loss limitations and collimation angelika drees, r. fliller, w. fu the rhic collimation sytem:...
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Loss Limitations and CollimationAngelika Drees, R. Fliller, W. Fu
The RHIC Collimation Sytem: history and overview
RHIC Loss Limitations Operational limits Beam Dumps/Quench Limits Soil Activation Exp. Background
Summary
Angelika DreesICFA-HB2004 Workshop, Bensheim, Oct. 18, 2004
RHIC Collimator Configuration
RHIC was originally built with a 1-stage collimation system only:
1 dual plane h/v scraper with 45 cm copper jaws, linear motion in both planes, skew motion only in horizontal1 bent crystal collimator for studies in 1 ring (yellow) only
The system was upgraded after the 2003 run because of high experimental backgrounds and gap cleaning demands. Crystal approach proved non sufficient.
RHIC overview: collimation system 2004 and upgrade
(capped area)
New blue 2ndary vertical collimator (v2)
New yellow 2ndary vert. Collimator (v2)
2000-2003:1-stage system including bent crystal in 1 ring
2004:Traditional 2-stage system with 2 horizontal and 1 vertical secondary collimators
2005:Traditional 2-stage system with 2 horizontal and 2 vertical secondary collimators
Angelika DreesICFA-HB2004 Workshop, Bensheim, Oct. 18, 2004
Collimator Section Layout
In the shutdown 2003-2004 the collimation system was upgraded to a conventional 2-stage system including new individual secondary collimators for both planes. The new system was first used in the run 2004 for both, Au and protons.
Angelika DreesICFA-HB2004 Workshop, Bensheim, Oct. 18, 2004
Collimator Design
cross-section of the primary collimator (dual plane)
H
V
H
45 cm copper jawsOne side onlyRotatable, positioning: few m
Angelika DreesICFA-HB2004 Workshop, Bensheim, Oct. 18, 2004
Crystal Collimation Attempt during Fill 3061 (Au)
• STAR “Yellow Halo” signal during Crystal Collimation attempt scraper is closer to the beam toward the bottom of graph
crystal is at 13.6 mm from beam center and channeling
• Scraper alone is more effective than the crystal and scraper together
… If you want to know why we pulled out the crystal and installed traditional secondaries …
Angelika DreesICFA-HB2004 Workshop, Bensheim, Oct. 18, 2004
Loss Limitations
Operational limit: keep allowable loss budget (radiation safety), monitored hour by hour
Quench limit:magnet quenches due to accidental local losses during ramp/store => BLMsmagnet quenches at beam dump due to debunched beam => gap cleaning
Soil activation (not under radiation protection), depends on integrated yearly losses
Experimental backgrounds: need ‘clean’ beams to allow good signal/noise ratio in experiments and keep false trigger rate small (dead time)
Angelika DreesICFA-HB2004 Workshop, Bensheim, Oct. 18, 2004
Operational Limits: Monitoring of Beam Loss/hour
dirty dumpalarm level is reset
alarm level
max.
1 hour
After every dump (operational or accidental) all RHIC loss monitors are analyzed. If dump appears to be “dirty”, i.e. unmasked loss monitors register losses above a certain level, injection into RHIC will be blocked for 1 hour (see figure).
Angelika DreesICFA-HB2004 Workshop, Bensheim, Oct. 18, 2004
Loss Limitations
Operational limit: keep allowable loss budget (radiation safety), monitored hour by hour
Quench limit:
magnet quenches due to accidental local losses during ramp/store => BLMs
magnet quenches at beam dump due to debunched beam => gap cleaning
Soil activation (not under radiation protection), depends on integrated yearly losses
Experimental backgrounds: need ‘clean’ beams to allow good signal/noise ratio in experiments and keep false trigger rate small (dead time)
Angelika DreesICFA-HB2004 Workshop, Bensheim, Oct. 18, 2004
• fill 4118
Accidental Local Losses: BLM thresholds
Magnet quenches can be caused by fast (ms) type losses.=> BLM trip levels (thresholds)
• fill 4198 Continuous scraping/losses cause magnet quenches as well. Trip levels don’t help. software integration or collimators
Angelika DreesICFA-HB2004 Workshop, Bensheim, Oct. 18, 2004
Collimators as limiting Aperture during Store
It is difficult to maintain limiting aperture position at all times – especially during ramp!
