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CSR-Driven Longitudinal Instability – Comparison of Theoretical and Experimental Results. Peter Kuske, Helmholtz-Zentrum Berlin, Germany. 3 rd Low Emittance Ring Workshop, 8 th -10 th July, 2013, Oxford, UK. Content of the Talk.

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CSR-Driven Longitudinal Instability –

Comparison of

Theoretical

and Experimental Results

Peter Kuske, Helmholtz-Zentrum Berlin, Germany

3rd Low Emittance Ring Workshop, 8th-10th July, 2013, Oxford, UK


Content of the talk
Content of the Talk

Introduction - Detection of CSR and Signals of Unstable Beams

Theoretical Predictions

Comparison of Predictions with Observations

IV. Summary

2


I 1 observation of csr @ bessy ii
I.1 Observation of CSR @ BESSY II

K. Holldack, et al., THPKF013, EPAC‘04


I 2 observation of csr @ anka mls atf
I.2 Observation of CSR @ ANKA, MLS, ATF, …

S. De Santis, et al., THPCH067, EPAC‘06

Hot Electron Bolometer

A.-S. Müller, et al., TU5RFP027, PAC‘09


I 3 observation of csr @ diamond
I.3 Observation of CSR @ Diamond

G. Rehm, et al., TUPD32,DIPAC‘09


I 4 observations @ diamond
I.4 Observations @ Diamond

R. Bartolini, et al., THPC068, IPAC‘11



I 6 observations @ mls
I.6 Observations @ MLS

G. Wüstefeld, et al., WEPA015, IPAC‘10


I.7 Observations @ BESSY II

BESSY II, Fsyno=1 kHz, o~1.5 ps

Many modes visible in the Fourier transformed CSR

9

9


II.1 Shielded CSR-Impedance

d=2h, plate separation

Broad band resonator with low Q:

ANKA: Fres ~ 127 GHz

BESSY II: Fres ~ 100 GHz

MLS: Fres ~ 44 GHz

R.L. Warnock, PAC'91,

PAC1991_1824, http://www.JACoW.org

10



Ii 3 shielded csr wake bessy ii
II.3 Shielded CSR-Wake – BESSY II

Scsr ~ 0.5 + 0.12·X(Bane, et al., IPAC’10)


Ii 3 shielded csr wake bessy ii1
II.3 Shielded CSR-Wake – BESSY II


Ii 3 shielded csr wake bessy ii2
II.3 Shielded CSR-Wake – BESSY II


II.4 Shielded CSR-Wake


Ii 5 frequency of first unstable mode vs norm 0
II.5 Frequency of First Unstable Mode vs. norm. 0

BBR-Wake:

variation of Fres with constant o and

Shielded CSR-Wake:

Fres given by geometry and variation of o through α


Ii 6 bunch length inst at instability threshold
II.6 Bunch Length, inst, at Instability Threshold

Broad-Band-Resonator Impedance Shielded CSR Impedance



III.1 First Unstable Modes BESSY II

Slope agrees with resonance Fres~100 GHz

19

19


III.2 CSR-Threshold Currents for BESSY II

In fair agreement with predictions – bunch lengthening explains shift

Solid black line: K.L. Bane, et al., Phys. Rev. ST-AB 13, 104402 (2010)

20

20


III.3 CSR-Threshold Currents for the MLS

Solid black line: K.L. Bane, et al., Phys. Rev. ST-AB 13, 104402 (2010)

21

21


III.4 CSR-Threshold Currents Observed at ANKA

Observation at ANKA: Stability between 45 μA and 60 μA, below and above the beam is longitudinally unstable


Shielded csr impedance at anka

III.5 Parameters for ANKA

RMS bunch length: σ = 1.953 ps

Bending radius: ρ = 5.593 m

Height of dipole chamber: 2·h = 0.032 m

Energy: E = 1.3 GeV

Momentum compaction factor: α = 2.033 10-4

Accelerating voltage: Vrf = 1.8 MV

Longitudinal damping time: Τlong = 10.6 ms

Synchrotron frequency: Fsyn = 7.8 kHz

Damping/excitation parameter:β = 1.93 10-3

CSR-impedance has first maximum at:

Fres = c·(/24)1/2·ρ1/2·h-3/2 = 126.7 GHz

Shielding parameter (à la Bane, et al.):

Χ = c·σ·ρ1/2·h-3/2 = 0.684 or 2Fres·σ = 1.555

Shielded CSR-Impedance at ANKA

Decscription of the code: P. Kuske, “CSR-DRIVEN LONGITUDINAL SINGLE BUNCH INSTABILITY THRESHOLDS”, WEOAB102, IPAC’13, Shanghai, China


Beam is longitudinally stable below 20 a and between 48 a and 59 a

III.6 Result of the Simulation

Beam is longitudinally stable below 20 μA

and between ~48 μA and ~59 μA


III.7 Result of the Simulation:

Normalized Energy Spread vs. Time

Stability – thin lines, normalized energy spread = 1.0

Oscillations – thick lines, normalized energy spread > 1.0

Bursts – more or less regular, saw tooth instability


III.8Result of the Simulation:

CSR-Signal

60 μA


III.8Result of the Simulation:

CSR-Signal

48 μA


IV.1Summary of the Results for ANKA

  • Very good agreement between the observations at ANKA and the results of the numerical solution of the VFP-equation:

  • Correct predictions for the regions of longitudinal stability and instability

  • Correct predictions for the dominant mode of the longitudinal instability:

  • Dipole mode at 20 μA, above ~36 μA quadrupole mode with a shift to lower frequencies as the current is increased

  • Quadrupole mode above 60 μA shifting upwards with increasing bunch charge


Iv 2 status of csr driven longitudinal instability
IV.2 Status of CSR-Driven Longitudinal Instability

Example for ANKA: 0=5.5 ps, Fsyn=8.5 kHz, Vrf=0.7 MV (E=1.3 GeV, α=6.24e-4):

2Fres·0 = 4.38  Finst ~4.5·Fsyn~ 38 kHz

 Scsr ~ 0.8, Ithr ~ 0.32 mA

V. Judin, et al., TUPPP010, IPAC’12



Predictions using the shielded CSR-wake are in surprisingly good agreement with measurements at BESSY II, MLS, ANKA, and other storage rings.

If the bunch length is known then we can estimate the shielding parameter or normalized resonance frequency and predict the threshold current.

The observed resonance-like features show the importance of the vertical gap of the dipole vacuum chamber.

Increasing the cavity voltage gradient will not necessarily lead to higher threshold currents for shorter bunches – consequences for BESSY-VSR.

CSR-driven longitudinal single bunch instability thresholds are of no concern for low emittance rings – coupled multi bunch instabilities could be an issue because of the tube-like vacuum chambers.

If bunch length and Finst are known then we can determine the dominant frequency of the impedance.

Longitudinal modes below the instability thresholds can be observed - with sensitive detectors.

The support of Dennis Engel is acknowledged.

IV.4 Summary

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