN, Oct. 30, 2015
I – Introduction II – Simulations of pp operations III – Loss maps from tracking IV – Machine protection studies V - Conclusion
Dedicated data set: simulations aim at reproducing the loss pattern generated by the V1 jaw scraping the Blue beam:
IR8
IR6
Collimation - WP5 Tracking Workshop, CERN, Oct. 30, 2015

7. Collimation - WP5 Tracking Workshop, CERN, Oct. 30, 2015
I – Introduction II – Simulations of pp operations III – Loss maps from tracking IV – Machine protection studies V - Conclusion
Result of simulations: list locations of direct proton losses, i.e. elements in which the transverse coordinates of tracked protons get larger than the available mechanical aperture
The aperture model allows spotting proton losses with a 10 cm resolution, while in the machine loss monitors are only installed at predetermined locations, mostly looking in the horizontal plane and are color blind (i.e. measure and display losses coming from both beam lines at the same time).
Real machine conditions like orbit perturbations and β-beating are derived from logged datasets and inserted into the tracking model.
Collimation - WP5 Tracking Workshop, CERN, Oct. 30, 2015

8. Good agreement overall on loss locations !!
I – Introduction II – Simulations of pp operations III – Loss maps from tracking IV – Machine protection studies V - Conclusion
Interpolated loss locations compared to amplitude of BLM signal:
Good agreement overall on loss locations !!
Collimation - WP5 Tracking Workshop, CERN, Oct. 30, 2015

9. Collimation - WP5 Tracking Workshop, CERN, Oct. 30, 2015
I – Introduction II – Simulations of pp operations III – Loss maps from tracking IV – Machine protection studies V - Conclusion
Zoom in collimation system region:
Intensity of the loss signal at ~700 m is due to the showers from the collimators
Collimation - WP5 Tracking Workshop, CERN, Oct. 30, 2015

10. Collimation - WP5 Tracking Workshop, CERN, Oct. 30, 2015
I – Introduction II – Simulations of pp operations III – Loss maps from tracking IV – Machine protection studies V - Conclusion
Deriving a scaling law from the statistics shown remains complicated since the tracking tools are designed to show only the locations where the protons scattered by the collimation system are lost, while particle showers induced by the proton-matter inelastic interactions in each collimator are also seen by the BLMs
=> need to include FLUKA/GEANT simulations to those results!
one can consider that each BLM can only "see" beam losses up to a certain distance upstream of it (taken as 5m here).
Collimation - WP5 Tracking Workshop, CERN, Oct. 30, 2015

11. Collimation - WP5 Tracking Workshop, CERN, Oct. 30, 2015
I – Introduction II – Simulations of pp operations III – Loss maps from tracking IV – Machine protection studies V - Conclusion
Zoom in the STAR experiment region:
TRIPLET
TRIPLET
losses in STAR take place in front of the Q3 triplet magnet
Collimation - WP5 Tracking Workshop, CERN, Oct. 30, 2015

12. Collimation - WP5 Tracking Workshop, CERN, Oct. 30, 2015
I – Introduction II – Simulations of pp operations III – Loss maps from tracking IV – Machine protection studies V - Conclusion
Main issue for machine protection: damage to the detectors from abort system pre-fires.
Location of abort system w.r.t. experiments:
=> 4 arcs <Abort ↔ STAR> for Blue beam, but only 1 arc <Abort ↔ PHENIX>!!
Damage to the PHENIX MPC pre-amplifiers after a pre-fire event:
BEAM ABORT RF
Collimation - WP5 Tracking Workshop, CERN, Oct. 30, 2015

