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RF Distribution System Insights for MICE Collaboration Meetings

Learn about RF distribution, triode circuits usage, and recommendations from RAL, CERN, Brookhaven, and Fermilab for the MICE project at the collaboration meetings held in October 2007 and 2008.

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RF Distribution System Insights for MICE Collaboration Meetings

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  1. MICE RF distribution system Andrew Moss ASTeC Collaboration meeting, DL, 19th – 23th October 2008

  2. Contents • RF distribution • Advice on using the triode circuits • RAL • CERN • Brookhaven • Fermilab • Coax power handling • Waveguide system design • discussion Collaboration meeting, RAL, 7th – 10th October 2007

  3. Triodes • Advice has been sought from RAL, CERN, Fermilab and Brookhaven on how triodes of this type are connected to accelerating structures • In general, the triode is very tolerant of reflected power • However methods are used in certain cases to increase the isolation between amplifiers and cavities • Also methods are used to balance RF power to multiple accelerating structures driven by one active device Collaboration meeting, RAL, 7th – 10th October 2007

  4. RAL • On the ISIS linac, the TH116 triode circuits use 12 inch output coax • The amplifiers use an output tap on the grid circuit to take off RF power, this is considered the best approach to maximise RF power transfer • The RF power is taken through a 12 inch coaxial motorised trombone to set the phase for each linac tank • No device to isolate the reflected power is used, although the trombone phase shifter may well be shifting reflecting power and reducing standing waves • ISIS runs at 50Hz 1mSec ? Considerably higher PRF than MICE will Collaboration meeting, RAL, 7th – 10th October 2007

  5. CERN • At CERN the TH170 triode is used • This is a direct plug in replacement for the 116 but has different characteristics in terms of gain and output power • The CERN amplifiers use 2 output taps (6 inch coax) to couple power out of the amplifier circuit • A multitude of pulse regimes are used with these circuits • The circuits are connected to linac tanks without reflected power protection • CERN are doubtful that > 1.8MW can be extracted from the circuits Collaboration meeting, RAL, 7th – 10th October 2007

  6. Brookhaven • At Brookhaven triodes are used to power Linacs • The RF distribution system includes high power trombones to move reflected power away from the amplifier and hybrids are used to split power to differing sections of the Linac • They would recommend that the MICE RF systems follows this same format, it allows for more isolation and adjustability of the coax distribution system Collaboration meeting, RAL, 7th – 10th October 2007

  7. Fermilab • At Fermilab the MTA is being operated now with the MICE cavity • System consists of an amplifier circuit some distance away, with a 9 inch coax feed line • In the cavity test area a motorised trombone is used to set phase into the cavity, and also to ‘move resonance away from the amplifier’ • Power is split close to the cavity using a 3dB splitter and the coax size reduced first to 6 inch and then to 4 inch coax, which is pressurised with SF6 • Fermilab recommend the same approach for MICE, however they also recommend using a hybrid splitter rather than the 3dB type, this will isolated the cavity couplers to 30dB with respect to each other Collaboration meeting, RAL, 7th – 10th October 2007

  8. MICE 116 Amplifiers • The amplifiers from Berkeley have a have single 9”coax output, this is a capacitive output coupling, not sure what the power limit of this style of output is • 2 CERN style large amplifiers have twin 6” coax outputs, these are a direct grid tap , generally a better way to couple power out , however CERN are doubtful at achieving more than 1.6MW without issues in the coax lines • when are these going to arrive ? • May need to adapt them to suit our systems/needs in terms of controls and electrical interface Collaboration meeting, RAL, 7th – 10th October 2007

  9. MICE cavity • The MICE cavity will have a fill time of <200uSec • This means that at the start of each pulse, total reflection of power will occur • Cavity filling time is shown • A triode can tolerated this reflection Collaboration meeting, RAL, 7th – 10th October 2007

  10. Coax • Coax system can be designed with minimum phase error at cavity inputs • Cavity input couplers used to match up coax feeds • ‘Flexible’ sections used to take up misalignments in coax • Copper or Aluminium section possible, aluminium easier to work with – lighter ! • Cavity couplers need SF6 (used on MTA couplers) to stand off RF voltages • For the MICE coax system suggest using nitrogen pressurised to 1 bar, coax system will provide a slow leak Collaboration meeting, RAL, 7th – 10th October 2007

