R. M. Jones (ESGARD OMIA/SRF-AS FP7, CERN, ) 11 1 High Precision SC Cavity Alignment/Diagnostics/BPM with HOM Measurements Nicoleta Baboi, Olivier Napoly, Ursula van Rienen Roger M. Jones DESY, CEA Saclay, Univ. of Rostock, Univ. of Manchester/ Cockcroft Inst.

R. M. Jones (ESGARD OMIA/SRF-AS FP7, CERN, ) 22 2

33 3  HOM based monitors can radically contribute to the improvement of the beam quality in existing accelerator facilities FLASH at DESY and ERLP at Daresbury Laboratory, as well as the stretched wire test HOM characterisation facility at the Cockcroft Institute, CMTF at DESY, and further facilities using superconducting cavities such as the future XFEL, ILC and 4GLS.  The existing international work with SLAC and FNAL, USA, and KEK, Japan is expected to continue as it has important implications on maintaining beam quality in the development of the 16,000 or more main linac accelerating cavities for the ILC.  Participating institutes: - DESY, Germany - Cockcroft Institute, UK - Dept. of Physics and Astronomy, University of Manchester, UK - CEA, Saclay, France - Institute of General Electrical Engineering, University of Rostock, Germany - ASTeC, Daresbury Laboratory, UK HOMs in SC Accelerator Cavities

R. M. Jones (ESGARD OMIA/SRF-AS FP7, CERN, ) 44 4 HOMs in SC Accelerator Cavities In addition to the fundamental accelerating mode, SC cavities support a spectrum of higher order modes In accelerator physics HOM modes are generally considered a source of problems: Beam breakup, emittance dilution, HOM heating, etc. TE111-6 TM011-3

R. M. Jones (ESGARD OMIA/SRF-AS FP7, CERN, ) 55 5 HOMs for Diagnostics Here we attempt to harness the HOMs –Beam Diagnostics (position and phase) –Structure Diagnostics (cavity and cell alignment) –Cavity diagnostics (cavity shape).

R. M. Jones (ESGARD OMIA/SRF-AS FP7, CERN, ) 66 6 Why Use HOM Modes Beam Diagnostics: –HOM Modes must be coupled, and signals brought out to room temperature for damping Beam line and Cryogenic components already present Electronics is low cost –Large fraction of the linac length is occupied by structures –Measures beam relative to structures Structure Diagnostics: –HOM modes measure interior of structure –Other considerations There is enough information provided with each band of the structure to allow detailed information on the cavities to be recovered –cell to cell alignment HOM mode centers can be offset by couplers – not directly tied to cavity center (typically at 100 micron level).

R. M. Jones (ESGARD OMIA/SRF-AS FP7, CERN, ) 7 HOM based Beam Position Monitors (HOMBPM)  Initial electronics have been developed for single bunch and installed at FLASH allowing the beam to be centered to within 5  m.  Method needs to be verified with additional modes  Multi-bunch issues need to be understood.  The 3.9 GHz bunch shaping cavities being installed in FLASH and XFEL can readily dilute the beam emittance –important to instrument with electronics modules to diagnose the beam position and improve the emittance. 2. HOM based phase monitors (HOMPM)  Measure phase of RF injected into the cavities with respect to the phase of beam.  Proof of principle tests revealed the potential of a HOM-based method.  Additional LLRF development at FLASH needed. Aspects of HOMs in SC Accelerator Cavities

R. M. Jones (ESGARD OMIA/SRF-AS FP7, CERN, ) 8 HOM Cavity Diagnostics and ERLP (HOMCD)  HOM spectrum allows one to ascertain the cavity alignment and cell geometry.  Will investigate: - mechanical deviations of individual cells from the ideal geometry, - cell-to-cell misalignment, - deformation of fields by couplers.  This part of the project requires beam-based measurements at FLASH, DESY and ERLP, Daresbury, and RF-based measurements using the wire test facility at the Cockcroft Institute. 4. HOM Distributions and Geometrical Dependences (HOMDG)  Combining finite element and S-matrix cascading techniques allows the eigenmodes in multiple accelerating cells and cavities to efficiently modeled. The University of Rostock and the University of Manchester have developed a suite of codes.  Will apply these powerful computing methods in order to specify allowable tolerances on fabrication and alignment of the TESLA cells and cavities for future colliders and light sources. 5. HOM studies for Proton Injectors (HOMPI)  HOMS also impact the beam emittance and cause heating of the cavity in proton machines  Has the potential to benefit from similar HOM measurement diagnostics

