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RF limitations while running at high intensities in the injectors

RF limitations while running at high intensities in the injectors. Collected by E. Jensen, BE-RF. Introduction. RF limitations: Voltage necessary to create a certain accelerating bucket area – this becomes often a power limit

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RF limitations while running at high intensities in the injectors

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  1. RF limitations while running at high intensities in the injectors Collected by E. Jensen, BE-RF ATOP days 4.-6. 3. 2009 RF limitations

  2. Introduction • RF limitations: • Voltage necessary to create a certain accelerating bucket area – this becomes often a power limit • Beam loading (BL) will increase the power needs by (at least) the power transferred to the beam (good for higher efficiency) • But BL also leads to an induced voltage that interacts with the various beam and cavity control loops, which may become unstable. • High intensity beams will get closer to a number of stability limits (or instability thresholds). This is RF related since the system dealing with these are often RF systems. ATOP days 4.-6. 3. 2009 RF limitations

  3. Linac 2 ATOP days 4.-6. 3. 2009 RF limitations

  4. Linac 2 • Linac 2 can produce 180 mA (190 mA in TRA10 were obtained in MD’s in 1994) • It produces regularly 150 – 170 mA for all requested intensity ranges. • Rien à signaler … M. Vretenar ATOP days 4.-6. 3. 2009 RF limitations

  5. PSB ATOP days 4.-6. 3. 2009 RF limitations

  6. PSB RF systems (for each ring) + transverse damper • The PSB regularly produces very high intensity beams for Isolde, (3.5 · 1013) not limited by the RF systems. Krusche, Paoluzzi: http://cern.ch/AccelConf/e98/PAPERS/TUP03H.PDF ATOP days 4.-6. 3. 2009 RF limitations

  7. PSB RF limitations at high intensity • C04 power limitations (for faster cycle) • C04: “High Loss Mode” (not intensity related) • Instability at low C04 voltage and high BL • “Ring 4 problem” (transverse plane) • C02 beam loading ATOP days 4.-6. 3. 2009 RF limitations

  8. PSB: Power limitations • During the tests for 900 ms cycling (2005), power limits were observed in the air-cooled C04 system. • It was concluded that – for reliable operation with 900 ms – the C04 system would require a fundamental upgrade (water-cooling). For even shorter cycles, this would become mandatory. Haase, Paoluzzi: http://cdsweb.cern.ch/record/877814/files/ab-note-2005-037.pdf ATOP days 4.-6. 3. 2009 RF limitations

  9. PSB C04: “High Loss Mode” Limit (for completeness – not intensity related) • The C04 were constructed from the old C08 system – the ferrites are not optimal. • When too long at a fixed frequency, the ferrites go into “High Loss Mode”. • This figures shows the time for the losses to increase by 20%. Paoluzzi: https://edms.cern.ch/file/593255/1/RFN2000-013.pdf ATOP days 4.-6. 3. 2009 RF limitations

  10. PSB: Instability at low C04 voltage and high BL In MD’s in 2007 and 2008, a stability threshold was observed when increasing the beam-loading (BL) by lowering the C04 voltage. This leads to phase and amplitude coupling and a complex interplay between C02 and C04 control loops. Above threshold voltage Below threshold voltage Note in preparation: Blas, Findlay, Haase, Paoluzzi, Pedersen ATOP days 4.-6. 3. 2009 RF limitations

  11. Strong detuning by BL Y locus of cavity admittance IG/V seen by tuning loop: φL I0/V cavity tuning angle: φZ IB/V ω If IB /I0 becomes large, small perturbations of IB will have large effects. ATOP days 4.-6. 3. 2009 RF limitations

  12. “Ring 4 problem” • Only in outer rings, mainly ring 4, at high intensity on the flat top, one observes: • shaving of trailing edge of the bunch, • large transverse intra-bunch excursions, • often with synchro-loop transients and during extraction bump (5 ms before extraction), • beam loss. • Problem is still under investigation: • Influence of transverse feedback settings checked, • voltages and loops behaviour checked and excluded • A likely explanation: • There exists a transverse impedance, particularly high in ring 4, which the transverse damper can cope with only marginally. • The additional power demand due to the perturbations (synchro-loop, ejection bump) leads to saturation of the transverse damper amplifier. Blas, Chanel, Findlay, Hanke, Mikulec, Quatraro, Rumolo ATOP days 4.-6. 3. 2009 RF limitations

