psb injection sequencing in the linac 4 era

PSB Injection Sequencing in the Linac 4 era Alfred BLAS

1

PSB Injection Sequencing

in the Linac 4 EraSynthesized information from:

M.E. Angoletta, P. Baudrenghien, C. Carli, A. Findlay, T. Fowler, F. Gerigk,A. Lombardi, M. Paoluzzi, F. Pedersen, L. Sermeus, R. Scrivens

07/01/2009


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The Linac 4 H- beam is injected in the PSB at 160 MeV/c (v=157mm/ns)instead of 50 MeV/c with Linac 2 H+.

Radio-activation of the PSB can be limited by chopping the beam at 3 MeV/c during “dead times” of the multi-turn injection process.

“Dead times” of the multi-turn injection process means:

•Switching time from one ring to the following (distributor rise time 500 ns)

•Out-of-bucket beam injection

Outline

07/01/2009


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Linac 4 beam from the source

Time

High Voltage

< 1.2 ms0.3 ms

0.15

ms

0.15

ms

Beam segment

RF transient

RF ON segment

HV ON segment

from RF Structures for Linac 4, F. Gerigk, PAC 07

from P. Baudrenghien

07/01/2009

SourcePre-chopper

LEBT

45 keV

Distri

180 m

4*rf

Chopper Amplitude modulatedfor energy modulation (painting)


4

•The beam out of the source is chopped by a pre-chopper until it stabilizes (< 1ms).

•When this is achieved the beam flies through a LEBT which requires 20 μs to let through the full intensity (space charge compensation).

•The beam is then accelerated in the RFQ after which the fast chopper selects the destination: either distributor at the end of the Linac or the 3 MeV dump. This dump position can only be hold for 1 μs.

• Just before the distributor the beam flies through the last two PIMS cells where the voltage can be modulated. This modulates the beam energy and the beam flight time towards the PSB.

•The beam then flies through the distributor which selects its final destination in always the same sequence from Head dump to Tail dump via the 4 PSB rings from ring 4 to ring 1 (top-down). The Linac-to-ring distance varies depending on the selected ring.

Beam generation

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Elements affecting the Chopper timing

1 – The distributor (source T. Fowler, L. Sermeus)

The distributor selects the destination of the Linac beam into the PSB.5 positions selected by 5 magnets that are adding up their fieldRest position = head dump no kick1st position = ring 4 1st magnet fired2nd position = ring 3 1st + 2nd magnets fired3rd position = ring 2 1st + 2nd + 3rd magnets fired4th position = ring 1 1st + 2nd + 3rd +4th magnets fired5th position = Tail dump 1st + 2nd + 3rd + 4th + 5th magnets fired

These 5 magnets should stay active until the end of the Linac pulse (total length 450 μs, see slide 8)

Kicker rise time (10-90%) = 500 nsEstimated value with solid state switches;Could be diminished to 200 ns with thyratronsMore information in June 2009

The beam travel time within the injection channels is different from ring to ring and this affects the beam-to-bucket synchronization

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Elements affecting the Chopper timing

2 – Last 2 PIMS cells (source F. Gerigk)

•The rf amplitude is modulated to modulate the beam output energy (bucket painting) .•From the central 160 MeV/c , an extra ΔE from -1.2 MeV to +1.2 MeV is added.•For each cell, this means a Klystron power ranging from 0.51 MW to 0.84 MW (>1.25 MW available)•The amplitude change rate is limited by the klystron’s power.•For ΔE a single sweep from -1.2 MeV to +1.2 the required time is 8.7 us for 1MW available•For a lower ΔE the sweep time can be decreased proportionnaly (4.35 us from -0.6 to +0.6MeV)

Energy modulation means beam flight time modulation : +/- 3.4 ns difference over the 180 m separating PIMS and PSB injection Foil (source A. Lombardi)This will affect the beam phasing with respect to the debuncher and to the rf buckets.

