Bingxin Yang

1/24/2008

Test and Calibration Plan for LCLS-BLM at the APS

2

Objectives and outline

Objective for the BLM test / calibration at the APS

1. Validate high-energy shower simulation for relevant geometry.2. Calibration of BLM?

Outline

1. Test and calibration of LCLS-BLM with a single electron• Single-electron calibration procedures• Statistical analysis of BLM signal and pulse height spectrum

2. Reality check: experience with the APS Cherenkov detectors• Control of APS beam loss rate• Cherenkov detector measurements: pulse height, length, and charge

3. Preparation for test and calibration of LCLS-BLM at the APS• Progress to date• Other beam loss scenarios

4. Proposed calibration scheme for the LCLS-BLM• Scattering foils and energy question

3

Testing LCLS-BLM with a single electronSimple procedures for testing LCLS-BLM using one electron

• Store beam current < 0.5 mA in APS storage ring. Count rate < 10 K (c/s).• Measure the pulse height spectrum of the PMT signal• Scan stored beam current / beam loss rate and record pulse height spectra. The peaks

from n-shower-particle events are proportional to n-power of the loss rate.• Identify peak for single-electron scattering event and calculate expection value: V1.• Calibration for the BLM: PMT charge for one APS-electron = CAV1 , where CA is inverse of

the charge amplifier calibration factor.• Exchange rate from a standard APS electron to LCLS electrons needs to be performed

with computer simulation (Jeff Dooling et. al.).

Under what conditions will these procedures work?

0 0.005 0.01 0.015 0.02 0.025 0.030

0.005

0.01

0.015

0.02

0.025

F ni ns0 m0 V0y

ni q0 V0x

ns0 6

m0 7

V0 0.0126

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Statistics of BLM signal originated from a single electron

0

0( )!

s

s

nn

s ss

eP n n

n

A statistical model for shower detection process

• A Large number of shower particles are created (NS >> 1). Assume that the number of shower particles intercepted by BLM is given by Poisson distribution (right, ns0 = average number of shower particle intercepted).

• Each intercepted shower particle creates many Cherenkov photons, which in turn generates m0 photoelectrons at the PMT cathode, on average. # of photoelectrons are given by Poisson distribution (right, m0 = average # of photoelectrons generated by one shower particle).

• The distribution of total # of photoelectrons, n, and the PMT signal charge generated, nq0:

0

1 0!

mme

P m mm

0 0

0 0 01

( )! !

sn kmnk

sk

e ef nq n km

k n

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Impact of “collection efficiency” of the BLM

Conclusion: High collection efficiency, ns0 >> 1, is highly desirable.

For high collection efficiency: ns0 >> 1, the spectrum is peaked around V0 = nso * m0 * q0

0 0.005 0.01 0.015 0.02 0.025 0.030

0.01

0.02

0.03

0.04

F ni ns0 m0 V0y

ni q0 V0x

ns0 3

m0 7

V0 6.3 103

0 0.005 0.01 0.015 0.02 0.025 0.030

0.005

0.01

0.015

0.02

0.025

F ni ns0 m0 V0y

ni q0 V0x

ns0 6

m0 7

V0 0.0126

0 0.005 0.01 0.015 0.02 0.025 0.030

0.005

0.01

0.015

0.02

F ni ns0 m0 V0y

ni q0 V0x

ns0 8

m0 7

V0 0.0168

For low collection efficiency, ns0 <= 1, the spectrum is dominated by photoelectron distribution P1

0 0.005 0.01 0.015 0.02 0.025 0.030

0.005

0.01

0.015

F ni ns0 m0 V0y

ni q0 V0x

ns0 0.1

m0 7

V0 2.1 104

0 0.005 0.01 0.015 0.02 0.025 0.030

0.01

0.02

0.03

0.04

F ni ns0 m0 V0y

ni q0 V0x

ns0 0.3

m0 7

V0 6.3 104

0 0.005 0.01 0.015 0.02 0.025 0.030

0.02

0.04

0.06

F ni ns0 m0 V0y

ni q0 V0x

ns0 1

m0 7

V0 2.1 103

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Reality check: Control beam loss rate in the APS-SR

Extrapolate from operation experience of the APS storage ring: • At 324-bunch user run, stored beam has 0.3 mA current per bunch,

lifetime ~ 50 hours

• Assuming gas scattering dominates and lifetime = 50 hours with 1-bunch 0.1 mA.

• Tracking studies by M. Borland and L. Emery estimated that ~ 1% of total loss occur at each normal ID chamber (non-limiting aperture). Hence the single electron deposit rate at a normal ID chamber is ~ 128 hits/sec, comparable to the beam frequency of the LCLS.

• In fact, a higher loss rate is more desirable for better efficiency collecting data. The bottle neck is defined by the charge amplifier output pulse width of ~100s.