R. M. Jones (ESGARD OMIA/SRF-AS FP7, CERN, ) 99 9 Work PackageNameCoordinating Institute/Univ. 1HOMBPMCockcroft/Univ. Manchester 2HOMPMDESY 3HOMCDCockcroft/Univ. Manchester 4HOMDGUniv. Rostock 5HOMPICEA, Saclay Aspects of HOMs in SC Accelerator Cavities

R. M. Jones (ESGARD OMIA/SRF-AS FP7, CERN, )  Response of HOM modes to beam

R. M. Jones (ESGARD OMIA/SRF-AS FP7, CERN, )  11 11

R. M. Jones (ESGARD OMIA/SRF-AS FP7, CERN, )  HOM modes in FLASH SC Structures  Monopole, Dipole, and higher order bands  Near speed of light modes have strongest coupling to beam.  Frequencies from simulations in  R. Wanzenberg “Monopole, Dipole and Quadrupole Passbands of the TESLA 8-cell Cavity”,TESLA  Monopole bands  1.28 to 1.3 GHz  Includes accelerating mode at 1.3GHz (R/Q 511 Ohms)  Low R/Q for other modes  2.38 – 2.45 GHz  Some modes with R/Q ~75 Ohms.  Used here for phase measurements  Dipole Bands  GHz  TE111-6, at 1.7GHz has strong coupling to beam  Used for beam position measurements  GHz  2 modes also have strong coupling to beam  Not currently used for position measurements, but available

R. M. Jones (ESGARD OMIA/SRF-AS FP7, CERN, )  HOM-based Studies at FLASH Center beam in TESLA accelerating cavity/module –reduce transverse emittance growth Cavity alignment in cryo-modules with 8 cavities –module misalignment wrt other accelerator components Beam position monitoring Relative RF phase measurement Cryo-module with 8 cavities

R. M. Jones (ESGARD OMIA/SRF-AS FP7, CERN, )  dump bunch compressors collimator section bypass line 5 accelerating modules with 8 cavities each gun undulators FEL beam 1.3GHz SC, typically MeV, 1 nC charge for FLASH/XFEL HOMs generated in accelerating cavities must be damped. These HOMs may also be monitored to obtain beam/cavity info Forty cavities exist at FLASH. Couplers/cables already exist. Electronics installed to monitor HOMs (wideband and narrowband response).

R. M. Jones (ESGARD OMIA/SRF-AS FP7, CERN, )  HOM Signals Dipole Mode 1.7GHz 4 MHz frequency span shown Monopole Modes 100 MHz frequency span shown Broadband system data

R. M. Jones (ESGARD OMIA/SRF-AS FP7, CERN, )  Narrow-band Measurements ~1.7 GHz tone added for calibration purposes. Cal tone, LO, and digitiser clock all locked to accelerator reference. Dipole modes exist in two polarisations corresponding to orthogonal transverse directions. The polarisations may be degenerate in frequency, or may be split by the perturbing affect of the couplers, cavity imperfections, etc. Difficult to determine their frequencies precisely.

R. M. Jones (ESGARD OMIA/SRF-AS FP7, CERN, )  Analysis of Narrowband Signals – Beam Position Small differences in each cavity lead to differing values of the frequency split, etc. With eighty signals to analyse (40 cavities with 2 couplers each), it is difficult to find the frequency and Q for each one. Instead, use SVD to find major “modes” for each cavity. –Must find ≥4 SVD modes as the beam has 4 transverse degrees of freedom. Find correlation between the amplitude of each of these modes and the transverse position of the beam.

