NLC IP Layout Issues

NLC - The Next Linear Collider Project

NLC IP Layout Issues

Jeff Gronberg/LLNL

MAC Collaboration Meeting

June 1, 2000


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*


1995-99 Beam Delivery Layout

NLC Beam Delivery Magnet Layout

-30

-20

-10

0

10

20

30

-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


2000 Beam Delivery Layout

NLC Beam Delivery Magnet Layout

-10

-8

-6

-4

-2

0

2

4

6

8

10

-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


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


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


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


Luminosity Monitor Detail


-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0 2 4 6 8 10 12

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0 2 4 6 8 10 12

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


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


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


Energy Distributions

Radiative Bhabhas125K per bunch @ <E>=370 GeV


Extraction Line150 m long with common and e- dump

1

10

100

1000

10000

100000

0 100 200 300 400 500

Beam Energy

2.1% of beam with77 kWatts has E<250 GeV

NLC 1000 B

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Neutrons from the Beam Dump


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


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


Luminosity Loss vs. Jitter


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


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

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NLC IP Layout Issues

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