Testing with beams in CTF3: breakdown kick and advanced diagnostics

1. Testing with beams in CTF3: breakdown kick and advanced diagnostics Testing with beam in CTF3 - W. Farabolini

2. Contents • Two Beam test stand equipments and tools • Beam used for structures RF diagnostics • Energy gain / spread measurement and optimization • RF power measurements • BD detection • Test bench for beam diagnostics • Wake Field Monitors study • Beam kicks study • Beam shape distortion and multipolar field modeling • Conclusion Testing with beam in CTF3 - W. Farabolini

3. The Two Beam Test Stand Spectrometer lines Drive beam (24 Amps) Variable phase shifters & On/Off mechanisms Correctors Wake Field Monitors Dipoles Quadrupoles RF couplers Probe beam (1 Amp) Germana Riddone Ion analyzer Screens BPMs FCU PMTs Water thermal probes and flow meters Franck Peauger - IRFU Testing with beam in CTF3 - W. Farabolini

4. Operational models for beam optimization From Quad scan… For beam focusing: Current in quadrupoles -> beam enveloppe. For beam trajectory: Current in correctors -> beam position on BPMs … to beam optimization. Testing with beam in CTF3 - W. Farabolini

5. Tuning frequency validation of the structures LO = 11994.2 - 10 MHz LO = 11994.2 MHz LO = 11994.2 - 1 MHz LO = 11994.2 + 2 MHz LO = 11994.2 + 10 MHz LO = 11994.2 + 1 MHz • RF output generated by a short beam pulse (3 ns: 5 bunches) is down-mixed with a local reference oscillator -> structure resonant frequency . • Nominal tuning: 11.9942 GHz checked (accuracy < 1 MHz). Testing with beam in CTF3 - W. Farabolini 5

6. RF production with longer pulses Pulse 150 ns LO = 11894.2 MHz Pulse 194 ns LO = 11994.2 MHz Extracted Faraday cup acts as a button pick-up • RF output frequency forced by the probe beam pulse frequency • RF output rising time = ACS filling time (65 ns) • RF output rising time + sustain time = pulse length • RF output falling time = ACS filling time (65 ns) • Delays between the (RF couplers , Faraday cup, BPMs, PMTs, WFMs) -> instrumentation calibration Testing with beam in CTF3 - W. Farabolini

7. Energy gain measurement • Accurate measurement of the energy gain despite CALIFES beam energy fluctuations. • Double pulsing method for energy gain lower than 30 MeV • Accelerated /non-accelerated beam -> dipole strength to be adapted • Califes beam energy fluctuation +/- 2 MeV , period around 150 s (temperature oscillations ?) • Sinusoidal function fit -> valid at least during 30 minutes Testing with beam in CTF3 - W. Farabolini

8. Energy gain optimization • PETS On/Off mechanism • Timing between drive beam pulse and probe beam pulse. • 2 phase shifters (RF/ probe beam phase and inter-structures phase) Drive beam and probe beam detected by PMT RF power control Structures phase in opposition • Inter-structures phase shifter position set for no acceleration whatever Drive Beam / Califes phase. • This phase is then shifted by 180 deg -> accelerating crest. Testing with beam in CTF3 - W. Farabolini

9. Energy gain as function of RF power check Power fluctuations Phase scan Energy gain lower than the nominal one -> uncertainties in the calibration of the RF chains ? • Califes / Drive beam phase scanned over 360 deg of 12 GHz Testing with beam in CTF3 - W. Farabolini

10. Thermal method for RF power measurements Finite differences thermal model of structure and cooling circuit. 0.02 oC Water cooling circuit • Inlet/outlet water temperature difference -> mean RF power deposited • 10 % discrepancy factor found (power overvalued by the RF couplers) Testing with beam in CTF3 - W. Farabolini

11. Reviewed power and energy spread Structures performances much closer to the nominal Energy spread maximal at the zero crossing due the phase extension of the bunch on the 12 GHz period -> bunch length measurement method. Testing with beam in CTF3 - W. Farabolini

