RF LIMITATIONS WHILE RUNNING AT HIGH INTENSITIES IN THE INJECTORSCollected by E. Jensen, BE-RF

1ATOP days 4.-6. 3. 2009 RF limitations

Introduction RF limitations:

Voltage necessary to create a certain accelerating bucket area – this becomes often a power limit

Beam loading (BL) will increase the power needs by (at least) the power transferred to the beam (good for higher efficiency)

But BL also leads to an induced voltage that interacts with the various beam and cavity control loops, which may become unstable.

High intensity beams will get closer to a number of stability limits (or instability thresholds). This is RF related since the system dealing with these are often RF systems.

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LINAC 2

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Linac 2

Linac 2 can produce 180 mA (190 mA in TRA10 were obtained in MD’s in 1994)

It produces regularly 150 – 170 mA for all requested intensity ranges.

Rien à signaler …

4ATOP days 4.-6. 3. 2009 RF limitationsM. Vretenar

PSB

5ATOP days 4.-6. 3. 2009 RF limitations

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PSB RF systems (for each ring)

Name

harmonic h

f range [MHz]

peak voltage

remarks:

C02 1 0.6 … 1.8 8 kV accelerationC04 2 1.2 … 3.9 8 kV acceleration, bunch

shapingC16 8, 9 6 … 17 6 kV controlled long. BU

Krusche, Paoluzzi: http://cern.ch/AccelConf/e98/PAPERS/TUP03H.PDF

• The PSB regularly produces very high intensity beams for Isolde, (3.5 · 1013) not limited by the RF systems.

6

+ transverse damper

PSB RF limitations at high intensity C04 power limitations (for faster

cycle) C04: “High Loss Mode” (not intensity

related) Instability at low C04 voltage and

high BL “Ring 4 problem” (transverse plane) C02 beam loading

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PSB: Power limitations

• During the tests for 900 ms cycling (2005), power limits were observed in the air-cooled C04 system.

• It was concluded that – for reliable operation with 900 ms – the C04 system would require a fundamental upgrade (water-cooling). For even shorter cycles, this would become mandatory.

Haase, Paoluzzi: http://cdsweb.cern.ch/record/877814/files/ab-note-2005-037.pdf 8ATOP days 4.-6. 3. 2009 RF limitations

PSB C04: “High Loss Mode” Limit The C04 were constructed from the

old C08 system – the ferrites are not optimal.

When too long at a fixed frequency, the ferrites go into “High Loss Mode”.

This figures shows the time for the losses to increase by 20%.

Paoluzzi: https://edms.cern.ch/file/593255/1/RFN2000-013.pdf

(for completeness – not intensity related)

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PSB: Instability at low C04 voltage and high BLAbove threshold voltage

Below threshold voltage

Note in preparation: Blas, Findlay, Haase, Paoluzzi, Pedersen

In MD’s in 2007 and 2008, a stability threshold was observed when increasing the beam-loading (BL) by lowering the C04 voltage. This leads to phase and amplitude coupling and a complex interplay between C02 and C04 control loops.

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Strong detuning by BL

IG/V

IB/V

ω

seen by tuning loop: φL

cavity tuning angle: φZ

Y

If IB /I0 becomes large, small perturbations of IB will have large effects.

I0/V

locus of cavity admittance

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“Ring 4 problem” Only in outer rings, mainly ring 4, at high intensity on

the flat top, one observes: shaving of trailing edge of the bunch, large transverse intra-bunch excursions, often with synchro-loop transients and during extraction

bump (5 ms before extraction), beam loss.

Problem is still under investigation: Influence of transverse feedback settings checked, voltages and loops behaviour checked and excluded

A likely explanation: There exists a transverse impedance, particularly high in

ring 4, which the transverse damper can cope with only marginally.

The additional power demand due to the perturbations (synchro-loop, ejection bump) leads to saturation of the transverse damper amplifier.Blas, Chanel, Findlay, Hanke, Mikulec, Quatraro, Rumolo

12

PSB: Beam loading

The current limitation of the C02 system would lead to a maximum ΦS of 18°, whereas 22° would be needed for the present cycle. This would indicate an intensity limit of 1.65 · 1013 per ring.