9

45

0.1 2.3 10_ 1.28 10 0.047

50 1.8 10 sec sec

mA e e eLoss rate

hour turn

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Estimate of Cherenkov detector signal strength

PMT pulse is generated by a single shower particle: • Frank-Tamm formula for Cherenkov radiation can be used to estimate energy deposit of an electron traversing the entire thickness of the radiator:

This yields 640 eV/cm for wavelength region 300 – 500 nm.

• For radiator thickness = 1.2 cm, we have ~ 240 photons, with 20% optical efficiency and ~ 15% quantum efficiency, we get 7 photoelectrons/shower-particle.

• For PMT-HV = 900 V, gain = 1.5 × 106, each photoelectron produces ~ 0.25 pC. Each shower particle produces ~ 1.7 pC, on average.

2

2 2 22 1

2 1 1 11

4 2PMT

ceE

x n

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APS Cherenkov detector measurements

Construction of the APS Cherenkov detector:• 8 mm quartz radiator enclosed by 15 mm thick lead can. • Located at 2.3 m downstream of chamber entrance, 0.1 radian off-axis.• Detector has very low collection efficiency. Pulse height spectrum is

dominated by photoelectron statistics. • Estimate = 4 – 5 photoelectrons, or ~ 1 pC PMT change per shower

particle.Measurements with the APD detector would help us get familiar with the PMT and compare its signal with the above estimates from statistical analysis.

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Weakest PMT pulses: height, length and charge

Pulses of lowest amplitude can be observed during user operation using a scope.• Pulse width is about 2.5 – 3.5 ns FWHM.• The pulse shown in the following example carries a charge of 0.034 (V) / 50

(ohm) * 2.5 (ns) = 1.7 pC, consistent with an event for 7 photoelectrons. • Typical pulse height ranges from 10 mV (2 photoelectron) to 100 mV (20

photoelectron). • No detailed pulse height analysis was performed due to a lack of equipment.

Conclusion: Signal estimate is OK.

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Most intense PMT pulses: height, length and chargePulses of highest amplitude can be observed when dumping a 19-mA single bunch beam.

• PMT-HV = 750 V. Gain reduced by a factor of four.

• Pulse train recorded with 5 GS/s scope. Height = 7 V.

• PMT is heavily saturated and maximum pulse width > 20 ns.

• Maximum charge per pulse is 6 nC!Conclusion: > 104 dynamic range for single pulse charge.

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Preparation for testing the LCLS-BLM in the APS-SR

Planning and discussion has many participants: Jim Bailey, Jeff Dooling, Marion White, Bill Berg, Glenn Decker, Liz Moog, Tony Pietryla, Eric Norum, Isaac Vasserman, …

Status:

Physics: Concept development still in progress and in flux. Program / script to be developed.

Mechanical support: Version 0.0 made and tested. Approved by APS ID group with suggestions. Improvement expected: better protect ID chamber.

Electronics: Charge amplifier work in progress (other talks). Spectroscopy amplifier: ANL or eBay ($200). Pulse height analyzer: to be specified Cables: to be specified and installed.

BLM itself: Expected in March.

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Other test / calibration scenarios

1. Beam dump• Stored beam from 0.1 mA to 19.2 mA. • FWHM of the lost charge pulse is 14 turn.• 3 × 108 to 6 × 1010 electrons hit the wall in a single turn.• Pulse spacing 3.6 s, not resolved by the charge amplifier.• Distribution among sectors to be studied.

2. Kicker-induced beam loss• Use controlled kick to perturb the stored beam. Motion-related loss

lasts about 1 ms, or 200 – 300 turns.• Loss can be controlled from 105 to 107 per turn.• Pulse spacing 3.6 s, not resolved by the charge amplifier.• Distribution of lost particles are to be studied.

3. Injected beam• Injector sends 0.2 – 2 nC ( 109 – 1010 electrons) into the storage ring.• A faction of them can be scraped on the ID chamber using steering.• A systematic measurement technique needs to be developed.

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Summary1. Single-electron test

If simulation or experiment shows that the BLM intercept more than one shower particle per hit, the test will work, at least in principle.

The PMT signal will be in the range of 10 – 100 pC per pulse, as scaled from the APS Cherenkov detector measurements.

2. Other measurements If we intercept less than one shower particle per 7-GeV electron, we will

need to have additional measurements / knowledge about the lost beam. We will continue to develope concept and plans to use three other beam

loss scenarios: Kicker-induced beam loss (105 – 107 e/turn). Injection, where the storage ring is treated as a long transport line

after the injectors Beam dump (1010 e/turn)

3. LCLS calibration foil Proposal / request for simulation of the calibration foil was made last

March. We hope to see some results soon.