R. M. Jones (ESGARD OMIA/SRF-AS FP7, CERN, )  Analysis of Narrowband Signals – Beam Position Resolution of position measurement. –Predict the position at cavity 5 from the measurements at cavities 4 and 6. –Compare with the measured value. X resolution –9 microns Y resolution –4 microns

R. M. Jones (ESGARD OMIA/SRF-AS FP7, CERN, )  HOM-BPMs Calibration based on SVD Resolution achieved: –~5/10  m rms (Y/X) single bunch, limited by LO Issues: –improve resolution –multi-bunch: individual bunches measurable with lower resolution (f rep ≤ 1MHz at FLASH) speed issues –suitability of various HOMs –alternative electronics –electronics for 3.9GHz cavities needed

R. M. Jones (ESGARD OMIA/SRF-AS FP7, CERN, )  Cavity Alignment Within a Structure Find correlation between HOM mode amplitudes and position (from conventional BPMs). Calculate position corresponding to minimum HOM power in cavity Ideally would like to find position and angle corresponding to minimum HOM power for each cavity –Need further beam position / angle scans to cover full range Plot position for minimum HOM power for multiple data runs

R. M. Jones (ESGARD OMIA/SRF-AS FP7, CERN, )  Measurement of Cavity Alignment in Cryo-Modules at FLASH –measurement made for two FLASH cryo-modules –alignment better that specs (300 mm rms) results from various measurements (mm)

R. M. Jones (ESGARD OMIA/SRF-AS FP7, CERN, )  Monopole Modes for Beam Phase Measurement  For FELs and the Linear Collider beam phase relative to RF is important at the < 0.1 degree L-band (<200 femtosecond level)  Monopole HOM mode phase determined by beam arrival time  HOM couplers measure both HOM and fundamental RF fields in cavity  High power accelerating fields blocked by superconducting filter, but significant power leaks through  Can use same coupler, cable, and electronics to measure accelerator RF and HOM modes  Allows low drift measurement of beam vs. RF phase.

R. M. Jones (ESGARD OMIA/SRF-AS FP7, CERN, )  HOM-based Phase Measurement Measure RF phase wrt beam –compare monopole modes from HOM-band with fundamental 1.3GHz mode (leaking through HOM coupler), through same cable Proof of principle made –noise: 0.08deg in relative phase measurement –potential for better resolution with specialized electronics –absolute phase measurement in progress

R. M. Jones (ESGARD OMIA/SRF-AS FP7, CERN, )  HOM Studies for Proton Injectors (SPL) HOM in 704 MHz SC cavities: – Analysis of the effect on the beam emittance, – R/Q dependence on cavity  and beam velocity – Study of the possible means of compensation, – Specification of solution (e.g. HOM dampers). Cavity/beam alignment: – Development of a measurement technique using dipole modes (pick up design, simulation) – Development of a correction method exploiting the measurement technique.

R. M. Jones (ESGARD OMIA/SRF-AS FP7, CERN, )  Frequency Domain S 11 Simulations for Several Wire Offsets  Stretched wire facility at Cockcroft used to measure 3.9 GHz Crab cavities  Resonances located at dipole modes  Area under S 21 ~ Z l (beam impedance)  Fourier transform enables wake-field and kick factors to be calculated  Bench-top measurement allows rapid determination of cavity modes (sync. freqs and kick factors).  Also allows cavity alignment to be determined.  Use method to determine modes in main linac cavities (1.3 GHz) Crab Cavity Measurement Set-Up Synergy with Crab Cavity Measurement

R. M. Jones (ESGARD OMIA/SRF-AS FP7, CERN, )  Simulations of Trapped Modes in RE Cavities Cornell Single-Cell RE Cavity Parameters for GdfidL Simulations 9-Cell RE Cavity Trapped Modes in RE Cavity

R. M. Jones (ESGARD OMIA/SRF-AS FP7, CERN, )  Wake-fields and Beam Dynamics in RE Cavities Wake-field Along Bunch Train Emittance Dilution Vs Percentage Change In Bunch Spacing Brillouin Diagram For RE Cavity  Solid line correspond to single-cell infinite periodic structure  Circles correspond to simulations made with 9 cell cavity (4.5 cells with E-wall and H-wall simulated separately) Multi-Cell Kick Factors  A small change in the bunch spacing (corresponding to a systematic error in the cels freqs) results in disturbingly large emittance dilution. For details see Jones et al, Linac 2006