Recent results with the new digital beam control tested in ring 4 are very encouraging. The upgrade to D-LLRF is essential to eventually get closer to this intensity limit ( see M.E. Angoletta’s APC presentation 27/2/09).

Also the beam stability with low C04 voltage/high BL should profit from this upgrade.

http://indico.cern.ch/materialDisplay.py?contribId=8&materialId=slides&confId=46255

In the recent “Review on PS Booster with Linac4”, A. Blas investigated the feasibility of 2 · 1013 per ring. He found that

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PS

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PS RF systemsName

harmonic h

count frequency range [MHz]:

peak voltage

remarks:

C10 7, 10, …, 21

10+1 2.7 … 10.01 1 … 20 kV acceleration, RF “gymnastics”

C20 28, 42 1+1 13 or 20  15 kV LHC 75 ns & 50 ns bunching

C40 84 1+1 40 3 … 350 kV LHC bunching, bunch compression

C80 168 2+1 80 350 kV LHC bunch compression

C200 420, 433 4+2 200 30 kV re-bunching, controlled long. BU

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+ transverse damper (2 x 6 kW)

PS RF limitations at high intensity Phase drift at low voltages due to beam

loading Over-current at γtr crossing with high BL Beam losses at γtr (partly RF related) Insufficient transient beam-loading

compensation for an asymmetrically filled machine

Coupled bunch instabilities above γtr. (For LHC 25 and 50 ns beams). Ok up to nominal.

Bunch lengthening due to residual impedance of 40 and 80 MHz cavities.

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Phase drift at low voltages due to BLN = 0.7 · 1013 ppp N = 1.4 · 1013 ppp

voltage voltage

phaseradial positionradial position

phase

H. Damerau

When reducing the voltage, the relative BL gets stronger. It leads to the slow phase drift, which the AC coupled phase loop cannot correct. An intensity dependent phase drift is observed, which in turn acts also on the radial position.

beambeam

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Over-current at γtr crossing with high BL A phase switch is necessary at γtr . In the presence of strong BL, this fast

phase change requires large power, resulting in a significant, but short increase in anode current, which made the tube protection trip.

In 2008, the tube protection circuit was modified to cope (increased lag time from 500 µs to a few ms) – should be OK now.

18ATOP days 4.-6. 3. 2009 RF limitationsC. Rossi

Beam losses at γtr (RF related ?)Observed: a large excursion of MRP when crossing γtr (and switching RF phase), too fast for the radial loop to correct.Possible explanation: The PU’s see not only the mean radial position, but also the orbit distortions. An additional PU has recently been installed in SS76 – hold your breath for results in 2009!

S. Aumon, S. Giladroni, J. Belleman

260 280 300 320 340 360 380

-4.50

-4.00

-3.50

-3.00

-2.50

-2.00

-1.50

-1.00

-0.50

0.00

MRP vs. C-timing

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PS: Coupled bunch instabilityGrowth of a coupled bunch (CB) instability

Observed on LHC type beams.

At present, two C10 cavities are used as longitudinal kicker in the CB feedback; this works OK up to nominal intensity.

Most probable source: the impedance of the C10 cavities themselves.

beam peak detectedtransition

H. Damerau et al. : http://cdsweb.cern.ch/record/1055555/files/ab-2007-051.pdf

Beam PU signal around 5th harmonic, down-converted

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PS: Transverse damper System dimensioned for injection error

damping (6 kW per plane) – OK during the cycle.

Somewhat counter-intuitively (for me), higher brilliance (same intensity with smaller ε) will slow down incoherent betatron motion and thus leave more time for the transverse damper before filamentation smears out the transverse beam.

A. Blas, G. Rumolo, E. Benedetto 21ATOP days 4.-6. 3. 2009 RF limitations

PS: Residual impedance of 80 MHz cavities Bunch length (4 σ Gaussian fit) vs. bunch #

using 2 (blue) and 3 (red) 80 MHz cavities.

Bunches near the end of the batch are longer, due to residual impedance of 40/80 MHz systems.

This effect is more pronounced with all 3 cavities.