Angelika DreesICFA-HB2004 Workshop, Bensheim, Oct. 18, 2004
Collimator use during the Ramp
• Losses around RHIC on 2 ramps during FY01
top: beam scraping at abort kickers
center: scrapers inserted
bottom: lattice
• Using the collimators reduced losses at abort kickers by x100
Angelika DreesICFA-HB2004 Workshop, Bensheim, Oct. 18, 2004
Loss Limitations
Operational limit: keep allowable loss budget (radiation safety), monitored hour by hour
Quench limit:
magnet quenches due to accidental local losses during ramp/store => BLMs
magnet quenches at beam dump due to debunched beam => gap cleaning
Soil activation (not under radiation protection), depends on integrated yearly losses
Experimental backgrounds: need ‘clean’ beams to allow good signal/noise ration in experiments and keep false trigger rate small (dead time)
Angelika DreesICFA-HB2004 Workshop, Bensheim, Oct. 18, 2004
Gap Cleaning: Motivation and Method
gap cleaning is necessary due to the extensive debunching of HI beams (quenching risk)
method:debunched beam is excited transversely (continuously during the store: 1Hz) using damping kickers
the collimation system absorbs the large amplitude particles (in addition to halo)
debunched beam should be lost in collimator area, efficiency relies on collimator performance to avoid increase of exp. backgrounds
Angelika DreesICFA-HB2004 Workshop, Bensheim, Oct. 18, 2004
Excitation frequency for Cleaning
default procedure:excite around betatron tune of bunched beam (measured automatically at the end of ramp)
optional procedure:perform automated tune scan to find resonant tune of debunched beam and excite with this frequency
Angelika DreesICFA-HB2004 Workshop, Bensheim, Oct. 18, 2004
fill 4471, Feb. 06 2004
start of cleaning
~5 109
56 x 56
Cleaning ON vs. cleaning OFF:
yellow:cleaning off, started around 3:30 to allow clean beam dump
blue:cleaning on, debunched beam is continuously excited and absorbed by collimators
Angelika DreesICFA-HB2004 Workshop, Bensheim, Oct. 18, 2004
1 Hz excitation on standard BLMs:
Normal cleaning levels are < 1% of tripLevel => acceptable
Angelika DreesICFA-HB2004 Workshop, Bensheim, Oct. 18, 2004
Debunching rate clearly correlated with bunch current at the beginning of the store. For 4h store length, an upper limit of 1.4*109 ions per bunch results to maintain less than 5*109 debunched beam at the end of store (for safe beam dump).
Remaining debunching with Cleaning ON:
Angelika DreesICFA-HB2004 Workshop, Bensheim, Oct. 18, 2004
Loss Limitations
Operational limit: keep allowable loss budget (radiation safety), monitored hour by hour
Quench limit:
magnet quenches due to accidental local losses during ramp/store => BLMs
magnet quenches at beam dump due to debunched beam => gap cleaning
Soil activation (not under radiation protection), depends on integrated yearly losses
RHIC has to operate such that leachate (from activated soil) cannot contain concentrations of 3H or 22Na that exceed 5% of the drinking water standard
Method: either cap the collimator area (prevent leaching) or use removable soil samples inside tunnel near collimators/dumps to monitor activation level. 22Na is easily measured.
Fill 4581: Au ions (100 GeV), blue gap cleaning was off
The debunched beam is lost atthe collimators:deb.rate = 2*10^7 ions/min
loss rate (total beam) is:loss rate = 3*10^7 ions/min(lost anywhere: triplets, dump ...)
total: 2-5*10^7 ions/min
Fill 4471, Feb. 06 2004, Au ions, 100 GeV
The debunched beam is lost atthe collimators:deb.rate = 2.3*10^7 ions/min
loss rate (total beam) is:loss rate = 3*10^7 ions/min
total (at collimators): 2.5-5*10^7 ions/min
Fill 4870, Mar 24 2004, Au ions, 100 GeV
The debunched beam is lost atthe collimators:deb.rate = 2.4*10^7 ions/min
loss rate (total blue beam) is:3*10^7 ions/min
loss rate (total yellow beam) is:8*10^7 ions/min
total (at blue collimators): 2.5-5*10^7 ions/min
total (at yellow collimators):2.5-10*10^7 ions/min
Beam losses with proton beams
Typical “bad” store:
Loss rate: 10 10^11 = 5 10^9 p/min
Angelika DreesICFA-HB2004 Workshop, Bensheim, Oct. 18, 2004
Assumptions for Soil Activation Calculation
Au Operation:
total loss/min (from gap cleaning): 2*107 ions/min
assume all is lost at collimators, 10% at new V2
-> 2*106 ions/min during the store
assume average of 100 h/week at store (best week last run)
add 0-3 106 ions/min @ v2 => 2-5*106 ions/min
proton Operation:
total loss/min (from bad lifetime store during high intensity run): 5*109 p/min
assume all is lost at collimators, 10% at new v2
-> 5*108 p/min during the store
assume average of 80 h/week at store (best avg. last run)
Angelika DreesICFA-HB2004 Workshop, Bensheim, Oct. 18, 2004
Calculate Soil Activation (Na-22) from Loss Data
Calculate for v2:given the assumptions and using an estimation with the bare collimator/no shielding (there is some concrete shielding @ v2), for Na-22, assuming 15 weeks for each of the Au and proton ions (ie. 30 weeks in total) we get:
8.7 (18) pCi/l and 11.0 pCi/lfor the case of Au-ions and protons respectively. This adds up to
19.7 (29) pCi/l (after 1year), which is just the same (or x1.5 of) the 5% drinking water limit (20 pCi/l) for Na-22.
