1. ESRF Experimental Contribution Towards Working Group Introduction
REVOL Jean-Luc

2. ESRF Experimental Contribution Towards Working Group Introduction
Goal of this talk:
Interdependence
Longitudinal/transverse/single-bunch/multi-bunch
based on ESRF observations
Longitudinal Single
Transverse Single
Transverse Multi.
Longitudinal Multi.
Vertical
Horizontal
WG1
WG2
Longitudinal Single
Transverse Single
Transverse Multi.
Longitudinal Multi.
Vertical
Horizontal
10/2/2019
Beam Instability Workshop; Monday,13 March ; Revol Jean-Luc

3. ESRF Experimental Contribution Towards Working Group Introduction
Goal of this talk:
Interdependence
Longitudinal/transverse/single-bunch/multi-bunch
based on ESRF observations
IMPEDANCE MODELING
WITH BEAM
Resistive wall contribution to
single bunch dynamics
Broad Band contribution to
multibunch dynamics
Effect of partial filling on multibunch transverse instabilities
Transverse feedbacks
WG1
Transverse Single
Transverse Single
Longitudinal Single
Longitudinal Single
Vertical
Vertical
Horizontal
Horizontal
Vertical
Vertical
Horizontal
Horizontal
Transverse Multi.
Transverse Multi.
Longitudinal Multi.
Longitudinal Multi.
WG2
10/2/2019
Beam Instability Workshop; Monday,13 March ; Revol Jean-Luc

4. Longitudinal /Single Bunch
Bunch lengthening
Recent measurements show oscillating values.
Is it a sign of a Saw-tooth mechanism ?
Stable turbulent regine above threshold
3.5 mA
At zero current, discrepancy between theoretical bunch length and measured bunch length > 20 %
10/2/2019
Beam Instability Workshop; Monday,13 March ; Revol Jean-Luc

5. Longitudinal /Single Bunch
Measurement of energy spread based on two emittance measurements one in a dispersive section and one in a non dispersive section
Microwave regime starts at 3.5 mA for the ESRF.
From bunch lengthening and energy spread (simulated by tracking code)
BBR model for the longitudinal impedance
fres = 30 GHz, Rs = 42 k, Q=1
3.5 mA
10/2/2019
Beam Instability Workshop; Monday,13 March ; Revol Jean-Luc

6. Transverse / Single Bunch / BBR
Z (M  )
Transverse / Single Bunch / BBR
Z ( M ) 20
R/Q
==> no effect of the resistive impedance ==> Positive real part compensates exactly the negative real part
Simulation of the mode coupling with a BBR Model 10
Real (Z)
f (GHz)
-40
-20 20 40
-10
Imag (Z)
Mode 0 and mode -1 interact with the imaginary positive part of the impedance ==> defocusing
-20
Zero Chromaticity
m=0
Detuning of Mode 0 = f (R/Q)
Detuning of mode -1 = f( fr)
Mode with larger m are much less detuned
m=-1
m=-1
m=-1
m=-2
m=-2
f (GHz)
-40
-20 20 40
10/2/2019
Beam Instability Workshop; Monday,13 March ; Revol Jean-Luc

7. Transverse / Single Bunch / Mode merging
Z (M  )
Transverse / Single Bunch / Mode merging
Z ( M ) 20
R/Q
Simulation of the mode coupling with a BBR Model 10
Real (Z)
f (GHz) s
-40
-20 20 40
-10
Imag (Z)
Q Threshold
Fr = 22 GHz
R/Q= 13.5 M
-20
0.8 mA
IThreshold
m=0
Fit of the model to
the experimental data
using Moses
m=-1
m=-1
m=-1
m=-2
m=-2
f (GHz)
-40
-20 20 40
10/2/2019
Beam Instability Workshop; Monday,13 March ; Revol Jean-Luc

8. Transverse / Single Bunch
impedance 30
Z ( M ) 20
R/Q
Simulation of the mode coupling with a RW Model
For the ESRF the modeling of the mode merging could be obtained with a Resistive Wall model only!!! 10
f (GHz)
-40
-20 20 40
-10
Imag (Z)
But this resistive wall model
should be much larger than
the theoretical one !!!!
(Factor 10)
-20
Zero Chromaticity
m=0
m=-1
m=-1
m=-1
From mode merging, We cannot conclude on the broadband shape
BBR + RW
m=-2
m=-2
f (GHz)
-40
-20 20 40
10/2/2019
Beam Instability Workshop; Monday,13 March ; Revol Jean-Luc

