1. Operating Experience with Electron Cloud Clearing Electrodes at DAFNE
La Biodola, Isola d’Elba, Italy,
5-9 June 2012
Operating Experience with Electron Cloud Clearing Electrodes at DAFNE
M. Zobov, LNF-INFN, Frascati, Italy
With contributions of
D.Alesini, A.Drago, A.Gallo, S.Guiducci, C.Milardi, A.Stella (LNF-INFN, Frascati);
S.De Santis (LBNL, Berkeley); T.Demma (LAL, Orsay); P.Raimondi (ESRF, Grenoble)

2. OUTLINE DAFNE Collider and e-Cloud Effects Clearing Electrodes
Design
Installation
Beam coupling impedance
Experimental Measurements with Electrodes
Shift and spread of betatron tunes
Growth rate of instabilities
Beam dimension variations
Vacuum chamber HOM frequency shifts
Currents delievered by voltage generators
Conclusions

3. The DANE Collider BTF Energy per beam 510 [MeV] Machine length 97 [m]
Max. beam current
(KLOE run)
2.5(e-) 1.4(e+) [A]
N of colliding bunches
100
RF frequency
[MHz]
RF voltage
200[kV]
Harmonic number
120
Bunch spacing
2.7[ns]
Max ach. Luminosity
(SIDDHARTA run)
4.51032 [cm-2s-1]
BTF

4. anomalous vacuum pressure rise
e-Cloud in DAFNE
The worst case:
Aluminium vacuum chamber
Shortest bunch separation of 2.7 ns
Al wiggler and dipole chamber
anomalous vacuum pressure rise
larger positive tune shift
Very fast horizontal instability not explainable by parasitic HOMs
tune shifts along the bunch train

5. Solenoids Off 28/05/20122
VUGPS203
VUGPL201
VUGI2001

6. Installation of Electrodes
To mitigate the e-cloud instability copper electrodes have been inserted in all dipole and wiggler chambers of the machine and have been connected to external dc voltage generators.
The dipole electrodes have a length of 1.4 or 1.6 m depending on the considered arc, while the wiggler ones are 1.4 m long.

7. Simulation of e-Cloud Suppression
Electric field as computed by POISSON
Simulation of electron cloud evolution using ECLOUD (CERN)
With a dc voltage of V applied to each electrode we expected a reduction of the electron cloud density by two orders of magnitude !
Time (ns)

8. Clearing Electrodes Realization
The electrodes have a width of 50 mm, thickness of 1.5 mm and their distance from the chamber is about 0.5 mm. This distance is guaranteed by special ceramic supports (made in SHAPAL), distributed along the electrodes. This ceramic material is also thermo-conducting in order to partially dissipate the power released from the beam to the electrode through the vacuum chamber. The supports have been designed to minimize the beam coupling impedance and to simultaneously sustain the strip. The distance of the electrode from the beam axis is 8 mm in the wigglers and 25 mm in the dipoles.
The electrodes have been inserted in the vacuum chamber using a dedicated tool allowing the electrode to be inserted without damages the Al chamber. They have been connected to the external dc voltage generators modifying the existing BPM flanges.

9. Electrode Impedance Evaluation
Resistive wall
It is due to a finite conductivity of the electrode. Wiggler case: 112 W/m2.
Such power density would result in electrode heating under vacuum up to C.
Strip-line Impedance
Two extreme cases have been simulated:
Perfectly matched: broad-band impedance, in this case the loss factor can be used for the power loss evaluation. Loss factor (1.6x109 V/C) is a factor 3 higher than that of the resistive walls but part of power is dissipated in the external load
Short-circuited: situation is less predictable. The released power can be much higher in this case if one of the narrow peaks coincides with one of the RF frequency harmonics. Electrode length has been properly chosen. Moreover, we used thermo-conducting dielectric material as supports (SHAPAL).

10. Impedance (Simulation and Measurements)
Numerical Simulations
Bench Measurements
RF harmonics

11. Bunch Length Measurements
electrons
positrons
Surprisingly, the impedance (at least, its inductive part) seems to be somewhat higher for the electron ring where there are no electrodes!

