Transcript Calibration System/Method

Test and Calibration Plan for
LCLS-BLM at the APS
Bingxin Yang
1/24/2008
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
2
Testing LCLS-BLM with a single electron
Simple 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?
0.025
n s0  6
m0  7
0.02

F n i  n s0  m0
V0  0.0126
0.015
V0y
0.01
0.005
0
0
0.005
0.01
0.015
0.02
0.025
0.03
n i  q 0  V0x
3
Statistics of BLM signal originated from a single electron
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:
e  ns 0 ns
P(ns ) 
ns 0
ns !
e m0 m
P1  m  
m0
m!
e ns 0 k e km0
n
f (nq0 )  
ns 0
km
 0
k
!
n
!
k 1

4
Impact of “collection efficiency” of the BLM
0.015
0.04
0.06
n s0  0.1
m0  7
V0  2.1  10

F ni  ns0  m0


F ni  ns0  m0
m0  7
m0  7
0.03
4
0.01
n s0  1
n s0  0.3
4
V0  6.3  10


V0y
V0y
V0y

F ni  ns0  m0
0.02
3
V0  2.1  10
0.04
0.02
0.005
0.01
0
0
0
0.005
0.01
0.015
0.02
0.025
0.03
0
0
0.005
0.01
0.015
0.02
0.025
0.03
0
0.005
0.01
ni q0  V0x
0.015
0.02
0.025
0.03
ni q0  V0x
ni q0  V0x
For low collection efficiency, ns0 <= 1, the spectrum is dominated by photoelectron distribution P1
0.02
0.025
0.04
m0  7
0.02
m0  7
0.03
3

F ni  ns0  m0
V0  6.3  10

n s0  8
n s0  6
n s0  3

F ni  ns0  m0
V0  0.0126
0.015
m0  7
0.015

F ni  ns0  m0
V0  0.0168

0.01
0.02
V0y
V0y
V0y
0.01
0.005
0.01
0
0.005
0
0
0.005
0.01
0.015
0.02
ni q0  V0x
0.025
0.03
0
0.005
0.01
0.015
0.02
ni q0  V0x
0.025
0.03
0
0
0.005
0.01
0.015
0.02
0.025
0.03
ni q0  V0x
For high collection efficiency: ns0 >> 1, the spectrum is peaked around V0 = nso * m0 * q0
Conclusion: High collection efficiency, ns0 >> 1, is highly desirable.
5
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.
0.1 mA
2.3 109  e 
 e 
4 e 
Loss _ rate 


1.28

10

0.047




50  hour  1.8 105  sec 
sec
turn




• 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 ~100ms.
6
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:
m  2 ce  
1  1
1 
 E 

1



2 
2
2 
 x 
4
2
 n   2 1 
PMT
This yields 640 eV/cm for wavelength region 300 –
500 nm.
2
• 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.
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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.
8
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.
9
Most intense PMT pulses: height, length and charge
Pulses 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.
10
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.
11
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 ms, 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 ms, 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.
12
Summary
1. 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.
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