9. Transverse / Single Bunch20 30
-20
-10 10
-40
-20 20 40
With the negative chromaticity
Mode 0 is strongly unstable
Thanks to the amplitude dependant tune spread
0.8 mA can be stored independently of the chromaticity
f (GHz)
Mode 0 shift with negative chromaticity
v = -0.5
v = -0.8
v = 0
-40
-30
-20
-10 10 20 30 40 50
ESRF reduced chromaticity should be multiplied by 14.39
Maximum interaction
at v = -0.6
10/2/2019
Beam Instability Workshop; Monday,13 March ; Revol Jean-Luc

10. Transverse / Single Bunch20 30
-20
-10 10
-40
-20 20 40
f (GHz)
Mode 0 shift with negative chromaticity
v = -0.5
v = -0.8
v = 0
@ Measurement of Growth rate in the negative chromaticity
-40
-30
-20
-10 10 20 30 40 50
Maximum interaction
at v = -0.6
Stabilization by amplitude dependent tune spread induces a saw-tooth effect
10/2/2019
Beam Instability Workshop; Monday,13 March ; Revol Jean-Luc

11. Transverse / Single Bunch20 30
-20
-10 10
-40
-20 20 40
f (GHz)
Growing Time
Mode 0 shift with negative chromaticity v
v = -0.8
v = -0.5
v = 0
-40
-30
-20
-10 10 20 30 40 50
At high chromaticity
==> No contribution at low frequency of the unstable mode
==> need of a high frequency pick-up
Maximum interaction
at v = -0.6
10/2/2019
Beam Instability Workshop; Monday,13 March ; Revol Jean-Luc

12. Transverse / Single Bunch20 30
-20
-10 10
Probing the real part of the impedance at negative chromaticity ==> no effect coming from the RW
-40
-20 20 40
f (GHz)
Expected trend
from simulation
with a BBR
Fr = 22 GHz R/Q= 13.5 M
Growing Time
Mode 0 shift with negative chromaticity v
v = -0.8
v = -0.5
v = 0
The too long observed growing time (factor 4) is in favor or Reducing the strength of the BBR (R/Q)
obtained from mode merging
(compensated by a contribution from the RW)
-40
-30
-20
-10 10 20 30 40 50
Maximum interaction
at v = -0.6
10/2/2019
Beam Instability Workshop; Monday,13 March ; Revol Jean-Luc

13. Transverse / Single Bunch20 30
-20
-10 10
An attempt was made to
measure head-tail damping as a function of the single bunch current
In the positive chromaticity regime
-40
-20 20 40
f (GHz)
Despite our efforts, no valid data could be obtained
Many difficulties in practice due to :
@ Chromatic modulation
@ Amplitude dependent tune spread
Mode 0 shift with negative chromaticity
v = 0
v = -0.5
v = -0.8
-40
-30
-20
-10 10 20 30 40 50
Maximum interaction
at v = -0.6
10/2/2019
Beam Instability Workshop; Monday,13 March ; Revol Jean-Luc

14. Transverse / Multibunch30 -5 -4 -3 -2 -1 1 2 3 4 5
-10 10
f (GHz)
* frf
Z ( M )
Resistive Wall
Impedance 20
Transverse multibunch
instabilities are
associated to narrow band impedance 10
-40
-20 20 40
-10
Multibunch beam spectrum
-20
Zero Chromaticity
-1.4
-1.05
-0.7
-0.35
0.35
0.7
1.05
1.4
f (GHz)
m=0
n = 991
1.75 -4 -3 -2 -1 1 2 3 4
* frf
m=0
Coupled bunch number = 0…. 991
m=-1
m=-1
m=-1
m=-2
m=-2
f (GHz)
-40
-20 20 40
10/2/2019
Beam Instability Workshop; Monday,13 March ; Revol Jean-Luc

15. Transverse / Multibunch30
Z ( M ) 20
-10 -5 5 10 -4 -3 -2 -1 1 2 3 4
* frf 10
-40
-20 20 40
-10
-20
Chromaticity
0.3
m=0
m=-1 -4 -3 -2 -1 1 2 3 4
* frf
n = 991
Fs spacing between the two lines
m=0
m=-1
m=-1
m=-1
m=-2
m=-2
f (GHz)
-40
-20 20 40
10/2/2019
Beam Instability Workshop; Monday,13 March ; Revol Jean-Luc

16. Transverse / Multibunch
Observation in uniform filling
Transverse / Multibunch
991
mode 0
mode -1
Red: Chi V =0.3
Blue: Chi V =0.35 Fs
20 dB
Mode 991
Only two unstable modes
991
990 F0 2
30 db
Chiv=0.3 Fo
Uniform Filling
990 1 2 3 4 5 6 7 8 9 10 11 12 F0 F0 F0 F0 F0 F0 F0 F0 F0 F0 F0 F0
Chromaticity
0.3
10/2/2019
Beam Instability Workshop; Monday,13 March ; Revol Jean-Luc

