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Superfast BPM Processor Scheme and First Results

Superfast BPM Processor Scheme and First Results. Stephen Molloy, QMUL 2 nd Mini-Workshop for Nano-Project at ATF. FONT at ATF. Micron-level stabilisation of the ATF extraction line beam. Measure position of start of train, and correct the end. To be accomplished within 56ns train.

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Superfast BPM Processor Scheme and First Results

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  1. Superfast BPM Processor Scheme and First Results Stephen Molloy, QMUL 2nd Mini-Workshop for Nano-Project at ATF

  2. FONT at ATF • Micron-level stabilisation of the ATF extraction line beam. • Measure position of start of train, and correct the end. • To be accomplished within 56ns train. • Latency must be kept low. • BPM processor should work in <5ns.

  3. Processor Design • Obtain difference from hybrid and mix with 714MHz from ATF control system. • Low-pass filter with 200MHz cutoff • 5-pole Chebyshev chosen due to low latency, and strong out of band fall-off.

  4. Issues • Non-zero hybrid isolation causes zero-offset. • BPM centre can be moved with variable attenuation if necessary. • Stripline -> hybrid cables must be matched in time to better than ~50ps. • This is possible, and has been achieved. • 714MHz LO input to the mixer should be very phase stable with respect to the beam. • A lot of power at the beam bunching frequency. • Low-pass filter must limit this to a very small value before signal reaches feedback amplifier.

  5. Phase Stability of 714MHz

  6. Simulated Output - 100μm

  7. Comments • Positive points • Simulated output is linear with beam position. • Hybrid common-mode residual shifts BPM centre in a predictable way. • Imperfect cable lengths (within achievable limits) merely shift BPM centre. • Mixer leakage easily reduced by low-pass filter. • Potential problems • Relatively large amount of power at beam bunching frequency remains after low-pass filter. • Predicted latency of entire system is ~6ns. • Remember latency should be <5ns for feedback experiment.

  8. Alternative Filtering • 3 or 4-pole Bessel band-pass filter before mixer. • Centred at 714MHz, bandwidth ~400MHz. • Reduces 357MHz entering mixer. • Reduces out of band power entering mixer, thus increasing dynamic range of BPM. • Less 357MHz means low-pass filter requirements are relaxed. • Less poles results in faster filter. • 3-pole Chebyshev with ~170MHz cut-off. • Total latency should be equal to or less than the previous scheme as the band-pass poles have a larger bandwidth than the low-pass poles

  9. Simulated Response of 3-pole Bessel BPF

  10. Simulated Output - 100μm

  11. Simulated Output - 1μm

  12. Comments • 357MHz beam bunching is not observed. • Power at 714MHz • Due to mixer leakage. • Level equivalent to DC output when beam has 1μm displacement. • Predicted latency ~4.5ns • Longer by ~0.5ns when 4-pole band-pass filter is used.

  13. Recent Beam Tests • Single bunch tests • Verified cable lengths were correct. • Stepped through each component in turn and verified signal. • Found correct LO phase. • Verified output was correlated with beam position. • Measured latency.

  14. Recent Beam Tests • Multi-bunch tests • Calibrated each of three processors using corrector magnets. • Recorded many extraction pulses to measure resolution.

  15. Reminder!

  16. Single Bunch - Raw Signals

  17. Single Bunch - After hybrid

  18. Single Bunch - After Band-Pass Filter

  19. Single Bunch – After Mixer

  20. Single Bunch – Final Difference Output

  21. Single Bunch - Corrector Sweep

  22. Latency Measurement • Triggered the scope with a sum signal. • Recorded raw stripline signal, and final output of processor. • Results • 3-pole band pass scheme – 4.2+-0.2ns • 4-pole band pass scheme – 7.3+-0.2ns • No band pass, 5-pole low pass – 6.1+-0.2ns

  23. Multibunch – 3-pole band-pass filter

  24. Multibunch – 4-pole band-pass filter

  25. Multibunch – No bandpass, 5-pole lowpass

  26. Calibration Run – BPM13

  27. Bunch charge during Dec 9th shift

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