FERMI LINAC O VERVIEW The 1.5 GeV 50 Hz Upgrade July 2013 C. Serpico

FERMI LINAC

FERMI LINAC

OVERVIEWOVERVIEW

The 1.5 GeV 50 Hz Upgrade

The 1.5 GeV 50 Hz Upgrade

July 2013

C. Serpico

OUTLINEOUTLINE

FERMI Linac LayoutFERMI Linac Layout

Operating Paramenters until RUN15Operating Paramenters until RUN15

Energy and Rep. Rate UpgradeEnergy and Rep. Rate Upgrade

BTW Accelerating StructuresBTW Accelerating Structures

Phase modulation techniquePhase modulation technique

May 2013: the RF conditioningMay 2013: the RF conditioning

Vacuum bi-directional couplers faultsVacuum bi-directional couplers faults

1.5 GeV @ 50 Hz1.5 GeV @ 50 Hz

Linac Operating ConditionLinac Operating Condition

ConclusionsConclusions

Some comments…Some comments…

SummarySummary

What is the next?What is the next?

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LINAC LAYOUT 1/4LINAC LAYOUT 1/4

Linac Linac SectionSection

# of # of Str.Str.

TypeType

InjectorInjector 22 S0A-S0BS0A-S0B

Linac1Linac1 44 CERNCERN

Linac2Linac2 33 CERNCERN

Linac3Linac3 22 BTWBTW

Linac4Linac4 55 BTWBTW

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S0A – S0B accelerating structuresS0A – S0B accelerating structures

Geometric Geometric ParametersParameters

ValueValue UnitsUnits

Number of CellsNumber of Cells 9393Cells' LengthCells' Length 33.3333.33 mmmmTotal LengthTotal Length 3099.693099.69 mmmmMin. Iris RadiusMin. Iris Radius 8.028.02 mmmmMean Iris RadiusMean Iris Radius 9.739.73 mmmm

Acc. Str. RF Acc. Str. RF ParametersParameters


TypeType MixedMixedOperating ModeOperating Mode 2/3 pi2/3 piOperating Operating FrequencyFrequency 2998.012998.01 MHzMHzQ factorQ factor 1410014100R0R0 67.1067.10 Mohm/mMohm/mTauTau 0.600.60 NeperNeperFilling TimeFilling Time 0.8980.898 µsµs

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CERN accelerating structureCERN accelerating structure



Number of CellsNumber of Cells 134134Cells' LengthCells' Length 33.3333.33 mmmmTotal LengthTotal Length 4466.224466.22 mmmmMin. Iris RadiusMin. Iris Radius 8.028.02 mmmmMean Iris RadiusMean Iris Radius 9.739.73 mmmm



TypeConst. Grad.

Operating Mode 2/3 piOperating Frequency 2998.01 MHzQ factorQ factor 1400014000R0R0 65.0065.00 Mohm/mMohm/mTauTau 0.840.84 NeperNeperFilling TimeFilling Time 1.2551.255 µsµs

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BTW accelerating structuresBTW accelerating structures



Number of CellsNumber of Cells 162162Cells' LengthCells' Length 37.5037.50 mmmmTotal LengthTotal Length 6075.006075.00 mmmmIris RadiusIris Radius 5.005.00 mmmm



TypeTypeConst. Const. Imp.Imp.

Operating ModeOperating Mode 3/4 pi3/4 piOperating Operating FrequencyFrequency 2998.012998.01 MHzMHzQ factorQ factor 1160011600R0R0 70.0070.00 Mohm/mMohm/mTauTau 0.610.61 NeperNeperFilling TimeFilling Time 0.7530.753 µsµs

SLED RF SLED RF ParametersParameters


Operating Operating FrequencyFrequency 2998.012998.01 MHzMHzQ factorQ factor 190000190000BetaBeta 1010Filling TimeFilling Time 1.8321.832

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K1K1 GUNGUN 4.84.8K2K2 L0_1L0_1 47.047.0K2K2 L0_2L0_2 47.047.0K3K3 L1_1L1_1 55.255.2K3K3 L1_2L1_2 55.255.2K4K4 L1_XL1_X -20-20K5K5 L1_3L1_3 55.255.2K5K5 L1_4L1_4 55.255.2K6K6 L2_1L2_1 57.557.5K6K6 L2_2L2_2 57.557.5K7K7 L2_3L2_3 52.052.0K8K8 L3_1L3_1 118.0118.0K9K9 L3_2L3_2 127.0127.0K10K10 L4_1L4_1 116.0116.0K11K11 L4_2L4_2 119.0119.0K12K12 L4_3L4_3 119.0119.0K13K13 L4_4L4_4 117.0117.0K14K14 L4_5L4_5 121.0121.0

OPERATING PARAMENTERS UNTIL OPERATING PARAMENTERS UNTIL

RUN15RUN15

The Linac has been operated at 10 Hz with an overall energy gain of about The Linac has been operated at 10 Hz with an overall energy gain of about 1270 MeV.1270 MeV.

