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System for High Resolution Detection of Beam Induced Transients in RF Signals

System for High Resolution Detection of Beam Induced Transients in RF Signals. P. Pawlik, M. Grecki , S. Simrock. Introduction. The goal of the accelerator is to provide particle beam with low spatial and energy dispersion

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System for High Resolution Detection of Beam Induced Transients in RF Signals

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  1. System for High Resolution Detection of Beam Induced Transients in RF Signals P. Pawlik, M. Grecki, S. Simrock

  2. Introduction • The goal of the accelerator is to provide particle beam with low spatial and energy dispersion • The goal of the rf system is maintaining the phase and amplitude of the rf signal within a given tolerance with respect to the beam • The measurement and adjustment of the phase of the accelerating field with respect to the beam phase is essential for the operation of an accelerator max(Vacc) => beam=0 - on crest acceleration

  3. Requirements for beam phase calibration • The actual requirements depend on uncorrelated detuning errors induced by microphonics and Lorentz force detuning • few degree in RF phase - initial coarse phasing of the linac • up to 0.1 degree - the bunch compressor of the X-FEL (that can be achieved by observation of parameters such as bunch length or beam arrival time) • 1 degree in phase and 1% in gradient in presence of microphonics of up to 10 degrees - vector-sum calibration for the European X-FEL (to guarantee a stability of the vector-sum of 32 cavities of about 0.01% in amplitude and 0.01 degree in phase) • The required precision for the beam phase measurement should be obtained possible with very small beam current to avoid unnecessary activation of the beam line or even melting a hole in the beam line

  4. Methods Of Beam Phase Calibration • The measurements of the beam energy change with the beam diagnostics (spectrometer). At maximum energy gain the beam will be operated on crest (beam=0) of the accelerating field • The measurements of the changes in gradient with and without beam during open loop operation • In closed loop operation the additional measurement of the forward or reflected wave is needed • The measurements of single bunch transients - for bunch charge of 8nC (ILC), 1nC (X-FEL) or even lower (down to 100pC) • for European X-FEL operated at gradients of up to 25MV/m a single bunch with a charge of 1nC will induce a transient of the order of 2e-4 in respect to the RF field, which must be resolved with a precision of a few percent. This implies a relative accuracy of the order of a few 1e-6

  5. Single bunch going through the cavity induces a vector change in the RF field. Phase of this vector field change called transient depends on the phase between beam and the RF Field. For 0º of phase between beam and RF Field (on crest acceleration) we get 180º phase difference between transient vector and RF field vector in the cavity. Single Bunch Induced Transients

  6. Filtration: Subtraction of actual signal from delayed one, by precisely adjusting phase difference at the input of second combiner to 180º and magnitudes to the same value. To achieve 100dB attenuation precision better then 0.1° and 1e-5 in magnitude is required. Amplified output of the filter by 75dB has information about transient vector as a pulse with width equal 100ns (delay of the delay line) Beam Induced Transient Detection

  7. Beam Induced Transient Detection Hardware

  8. The procedure of filter adjustment Set min. gain adjust phase and attenuation to obtain min. output signal increase gain N finish? Y do the measurement

  9. To detect transients that are more then 60dB smaller then the carrier, low noise method is required Phase detection with AD8347 Analog Devices IQ demodulator failed (noise was too large) The application of Schottky diode based vector detector was succesful Low Noise Magnitude And Phase Detection Based on The Low of Cosines

  10. Measurement Results (1) 1nC Single Bunch Induced Transient 2nC Single Bunch Induced Transient

  11. Measurement Results (2) Measurement results of calculated beam phases from captured single bunch induced transients for 3 different bunch charges. Measurements at cavity 3 module ACC1. Expected value  -10°. Measurement of 3nC single bunch induced transient (phase calibrated for direct beam phase calculation).

  12. Sample5 Expected beam phase = 45 ° Measured phase = 41.6 ° Measurement error = -3.4 ° Measured magnitude = 4.09e-4 Results of new measurements (2.06.2005)

  13. Measurement Conditions For 110nC Transients: Module ACC1, cavity 3 Gradient 14.75 MV/m 3.67nC bunch charge 30 bunches Feedback off Measurement Conditions For 2nC Transients: Module ACC1, cavity 3 Gradient 14.75 MV/m 2nC bunch charge 1 bunch Feedback off Results of new measurements (2.06.2005) Beam Phase Measurement Results Transient Magnitude Measurement Results

  14. Conclusions And Outlook • Conclusions • Presented method is promising for amplitude and phase calibration in linear accelerators • Beam phase measured with this method differ only slightly from expected values • Automation of the filter will make measurements more convenient and less time consuming • Temperature stabilization will make measurements more precise and robust • Work In Progress • Automatic filter adjustment hardware • Connection to DOOCS • Temperature stabilization of Hardware • Plans For Next Tests • Automation of measurements with Matlab scripts (Measurements may take only several minutes) • Measurements in a different module ACC5 • Measurements in different cavities

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