Linac 4 Timing
Summary of discussions held with
M.E. Angoletta, P. Baudrenghien, C. Carli, M. Chanel, A. Findlay
Alfred Blas, Flemming Pedersen
L4-PSB working group meeting 25/06/2009
1
Justification for Longitudinal Painting in PSB
1/3
During it’s 37 years long history, transverse space charge at low energy has been a dominant
cause of intensity (Isolde, CNGS, AD) and brightness (LHC) performance limitations in the
PSB.
One very important parameter is the bunching factor at low energy:
Make the bucket long (reduce fs), add a second harmonic RF
Make bunches as long as possible: fill the bucket as much as possible (but without losses)
Avoid bunch shape oscillations and associated oscillations in bunching factor
Modify the potential well to make bunches flatter (= add second harmonic RF)
Modify the injected phase space distribution in the Linac or prior to capture in PSB.
Modified potential well.
During the h = 5 era, it was first studied with a h = 10 prototype in 19801 and all 4 C16
systems were added in 19822.
During the LHC injector conversion project in 97/983, the dual harmonic operation was
retained as we entered the h = 1 & 2 era in the PSB to maximize the brightness of the
LHC beams in the PSB
1) Shaping of Proton Distribution for Raising the Space-Charge Limit of the CERN PS Booster, XIth Int. Conf. on High Energy Accelerators, CERN,
Geneva, 1980 or CERH/PS/BR 80-14
2) A Second Harmonic (6-16 MHz) RF System with Feedback-Reduced Gap Impedance for Accelerating Flat-topped Bunches in the CERN PS Booster
http://cern.ch/AccelConf/p83/PDF/PAC1983_3499.PDF
3) Conversion of the PS Complex as LHC Proton Pre-Injector, http://cern.ch/AccelConf/pac97/papers/pdf/9W011.PDF
Alfred Blas, Flemming Pedersen
L4-PSB working group meeting 25/06/2009
2
Justification for Longitudinal Painting in PSB
2/3
Modified phase space distribution (hollow distribution, depressed central
density)
The concept was proposed analytically as a sum of two elliptical
momentum distributions in 19794
Machine studies were done in 79/801 by using the 400 MHz
debuncher of Linac2 with 180 degree phase change as well as empty
bucket deposition using PSB 3 MHz rf.
Instabilities associated with closing of the phase loop prevented this
from being used operationally; mechanism not well understood at
that time.
In the h = 1&2 era, this method was further pursued with machine
studies in 995 using the 16 MHz C16 system to inject an empty
bucket into the coasting beam prior to RF capture (requires dB/dt =
0 at injection).
Numerical computations of longitudinal bunched beam transfer
functions of bunches with depressed central density by Shane
Koscielniak (TRIUMF) contributed significantly to the understanding
of the mechanism of the instability.
4) Bunches with Local Elliptic Energy Distributions http://cern.ch/AccelConf/p79/PDF/PAC1979_3526.PDF
5) New Technique for Bunch Shape Flattening http://cern.ch/AccelConf/p99/PAPERS/TUBL7.PDF
Alfred Blas, Flemming Pedersen
L4-PSB working group meeting 25/06/2009
3
Justification for Longitudinal Painting in PSB
3/3
Longitudinal painting in the Linac4 era
The increased injection energy (160 MeV) reduces the transverse space charge by a factor of 2
The 3 MeV chopper and H- charge exchange injection permits loss-free injection into a
moving bucket at full dB/dt which means reduced time at low energy.
Turn by turn energy modulation and turn-by-turn timing control of the 3 MeV chopper timing
permits to inject a longitudinal phase space distribution very close to being the stationary
distribution such that harmful bunch shape oscillations and losses are avoided and obtain a
good bunching factor at low energy which improves both brightness and intensity
The digital chopper timing control system with associated application and controls software
layers must be conceived and planned to have turn-by-turn timing control ready from day one
of operation of PSB with Linac4
This permits the LHC (brightness, reduced filling time) and other CERN users (intensity) to
rapidly obtain maximum profit from the Linac4 upgrade
To fully profit from this longitudinal painting (depressed central density, intensity control)
several chopper on/off events must be foreseen within a single turn
The PSB will then finally have the longitudinal phase space density control tool we have
always wished we had for the last 37 years!!
5) Linac4, a New Injector for the CERN PS Booster http://cern.ch/AccelConf/e06/PAPERS/TUPLS057.PDF
Alfred Blas, Flemming Pedersen
L4-PSB working group meeting 25/06/2009
4
Linac 4 - PSB
 2μs
Pre-chopper
+LEBT
Debuncher
RF Feedforward
Distri
From F. Gerigk
4*rf
180 m
Source
45 keV
Chopper
Amplitude modulated
for energy modulation (painting)
RF feed-forward, energy modulator,
debuncher, distributor and rf have to
be in phase with the Chopper
from P. Baudrenghien
High
Voltage
HV ON segment
RF ON segment
RF
transient
Beam segment
Alfred Blas, Flemming Pedersen
0.3 ms
L4-PSB working group meeting 25/06/2009
< 1.2 ms
0.15 ms
0.15 ms
Time
5
Chopper + Feed-Forward timing
PSB beam demand = some number of Linac 4 bunches (352.2 MHz).
