1. Kicker and RF systems for Damping Rings
CLIC Technical Committee
Kicker and RF systems for Damping Rings
Fanouria ANTONIOU, Mike BARNES, Tony FOWLER, Alexej GRUDIEV, Yannis PAPAPHILIPPOU
February3rd, 2009

2. Design optimisation for CDR (2010) M. Korostelev, PhD thesis, 2006
Outline
CLIC damping rings (DR) design goals and challenges
Design parameters’ evolution
Lattice choice, optics revision and magnet design
Wiggler design and power absorption
Non-linear dynamics
Low emittance tuning
e-cloud and other collective effects (IBS)
Diagnostics
CLIC DR activities
Summary
CLIC parameter note 2005
CLIC parameter note 2008
Design optimisation for CDR (2010)
M. Korostelev, PhD thesis, 2006
03/02/09
CTC, YP

3. CLIC damping ring layout
M. Korostelev, PhD thesis, EPFL 2006

4. DR bunch structure and timing
CLIC DR timing parameters
Old
(2005)
New
(2007)
Repetition rate [Hz]
150 50
Number of bunches
110
312
Number of trains 4 1
Bunch spacing [ns]
0.533
0.500
Revolution Time [μs]
1.2
>1.2
Machine pulse [ms]
6.67 20
H/V/L damping times [ms]
2.8/2.8/1.4
1.5/1.5/0.76
Reduction of repetition rate from 150 to 50Hz leaves enough time for the emittances to reach their equilibrium
Bunch spacing increased almost to same level as for the interleaved train scheme
Interleaved train scheme abandoned
Extraction kicker rise time relaxed
Injection and extraction process simplified
312 bunches with 0.5ns spacing, fill 13% of the rings 4

5. DR injection/extraction optics
Injection and extraction system placed at the same area, upstream of the super-conducting wigglers
Kickers placed at maximum beta functions for minimum deflection angle
Septa and kickers share the same cell
Additional cells to be added in order to increase available space for elements and protection system
Phase advance between injection (extraction) septa and kickers of around π/2
M. Korostelev, PhD thesis, EPFL 2006
03/02/09
CTC, YP

6. Septa parameters
DR septa parameters
SEP-1
SEP-2
Effective length [m]
0.4
0.5
Bending angle [mrad] 13 42
Field integral [T.m]
0.11
0.34
Blade thickness [mm] 5
Same parameters for inj/ext elements due to optics mirror symmetry
Septum parameters are scaled from NLC damping rings
Two DC modules with blade thickness of 5 and 13mm
Effective length can be increased to 2m if additional cells are added
Larger septa blade thicknesses and smaller peak field

7. Septa and kicker parameters
Rise and fall time significantly increased
Effective length can be increased to 2m
Smaller peak field
Kicker stability refers to field uniformity and pulse-to-pulse stability
Tolerance of 0.1σ beam centroid jitter
DR kicker parameters
Old
New
Rise and fall time [ns] 25
1000
Flat top [ns]
142
~160
Repetition rate [Hz]
150 50
Effective length [m]
0.4
0.4-2
Aperture [mm] - 20
Kick [mrad]
2.45 3
Field [Gauss]
500
<610
Kicker stability
inj
ext
Second kicker in transfer phase advance of π for jitter compensation (as in NLC)
Third kicker, delay line and RF deflector are removed
Kickers’ impedance issues should be adressed during the design (budget of a few MΩ/m in transverse and a few Ω in longitudinal)

8. DR injection/extraction lattice
Most critical the e+ PDR
Injected e+ emittance ~ 2 orders of magnitude larger than for e-, i.e. aperture limited if injected directly into DR
PDR for e- beam necessary as well
A “zero current” linac e- beam (no IBS) would need ~ 17ms to reach equilibrium in DR, (very close to repetition time of 20ms)
PDR main challenges
Large input momentum spread necessitates large longitudinal acceptance for good injection efficiency
Polarised positron stacking time long compared to repetition rate (need fast damping and/or staggered trains)
PDR Extracted Parameters
CLIC
NLC
Energy [GeV]
2.424
1.98
Bunch population [109]
7.5
Bunch length [mm] 10
5.1
Energy Spread [%]
0.5
0.09
Hor. Norm. emittance [nm]
63000
46000
Ver. Norm. emittance [nm]
1500
4600
Injected Parameters e- e+
Bunch population [109]
4.4
6.4
Bunch length [mm] 1 5
Energy Spread [%]
0.1
2.7
Hor.,Ver Norm. emittance [nm]
100 x 103
9.3 x 106
03/02/09
CTC, YP