C – Sections:C – Sections:~ 55 MeV/section~ 55 MeV/section

S – Sections:S – Sections:~ 120 MeV/section~ 120 MeV/section

Energy Budget on 06/2012Energy Budget on 06/2012

RF Plant Klystron Output Power [MW]

Pulse Width [µs]

K2 K2 K7 K7 ~32~32 2.02.0

K8 K8 K14 K14 ~22~22 3.03.0

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In May 4 weeks were foreseen for testing the power plants operation In May 4 weeks were foreseen for testing the power plants operation at 50 Hz repetition rate and the RF conditioning up to a power level at 50 Hz repetition rate and the RF conditioning up to a power level

corresponding to 1.5 GeV beam energy.corresponding to 1.5 GeV beam energy.

ENERGY AND REP. RATE UPGRADEENERGY AND REP. RATE UPGRADE

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ENERGY AND REP. RATE UPGRADEENERGY AND REP. RATE UPGRADE

To get 1.5 GeV beam energy at the end of the Linac the 7 BTW S-type To get 1.5 GeV beam energy at the end of the Linac the 7 BTW S-type accelerating structures have to be pushed up to an energy gain of about accelerating structures have to be pushed up to an energy gain of about

155 MeV/section (corresponding to a gradient of 26 MV/m). 155 MeV/section (corresponding to a gradient of 26 MV/m).

Linac3: Linac3: two sections Linac4: Linac4: presently five

sections

From old Elettra From old Elettra injector.injector.

Both Linac 3 and Both Linac 3 and Linac 4 RF power Linac 4 RF power distribution systems distribution systems are equipped with are equipped with SLED systems.SLED systems.

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BTW ACCELERATING STRUCTURESBTW ACCELERATING STRUCTURES

S-type structures are Backward Traveling Wave (BTW) structures comprised of S-type structures are Backward Traveling Wave (BTW) structures comprised of 162 nose cone cavities coupled magnetically. 162 nose cone cavities coupled magnetically.

TypeType Const. gradient, Const. gradient, mag. coupledmag. coupled

ModeMode 33ππ/4/4

FrequenFrequencycy

2998.012998.01 MHzMHz

LengthLength 6.156.15 mm

QQ 1100011000

RRshsh 71-7371-73 MOhMOhm/mm/m

Filling Filling timetime

0.7470.747 µsµs

Past operation of the BTW accelerating Past operation of the BTW accelerating structures showed that such structures structures showed that such structures suffered heavy breakdown phenomena suffered heavy breakdown phenomena when pushed to high gradient when pushed to high gradient (no phase (no phase modulation implemented)modulation implemented).. Single cell simulation shows that surface Single cell simulation shows that surface electric field on the magnetic coupling slot electric field on the magnetic coupling slot becomes of the same magnitude and even becomes of the same magnitude and even higher than the surface electric peak field higher than the surface electric peak field on the nose.on the nose.

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THE PHASE MODULATION TECHNIQUETHE PHASE MODULATION TECHNIQUE

SLED operation with Phase Modulation:SLED operation with Phase Modulation:Amplitude of klystron output is kept constant during the RF Pulse95º Phase Shift 0.77 µs before the end of the pulsePhase changed gradually (slope = 149º/µs)Phase kept constant during the last 200 ns of the RF pulse

Note: Phase offset and Phase Slope Parameters adjustable by LLRF

Am

pP

hB

eam

t

t

t

0.5 1 1.5 2 2.5 3 3.5 4 4.5

0

0.05

0.1

0.15

0.2Sled Phase Modulation - 3 us

t (us)

Am

p (

a.u

.)