17. multi: n = 991 I= 50 mA ==> single : m=0
Uniform filling
Transverse / Multibunch
Coupled bunch mode 991 threshold
Mode -2
991
Mode -1
Mode 0
multi: n = 991 I= 50 mA ==> single : m=0
0.15
Vertical emittance
blow up
20 dB Threshold
In uniform filling, RW is a rather smooth instability,
what is the criteria for the threshold ???
10/2/2019
Beam Instability Workshop; Monday,13 March ; Revol Jean-Luc

18. Transverse / Multibunch
Black is the measured curve
10/2/2019
Beam Instability Workshop; Monday,13 March ; Revol Jean-Luc

19. Transverse / Multibunch
Resistive wall only, largely underestimates the threshold curve
Fit of the threshold curve of mode 991 in uniform filling
==> simulation of the interaction of the beam spectrum with an impedance
10/2/2019
Beam Instability Workshop; Monday,13 March ; Revol Jean-Luc

20. Transverse / Multibunch
Resistive wall only, largely underestimates the threshold curve
Fit of the threshold curve of mode 991 in uniform filling
==> simulation of the interaction of the beam spectrum with an impedance
The addition of a broadband contribution helps to increase the threshold which gives more damping with the increased chromaticity
The threshold at zero chromaticity is not sensitive to BBR.
 b = 8 mm is deduced.
Good agreement with beff = 7.3 mm from the evaluation of resistive wall components from the low gap ID vessels
Which is also in favour of reducing the shunt impedance obtained with the mode merging
The obtained combination of BBR+RW
applied to mode-merging calculation gives a rather good reproduction
Keeping fres to 22 GHz
==> R/Q = 6.8 M
is needed for the fit
10/2/2019
Beam Instability Workshop; Monday,13 March ; Revol Jean-Luc

21. Transverse / Multibunch30
Z ( M ) 20 10
f (GHz)
-40
-20 20 40
-10
Broadband impedance
strongly helps
stabilize the narrow band resistive wall impedance
-20
Chromaticity
0.25
m=0
m=-1
m=-1
m=-1
m=-2
m=-2
f (GHz)
-40
-20 20 40
10/2/2019
Beam Instability Workshop; Monday,13 March ; Revol Jean-Luc

22. Transverse / Multibunch30
Z ( M ) 20 10
No sign of transverse HOM induced by the cavity at ESRF. (Narrow band impedance )
What is the mechanism to cure transverse HOM?
In 97 the installation of a new pair of cavities modified neither the SINGLE BUNCH dynamics nor the MULTIBUNCH TRANSVERSE dynamics
f (GHz)
-40
-20 20 40
-10
-20
Chromaticity
0.25
m=0
m=-1
m=-1
m=-1
m=-2
m=-2
f (GHz)
-40
-20 20 40
10/2/2019
Beam Instability Workshop; Monday,13 March ; Revol Jean-Luc

23. Transverse / Multibunch30
Z ( M ) 20 10 10 5
f (GHz)
-40
-20 20 40 n n
+ frf n
- frf
-10 -5
-20
-10
Chromaticity
0.25
Broadband impedance
strongly helps
stabilize the narrow band transverse HOM instabilities
m=0
m=-1
m=-1
m=-1
m=-2
m=-2
f (GHz)
-40
-20 20 40
10/2/2019
Beam Instability Workshop; Monday,13 March ; Revol Jean-Luc

24. Transverse / Multibunch30
What is the status between impedance modeling and measurement with beam??
At ESRF a new modeling campaign is underway
Z ( M ) 20 10
f (GHz)
-40
-20 20 40
-10
Chromaticity is a very efficient tool to cure transverse instabilities.
Nevertheless, the impact is strong on the LIFETIME
mainly for single bunch.
-20
Chromaticity
0.25
m=0
m=-1
m=-1
m=-1
m=-2
Other transverse impedance effects at ESRF ???
m=-2
f (GHz)
-40
-20 20 40
10/2/2019
Beam Instability Workshop; Monday,13 March ; Revol Jean-Luc

25. Transverse / Multibunch
What is the status between impedance modeling and measurement with beam??
At ESRF a new modeling campaign is underway
986
Uniform 124 mA Vertical
Chih = 0.2 ChiV =0.19
Bump at 5 F0
991
990
Resistive wall
Chromaticity is a very efficient tool to cure transverse instabilities.
Nevertheless, the impact is strong on the LIFETIME
mainly for single bunch.
Other transverse impedance effects at ESRF ???
10/2/2019
Beam Instability Workshop; Monday,13 March ; Revol Jean-Luc

