Measurement of the Beam

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S. De Santis “Measurement of the Beam Longitudinal Profile in a Storage Ring by Non
-
Linear Laser Mixing”
-

BIW 2004 May, 5th

Measurement of the Beam
Longitudinal Profile in a Storage
Ring by Non
-
Linear Laser Mixing


J.
-
F. Beche, J. Byrd,
S. De Santis
, P. Denes, M. Placidi,W. Turner, M. Zolotorev

Lawrence Berkeley National Laboratory

S. De Santis “Measurement of the Beam Longitudinal Profile in a Storage Ring by Non
-
Linear Laser Mixing”
-

BIW 2004 May, 5th

All
-
in
-
One Tool for Synchrotron Rings



Online measurement of
bunch length

and
shape



Bunch

current



including nominally unfilled
RF buckets (“ghost bunches”)


Synchronous bunch position



Fast
: the results shown were accumulated
in seconds/minutes



Very
wide dynamic range

(10
4
)


S. De Santis “Measurement of the Beam Longitudinal Profile in a Storage Ring by Non
-
Linear Laser Mixing”
-

BIW 2004 May, 5th

Summary


Longitudinal Density Monitor overview


what it does, how it does it


The hardware


Experimental runs at the ALS


Applications to LHC and NLC

S. De Santis “Measurement of the Beam Longitudinal Profile in a Storage Ring by Non
-
Linear Laser Mixing”
-

BIW 2004 May, 5th

A Brief History



Original concept: femtoslicing experiments
at the ALS (R. Schoenlein and
M.Zolotorev, 2000)


Prototype:
L
HC
A
ccelerator
R
esearch
P
rogram (2002
-
03)


Longitudinal dynamics studies in damping
rings (?)

S. De Santis “Measurement of the Beam Longitudinal Profile in a Storage Ring by Non
-
Linear Laser Mixing”
-

BIW 2004 May, 5th

An Optical Sampling Scope

Bunch

Synchrotron

Photons

1

Laser

Photons

2

Mixed

Photons

1
+

2

PMT

Non
-
linear

crystal

[BBO]

laser pulse length << bunch length


1
+

2

= visible wavelength

S. De Santis “Measurement of the Beam Longitudinal Profile in a Storage Ring by Non
-
Linear Laser Mixing”
-

BIW 2004 May, 5th

Scanning the Bunch

Piezo
-
ceramic

positioner



LHC: 40 MHz

ALS: 71 MHz

Laser

L

C/2L = 40 (or 71) MHz

Adjust


by changing

path length (phase

modulation)

100
-
200 mW diode
-
pumped laser

ALS: 10 Hz

ALS: 50 fs, LHC: 50 ps (10 W)

LHC: 22 bins (std. mode)

ALS: 32 bins

S. De Santis “Measurement of the Beam Longitudinal Profile in a Storage Ring by Non
-
Linear Laser Mixing”
-

BIW 2004 May, 5th

Schematic (timing)

Laser

Synchrotron Light

PMT

PD

ADC

ADC

DAC

Board

Timing

Cavity

Control

Processing

+Interface

(USB to PC)

BBO

Laser

Timing

Filter

Ring main RF (PLL)

Orbit clock

S. De Santis “Measurement of the Beam Longitudinal Profile in a Storage Ring by Non
-
Linear Laser Mixing”
-

BIW 2004 May, 5th

LDM


Electronics Board Layout


Architecture


Different Boards


#1 : Phase modulation Generator


#2 : Phase Information Digitization


#3 : PMT pulse digitization


#4 : Delay generators/timing


#5 : Digital Backend (Storage and
USB)



1

2

3

4

To Laser Phase Offset

From PMT

From Photodiode

PC

5

S. De Santis “Measurement of the Beam Longitudinal Profile in a Storage Ring by Non
-
Linear Laser Mixing”
-

BIW 2004 May, 5th

Electronics
-

I.

Mother Board with
71MHz clock board


USB Control and
histogram/average
is fully operational

S. De Santis “Measurement of the Beam Longitudinal Profile in a Storage Ring by Non
-
Linear Laser Mixing”
-

BIW 2004 May, 5th

Electronics
-

II.

