1. ‘Collective effects’ activities at GSI and TUD as part of the EU-design study ‘DIRACsecondary beams’
A. Al-khateeb, O. Chorniy, R. Hasse, V. Kornilov, O. Boine-F. (GSI-HSSP), U. Blell (GSI-BEN)
P. Hülsmann (GSI-HF), E. Arevalo, B. Doliwa, W. Müller, Th. Weiland (TUD-TEMF)
Collective instabilities and ‘impedance budget’ (GSI):
Longitudinal studies and rf cycle optimization: LOBO code and ESR/SIS experiments
(talk by O. Boine-F.)
Transverse instabilities with space charge: Analytic work, PATRIC code, SIS experiments
(talks by V. Kornilov and O. Boine-F.)
Feedback schemes and experimental verification
Ring impedances (TEMF/GSI):
Analytic work on longitudinal/transverse resistive wall impedances and shielding
(talk by A. Al-khateeb)
3D simulation studies and parameterized impedance models for kickers and collimators
(talk by B. Doliwa)
SIS 18/100 impedance library

2. Reference cases for the SIS 18/100 ‘impedance budget’ and the role of space charge
SIS 18/100 rf cycle
space charge tune shift:
Longitudinal space charge
parameter:
Reference case
a) accumulation
U28+ reference cases for SIS 100:
Reference case b) pre-compression
U73+ reference cases for SIS 18:
Reference case c) compressed bunch

3. Control of collective effects SIS 18/100 challenges
Why can’t we simply extrapolate stability limits from existing (proton) machines ?
Low momentum spreads needed for bunch compression:
Tolerable longitudinal phase space dilution factor < 3.5
‘Loss of Landau’ damping due to space charge tune shifts
‘Thick beams’: E.g. aperture filling factor of 60 % in SIS 100.
Low frequencies (and energy): Revolution frequencies 216 kHz (SIS 18), 150 kHz (SIS 100)
Resistive thin pipe walls (fast ramping synchrotrons) in the dipoles: 0.03 mm
Densely distributed Ferrite loaded kickers and rf cavities (Ferrite and MA).
Distributed combined pumping/collimation system
Goals: estimation of impedance budgets, reliable impedance models, determine
feedback requirements and schemes, benchmark with SIS/ESR experiments

4. Observation of collective instabilities Longitudinal bunch instabilities with e-cooling in the ESR
Beam parameter: Xe48+, 340 MeV/u, Nb≈108, e-cooling rate ≈10 s-1
Unstable bunch oscillations above approx. 10 mA peak current (∑≈0.3)
Similar observations in the SIS (with e-cooling).
No feedback system in SIS/ESR yet. Double rf system will be installed in SIS 18.
Observed saturated bunch oscillation
Questions:
‘Loss of Landau damping’ ? (Dipole mode: ∑th≈0.1)
Driving impedance ?
Can we increase Landau damping (double rf) ?
Other cures (passive/active feedback,...) ?
(Dedicated ESR beam time in April.)

5. Observation of collective instabilities coasting beam transverse resistive wall instability in SIS 18
beam current vs. time
talk by V. Kornilov
beam parameters (dc):
Ar18+
11.4 MeV/u (injection)
N0=6x1010
∆Qy≈-0.1
Present situation:
Transverse instabilities
are routinely observed
in SIS18 during high intensity
operation with and without
electron cooling.
A digital transverse feedback
system has been installed in order
to damp instabilities in coasting
and in bunched beams.
Presently the system is not able
to damp the instabilities. We plan
to obtain the characteristics of
the feedback system using
transverse BTF measurements.
beam offset vs. time

6. Status of the studies and open questions SIS 18/100 impedance budget and intensity limits
Intensity limitations and ‘impedance budget’:
Detailed studies of Landau damping with space charge.
Simulation codes: LOBO code (longitudinal) and PATRIC (3D, still being benchmarked)
Experimental studies in SIS/ESR on instability thresholds and damping mechanisms.
Impedance studies:
Analytic expressions for resistive wall impedances (with coatings) and shielding.
3D study of the SIS 18 kicker impedance: parameterized model.
To do (next step):
Verification of impedance models and feedback system in SIS 18 (BTF measurements).
3D code benchmarking with dispersion relations and experiments
Questions:
Feedback requirements: Needed if a mode is undamped because of space charge ?
Intensity limitations: Determined by the remaining undamped modes ?
Most relevant instabilities/modes: resistive wall, microwave, headtail-type, e-cloud driven.
Passive feedback to restore Landau damping ?
Restoring Landau damping with double rf wave and octupoles ?
Contribution of the distributed collimator system to the broadband impedance ?
SIS 100 kicker impedances ?

7. Discussion: CERN/GSI joint work First ideas
Impedances:
Comparison of resistive wall formulas for SIS 18/100 and LHC.
Comparison of kicker impedance models (CERN MKE kickers and SIS 100 kickers).
Broadband impedance model for SIS 18/100 and CERN rings.
......
Code development and benchmarking:
Define common benchmarks for HEADTAIL and PATRIC with space charge and impedances.
.....
Intensity limits and related studies:
‘Impedance budget’ with space charge effects: Role of the ‘loss of Landau damping’
Restore Landau damping: double rf, octupoles, inductive inserts, active feedback...
Emittance growth below instability thresholds.
E-clouds induced instabilities (in connection with space charge) in SIS 100 and LHC
Experiments:
Longitudinal BTF (and Schottky) with space charge and double rf (PSB)
Headtail-type instabilities with space charge (PS, SPS)
Verification of thresholds for ‘loss of Landau damping’ longitudinal and transverse (SIS, PS).

8. Discussion: more ideas