  11. RF power into Arcs • RF power likes nothing better than to dump itself into an arc ! • Once an arc has imitated, all of the power will dissipate into that area potentially causing damage to surfaces, and if the problem is not detected, burning of the coax system will result and possibly ‘holes’ • Filling the coax system with pressured gas (nitrogen, SF6) will increase breakdown resistance and provide a useful interlock to stop running in the event of a ‘hole’ • Other interlocks to protect against these issues include arc detectors and RF power level comparators Collaboration meeting, RAL, 7th – 10th October 2007

  12. Coax power handling Peak power level 3MW Average power 130kW From Mega Industries Collaboration meeting, RAL, 7th – 10th October 2007

  13. Spinner 6 1/8 coax parameters Collaboration meeting, RAL, 7th – 10th October 2007

  14. Peak power handling More conservative numbers* From Andrews 1.5 MW Collaboration meeting, RAL, 7th – 10th October 2007

  15. Collaboration meeting, RAL, 7th – 10th October 2007

  16. Waveguide feeder • Cavity is fed from both sides, flexible sections to take up alignment errors, couplers can be used to adjust incoming phase (by how much?) • Probe style phase shifters could be used to adjust further if necessary, limited range but remote control possible – relatively small • Advice suggests that hybrid power splitters be used rather than 3dB types, greater isolation between the output arms and hence the cavity couplers, these items are not that expensive • Advice suggests that a trombone phase shifter be used on the output of the tube to move any resonance out of band, these are large devices Collaboration meeting, RAL, 7th – 10th October 2007

  17. Feeder design • Using this design is more costly, larger in terms of space required but a safer design - easier to set up, more adjustable, greater tolerance to problems • This will more closely follow the MTA design, which is proven, expert advice says it should be done this way • The hybrid will isolate the amplifier in a limited manner, phase combined signals coming back at the hybrid will be split into the load and back into the input arm – advice suggests triode not concerned with reflected power anyway- may simply help to make system more robust Collaboration meeting, RAL, 7th – 10th October 2007

  18. MICE hall GA for DL/LBNL Amplifiers Collaboration meeting, RAL, 7th – 10th October 2007

  19. MICE hall GA for CERN Amplifiers Collaboration meeting, RAL, 7th – 10th October 2007

  20. Block diagram of amplifier system Collaboration meeting, RAL, 7th – 10th October 2007

  21. Waveguide distribution Collaboration meeting, RAL, 7th – 10th October 2007

  22. Hybrids and loads • Using hybrids requires a reject load, this will need to be rated to 1MW for voltage stand off = large device! • Possible to find another type, none find so far 50/50 Water/glycol coolant 83 inch’s long 12 inch input port Collaboration meeting, RAL, 7th – 10th October 2007

  23. RF testing in the MICE hall • RF testing will be possible with the MICE hall searched and interlocked, RF personnel on limited access • For amplifier testing, coax switches will be used to isolate cavity and direct power into the test load • These switches will be interlocked by the PSS system, only allowing RF test mode when all cavities are removed from the system • Possible that one load can be used with a combiner, or a pair of loads to simulate cavity Collaboration meeting, RAL, 7th – 10th October 2007

  24. LLRF • Possibility of using Larry Doolittle’s LLRF4 board for monitoring and controlling mice amplifier chain • In house development possible, however DL RF group has a lot to do already • Outside help possible at a cost, need to check on more details • Money is in the budget for LLRF at ~ 20K per cavity system • This would provide all the measurement of power levels, digital interlocks and controlling functions for the amplifier chain • Power supplies are expected to look after them selves • May leave a few interlocks for the main control system that would be hardwired into the amplifier enable line Collaboration meeting, RAL, 7th – 10th October 2007

  25. Discussion • Amplifier issues • What is needed for the CERN contribution • Hall issues, plan to finalise hall layout by July 09 • Power supply issues • Simply a matter of work effort to finish them • Waveguide issues • The current design is back to the more complex, larger system with trombones and hybrids/loads, will it fit ? • On the positive side this will provide a more robust system that has already been demonstrated at the MTA • Cost, all of the components are in the original budget • Planning issues • Plan for 2009 is tight but we thin achievable • Electrical plan for stage 6 available, includes RF amplifier milestones Collaboration meeting, RAL, 7th – 10th October 2007

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