26. Transverse / Multibunch
Experiments performed showed a
STRONG STABILIZING EFFECT OF
PARTIAL FILLING OF THE RING
ON THE RESISTIVE WALL INSTABILITIES
* and also of the “bump”
Even a quasi uniform filling was more stable
m = 0 and n =991
Resistive Wall effect
“Bump” effect
Partial Filling Threshold
Is stabilization coming from longitudinal?
In uniform, the use of the RF modulation for longitudinal Landau damping
did not change the threshold
50 mA Multibunch
10/2/2019
Beam Instability Workshop; Monday,13 March ; Revol Jean-Luc

27. Transverse / Multibunch
Large incoherent betatron tune shifts observed are suspected of coming from an ASYMMETRY of resistive wall chamber cross sections.
Horizontal
Qs
Is stabilisation due to an intra-beam tune spread arising from the current dependent tune shift resulting from the differently populated bunches ??
Vertical
Quantification of this effect is on the way
(R. Nagaoka et al).
10/2/2019
Beam Instability Workshop; Monday,13 March ; Revol Jean-Luc

28. Brutal apparition of the instability at 0.08
Transverse / Multibunch
Partial filling pattern 2*1/3
ChiV = 0.08
200 mA
could be reached in 2*1/3
with a chromaticity of 0.08
instead of 0.4 (operation)
and with a low emittance of 14 pm
(at 0,07, the beam exploded)
@ Feedback in 1/3 did not work.
Brutal apparition of the instability at 0.08
ChiV = 0.08
Stabilization of all the lines by a 2 modes feedback
(additional lines are induced by the filling pattern)
Mode 991 and 990 Feedback ON
Vertical emittance = 13 pm
Brutal apparition of the instability at 0.03
(Feedback ON)
ChiV = 0.03
2 F0
12 F0
I = 50 mA
10/2/2019
Beam Instability Workshop; Monday,13 March ; Revol Jean-Luc

29. To get low emittance. 200 mA in uniform at V = 0.1 instead of 0.5
Transverse / Multibunch
200 mA in uniform at V = 0.1 instead of 0.5
200 mA
could be reached in 2*1/3
with a chromaticity of 0.08
instead of 0.4 (operation)
and with a low emittance of 14 pm
(at 0,07, the beam exploded)
@ Feedback in 1/3 did not work.
But still a lot of lines to stabilize to reach the low emittance !!
Coming from the 5 f0 bump??
How many modes should we feedback ??
To get low emittance.
Is it achievable with a bunch by bunch feedback??
10/2/2019
Beam Instability Workshop; Monday,13 March ; Revol Jean-Luc

30. To get low emittance. Single bunch transverse feedback
Transverse / Single bunch
Transverse / Multibunch
Single bunch transverse feedback
is working on mode merging at
ESRF
Feedback allows to increase the current by a factor 5 !!
==> 0.7 * 5 = 3.5 mA !!!!
With Empirical setting of the feedback parameters (most probably resistive)
200 mA
could be reached in 2*1/3
with a chromaticity of 0.08
instead of 0.4 (operation)
and with a low emittance of 14 pm
(at 0,07, the beam exploded)
@ Feedback in 1/3 did not work.
But in single bunch,
this performance is not competitive
with the chromaticity
17 mA at 0.9
(and bunch lengthening)
How many modes should we feedback ??
To get low emittance.
Is it achievable with a bunch by bunch feedback??
10/2/2019
Beam Instability Workshop; Monday,13 March ; Revol Jean-Luc

31. Feedback in single bunch at higher chromaticity
mode m
At 3 mA:
Without feedback, strong instability at v = 0.3 which could be damped by the feedback
6 mA:
Could be reached with feedback, at v = 0.3 (instead of 0.5 without)
Then we get two strong unstable modes which could not be damped independently
(empirical setting of the phase to optimize the damping of both)
3 mA
134 kHz
mode m
mode m-1
Feedback is less and less efficient with increased chromaticity
and the emittance is strongly affected
A feedback acting independently on two single bunch modes is under design
6 mA
132 kHz
10/2/2019
Beam Instability Workshop; Monday,13 March ; Revol Jean-Luc

32. Conclusion
WG1
WG2
Why is the ESRF longitudinal turbulent regime stable?
What are the experimental methods to probe the machine impedance?
What is the comparison between experimental model and simulation?
How far, RW should be considered in single bunch ?
How far, BBR should be considered in multibunch?
What is the multibunch transverse stabilizing mechanism in partial filling?
What is the single bunch dynamics at high chromaticity?
Could we use an harmonic cavity to increase the single bunch intensity threshold?
What is the limitation of the transverse feedback in single bunch?
How many modes should we consider for a transverse feedback in multibunch?
Is transverse feedback compatible with very low emittances ?
And much more questions will be asked during the working group discussions !!!!
10/2/2019
Beam Instability Workshop; Monday,13 March ; Revol Jean-Luc