9. Arc and wiggler cell
TME arc cell chosen for compactness and efficient emittance minimisation over Multiple Bend Structures used in light sources
Large phase advance necessary to achieve optimum equilibrium emittance
Very low dispersion
Strong sextupoles needed to correct chromaticity
Impact in dynamic aperture
Very limited space
Extremely high quadrupole and sextupole strengths
FODO wiggler cell with phase advances close to 90o giving
Average β’s of ~ 4m and reasonable chromaticity
Quad strength adjusted to cancel wiggler induced tune-shift
Limited space for absorbers
03/02/09
CTC, YP

10. New arc cells optics Alternative cell based on SUPERB lattice
S. Sinyatkin, et al., BINP
P. Raimondi (INFN-LNF)
Alternative cell based on SUPERB lattice
Using 2 dipoles per cell with a focusing quadrupole in the middle
Good optics properties
To be evaluated for performance when IBS is included
New arc cell design
Increasing space between magnets, reducing magnet strengths to realistic levels
Reducing chromaticity, increasing DA
Even if equilibrium emittance is increased (0 current), IBS dominated emittance stays constant!
Dipoles have quadrupole gradient (as in ATF!).
03/02/09
CTC, YP

11. CLIC DR RF system 1) Main issues: Frequency: 2 GHz
Highest peak and average power
Very strong beam loading transient effects (beam power of ~5 MW during 156 ns, no beam power during the other 1060 ns)
Small stored energy at 2 GHz
High energy loss per turn at relatively low voltage results in big sin φs = 0.95 (see also LEP)
Wake-fields
Pulsed heating related problem (fatigue, …)
2) Recommendations:
Reduce energy loss per turn and/or increase RF voltage
Consider 1GHz frequency (RF system becomes conventional, RF power reduced, but delay loop for recombination is necessary and emittance budget is tight)
A. Grudiev (CERN)
03/02/09
CTC, YP

12. RF for CLIC DR A very first look
Alexej Grudiev

13. Outline CLIC DR energy acceptance Scaling NLC DR RF system to CLIC *
Traveling versus Standing wave system
RF source issue
Conclusions
* Parameters of NLC DR RF are taken from “Collective effects in the NLC damping ring design” T. Raubenheimer ,et. al., PAC95

14. Energy acceptance ±0.8 ±1.7 ±2.6 CLIC DR parameters*
Circumference: C [m]
365.2
Energy : E [GeV]
2.42
Momentum compaction: αp
0.8x10-4
Energy loss per turn: U0 [MeV]
3.9
Maximum RF voltage: Vrf [MV]
4.115
RF frequency: frf [GHz]
2.0
Energy acceptance versus rf voltage
Vrf [MV]
ΔE/E [%]
4.115
±0.8
4.5
±1.7 5
±2.6
* From Yannis CLIC pars WG, 2/10/07

15. Scaling of NLC DR RF cavity
NLC DR RF cavity parameters
Frequency: f[GHz]
0.714
Shunt impedance: R [MΩ] 3
Unloaded Q-factor: Q0
25500
Aperture radius: r [mm] 31
Max. Gap voltage: Vg [kV]
500
Scaled RF cavity parameters
Frequency: f[GHz] 2
Shunt impedance: R [MΩ]
1.8
Unloaded Q-factor: Q0
15400
Aperture radius: r [mm] 11
Max. Gap voltage: Vg [kV]
180
Five 1 MW CW klystrons feeding 5 SW 5-cells accelerating structures would do it.
ηrf-to-beam < 30%
Total length ≥ 2m
Calculated RF cavity parameters
Number of cavities: N
Vrf/Vg ~ 23
Total wall losses: POhm [MW]
Vrf2/2NR ~ 0.2
Peak beam current: Ib [A]
Qb*f ~ 1.3
Peak beam power: Pb [MW]
U0*Ib ~ 5
Loaded Q-factor: Qext
Q0*POhm /Pb~ 620
Filling time: Tf [ns]
Qext/f ~ 310