12. Reliable breakdown detection on 2 ACSs • Two criteria used: Reflected Power and Missing Energy • Miss = Enerin – Enerout x attenuation • Data are post processed with adaptative thresholds. • Thresholds = mean + 3.72 s • [ PGauss(X>3.72s) = 10-4] • Compromise between Detection prob. and False Alarm prob. • A BD sometime triggers the other structure BD. • Reflected power and Missing energy are data logged for each RF pulse • Faraday cup and Photomultiplier tube activity also used to confirm BD Testing with beam in CTF3 - W. Farabolini

13. Test bench for beam diagnostics Rui Pan (PhD student), Electro-0ptical Bunch Profile Measurement at CTF3 IPAC’13 MOPME077. Inside CLEX optical tables for laser beam injection F. Cullinan (PhD student), J. Towner A Prototype Cavity Beam Position Monitor for the CLIC Main Beam, IBIC'12 MOPA18 Position and beam charge linearity Sophie Mallows(PhD student),A fiber Based BLM System Research and Development at CERN, HB2012 THO3C05  Testing with beam in CTF3 - W. Farabolini

14. Wake Field Monitors as BPMs F. Peauger - IRFU WFM signals from a PB pulse 18 GHz on diodes 0.12 mm 24 GHz on log detectors WFM signals without 12 GHz RF power WFM signals with RF power • Two types of WFM installed on the structures : (HOMs: 18 GHz and 24 GHz). • Resolution already better than 20 mm. • First successful results: realignment of the ACSs tank. • Robustness with nominal 12 GHz RF power (42 MW) still under investigation Testing with beam in CTF3 - W. Farabolini

15. Breakdown beam kick studyPhD research of A. Palaia Screen MTV 790 cavity BPM CA.BPM0745V 0.68 mm 0.75 mm w/o BD With BD • Average measured kick to the beam orbit : 29 +/- 14 keV • Kicks angle measured not isotropic, not clear why Testing with beam in CTF3 - W. Farabolini

16. Beam observed on MTV0790 • Beam kicks during acceleration observed, especially when beam is passing off-axis through the 12 GHz structures. • Beam shape can also be distorted accelerated non accelerated Hor. Position [mm] Vert. Pos. [mm] Non-accelerated (left) and accelerated (right) beam shapes observed on the straight line screen, 4.75 m downstream the ACS Horizontal beam kick during scan in horizontal positions within the ACSs Testing with beam in CTF3 - W. Farabolini

17. Observation of octupolar shapes • Used of a non-focused beam to fully observe beam shape distortion -> full structure aperture covered (4.7 mm bore diameter). • The octupolar beam shape changes from positive to negative at the RF crest phases. On crests (accelerating or decelerating) At zero-crossing (rising RF power side), 25 MW At zero-crossing (falling RF power side) Without RF power Testing with beam in CTF3 - W. Farabolini

18. Modeling of the octupolar fields A. Grudiev Dipolar field Quadrupolar field Panofsky-Wenzel (PW) theorem or Sextupolar field Lorenz Force (LF) Comparison b(4) @Vx=1V LF: 0.17 +3.23i [mTm/m2] PW: 0.22 +3.22i [mTm/m2] Octupolar field Testing with beam in CTF3 - W. Farabolini

19. Consequences A. Grudiev for Vz = 22.8 MV; Pin = 46.5 MW Beam spots in the structure Beam spots on the screen Testing with beam in CTF3 - W. Farabolini

20. Conclusion • A facility with a well controlled beam and a full set of diagnostics is an important tool for testing RF structures. • In addition it attracts many users and PhD students who develop innovative diagnostics. • But of course it requires significant resources for operation and maintenance. I would like to thank all of them, CERN staff and collaborators, for their constant effort in running CTF3. Testing with beam in CTF3 - W. Farabolini

21. Testing with beam in CTF3 - W. Farabolini

22. Detail of the computations A. Grudiev Accelerating gradient: Accelerating voltage: Multipole expansion in vacuum only: Panofsky-Wenzel (PW) theorem: Gives an expression for multipolar RF kicks: Lorenz Force (LF): Gives an expression for kick directly from the RF EM fields: Which can be decomposed into multipoles: Equating the RF and magnetic kicks, RF kick strength can be expressed in magnetic units: Testing with beam in CTF3 - W. Farabolini