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The Chopper (source M. Paoluzzi)

•Trise/fall < 3.6 ns

•25 ns < TON < 1000 ns

•TOFF > 40 ns

Values to be checked with the new version of the amplifier to be tested in January 2009The reproducibility of the response time from trigger to kick remains a question mark

Linac bunch length out of the RFQ 280 ps

Chopper IN/OUT

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The (slow) pre-Chopper (source Richard Scrivens)

•Trise < 1ms

•Tfall < 2μs

•Tstable > 1 ms

After Tfall , the beam starts to feed the downstream LEBT (beam transport) which requires 20 us to stabilize (space charge compensation) and deliver the full current

“The pre-chopper is used once per cycle; it doesn’t act at the end of the cycle” “the beam OFF duration is not unlimited”

07/01/2009

Pre-chopperUpstream the RFQ at 45 keV/c

0

10

20

30

40

50

60

70

80

90

-1200 -700 -200 300 800Time (us)

Prechopper

Plasma RF Power / Beam From Source

Beam into RFQ

TstableTrise

0

10

20

30

40

50

60

70

80

90

-80 -60 -40 -20 0 20 40Time (us)

Prechopper

Plasma RF Power / Beam From Source

Beam into RFQ

Tfall

The beam into RFQ rise time is more likely around 20 μs

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Bucket Painting (source C. Carli)


BeamRevolution reference tics

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Consequences of the B field increase during injection

•Injection from the 1st turn into R4 until the last turn into R1 can last 401 μs (4 x 100 turns/ring + 3 x 500 ns distributor rise time)

•Such an injection duration associated with a non-zero Bdot leads to an horizontal injection error if the Linac energy remain constant and if the field in each ring is not compensated for.

•For Bdot = 0.1 T/s, the field increase during 400 μs will be 0.4 Gauss or 4 tics of the 0.1 Gauss B-train.

•With dp=0 => and (dp = 0 frequency law followed by the rf)

( BINJ 160MeV = 2311 G R = 25 m p = 570 MeV/c ΔE [eV] = 0.519. Δp[eV/s] . c)

=> with Bdot = 0.1T/s and the maximum number of turns =>ΔR= -0.26 mm.

This corresponds to a 1 mm error for a presently typical 0.4 T/s at injection with a total ΔB = 1.6 Gauss .

If we can increase the Linac energy by Δp such that dp/p = dB/B in order to have ΔR = 0we would need Δp = 0.11 MeV for ΔB = 1.6 Gauss, and the frequency law should be adapted accordingly.

07/01/2009

B

dB

R

dR

tr

2

1

5.162 tr

B

dB

f

df

tr

2

1

17.1


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Consequences of the B field increase during injection

•If B (0.4 T/s) R (-1mm for 100 turns in each ring) and frequency law =

•If B (0.4 T/s) and p (0.11 MeV) ΔR = 0 and frequency law =

•In both cases with ΔB = 1.6 Gauss, the revolution frequency swing would in the order of 700 Hz

07/01/2009

B

dB

f

df

B

dB

f

df 2

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Injection scenari (under construction!!)

PSB Injection Sequencing in the Linac 4 era Alfred BLAS07/01/2009

The injection can be made in different ways:One fact is accepted: the field will increase during injection.

If we want the same conditions in each ring, we need to apply a Field compensation with the Bdl magnets. What is the possible amplitude of this correction?Would we like to make the injection at a fixed frequency? Can we make a B-field correction with a slope that compensates the one of the main magnets?

With the B-field compensation, the conditions are identical during the injection but are different from one ring to the other during the catch-up process that follows.

One different approach is to play with the linac energy. The beam energy is increased at a rate that corresponds to the Field increase ΔR=0 and so is the rf. We need to check what the head room is in term of energy increase???

One practical aspect is that having a single rf reference would ease the synchronization of all the linac 4 and PSB elements.

Do we accelerate during injection or do we not?

If we don’t:•If all rings are synchronized to a single reference following a Δp=0 law, the price to pay will be a radial offset (1 mm max for 0.4 T/s)•If we don’t want this radial offset, we can increase the Linac energy at the same rate as B (0.11 MeV max in the most demanding case with 0.4 T/s). The single rf reference will follow this time a different frequency law

•If we don’t want to increase the Linac energy, we can compensate the field in each ring (max 1.6 Gauss with 0.4 T/s) using the Bdl correction. A single rf reference can be applied but it should stay at a fixed value!