DAC Analog board for
laser phase offset
modulation

Track and Hold board with self
trigger for PMT pulse detection
(only one bit is used in
single photon
counting mode
)

Actual Laser phase
offset digitization board

S. De Santis “Measurement of the Beam Longitudinal Profile in a Storage Ring by Non
-
Linear Laser Mixing”
-

BIW 2004 May, 5th

DAQ PCI6534 National Instruments


Standard DAQ Board


Function Library Labview, C/C++ compatible


Quad 8
-
bit I/O board, DMA transfer rates up to
20MHz (8, 16 or 32
-
bit)

S. De Santis “Measurement of the Beam Longitudinal Profile in a Storage Ring by Non
-
Linear Laser Mixing”
-

BIW 2004 May, 5th

Tests at the ALS

Bucket spacing 2 ns

Bunch width ~50 ps

“Camshaft” pulse

328 RF buckets

276+1 filled

Gap

(LHC parameters)

(2808/35640)

(280
-
620 ps)

(2.5 ns)

S. De Santis “Measurement of the Beam Longitudinal Profile in a Storage Ring by Non
-
Linear Laser Mixing”
-

BIW 2004 May, 5th

bend

magnet

mirror

e
-
beam

Laser System

BBO


2
= ~2 eV


1
= 1.55 eV


1
+

2

delay

slit

Optical layout at the ALS


L
= 800 nm,

S
= 638 nm


㴳㔵=

Conversion efficiency is
proportional to the laser power


density and is
optimized for


=

S

S. De Santis “Measurement of the Beam Longitudinal Profile in a Storage Ring by Non
-
Linear Laser Mixing”
-

BIW 2004 May, 5th

First Experimental Data (Nov. 2002)

First data (Peak Height
distribution)

Electronics Setup

S. De Santis “Measurement of the Beam Longitudinal Profile in a Storage Ring by Non
-
Linear Laser Mixing”
-

BIW 2004 May, 5th

ALS Bunch Profile in Time

0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0
100
200
300
400
500
600
Time [ns]
Counts/Bunch Passage
SET3

S. De Santis “Measurement of the Beam Longitudinal Profile in a Storage Ring by Non
-
Linear Laser Mixing”
-

BIW 2004 May, 5th

Zoom in...

0.00
0.01
0.01
0.02
0.02
0.03
0.03
0.04
0.04
0.05
-2
0
2
4
6
8
10
12
Time [ns]
Counts/Bunch Passage
SET3

S. De Santis “Measurement of the Beam Longitudinal Profile in a Storage Ring by Non
-
Linear Laser Mixing”
-

BIW 2004 May, 5th

Compress Scale...

0.00
0.01
0.01
0.02
0.02
0.03
0.03
0.04
0.04
0.05
0
1
2
3
4
5
6
Bunch (270 ps)
Counts/Bunch Passage
SET3

S. De Santis “Measurement of the Beam Longitudinal Profile in a Storage Ring by Non
-
Linear Laser Mixing”
-

BIW 2004 May, 5th

Large dynamic range

0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
159
160
161
162
163
164
165
166
167
168
169
170
Bunch (270 ps)
Counts/Bunch Passage
SET3

S. De Santis “Measurement of the Beam Longitudinal Profile in a Storage Ring by Non
-
Linear Laser Mixing”
-

BIW 2004 May, 5th

0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
159
160
161
162
163
164
165
166
167
168
169
170
Bunch (270 ps)
Counts/Bunch Passage
Details

0.000
0.001
0.002
0.003
329.9
329.95
330
330.05
330.1
Time [ns]
Counts/Bunch Passage
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
327.9
327.95
328
328.05
328.1
Time [ns]
Counts/Bunch Passage
S. De Santis “Measurement of the Beam Longitudinal Profile in a Storage Ring by Non
-
Linear Laser Mixing”
-

BIW 2004 May, 5th

Background

0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
80
90
100
110
120
130
140
150
Bunch Number
Total Counts/bunch/passage
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
129
130
131
132
Bunch Number
Counts/bunch/turn
Camshaft/Background ~ 10
3