  13. PSB: Beam loading In the recent “Review on PS Booster with Linac4”, A. Blas investigated the feasibility of 2 · 1013 per ring. He found that • The current limitation of the C02 system would lead to a maximum ΦS of 18°, whereas 22° would be needed for the present cycle. This would indicate an intensity limit of 1.65 · 1013 per ring. • Recent results with the new digital beam control tested in ring 4 are very encouraging. The upgrade to D-LLRF is essential to eventually get closer to this intensity limit ( see M.E. Angoletta’s APC presentation 27/2/09). • Also the beam stability with low C04 voltage/high BL should profit from this upgrade. http://indico.cern.ch/materialDisplay.py?contribId=8&materialId=slides&confId=46255 ATOP days 4.-6. 3. 2009 RF limitations

  14. PS ATOP days 4.-6. 3. 2009 RF limitations

  15. PS RF systems + transverse damper (2 x 6 kW) ATOP days 4.-6. 3. 2009 RF limitations

  16. PS RF limitations at high intensity • Phase drift at low voltages due to beam loading • Over-current at γtrcrossing with high BL • Beam losses at γtr (partly RF related) • Insufficient transient beam-loading compensation for an asymmetrically filled machine • Coupled bunch instabilities above γtr. (For LHC 25 and 50 ns beams). Ok up to nominal. • Bunch lengthening due to residual impedance of 40 and 80 MHz cavities. ATOP days 4.-6. 3. 2009 RF limitations

  17. Phase drift at low voltages due to BL N = 0.7 · 1013ppp N = 1.4 · 1013ppp voltage voltage phase phase radial position radial position beam beam When reducing the voltage, the relative BL gets stronger. It leads to the slow phase drift, which the AC coupled phase loop cannot correct. An intensity dependent phase drift is observed, which in turn acts also on the radial position. H. Damerau ATOP days 4.-6. 3. 2009 RF limitations

  18. Over-current at γtrcrossing with high BL • A phase switch is necessary at γtr. • In the presence of strong BL, this fast phase change requires large power, resulting in a significant, but short increase in anode current, which made the tube protection trip. • In 2008, the tube protection circuit was modified to cope (increased lag time from 500 µs to a few ms) – should be OK now. C. Rossi ATOP days 4.-6. 3. 2009 RF limitations

  19. Beam losses at γtr (RF related ?) Observed: a large excursion of MRP when crossing γtr (and switching RF phase), too fast for the radial loop to correct. Possible explanation: The PU’s see not only the mean radial position, but also the orbit distortions. An additional PU has recently been installed in SS76 – hold your breath for results in 2009! MRP vs. C-timing S. Aumon, S. Giladroni, J. Belleman ATOP days 4.-6. 3. 2009 RF limitations

  20. PS: Coupled bunch instability Growth of a coupled bunch (CB) instability • Observed on LHC type beams. • At present, two C10 cavities are used as longitudinal kicker in the CB feedback; this works OK up to nominal intensity. • Most probable source: the impedance of the C10 cavities themselves. transition beam peak detected Beam PU signal around 5thharmonic, down-converted H. Damerau et al. : http://cdsweb.cern.ch/record/1055555/files/ab-2007-051.pdf ATOP days 4.-6. 3. 2009 RF limitations

  21. PS: Transverse damper • System dimensioned for injection error damping (6 kW per plane) – OK during the cycle. • Somewhat counter-intuitively (for me), higher brilliance (same intensity with smaller ε) will slow down incoherent betatron motion and thus leave more time for the transverse damper before filamentation smears out the transverse beam. A. Blas, G. Rumolo, E. Benedetto ATOP days 4.-6. 3. 2009 RF limitations

  22. PS: Residual impedance of 80 MHz cavities • Bunch length (4 σ Gaussian fit) vs. bunch # using 2 (blue) and 3 (red) 80 MHz cavities. • Bunches near the end of the batch are longer, due to residual impedance of 40/80 MHz systems. • This effect is more pronounced with all 3 cavities. • Bunches # 50+ are shorter due to coupled-bunch feed-back. Damerau: http://cdsweb.cern.ch/record/1141522/files/AB-Note-2008-052-MD.pdf ATOP days 4.-6. 3. 2009 RF limitations