R. M. Jones (ESGARD OMIA/SRF-AS FP7, CERN, )  Coupled S-Parameter Computation Coupled S parameter Calculation  Calculate individual cells precisely  Exploit technique developed to cascade individual cells  Obtain S-matrix and resonance modes of complete structure  Already made calculations on modules to investigate trapped modes  Needs experimental verification at FLASH

R. M. Jones (ESGARD OMIA/SRF-AS FP7, CERN, )  Multi-Bunch Operation (Future) For low repetition rate machines (bunch rate comparable to HOM mode decay time) –System is linear –Data acquisition system can be synchronous with bunch rate –Possible to de-convolve signal to get single bunch response Measure single bunch response Subtract signals from previous bunches to measure beam on each bunch. –Initial results at FLASH indicate this is succcessful. –Will allow procedure to be used for XFEL and ILC. For high repetition rate (near CW) machines –Need signal at HOM frequency –In principal can slightly modulate beam intensity at HOM frequency –Requires additional study

R. M. Jones (ESGARD OMIA/SRF-AS FP7, CERN, )  Cavity Diagnostics (Future)  Using broadband system can measure HOM mode amplitudes and phases as a function of beam orbit for all cavities in a structure  Cavity shape diagnostic  Measure relative centers of Dipole (and possibly Quadrupole) HOMs  Possibly measure Lorentz force detuning in high gradient structures?

R. M. Jones (ESGARD OMIA/SRF-AS FP7, CERN, )  Summary HOM Diagnostics (1)  Demonstrated –Cavity alignment measurement with ~5/10 micron reproducibility –HOM based feedback to minimize dipole HOM power –Beam position with 5 micron, 5 micro-radian resolution Potential for significant improvement –Beam phase vs. main RF phase measurement at the <0.1 degree level  Future –Verify initial results obtained on a single mode with higher bands –Multi-bunch operation is needed –Cavity diagnostics on the implications of geometrical distortions and cavity misalignments on mode distribution –3.9 GHz being installed at FLASH have potential to damage beam emittance –Emittance dilution can be minimised with this technique

R. M. Jones (ESGARD OMIA/SRF-AS FP7, CERN, )  Summary HOM Diagnostics (2) Inter-lab,University Collaboration –Entails 3 labs and 2 Universities and we will exploit the synergy –Supports the existing infrastructure of these labs. –We are already engaged in optimising the financial demands of this WP Acknowledgements  Joe Frisch, Nicoleta Baboi, Olivier Napoly,Ursula van Rienen, Steve Molloy, Ian Shinton, Chris Glasman for various discussions and materials for this presentation.  Prof. Swapan Chattopadhyay, Director of Cockcroft Inst, for supporting the goals of this project.

R. M. Jones (ESGARD OMIA/SRF-AS FP7, CERN, )  Additional Slides

R. M. Jones (ESGARD OMIA/SRF-AS FP7, CERN, )  HOM Measurement Systems: 2 Systems in Use  Broadband system –2.5 GHz bandwidth, 5 Gs/s 8 bit digitization, 4 simultaneous channels –Based on high bandwidth oscilloscopes –Used with 200MHz bandwidth filter for monopole measurements  Narrowband system –20MHz Bandwidth, 108Ms/s 14 bit digitization, 80 simultaneous channels –Frequency centered on TE111-6 mode (1.7 GHz) –Based on frequency down conversion electronics –Used for dipole mode measurements

R. M. Jones (ESGARD OMIA/SRF-AS FP7, CERN, )  Theoretical best position resolution is ~6 nm. Given the cable losses, the protective attenuator in the circuit, and the noise factor, the theoretical res is ~130 nm. The theoretical angle resolution is ~2 u.rad. Resolution Energy coupled into a mode: Minimum detectable energy is dominated by thermal noise: The measured resolutions are ~5 um and ~150 u.rad There are two main reasons for this disparity: The amplitude of the HOMs are proportional to the beam charge, so the signals must be normalised by the toroid output. The FLASH toroids have a noise of ~0.6%, and so contribute strongly to the measurement error. The LO signal in the electronics has a phase error of ~1 degree. This causes mixing between position and angle, and leads to a degradation in their resolution.