cern 11-12 mar. 2002 luminosity and longitudinal density w.c. turner 1 luminosity optimization and...

CERN11-12 Mar. 2002

Luminosity and longitudinal densityW.C. Turner

1

Luminosity optimization and longitudinal density instrumentation

W.C. TurnerLBNL

CERN11-12 Mar. 2002

CERN11-12 Mar. 2002


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TAN and TAS absorbers in IPs 1 and 5

• The TAN and TAS each absorb ~200W of forward collision products

• Instrument the TAN and TAS to measure the luminosity of colliding bunch pairs with 40MHz resolution

• Applications– Bringing beams into collision– Use in feedback loop to maintain optimum luminosity– Segment to measure crossing angle and IP position

CERN11-12 Mar. 2002


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Constraints

• Very high peak radiation fluxes and high induced activation over many years of operation, 170 MGy (17GRad)/oper yr

• Size limited to 80 x 80 mm2 by beam-beam separation at the TAN

• ~ 25 ns clearing time between bunch crossings• Sensitivity to a single pp interaction with good S/N ratio, ~

270 mips in 40 x 40 mm2

Particle type Peak Flux(cm-2sec-1) Charged hadrons 4.7x108

Electron/ positron 7.5x1010

Photons 1.1x1012

Neutrons 4.6x109

CERN11-12 Mar. 2002


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The technical solution ...

• Segmented, multi-gap, pressurized gas ionization chamber constructed of rad hard materials

• 3-11 atmospheres Ar + 2%N2 gas mixture, e- drift velocity 3.2 cm/s

• Low noise bi-polar transistor pre-amplifier “cold” cable termination, ENC ~ 1,824 e-

• Pulse shaper, = 2.5 ns

• 3 m radiation hard cable between ionization chamber and front end electronics, radiation dose to electronics < 100 Gy/oper yr

• S/N ~ 5 for single pp interaction

CERN11-12 Mar. 2002


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Ionization chamber

• 60 gaps, 10 parallel x 6 series

• segmented into quadrants

• 0.5 mm gap spacing

• copper, ceramic construction

CERN11-12 Mar. 2002


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40 MHz capability of Luminosity Instrumentation

• Peaking time is less than the 25 ns bunch spacing• Pulse train obtained by superposition of single pulses

• Un zipped amplitudes track the raw pulse heights

0 50 100 150 200 250 300 350

0

1

2

3

4

5

6

Step 25k

mV

olts

nsec

1 to 10 2 to 10 3 to 10 4 to 10 5 to 10 6 to 10 7 to 10 8 to 10 9 to 1010

0 50 100 150 200 250 300

4.2

4.3

4.4

4.5

Step 25k

mV

olts

nsec

Raw Peak, mVolts Unzipped peak, mVolts

CERN11-12 Mar. 2002


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The right-left asymmetry ratio is a sensitive function of the crossing angle

-4 -2 0 2 4-1.0

-0.5

0.0

0.5

1.0

asym

met

ry r

atio

center of profile - cm

- TAN 142 m from IP, xing angle = 150 mrad

– Measurement of the asymmetry ratios at the positions of the TAS and TAN on both sides of an IP may allow determination of IP pos. and xing angle- MARS simulations of D1 collimation and ATLAS/CMS magnetic fields are needed

CERN11-12 Mar. 2002


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Radiation hardness test

• On LHC the hadron energy incident on the TAN is ~ 1 GJ/yr; neutrons, <E> = 2 TeV• Simulate hadron showers with equivalent energy

flux of SPS 450 GeV protons• Compress 1 yr radiation exposure on LHC to ~ 1 wk

on SPS extracted beam– Assume one slow 5.8 spill every 15.8s => ~5x1011 p/spill, reasonable

• Periodically insert a collimator to reduce intensity to ~ 106p/spill to verify single proton shower operation

• Investigating possibilities in ENH1 area (p0 and M2)