  23. SPS ATOP days 4.-6. 3. 2009 RF limitations

  24. SPS 200 MHz – 4 TW cavities SIEMENS (2 lines ,28 tetrodes) “Philips” (2 lines , 72 tetrodes) Each line feeds one cavity ATOP days 4.-6. 3. 2009 RF limitations

  25. The lines of the TWC200 system SIEMENS, 2 TX per line, 2 lines “Philips”, 2 TX per line, 2 lines … … 1 line sketched 1 line sketched Total voltage (2 cavities): 4.1 MV w/o BL, ≈ 3.3 MV with nominal BL (1.15 1011 ppb). With new tubes: 350 kW/TX Total voltage (2 cavities): 5.5 MV w/o BL, ≈ 4.2 MV with nominal BL (1.15 1011 ppb). With new tubes: 400 kW/TX ATOP days 4.-6. 3. 2009 RF limitations

  26. SPS 800 MHz System is essential at high intensity to cope with the dominant coupled bunch instability! This system is dying! “Valvo” klystrons (YK1198) are very old. These klystrons have not been built any more for decades! Of 16 existing tubes, 10 are broken, 6 are operational. Equally the transformers in their power supplies are at the end of their lifetime. The cavities are OK. We managed “just” (with a lot of personal commitment of some people) to supply the necessary 800 MHz voltage (700 kV) thru 2008. ATOP days 4.-6. 3. 2009 RF limitations

  27. Transverse Damper SPS 4 x 25 kW RF System runs stably RAS ATOP days 4.-6. 3. 2009 RF limitations

  28. SPS Limitations • Voltage and power OK up to ultimate LHC intensity for nominal cycle (7.5 s). For faster cycles, see next slide for maximum current! • Coupled bunch instability requires 800 MHz system, which is presently running marginally! • Power 200 MHz: • coaxial lines limited to 750 kW • ageing tetrodes (more wear with CNGS operation!) • power couplers: ceramics upgraded, but still limited (transition coupler – cavity). • Losses at flat bottom at high intensity – presently not well understood (e-cloud?). E. Chapochnikova ATOP days 4.-6. 3. 2009 RF limitations

  29. SPS TWC200: Accelerating voltage in one 5-section cavity Vacc/MV nominal LHC (1.15 1011 ppb, 25 ns) 750 kW 550 kW 450 kW 350 kW Ib/A (200 MHz component) Extrapolated from: G. Dôme: “The SPS Acceleration System”, CERN-SPS/ARF/77-11 ATOP days 4.-6. 3. 2009 RF limitations

  30. Interventions – let the statistic speak! # of interventionsper week outsidenormal working hours mentioned in D. Manglunki’s talk weeks with piquet service • From 2005, limited resources forced us to reduce some maintenance work. • CNGS type beams result in more wear and thus reduce tube-lifetime (16! tubes broken in 2008!) • This is more related to maintenance and high power than high intensity E. Montesinos ATOP days 4.-6. 3. 2009 RF limitations

  31. SPS 800 MHz system upgrade programme The upgrade to more modern IOT is in full swing (white paper). IOT’s are used widely for digital TV transmitters (DVB-T). At present in the “Market Survey” state. Modular: Each line will be composed of 4 identical PA “cubicles” A cubicle can produce 60 kW, 4 cubicles will make up one transmitter, a total of 2 transmitters is needed. Present planning: 1 (test) cubicle in 2009, 4 cubicles in 2011, completion in 2013. ATOP days 4.-6. 3. 2009 RF limitations

  32. Summary “RF Limitations” • All systems OK up to nominal LHC. • Main Issues: • PSB C04 with strong beam loading at low voltage • PSB transverse damper installed power marginal • PS C10 with strong beam loading at low voltage • PS around transition – under investigation • PS coupled bunch instability and residual impedances • SPS 200 MHz: frequent interventions, requires regular maintenance • SPS 800 MHz – renovation underway (IOT based) ATOP days 4.-6. 3. 2009 RF limitations

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