Effect of kicker impedance on beam - longitudinal

E. Shaposhnikova, LIU-SPS Review, 20.03.2013

In collaboration with T. Argyropoulos, T. Bohl, J.E. Muller, H. Timko + help in beam measurements from OP shifts, PS and PSB expertsAcknowledgements: impedance calculations of C. Zannini

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Outline

• Present MKE kickers– Effect of serigraphy– Beam measurements

• Open MKE kickers– Beam out– Beam in

• Impedance• Beam energy dependence• Time scale

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Longitudinal impedance

• Broad-band → single bunch effects• Narrow-band → coupled bunch instabilities (2Q/ωr vs bunch spacing)

• Reactive part (broad-band and narrow-band) → loss of Landau damping, instability threshold

• Resistive part → instability growth rate, element heating, outgassing

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SPS impedance budget(in single bunch simulations)

• 200 MHz TW RF system: – Fundamental: Rsh=4.5 MΩ, Q=140, R/Q=32 kΩ– HOM: fr=629 MHz, Rsh=600 kΩ, Q=500, R/Q=1.2 kΩ

• 800 MHz RF system: Q=300, R/Q=65 kΩ• 19 kickers (shielded & unshielded) + ZS impedance• Space charge: ImZ/n = - 1.0 Ω• “1.4 GHz impedance” Rsh > 30 kΩ

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Synchrotron frequency shift

•1999-2001: SPS impedance reduction - factor 2.5 decrease in slope in agreement with reduction of ImZ/n from 12 to 5Ω •2003-2006: impedance increase due to re-installation of 8 MKE – main contribution to longitudinal broad-band impedance budget •2007-2013: impedance decreasedue to serigraphy of MKEs => seen in MDs in 2013

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MKE kicker shieldingImaginary part of ZPrinted strips in MKE-L10

Interdigital comb structure 20mm spacing surface

discharge

F. Caspers, T. Kroyer, M. Barnes , E. Gaxiola et al.

Longitudinal Re[Z]

Transverse Re[Zh]

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Synchrotron frequency shift

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Difficult measurements: increase of the slope was measured in 2007 instead of expected reduction due to the removal of one MKE kicker and shielding of another + 2cells

?

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Total SPS reactive impedance(can be measured via synchrotron frequency shift)

Reactive impedance of all kickers in the SPS (without ZS), calculated by B. Salvant (using Tsutsui formula) → significant increase due to MKE installation

Now we expect to be close to the 2001 situation

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2013

Synchrotron frequency shift:preliminary results for 2013

The slope is smaller than in 2008 (-2 < b < -1)

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T. Argyropoulos et al9

Measured and calculated slope |b| as a function of bunch length:

space charge term is larger for bright bunches (-1 Ohm for round beam pipe and b/a=5)

January 2013

→ similar results for 2001 and 2013 as expected!

-0.2Ω SC

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Total reactive impedance

MKE contribution above 50 MHz is small,but we need to verify the difference 2008 2013 due to kicker models

Total ImZ/n for 2008

Total ImZ/n for 2012

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Contribution of present MKEs to impedance of all kickers

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C. Zannini

MKEs (0.7Ω ): ~ 1/5 of ImZ/n=3.6Ω in range (50 – 500) MHz

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Coupled bunch instability and loss of Landau damping

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Threshold impedances for nominal LHC intensity

Minimum threshold on the flat top8 MKEs with serigraphy: resonant peak at 48 MHz with Rsh=25 kΩ

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Frequency dependence of threshold

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S2

Open MKELongitudinal impedance for beam out

Case1: S2=0 cm

Case2: S2=2 cm

Case3: S2=3.8 cm

C. Zannini

~ 5 times more for 5 open MKEs, but still very small

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Open MKE “beam in”Comparison with present MKE without serigraphy

~ 3 smaller ImZ/n for open MKE

C. Zannini

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Open MKE “beam in”Comparison with present MKE with serigraphy

C. Zannini

Open MKE: ~4 times higher ImZ/n ~5 times higher ReZ

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Total longitudinal MKE impedance

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Open MKEs: ~3 times higher ImZ/n ~3 times higher ReZ

C. Zannini

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Longitudinal multi-bunch instabilitySingle 200 MHz RF system

50 ns beam, 1.6x1011 p/b 25 ns beam, 1.2x1011 p/b

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Instability starts at energy ~ 1/Ntot => multi-bunch effectBut single batch with 2-3x1010 p/b is still unstable on flat top (450 GeV/c)

T. Argyropoulos

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Open MKE kickers

• No problem for “beam out” (much less than present MKEs with serigraphy)

• “Beam in” – significant impedance, higher than now• “Beam in” is only 100 ms – is it long or short?

– Minimum thresholds on flat top– Q20, 7 MV, 450 GeV: Ts=3.3 ms, dfs/fs=0.2 (2RF) =>

instability growth time > 1 ms should be damped due to synchrotron frequency spread (simple Sacherer’ criterion)

• Beam quality is measured by the SPS BQM 50 ms before beam extraction => interlock

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Double RF system and Q20

50 ns, 1.9x1011 p/b 25 ns, 1.2x1011

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=> Still controlled emittance blow-up needed for stability

T. Argyropoulos

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SummaryPresent kickers:•The positive effect of serigraphy on the present MKE kickers can be clearly seen from beam measurements. Contribution to the impedance budget is much smaller (~ 1/5 of ImZ/n of all kickers for f> 50 MHz and 1/2 below). However possible measures to reduce peak at 48 MHz should be studied New kickers:•Relative contribution of open MKE with “beam out” is very small in the present SPS impedance model•Effect of open MKE with “beam in” is more important than now •If beam is unstable – it will be extracted to the LHC?•We need to have a very accurate SPS impedance model to predict the effect of the open MKE impedance – ongoing benchmarking of impedance calculations with particle simulations and beam measurements

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