13. Collimation - WP5 Tracking Workshop, CERN, Oct. 30, 2015
I – Introduction II – Simulations of pp operations III – Loss maps from tracking IV – Machine protection studies V - Conclusion
Simple tracking results showing the impact of abort pre-fires:
Studies on the history and statistics of pre-fire events show that severe damage to PHENIX occurs during asymmetric running conditions only: Run12 Cu-Au and Run15 p-Au.
From the Report of the Task Force on Experimental Protection (July 2015): “Bench studies have been able to reproduce this failure mode with signal size equivalent to approximately 30 TeV of energy (i.e. the energy of 1.5 Au ions at 100 GeV/nucleon) deposited into a single MPC crystal.”
=> it is critical to intercept all bunches at risk to reach the machine aperture!!
Collimation - WP5 Tracking Workshop, CERN, Oct. 30, 2015

14. Collimation - WP5 Tracking Workshop, CERN, Oct. 30, 2015
I – Introduction II – Simulations of pp operations III – Loss maps from tracking IV – Machine protection studies V - Conclusion
Solution 1: use the arcs immediately around IR10 to lose kicked bunches in the cold aperture:
Evidence of the protection added:
Collimation - WP5 Tracking Workshop, CERN, Oct. 30, 2015

15. Collimation - WP5 Tracking Workshop, CERN, Oct. 30, 2015
I – Introduction II – Simulations of pp operations III – Loss maps from tracking IV – Machine protection studies V - Conclusion
Solution 2: install additional movable jaws to intercept kicked bunches before they reach STAR/PHENIX
First (not yet very detailed) Geant4 studies have shown that the deposited energy in one MPC tower (lead glass with 2.2 cm x 2.2 cm cross section) in an event where the primary beam hits the mask upstream of PHENIX would always exceed the (old) preamp limits. But we HAVE survived MANY prefires!
Studies need to be expanded with more details added (e.g. magnetic field, transverse offsets).
installed one arc downstream of the abort system: IR12 for Blue, IR8 for Yellow
Collimation - WP5 Tracking Workshop, CERN, Oct. 30, 2015

16. Collimation - WP5 Tracking Workshop, CERN, Oct. 30, 2015
I – Introduction II – Simulations of pp operations III – Loss maps from tracking IV – Machine protection studies V - Conclusion
Comparing the performance of each solution – looking at a Yellow pre-fire:
before damage
after 1st event
mask only
Collimation - WP5 Tracking Workshop, CERN, Oct. 30, 2015

17. Collimation - WP5 Tracking Workshop, CERN, Oct. 30, 2015
I – Introduction II – Simulations of pp operations III – Loss maps from tracking IV – Machine protection studies V - Conclusion
Comparing the performance of each solution – looking at a Yellow pre-fire:
before damage
after 2nd event
bump only
Collimation - WP5 Tracking Workshop, CERN, Oct. 30, 2015

18. Collimation - WP5 Tracking Workshop, CERN, Oct. 30, 2015
I – Introduction II – Simulations of pp operations III – Loss maps from tracking IV – Machine protection studies V - Conclusion
SixTrack studies have been performed with polarized protons in the context of benchmarking the software with live RHIC data => allowed demonstrating the prediction accuracy of the collimation tracking module!
Tracking simulations with heavy ions possible, albeit not with SixTrack at the time of the study.
Recreating loss patterns that match regular machine operations can be difficult since the design goal of the RHIC collimation system is background control for STAR/PHENIX experiments => study accident scenarios, especially in the context of the planned upgrade to both detectors for both symetric (pp-pp, A-A) and asymetric (p-A) running!
During the p-Au program of Run15, nearly all of PHENIX’s MPC detector pre-amplifiers were destroyed after two abort system pre-fire events, essentially blinding the detector for the rest of the run.
Additional masks have been designed and installed in the RHIC ring: tanks with two-sided, movable jaws to intercept bunches kicked by abort system pre-fires. Lattices are being optimized to provide a better phase advance between the abort system and the masks.
Currently: detailed studies that include Geant4 tracking of the secondary showers from inelastic interactions inside of the mask’s jaws are being performed to predict how much energy can still reach the detectors => additional considerations for machine protection on the detector’s side?
Collimation - WP5 Tracking Workshop, CERN, Oct. 30, 2015