Bunches # 50+ are shorter due to coupled-bunch feed-back.

Damerau: http://cdsweb.cern.ch/record/1141522/files/AB-Note-2008-052-MD.pdf22ATOP days 4.-6. 3. 2009 RF limitations

SPS

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SPS 200 MHz – 4 TW cavities

SIEMENS (2 lines ,28 tetrodes)

“Philips” (2 lines , 72 tetrodes)

Each line feeds one cavity24ATOP days 4.-6. 3. 2009 RF limitations

The lines of the TWC200 systemSIEMENS, 2 TX per line, 2 lines

“Philips”, 2 TX per line, 2 lines

Total voltage (2 cavities): 4.1 MV w/o BL, ≈ 3.3 MV with nominal BL (1.15 1011 ppb). With new tubes: 350 kW/TX

Total voltage (2 cavities): 5.5 MV w/o BL,≈ 4.2 MV with nominal BL (1.15 1011 ppb).With new tubes: 400 kW/TX

… …

1 line sketched 1 line sketched

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SPS 800 MHzSystem is essential at high intensity to cope with the dominant coupled bunch instability!This system is dying!“Valvo” klystrons (YK1198) are very old. These klystrons have not been built any more for decades! Of 16 existing tubes, 10 are broken, 6 are operational.Equally the transformers in their power supplies are at the end of their lifetime. The cavities are OK.We managed “just” (with a lot of personal commitment of some people) to supply the necessary 800 MHz voltage (700 kV) thru 2008.

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Transverse Damper SPS• 4 x 25 kW RF• System runs

stably• RAS

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SPS Limitations Voltage and power OK up to ultimate LHC

intensity for nominal cycle (7.5 s). For faster cycles, see next slide for maximum current!

Coupled bunch instability requires 800 MHz system, which is presently running marginally!

Power 200 MHz: coaxial lines limited to 750 kW ageing tetrodes (more wear with CNGS operation!) power couplers: ceramics upgraded, but still limited

(transition coupler – cavity). Losses at flat bottom at high intensity –

presently not well understood (e-cloud?).

28ATOP days 4.-6. 3. 2009 RF limitationsE. Chapochnikova

0.2 0 .4 0 .6 0 .8 1 .0

1 .5

2 .0

2 .5

SPS TWC200: Accelerating voltage in one 5-section cavity

Vacc/MV

350 kW

450 kW

550 kW

750 kW

Ib/A (200 MHz component)

nominal LHC (1.15 1011 ppb, 25 ns)

Extrapolated from: G. Dôme: “The SPS Acceleration System”, CERN-SPS/ARF/77-11 29ATOP days 4.-6. 3. 2009 RF limitations

Interventions – let the statistic speak!

0123456789

101112

Run 2002 Run 2003 Run 2004 Run 2006 Run 2007 Run 2008Run 2005

CNGS

# of interventionsper week outsidenormal working hours

weeks with piquet service

• From 2005, limited resources forced us to reduce some maintenance work.

• CNGS type beams result in more wear and thus reduce tube-lifetime (16! tubes broken in 2008!)

• This is more related to maintenance and high power than high intensityE. Montesinos 30ATOP days 4.-6. 3. 2009 RF limitations

mentioned in D. Manglunki’s talk

SPS 800 MHz system upgrade programme• The upgrade to more

modern IOT is in full swing (white paper).

• IOT’s are used widely for digital TV transmitters (DVB-T).

• At present in the “Market Survey” state.

• Modular: Each line will be composed of 4 identical PA “cubicles”

• A cubicle can produce 60 kW, 4 cubicles will make up one transmitter, a total of 2 transmitters is needed.

• Present planning: 1 (test) cubicle in 2009, 4 cubicles in 2011, completion in 2013.

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Summary “RF Limitations” All systems OK up to nominal LHC. Main Issues:

PSB C04 with strong beam loading at low voltage PSB transverse damper installed power marginal PS C10 with strong beam loading at low voltage PS around transition – under investigation PS coupled bunch instability and residual

impedances SPS 200 MHz: frequent interventions, requires

regular maintenance SPS 800 MHz – renovation underway (IOT based)

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