If we do:•Each ring has its specific rf following a ΔR=0 law, The proper inter ring rf phasing during ring switching can be pre-established from initial conditions with initial phases and frequencies being set at a specific time, and from then on, the sequence is programmed (we don’t follow the measured Btrain )

B

dB

f

df2


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Information flow-chart

Bucket Shape

VH1

VH2

Stable φ

REV refDistri. Position

Linac Energy offset

Distributor trig

Window generation

+ Gating

ChopperON/OFF

RFFeed-

Forward

•The distributor position impacts the beam flight distance•The energy offset impacts the beam flight time and the bucket boundaries

07/01/2009

Bucket Position


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Bucket Painting

ΔT1 = f(ring selected)ΔT2 = f(ΔE, φS , V1, V2)ΔT3 = f(ΔE, φS , V1, V2)ΔT4 = f(ΔE, φS , V1, V2)ΔT5 = f(ΔE, φS , V1, V2)

Rev. reference

h2 Buckets limits

h1 Bucket limits

07/01/2009

•The chopper ON/OFF control would require revolution tics synchronous with the rf buckets + 5 timing values for each turn in each ring. These values are calculated at an application level and sent to a hardware register for each cycle (ppm).


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Chopping during change of injected ring1. All rings in rf phase

RevolutionReference

Distributor fieldRing X-1

Ring X

Chopping during one period to maintain rf synchronism

T > 1 us -> too long

This approach with all rings synchronized on the same signal is not viableIt requires too long a chopping time for the actual circuit capability

07/01/2009


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Ring X reference

Distributor field Ring X-1

Ring X

Force Chopper-ON + changeChopper REV ref

With the REV reference signal delayed by 500 ns or 5/10th of a turn (distributor rise time = 500 ns) for the rings in descending order, the chopping will be limited in time according to the specificationsNB: the diagram is valid for a whole number of turns injected in ringX . If additional 1/10th of a turn are required, the REV reference of the following ring should be delayed accordingly!!!

Ring X-1 reference

Chopping OFF

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Chopping during change of injected ring1. Rf phase different from one ring to another


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Reference signals

PSBInj.REF

Source

Rev

Number of turns& VPROG H1

Rev R4Rev R3RevR2RevR1

SDis R4SDis R3SDisR2SDisR1SDisTail

SDis represented for 1.0 turn injected in Ring 4

1.5 turn in Ring 31.0 turn in Ring 2

0.1 turn in Ring 1

Revolution signal delayed by some 1/10th of a period with respect to the previous ring. The first 5/10th comes for the distr. Delay and the

other 1 to 9/10th for the decimal part of the nb of turns in the previous ring

10*Rev

5/10th of TREV = Distr. Rise time

LLRF

Rev signals follow a Δp = 0 law during injectionNOT ΔR = 0 !!!!

rf R4rf R3rf R2rf R1

To rf synchro= h1 (or h2 if VPROG H1 = 0)

To Distr. and

Chopper Control

NB: All signals in phase with 10*RevOK for counters!!

BIX. SINJ

07/01/2009

ToChopper Control

Chopper ON (high) – OFF (low) {high => no beam to PSB}

Linac rfTo chop in

synchronism with the bunches


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Chopper Control

Chopperand

Energy Modulator

Control

Rev R4Rev R3RevR2RevR1

SDis R4SDis R3SDisR2SDisR1

SDisTail

ΔE R4ΔE R3ΔE R2ΔE R1

To Chopperand RF feed-forward

Chopper ON/OFF

PIMSΔE

Analog modulation

PIMS voltage modulation

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5 timing values for

each injected turn


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Chopper Control

Chopperand

Energy Modulator

Control

To Chopperand RF feed-forward

Chopper ON/OFF

PIMSΔV

Analog modulation

ΔE4 rings

07/01/2009

5 timing values for

each injected turn

PSBInj.REF

Source

Rev

10*RevLLRF

BIX. SINJ

Number of turns& VPROG H1

Rf synchro4 rings

SDIS4 rings

Linac rf


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Injection sequencing - Possible setup

Chopper ON/OFF

Voltage modulation

ΔE4 rings

07/01/2009

5 timing values for each injected turn

Rev + 10* REV

BIX. SINJ

Number of turns+ VH1

BIXi.SDIS

SourcePre-chopper

LEBT

45 keV

Distri

180 m

4*rf

Dedicated Injection rf

source in BOR

InjectionSequencing

control

To Linac rf feed-forward

Rf for synchro, 4 rings

Application

Linac rf

Could be a programmable fixed frequency

source at the ISC level

psb injection sequencing in the linac 4 era

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