NLC IP Layout Issues
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Transcript of NLC IP Layout Issues
NLC - The Next Linear Collider Project
NLC IP Layout Issues
Jeff Gronberg/LLNL
MAC Collaboration Meeting
June 1, 2000
NLC - The Next Linear Collider Project
Old FF : Remotely correct chromaticity of final doublet
•Long and length depends on Energy
•2m L*, inside detector, Length scales with L*
•Adequate but limited energy bandwidth
• “classical” telescope design, tested at FFTB
•Studied since <1995
•Magnets engineered
Today’s report is on Yesterday’s FF Design
CCX,CCY L*
New FF : Locally correct chromaticity of final doublet
•Short and length roughly independent of Energy
•Length roughly independent of L*
•Larger energy bandwidth and dynamic aperture (easier collimation)
•Transverse separation of 2 IPs more difficult
•Collimation system closer to the IP
•No magnets engineering yet
L*
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1995-99 Beam Delivery Layout
NLC Beam Delivery Magnet Layout
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-20
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-6000 -4000 -2000 0 2000 4000 6000
e- e+
Collimation Collimation
Interaction Region 1
IR2
IR Transport = Final Focus
Extraction
Big Bend=10 mrad
Interaction Point 2
NLC - The Next Linear Collider Project
2000 Beam Delivery Layout
NLC Beam Delivery Magnet Layout
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-2500 -2000 -1500 -1000 -500 0 500 1000 1500 2000 2500
e- e+
Collimation Collimation
Interaction Region 2
IR1
IR Transport = Final Focus
Extraction
Big Bend=10 mrad
Interaction Point 1
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Basic Issues
Bunch Spacing: 2.8 ns / 84 cm (or 1.4 ns / 42 cm)
Need some crossing angle to avoid unwanted collisions before bunch gets to the IP, 4-30 mrad
Beam-beam effects: e+e- pair production, disrupted beam energy, beamstrahlung photons, hadron production and machine backgrounds
Masks, L*, collimation depth, halo assumptions, spoilers,...
Small spot sizes: x /y = 235 nm / 3.9 nm
Need to control position & motion of final quads and/or position of the beam
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Basic Issue #1Bunch Structure
NLC: 2.8 ns (or 1.4 ns), 95 bunches•Crossing angle to avoid unwanted collisions before bunch gets to the IP
•Integrate 95 bunches of background before clear
•120 Hz “slow” feedback system for drifts; other means for high frequency jitter
TESLA: 337 ns, 2820 bunches•0 degree crossing angle
•Detectors clear after 1 bunch worth of backgrounds (except maybe VXD)
•“Slow” feedback system (80 pulses <=> 100 sigma offset) on beam-beam deflection signal corrects beam position for all drifts and jitters
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Crossing Angle Considerations~ 4-30 mrad
Crab Cavity:
•Transverse RF cavity on each side of IP rotates bunches so they collide head on
•relative timing accuracy determines maximum crossing angle
Beam Steering: •Transverse component of detector solenoid steers beams and causes spot size blow up
•Handle with clever upstream beam steering gymnastics
Transverse Space for (easier) extraction line
Non cylindrically symmetric geometry for inner detectors
If crossing angle comes from “Big Bend”•get extra protection from muons•pay for extra optics to deal with dispersion
Current design choice = 20 mrad
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Luminosity Monitor Detail
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-0.4
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LCD Small Detector with L* =2m CD1 OpticsPlan View
M1
M2Q1 Q1-SC Q2
Q1-EXT
10 mrad
Support Tube
Lum
RF Shield-10 mrad
Tunnel Wall
Beam Pipe
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Elevation View•Iron magnet in a SC Compensating magnet
•8 mrad crossing angle
•Extract beam through coil pocket
•Vibration suppression through support tube
JLC IR
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Basic Issue#2: BackgroundsWell Studied by ALL GROUPS: Not a Problem
Machine Backgrounds:•Synchrotron Radiation•Muons Production at collimators•Direct Beam Loss
•Beam-Gas•Collimator edge scattering
•Neutron back-shine from Dump•Extraction Line Losses
IP Backgrounds:•Beam-Beam Interaction
•Disrupted primary beam•Beamstrahlung photons•e+,e- pairs from beams. interactions•Hadrons from beams. interactions
•Radiative Bhabhas
“Bad”, get nothing in exchange
1) Don’t make them
2) Keep them from IP if you do
“Good”, scale with luminosity
1) Transport them away from IP
2) Shield sensitive detectors
3) Timing
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Energy Distributions
Radiative Bhabhas125K per bunch @ <E>=370 GeV
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Extraction Line150 m long with common and e- dump
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Beam Energy
2.1% of beam with77 kWatts has E<250 GeV
NLC 1000 B
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Neutrons from the Beam Dump
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Muon Backgrounds(Lew Keller)
Beam Halo: •10-6 (calculated)•10-3(current NLC assumption)•10-2(worst SLC, ZDR assumption)
•Pre-linac (8 GeV) Collimation: Damping ring extraction root of all evil at SLC
•Collimation system “depth”: 8 x 40 y
•Loosen it and get less muons and an easier optical lattice
•Increase it and get more synchrotron radiation (for the same halo)
•Big Bend: buys ~ x5 in muon protection
•Distance from IP: more is better for muons: 30x more muons in new, shorter FF
•Spoilers: 9m long tunnel-filling toroids bend muons away from IP, endcaps see <1/train
•spoilers buy x2000 @ 250 GeV/beam and x500 @ 500 GeV/beam
•Let the detector eat more:•Design goal of 1 muon in detector/train is achievable, with some pain•What is real detector limit? No spoilers leave <6 muons per square m of endcap
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Synchrotron Radiation(Stan Hertzbach)
Goal: No SR hits inner bore of Q1 or Be Ring Mask protecting VXD-L1
Assumptions: Flat beam halo assumed to fill collimation aperture 8 x 40 y
•At IP, SR due only to halo particles spraying in the final doublet
•Tighter/looser collimation limits what is hit
•Assumed halo (0.1%) tells you power deposited
Incoming Aperture: 5.9mm radial stay-clear on Q1 (5.4 drawn on plots)
Exit Apertures: 10mm radius beam-pipe, then increasing
ResultsSR masks at 11m (y) and 12m (x) keep upstream SR away from Q1 and Q2
Q1 aperture just big enough so that 8 x 40 y collimation works
Looser collimation or smaller apertures would require masks closer to the IP
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Luminosity Loss vs. Jitter
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Basic Issues #3Vibration
Control position & motion of final quads and/or position of the beam to achieve x /y = 235 nm / 3.9 nm
•Get a seismically quiet site
•Don’t screw it up: Pumps, compressors, fluids
•Good magnet and detector engineering: Light, stiff Q1
•Tie to “bedrock”: get lenses outside detector as soon as possible
•Slow feedback: based on 120 Hz, controls frequencies < ~ 3-5 Hz
•Fast (10-20 ns) feedback on front of bunch train corrects the (guaranteed correlated) train offset
•“Actively” tie lenses inside detector to bedrock:
•Optical Interferometer + piezo-mover or Inertial sensor + piezo-mover
•The less cantilevering and the best lines-of-sight through the detector the better
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Conclusions
Each potential problem in the beam delivery/IR/detector area has a variety of possible solutions that we have only begun to investigate
Many of these are independent of the machine technology
Working groups have been actively collaborating to resolve issues through meetings (next: FF/IR workshop, Daresbury, June 2000) and personnel exchange