MeasuredCavity Input Field

3µs pulse3µs pulse

EEpk-cavpk-cav/E/Epk-klypk-kly= 1.83 = 1.83 (field ratio)(field ratio)

PPpk-cavpk-cav/P/Ppk-klypk-kly= 3.34= 3.34 (power (power ratio)ratio)

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Even if the RF pulse shape is not flat, for a 3 µs overall pulse width the energy gain loss with respect to the normal operation

is just 3.3 %

THE PHASE MODULATION TECHNIQUETHE PHASE MODULATION TECHNIQUE

RF width [µs]

Epk-cav/Epk-kly

[field ratio]Gain Facto

r

Normal Normal OperationOperation 3.03.0 2.52.5 1.51.5

Phase Phase ModulationModulation 3.03.0 1.831.83 1.451.45

Phase Phase ModulationModulation 4.04.0 2.12.1 1.651.65

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

x 10-6

0

0.05

0.1

0.15

0.2

0.25

Cavity Field Comparison - Normal Sled Operation and Phase Modulation

t (us)

Am

p (

a.u

.)

Normal Sled - 3us

Phase Mod - 3 usPhase Mod - 4 us

3.03.0

3.03.0

1.51.5

1.451.45

Normal Normal OperationOperation

Phase Phase ModulatioModulationn

2.52.5

1.831.83

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MAY 2013: THE RF CONDITIONINGMAY 2013: THE RF CONDITIONING

From K2 to K7: From K2 to K7: ((S0A-S0B & CERN sectionsS0A-S0B & CERN sections))Same Klystron Output Power as before (~32 MW)Same Pulse Length as before Repetition rate from 10 Hz to 50 Hz

From K8 to K14: From K8 to K14: ((equippedequipped with SLED with SLED systems)systems)Klystron Output Power increased up to 32 MW (from the past 22 MW operation)Pulse Length up to 3.0 µsInput power to the structure increased from 62 MW up to 91 MW peak. Repetition rate from 10 Hz to 50 Hz

RF conditioning started on 6RF conditioning started on 6thth of May and it took about 4 full of May and it took about 4 full days to get to the target operating parameters.days to get to the target operating parameters.

RF conditioning started on 9RF conditioning started on 9thth of May. of May.

Kly Output Power [MW]

Pulse Width

[µs]

Rep Rate

[Hz]

Strucutre Input Power [MW]

Up to RUN 15

~22~22 3.03.0 1010 6262

RUN 16 ~32~32 3.03.0 5050 9191

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All 3 directional couplers were installed between the output of the SLED system and the input coupler of the accelerating structures.

Vacuum leaks were detected only on the 50dB pick-up ceramic windows.

Cracks on ceramic windows were experienced on directional couplers produced by 2 different firms.

During RF conditioning we experienced 3 major faults (on two different plants) on bidirectional couplers as a consequence of breakdown events in accelerating structures.


Vacuum Bi-directional Couplers FaultsVacuum Bi-directional Couplers Faults

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Bethe hole coupler type:

~60 dB coupling on Forward Port

~50 dB coupling on Backward Port

~23 dB directivity


Vacuum Bi-directional CouplersVacuum Bi-directional Couplers

Changing the RF design of the coupler is needed!

Bethe hole coupler with lower coupling on the Reflected Port (~60 dB)(the available length is 159 mm, mandatory!)

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1.5 GEV @ 50 HZ1.5 GEV @ 50 HZ

On 31st of May an energy of 1.503 GeV (on crest) at 50 Hz repetition rate was reached.

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LINAC OPERATING CONDITIONLINAC OPERATING CONDITION

According to the FERMI Commissioning Plan, the commissioning of FEL2 line with a 1.5 GeV beam energy was foreseen starting from 10th of June.

On 31st of May, 1.5 GeV beam energy (on crest) was reached with the Linac operating at 50 Hz.While operating Linac with the above parameters, the breakdown rate was high to guarantee the desired uptime for the FEL2 line commissioning.

To guarantee a reliable operation of the FERMI Linac during FEL2 commissioning in June , on 3rd of June it was agreed to bring the Linac back to 10 Hz repetition rate

while still pushing the energy gain at 1.46 GeV.

Commissioning of FEL2 during JuneCommissioning of FEL2 during June

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A comparison between the 50 Hz and the 10 Hz A comparison between the 50 Hz and the 10 Hz operationoperation

Faults rate and faults rate distribution before and just after switching back to 10 Hz on a 24 hours basis.