Compare with Measurement (from soil sample): activation level at primary collimator calculated: 200 pCi/l. measured: 13 pCi/l assuming 100% of all losses at primary collimator is conservative the 15+15 weeks of running is conservative the operating efficiency is overestimated distance collimator-soil is underestimated
Angelika DreesICFA-HB2004 Workshop, Bensheim, Oct. 18, 2004
Loss Limitations
Experimental backgrounds: need ‘clean’ beams to allow good signal/noise ratio in experiments and keep false trigger rate small (dead time).
Collimator positioning should be reliable and efficient for background reduction
Time at the beginning of the store is precious because this has the highest luminosity
-> Collimation and positioning of collimators should be FAST (and precise!)
Angelika DreesICFA-HB2004 Workshop, Bensheim, Oct. 18, 2004
Collimation during Fill 1759 (Au)
• FY01 run has low beam current intensities
• 1-stage system
• only PHENIX benefits (some) from collimation
• there is generally no/little need for collimation during the FY01 run
Angelika DreesICFA-HB2004 Workshop, Bensheim, Oct. 18, 2004
Collimation during Fill 3094 in FY03 (d-Au)
• All experiments but PHOBOS benefit from collimation
• vertical scraper retraction (vert. lines) clearly raises background
• reduction rates are between 2 and 5
Angelika DreesICFA-HB2004 Workshop, Bensheim, Oct. 18, 2004
Automatic Steering Algorithm
RHIC has 5 jaws per ring, most allow both, linear motion and angular motion (to parallelize with beam). Potentially time consuming!
=> 18 degrees of freedom (+ 4 more next run)
Requires automation (3 steps):1. Move to STDBY position
(based on BPM readings)2. Move Closer to beam
(based on loss monitor feedback, serial)
3. Remove Halo/Store (based on lattice functions, parallel)
Angelika DreesICFA-HB2004 Workshop, Bensheim, Oct. 18, 2004
Collimation during Fill 4854 (Au) in the blue ring
Serial collimator steering (mode: Move Closer), following parallel mode does not improve backgrounds.
Vertical lines denote when each collimator moves. Background improvement approx. x6.
Note: secondary vertical collimator quite efficient.
Angelika DreesICFA-HB2004 Workshop, Bensheim, Oct. 18, 2004
Collimation during Fill 4436 (Au)
Parallel collimator steering (mode: “Store”), using lattice fct. and assumed emittances.
Vertical lines denote when collimators start and stop moving simultaneously. Background improvement approx. x10.
Note: procedure stops automatically when desired background levels are reached.
Angelika DreesICFA-HB2004 Workshop, Bensheim, Oct. 18, 2004
Fill 32541-stage
Fill 48542-stage
Compare RHIC data from 1-stage and 2-stage collimation system
Angelika DreesICFA-HB2004 Workshop, Bensheim, Oct. 18, 2004
Comparison of Background Reduction Rates
Background signal
d-Au 2003
* = 3m
Au-Au 2004
*=3m
STAR Blue 5 11
STAR Yellow 1 3
PHENIX Blue 4 11
PHENIX Yellow
1 4
Average ratio of uncollimated to collimated background for the STAR and PHENIX detectors (sensitive to beam direction) over 6 stores in 2003 and 2004. In both cases PHOBOS had a * of 3m.
Angelika DreesICFA-HB2004 Workshop, Bensheim, Oct. 18, 2004
Summary
Operational Limitabout 1 normal ‘fill’ /hour in uncontrolled areas
Magnet Quenchesstore/ramps: BLM thresholds pull permit (fast and slow modes)
beam dump: continuous gap cleaningSoil activation
maintain 5% of DWS (i.e. 20 pCi/l 22Na) by caps or soil sample monitoring in uncapped areas
Exp. Backgroundtraditional 2-stage collimation system achieves average background reduction of x10
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