Longitudinal impedance identification

T. Argyropoulos, T. Bohl, J. E. Muller, H. Timko, E. ShaposhnikovaThanks to H. Damerau for preparing the beam

LIU-SPS BD WG 30/08/2012

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The increase of the average bunch length of the batches along the flat bottom

Observations of high frequency pattern on the nominal SPS bunches at flat bottom during the single bunch measurements for threshold instabilities

Motivation

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Same method used in the past (Measuring the Resonance Structure of Accelerator Impedance with Single Bunches, T. Bohl, T. Linnecar, and E. Shaposhnikova, PRL, 1998)

Experimental conditions: Long single proton bunches (εL(90%) ~ (0.23 – 0.26) eVs – τinj~ (25-30) ns) Small momentum spread (to be more unstable and debunch slowly) Intensity scan from 0.5x1011 to 2.0x1011 p SPS RF off

Method:Acquisitions of beam profiles for a period of ~90 ms after injection each 10 turns.followed by a Fourier analysis The presence of resonant impedances with high R/Q and low Q leads to line density modulation at the resonant frequencies

Video of bunch profile

1.4 GHz

Example with Np=1.58x1011 and τinj(4σ) = 28.5 ns

Measurements

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Example at average (90 ms) Np= 8x1010

Projection

Mountain range

Mode Amplitude vs Time

Contour plot

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Comparison with old measurements

Np = 8x1010

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Projection Contour plotT. Bohl, 2007

2012

Simulations

SPS long. impedance model: TWC 200 TWC 200 HOM (629 MHz) TWC 800 all kickers + resistive impedance (C. Zannini)

Initial particle distribution: Saved at PS (tomoscope – for low intensity smaller bunch length) Simulated (with ESME) until extraction (Helga)

Simulations here have been done with my code (in Matlab): (Helga is currently working with HEADTAIL)

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Comparison with Simulations (preliminary)Simulations with SPS long. impedance model + fr = 1.4 GHz – R/Q = 58 kOhms – Q = 7 for

Np = 1x1011

Good agreement but more fine tuning is needed

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SummaryMode amplitude ratio of 1.4/0.2

versus IntensitySimulations were done with the SPS impedance model + fr = 1.4 GHz – R/Q = 35 kOhm – Q = 20

Strong peak at 1.4 GHz observed in the beam spectrum

Threshold around Np~8x1010 p: observed also in old measurements (2001 and 2007) this impedance was always there!

Impedance evaluation through simulations Using low Q and high R/Q more fine tuning is necessary

Any ideas about the possible impedance sourcewould be very welcome!