CERN11-12 Mar. 2002


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Longitudinal density measurement

• Applications

– Debunched beam at injection– Population in abort gap– Ghost bunches– Beam centroid and shape

• Requirements (ref. C. Fischer, LHC-BSRL-ES-0001.00 rev 0.1 draft, 11 Nov 01)

Function Beam energy TeV

Nominal peak density*, p/ps

Resolution, p/ps

Integration time

Debunched beam

0.45 1.4x108 4x105 ~10 sec

Abort gap population

7.0 2.3x108 21x104 ~ 100 ms

Ghost bunches 7.0 2.3x108 2x105 ~ 10 sec Tails 7.0 2.3x108 21x104 ~ 10 sec Bunch core 7.0 2.3x108 ~2x106 ~ 1 msec * Nb = 1011

CERN11-12 Mar. 2002


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Several methods are being investigated

1. Non-linear mixing of “synchrotron-undulator” radiation with a cw laser oscillator

2. A photodiode array with TDCs3. Gated PMT for abort gap

• All utilize radiation from a 2-period undulator and D3 in IR4

RF system

RF system

SC undulator

Extraction mirror

Q7 Q6 Q5 D4 D3

CERN11-12 Mar. 2002


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Extraction mirror and photon flux

500<<1000nm 1.24<E<2.48 eV

Beam Energy TeV

No. photons/passage 1011 protons

0.45 3.8x105 0.75 5.2x105

1 2.8x105 2 4.3x104 7 5.4x103

Beam

Photons Mirror, 10mm x 50mm

5mm

30mm = 15V

10m

D3Undulator V

1.48mrad

Ref. Laurette PoncePhD thesis in prep.

Side View

CERN11-12 Mar. 2002


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~ 50ps

Rf clock,40 MHz

Delay generator

Laser~ 10W

40MHz, 50ps

+

PMT

Synchrotron, wiggler radiation

Crystal Filter

Proton, 208Pb+82 bunchrms length ~ 250ps

3564 50ps samples/turn25,000 ps nominal bunch separation500 turns to map entire ring50 turns to map nominal bucketsSynchrotron period = 535 turns

Non linear mixing of “synchrotron-undulator” radiation with a pulsed laser

CERN11-12 Mar. 2002


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Laser mixing abort gap integration time

• At 7 TeV, 5,400/1011 ~ 5.4x10-8 photons/p passage, / = 1

• QE*/~ 10-2

• Res req’d 2x104p/ps x 50ps x 119 samples/rev= 1.2x108 p/rev

• Counts/rev = 5.4x10-8 x 10-2 x 1.2x108 = 6.5x10-2

• Integration time to get 42 counts = 61.5 rev

= 5.5 ms << 100 ms• Alternatively in 100 ms get 738 photons

CERN11-12 Mar. 2002


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Photodiode array measurement of bunch tails

• Single photon counting used to get 104 dynamic range

time

No. photons

10 x 10 diode arrayGrey filter

TDC100 ch

1011 protons7 TeV

QE ~ 1P(0) = 0.90~ 100x0.1 = 10 photons/rev

• Integration time ~ 104/10 ~ 103 rev

~ 90 ms << 10 s

5,400 photons/rev0.5 < < 1.0

CERN11-12 Mar. 2002


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Summary

• Luminosity monitor– Data from SPS experiments indicate ionization chamber

will work at 40 MHz– 40MHz bunched beam and high radiation experiments in

2003, 2004 are highly desirable– MARS simulations needed to investigate: D1 collimation,

ATLAS and CMS magnetic fields, sensitivity of TAS to transverse position of IP

• Longitudinal density – In early stage of development– Several applications and possibilities– May want to consider specialized instruments rather than

one to do all– Laser mixing and photodiode approaches to be

investigated on ALS at LBNL in 2002

cern 11-12 mar. 2002 luminosity and longitudinal density w.c. turner 1 luminosity optimization and...

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