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A comparison between the 50 Hz and the 10 Hz A comparison between the 50 Hz and the 10 Hz operationoperation

Vacuum trends before and just after switching back to 10 Hz

K14K14

50 Hz 50 Hz OperatiOperati

onon


onon

3.0e-9 mbar3.0e-9 mbar

1e-9

1e-8

1e-7

1e-6


onon


onon

K8K8

1.5e-9 mbar1.5e-9 mbar

1e-9

1e-8

1e-7

Most of the vacuum spikes have been detected by ion pumps closer to the structures input: breakdown events take place in structures input

couplers and structures cells.

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CONCLUSIONS

CONCLUSIONS

SOME COMMENTS…SOME COMMENTS…

BTW accelerating structures were rated for working up to 35 MV/m accelerating gradient at 10 Hz repetition rate.

While operating as Elettra injector, such structures suffered heavy breakdown phenomena if pushed to gradient of 25-26 MV/m.At that time no Phase Modulation technique was implemented.

In September 2011 Phase Modulation techinque was implemented. Some tests were done at 10 Hz on one plant: an overall energy gain of 165 MeV (~27.5 MV/m) was reached without any major issue.


…making a long story short …

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In May 2013, four weeks of RF conditioning were foreseen to reach a gradient of 25-26 MV/m on each BTW structure while operating for the first time at 50 Hz repetition rate.

In May 2013 Just after reaching 1.5 GeV (with a gradient of 25 MV/m on each BTW structure) at 50 Hz repetition rate we had the third directional coupler fault.

We decided to reduce the gradient to about 23 MV/m on BTW structures and switch back to 10 Hz repetition rate.


…making a long story short …

Is 26 MV/m @ 50 Hz a real limit for our BTW structures?

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Breakdown diagnosticBreakdown diagnostic

On all the plants Bethe hole bi-directional couplers are installed.At the output of the klystrons

On all the plants the backward power signal is used to shut-off the RF in case of breakdown events to protect the klystron itself. At the input of the accelerating structures

On RF plants equipped with SLED system, the forward power signal is just used for tuning the SLED cavity.

On RF plants equipped with SLED system, bi-directional couplers installed at the output of the klystron are not able to detect any breakdown event originated from the RF structure.

RF breakdown are detected using signals coming from ion pumps installed in the system.

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SUMMARYSUMMARY

Increase of operational margin at 1.5 GeV possible with the installation of the 2 accelerating structures already foreseen

in the machine layout

1.5 GeV Linac energy (on crest) has been reached at 50 Hz repetition rate.

Stable operation of the Linac with about 1.46 GeV (on crest) beam energy at 10 Hz repetition rate.

1. Phase ModulationImprove pulse shape flatness

2. New Bi-directional Couplers Bethe hole couplers? Mechanical length of 159 mm is mandatory

3. Improve breakdown diagnostic

UpgradesUpgrades

AchievementsAchievements

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WHAT IS THE NEXT…WHAT IS THE NEXT…

The Machine Advisory Committee (MAC meeting, 13-14 June 2013) strongly suggested to consider the possibility of installing a SLED system on CERN-type

structures.

SLED on CERN-type accelerating structures?SLED on CERN-type accelerating structures?



Total Length ~4.5 mQ factorQ factor 1410014100R0R0 65.0065.00 Mohm/mMohm/mTauTau 0.840.84 NeperNeperFilling TimeFilling Time 1.2551.255 µsµs

SLED RF SLED RF ParametersParameters


Q factorQ factor ~100000~100000BetaBeta 66Filling TimeFilling Time 1.51.5 µsµs

PPklyklyRF RF

PulsePulseEnergEnergy Gainy Gain

GradienGradient t

[MW] [µs] [MeV] [MV/m]

NO SLED NO SLED operatiooperationn

3636 4.04.0 5858 ~13~13

SLED SLED operatiooperationn

3636 4.04.0 7676 ~17~17

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About 120 MeV more with a SLED system installed on 7 CERN type accelerating structures.

About 700 k€

SLED SLED operatiooperationn

~17~17

WHAT IS THE NEXT…WHAT IS THE NEXT…

All 6 meter long, BTW accelerating sections are 20 years old structures. A reliable solution to replace them is under investigation.

Each structure could be replaced by one accelerating module endowed with three 2-meter long accelerating structures.

Keeping the same power plant, new accelerating modules could guarantee a slightly higher energy gain.

With a 80 MW peak power klystron (commercially available) a 40 MV/m accelerating gradient could be achieved.

A preliminary ideaA preliminary idea

…high gradient 2-m long structures…

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THANK YOUTHANK YOU

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FERMI LINAC O VERVIEW The 1.5 GeV 50 Hz Upgrade July 2013 C. Serpico