16. TW versus SW acc. structure
Several fully beam loaded travelling wave accelerating structures with shorter filling time ~20ns could increase efficiency significantly but only at fixed (nominal) current and voltage.
SW structure would require tunable coupler in order to change the loaded Q-factor and maintain efficiency when changing beam current
In summary, both systems are possible

17. RF power source In case of a klystron,
It must be pulsed with DR revolution frequency of ~1 MHz repetition rate in order to maintain efficiency OR
a better option would be to do pulse compression on each turn, this will also reduce peak power requirements on klystrons at the expenses of pulse compression efficiency (~70%) though
And it must have certain bandwidth in order to be able to shorten the filling time to increase efficiency (more of an issue in TWS).
Tf~50ns => df~20MHz => df/f~1%
IOTs are better choice from the point of view efficiency and bandwidth (-> efficiency). But they have less power per tube and lower gain (two stages will be required). An R&D item at 2 GHz.
Solid state rf power amplifier showed 50% efficiency from the plug at 500 MHz (SOLEIL). BUT Efficiency and power at 2 GHz -?

18. Vacuum Electron Device Limitations for High-Power RF Sources
Considerable part of former klystron domain claimed by IOTs
Why?
Frequency (GHz)
Heinz Bohlen,Thomas Grant, CPI

19. Wakefields of the rf system
Loss/kick factors of NLC DR rf cavity for bunch length of 3.3 mm and aperture radius of 31 mm from the reference:
Total loss factor: kl = 1.7 V/pC
Transverse kick factor: kt = 39.4 V/pC/m
Scaling to CLIC DR rf cavity for bunch length of 3.3 mm and aperture radius of 11 mm:
Total loss factor: ~1/d: kl ~ 4.8 V/pC per cell
Transverse kick factor: ~1/d3: kt ~ 873 V/pC/m per cell
Number of cavities (cells) is also higher for CLIC: ~23
In summary: it is ~10 times higher for longitudinal wake and ~100 times higher for transverse wake for the whole rf system
One good thing is that at 2 GHz HOM damping is more compact and could be done more efficient. Q-factor of HOM could of the order of few tens or so.

20. CLIC DR RF system issues
Frequency: 2 GHz
Highest peak power
High average power
Very strong beam loading transient effects:
Peak beam power of ~5 MW during 156 ns
No beam power during the other 1060 ns
Small stored energy at 2 GHz
High energy loss per turn at relatively low voltage results in big sin φs = 0.95 (any examples of operation ?)
Wakefields
Pulsed heating related problem (fatigue, …)

21. Recommendations Reduce energy loss per turn
This will help anyway
Consider frequency reduction down to 1 GHz
It makes rf system a conventional high power rf system (other DRs, B-factories, etc.)
One can take advantage of a superconducting RF system
Reduce beam peak power by 2, so the SR peak power, less pulsed heating, less rf peak power, etc.
It makes life easier for positron capture
BUT
Recombination of bunches is necessary at extraction or in a separate delay loop. Potential impact on the beam emittance must be addressed.

22. Summary
Detailed design of the CLIC damping rings, delivering target emittance with the help of super-conducting wigglers
Prototype to be built and tested at ANKA synchrotron
Radiation absorption protection
Collective effects evaluation including electron cloud and fast ion instability
Lattice revision with respect to space and magnet parameters
Parameter scan for conservative beam emittances for 500GeV collider
Active collaboration with ILC, test facilities, B-factories, synchrotron light sources and other interested institutes
Critical items for the performance of the damping rings
Super-conducting wigglers
E-cloud and fast ion instability
Low emittance tuning
Intra-beam scattering
03/02/09
CTC, YP