12. Horizontal Tune Shift I+  550 mA
The frequency shift of the horizontal tune line is
 +20 kHz switching off all the electrodes.
This corresponds to a difference in the horizontal tune of 

13. Horizontal Bunch-by-Bunch Tune Spread Measured by the Feedback System
OFF ON
OFF
DAFNE e+ beam: 100 bunches, spaced by 2.7ns with 20 buckets gap Turning on the electrodes in 4 wigglers and 2 dipoles (not all) horizontal tune spread decreases ON

14. ..the effect of the electrodes is seen also on the vertical plane...
OFF

15. Vertical Size Variation
All
off
3 on
5 on
8 on
10 on
12 on
Vertical beam size enlargement is clearly observed on the Synchrotron Light Monitor while turning off the clearing electrodes progressively

16. Horizontal Instability Growth Rate Measurements Using Bunch-by-Bunch Feedback
Applied voltages were
0V, 70V and 140V
Mode 0 -1
Mode = -1 is unstable

17. Vacuum Chamber HOM Shifts: Measurement Setup
The e-cloud plasma can interact with RF waves transmitted in the vacuum chamber changing the phase velocity of such waves. A similar approach can be used in case of resonant waves in the vacuum chamber. Even in this case the e-cloud changes the electromagnetic properties of vacuum and this can result in a shift of the resonant frequencies of vacuum chamber trapped modes.
Transmission coefficient between two buttons
Button pickups
Beam OFF
Resonant TE-like modes are trapped in the DANE arcs and can be excited through button pickups. The lower modes have frequencies between 250 and 350 MHz.
Beam power spectrum lines
Beam ON

18. Vacuum Chamber HOM Shifts: results
The analysis of data, up to now, gave the following results:
all modes have a positive frequency shift as a function of the positron beam current and, with 800 mA, it is between 100 and 400 kHz depending on the modes we are considering;
for almost all modes we can partially cancel the frequency shift switching on the electrodes;
the quality factor of the modes decreases with positron current;
The fact that for some modes the shift does not depend on the electron voltage could depend by the fact that these modes are localized in different places of the arc and also in regions not covered by electrodes.
In principle, from these shifts it is possible to evaluate the e-cloud density applying the formula given in [J. Sikora et al., MOPPR074, IPAC12]
An identification of resonant mode location is still in progress and is not trivial due to the complex 3D geometry of the arc chamber

19. Current delivered by voltage generators
The voltage generators connected to the electrodes absorbs the photo-electrons.
In the present layout one voltage generator is connected to three electrodes of one arc (i.e. one wiggler and two dipoles).
The current delivered by the generator has been measured as a function of the generator voltage and for different beam currents.
Possible explanation???
Current supplied by the generator IVDCne-
e-cloud density ne-IB-VDC.
Combining the two previous relations we obtain that IVDCIB-V2DC,
The e-cloud is completely absorbed when I0. In all other situations there is still an e-cloud density. Fitting these curves and scaling their behaviour up to currents >1A, one discover that a voltage of the order of 250 V is no longer adequate to completely absorb the e-cloud when IB>1A. So the applied voltage has to be increased.

20. ... also during the current DAFNE run we have exceeded 1 A in the positron ring...

21. Conclusions Acknowledgment
Metallic e-cloud clearing electrodes have been inserted in all the dipole and wiggler mangets and are used now in routine operations of the DAFNE collider.
They are found to be very useful to reduce the horizontal positron beam instability strength; to decrease the betatron tune shift and tune spread inside the bunch trains; to suppress the vertical size blowup of the positron beam.
As the result, with the electrodes switched on it is possible to store higher positron beam current, to achieve higher luminosity and to have more stable overall collider performance.
Acknowledgment
The research leading to these results has received funding from the European Commission under the FP7 project HiLumi LHC, GA no , co-funded by the DoE, USA and KEK, Japan and from the European Commission FP7 Program EuCARD, WP11.2,GA no