S. De Santis “Measurement of the Beam Longitudinal Profile in a Storage Ring by Non
-
Linear Laser Mixing”
-

BIW 2004 May, 5th

Synchronous Phase Transients

-0.02
-0.01
0.00
0.01
0.02
0
50
100
150
200
250
300
Bunch Number
t
0
offset [ns]
SET3

S. De Santis “Measurement of the Beam Longitudinal Profile in a Storage Ring by Non
-
Linear Laser Mixing”
-

BIW 2004 May, 5th

Bunch Length

0
5
10
15
20
25
0
50
100
150
200
250
300
Bunch Number
RMS Bunch Width [ps]
SET3

S. De Santis “Measurement of the Beam Longitudinal Profile in a Storage Ring by Non
-
Linear Laser Mixing”
-

BIW 2004 May, 5th

A Longitudinal Monitor for LHC



450 GeV
-

7 TeV



Untrapped beam fraction



Protons in the abort gap



Longitudinal bunch tails



“Ghost bunch” population



Etc.

S. De Santis “Measurement of the Beam Longitudinal Profile in a Storage Ring by Non
-
Linear Laser Mixing”
-

BIW 2004 May, 5th

LHC Applications of LDM


Bunch core measurement (std. mode)


Tails and “ghost bunches” (HS mode)


10 W laser @ 1064 nm


Laser pulse period: 25 ns


Laser pulse length: 50 ps


LHC length: 88.9 µs


Photons/bunch/turn: 10’s (10
2

gain @ 450 nm) at
full current

S. De Santis “Measurement of the Beam Longitudinal Profile in a Storage Ring by Non
-
Linear Laser Mixing”
-

BIW 2004 May, 5th

Bunch core measurement


Measure bunch population at
±
2
s

with 1%
accuracy in a time short compared to the
synchrotron period (~42 ms @ 7 TeV)


22 bins required (50 ps x 22 ≈
±
2
s
).


Time required to map the core once: 22 turns ≈ 2 ms


Expected population: 2∙10
8
-

2∙10
6

p/ps.


Accuracy: 0.5
-
5.5%

S. De Santis “Measurement of the Beam Longitudinal Profile in a Storage Ring by Non
-
Linear Laser Mixing”
-

BIW 2004 May, 5th

Tails and “ghost bunches” measurement


Measure bunch populations as small as 2∙10
4

p/ps all around the ring (causing background
in experiments) with ~50% accuracy.


Entire ring is mapped in 50 ps slices. Minimum number of
turns: 500 (3556 slices/turn).


At 7 TeV, (2
±
0.8)∙10
4

p/ps give 3.2
±
1.8 counts in a 50 ps
slice.


1000 turns (< 100 ms) are required for the 50% accuracy as
per specs.

LDM is the ideal instrument...

S. De Santis “Measurement of the Beam Longitudinal Profile in a Storage Ring by Non
-
Linear Laser Mixing”
-

BIW 2004 May, 5th

NLC Damping Rings Parameters



E = 1.9 GeV



C = 300 m



s
z
= 12 ps



∆t
b
= 1.4 ns



N
b
= 576/714 (
3

trains, laser @ 71 MHz can sample 20


bunches x train)




S
= 0.0035 (~300 turns)



Long. damping time = 2.6 ms (2600
/3

turns)

Longitudinal

dynamics

in

the

machine

never

reaches

a

steady

state

+

short

bunches

the

LDM

can

be

a

valuable

tool

for

understanding/monitoring

the

damping

rings
.

S. De Santis “Measurement of the Beam Longitudinal Profile in a Storage Ring by Non
-
Linear Laser Mixing”
-

BIW 2004 May, 5th

Conclusions: Pros and Cons of LDM


Fast sampling rate (laser cavity frequency)


High dynamic range


High time resolution (laser pulse length)


Not limited to optical wavelengths


Requires multi
-
turn sampling


Requires synchrotron light


Especially suited for dynamics studies in damping rings