Intensity precision:
No jitter, 3 bunches doesn’t mean 3 +/- 1 bunch(es).
Time precision:
One L4 bunch at time t doesn’t mean 1 bunch at time t + TL4_RF
This one Linac 4 rf period uncertainty is only accepted as a global
shift of the one-revolution bunch train.
Absolute Time reference:
Reference clock:
Alfred Blas, Flemming Pedersen
Could be time zero of the entire injection process (start
injection re-synchronized with the revolution reference)
Could be “start injection” + each revolution reference tic
These constraints imply that the chopper should use the L4_RF or a
(sub) multiple.
L4-PSB working group meeting 25/06/2009
6
Chopper + Feed-Forward timing
Single absolute time reference = start injection
Pros:
No intensity jitter
No time jitter (just a single jitter of one L4 rf period for the global filling
pattern)
Cons:
any error is integrated over the entire injection process duration (≠ FECs to
be well synchronized when changing L4 fRF or PSB fINJ REF )
The entire injection process lasts < 400 μs = 140k TL4-RF = 18 bit value
Time
Value
Alfred Blas, Flemming Pedersen
L4-PSB working group meeting 25/06/2009
Event Number
7
Chopper + Feed-Forward timing
Dual absolute time reference = start injection + revolution reference
=> L4 rf re-synchronized at each revolution
Pros:
No intensity jitter
Any timing program error is effective only within one pair of revolutions
Cons:
One L4 rf period jitter for the global filling pattern within one revolution
Need to treat a pair of revolution periods for each revolution filling pattern
as the actual rf overlaps the reference during acceleration
Alfred Blas, Flemming Pedersen
L4-PSB working group meeting 25/06/2009
8
Chopper + Feed-Forward timing
Triple absolute time reference = start injection + revolution reference + multiple of the reference
=> L4 rf re-synchronized at each Beam ON-OFF
Pros:
Any timing program error is effective only within one revolution
Cons:
One L4 rf period jitter for any Beam ON-OFF event (intensity jitter)
Most complicated
Alfred Blas, Flemming Pedersen
L4-PSB working group meeting 25/06/2009
9
Functions for the Modulator + debuncher
The function generator will provide the set point value to the energy modulator (Voltage
amplitude modulation) and to the debuncher (phase and ? amplitude modulation). The
typical signal is a triangle with a >20 μs period.
Requirements :
Alfred Blas, Flemming Pedersen
New value every revolution. Both the Modulator and the
debuncher need to have the same final response. Staircase signal
acceptable when both modulator and debuncher have a high BW
(staircase response also = flat line painting). When one or both of
the response times are long (>10ns) the homogeneity of the
painting will be affected by parasitic exponentials. This would
require additional interpolated values.
L4-PSB working group meeting 25/06/2009
10
Functions for the Modulator + debuncher
Requirements :
New value every revolution. Both the Modulator and the debuncher
need to have the same final response. Staircase signal acceptable
when both modulator and debuncher have a high BW (staircase
response also = flat line painting). When one or both of the
response times are long (>10ns) the homogeneity of the painting will
be affected by parasitic exponentials. This would require an addition
of interpolated values to obtain a smooth ramp.
Time precision:
one L4 rf period (although not critical)
Absolute Time reference:
Reference clock:
Alfred Blas, Flemming Pedersen
Time zero of the entire injection process (start injection
re-synchronized with the revolution reference)
Injection revolution reference
L4-PSB working group meeting 25/06/2009
11
Block diagram
 2μs
Pre-chopper
Source +LEBT
Chopper
Energy
modulation
Linac 4
Debuncher
20 m
Linac rf
feed-forward
Distri
4*rf
160 m
Function
Generator
FMC3 (CO)
45 keV
BIXi.RF_PHASE + dF/dT
TON-OFF Chopper
Chopper
Control
Application
L4 rf
CTRV
Timing
CO
BIX.SInjChop
Rev
Inj. Ref
Source
h2
1/2
BIXi.SDIS
BIXi.SInjRF
h1
SP2T
Inj. rf reference (h1 or h2)
BIXi.SDIS
Alfred Blas, Flemming Pedersen
L4-PSB working group meeting 25/06/2009
12
Block diagram
 2μs
Pre-chopper
Source +LEBT
Chopper
Linac 4
Energy
modulation
Debuncher
20 m
Distri
4*rf
160 m
45 keV
Linac4
Trigger(s), possible RF train(s) and E-modulation
PSB
Low level RF and
standard timing
modules
Low level RF,
timings ….
Application
Provides data for chopper & E-modulation well in
advance (different data for different USER’s for PPM)
Slide from C. Carli
Aim of these discussions: discuss and agree on an interface between these three parties
Alfred Blas, Flemming Pedersen
L4-PSB working group meeting 25/06/2009
13
Security issues
Requirements :
The Linac 4 beam should not reach high (Beam-OFF at the
chopper level) energy when:
the Distributor targets the Head Dump
the Distributor is in a transition state
the Distributor targets the Tail Dump
The Tail Dump position of the distributor should be removed, at
the kicker level, as the tail Dump is unlikely to withstand a long L4
pulse.
Alfred Blas, Flemming Pedersen
L4-PSB working group meeting 25/06/2009
14